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APR 2 1 mn
i-
THE CELL
IN DEVELOPMENT AND INHERITANCE
Columbia ^Snibcrsito Biological Scries.
«
EDITED BY
HENRY FAIRFIELD OSBORN
AND
* EDMUND B. WILSON,
1. FROM THE CREEKS TO DARWIN.
By Henry Fairfield Osborn, Sc.D. Princeton.
2. AMPHIOXUS AND THE ANCESTRY OF THE VERTEBRATES.
By Arthur Willey. B.Sc. Lond. Univ.
3. FISHES. LIVINC AND FOSSIL. An Introductory Study.
By Bashford Dean, Ph.D. Columbia.
4. THE CELL IN DEVELOPMENT AND INHERITANCE.
By Edmund B. Wilson, Ph.D. J.H.U.
5. THE FOUNDATIONS OF ZOOLOGY.
By William Keith Brooks.
COLUMBIA UNIVERSITY BIOLOGICAL SERIES. IV
THE CELL
IN
Development and Inheritance
BY
EDMUND B. WILSON, Ph.D.
PROFESSOR OF ZOOLOGY, COLUMBIA UNIVERSITY
SECOND EDITION REVISED AND ENLARGED
" Natura nusquam magis est tota quam in minimis
PLINY
THE MACMILLAN COMPANY
LONDON: MACMILLAN & CO., LTD. 3>3oV^vv\>A\— 1904
AN rights reserved
Copyright, 1896, By the MACMILLAN COMPANY.
Copyright, 1900, By the MACMILLAX COMPANY.
Set up and electrotyped October, 1896. Reprinted September, 1897; September, 1898.
New edition, revised, set up and electrotyped January, 1900; March, 1902 ; June, 1904.
XorSuooti 39rc33
J. S. Gushing & Co. — Berwick & Smith Norwood, Mass. U. S. A.
Co mo fxitnH THEODOR BOVERI
144370
PREFACE
This volume is the outcome of a course of lectures, delivered at Columbia University in the winter of 1892-93, in which I endeavoured to give to an audience of general university students some account of recent advances in cellular biology, and more especially to trace the steps by which the problems of evolution have been reduced to problems of the cell. It was my first intention to publish these lectures in a simple and general form, in the hope of showing to wider circles how the varied and apparently heterogeneous cell- researches of the past twenty years have grown together in a coherent group, at the heart of which are a few elementary phe- nomena, and how these phenomena, easily intelligible even to those having no special knowledge of the subject, are related to the problems of development. Such a treatment was facilitated by the appearance, in 1893, of Oscar Hertwig's invaluable book on the cell, which brought together, in a form well designed for the use of special students, many of the more important results of modern cell-research. I am glad to acknowledge my debt to Hert- wig's book ; but it is proper to state that the present volume was fully sketched in its main outlines at the time the Zelle mid Gcwcbc appeared. Its completion was, however, long delayed by investiga- tions which I undertook in order to re-examine the history of the centrosomes in the fertilization of the ^gg, — a subject which had been thrown into such confusion by Fol's extraordinary account of the " Quadrille of Centres " in echinoderms that it seemed for a time impossible to form any definite conception of the cell in its relation to inheritance. By a fortunate coincidence the same task was inde- pendently undertaken, nearly at the same time, by several other investigators. The concordant results of these researches led to a decisive overthrow of Fol's conclusions, and the way was thus cleared for a return to the earlier and juster views founded by Hertwig, Strasburger, and Van Beneden, and so lucidly and forcibly developed by Boveri.
The rapid advance of discovery in the mean time has made it seem desirable to amplify the original plan of the work, in order to render it useful to students as well as to more general readers ; and to this end it has been found necessary to go over a considerable
vii
Vlll PREFACE
part of the ground already so well covered by Hertwig.i This book does not, however, in any manner aim to be a treatise on general histology, or to give an exhaustive account of the cell. It has rather been my endeavour to consider, within moderate limits, those features of the cell that seem more important and suggestive to the student of development, and in some measure to trace the steps by which our present knowledge has been acquired. A work thus limited neces- sarily shows many gaps ; and some of these, especially on the botani- cal side, are, I fear, but too obvious. On its historical side, too, the subject could be traced only in its main outlines, and to many investigators of whose results I have made use it has been impossible to do full justice.
To the purely speculative side of the subject I do not desire to add more than is necessary to define some of the problems still to be solved ; for I am mindful of Blumenbach's remark that while Drelin- court rejected two hundred and sixty-two "groundless hypotheses" of development, ** nothing is more certain than that Drelincourt's own theory formed the two hundred and sixty-third." ^ i have no wish to add another to this list. And yet, even in a field where standpoints are so rapidly shifting and existing views are still so widely opposed, the conclusions of the individual observer may have a certain value if they point the way to further investigation of the facts. In this spirit I have endeavoured to examine some of the more important existing views, to trace them to their sources, and in some measure to give a critical estimate of their present standing, in the hope of finding suggestion for further research.
Every writer on the cell must find himself under a heavy obliga- tion to the works of Van Beneden, Oscar Hertwig, Flemming, Stras- burger, and Boveri ; and to the last-named author I have a special sense of gratitude. I am much indebted to my former student, Mr. A. P. Mathews, for calling my attention to the importance of the recent work of physiological chemists in its bearing on the problems of synthetic metabolism. The views developed in Chap- ter VII. have been considerably influenced by his suggestions, and this subject will be more fully treated by him in a forthcoming work ; but I have endeavoured as far as possible to avoid anticipating his own special conclusions. Among many others to whom I am indebted for kindly suggestion and advice, I must particularly mention my ever helpful friend. Professor Henry F. Osborn, and Professors J. E. Humphrey, T. H. Morgan, and F. S. Lee.
In copying so great a number of figures from the papers of other
1 Henneguy's Le(;ons sur la cellule is received, too late for further notice, as this volume is going through the press. ^ Allen Thomson.
PREFACE ix
investigators, I must make a virtue of necessity. Many of the facts could not possibly have been illustrated by new figures equal in value to those of special workers in the various branches of cytological research, even had the necessary material and time been available. But, apart from this, modern cytology extends over so much debatable ground that no general work of permanent value can be written that does not aim at an objective historical treatment of the subject; and I believe that to this end the results of investigators should as far as practicable be set forth by means of their original figures. Those for which no acknowledgment is made are original or taken from my own earlier papers.
The arrangement of the literature lists is as follows. A general list of all the works referred to in the text is given at the end of the book (p. 449). These are arranged in alphabetical order, and are referred to in the text by name and date, according to Mark's con- venient system. In order, however, to indicate to students the more important references and partially to classify them, a short separate list is given at the end of each chapter. The chapter-lists include only a few selections from the general list, comprising especially works of a general character and those in which reviews of the special literature may be found.
E. B. W.
Columbia University, New York, July, 1896.
PREFACE TO THE SECOND EDITION
Since the appearance of the first edition of this work, in 1896, the aspect of some of the most important questions with which it deals has materially changed, most notably in case of those that are f ocussed in the centrosome and involve the phenomena of cell-division and fertilization. This has necessitated a complete revision of the book, many sections having been entirely rewritten, while minor changes have been made on almost every page.
In its first form, the work was compressed within limits too nar- row for a sufficiently critical treatment of many disputed subjects. It has therefore been considerably enlarged, and upwards of fifty new illustrations have been added. The endeavour has, however, still been made to keep the book within moderate Hmits, even at some cost of comprehensiveness ; and the present edition aims no more than did the first to cover the whole vast field of cellular biology. Its limita- tions are, as before, especially apparent in the field of botanical cytology. Here progress has been so rapid that, apart from the dif- ficulty experienced by a zoologist in the attempt to maintain a due sense of proportion in reviewing the subject, an adequate treatment would have required a separate volume. I have therefore, for the most part, considered the cytology of plants in an incidental way, endeavouring only to bring the more important phenomena into rela- tion with those more fully considered in the case of animals.
The steady and rapid expansion of the literature of the general subject renders increasingly difficult the historical form of treatment and the citation of specific authority in matters of detail. This plan has nevertheless still been followed as far as possible, despite the increased bulk of the book and the encumbrance of the text with references thus occasioned, in the hope that these disadvantages will be outweighed by increased usefulness of the work. I beg the reader to remember, however, that no approach to a complete history of cytology and experimental embryology could be attempted, save in a work of far greater proportions, and that it has been necessary
XI
xii PREFACE TO THE SECOXD EDITION
to pass by, or dismiss with very brief mention, many works to which space would gladly have been given.
Recent research has yielded many new results of high interest, conspicuous among them the outcome of experiments on cell-division, fertilization, and regeneration ; and they have cleared up many special problems. Broadly viewed, however, the recent advance of discovery has not, in the author's opinion, tended to simplify our conceptions of cell-life, but has rather led to an emphasized sense of the diversity and complexity of its problems. " One is sometimes tempted to con- clude," was recently remarked by a well-known embryologist, " that every <t%g is a law unto itself ! " The jest, perhaps, embodies more of the truth than its author would seriously have maintained, express- ing, as it does, a growing appreciation of the intricacy of cell-phe- nomena, the difficulty of formulating their general aspects in simple terms, and the inadequacy of some of the working hypotheses that have been our guides. It is in the full recognition of such inade- quacy, when it exists, and of the danger of hasty generalization in a subject so rapidly moving as this, that our best hope of progress lies.
My best thanks are again due to many friends for helpful criti- cism, suggestion, and other aid ; and I am indebted to Professor Ulric Dahlgren for the beautiful preparation imperfectly represented by Fig. 59 (from a direct photograph); to F. Emil, E. M. Van Harlin- gen, and Dr. G. N. Calkins, for aid in the preparation of new illus- trations ; and to Messrs. Ginn & Co. for the electrotypes of Figs. 1 1, 12, and 1 88, from the Wood's Holl Biological Lectures for 1899.
Columbia University, December 7, 1S99.
Postscript. — Of especial importance for some of the discussions in Chapters I., V., and VII. are F'ischer's extensive work on protoplasm (see Literature, I.) and Strasburger's new researches on reduction (see Literature, V.), both received while this volume was in press and too late for more than a passing mention in the text.
March, 1900.
TABLE OF CONTENTS
INTRODUCTION
PAGE
List of Figures xvii
Historical Sketch of the Cell-theory; its Relation to the Evolution-theory. Earlier Views of Inheritance and Development. Discovery of the Germ-cells. Cell- division, Cleavage, and Development. Modern Theories of Inheritance. Lamarck,
Darwin, and Weismann ........... i
Literature ............... 14
CHAPTER I General Sketch of the Cell
A. General Morphology of the Cell .......... 19
B. Structural Basis of Protoplasm .......... 23
C. The Nucleus ............. 30
1. General Structure . . . . . . . . . . -31
2. Finer Structure of the Nucleus . . . . . . . , -37
3. Chemistry of the Nucleus .......... 41
D. The Cytoplasm ............. 41
E. The Centrosome ............. 50
F. Other Cell-organs ............. 52
G. The Cell-membrane ............ 53
H. Polarity of the Cell 55
I. The Cell in Relation to the Multicellular Body 58
Literature, I. 61
CHAPTER II Cell-division
A. Outline of Indirect Division or Mitosis
B. Origin of the Mitotic Figure
C. Details of Mitosis
1. Varieties of the Mitotic Figure
(a) The Achromatic Figure (^) The Chromatic Figure
2. Bivalent and Plurivalent Chromosomes
3. Mitosis in the Unicellular Plants and Animals
4. Pathological Mitoses .....
65 72
77
78
78 86
87 88
97
xni
XIV
TABLE OF COX TEXTS
D. The xMechanism of Mitosis .....
1. function of the .Aniphiaster
(/7) Theory of Kil)rillar Contractility {b) Other Facts and Theories
2. Division of the Chromosomes
E. Direct or Amitotic Division .....
1. General Sketch ......
2. Centrosome and Attraction-sphere in .Xmitosis
3. Biological Significance of .Vmitosis
F. Summary and Conclusion ..... Literature, II. .
PAGE
100
100 100 106
I \2 114 114
'•5 116
119
121
CIIAPTKR III The Germ-cells
A. The Ovum ....
1. The Nucleus
2. The Cytoplasm .
3. The Egg-envelopes li. The Spermatozoon
1. The Flagellate Spermatozoon
2. Other Forms of Spermatozoa
3. Paternal Germ-cells of Plants
C. Origin of the Germ-cells
D. Growth and Differentiation of the Cierm-cells
1. The Ovum ....
(a) Growth and Nutrition
{b) Differentiation of the Cytoplasm.
{c) Yolk-nucleus .
2. Origin of the Spermatozoon
3. Formation of the Spermatozoids in Plants
E. * Staining-reactions of the Germ-nuclei Literature, III. .....
Deposit of Deutoplasm
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CHAPTER IV Fertiliz.\tion of the Ovi
A. General Sketch .......
1. The Germ-nuclei in Fertilization
2. The Achromatic Structures in Fertilization .
B. Union of the Germ-cells .....
1. Immediate Results of Union
2. Paths of the Germ-nuclei ....
3. Union of the Germ-nuclei. The Chromosomes
C. The Centrosome in Pertilization ....
D. Fertilization in Plants ......
E. Conjugation in Unicellular Forms
F. Summary and Conclusion ..... Literature, IV. ........
M
180
iSi
185
196
200
202
204
208
215 222
229 231
TABLE OF CONTENTS
XV
CHAPTER V
Reduction of the Chromosomes, Oogenesis and Spermatogenesis
PAGE
A. Genera' Outline ............. 234
1, Reduction in the Female. The Polar Bodies ...... 236
2, Reduction in the Male. Spermatogenesis . . . . . . .241
3, Weismann's Interpretation of Maturation ....... 243
B. Origin of the Tetrads 246
1. General Sketch ............ 246
2. Detailed Evidence ........... 248
C. Reduction without Tetrad-formation ......... 258
D. Some Peculiarities of Reduction in the Insects . . . . . . -271
E. The Early History of the Germ-nuclei ......... 272
F. Reduction in Unicellular Forms . . . . . . . . . -277
G. Maturation of Parthenogenetic Eggs 2S0
Appendix
1. Accessory Cells of the Testis ......... 284
2. Amitosis in the Early Sex-cells ......... 285
H. Summary and Conclusion ........... 285
J- iterature, V. .............. 287
CHAPTER VI
t
Some Problems of Cell-organization
A. The Nature of Cell-organs
B. Structural Basis of the Cell
C. Morphological Composition of the Nucleus
I. The Chromatin . . . . . • - .
{a) Hypothesis of the Individuality of the Chromosomes (/;) Composition of the Chromosomes ....
D. Chromatin, Linin, and Cytoplasm
E. The Centrosome . .■ .
F. The Archoplasmic Structures
1. Hypothesis of Fibrillar Persistence
2. The Archoplasm Hypothesis
3. The Attraction-sphere
G. Summary and Conclusion Literature, VI
291
293
294
294
294
301
302
304
31^ 316
3>S
327 328
CHAPTER VII
Some Aspects of C-iLL-cHEMisTRV and Cell-phvsiology
A. Chemical Relations of Nucleus ana Cytoplasm
1. The Proteids and their Allies
2. The Nuclein Series ....
3. Staining-reactions of the Nuclein Series
330
332 334
XVI
TABLE OF COXTENTS
B. Physiological Relations of Nucleus and Cytoplasm
1. Experiments on Unicellular Organisms
2. Position and Movements of the Nucleus
3. The Nucleus in Mitosis
4. The Nucleus in Fertilization
5. The Nucleus in Maturation
C. The Centrosome .....
D. Summary and Conclusion Literature. VII
PAGE 340
o:)
I
J3- 353 354 358 359
CHArrr.R viii
Cell-division and Development
A. Geometrical Relations of Cleavage-forms
B. Promorphological Relations of Cleavage
1. Promorphology of the Ovum
(^/) Polarity and the Egg-axis
(<^) Axial Relations of the Primary Cleavage-planes
(r) Other Promorphological Characters of the Ovum
2. Meaning of the Promorphology of the Ovum
C. Cell-division and Growth .......
Literature, VIIL . .
362 37S 378 37S
379 382
3S4 394
CHAPTER IX Theories of Lmiekitance and Development
A. The Theory of Germinal Localization .
B. The Idioplasm Theory .....
C. Union of the Two Theories ....
D. The Roux-Weismann Theory of Development
E. Critique of the Roux-Weismann Theory
F. On the Nature and Causes of Differentiation
G. The Nucleus in Later Development H. The External Conditions of Development
I. Development, Inheritance, and Metabolism . J. Preformation and Epigenesis. The Unknown Factor in Development Literature, IX.
Glossary
General Literature- list Lndex of Authors . Index of Subjects .
397 401
403 404
407
413
425 428
430 431 434
437
449
471
477
LIST OF FIGURES
INTRODUCTION
1. Epidermis of larval salamander
2. Section of growing root-tip of the onion
3. Avioeba Proteus . . . . .
4. Cleavage of the ovum in Toxopneustes .
5. Diagram of inheritance . .
PAGE
4 1 1
CHAPTER I
6. Diagram of a cell ......•••
7. Spermatogonia of salamander ......
Group of cells, showing cytoplasm, nucleus, and centrosomes Living cells of salamander larva, showing fibrillar structure . Alveolar or foam-structure of protoplasm, according to BiitschU Structure of protoplasm in the echinoderm e^ Aster-formation in alveolar protoplasm . Nuclei from the crypts of Lieberkiihn . Special forms of nuclei ....
Scattered nucleus in Trachelocerca Scattered nucleus in Bacteria and Flagellata Ciliated cells . Cells of amphibian pancreas
8.
9. 10.
II.
12.
13- 14.
15- 16.
17- 18.
19.
20.
21.
22.
2^.
Nephridial cell of Clepsine
Nerve-cell of frog .
Diagram of dividing cell
Diagrams of cell-polarity
Centrosomes in epithelium and in blood-corpuscles
18 20 21
24 26
27 28
32 35 37 39 43 44 45 47 49 56 57
CHAPTER 11
24. Remak's scheme of cell-division
25. Diagram of the prophases of mitosis
26. Diagram of later phases of mitosis
27. Prophases in salamander-cells
28. Metaphase and anaphases in salamander-cells
29. Telophases in salamander-cells
30. Mid-body and cell-plate in cells of Umax
31. Middle phases of mitosis in Ascaris-Qgg%
32. Mitosis in Stypocaulon ....
xvii
64 66
69
73
75 76
79 80 Si
XVlll
LIST OF FIGURES
FIG.
2,2i- Mitosis in Erysiphe .......
34. Mitosis in pollen-mother-cells of lily, according to Guignard
36. Mitosis in spore-cells of Eqiiisetiim
37. Heterotypical mitosis
38. Mitosis in Infusoria
39. Mitosis in Etiglypha
40. Mitosis in Euglena
41. Mitosis in Acanthocystis
42. Mitosis in Noctiliica
43. Mitosis in Paramaba
44. Mitosis in Actinospharium
45. Mitosis in Actinosphivriiim .
46. Pathological mitoses in cancer-cells
47. Pathological mitosis caused by poisons
48. \'an Beneden's account of astral systems in Ascaris
49. Leucocytes .....
50. Pigment-cells ....
51. Heidenhain's model of mitosis
52. Mitosis in the egg of Toxopneusies
53. Pathological mitoses in polyspermy
54. Nuclei in the spireme-stage .
55. Early division of chromatin in Ascaris
56. Amitotic division ....
PAGE
83 84
85
87
89 90
91 92
93
95 96
97 98
99 100 102 103 104 107 109 112
"3
"5
CHAPTER III
57. Volvox
58. Ovum of Toxopneustss .
59. Ovum of the cat ....
60. Ovum of Nereis ....
61. Germinal vesicles of Unio and Epeira
62. Insect-egg .....
63. Micropyle in Argonauta
64. Germ-cells of Volvox
65. Diagram of the flagellate spermatozoon
66. Spermatozoa of fishes and amphibia
67. Spermatozoa of birds and other animals
68. Spermatozoa of mammals
69. Unusual forms of spermatozoa
70. Spermatozoids of Chara
71. Spermatozoids of various plants
72. Germ-cells of Cladonema
73. Primordial germ-cells of Ascaris .
74. Primordial germ-cells of Cyclops .
75. Ovarian ova and follicles of Helix
76. Egg and nurse-cells in Ophryotrocha
77. Ovarian eggs of insects .
78. Young ovarian eggs of various animals
79. Young ovarian eggs of birds and mammals
80. Ovarian eggs of spider, earthworm, ascidian, showing yolk-nucleus
^^2, 126
127
129
130
132
134 135 136 138 140 141 142
143 146
147 149
152 153 154
155 157
LIST OF^FIGURES
XIX
FIG.
8i. Ova.na.n eggs oi Limti/tis and Po/yzoniuf/i ....
82. Formation of the spermatozoon in Ajiasa ....
83. Transformation of the spermatids of the salamander
84. P'ormation of the spermatozoon in Salamandra and Amphinma
85. The same in Helix and in elasmobranchs ....
86. The same in mammals .......
87. Formation of spermatozoids in cycads .....
88. Formation of spermatozoids in cryptogams ....
PAGE
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CHAPTER IV
89. Fertilization of Physa .
90. Fertilization of Ascaris
91. Germ-nuclei of Nematodes ,
92. Fertilization of the mouse .
93. Fertilization of Pterotrachea
94. Entrance and rotation of sperm-head in Toxopneustes
95. Conjugation of the germ-nuclei in Poxopnensies .
96. Diagrams of fertilization .....
97. Fertilization of Nereis ......
98. Fertilization of Cyclops .....
99. Fertilization and persistence of centrosomes in Thalassema
100. Entrance of spermatozoon into the egg
101. Pathological polyspermy ....
102. Polar rings of Clepsine ....
103. Paths of the germ-nuclei in Toxopneustes
104. Fertilization of Myzostoma ....
105. Fertilization of Pihilaria ....
106. Penetration of the pollen-tube in angiosperms
107. Fertilization of the lily ....
108. Fertilization in Zamia ....
109. Diagram of conjugation in Infusoria
110. Qoxi]\xgzX\ox\ oi Pa ramie ciiim
111. Conjugation of Vorticella ....
112. Con]\xgz\.\on o( Noctiltcca ....
113. Co\\]\xg2ii\on oi Spirogyra ....
180
184 185 186 187 189 190 191
193 195 197 199
201 203 209 216 217 219 220 22 ? 225 226 227 228
CHAPTER V
114. Polar bodies in Toxopneustes
115. Genesis of the egg
116. Diagram of formation of polar bodies
117. VoXaxhoCixtsm. Ascaris
118. Genesis of the spermatozoon
119. Diagram of reduction in the male
120. Spermatogenesis of .'^5<r<7;-?5
121. Diagrams illustrating tetrad-formation
122. Tetrads of Gryllotalpa
123. Tetrads and polar bodies in Cyclops
o o
234 235 237 239
240 242
244
247
249
250
XX
LIST OF FIGURES
FIG.
124.
125.
126.
127.
128.
129.
130.
132. 134.
135- 136.
137- 138.
139- 140.
141.
142.
Diagrams of tetrad-formation in arthropods
Germinal vesicles and tetrads
Maturation in Auasa ....
Maturation in Anasa ....
Diagrams of reduction
Maturation in Thalassema .
Maturation in Thalassema and Zirphiza
Maturation in Salamamira .
The maturation-divisions in angiosperms
Maturation in I. ilium ....
Maturation in Liliiim ....
Diagrams of reduction in the flowering plants
Ovary of Canthocamptus ....
Polar spindles without centrosomes
Polar bodies in Actinophrys
Polar bodies in Acti)wsphiErinm .
Conjugation and reduction in Closteriiim
First type of parthenogenetic maturation in Arlemia
Second type of parthenogenetic maturation in Arlemia
PAGE
252 254 255 259 260
261 262 264 266 268 270
273 276
278
278
279
282
28 -.
CHAPTER VI
143. Abnormalities in the fertilization of ^j<r^;-7,f
144. G'xzni Qvnhxyo oi A scaris ......
145. Individuality ofchromosomes in Ascuris
146. Independence of chromosomes in fertilization of Cyclops
147. I lybrid fertilization of /^5frt'r;.y .....
148. Mitosis with intranuclear centrosome in Ascaris .
149. Abnormal mitoses in Plemerocallis ....
150. Centrosomes in Chctlopteriis and Cerehratiiliis
151. Artificially produced asters and centrosomes in echinoderms
152. Diagram of different types of centrosome and centrosphere
153. P(j]ar mitoses in Diaiilula .......
154. Astral systems in Ufiio .......
155. Astral systems in Cerebratiilus and Thalassema .
156. Structure of the aster in spermatogonium of salamander
295 296 297 298 300
305 306
307 308
310
^12
3^3 320
326
CHAPTER VII
157. History of chromosomes in the germinal vesicle of sharks
158. Nucleated and enucleated fragments oi Slylonychia
159. Regeneration in Slenlor ......
160. Nucleated and enucleated fragments of . -////«'/'<? .
161. Nucleated and enucleated fragments of plant-protoplasm
162. Position of nuclei in plant-cells .
163. Ovzxy o{ Forjictila ....
164. Normal and dwarf larvse of sea-urchins
165. Supernumerary centrosome in Ascaris
166. Cleavage of dispermic egg of Toxopneiisles
167. Centrosomes and cilia ....
339 342 343 344 345 347 349 352
355 356 357
LIST OF FIGURES
XXI
FIG.
i68. 169. 170. 171. 172.
173- 174.
175-
176.
177. 178. 179. 180. iSi.
CHAPTER VIII
Geometrical relations of cleavage-planes in plants
Cleavage of Synapta ....
Cleavage of Polygordiiis
Cleavage of Nereis ....
Variations in the third cleavage .
Meroblastic cleavage in the squid
Rudimentary cells in Aricia
Teloblasts of the earthworm
Contradiction of Hertwig's rule in Ascaris
Bilateral cleavage in tunicates
Bilateral cleavage in Loligo .
Eggs of Loligo .....
Eggs and embryos of Corixa
Variations in axial relations of Cyclops
PAGE
365 367 369 370 11^
■\m -% J/ J
374 376 380
381 382
383 385
CHAPTER IX
182. Half-embryos of the frog ....
183. Half and whole cleavage in sea-urchins
184. Normal and dwarf gastrulas of .-iw//z/^jr«5 .
185. Dwarf and double embryos of ^;;/////£'a-?^5' .
186. Cleavage of sea-urchin eggs under pressure .
187. Cleavage of A^'^rm-eggs under pressure
188. Diagrams of cleavage in mollusks and polyclades
189. Partial larva; of ctenophores
190. Partial cleavage in Ilyanassa
191. Double embryos of frog ....
192. Cleavage in Crepidula ....
193. Normal and modified larvre of sea-urchins .
194. Regeneration in coelenterates
400 407 40S 409 411 412
414
418 420 421 424 428 429
INTRODUCTION
-OO^iO^OO-
" Jedes Thier erscheint ah eine Sunwie vitaler Einheiten, von denen jede den vollen Charakter des Lebens an sich tragt." \'lRCHO\v.i
During the half-century that has elapsed since the enunciation of the cell-theory by Schleiden and Schwann, in 1838-39, it has become ever more clearly apparent that the key to all ultimate biological problems must, in the last analysis, be sought in the cell. It was the cell-theory that first brought the structure of plants and animals under one point of view, by revealing their common plan of organization. It was through the cell-theory that Kolliker, Remak, Nageli, and Hof- meister opened the way to an understanding of the nature of embryo- logical development, and the law of genetic continuity lying at the basis of inheritance. It was the cell-theory again which, in the hands of Goodsir, Virchow, and Max Schultze, inaugurated a new era in the history of physiology and pathology, by showing that all the various functions of the body, in health and in disease, are but the outward expression of cell-activities. And at a still later day it was through the cell-theory that Hertwig, Fol, Van Beneden, and Strasburgcr solved the long-standing riddle of the fertilization of the ^gg and the mechanism of hereditary transmission. No other biological generali- zation, save only the theory of organic evolution, has brought so many apparently diverse phenomena under a common point of view or has accomplished more for the unification of knowledge. The cell-theory must therefore be placed beside the evolution-theory as one of the foundation stones of modern biology.
And yet the historian of latter-day biology cannot fail to be struck with the fact that these two great generalizations, nearly related as they are, have been developed along widely different lines of research, and have only within a very recent period met upon a common ground. The theory of evolution originally grew out of the study of natural history, and it took definite shape long before the ultimate structure of living bodies was in any degree comprehended. The evolutionists of the Lamarckian period gave little heed to the finer details of internal organization. They were concerned mainly with the more
^ Cellularpathologie, p. 12, 1S5S.
B I
2 IXTRODUCTION
obvious characters of plants and animals — their forms, colours, habits, distribution, their anatom\' and embryonic development — and with the systems of classification based upon such characters ; and long afterward it was, in the main, the study of like characters with reference to their historical origin that led Darwin to his splen- did triumphs. The study of microscopical anatomy, on which the cell-theory was based, lay in a different field. It was begun and long carried forward with no thought of its bearing on the origin of living forms ; and even at the present day the fundamental problems of organization, with which the cell-theory deals, are far less accessible to historical inquiry than those suggested by the more obvious external characters of ])lants and animals. Only within a few years, indeed, has the ground been cleared for that close alliance between students of organic evolution and students of the cell, which forms so striking a feature of latter-day biology and is exerting so great an influ- ence on the direction of research. It has, therefore, only recently become possible adequately to formulate the great problems of devel- opment and heredity in the terms of cellular biology — indeed, we can as vet do little more than so formulate them. Yet the fact that these two great lines of research, both concerned with the deeper problems of life, yet having their beginnings so far apart, have at length converged to a meeting-point, is one of the more striking evidences of progress that modern biology has to show ; and it sufficiently justifies an attempt to treat the cell from the standpoint of the general student of development.
Let us at the outset briefly outline the cell-theory as thus regarded, and indicate the manner of its historical connection with the general problems of evolution.^
^ Schleiden and Schwann are universally and justly recognized as the founders of the cell- theory; hut like every other great generalization the theory was based on a long series of earlier investigations l)eginniiig with the memorable microscopical researches of l.eeuwen- hoek, Mali)ighi, lIo(>ke, and (jrew in the second lialf of the seventeenth century.
Wolff, in the Theoria Cenerationis (1759), clearly recognized the "spheres" and "vesi- cles" composing the embryonic parts both of animals and of plants, though without grasping iheir real nature or mode of origin, and his conclusions were developed by the botanist Mirbel at the beginning of the i^resent century. Nearly at the same time (1805) Oken fore- shadowed the cell-theory in the form that it assumed with Schleiden and Schwann; but his conception of " Urschleim " and " Hlaschen " can hardly be regarded as more than a lucky guess. A still closer approximation to the truth is fuuntl in the works of 'ruri)in (1826), Meyen (1830), Raspail (1831), and Dutrochet (1837); '^"^^ these, like others of the same period, only paved the way for the real founders of the cell-theory. Among other immedi- ate predecessors f)r contemporaries of Schleiden and Schwann should be especially mentioned Robert Brown, Dujardin, Johannes Miiller, I'urkinje, Hugo von Mohl, Valentin, Unger, Nageli, and Henle. The significance of Schleiden's, and especially of Schwann's, work lies in the thorough and comprehensive way in which the problem was studied, the philosophic breadth with which the conclusions were developed, and the far-reaching influence which they exercised upon subsequent research. In this respect it is hardly too much to com- pare the Mikroikopische L'ntersiichnngcn with the Origin of Species.
INTR OD UC TION 3
During the past thirty years the theory of organic descent has been shown, by an overwhelming mass of evidence, to be the only tenable conception of the origin of diverse living forms, however we mav conceive the causes of the process. While the study of general zoology and botany has systematically set forth the results, and in a measure the method, of organic evolution, the study of microscopical
a
X
Fig i._ A portion of the epidermis of a larval salamander {Amblystoma) as seen in slightly oblique horizontal section, enlarged 550 diameters. Most of the cells are polygonal m form, con- tain large nuclei, and are connected by delicate protoplasmic bridges. Above v is a branched, dark pigment-cell that has crept up from the deeper layers and lies between the epidermal ceLs. Three of the latter are undergoing division, the earliest stage {sp,rcme) at a, ^J l-';»«^r/»-'f ("^"^"^ figure in the anaphase) at b, showing the chromosomes, and a final stage {telophase), showing fission of the cell-body, to the right.
anatomy has shown us the nature of the material on which it has operated, demonstrating that the obvious characters ot plants and animals are but varving expressions of a subtle interior organization common to all. In its broader outlines the nature of this organiza- tion is now accurately determined; and the ''cell-theory," by which it is formulated, is, therefore, no longer of an inferential or hypo-
^ INTRODUCriOy
thetical character, but a generalized statement of observed fact which may be outlined as follows : — *
In all the higher forms of life, whether plants or animals, the body may be resolved into a vast host of minute structural units known as cells, out of which, directly or indirectly, every part is built (Figs. 1,2). The substance of the skin, of the brain, of the blood, of the bones or muscles or any other tissue, is not homogeneous, as it appears to the unaided eye, but is shown by the microscope to be an aercrretrate composed of innumerable minute bodies, as if it were a
Fig. 2. — General view of cells in the growing root-tip of the onion, from a longitudinal section, enlarged 800 diameters.
a. non-dividing cells, with chromatin-network and deeply stained nucleoli ; b. nuclei preparing for division (spireme-stage) ; <r. dividing cells showing mitotic figures; e. pair of daughter-cells shortly after division.
colony or congeries of organisms more elementary than itself. The name cells given to these bodies by the early botanists, and ulti- mately adopted by nearly all students of microscopical anatomy, was not happily chosen ; for modern studies have shown that although the cell may assume the form of a hollow chamber, as the name indicates, this is not one of its characteristic or even usual features. Essentially the cell is a minute mass of protoplasm, a substance long since identified by Cohn, Leydig, Max Schultze, and De Bary as the essential active basis of the organism, afterward happily characterized
INTRODUCTION
5
by Huxley as the *' physical basis of life," and at the present time universally recognized as the immediate substratum of all vital activity.^ Endlessly diversified in the details of their form and struc- ture, these protoplasmic masses nevertheless possess a characteristic type of organization common to them all; hence, in a certain sense, they may be regarded as elementary organic units out of which the body is compounded. This composite structure is, however, character-
3 ''.>;;
Fig. 3. — Amcela Proteus, an animal consisting of a single naked cell, x 280. (From Sedgwick and Wilson's Biology.)
n. The nucleus; iv.v. water-vacuoles ; c.v. contractile vacuole ; f.v. food-vacuole,
istic of only the higher forms of life. Among the lowest forms at the base of the series are an immense number of microscopic plants and animals, famiUar examples of which are the bacteria, diatoms, rhizo- pods, and Infusoria, in which the entire body consists of a single cell (Fig. 3), of the same general type as those which in the higher multi- cellular forms are associated to form one organic whole. Structurally, therefore, the multicellular body is in a certain sense comparable with a colony or aggregation of the lower one-celled forms.- This com-
1 The word protoplasm is due to Purkinje (1840), who applied it to the formative sub- stance of the animal embryo and compared it with the granular material of vegetable "cambium." It was afterward independently used by \\. von Mohl (1846) to designate the contents of the plant-cell. The full physiological signiticance of protoplasm, its identity with the "sarcode" (Dujardin) of the unicellular forms, and its essential similarity in plants and animals, was first clearly placed in evidence through the classical works of Max Schultze and De Bary, beside which should be placed the earlier works of Dujardin, L nger, Nageli, and Mohl, and that of Cohn, Huxley, Virchow, Leydig, Brucke, Kuhne, and Beale.
2 This comparison must be taken with some reservation, as will appear beyond.
6 IXTRODUCTIOX
parison is not less suggestive to the physiologist than to the mor- phologist. In the one-celled forms all of the vital functions are performed by a single cell. In the multicellular forms, on the other hand, these functions are not ecjualh- i)erformed b)- all the cells, but are in varving degree distributed among them, the cells thus falling into physiological groups or tissues, each ot which is especially de- voted to the performance of a specific function. Thus arises the "physiological division of labour" through which alone the highest development of vital activity becomes possible ; and thus the cell becomes a unit, not merely of structure, but also of function. luich bodilv function, and even the life of the organism as a whole, may thus in one sense be regarded as a resultant arising through the inte- gration of a vast number of cell-activities ; and it cannot be adequately investigated without the study of the individual cell-activities that lie at its root.^
The foregoing conceptions, founded by Schwann, and skilfully developed by Kolliker, Siebold, Virchow, and Haeckel, gave an im- pulse to anatomical and physiological investigation the force of which could hardly be overestimated; yet they did not for many years measurably affect the more speculative side of biological inquiry. The Origin of Species, published in 1859, scarcely mentions it; nor, with the important exception of the theory of pangenesis, did Darwin attempt at any later period, to bring it into any very definite relation to his views. The initial impulse to the investigations that brought the cell-theory into definite contact with the evolution-theory was given nearly twenty years after the Origin of Species, by researches on the early history of the germ-cells and the fertilization of the ovum. Begun in 1873-74 by Auerbach, Fol, and Butschli, and eagerly followed up b\- Oscar Hertwig, Van Beneden, Strasburger, and a host of later workers, these investigations raised wholly new questions regarding the mechanism of development and the role of the cell in hereditary transmission. Through them it became for the first time clearly apparent that the general problems of embryology, heredity, and evolution are indissolubly bound up with those of cell- structure, and can only be fully apprehended in the light of cytologi- cal research. As the most significant step in this direction, we may re£:ard the identification of the cell-nucleus as the vehicle of inheri-
1 Cf. pp. 58-61. " It is to the cell that the study of every bodily function sooner or later drives us. In the muscle-cell lies the problem of the heart-beat and that of muscular con- traction ; in the gland-cell reside the causes of secretion ; in the epithelial cell, in the white blood-cell, lies the problem of the absorption of food, and the secrets of the mind are hidden in the ganglion-cell. ... If then physiology is not to rest content with the mere extension of our knowledge regarding the gross activities of the human body, if it would seek a real explanation of the fundamental phenomena of life, it can only attain its end through the study of cell-physiology'''' (Verworn, Alkemeine Fhysiologie, p. 53, 1895).
INTRODUCTION y
tance, made independently and almost simultaneously in 18S4-85 by -Oscar Hertwig, Strasburger, Kolliker, and Wcismann/ while nearly at the same time (1883) the splendid researches of Van Beneden on the early history of the animal Qgg opened possibilities of research into the finer details of cell-phenomena of which the early workers could hardly have dreamed.
We can only appreciate the full historical significance of the new period thus inaugurated by a glance at the earlier history of opinion regarding embryological development and inheritance. To the modern student the germ is, in Huxley's words, simply a detached living por- tion of the substance of a preexisting living body ^ carrying with it a definite structural organization characteristic of the species. By the earlier embryologists, however, the matter was very differently re- garded ; for their views in regard to inheritance were vitiated by their acceptance of the Greek doctrine of the equivocal or spontaneous generation of life ; and even Harvey did not escape this pitfall, near as he came to the modern point of view. " The Qgg,'' he savs, " is the mid-passage or transition stage between parents and offspring, between those who are, or were, and those who are about to be ; it is the hinge or pivot upon which the whole generation of the bird revolves. The Qgg is the terminus from which all fowls, male and female, have sprung, and to which all their lives tend — it is the result which nature has proposed to herself in their being. And thus it comes that individuals in procreating their like for the sake of their species, endure forever. The egg, I say, is a period or por- tion of this eternity." ^
This passage appears at first sight to be a close approximation to the modern doctrine of s^erminal continuitv about which all theories of heredity are revolving. In point of fact, however, Harvey's view is only superficially similar to this doctrine ; for, as Huxley pointed out, it was obscured by his belief that the germ might arise *' spontaneously," or through the influence of a mysterious '' calidiun innaUmi,'' out of not-living matter."* Neither could Harvey, great physiologist and embryologist as he w^as, have had any adequate con- ception of the real nature of the ^gg and its morphological relation to
1 It must not be forgotten that Haeckel expressed the same view in 1866 — only, how- ever, as a speculation, since the data necessary to an inductive conclusion were not obtained until long afterward. "The internal nucleus provides for the transmission of hereditary characters, the external plasma on the other hand for accommodation i)« adaptation to the external world" {Gen. MorpJi., pp. 2S7-289).
2 Evolution in Biology, 1878; Science and Culture, p. 291. ^ De Generatione, 1651; Trans., p. 271.
^ Whitman, too, in a brilliant essay, has shown how far Harvey was from any real grasp of the law of cenetic continuitv. which is well characterized as the central fact of modern biology. Evolution and Epigenesis, Wood's HoU Biological Lectures, 1894.
8 JNTR OD UC TION
the body of which it forms a part, since the cclkilar structure of Uving things was not comprehended until nearly two centuries later, the spermatozoon was still undiscovered, and the nature of fertilization was a subject of fantastic and baseless speculation. For a hundred years after Harvey's time embryologists sought in vain t(^ penetrate the mysteries enveloping the beginning of the individual life, and despite their failure the controversial writings of this period form one of the most interesting chapters in the history of biology. By the extreme " evolutionists " or " prceformationists " the egg was believed to contain an embryo fully formed in miniature, as the bud contains the flower or the chrysalis the butterfly. Development was to them merely the unfolding of that which already existed ; inheritance, the handing down from parent to child of an infinitesimal re])roduction of its own body. It was the service of Bonnet to push this concep- tion to its logical consequence, the theory of eiJiboitciiicjit or encase- ment, and thus to demonstrate the absurdity of its grosser forms, pointing out that if the egg contains a complete embryo, this must itself contain eggs for the next generation, these other eggs in their turn, and so ad infinitum, like an infinite series of boxes, one within another — hence the term cniboitemcnt. Bonnet himself renounced this doctrine in his later writings, and Caspar Friedrich Wolff ( 1759) led the way in a return to the teachings of Harvey, showing by pre- cise actual observation that the egg does not at first contain any formed embryo whatever ; that its structure is wholly different from that of the adult; that development is not a mere process of unfolding, but involves the continual formation, one after an- other, of new parts, previously non-existent as such. This is some- what as Harvey, himself following Aristotle, had conceived it — a process of cpigcncsis as opposed to evolution. Later researches established this conclusion as the very foundation of embryological science.
But although the external nature of development was thus deter- mined, the actual structure of the egg and the mechanism of inheri- tance remained for nearly a century in the dark. It was reserved for Schwann (1839) and his immediate followers to recognize the fact, conclusively demonstrated by all later researches, that tJic egg is a cell having the same essential structure as other cells of the body. And thus the wonderful truth became manifest that a single cell may contain within its microscopic compass the sum-total of the heritage of the species. This conclusion first reached in the case of the female sex was soon afterward extended to the male as well. Since the time of Leeuwenhoek (1677) it had been known that the sperm or fertilizing fluid contained innumerable minute bodies endowed in nearly all cases with the power of active move-
INTRODUCTION
ment, and therefore regarded by the early observers as parasitic animalcules or infusoria, a view which gave rise to the name sperma- tozoa (sperm-animals) by which they are still generally known. ^ As long ago as 1786, however, it was shown by Spallanzani that the fertilizing power must lie in the spermatozoa, not in the liquid in which they swim, because the spermatic fluid loses its power when filtered. Two years after the appearance of Schwann's epoch-mak- ino- work Kolliker demonstrated (1841) that the spermatozoa arise directly from cells in the testis, and hence cannot be regarded as parasites, but are, like the ovum, derived from the parent-body. Not until 1865, however, was the final proof attained by Schweigger- Seidel and La Valette St. George that the spermatozoon contains not only a nucleus, as Kolliker believed, but also cytoplasm. It was thus shown to be, like the ^^,g, a single cell, peculiarly modified in structure, it is true, and of extraordinary minuteness, yet on the whole morphologically equivalent to other cells. A final step was taken ten years later (1875), when Oscar Hertwig established the all-important fact that fertilization of the egg is accomplished by its union with one spermatozoon, and one only. In sexual repro- duction, therefore, each sex contributes a single cell of its own body to the formation of the offspring, a fact which beautifully tallies with the conclusion of Darwin and Galton that the sexes play, on the whole, equal, though not identical parts in hereditary trans- mission. The ultimate problems of sex, fertilization, inheritance, and development were thus shown to be cell-problems.
Meanwhile, during the years immediately following the announce- ment of the cell-theory, the attention of investigators was especially focussed upon the question : How do the cells of the body arise .? The origin of cells by the division of preexisting cells was clearly recognized by Hugo von Mohl in 1835, though the full significance of this epoch-making discovery was so obscured by the errrors of Schleiden and Schwann that its full significance was only perceived long afterward. The founders of the cell-theory were unfortunately led'to the conclusion that cells might arise in two different ways, viz. either by division or fission of a preexisting mother-cell, or by "Iree cell-formation," new cells arising in the latter case not from pre- existing ones, but by crystallizing, as it were, out of a formative or nutritive substance, termed the " cytoblastema " ; and they even beheved the latter process to be the usual and typical one. It was only after many years of painstaking research that " free cell- formation " was absolutely proved to be a myth, though many of
iThe discovery of the spermatozoa is generally accredited to Ludwig Hamm. a pupil of Leeuwenhoek (1677). though Ilartsoeker afterward claimed the ment of havmg seen them as early as 1674 (Dr. Allen Thomson).
10 IXTRODUC TION
Schwann's immediate followers threw doubts upon it,^ and as early as 1855 Virchow positively maintained the universality of cell-divi- sion, contending that ever}- cell is the offs})ring of a preexisting parent-cell, and summing up in the since famous aphorism, " oniuis celliila c Cillula.^''^ At the ]:)resent day this conclusion rests upon a foundation so firm that we arc justified in regarding it as a universal law of development.
Now, if the cells of the body always arise by the division of pre- existing cells, all must be traceable back to the fertilized egg-cell as their common ancestor. Such is, in fact, the case in every plant and animal whose development is accurately known. The first step in development consists in the division of the <:.^^ into two j^arts, each of which is a cell, like the <t^^^ itself. The two then divide in turn to form four, eight, sixteen, and so on in more or less regular progres- sion (Fig. 4.) until step by step the f^gg has split up into the multitude of cells which build the body of the embryo, and finally of the adult. This process, known as the cleavage or segmentatioji of the ^^^^ was observed long before its meaning was understood. It seems to have been first definitely described in the case of the frog's ^^,^,, by Prevost and Dumas ( 1824), though earlier observers had seen it; but at this time neither the ^g'g nor its descendants were known to be cells, and its true meaning was first clearly perceived by Bergmann, Kolliker, Reichert, Von Baer, and Remak, some twenty years later. The interpretation of cleavage as a process of cell-division was fol- lowed by the demonstration that cell-division does not begin with cleavage, but can be traced back into the foregoing goieration ; for the egg-cell, as well as the sperm-cell, arises by the division of a cell pre- existing in the parent-body. // is therefore derived by direct descent from an egg-cell of the foregoing generation, and so on ad infinitnni. Embryologists thus arrived at the conception so vividly set forth by Virchow in 1858-'^ of an uninterrupted series of cell-divisions extend- ing backward from existing plants and animals to that remote and unknown period when vital organization assumed its present form. Life is a continuous stream. The death of the individual involves no breach of continuitv in the series of cell-divisions bv which the life of the race flows onwards. The individual body dies, it is true, but the germ-cells live on, carrying with them, as it were, the traditions of the race from which they have sprung, and handing them on to their descendants.
1 Among these may be especially mentiDncd Mohl, L'ngcr, Nageli, Martin liarry, Goodsir, and Remak.
2 Arch, fur Path. Anat., VIII.. p. 23, 1851;.
3 See the quotation from the original edition of the Celhdarpathologie at the head of Chapter II., p. 63.
INTRODUCTION'
II
We have thus arrived at the form in which the problems of heredity and development confront the investigator of the present day. It remains to point out more clearly how they are related to the general problems of evolution and to those post-Darwinian discussions in which Weismann has taken so active a part. All theories of evolu-
B
C
D
F
G ^ ^
Fig 4. — Cleavage of the ovum of the sea-urchin Toxopncustes, X 33°. ^''oni ''f^^- ^''^^ suc- cessive divisions up to the i6-cell stage (//) occupy about two hours. / is a section of the embryo (blastula) of three hours, consisting of approximately 128 cells surrounding a central cavity or blastocoel.
tion take the facts of variation and heredity as fundamental postulates, for it is by variation that new characters arise and by heredity that they are perpetuated. Darwin recognized two kinds of variation, both of which, being inherited and maintained through the conserving action of natural selection, might give rise to a permanent transfor- mation of species. The first of these includes congenital or mborn
12 INTR OD UCTIOJV
variations, i.e. such as appear at birth or are developed "spontane- ously," without discoverable connection with the activities of the organism itself or the direct effect of the environment upon it, though Darwin clearly recognized the fact that even such variations must indirectly be due to changed conditions acting upon the parental organism or on the germ. In a second class of variations were placed the so-called acquired characters, i.e. definite effects directly produced in the course of the individual life as the result of use and disuse, or of food, climate, and the like. The inheritance of congen- ital characters is now universally admitted, but it is otherwise with acquired characters. The inheritance of the latter, now the most debated question of biology, had been taken for granted by Lamarck a half-century before Darwin ; but he made no attempt to show how such transmission is possible. Darwin, on the other hand, squarely faced the physiological requirements of the problem, recognizing that the transmission of acquired characters can only be possible under the assumption that the germ-cell definitely reacts to all other cells of the body in such wise as to register the changes taking place in them. In his ingenious and carefully elaborated theory of pangenesis,^ Darwin framed a provisional physiological hypothesis of inheritance in ac- cordance with this assumption, suggesting that the germ-cells are reservoirs of minute germs or gemmules derived from every part of the body ; and on this basis he endeavoured to explain the trans- mission both of acquired and of congenital variations, reviewing the facts of variation and inheritance with wonderful skill, and buildinc: up a theory which, although it forms the most speculative and hypo- thetical portion of his writings, must always be reckoned one of his most interesting contributions to science.
In the form advocated by Darwin the theory of pangenesis has been generally abandoned in spite of the ingenious attempt to remodel it made by Brooks in 1883.- In the same year the whole aspect of the problem was changed, and a new'period of discussion inaugurated by Weismann, who put forth a bold challenge of the entire Lamarckian principle.'^ " I do not propose to treat of the whole problem of hered- ity, but only of a certain aspect of it, — the transmission of acquired characters, which has been hitherto assumed to occur. In taking this course I may say that it was impossible to avoid going back to the foundation of all phenomena of heredity, and to determine the sub- stance with which they must be connected. In my opinion this can only be the substance of the germ-cells ; and this substance trans-
1 Variation of Animals and Plants, Chapter XXVII.
2 The Law of Heredity, Baltimore, 1883.
3 Ueber Vererbtmg, 1883. See Essays upon Heredity, I., by A. Weismann, Clarendon Press, Oxford, 1889.
INTRODUCTION
fers its hereditary tendencies from generation to generation, at first unchanged, and always uninfluenced in any corresponding manner, by that which happens during the life of the individual which bears it. If these views be correct, all our ideas upon the transformation of species require thorough modification, for the whole princij^le of evolution by means of exercise (use and disuse) as professed by La- marck, and accepted in some cases by Darwin, entirely collapses" {I.e., p. 69).
It is impossible, he continues, that acquired traits should be trans- mitted, for it is inconceivable that definite changes in the body, or "soma," should so affect the protoplasm of the germ-cells as to cause corresponding changes to appear in the offspring. How, he asks, can the increased dexterity and power in the hand of a trained piano- player so affect the molecular structure of the germ-cells as to pro- duce a corresponding development in the hand of the child 'i It is a physiological impossibility. If we turn to the facts, we find, W'eis- mann affirms, that not one of the asserted cases of transmission of acquired characters will stand the test of rigid scientific scrutiny. It is a reversal of the true point of view to regard inheritance as taking place from the body of the parent to that of the child. The child inherits from the parent germ-cell, not from the parent-body, and the germ-cell owes its characteristics not to the body which bears it, but to its descent from a preexisting germ-cell of the same kind. Thus the body is, as it were, an offshoot from the germ-cell (Fig. 5). As
Line of succession.
\£) Line of inheritance.
G
Fig. 5. — Diagram illustrating Weismann's theory of inheritance.
G. The germ-cell, which by division gives rise to the body or soma (5) and to new germ-cells (G) which separate from the soma and repeat the process in each successive generation.
far as inheritance is concerned, the body is merely the carrier of the germ-cells, which are held in trust for coming generations.
Weismann's subsequent theories, built on this foundation, have given rise to the most eagerly contested controversies of the post- Darwinian period, and, whether they are to stand or fall, have played a most important part in the progress of science. For^aside_fromjhe truth or error of his special theories, it has been Weismann's great service to place the keystone between the work of the evolutionists and that of the cytologists, and thus to bring the cell-theory and the
14 I.\TK OD UC TION
evolution-theory into organic connection. It is from the point of view thus suggested that the present volume has been written. It has accordingly not been my primary object to dwell on the Diiuiitice of histology, still less to undertake an exhaustive description of all the modifications of cell-structure and cell-action ; and for these the stu- dent must refer to other and more extended treatises. Yet the broader questions with which we have to deal cannot ])rofitably be discussed apart from the concrete phenomena by which they are suggested, and hence a considerable part of the text is necessarily given over to descriptive detail ; but I hope that the reader will not lose sight of the relation of the part to the whole, or forget the primary intention of the work.
We shall follow a convenient, rather than a strictly logical, order of treatment, beginning in the first two chapters with a general sketch of cell-structure and cell-division. The following three chapters deal with the germ-cells, — the third with their structure and mode* of origin, the fourth with their union in fertilization, the fifth with the phenomena of maturation by which they are prepared for their union. The sixth chapter contains a critical discussion of cell-organization, completing the morphological analysis of the cell. In the seventh cha])ter the cell is considered with reference to its more fundamental chemical and physiological properties as a prelude to the examination of development which follows. The succeeding chapter approaches the objective point of the book by considering the cleavage of the ovum and the general laws of cell-division of which it is an expression. The ninth chapter, finally, deals with the elementary operations of development considered as cell-functions and with the theories of inheritance and development based upon them.
SOME GENERAL WORKS OX THE CELL-THEORY ^
Bergh. R. S. — Vork'sungen liber die Zelle und die einfachen Gewebe : Wiesbaden^ 1894.
Carnoy. J. B. — La Biologie Cellulaire : IJcrrc, 1884.
Delage, Yves. — La Structure du Protoplasma et les Theories sur THdrdditd et les grands Problemes de la Biologie Gcnerale : Paris, 1895.
Geddes & Thompson. — The Evolution of Sex : A'cw ]'ork, 1890.
Hacker. V. — Pra.xis und Theorie der Zellen- und Befruchtungslehre : Jena. 1899.
Henneguy. L. F. — Legons sur la Cellule : /^a?/s. 1896.
Hertwig. 0. — Die Zelle und die Gewebe: Fischer, Jeua, L. 1893. II., 1898. Trans- lation, published by Mactnillan, London and Neiv York^ 1895.
Hofmeister. Lehre von der Pflanzenzelle : Leipzii^, 1867.
Huxley. T. H. — Review of the Cell-theory: British and Foreign Medico-Chiriirgical Review, XIL, 1853.
^ See also Literature, T., p. 61.
INTRODUCTION
15
Minot. C. S. — Human Embryology: New York, 1892.
Remak. R. — Untersuchimgen iiber die Entwicklung der Wirbelthiere : Berlin, 1850-55.
Sachs, J. V. History of Botany. Translation: Oxford, \%<^q.
Schleiden, M. J. — Beitrage zur Phytogenesis : M'uller's Arclih\ 1838. Translation in Sydenham Soc, XII. Loudon, 1847.
Schwann. Th. — Mikroscopische Untersuchungen liber die Uebereinstimmung in der Structur und dem Wachsthum der Thiere und Pflanzen : Berlin, 1839. Trans- lation in Sydenham Soc. XII. London, 1847.
Tyson. James. — The Cell-doctrine, 2d ed. PJiiladelpJiia, 1878.
Virchow, R. — Die Cellularpathologie in ihrer Begriindung auf physiologische und pathologische Gewebelehre : Berlin^ 1858.
Weismann, A. — Essays on Heredity. Translation: First series, Oxford, 1891 ; Second series, Oxford, 1892. •
Id. — The Germ-plasm: Ne^u York, 1893.
CHAPTER I
GENERAL SKETCH OF THE CELL
" Wir haben gesehen, dass alle Organismen aus wesentlich gleichen Thcilen, namlich aus Zellen zusammengesetzt sind, dass diese Zellen nach wesentlich densellKTi Cieset/en sich bilden und wachsen, dass also diese Prozesse iiberall auch durch dieselben Krafte hervorge- bracht warden miissen." Schwann.^
In the passage quoted above Schwann expressed a truth which subsequent research has estabhshed on an ever widening basis ; and we have now^ more than ever reason to believe that despite unending diversity of form and function all cells may be brought into definite relation to a common morphological and physiological type. We are, it is true, still unable to specify all its essential features, and hence can give no adequate brief definition of the cell. For practical pur- poses, however, no such definition is needed, and we may be content with the simple type that has been familiar to histologists since the time of Leydig and Max Schultze.
It should from the outset be clearly recognized that the term "cell" is a biological misnomer; for cells only rarely assume the form implied by the word of hollow chambers surrounded by solid walls. The term is merely an historical survival of a word casually employed by the botanists of the seventeenth century to designate the cells of certain plant-tissues which, when viewed in section, give somewhat the appearance of a honeycomb.^ The cells of these tis- sues are, in fact, separated by conspicuous solid walls which were mistaken by Schleiden, followed by Schwann, for their essential i)art. The living substance contained within the walls, to which Hugo von Mohl gave the r\2imQ protoplasm^ (1846), was at first overlooked or was regarded as a waste-product, a view based upon the fact that m many important plant-tissues such as cork ox wood it may wholly disappear, leaving only the lifeless walls. The researches of Herg- mann, Kolliker, Bischoff, Cohn, Max Schultze, and many others
1 Uniersuchungen, p. 227, 1839.
2 The word seems to have been first employed by Robert Hooke, in 1665, to designate the minute cavities observed in cork, a tissue which he describcl as made up of •' httle boxes or cells distinct from one another " and separated In- solid walls.
3 The same word had been used by Purkinje some years before (1840) to designate the formative material of young animal embryos.
c 17
D. H. HILL LIBRARY
i8
GENERAL SKETCH OF THE CELL
showed, however, that most living cells are nut hollow but solid bodies, and that in many cases — for example, the colourless corpuscles of blood and lymph — they are naked masses of protoplasm not sur- rounded by definite walls. Thus it was proved that neither the vesicular form nor the presence of surrounding walls is an essential character, and that the cell-contents, i.e. the protoplasui, must be the seat of vital activity.
Within the protoplasm (Figs. 6 <S) lies a body, usually of definite rounded form, known as the nnclcus,'^ and this in turn often contains
Attraction-sphere enclosing two ccntrosomes
Nucleus -
r Plasmosome or
true
nucleolus
Chromatin-
nctwork
Linin-network
I Karyosome, net-knot, or chromatin- nucleolus
Plastids lying in the cytoplasm
Vacuole
Passive bodies fmeta- plasm or paraplasm) suspended in the cy- toplasmic mesh work
Fig. 6. — Diagram of a cell. Its basis consists of a meshwork containing numerous minute granules {microsomes) and traversing a transparent ground-substance. a
one or more smaller bodies or nucleoli. By some of the earlier workers the nucleus was supposed to be, like the cell-wall, of sec- ondary im])ortance, and many forms of cells were described as being devoid of a nucleus ("cytodes" of Haeckel). Nearly all later re- searches have indicated, however, that the characteristic nuclear material, whether forming a single body or scattered in smaller masses, is always present, and that it plays an essential part in the life of the cell.
Besides the presence of protoplasm and nucleus, no other struc- tural features of the cell are yet known to be of universal occurrence.
1 First described by Fontana in 1 781, and recognized as a normal element of the cell by Robert Brown in 1S33.
GENERAL MORPHOLOGY OF THE CELL
19
We may therefore still accept as valid the definition ^qven more than thirty years ago by Leydig and Max Schultzc, that a cell is a mass of protoplasm containing a nucleus} to which we may add Schultze's statement that both nucleus and protoplasm arise tlirougli the division of the corresponding elements of a preexisting cell. Nothing could be less appropriate than to call such a body a ''cell " ; yet the word has become so firmly established that every effort to replace it bv a better has failed, and it probably must be accepted as part of the established nomenclature of science.^
A. General Morphology of the Cell
The cell is a rounded mass of protoplasm which in its simplest form is approximately spherical. The form is, however, seldom realized save in isolated cells such as the unicellular plants and ani- mals or the egg-cells of the higher forms. In vastly the greater number of cases the typical spherical form is modified by unequal growth and differentiation, by active movements of the cell-substance, or by the mechanical pressure of surrounding structures, but true angular forms are rarely if ever assumed save by cells surrounded by hard walls. The protoplasm which forms its active basis is a viscid, translucent substance, sometimes apparently homogeneous, more fre- quently finely granular, and as a rule giving the appearance of a meshwork, which is often described as a spongelike or netlike '' reticu- lum." ^ Besides the active substance or protoplasm proper the cell almost invariably contains various lifeless bodies suspended in the meshes of the network; examples of these are food-granules, pig- ment-bodies, drops of oil or w^ater, and excretory matters. These bodies play a relatively passive part in the activities of the cell, being either reserve food-matters destined to be absorbed and built up into the living substance, or by-products formed from the proto- plasm as waste-matters or in order to play some role subsidiary to the actions of the protoplasm itself. The passive inclusions in the protoplasm maybe collectively designated as metaplasm (Hanstein) ox paraplasm {Yi\\^^^^x\ in contradistinction to the -AQixxc protoplas}n.
1 Leydig, Lchrlmch der Hisiologie, p. 9, 1857; Schultze, .7;r//. AiiaL u. IViys.,\^. 11, 1S61.
2 Sachs has proposed the convenient word cnergid {flora, '92, p. 57) to dcsijjnatc the essential living part of the cell, i.e. the nucleus with that portion of the active cyt«»plasm that falls within its sphere of influence, the two forming an organic unit both in a morpho- logical and in a physiological sense. It is to be regretted that this convenient and appro- priate term has not come into general use. (See also Flora, '95, p. 405. and cf. Kupfter ('96), Meyer ('96), and Kolliker ('97).)
3 Such meshworks are sometimes plainly visible in the living protoplasm (p. 44). It is always more or less an open question how far the appearances seen in hxed (coagulated) material correspond with the conditions existing in life. See pp. 42-46.
20
GENERAL SKETCH OE THE CELL
It is often difficult to distinguish between such metaplasmic bodies and the granules commonly supposed to be elements of the active protoplasm; indeed, as will appear beyond (p. 29), there is reason to believe that "protoplasmic" and "metaplasmic" granules cannot be separated by any definite limit, but are connected by various gradations. Among the lifeless products of the protoplasm must be reckoned also the ctli-ica!/ or lucnibrauc bv which the cell-body may
Fig. 7 — Spermatogonia of the salamander. [Meves.] Above, two cells showing large nuclei, with linin-threads and scattered chromatin-graniiles ; in each cell an attraction-sphere with two centrosomes. Below, three contiguous spermatogonia, showing chromatin-reticulum, centrosomes and spheres, and sphere-bridges.
be surrounded ; but it must be remembered that the cell-wall in some cases arises by a direct transformation of the protoplasmic substance, and that it often retains the power of growth by intussusception like living matter.
It is unfortunate that some confusion has arisen in the use of the word protoplasm. When Leydig, Schultze, Brlicke, De Bary, and other earlier writers spoke of "protoplasm," they had in mind only the substance of the cell-body, not that of the nucleus. Strasburger,
GENERAL MORPHOLOGY OF TLIE CELL
21
J. ♦il?.v-'.v. -. , .
^
c
z>
. ^^^-^-^'^lioiis cells showing the typical parts.
N.c1;or2XrreS'„:,'f lli^e,,:^ 'o^J sa,an,a„de...,arva. T.o ce„,.oso„,es a, ,he rish.
f?.'l' kT' ''T ''"'" ''''°P'='' ''>' ™'^"y' b"t not all. later writers eestio'n h ""I' "'"'t^'''"- '^-•'"S, however, at Flen,n,i„,'s u" gestion, been changed to ta,yop/as„r At the present time there fore, the word /;...^/.„,„ is used by some authors'(Hutsch i, H^ tw '"
22 GENERAL SKETCH OE THE CELL
Kolliker, etc.) in its orif^inul narrower sense (equivalent to Stras-. burger's cytoplasm), while perhaps the majority of writers have accepted the terminology of Strasburger and Flemming. On the whole, the terms cytoplasin and kiDyop/asui seem too useful to be rejected, and, without attaching too much importance to them, they will be employed throughout the present work. It must not, how- ever, be supposed that either of the words denotes a single homo- geneous substance; for, as will soon appear, both cytoplasm and karyoplasm consist of several distinct elements.
The nucleus is usually bounded by a definite membrane, and often appears to be a perfectly distinct vesicular body suspended in the cvtoplasm — a conclusion sustained by the fact that it may move actively through the latter, as often occurs in both vegetable and animal cells. Careful study of the nucleus during all its phases gives, however, reason to believe that its structural basis is similar to that of the cell-body ; and that during the course of cell-division, when the nuclear membrane usually disappears, cytoplasm and karyoplasm come into direct contmuity. Even in the resting cell there is good evidence that both the intranuclear and the extranuclear material may be structurally continuous with the nuclear membrane^ and among the Protozoa there are forms (some of the flagellates) in which no nuclear membrane can at any period be seen. For these and other reasons t/ic tcrtns ''nucleus^' and '' ccU-bQ,dy'' sJioiild probably be regarded as only topographical expressions denoting tzuo differentiated areas in a common structural basis. The terms karyoplasm and cytoplasm possess, however, a specific significance owing to the fact that there is on the whole a definite chemical contrast between the nuclear substance and that of the cell-body, the former being characterized by the abundance of a substance rich in phosphorus known as nuclein, while the latter contains no true nuclein and is especially rich in albuminous substances such as nucleo-albumins, albumins, globulins, and the like, which contain little or no phosphorus.
Both morphologically and physiologically the differentiation of the active cell-substance into nucleus and cell-body must be regarded as a fundamental character of the cell because of its universal, or all but universal, occurrence, and because there is reason to believe that it is in some manner an expression of the dual aspect of the fundamental process of metabolism, constructive and destructive, that lies at the basis of cell life. The view has been widely held that a third essen- tial element is the centrosome, discovered by Flemming and Van Beneden in 1875-76, and since shown to exist in a large number of other cells (Figs. 7, 8). This is an extremely minute body which
1 Conklin ('97, i). Obst ('99), and some others have described a direct continuity in the resting cell between the intranuclear and extranuclear ineshworks.
STRUCTURAL BASIS OF PROTOPLASM 23
is concerned in the process of cell-division and in the fertilization of the G.gg, and has been characterized as the " dynamic centre " of the cell. Whether it has such a significance, and whether it is in point of morphological persistence comparable with the nucleus, are ques- tions still sub judicCy which will be discussed elsewhere.^
B. Structural Basis of Protoplasm
As ordinarily seen under moderate powers of the microscope, proto- plasm appears as a more or less vague granular substance which shows as a rule no definite structure organization. More precise examination under high powers, especially after treatment by suitable fixing and staining reagents, often reveals a highly complex structure in both nucleus and cytoplasm. Since the fundamental activities of protoplasm are everywhere of the same nature, investigators have naturally sought to discover a corresponding fundamental morpho- logical organization common to all forms of protoplasm and under- lying all of its special modifications. Up to the present time, however, these attempts have not resulted in any consensus of opinion as to whether such a common organization exists. In many forms of proto- plasm, both in hfe and after fixation by reagents, the basis of the structure is a more or less regular framework or niesJiwork, consisting of at least two substances. One of these forms the substance of the meshwork proper; the other, often called the ground-substance (also cell-sap, enchylema, hyaloplasma, paramitome, interfilar substance, etc.), 2 occupies the intervening spaces. To these two elements must be added minute, deeply staining granules or " microsomes " scattered along the branches of the meshwork, sometimes quite irregularly, sometimes with such regularity that the meshwork seems to be built of them. Besides the foregoing three elements, which we may pro- visionally regard as constituting the active substance, the protoplasm almost invariably contains various passive or metaplasmic substances in the form of larger granules, drops of liquid, crystalloid bodies, and the like. These bodies, which usually lie in the spaces of the mesh- work, are often difficult to distinguish from the microsomes lying in the meshw^ork proper — indeed, it is by no means certain that any adequate ground of distinction exists.^
From the time of Frommann and Arnold ('65-'67) onwards, most of the earlier observers regarded the meshwork as a fibrillar structure, either forming a continuous network or reticulum somewhat like the fibrous network of a sponge ("reticular theory " of Klein, Van Bene- den, Carnoy, Heitzmann), or consisting of disconnected threads,
1 Cf. pp. 304. 354. ' Q'- ^'l^^ssary. ^ Cf. p. 29.
24
GENERAL SKETCH OF THE CELL
••^
jD
/
[ TK
IS ^
Fig- 9- — Living cells of salamander-larva. [Flemminc^..]
A. Group of epidermal cells at different foci, showing protoplasmic bridges, nuclei, and cyto- plasmic fibrillae; the central cell with nucleus in the spireme-stage. B. Connective tissue cell.
C. Epidermal cell in early mitosis (segmented spireme) surrounded by protoplasmic bridges.
D. Dividing cell. E.F. Cartilage-ceils with cytoplasmic fibrillas (the latter somewhat exaggerated in the plate).
STRUCTURAL BASIS OF PROTOPLASM 2$
whether simple or branching (''filar theory" of Flcmming), and the same view is widely held at the present time. The meshwork has received various names in accordance with this conception, among which may be mentioned reticulum, thrcad-ivork, spongioplasm, mitomc, filar subslance} all of which are still in use. Under this view the " granules " described by Schultze, Virchow and still earHer observers have been variously regarded as nodes of the network, optical sec- tions of the threads, or as actual granules (" microsomes ") suspended in the network as described above.
Widely opposed to these views is the " alveolar theory " of Butschli, which has won an increasing number of adherents. Butschli regards protoplasm as having a foam-like alveolar structure ("W'aben- struktur"), nearly similar to that of an emulsion (Fig. lo), and he has shown in a series of beautiful experiments that artificial emul- sions, variously prepared, may show under the microscope a marvel- lously close resemblance to living protoplasm, and further that drops of oil-emulsion suspended in water may even exhibit amoeboid changes of form. To restate Biitschli's view, protoplasm consists of separate, closely crowded minute drops^ of a liquid alveolar substance suspended in a continuous interalvcolar substance, likewise liquid, but of different physical nature. The latter thus forms the walls of closed chambers or alveoli in which the alveolar drops lie, just as in a fine emulsion the emulsifying liquid surrounds the emulsified drops. The appear- ance of a network in protoplasm is illusory, being due to optical sec- tion of the interalvcolar walls or partitions as viewed at any given focus of the microscope. As thus seen, the walls themselves appear as fibres, while the "spaces of the network" are in like manner oi)ti- cal sections of the alveoli, the alveolar substance that fills them corresponding to the ''ground substance." As explained beyond/^ Butschli interprets in like manner the radiating systems or asters formed during cell-divison, the astral rays (usually considered as fibres) being regarded as merely the septa between radially arranged
alveoH (Fig. lo).
The two (three) general views above outlined may be designated respectively as thQ fibrillar (reticular or filar) and alveolar \.\\cox\c^ of protoplasmic structure ; and each of them has been believed by some of its adherents to be universally applicable to all forms of protoplasm. Beside them may be placed, as a third general view, Xh^ granular theory especially associated with the name of Altmann, by whom it has been most fully developed, though a number of earlier writers have held similar views. According to Altmann's view, which apart from its theoretical development approaches in
1 See Glossary.
2 Measuring on an average about .ooi mm. in diameter. ^ Cf. p. no.
26
GENERAL SKETCH OF THE CELL
some respects that of Biitschli, protoplasm is compounded of innu- merable minute granules which alone form its essential active basis ; and while fibrillar or alveolar structures may occur, these are of only secondary importance.
r
1
Fig. 10. — Alveolar or foam-structure of protoplasm, according to Biitschli. [BuTSCllLl.]
A. Epidermal cell of the earthworm. B. Aster, attraction-sphere, and centrosome from sea- urchin egg. C. Intracapsular protoplasm of a radiolarian ( Thalassicolla) with vacuoles. D. Peripheral cytoplasm of sea-urchin egg. E. Artificial emulsion of olive-oil, sodium chloride, and water.
It is impossible here adequately to review the many combinations and modifications of these views which different investigators have
STRUCTURAL BASIS OF PROTOPLASM
27
made.^ On the whole, the present drift of opinion is toward the conclusion that none of the above interpretations has succeeded in the attempt to give a universal formula for protoplasmic structure ; and many recent observers have reached the conclusion, earlier advo- cated by Kolliker ('89), that the various types described above are connected by intermediate gradations and may be transformed one into another, in different phases of cell-activity. Unna ('95), for example, endeavours to show how an alveolar structure may pass into a sponge-like or reticular one by the breaking down of the inter-
a
.-. .' "■•.#^. i ''. .'*^. zer, "^ -•"t»*'/*'w"*-v'**-** ^> '*^m'l '.
005;^
^.;
^
o:°Ao:„-D,
o
0°b-,-?.-lio°voQ,
00. -o-.... o'o •""•" o"
■?.?o
Fig. II.— ('?) Protoplasm of the egg of the sea-urchin {Toxopneustes) in section showing meshwork of microsomes; {b) protoplasm from a living star-fish egg {Astcrias) showing alveolar spheres with microsomes scattered between them ; {c) the same in a dying condition atler crush- ing the egg ; alveolar spheres fusing to form larger spheres ; {d) protoplasm from a young ovarian egg of the same. (All the figures magnified 1200 diameters.)
alveolar walls. Flemming, for many years the foremost and most consistent advocate of the fibrillar theory, now admits that protoplasm may be fibrillar, alveolar, granular, or sensibly homogeneous,^ and that we cannot, therefore, regard any one of these types of structure as absolutely diagnostic of the living substance. In plant-cells Strasburger^ and a number of his pupils maintain that the "kino- plasm" (p. 322) or filar plasm, from which the spindle-fibres and astral rays are formed, is fibrillar, while the " trophoplasm " or alveolar plasm forming the main body of the cell is alveolar, the former, however, assuming the fibrillar structure, as a rule, only during the mitotic activity of the cell. My own long-continued studies on various forms of protoplasm have likewise led to the con- clusion that no universal formula for protoplasmic structure can be
1 For full discussion, with literature list, see Flemming, '82, '97. ^ '97. 2. and Butschli.
'92, 2, '99.
2 '97, I, p. 260.
3 '95. '97, 3. '^8.
28
GENERAL SKETCH OF THE CELL
given. ^ In that classical object, the echinoderm-egg, for example, it is easy to satisfy oneself, both in the living cell and in sections, that the protoplasm has a beautiful alveolar structure, exactly as described by Hutschli in the same object (Fig. 1 1 ). This structure is here, however, entirely of secondary origin ; for its genesis can be traced step by step during the growth of the ovarian eggs through the deposit of minute drops in a homogeneous basis, which ultimately gives rise to the interalveolar walls. In these same eggs the astral systems formed during their subsequent division (Fig. 12) are, I
.. .■•.,••• .»•• ''/ '■. /' V<i *■•.•.''. >"••
: " vr^t'-.- :••••. >-./•; *>'••-'"•.•< :••-..••..••->
Fig. 12. — Section of sea-urchin egg (Toxopficustcs), li minutes after entrance of the sperma- tozoon, showing alveoli and microsomes, sperm-nucleus, middle piece, and aster (about 2000 diameters).
believe, no less certainly fibrillar ; and thus we see the protoplasm of the same cell passing successively through homogeneous, alveolar, and fibrillar phases, at different periods of growth and in different conditions of physiological activity. There is good reason to regard this as typical of protoplasm in general. BiJtschli's conclusions, based on researches so thorough, j^rolonged, and ingenious, are entitled to great weight ; yet it is impossible to resist the evidence that fibrillar and granular as well as alveolar structures are of wide occurrence ; and while each may be characteristic of certain kinds of
1 Wilson, '99.
STRUCTURAL BASIS OF PROTOPLASM
29
cells, or of certain physiological conditions,^ none is common to all forms of protoplasm. If this position be well grounded, we must admit that the attempt to find in visible protoplasmic structure any adequate insight into its fundamental modes of physiological activity has thus far proved fruitless. We must rather seek the source of these activities in the ultramicroscopical organization, accepting the probability that apparently homogeneous protoplasm is a complex mixture of substances which may assume various forms of visible structure according to its modes of activity.
Some of the theoretical speculations regarding the essential nature of that organization are discussed in Chapter VI., but one q2iasi-X\\(to- ' retical point must be here considered. Much discussion has been given to the (question as to which of the visible elements of the proto- plasm should be regarded as the "living" substance proper; and the diversity of opinion on this subject may be judged by the fact that although many of the earlier observers identified the "reticulum " as the living element, and the ground-substance as Ufeless, others, such as Leydig and Schafer, held exactly the reverse view, while Altmann insisted that only the " granules " were alive. Later discussions have shown the futility of this discussion, which is indeed largely a verbal one, turning as it does on the sense of the word "living." In practice we continually use the word "living" to denote various degrees of vital activity. Protoplasm deprived of nuclear matter has lost, wholly or in part, one of the most characteristic vital properties, namely, the power of synthetic metaboHsm ; yet we still speak of it as " living," since it still retains for a longer or shorter period such properties as irritability and the power of coordinated movement ; and, in like manner, various special elements of protoplasm may be termed " liv- ing " in a still more restricted sense. In its fullest meaning, however, the word "living" implies the existence of a group of cooperating activities more complex than those manifested by any one substance or structural element. I am therefore entirely in accord with the view urged by Sachs, Kolliker, Verworn, and other recent writers, that life can 'only be properly regarded as a property of the cell- system as a whole ; and the separate elements of the system would, with Sachs, better be designated as "active" or "passive," rather than as "living" or "lifeless." Thus regarded, the distinction
1 Thus the alveolar structure seems to be characteristic of Protozoa in general, and of the protoplasm of plant-cells when in the vegetative state, the fibrillar of nerve-cells and muscle-cells. The granular type is characteristic of some forms of leucocytes and gland- cells; but many of the granules in these cells are no doubt metaplasmic, and it is further very doubtful whether such a granular or "pseudo-alveolar" structure can be logically dis- tinguished from an alveolar (c/. Wilson, '99). In the pancreas-cell granular and hbr.llar structures alternate with the varying phases of secretory activity {r/. Mathews, '99).
30 GEXERAL SKETCH OF THE CELL
between "protoplasmic" and " nictai:)lasmic " substances, while a real and necessary one, becomes after all one of degree. I believe that we are probably justified in regarding the continuous substance as the most constant and active element, and that which forms the fundamental basis of the system, transforming itself into granules, drops, fibrilla?, or networks in accordance with varying physiological needs. ^
Thus stated, the question as to the relative activity of the various elements becomes a real and important one. It now seems probable that the substance of the meshwork (fibrillar or interalveolar structure) is most active in the processes of cell-division, in contractile organs such as cilia and muscle-fibres, and in nerve-cells ; but the ground- substance, while apparently the most frequent seat of metaplasmic deposits, is certainly also the seat of active chemical changes. This subject has, however, not yet been sufficiently investigated.
C. The Nucleus
A fragment of a cell deprived of its nucleus may live for a consid- erable time and manifest the power of coordinated movement without perceptible impairment. Such a mass of protoplasm is, however, devoid of the powers of assimilation, growth, and repair, and sooner or later dies. In other words, those functions that involve destructive metabolism may continue for a time in the absence of the nucleus ; those that involve constructive metabolism cease with its removal. There is, therefore, strong reason to believe that the nucleus plays an essential part in the constructive metabolism of the cell, and through this is especially concerned with the formative processes involved in growth and development. For these and many other reasons, to be discussed hereafter, the nucleus is generally regarded as a controlling
1 Wilson, '99. Cf. Sachs ('92. '95), Kulliker ('97), Meyer ('96), and Kupffcr ('96) on energids. Sachs sharply distinguishes between the energid {r^\x(^^\x% and protoplasm), which forms a living unit, and the passive ^x^^ix^xA-prodticts, placing in the former the nucleus, nucleolus, general cytoplasm, centrosome and plastids (chloroplasts and leucoplasts), and in the latter the starch-grains, aleurone-crystals, and membrane. Meyer carries the analysis further, classifying the active energid-elemcnts m\.o protoplasmatic and alloplasmatic organs, the former (nucleus cytoplasm, chromatophores, and perhaps the centrosomes) arising only by division, the latter (cilia, and according to Kolliker, also the muscle- and nerve-fibriiiee) formed by differentiation from the protoplasmatic elements. The passive energid-products {ergastic structures or " formed material " of Beale) are formed as enclosures (starch-grains, etc.), or excretions (membranes). These general views arc accejited by Kolliker; but none of these writers has undertaken to show how " alloplasmatic "' structures are to be distinguished from metaplasmic or ergastic. I believe Sachs' view to be in principle not only true but of high utility. Practically, however, it involves us in considerable difficulty, unless the terminology adopted above — itself directly suggested by and nearly agreeing with the usage of Sachs and Kolliker — be employed.
THE NUCLEUS
31
centre of cell-activity, and hence a primary factor in growth, develop- ment, and the transmission of specific qualities from cell to cell, and so from one generation to another.
I, General Strncticre
The cell-nucleus passes through two widely different phases, one of which is characteristic of cells in their ordinary or vegetative condi- tion, while the other only occurs during the complicated changes involved in cell-division. In the first phase, falsely characterized as the *' resting state," the nucleus usually appears as a rounded sac-like body surrounded by a distinct membrane and containing a conspicu- ous irregular network (Figs. 6, 7, 13), which is in some cases plainly visible in the living cell (Fig. 9). The form of the nucleus, though subject to variation, is on the whole singularly constant, and as a rule shows no very definite relation to that of the cell-body, though in elon- gated cells such as muscle-cells, in certain forms of parenchyma, and in epithelial cells (Fig. 49), the nucleus is itself often elongated. Typically spherical, it may, in certain cases, assume an irregular or amoeboid form, may break up into a group of more or less completely separated lobes (polymorphic nuclei. Fig. 49), sometimes forming an irregular ring (" ring-nuclei " of leucocytes, giant-cells, etc., Fig. 14. J)). It is usually very large in gland-cells and others that show a very active metabolism, and in such cases its surface is sometimes increased by the formation of complex branches ramifying through the cell
(Fig. 14, E). ^ ^
These forms seem in general to be fairly constant in a given species of cell, but in a large number of cases the nucleus has been seen in the living cell (cartilage-cells, leucocytes, ova) to undergo more or less active changes of form, sometimes so marked as to merit the name of amoeboid (Fig. 77). Perhaps the most remarkable deviations from the usual type of nucleus occur among the unicellular forms. In the dil- ate Infusoria the nuclei are massive bodies of two kinds, viz. a large macrormcleiis and one or more smaller viicroujiclci, both of which arc present in the same cell, the former kind being generally regarded as the active nucleus, the latter as a reserve nucleus from which at cer- tain periods new macronuclei arise (p. 224). The macronuclei show a remarkable diversity of form and structure in different species. Still more interesting are the so-called scattered or distributed nuclei, de- scribed by Butschli in flagellates and Bacteria, by Gruber in certain rhizopods and Infusoria, and by several authors in the Cyanophyccx (Figs. 15, 16). The nuclear material is here apparently scattered through the cell in the form of numerous minute, deeply stained gran- ules, which, if this identification is correct, represent the most primi-
32
GENERAL SKETCH OF THE CELL
tive known types of nucleus ; but this subject is still sub jiidice (p. 39). A transition from this condition to nuclei of the ordinary type appears to be L^iven in the nuclei of certain flagellates (e.g. CJii- Uwionas and Tracluhnotias), where the chromatin-granules are aggre- gated about a nucleolus-like body, but are not enclosed by a membrane.^ In considering the structure of the nucleus, as seen in sections, we must, as in the case of the cytoplasm, bear in mind the possibility, or
rather probability, that some of the elements described may be coagulation - products ; for the nucleus is in life comj^osed of liquid or semi-liquid substance, and Albrecht ('99) has recently shown that nuclei isolated in the fresh condition will flow together to form a single body. Most of the main features of the nucleus, both in the resting and in the dividing phases, have, however, been seen in life (Fig. 9), and the principal danger of mistaking artifacts for normal structures re- lates to the finer elements, con- sidered beyond.
In the ordinary forms of nuclei in their resting state the follow- ing structural elements may as a rule be distinguished (Figs. 6, 7, 10): —
Fig. 13. — Two nuclei from the crypts of ^^ q^^e liuclcav DUmbraUC, a
Lieberkiihn in the salamander. [Heidenhain.] ^^ -^ r i i t ^ n i • i
well-defined delicate wall which
The character of the chromatin-network • , i i
(^aj/r/4r^wa//«) is accurately shown. The upper glVCS the UUClCUS a Sharp COUtOUr
nucleus contains three plasmosomes or true -^^-^^ differentiates it clcarly from
nucleoli; the lower, one. A few fine linin-threads . t . •> -r-^'
{oxychromattn) are seen in the upper nucleus the SUrrOUuding Cytoplasm. ThlS
running off from the chromatin-masses. The wall SOmctimCS StaiuS but VCiy clear spaces are occupied by the ground-sub- ^|i n] ^,^,| ^aU SCarCcly be dif- stance. n ./ ' j
ferentiated from the outlying cytoplasm. In other and perhaps more frequent cases, it approaches in staining capacity the chromatin.
b. The nuclear rcticuliiDi. This, the most essential part of the nucleus, forms an irregular branching network or reticulum which con- sists of two very different constituents. The first of these, forming the general protoplasmic basis of the nucleus, is a substance known as lini7i
1 Calkins, '98, i.
THE NUCLEUS
33
(Schwarz), invisible until after treatment by reagents, which in sections shows a finely granular structure and stains like the cytoplasmic sub- stance, to which it is nearly related chemically (Figs. 7, 49). The second constituent, a deeply staining substance known as chromatiji (Flemming), is the nuclear substance /(^r excellence, for in many cases it appears to be the only element of the nucleus that is directly handed on by division from cell to cell, and it seems to have the power to pro- duce all the other elements. The chromatin often appears in the form of scattered granules and masses of differing size and form, which are embedded in and supported by the linin-substance (Figs. 7, 19). In some cases the entire chromatin-content of the nucleus appears to be condensed into a single mass which simulates a nucleolus ; for exam- ple, in Spirogyra and in various flagellates and rhizopods (e.g. Acti- jiospJicBviuni, Ai'cella) ; or there may be several such chromatin-masses, as in some of the Foraminifera and in Noctihica. More commonly the chromatin forms a more or less regular network intermingled with and more or less embedded in the linin, from which it is often hardly dis- tinguishable until the approach of mitosis, when a condensation of the chromatin-substance occurs.
In contradistinction to the other nuclear elements, chromatin is nut acted upon, or is but slowly affected, by peptic digestion. It may thus be easily isolated for chemical analysis, which shows it to consist mainly of luiclein, i.e. a compound in varying proportions of a complex phosphorus-containing acid known as inicleinic acid, with albumi- nous bodies such as histon, protamin, or in some cases albumin itself.' Upon this, as will be show^n in Chapter VL, probably depends the pro- nounced staining capacity when treated with the so-called *' nuclear stains " {e.g. hsematoxylin, methyl-green, and the basic tar-colours gen- erally) from which chromatin takes its name. This capacity always increases as the nucleus prepares for division, reaching a climax in the spireme- and chromosome-stages, and it is also very marked in con- densed nuclei such as those of spermatozoa. These variations are almost certainly due to varying proportions in the constituents of the nuclein, the staining capacity standing in direct ratio to the amount of nucleinic acid.
c. The nucleoli, one or more larger rounded or irregular bodies, suspended in the network, and staining intensely with many dyes. In some nuclei they are entirely absent. When present the nucleoli vary in number from one to five or more; and the number is otten inconstant in the same species of cell, and even varies in the same cell with varying physiological conditions. In the eggs of some animals, at certain periods of growth {e.g. lower vertebrates), the nucleus may contain hundreds of nucleoli. An interesting case is
1 See p. 334- D
^^ GENERAL SKETCH OF THE CELL
that of the subcutaneous gland-cells of Pisciola, the nuclei of which contain in early phases of secretion but a single nucleolus. During growth of the cell the nucleolus fragments, finally giving rise to several hundred nucleoli which then appear to migrate out into the cytoplasm, leaving but a single nucleolus to repeat the cycle. ^
The bodies known as nucleoli are of at least two different kinds. The first of these, the so-called true nucleoli or phuinosomcs (Figs. 6, 8, B, 13), are of spherical form, and are shown by the staining reactions to differ widely from chromatin, being in general sharply stained by dyes which, like eosin, orange or acid fuchsin, stain the linin and the general cytoplasm. The plasmosomes sometimes seem to have no envelope, but in many cases {e.g. in leucocytes) are surrounded by a thin layer that stains Hke chromatin. Nucleoli of a quite different type are the *' net-knots " (Netzknoten), chromatin- nucleoli, or karyosojucs, which agree in staining reaction with chro- matin and are doubtless to be regarded as merely a portion of the chromatin-network (Figs. 8, 49). These are sometimes spherical, more often irregular (Fig. 8), and often are hardly to be distinguished, except in size, from nodes of the chromatin-reticulum.''^ The relations between these two forms of nucleoli are far from certain, and the variations in staining reaction shown by true nucleoli render it not improbable that intermediate forms exist which may represent an actual transition from one to the other.-*^ In many of the Protozoa, as described beyond, the ''nucleolus" is shown by its behaviour during mitosis to be comparable with an attraction-sphere or centro- some ('Muicleolo-centrosome," Keuten); and even in higher forms there are some cells in which the centrosome is intranuclear
(Fig. 148).
There is good reason to believe that the chromatin-nucleoli are merely more condensed portions of the chromatin-network. since during cell-division they have the same history as the remaining portion of the chromatin-substance.* The nature of the true nucleoli is still imperfectly known. By some observers, including Flemming, O. and R. Hertwig, and Carnoy, they have been regarded as store- houses of material (para-nuclein, plastin) which contributes to the
^ Montgomery, '98, 2.
2 Flemining first called attention to the chemical difference between the true nucleoli and the chromatic reticulum ('82, pp. 138, 163) in animal-cells, and Zacharias soon afterward studied more closely the difference of staining reaction in plant-cells, showing that the former are especially coloured by alkaline carmine solutions, the latter by acid solutions. Other studies by Carnoy, Zacharias, Ogata, Rosen, Schwarz, Heidenhain, and many others show that the medullary substance (pyrenin) of true nuclei is coloured by acid tar-colours and other plasma stains, while the chromatin has a special affinity for basic dyes. Cf. p. 337.
3 For very full review of the literature of the nucleoli see Montgomery ( '98, 2).
4 Cf. p. 67.
THE NUCLEUS
35
formation of chromosomes during division, and hence may play an active role in the nuclear activity. Strasburger ( '95) likewise be- lieves them to contain a store of active material which, however, has no direct relation to the chromosomes but consists of " kinoplasm "
Fig. 14. — Special forms of nuclei.
A. Permanent spireme-nucleus, salivary gland of Chirowmus larva. Chromatin in a single thread, composed of chromatin-discs (chromomeres), terminating at each end m a true nucleolus or plasmosonie. [Balkianl]
B. Permanent spireme-nuclei. intestinal epithelium of dipterous larva Ptychcptfra. [\ AN
Gehuchten.] C. The same, side view.
D. Polymorphic ring-nucleus, giant-cell of bone-marrow of the rabbit; c. a group of ccntro-
somes or centrioles. [Heidenhain.]
E. Branching nucleus, spinning gland of butterfly-larva {Picris) . [KORSCHELT.J
(p. 322), from which arises the achromatic part of the division- figure (p. 82). On the other hand, Hacker ( '95, '99) ^^^^^ o^^er observers regard the nucleolar material as a passive by-product of the chromatin-activity destined to be absorbed by the active sub-
36 GENERAL SKETCH OF THE CELL
Stances. This is supported by the fact that in some forms of mitosis the nucleokis is at the time of division actually cast out of the nucleus into the cytoplasm, where it degenerates without further apparent function. This seems to constitute decisive evidence in support of Hacker's view as api)lied to certain cases ; but without further evidence it must remain doubtful whether it applies to
all.'
d. The groiiiid-siihstancc, nuclear sap, or kavyolympJi, a clear sub- stance occupying the interspaces of the network and left unstained by most of the dyes that colour the chromatin, the linin, or the plas- mosomes. By most observers the ground-substance is regarded as a liquid filling a more or less completely continuous space traversed by the nuclear network. By Biitschli, however, and some of his fol- lowers the nucleus is regarded as an alveolar structure, the walls of which represent the "netw^ork," while the ground-substance corre- sponds to the alveolar material. Nearly related with this is the view of Reinke ( 94) that the ground-substance consists of large pale frranules of 'Manthanin " or ** oedematin."
The configuration of the chromatic network varies greatly in dif- ferent cases. It is sometimes of a very loose and open character, as in many epithelial cells (Fig. i); sometimes extremely coarse and irregular, as in leucocytes (Fig. 49); sometimes so compact as to appear nearly or quite homogeneous, as in the nuclei of spermatozoa and in many Protozoa. In some cases the chromatin does not form a network, but appears in the form of a thread closely similar to the spireme-stage of dividing nuclei (r/. p. 65). The most striking case of this kind occurs in the salivary glands of dipterous \?.x\'^ {^Chiron 0- viHs\ where, as described by Balbiani, the chromatin has the form of a single convoluted thread, composed of transverse discs and termi- nating at each end in a large nucleolus (Fig. 14, A). Somewhat simi- lar nuclei (Fig. 14, B) occur in various epithelial cells of other insects (Van Gehuchten, Gilson), and also in the young ovarian eggs of cer- tain animals (r/. p. 273). In certain gland-cells of the marine isopod Anilocra it is arranged in regular rosettes (Vom Rath). Rabl, fol- lowed bv Van Gehuchten, Heidenhain, and others, has endeavoured to show that the nuclear network shows a distinct polarity, the nucleus having a *' pole " toward which the principal chromatin- threads converge, and near which the centrosome lies.- In many nuclei, however, no trace of such polarity can be discerned.
The network may undergo great changes both in physical con- figuration and in staining capacity at different periods in the life of the same cell, and the actual amount of chromatin fluctuates, sometimes to an enormous extent. Embryonic cells are in general
1 Cf. pp. 126-130. 2 cf. the polarity of the cell, p. 55.
THE NUCLEUS
17
characterized by the large size of the nucleus; and Zacharias has shown in the case of plants that the nuclei of meristem and other embryonic tissues are not only relatively large, but contain a larger percentage of chromatin than in later stages. The relation of these changes to the physiological activity of the nucleus is still imperfectly understood.^
2. Finer Stnictiire of tJie Nucleus
A considerable number of observers have raised the question whether the nuclear structures may not be regarded as aggregates of more elementary morphological bodies, though there is still no general agreement regarding their nature and relationships. The most definite evidence in this direction relates to the chromatic network. In the stages preparatory to division this network resolves itself into a definite number of rod-shaped bodies known as chroniosoines (Fig. 21), which split lengthwise as the cell divides. These bodies arise as aggregations of minute rounded bodies or microsomes to which various names have been g\vQx\{cJiroi)io- mcres, Fol ; ids, Weismann). They are as a rule most clearly visible and most regularly arranged during cell- division, when the chromatin is ar- ranged in a thread {spireme), or in separate cJironiosoines (Figs. 8, D, 53, B)\ but in many cases they are dis- tinctly visible in the reticulum of the scatteredchromatin-graniiles. [GRUBKK.]
"resting" nucleus ,(Fig. 54). It is,
however, an open question whether the chromatin-granules of the reticulum are individually identical with those forming the chromo- somes or the spireme-thread. The larger masses of the reticu-
Fig. 15. — An infusorian. Trachelo- cerca, with diffused nucleus consisting of
1 Both chromatin-granules and nucleoli have been seen in a considerable number of living cells (Fig. 9). Favourable objects for this purpose are according to Korschelt ('96) the silk- glands of caterpillars, where the whole nucleus may be seen to be filled with fine granules ("microsomes"), among which are scattered many larger granules (" macrosomes "). 'I he later studies of Meves ('97, i) make it probable that the latter are true nucleoli and the for- ?r chromatin-granules. Korschelt, however, regards the "macrosomes" as composed of romatin and the "microsomes" as representing the so-called "achromatic sulistance."
me ch
38 GENERAL SKETCH OE THE CELL
lum undoubtedly represent aggregations of such granules, but whether the latter completely fuse or remain always distinct is unknown. Even the chromosomes at certain stages appear perfectly homoge- neous, and the same is sometimes true of the entire nucleus, as in the spermatozoon. It is nevertheless possible that the chromatin-gran- ules have a persistent identity and are to be regarded as morpho- logical units of which the chromatin is built up.^
Heidenhain ('93, 94), whose views have been accepted by Rcinke, Waldeyer, and others, has shown that the "achromatic" nuclear net- work is likewise composed of granules, which he distinguishes as /tint/ianiu- or <u;jr//;v;;/^?//;/-granules from the has ic/iroiULi t i fi-gr3.\\\i\Q.s> of the chromatic network. Like the latter, the oxychromatin-granules are suspended in a non-staining clear substance, for which he reserves the term //;////. Both forms of granules occur in the chromatic network, while the achromatic network contains only oxychromatin. Thev are sharply differentiated by dyes, the basichromatin being coloured by the basic tar-colours (methyl-green, saffranin, etc.) and other true "nuclear stains"; while the oxychromatin-granules, like many cytoplasmic structures, and like the substance of true nucleoli (pyrenin), are coloured by acid tar-colours (rubin, eosin, etc.) and other "plasma stains." This distinction, as will appear in Chapter VI I., is possibly one of great physiological significance.
Still other forms of granules have been distinguished in the nucleus by Reinke ('94) and Schloter ('94). Of these the most important are the " oedematin-granules," which according to the first of these authors form the principal mass of the ground-substance or " nuclear sap " of Hertwig and other authors. These granules are identified by both observers with the " cyanophilous granules," which Altmann regarded as the essential elements of the nucleus. It is at present impossible to give a consistent interpretation of the morphological value and physiological relations of these various forms of granules. The most that can be said is that the basichromatin-granules are probably normal structures ; that they play a principal role in the life of the nucleus ; that the oxychromatin-granules are nearly related to them ; and that not improbably the one form may be transformed into the other in the manner suggested in Chapter VII.
The nuclear membrane is not yet thoroughly understood, and much discussion has been devoted to the question of its origin and structure. The most probable view is that long since advocated by Klein i^yd)) and Van Beneden ('83) that the membrane arises as a condensation of the general protoplasmic substance, and is part of the same structure as the linin-network and the cytoplasmic mesh- w^ork. Like these, it is in some cases "achromatic," but in other cases
1 Cf. Chapter VI.
THE NUCLEUS
39
it shows the same staining reactions as chromatin, or may be double, consisting of an outer achromatic and an inner chromatic layer. Ac- cording to Reinke, it consists of oxychromatin-granules like those of the linin-network.
Interesting questions are raised by a comparison of these facts with the conditions observed in some of the lowest organisms, such as the flagellates and lower rhizopods among animals and the
A
D
«, • /
B
H
O
F
%
'%)
7
E
or distrib
Fig i6.- Forms of Cyanophyce^, Bacteria, and Flagellates showing the so-called scattered
distributed nuclei. [.4-C BuTSCHLi; Z?-/^ Schewiakoff; G-y. Calkin^.]
A. Oscillaria. B. Chromafmrn, C. Bacterium lineola. D. Achrowatiuw. E. The same m
division. /; Fission of the granules. G. 7l'//-</w////.^ with central sphereandscatteredgninu.es.
H. Aggregation of the granules. /. Division of the sphere. J. Fission of the cell.
Cyanophyce^ and Bacteria among plants. In many ol these forms (Fig. i6) no distinct nucleus can be demonstrated, the cell consistmg of a mass of protoplasm in which are scattered numerous deeply staining granules. Many of these granules stam mtensely with hematoxylin and other " nuclear" dyes; like chromatm, they resist the action of peptic digestion, and in at least one case (the bacterium- like AckromatiHm, according to Schewiakoff, '93) they have the power of division like the chromatin-granules of higher forms. For these
40
GENERAL SKETCH OF THE CELL
reasons most observers (Biitschli, Gruber, Schewiakoff, Nadson, etc.) recrard them as true chromatin-f(ranules which rein-esent a scattered or distributed nucleus not differentiated as a definite morphological body. If this identification is correct, such forms probably give us the most primitive condition of the nuclear substance, which only in higher forms is collected into a distinct mass enclosed by a membrane; and the scattered granules are comparable to those forming the chro- matin-reticulum and chromosomes in the higher types. The identi- fication is, however, difficult, owing to the impossibility of actual chemical analysis; and Fischer (97) has shown in the case of the Bacteria and Cyanophycene that we cannot safely trust either the staining reactions or the digestion test, since the former are variable, while the latter does not differentiate the granules from some other cytoplasmic constituents.^ It is, however, certain that the staining power of chromatin in the higher forms varies with different condi- tions, and furthermore there is reason to believe that these granules may divide by fission. Besides these observations of Schewiakoff on AcJiromatiuin (see above), we have those of several authors on Infusoria, and more recently those of Calkins on flagellates, both pointing to the same conclusion. Balbiani, Gruber, Maupas, and others have described various Infusoria (^/r^i-Zj/Z^', Trac/icloccrca, HolosticJia, Urolcptns), as well as some rhizopods {Pcloniyxa), in which the body contains very numerous minute chromatin-granules of "nuclei" (Fig. 15), which Gruber {"^'J) showed to multiply by division. Balbiani ('61) long since showed that in Urostyla these bodies become concentrated toward the centre of the cell at the time of division, and Bergh ('89) demonstrated that they then fuse to form a macronucleus of the usual type, that elongates, assumes a fibrillar structure, and divides by fission. After division of the cell-body the macronucleus again fragments into minute scattered granules, which in this case certainly represent a distributed nucleus. In the flagellate Tctramitus Calkins ('98, i) likewise finds numerous scat- tered chromatin-granules, which at the time of division become aggre- gated into a single dividing mass (p. 92); while in other forms the mass (nucleus) persists as such without (yTracJiclomouas, Lagcnclla, CJiiloDionas) or with {Euglcna, Synura) a surrounding membrane.
Taken together, the foregoing facts, while certainly not conclusive, give good ground for the provisional acceptance of Biitschli's con- ception of the distributed nucleus, and indicate that nucleus and cytoplasm have arisen through the differentiation of a common protoplasmic mass. The nucleus, as Carnoy has well said,- is like a
^ It should be remembered that we have no unerring " chromatin-stain." Cf. p. 335. =^'84, p. 251.
THE CYTOPLASM ^I
house built to contain the chromatic elements, and its achromatic ele- ments (Unin, etc.) were originally a part of the general cell-substance. Moreover, as Carnoy points out, the house periodically goes to pieces in the process of mitotic division, the chromatin afterward "buildin"- for itself a new dwelling."
3. Chemist)')' of tJie Nucleus
The chemical nature of the various nuclear elements will be considered in Chapter VII., and a brief statement will here suffice. The following classification of the nuclear substances, proposed by Schwarz in 1887, has been widely accepted, thou'di open to criticism on various grounds.
1. Chromatin. The chromatic substance (basichromatin) of the network and of
those nucleoli known as net-knots or karyosomes.
2. Linin. The achromatic network and the spindle fibres arising from it.
3. Paratiniii. The ground-substance.
4. Pyrcniii or Parachroniatiii. The inner mass of true nucleoli.
5. Ainphipyrenin. The substance of the nuclear membrane.
Chromatin is probably identical with imclein (p. 332). which is a compound of nucleiiiic acid (a complex organic acid, rich in phosphorus) and albuminous sub- stances. In certain cases (nuclei of spermatozoa, and probably also the chromo- somes at the time of mitosis) the percentage of nucleinic acid is very large (p. 'Ji'}^'},). The /////;/ is supposed to be composed of "plastin'' — a substance identified -by Reinke and Rodewald ('81) and probably a nucleo-albumin or a related substance. '• Pyrenin " is related to plastin ; and Carnoy and Zacharias apply the latter word to the nucleolar substance, while O. Hertw'ig calls it paranuclein. " Amphipyrenin"" has no very definite meaning ; for the nuclear membrane sometimes appears to be of the same nature as the linin, w'hile in other cases it stains like chromatin. For cri- tique of the staining reactions see page 334.
D. The Cytoplasm
It has long been recognized that in the unicellular forms the cytoplasmic substance is often differentiated into two well-marked zones : viz. an inner medullary substance or cjuioplasui in which the nucleus lies, and an outer cortical substance or exoplasm (ectoplasm) from which the more differentiated products of the cytoplasm, such as cilia, trichocysts, and membrane, take their origin. Indications of a similar differentiation are often shown in the tissue-cells of higher plants and animals, ^ though it may take the form of a polar differen- tiation of the cell-substance, or may be wholly wanting. Whether the distinction is of fundamental importance remains to be seen ; but it appears to be a general rule that the nucleus is surrounded by
1 This fact was first pointed out in the tissue-cells of animals liy Kupffer ('75). and its importance has since been urged by Waldeyer, Reinke, and others. The cortical layer is by Kupffer termed paraplasm, by p'feffer hyaloplasm, by Tringsheim the Haiitschiclit. The medullary zone is termed by Y.^x^^'^^x protoplasm, sensii strictu; by Strasburger, K'dmer- plasma ; by ^'■k<g€(\, polioplasm.
42 GENERAL SKETCH OF THE CELL
protoplasm of relatively slight differentiation, while the more highly differentiated products of cell-activity are laid down in the more peripheral region of the cell, either in the cortical zone or at one end of the cell.^ This fact is full of meaning, not only because it is an expression of the adaptation of the cell to its external environment, but also because of its bearing on the problems of nutrition. ^ For if, as we shall see reason to conclude in Chapter VI I., the nucleus be immediately concerned with synthetic metabolism, we should expect to find the immediate and less differentiated products of its action in its neighbourhood, and on the whole the facts bear out this view.
The most pressing of all questions regarding the cytoplasmic structure is whether the sponge-like, fibrillar, or alveolar appearance is a normal condition existing during life. There are many cases, especially among plant-cells, in which the most careful examination has thus far failed to reveal the presence of a reticulum, the cyto- plasm appearing, even under the highest powers and after the most careful treatment, merely as a finely granular substance. This and the additional fact that the cytoplasm may show active streaming and flowing movements, has led some authors, especially among bota- nists, to regard the reticulum as non-essential and as being, when present, either a secondary differentiation of the cytoplasmic sub- stance specially developed for the performance of particular functions or a mere coagulation-product due to the action of fixatives. It has been shown that structureless proteids, such as egg-albumin and other substances, when coagulated by various reagents, often show a structure closely similar to that of protoplasm as observed in micro- scopical sections. Flemming ('82) long since called attention to the danger of mistaking such coagulation-products for normal structures as seen in fixed and stained material, and his warning has been emphasized by the later experiments of Berthold {"^6\ Schwarz i^^jX and especially of Butschli ('92, '98), Fischer ('94, '95, '99), and Hardy ('99). Butschli's extensive studies of such coagulation-phe- nomena show that coagulated or dried albumin, starch-solutions, gela- tin, gum arable, and other substances show a fine alveolar structure scarcely to be distinguished from that which he believes to be the normal and typical structure of protoplasm. Fischer and Hardy have likewise made extensive tests of solutions of albumin, peptone, and related substances, in various degrees of concentration, fixed and stained by a great variety of the reagents ordinarily used for the demonstration of cell-structures. The result was to produce a mar- vellously close simnlacniui of the appearances observed in the cell, alveolar, reticulated, and fibrillar structures being produced that often contain granules closely similar in every respect to those described as
1 Cf. p. 55. 2 See Kupfter ('90), pp. 473-476-
THE CYTOPLASM
43
"microsomes " in sections of actual protoplasm. After impregnating pith with peptone-solution and then hardening, sectioning, and stain- ing, the cells may even contain a central nucleus-like mass suspended in a network of anastomosing threads that extend in every direction outward to the walls, and give a remarkable likeness of a normal cell. These facts show how cautious we must be in judging the appear- ances seen in preserved cells, and justify in some measure the hesita-
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::.«.-V".»..*"'„^ti. ■
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Fig. 17. — Ciliated cells, showing cytoplasmic fibriilcTe terminating in a zone of peripheral microsomes to which the cilia are attached. [Engelmann.]
A. From intestinal epithelium of Anodonta. B. From gill of Anodonta. CD. Intestinal epi- thelium of Cyclas.
tion with which many existing accounts of cell-structure are received. The evidence is nevertheless overwhelmingly strong, as I believe, that not only the fibrillar and alveolar formations, but also the micro- somes observed in cell-structures, are in part normal structures. This evidence is derived partly from a study of the living cell, partly from the resfular and characteristic arrangement of the thread-work and
44
GENERAL SKETCH OF THE CELL
microsomes in certain cases. In many Protozoa, for example, a fine alveolar structure may be seen in the living jirotoplasm ; and Flem- ming as well as manv later observers has clearly seen fibrillar struc- tures in the living cells of cartilage, epithelium connective-tissue, and some other animal cells ( Fig. 9). Mikosch, also, has recently described ^;v7;///A7/' threads in living plant-cells.
Almost equally conclusive is the beautifully regular arrangement of the fibrillct' in ciliated cells ( Fig. 17, Fngelmann), in muscle-fibres and ncrve-hbres, and especially in the mitotic figure of dividing cells
B
C
D
Fig. 18. — Cells of the pancreas in Amphibia. [Mathfavs.]
A-C. Nectiiriis ; D. Rami. A and H represent two stages of the " loaded " cell, showing zymogen-granules in the-peripheral and fibrillar structures in tlie basal part of the cell. C shows cells after discharge of the granule-material and invasion of the entire cell by fibrillie. In D por- tions of the fibrillar material are coiled to form the mitosome (" paranucleus " or " Nebenkern "}.
(Figs. 2 1, 31), where they are likewise more or less clearly visible in life. A very convincing case is afforded by the pancreas-cells of Nccturiis, which Mathews has carefully studied in my laboratory. Here the thread-work consists of long, conspicuous, defmite fibrillae, some of which may under certain conditions be wound up more or less closely in a spiral mass to form the so-called Nebcjikern. In all these cases it is impossible to regard the thread-work as an accidental coagulation-product. In the case of echinoderm eggs, I have made ('99) a critical comparison of the living structure, as seen under powers
THE CYTOPLASM
45
of a thousand diameters and upwards, with the same ohject stained in thin sections after fixation by picro-acetic, subHmate-acetic, and
*•/.*••■; ■ ,.'-•,■
«I^-
^*^;s^SsyiiJ^i^/(^j^J^
Fig. 19. — Section through a nephridia! cell of the leech, Clepstite (drawn by Arnold Graf from
one of his own preparations).
The centre of the cell is occupied by a large vacuole, filled with a watery licinid. The cyto- plasm forms a very regular and distinct reticulum with scattered microsomes which become very large in the peripheral zone. The larger pale bodies, lying in the ground-substance, are cvcretory granules {i.e. metaplasm). The nucleus, at the right, is surrounded by a thick chromatic mem- brane, is traversed by a very distinct linin-network, contains numerous scattered chromatin- granules, and a single large nucleolus within which is a vacuole. Above are two isolated nuclei showing nucleoli and chromatin-granules suspended in the linin-threads.
Other reagents. The comparison leaves no doubt that the normal structures are in this case very perfectly preserved, thoui^h the sec- tions give at first sight an appearance somewhat different from that
46 GEXERAL SKE7CII OF THE CELL
of the living object, owing to differences of staining capacity. In these eggs the microsomes, thickly scattered through the alveolar walls, stain deeply (Figs. ii. i J ), while the alveolar spheres hardly stain at all. When, therefore, the stained sections are cleared in balsam, the contours of the alveolar spheres almost disappear, and the eye is caught by the walls, which give at first sight quite the appear- ance of a granular reticulum, as it has been in fact described by many observers. Careful study of the sections shows, however, that t lie form and arraui^cjuiut of all the elements is almost idcntieally t/ie same as in life.
This result shows that careful treatment by reagents in some cases at least gives a very faithful picture of the normal structure ; and while it should never be forgotten that in sections we are viewing coagulated material, much of which is liquid or semi-liquid in life, w^ should not adopt too pessimistic a view of the results based on fixed material, as I think some of the experimenters referred to above have done. Wherever possible, the structures observed in sections should be compared with those in the living material. When this is imprac- ticable we must rely on indirect evidence; but this is in many cases hardly less convincing than the direct.
It is a very interesting and important question whether living protoplasm that appears to the eye to be homogeneous does not really possess a structure that is invisible, owing to the extreme tenuity of the fibrillar or alveolar walls (as was long since suggested by Heitz- mann and Biitschli),^ or to uniformity of refractive index in the structural elements. It is highly prc^bable that such is often the case; indeed, Butschli has shown that such *' homogeneous " protoplasm in Protozoa may show a typical alveolar structure after fixation and staining. This explanation will not, however, apply to the young echinoderm eggs (already referred to at p. 28), where the genesis of the alveolar structure may be follow'ed step by step in the li\ing cell. The protoplasm here appears at first almost like glass, showing at most a sparse and fine granulation ; but after fixing and staining it appears as a mass of fine, closely crowded granules. This may indi- cate the existence of an extremely fine alveolar structure in life; but on the whole I believe that these granules are for the most part coagu- lation-products, since they cannot be demonstrated by staining ijitra vitam, and they very closely resemble the coagulation-granules found in structureless proteids like egg-albumin after treatment by the same reagents. In common with many other investigators, therefore, I believe that protoplasm may in fact be homogeneous dowji to the present limits of microscopical vision.
One of the must beautiful forms of cyto-reticulum with which I
1 Cf Butschli, '92, 2, p. 169.
THE CYTOPLASM
A7
am acquainted has been described by Bolsius and Graf in the ncnh ridial cells of leeches as shown in Fig. 19 (from a preparation by Dr. Arnold Graf). The meshwork is here of great di.stinctness and regularity, and scattered microsomes are found along its threads. It
Fig. 20. — Spinal ganglion-cell of the frog. [Lknhossek.] The nucleus contains a single intensely chromatic nucleolus, and a paler linin-network with rounded chromatin-granules. The cytoplasmic fibrillae are faintly shown passing out into the nerve-process below. (They are figured as far more distinct by Flemming.) Tlie dark cyto- plasmic masses are the deeply staining " chromophiiic granules" (.N'issi) of unknown function. (The centrosome, which lies near the centre of the cell, is shown in Fig. 8, C.) At the left, two connective tissue-cells.
appears with equal clearness, though in a somewhat different form, in many eggs, where the meshes are rounded and often contain food- matters or deutoplasm in the inter-spaces (Figs. 59, 60). In cartilage- cells and connective tissue-cells, where the threads can be plainly seen
48 GENERAL SKETCH OF THE CELL
in life, the network is loose and open, and appears to consist of more or less completely separate threads (F'ig. 9). In the cells of colum- nar epithelium, the threads in the peripheral part of the cell often assume a more or less parallel course, passing outwards from the central region, and giving the outer zone of the cell a striated appear- ance. This is very conspicuously shown in ciliated epithelium, the fibrill:^ corresponding in number with the cilia as if continuous with their bases (Fig. 17).^ In nerve-fibres the threads form closely set parallel fibrillce which may be traced into the body of the nerve-cell; here, according to most authors, they break up into a network in which are suspended numerous deeply staining masses, the "chromo- philic granules" of Nissl (Fig. 20).- In the contractile tissues the threads are in most cases very conspicuous and have a parallel course. This is clearly shown in smooth muscle-fibres and also, as Ballowite has shown, in the tails of spermatozoa. This arrangement is most striking in striped muscle-fibres where the fibrillae are extremely well marked. According to Retzius, Carnoy, Van Gehuchten, and others, the meshes have here a rectangular form, the principal fibrillar having a longitudinal course and being connected at regular intervals by transverse threads ; but the structure of the muscle-fibre is probably far more complicated than this account would lead one to suppo.se, and opinion is still divided as to whether the contractile substance is represented by the reticulum proper or by the ground-substance.
Nowhere, perhaps, is a fibrillar structure shown with such beauty as in dividing cells, where (Figs. 21, 31) the fibrillae group themselves in two radiating systems or asters, which are in some manner the immediate agents of cell-division. Similar radiating systems of fibres occur in amoeboid cells, such as leucocytes (Fig. 49) and pigment- cells (Fig. 50), where they probably form a contractile system by means of which the movements of the cell are performed.
The views of Biitschli and his followers, which have been touched on at p. 25, differ considerably from the foregoing, the fibrillae being regarded as the optical sections of thin plates or lamelte which form the walls of closed chambers filled by a more liquid substance. Butschli, followed by Rcinke, Eismond, Erlanger, and others, inter- prets in the same sense the astral systems of dividing cells which are regarded as a radial configuration of the lamellae about a central point (Fig. 10, B). Strong evidence against this view is, I believe,
1 The structure of the ciliated cell, as described by Engelmann, may be beautifully demon- strated in the funnel-cells of the nephridia and sperm-ducts of the earthworm.
2 The remarkable researches of Apathy ('97) on the nerve-cells of leeches have revealed the existence within the nerve-cell of networks far more complex and definite than was formerly supposed, and showing definite relations to incoming and outgoing fibrillae within the substance of the nerve-fibres.
THE CYTOPLASM
49
afforded by the appearance of the spindle and asters in cross-section. In the early stages of the ^g^ of Nereis, for example, the astral rays are coarse anastomosing fibres that stain intensely and are therefore very favourable for observation (Fig. 60). That they are actual fibres is, I think, proved by sagittal sections of the asters in which the rays are cut at various angles. The cut ends of the branching rays appear in the clearest manner, not as plates but as distinct dots, from which in obHque sections the ray may be traced inwards toward the centro- sphere. Druner, too, figures the spindle in cross-section as consisting of rounded dots, like the end of a bundle of wires, thou^-h these are connected by cross-branches (Fig. 28, F). Again, the crossin^r of
Centrospherc con- taining the cen- trosome.
Aster.
Spindle.
Chromosomes forming the equatorial plate.
Fig. 21. — Diagram of the dividing cell, showing the mitotic figure and its relation to the cyto- plasmic meshwork.
the rays proceeding from the asters (Fig. 128), and their beha^-iour in certain phases of cell-division, is difficult to explain under any other than the fibrillar theory.
We must admit, however, that the meshwork varies greatly in differ- ent cells and even in different physiological phases of the same cell ; and that it is impossible at present to bring it under any rule of uni- versal application. It is possible, nay probable, that in one and the same cell a portion of the meshwork may form a true alveolar structure such as is described by Biitschli, while other portions may. at the same time, be differentiated into actual fibres. If this be true the fibrillar or alveolar structure is a matter of secondary moment, and the essential features of protoplasmic organization must be sought in a more subtle underlying structure.^
1 See Chapter VI. E
50 GENERAL SKETCH OF THE CELL
Space would not sufifice for a comparative account of the endless modifications shown by the cytoplasmic substance in different forms of cells. Many of these arise through special differentiations of the active substance, the character of the structure thus being some- times so highly modified, as in the striated muscle-fibre, that it is difficult to trace its exact relation to the more usual forms. More commonly the cytoplasm is modified through the formation of passive or metaplasmic substances which often completely transform the original appearance of the cell. The most frequent of such modifi- cations arise through the deposit of liquid drops and *' granules " (many of the latter, however, being no doubt liquid in life). When the liquid drops arc of watery nature the cavities in which they lie are known as vacuoles^ which are especially characteristic of the pro- toplasm of plant-cells and of Protozoa. These may enlarge or run together to form extensive cavities in the cell, the protoplasm becom- ing reduced to a peripheral layer, or to strands and networks travers- ing the spaces ; while in some forms of unicellular glands the spaces may form branching canals traversing the protoplasm.
The vacuolization or meshlike appearance arising through the formation of larger vacuoles or the deposit of other metaplasmic material is not to be confounded with the primary protoplasmic struc- ture. When, however, smaller vacuoles or metaplasmic granules are evenly distributed through the protoplasm, a " pseudo-alveolar " struc- ture (Reinke) arises that can often hardly be distinguished from the " true " alveolar structure of Blitschli.^ Comparative study shows that all gradations exist between the "false " and the "true " alveolar structures and that no logical ground of distinction between the two exists.^ We thus reach ground for the conclusion that the coarser secondary alveolar or reticular formations are to be regarded as only an exaggeration of the primary structure, and that the alveolar mate- rial of *Biitschli's structure belongs in the same general category with the passive or metaplasmic substance.^
E. The Centrosome
The centrosome^ is usually an extremely minute body, or more commonly a pair of bodies, staining intensely with haematoxylin and
^ In the latter the alveolar spheres are, according to Biitschli, not more than one or two microns in diameter.
^ This has been demonstrated in the cells of plants by Craio ('96), and more recently by the writer ('99), in the case of echinoderm and other eggs.
^ QC p. 29.
* The centrosome was apparently first seen and described by Flemming in 1875, ^" ^^^ egg of the fresh-water mussel Anodonta, and independently discovered by Van Beneden, in
THE CENTROSOME 5 I
some other reagents, and surrounded by a cytoplasmic radiating aster or by a rounded mass known as the attr'actio}i-spJicrc (Figs. 8, 49, etc.). As a rule it lies in the cytoplasm, not far from the nucleus, and usually opposite an indentation or bay in the latter ; but in a few cases it lies inside the nucleus (Fig. 148). In epithelia the centro- somes (usually double) lie as a rule near the free end of the cell (Fig. 21)}
There is still much confusion regarding the relation of the centro- some to the surrounding structures, and this has involved a corre- sponding ambiguity in the terminology. We w^ill therefore only consider it briefly at this point, deferring a more critical account to Chapter VI. In its simplest form it is a single minute granule, which may, however, become double or triple (leucocytes, connective tissue- cells, some epithelial cells) or even multiple, as in certain giant-cells (Fig. 14, D\ and as also occurs in some forms of cell-division (Fig. 52). In some cases (Figs. 8, C, 120, 148) the ** centrosome " is a larger body containing one or more central granules or '* centrioles " (Boveri); but it is probable that in some of these cases the central granule is itself the true centrosome, and the surrounding body is part of the attraction-sphere. During the formation of the spermatozoon the centrosome undergoes some remarkable morphological changes (p. 171), and is closely involved in the formation of the contractile structures of the tail.
The nature and functions of the centrosome have formed the sub- ject of some of the most persistent and searching investigations of recent cytology. Van Beneden, followed by Boveri and many later workers, regarded the centrosome as a distinct and persistent cell- organ, which Hke the nucleus was handed on by division from one cell-generation to another. Physiologically it was regarded as being the especial organ of cell-division, and in this sense as the *' dy- namic centre" of the cell. In Boveri's beautiful development of this
the following year, in dycyemids. The name is due to Boveri ('88, 2, p. 68). Van Beneden's and Boveri's independent identification of centrosome in Ascaris as a permanent cell-organ ('87) was quicklv supported by numerous observations on other animals and on plants. In rapid succession the centrosome and attraction-sphere were found to be present m pig- ment-cells of fishes (Solger, '89, '90), in the spermatocytes of Amphil)ia (Hermann. Vto), hi the leucocytes, endothelial cells, connective tissue-cells, and lung-epithelium of salamanders (Flemming, '91), in various plant-cells (Guignard, '91), in the one-celled diatoms (Butschli. '91), in the giant-cells and other cells of bone-marrow (Heidenhain, Van Ban.heke, \ an der Stricht, '91), in the flagellate Noctiluca (Ishikawa, '91), in the cells of marine alg:v (Stras- burger, '92), in cartilage-cells (Van der Stricht, '92), in cells of cancerous growths j epitheli- oma, Lustig and Galeotti, '92), in the young germ-cells as already described, in gland-cells (Vom Rath, '95), in nerve-cells (Lenhossek, '95), in smooth muscle-hbres (Lenhossek, 99), and in embryonic cells of manv kinds (Heidenhain, '97)- •'^'any others have conhrmed and extended this list. Guignard's identification of the centrosomes in higher plants i. open to grave doubt (r/ p. 82). Q- P- 57-
52 GEXERAL SKETCH OF THE CELL
view it was regarded further as the especial fertiHzing element in the spermatozoon, which, when introduced into the fgg, endowed the latter with the power of division and development. Van Beneden's and Boveri's hypothesis, highly attractive on account of its simplicity and lucidity, is supported by many facts, and undoubtedly contains an element of truth ; yet recent researches have cast grave doubt upon its generality, and necessitate a suspension of judgment upon the entire matter. Many of the most competent recent workers on the cytologv of higher plants have been unable to find centrosomes, whether in the resting-cells, in the apparatus of cell-division, or dur- ing the process of fertilization, notwithstanding the fact that undoubted centrosomes occur in some of the lower plants. Among zoologists, too, an increasing number of recent investigators, armed with the best technique, have maintained the total disappearance of the cen- trosome at the close of cell-division or during the process of fertili- zation, agreeing that in such cases the centrosome is subsequently formed dc novo. Experimental researches, also, have given strong ground for the conclusion that cells placed under abnormal chemical conditions may form new centrosomes (p. 306). If these strongly supported results be well founded. Van Beneden's hypothesis must be abandoned in favour of the view that the centrosome is but a sub- ordinate part of the general apparatus of mitosis, and one which may be entirelv dispensed with. Thus regarded, the centrosome would lose somewhat of the significance first attributed to it, though still remaining a highly interesting object for further research.^
F. Other Organs
The cell-substance is often differentiated into other more or less definite structures, sometimes of a transitory character, sometimes showing a constancy and morphological persistency comj^arable with that of the nucleus and centrosome. From a general point of view the most interesting of these are the bodies known as /A? jr//V/jr ox proto- plasts{Y\g. 6), which, like the nucleus and centrosome, are capable of growth and division, and may thus be handed on from cell to cell. The most important of these are the cJirouiatopJiores or cJironioplastids, which are especially characteristic of plants, though they occur in some animals as well. These are definite bodies, varying greatly in form and size, which possess the power of growth and division, and have in some cases been traced back to minute colourless plastids or
^ Cf. pp. Ill, 304. Eisen ('97) asserts that in the blood of a salamander, Bairachoseps, the attraction-sphere (•' archosome ") containing the centrosomes may separate from the remainder of the cell (nucleated red corpuscles) to form an independent form of blood- corpuscle or " plasmocyte," which leads an active life in the blood.
OTHER ORGANS 53
leucoplastids in the embryonic cells. By enlargement and differen- tiation these give rise to the starch-builders (amyloplastids), to the chlorophyll-bodies (chloroplastids), and to other ])igment-bodies (chromoplastids), all of which may retain the power of division. The embryonic leucoplastids are also believed to multiply by division and to arise by the division of plastids in the parental organism ; but it remains an open question whether this is their only mode of origin, and the same is true of the more highly differentiated forms of plas- tids to which they may give rise.
The contractile or pulsating vacuoles that occur in most Protozoa and in the swarm-spores of many Algae are also known in some cases to multiply by division ; and the same is true, according to the researches of De Vries,. Went, and others, of the non-pulsating vacu- oles of plant-cells. These vacuoles have been shown to have, in many cases, distinct walls, and they are regarded by De Vries as a special form of plastid ("tonoplasts ") analogous to the chromatophores and other plastids. It is, however, probable that this view is only appli- cable to certain forms of vacuoles.
The plastids possess in some cases a high degree of morphological independence, and may even live for a time after removal from the remaining cell-substance, as in the case of the "yellow cells" of Radiolaria. This has led to the view, advocated by Brandt and others, that the chlorophyll-bodies found in the cells of many Protozoa and a few Metazoa {Hydra, Spongilla, some planarians) are in reality dis- tinct Alg^ living symbiotically in the cell. This view is probably correct in some cases, e.g. in the Radiolaria ; but it may be doubted whether it is of general appUcation. In the plants the plastids are. almost certainly to be regarded as differentiations of the protoplasmic
substance.
The existence of cell-organs which have the power of independent assimilation, growth, and division is a fact of great theoretical interest in its bearing on the general problem of cell-organization ; for it is one of the main reasons that have led De Vries, Wiesner, and many others to regard the entire cell as made up of elementary self-propa- gating units.
G. The Cell-membrane
The structure and origin of the cell-wall or membrane form a subject somewhat apart from our general purpose, since the wall belongs to the passive or metaplasmic products of protoplasm rather than to the living cell itself. We shall therefore treat it very briefly. Broadly speaking, animal cells are in general characterized by the slight development and relative unimportance of the cell-walls, while
54 GEXERAL SKETCH OE THE CELL
the reverse is the case in plants, where the cell-walls play a very important role. In the latter the wall sometimes attains a great thickness, usually displays a distinct stratification, and often has a complex sculj)ture. Such massive walls very rarely occur in the case of animal tissues, though the intercellular matrix of cartilage and bone is to a certain extent analogous to them, and the thick and often highly sculptured envelopes of some kinds of eggs and of various Protozoa may be placed in the same category.
It is open to question whether any cells are entirely devoid of an enclosing envelope; for even in such ** naked" cells as leucocytes, rhizopods, or membraneless eggs, the boundary of the cell is usually formed by a more resistant layer of protoplasm or " pellicle "(Biitschli) which may be so marked as to simulate a true membrane, as is the case, for example, in the red blood-corpuscles (Ranvier, Waldeyer, etc.). Such pellicles probably differ from true membranes only in degree ; but it is still an open question both in animals and in plants, how far true membranes arise by direct transformation of the periph- eral protoplasmic layer (the " Hautschicht " of botanists), and how far as a secretion-product of the protoplasm. In the case of animal cells, Leydig long since proposed ^ to distinguish between " cuticular " membranes, formed as secretions and usually occurring only on the free surfaces (as in epithelia), from *' true membranes " arising by direct transformation of the peripheral protoplasm. Later researches, including those of Leydig himself, have thrown so much doubt on this distinction that most later writers have used the term cuticular in a purely topographical sense to denote membranes formed only on one (the free) side of the ccll,^ leaving open the question of origin. The formation and growth of the cell-wall have been far more thor- oughly studied in plants than in animals, yet even here opinion is still divided. Most recent researches tend to sustain the early view of Nageli that the cell-wall is in general a secretion-product, though there are some cases in which a direct transformation of protoplasm into membrane-stuff seems to occur. ^ In the division of plant-cells the daughter-cells are in almost all cases cut apart by a cell-plate which arises in the protoplasm of the mother-cell as a transverse series of thickenings of the spindle-fibres in the equatorial region (Fig. 34). This fact, long" regarded by Strasburger and others as a proof of the direct origin of the membrane from the protoplasmic substance, is shown by Strasburger's latest work ('98) to be open to a quite different interpretation, the actual wall being formed by a splitting of the cell-plate into two layers between which the wall appears as a secretion-product. Almost all observers further are ao:reed that the formation of new membranes on naked masses of
1 Cf. '85, p. 12. "^ C/.O. Hertwig, '93. » Cf. Strasburger, '98.
POLARITY OF THE CELL
55
protoplasm produced by plasmolysis are likewise secretion-products, and that the secondary thickening of plant-membranes is produced in the same way. These facts, together with the scanty available zoological data, indicate that the formation of membranes by secre- tion is the more usual and typical process. ^
The chemical composition of the membrane or intercellular sub- stance varies extremely. In plants the membrane consists of a basis of cellidose, a carbohydrate having the formula CgHjoOg ; but this sub- stance is very frequently impregnated with other substances, such as silica, Hgnin, and a great variety of others. In animals the inter- cellular substances show a still greater diversity. Many of them are nitrogenous bodies, such as keratin, chitin, elastin, gelatin, and the like ; but inorganic deposits, such as silica and carbonate of lime, are common.
H. Polarity of the Cell
In a large number of cases the cell exhibits a definite polarity, its parts being symmetrically grouped with reference to an ideal organic axis passing from pole to pole. No definite criterion for the identi- fication of the cell-axis has, however, yet been determined ; for the general conception of cell-polarity has been developed in two differ- ent directions, one of which starts from purely morphological con- siderations, the other from physiological, and a parallelism between them has not thus far been fully made out.
On the one hand. Van Beneden ('83) conceived cell-polarity as a primary morphological attribute of the cell, the organic axis being identified as a line drawn through the centre of the nucleus and the centrosome (Fig. 22, A). With this view Rabl's theory ('85) of nuclear polarity harmonizes, for the chromosome-loops converge toward the centrosome, and the nuclear axis coincides with the cell- axis. Moreover, it identifies the polarity of the Qg^. which is so important a factor in development, with that of the tissue-cells; tor the egg-centrosome almost invariably appears at or near one pole of
the ovum.
Heidenhain ('94, '95) has recently developed this conception of polarity in a very elaborate manner, maintaining that all the struc- tures of the cell have a definite relation to the primary axis, and that this relation is determined by conditions of tension in the astral rays
1 Strasburger ('97, 3, '98) believes membrane-formation in general to be especially con- nected with the activity of the "kinoplasm," or tilar plasm of which he considers the " Haut- schicht," as well as the spindle-fibres, to be largely composed. In support ol this may be mentioned, besides the mode of formation of the partition-walls in the division of plant- cells, Harper's ('97) very interesting observations on the formation of the ascospores m Erysiphe (Fig. IZ), where the spore-membrane appears to arise directly from the astral rays.
56
GENERAL SKETCH OE THE CELL
focussed at the centrosome. On this basis he endeavours to explain the position and movements of the nucleus, the succession of division- planes, and many related phenomena.^
Hatschek {^^^) and Rabl ('89, 92), on the other hand, have ad- vanced a quite different hypothesis based on physiological considera- tions. By "cell-polarity" these authors mean, not a predetermined morphological arrangement of parts in the cell, but a polar differen- tiation of the cell-substance arising secondarily through adaptation of the cell to its environment in the tissues, and having no necessary relation to the polarity of Van Beneden (Fig. 22, B, C). This is
|
, |
\ |
|
r.. |
. ^ |
|
• • • • . • 0 |
• • • •• • • |
|
• 0 |
9 • • |
|
. • |
• • |
|
1 |
|
A
Van Beneden.
B C
Rabl, Hatschek.
Fig. 22. — Diagrams of cell-polarity.
A. Morphological polarity of Van Beneden. Axis passing through nucleus and centrosome. Chromatin-threads converging toward the centrosome. B.C. Physiological polarity of Rabl and Hatschek, Zj' in a gland-cell, C'in a ciliated cell.
typically shown in epithelium, which, as Kolliker and Haeckel long since pointed out, is to be regarded, both ontogenetically and phy- logenetically, as the most primitive form of tissue. The free and basal ends of the cells here differ widely in relation to the food- supply, and show a corresponding structural differentiation. In such cells the nucleus usually lies nearer the basal end, toward the source of food, while the differentiated products of cell-activity are formed either at the free end (cuticular structures, cilia, pigment, zymogen- granules), or at the basal end (muscle-fibres, nerve-fibres). In the non-epithelial tissues the polarity may be lost, though traces of it are often shown as a survival of the epithelial arrangement of the embryonic stages.
1 Cf. p. 105.
POLARITY OF THE CELL
57
But, although this conception of polarity has an entirely different point of departure from Van Beneden's, it leads, in some cases at least, to the same result ; for the cell-axis, as thus determined, may coincide with the morphological axis as determined by the position of the centrosome. This is the case, for example, with both the spermatozoon and the ovum ; for the morphological axis in both is also the physiological axis about which the cytoplasmic differentia- tions are grouped. Recent researches have further shown that the same is the case in many forms of epithelia, where the centrosomes lie in the outer end of the cell, often very near the surface.^ (Fig- '2-1)
A
B
|
WM) |
km ■ |
|
|
te^J-i |
'.'.i |
C • D
Fig. 23. — Centrosomes in epithelial and other cells. [A, D, ZiMMERMANN ; E, Heidenhain
and COHN; F, Heidenhain.]
A. From gastric glands of man ; dead cell at the left. B. Uterine epithelium, man. C. From human duodenum ; goblet-cell, with centrosome in the middle. D. Corneal epithelium of monkev. E. Epithelial cells from mesoblast-somites, embryo duck. F. Red blood-corpuscles from the duck- embryo. The centrosomes are double in nearly all cases.
and the recent observations of Henneguy ('98) and Lenhossek (98,1) give reason to believe that the ** basal bodies" to which the ciHa of ciliated epithelium are attached may be the centrosomes.- These facts are of very high significance; for the position of the centro- some, and hence the direction of the axis, is here obviously related to the cell-environment, and it is difficult to avoid the conclusion that the latter must be the determining condition to which the intracellular relations conform. When applied to the germ-cells, this conclusion becomes of high interest ; for the polarity of the Qgg is one of the
1 Zimmermann, '98; Heidenhain and Cohn, '97.
2 cf. p. 356.
58 GEXERAL SKETCH OF THE CELL
primary conditions of development, and we have here, as I beUeve, a clue to its determination.^
I. Till: Cell in Relation to the Multicellular Body
In analyzing the structure and functions of the individual cell we are accustomed, as a matter of convenience, to regard it as an inde- pendent elementary organism or organic unit. Actually, however, it is such an organism only in the case of the unicelkilar j)lants and animals and the germ-cells of the multicellular forms. When we consider the tissue-cells of the latter, we must take a somewhat dif- ferent view. As far as structure and origin are concerned the tissue- cell is unquestionably of the same morphological value as the one-celled plant or animal ; and i)i tliis sense the multicellular body is equivalent to a colony or aggregate of one-celled forms. Physi- ologically, however, the tissue-cell can only in a limited sense be regarded as an independent unit ; for its autonomy is merged in a greater or less degree into the general life of the organism. From this point of view the tissue-cell must in fact be treated as merely a localized area of activity, provided it is true with the complete apparatus of cell-life, and even capable of independent action within certain limits, yet nevertheless a part and not a whole.
There is at present no biological question of greater moment than the means by which the individual cell-activities are coordinated, and the organic unity of the body maintained ; for upon this question hangs not only the problem of the transmission of acquired charac- ters, and the nature of development, but our conception of life itself. Schwann, the father of the cell-theory, very clearly perceived this ; and after an admirably lucid discussion of the facts known to him (*39), drew the conclusion that the life of the organism is essentially a composite ; that each cell has its independent life ; and that " the whole organism subsists only by means of the reciprocal action of the single elementary parts." ^ This conclusion, afterward elaborated by Virchow and Haeckel to the theory of the ** cell-state," took a very strong hold on the minds of biological investigators, and is even now widely accepted. It is, however, becoming more and more clearly apparent that this conception expresses only a part of the truth, and that Schwann went too far in denying the influence of the totality of the organism upon the local activities of the cells. It would of course be absurd to maintain that the whole can consist of more than the sum of its parts. Yet, as far as growth and development are con-
^ Cf. pp. 384, 424. We should remember that the germ-cells are themselves epithelial products. 2 Untersuchungen, Trans., p. 181.
THE CELL LN RELATION TO THE MULTICELLULAR BODY 59
cerned, it has now been clearly demonstrated that only in a Hmited sense can the cells be regarded as cooperating units. Thcv are rather local centres of a formative power pervading the growing mass as a whole,^ and the physiological autonomy of the individual cell falls into the background. It is true that the cells may acquire a high degree of physiological independence in the later stages of embryological development. The facts to be discussed in the eighth and ninth chapters will, however, show strong reason for the conclu- sion that this is a secondary result of development, through which the cells become, as it were, emancipated in a greater or less degree from the general control. Broadly viewed, therefore, the life of the multicellular organism is to be conceived as a whole ; and the appar- ently composite character which it may exhibit is owing to a second- ary distribution of its energies among local centres of action. ^
In this light the structural relations of tissue-cells become a ques- tion of great interest ; for we have here to seek the means by which the individual cell comes into relation with the totality of the organ- ism, and by which the general equihbrium of the body is maintained. It must be confessed that the results of microscopical research have not thus far given a very certain answer to this question. Though the tissue-cells are often apparently separated from one another by a non-living intercellular substance, which may appear in the form of solid walls, it is by no means certain that their organic continuity is thus actually severed. Many cases are known in which division of the nucleus is not followed by division of the cell-body, so that multi- nuclear cells or syncytia are thus formed, consisting of a continuous mass of protoplasm through which the nuclei are scattered. Heitz- mann long since contended ( '73), though on insufficient evidence, that division is incomplete in nearly all forms of tissue, and that even when cell-walls are formed they are traversed by strands of protoplasm by means of which the cell-bodies remain in organic continuity. The whole body was thus conceived by him as a syncytium, the cells being no more than nodal points in a general reticulum, and the body forming a continuous protoplasmic mass.
This interesting view, long received with scepticism, has been to a considerable extent sustained by later researches, and though it still x^m-d^x^-^siibjiidice, has been definitely accepted in its entirety by some recent workers. The existence of protoplasmic cell-bridges between the sieve-tubes of plants has long been known ; and Tangl's dis- covery, in 1879, of similar connections between the endosperm-cells was followed by the demonstration by Gardiner, Kienitz-Gerloff, A. Meyer, and many others, that in nearly all plant-tissues the cell-walls
1 (7: Chapters VIII.. IX.
2 for a fuller discussion see pp. 388 and 413.
60 GENERAL SKETCH OF THE CELL
are traversed by delicate intercellular bridges. Similar bridges have been conclusively demonstrated by Ranvier, J^izzozero, Rctzius, Flem- ming, Pfitzner, and many later observers in nearly all forms of epithe- lium ( Fig. I ) ; and they are asserted to occur in the smooth muscle-fibres, in cartilage-cells and connective tissue-cells, and in some nerve- cells. Dendy ('88), Paladino ( '90), and Retzius ('89) have endeav- oured to show, further, that the follicle-cells of the ovary are connected by protoplasmic bridges not only with one another, but also with tJic oviiDi ; and similar protoplasmic bridges between germ-cells and somatic cells have been also demonstrated in a number of plants, e.g. by Goroschankin ( '83) and Ikeno ( '98) in the cycads and by A. Meyer ('96) in ]\)hox. On the strength of these observations some recent writers have not hesitated to accept the probability of Heitz- mann's original conception, A. Meyer, for example, expressing the opinion that both the plant and the animal individual are continuous masses of protoplasm, in which the cytoplasmic substance forms a morphological unit, whether in the form of a single cell, a multi- nucleated cell, or a system of cells. ^ Captivating as this hypothesis is, its full acceptance at present would certainly be premature ; and as far as adult animal tissues are concerned, it still remains unde- termined how far the cells are in direct protoplasmic continuity. It is obvious that no such continuity exists in the case of the corpuscles of blood and lymph and the wandering leucocytes and pigment-cells. In case of the nervous system, which from an a priori point of view would seem to be above all others that in which protoplasmic con- tinuity is to be expected, its occurrence and significance are still a subject of debate. When, however, we turn to the embryonic stages we find strong reason for the belief that a material continuity between cells here exists. This is certainly the case in the early stages of many arthropods, where the whole embryo is at first an unmistakable syncytium ; and Adam Sedgwick has endeavoured to show that in Pcripatns and even in the vertebrates the entire embryonic body, up to a late stage, is a continuous syncytium. I have pointed out ( '93) that even in a total cleavage, such as that of AnipJiioxus or the echi- noderms, the results of experiment on the early stages of cleavage are difficult to explain, save under the assumption that there must be a structural continuity from cell to cell that is broken by mechan- ical displacement of the blastomeres. This conclusion is supported by the recent work of Hammar ( '96, '97), whose observations on sea-urchin eggs I can in the main confirm.
Among the most interesting observations in this direction are those of Mrs. Andrews ('97),^ who asserts that during the cleavage
1 '96, p. 212. Cf. also the views of Hanstein, Strasburger. Russow, and others there cited. ^ Cf. also E. A. Andrews, '98, I, '98, 2.
THE CELL IN RELATION TO THE MULTICELLULAR BODY 6 1
of the echinoderm-egg the blastomeres ** spin " dehcate protoplasmic filaments, by which direct protoplasmic continuity is established between them subsequent to each division. These observations, if correct, are of high importance ; for if protoplasmic connections may be broken and re-formed at will, as it were, the adverse evidence of the blood-corpuscles and wandering cells loses much of its weight. Meyer ('96) adduces evidence that in Volvox the cell-bridges are formed anew after division ; and Flemming has also shown that when leucocytes creep about among epithehal cells they rupture the protoplasmic bridges, which are then formed anew behind them.^
We are still almost wholly ignorant of the precise physiological meaning of the cell-bridges ; but the facts indicate that they are not merely channels of nutrition, as some authors have maintained, but paths of subtler physiological impulse. Beside the facts determined by the isolation of blastomeres, referred to above, may be placed Townsend's recent remarkable experiments on plants, described at page 346. If correct, these experiments give clear evidence of the transference of physiological influences from cell to cell by means of protoplasmic bridges, showing that the nucleus of one cell may thus control the membrane-forming activity in an enucleated fragment of another cell. The field of research opened up by these and related researches seems one of the most promising in view; but until it has been more fully explored, judgment should be reserved regarding the whole question of the occurrence, origin, and physio- logical meaning of the protoplasmic cell-bridges.
LITERATURE. I •'-
Altmann, R.--Die Elementarorganismen und ihre Beziehungen zu den Zellen, 2d
ed. Leipzig, 1894. TAnnee Biologique. — /^^7;v>, 1895-96. (Full Reviews and Literature-lists.) Bohm and Davidoff . — Lehrbuch der Histologic des Menschen. Wiesbaden, 1895. Boveri, Th.— (See Lists IV., V.) Biitschli, 0. — Untersuchungen liber mikroskopische Schaume und das Protoplasma.
Zt'/^z^s/;^ (Engelmann), 1892. Id. — Untersuchungen liber Struktur. Leipzig, 1898. Carnoy, J. B. — La Biologie Cellulaire. /./Wvr. 1884. Engelmann, T. W. — Zur Anatomic und Pliysiologie der Flimmerzellen: Arch. ges.
Phvs., XXIII. 1880. , .^ ,
Erlanger, R. v. Neuere Ansichten liber die Struktur des Protoplasmas : ZooL
CentralbUUl-^j9' 1896. Fischer, A. Fixierung, Farbung und Bau des Protoplasmas. Jemu 1899. Flemming, W. Zellsubstanz, Kern und Zellteikmg. Leipzig, 1882. Id. ZeW^: MerkelundBonHet^sErgebnisse,\.-\'n. 1891-97- (Admirable reviews
and literature-lists.)
1 '95, pp. lo-i I ; '97, p. 261 . ' See also Introductory list, p. 14.
62 GENERAL SKETCH OF THE CELL
Heidenhain, M. — Uber Kern und Protoplasma : FestscJir. z. ^o-Jii/ir. Doctorjub. 7'Oh
7'. KolUkcr. Leipzig^ i^93- Klein, E. — Observations on the Structure of Cells and Nuclei : (J/^d^'i- Journ. Mic.
.SV/.. Will. 1878. Kolliker, A. — Handbucii der Gewebelehre, 6th ed. Leipzig, 1889. Leydig, Fr. — Zelle und Gewebe. Boiui. 1885. Schafer, E. A. — General Anatomy or Histology; in Quaifi's Anatof/ty, I., 2, loth
ed. London, 1891. Schiefferdecker & Kossel. — Die Gewebe des Menschlichen Korpers. Braunschweig,
I Sc) I . Schwarz, Fr. — Die morphologische und chemische Zusammcnsetzung des Proto-
plashias. Jyres/au, 1887. Strasburger, E. — Zcllbildung und Zellteilung. 3d ed. 1880. Id. — Das Botanische Practicum. 3d ed. Jena. 1897. Strasburger, Noll, Schenck, and Schimper. — Lehrbuch der Botanik, 3d ed. Jenay
1897. Strieker, S. — Handbuch der Lehre von den Geweben. Leipzig, 1871. Thoma, R. — Text-book of General Pathology and Pathological Anatomy: trans, by
Alex. Bruce. London, 1896. Van Beneden, E. — (See Lists II.. IV.) De Vries, H. — Intracellulare Pangenesis, /ena, 1889. Waldeyer, W. — Die neueren Ansichten liber den Bau und das Wesen der Zelle :
Deu'.sch. Med. IVoc/ienschr., Oct., Nov., 1895. Wiesner, J. — Die Elementarstruktur u. das Wachstum der lebenden Substanz :
U'ien, Holder. 1892. Wilson, E. B. — The Structure of Protoplasm: Journ. Morph., XV. Suppl. ; also
Wood's U oil Biol. Lectures, 1899. Zimmermann, A. — Beitrage zur Morphologic und Physiologic der Pflanzenzelle.
Tubingen, 1893. Id. — Die Morphologic und Physiologic des Pflanzlichen Zellkernes. Jena, 1896.
CHAPTER II
CELL-DIVISION
" Wo eine Zelle entsteht, da muss eine Zelle vorausgegangen sein, ebenso wie das Thier nur aus deni Thiere, die Pflanze nur aus der Pflanze entstehen kann. Auf diese Weise ist wenngleich es einzelne Punkte im Korper gibt, wo der strenge Nachweis noch nicht gelie- fert ist, doch das Princip gesichert, dass in der ganzen Reihe alles Lebendigen, dies mogen nun ganze Pflanzen oder thierische Organismen oder integrirende Theile derselben sein, ein ewiges Gesetz der coiithiuir lichen EntzvicklunghQ.%i&\i\.r ViKCHow.^
The law of genetic cellular continuity, first clearly stated by Vir- chow in the above words, has now become one of the primary data of biology, and the advance of research is ever adding weight to the conclusion that the cell has no other mode of origin than by division of a preexisting cell. In the multicellular organism all the tissue- cells arise by continued division from the original germ-cell, and this in its turn arises by the division of a cell preexisting in the parent-body. By cell-division, accordingly, the hereditary substance is split off from the parent-body ; and by cell-division, again, this substance is handed on by the fertilized egg-cell or oosperm to every part of the body arising from it.^ Cell-division is, therefore, one of the central facts of development and inheritance.
The first two decades after Schleiden and Schwann ('40-'6o) were occupied with researches, on the part both of botanists and of zool- ogists, which finally demonstrated the universality of this process and showed the authors of the cell-theory to have been in error in asserting the independent origin of cells out of a formative blastema.'"^ The mechanism of cell-division was not precisely investigated until long afterward, but the researches of Remak ('41), Kolliker (44), and others showed that an essential part of the process is a division of both the nucleus and the cell-body. In 1855 {I.e., pp. 174, 175), and again in 1858, Remak gave as the' general result of his researches the following synopsis or scheme of cell-division. Cell-division, he asserted, proceeds from the centre toward the periphery. It begins with the division of the nucleolus, is continued by simple constriction and division of the nucleus, and is completed by division of the cell-
1 Cellularpathologie, p. 25, 1858. 2 Cf. Introduction, p. 10.
3 For a full historical account of this period, see Remak, Untersttchungen iiher die Ent- wickhmg derWirbelthiere, 1855, PP- ^ 64-1 80. Also Tyson on the Cell-doctrine and Sachs's Geschichte der Botajiik.
63
64
CELL-DIVISION
body and membrane (Fig. 24). For many years this account was accepted, and no essential advance beyond Rcmak's scheme was made for nearly twenty years. A number of isolated observations were, however, from time to time made, even at a very early period, which seemed to show that cell-division was by no means so simple an operation as Remak believed. In some cases the nucleus seemed to disappear entirely before cell-division (the germinal vesicle of the ovum, according to Reichert, Von Ikier, Robin, etc.); in others to become lobed or star-shaped, as described by Virchow and by Remak himself (Fig. 24,/). It was not until 1873 that the way was opened for a better understanding of the matter. \\\ this year the discoveries of Anton Schneider, quickly followed by others in the same direction by Biitschli, Fol, Strasburger, Van Beneden, Flemming, and Hertwig, showed cell-division to be a far more elaborate process than had been
supposed, and to involve a com- plicated transformation of the nucleus to which Schleicher ('78) afterward gave the name of karyokincsis. It soon ap- peared, however, that this mode of division was not of universal occurrence ; and that cell-divi- sion is of two widely different types, which Van Beneden {'j^^ distinguished as fnii^Di 01 ta t io n , T?;„ . T^- ♦ ^- • • f n^-,^ .oUc ;^ Corresponding nearly to the
Fig. 24. — Direct division of blood-cells in J^ ^ J ^
the cmbrvo chick, illustrating Remak's scheme. simple proCCSS described by
[Remak.] Remak, and division, involving
a-e. Successive stasres of division; f. cell ,, t i. j
dividing by mitosis. ' -^ the morc Complicated process
of karyokincsis. Three years later Flemming ('79) proposed to substitute for these the terms direct and iJidircct division, which are still used. Still later ('82) the same author suggested the terms mitosis (indirect or karyokinetic division) and auiitosis (direct or akinetic division), which have rapidly made their w^ay into general use, though the earlier terms are often em- ployed. '
Modern research has demonstrated the fact that amitosis or direct division, regarded by Remak and his immediate followers as of uni- versal occurrence, is in reality a rare and exceptional process ; and there is reason to believe, furthermore, that it is especially char- acteristic of highly specialized cells incapable of long-continued multiplication or such as are in the early stages of degeneration, for instance, in glandular epithelia and in the cells of transitory embry- onic envelopes, where it is of frequent occurrence. Whether this
OUTLINE OF INDIRECT DIVISION 65
view be well founded or not, it is certain that in all the hifjjher and in many of the lower forms of Ufe, indirect division or mitosis is the typical mode of cell-division. It is by mitotic division that the germ- cells arise and are prepared for their union during the process of maturation, and by the same process the oosperm segments and gives rise to the tissue-cells. It occurs not only in the highest forms of plants and animals, but also in such simple forms as the rhizopods, flagellates, and diatoms. We may, therefore, justly regard it as the most general expression of the "eternal law of continuous develop- ment" on which Virchow insisted.
A. Outline of Indirect Division or Mitosis (Karyokixesis)
In the present state of knowledge it is somewhat difficult to give a connected general account of mitosis, owing to the uncertainty that hangs over the nature and functions of the centrosome. For the pur- pose of the following preliminary outline, we shall take as a type mitosis in which a distinct and persistent centrosome is present, as has been most clearly determined in the maturation and cleavage of various animal eggs, and in the division of the testis-cells. In such cases the process involves three parallel series of changes, which affect the nucleus, the centrosome, and the cytoplasm of the cell-body respectively. For descriptive purposes it may conveniently be divided into a series of successive stages or phases, which, however, graduate into one another and are separated by no well-defined limits. These are: (i) The Pi'opJiases, or preparatory changes; (2) the Mctaphasc, which involves the most essential step in the division of the nucleus ; (3) the Anaphases, in which the nuclear material is distributed ; (4) the Telophases, in which the entire cell divides and the daughter-cells are formed.
I. Prophases. — {a) The Nticletis. As the cell prepares for division, the most conspicuous fact is a transformation of the nuclear substance, involving both physical and chemical changes. The chromatin-sub- stance rapidly increases in staining-power, loses its net-like arrange- ment, and finally gives rise to a definite number of separate intensely staining bodies, usually rod-shaped, known as cliroiuosoines. As a rule this process, exemplified by the dividing cells of the salamander-epi- dermis (Fig. i) or those of plant-meristem (Fig. 2), takes place as fol- lows. The chromatin resolves itself little by little into a more or less convoluted thread, known as the 5/r/;/(Knauel)or spireme, and its sub- stance stains far more intensely than that of the reticulum (Fig. 25). The spireme-thread is at first fine and closely convoluted, forming the close spireme." Later the thread thickens and shortens and the
n
66
CELL-DIVISION
convolution becomes more open ('* open spireme"). In some cases there is but a single continuous thread ; in others, the thread is from
E
Fig. 25. — Diagrams showing the prophases of mitosis.
A. Resting cell with reticular nucleus and true nucleolus; at c the attraction-sphere containing two centrosomes. B. Early prophase ; the chromatin forming a continuous spireme, nucleolus still present : abo%-e, the amphiaster {a). C. D. Two different types of later prophases. C. Disappear- ance of the primary spindle, divergence of the centrosomes to opposite poles of the nucleus (exam- ples, some plant-cells, cleavage-stages of many eggs). D. Persistence of the primary spindle (to form in some cases the " central spindle"), fading of the nuclear membrane, ingrowth of the astral rays, segmentation of the spireme-thread to form the chromosomes (examples, epidermal cells of salamander, formation of the polar bodies). E. Later prophase of type C\ fading of the nuclear membrane at the poles, formation of a new spindle inside the nucleus; precocious splitting of the chromosomes (the latter not characteristic of this tvpe alone). E. The mitotic figure established; e.f. the equatorial plate of chromosomes. {€/. Figs. 21, 27, 32, etc.)
OUTLINE OF INDIRECT DIVISION 6/
its first appearance divided into a number of separate pieces or seg- ments, forming a segmented spireme. In either case it ultimately breaks transversely to form the chromosomes, which in most cases have the form of rods, straight or curved, though they are sometimes spher- ical or ovoidal, and in certain cases may be joined together in the form of rings. The staining-povver of the chromatin is now at a maxi- mum. As a rule "the nuclear membrane meanwhile fades away and finally disappears, though there are some cases in which it persists more or less completely through all the phases of division. The chromosomes now lie naked in the cell, and the ground-substance of the nucleus becomes continuous with the surrounding cytoplasm
(Fig. 25, A£,/^V
The remarkable fact has now been established with high probability that eveiy species of plant or animal has a fixed and characteristic niDH- ber of cJiromosomes, zvhicJi regularly recurs in the division of all of its cells ; and in all forms arising by sexual reproduction the number is even. Thus, in some of the sharks the number is 36 ; in certain gas- teropods it is 32 ; in the mouse, the salamander, the trout, the lily, 24 ; in the worm Sagitta, 18 ; in the ox, guinea-pig, and in man - the num- ber is said to be 16, and the same number is characteristic of the onion. In the grasshopper it is 12; in the hepatic Pallavicinia and some of the nematodes, 8 ; and in Ascaris, another thread-worm, 4 or 2. In the crustacean Artemia it is 168.^ Under certain conditions, it is true, the number of chromosomes may be less than the normal in a given species; but these variations are only apparent exceptions (p. %'j). The even number of chromosomes is a most interesting fact, which, as will appear hereafter (p. 205 ), is due to the derivation of one-half the number from each of the parents.
The nucleoli differ in their behaviour in different cases. Net-knots, or chromatin-nucleoli, contribute to the formation of the chromosomes; and in cases such as Spirogyra {ViQ.\m\Q.x, "^6, and Moll, '93 ) or .-^r//- nosphcerium (R. Hertwig, '99), where the whole of the chromatin is at one period concentrated into a single mass, the whole chromatic figure thus appears to arise from a "nucleolus." True nucleoli or plasmo- somes sooner or later disappear ; and the greater number of observers agree that they do not take part in the chromosome-formation. In a considerable number of forms {e.g. during the formation of the j^olar
1 The spireme-formation is by no means an invariable occurrence in mit»)sis. In a consid- erable number of cases the chromatin-network resolves itself ilirectly into tlie chromosomes, the chromatic substance becoming concentrated in separate masses which never form a con- tinuous thread. Such cases are connected by various gradations with the " segmented spi-
reme."
2 Flemmino believes the number in man to be considerably greater than 16. ^ For a more complete list see p. 206.
6 8 CELL-Di I vs/oy
bodies in various eggs) the nucleolus is cast out into the cytoplasm as the spindle forms, to persist as a " metanucleus " for some time before its final disappearance (Fig. 104). More commonly the nucleolus fades away /;/ s/t//, sometimes breaking into fragments meanwhile, while the chromosomes and spindle are forming. The fate of the material is in this case only conjectural. An interesting view is that of Strasburger ('95, '97), who suggests that the true nucleoli are to be regarded as storehouses of " kinoplasmic " material, which is either directly used in the formation of the spindle, or, upon being cast out of the nucleus, adds to the cytnj-)lasmic store of " kinoplasm " avail- able for future mitosis.
{d) The AinpJiiastcr. Meanwhile, more or less nearly parallel with these changes in the chromatin, a complicated structure known as the avipJiiastcr {Yo\, '77) makes its appearance in the position formerly occupied by the nucleus (Fig. 2^,B-F). This structure consists of a fibrous spindle-shaped bodv, the spindle, at either pole of which is a star or aster formed of rays or astral fibres radiating into the sur- rounding cytoplasm, the whole strongly suggesting the arrangement of iron filings in the field of a horseshoe magnet. The centre of each aster is occupied by a minute body, known as the centrosoine (Boveri, 'ZZ\ which may be surrounded by a spherical mass known as the ^rw/rt^j/Z/^/r (Strasburger, '93). As the amphiaster forms, the chro- mosomes group themselves in a plane passing through the equator of the spindle, and thus form what is known as the equatorial plate.
The amphiaster arises under the influence of the centrosome of the resting cell, which divides into two similar halves, an aster being developed around each while a spindle stretches between them (Figs. 25, 27). In most cases this process begins outside the nucleus, but the subsequent phenomena vary considerably in different forms. In some forms (tissue-cells of the salamander) the amj^hiaster at first lies tangentially outside the nucleus, and as the nuclear membrane fades away, some of the astral rays grow into the nucleus from the side, become attached to the chromosomes, and finally pull them into posi- tion around the equator of the spindle, which is here called the ecu- tral spiudle (Figs. 25, D, F ; 27). In other cases the original spindle disappears, and the two asters pass to o]:>])osite poles of the nucleus (some plant mitoses and in many animal-cells). A spindle is now formed from ravs that grow into the nucleus from each aster, the nuclear membrane fading away at the poles, though in some cases it may be pushed in by the spindle-fibres for some distance before its disappearance (Figs. 25, 32). In this case there is apparently ho central spindle. In a few exceptional cases, finally, the amphiaster may arise inside the nucleus (p. 304).
The entire structure, resulting from the foregoing changes, is
OUTLINE OF I.XDIRECT DIVISION
69
known as the karyokiiictic or mitotic figure. It may be described as consisting of two distinct parts; namely, i, the chromatic figure, formed by the deeply staining chromosomes ; and, 2, the achromatic figure, consisting of the spindle and asters which, in general, stain but slightly. The fibrous substance of the achromatic figure is gener-
Fig. 26. — Diagrams of the later phases of mitosis.
G. Metaphase; splitting of the chromosomes {e.p.^. ». The cast-off nucleolus. //. Ana- phase ; the daughter-chromosomes diverging, between them the interzonal-fibres (/./.). or central spindle; centrosomes already doubled in anticipation of the ensuing -division. /. Late anaphase or telophase, showing division of the cell-body, mid-body at the equator of the spindle and bcgm- ning reconstruction of the daughter-nuclei. J. Division completed.
ally known as archoplasm (Boveri, '88), but this term is not applied to the centrosome within the aster.
2. Metaphase. — TYiO, prophases of mitosis are, on the whole, pre- paratory in character. The metaphase, which follows, forms the initial phase of actual division. Each chromosome splits lengthwise into two exactly similar halves, which afterward diverge to opposite poles of the spindle, and here each group of daughter-chromosomes
70
CELL-DIVISIOX
finally gives rise to a daiii^^hter-nucleus (Fig. 26). In some cases the splitting of the chromosomes cannot be seen until they have grouped themselves in the equatt)rial plane of the spindle ; and it is only in this case that the term " metaphase " can be applied to the mitotic figure as a whole. In a large number of cases, however, the splitting may take place at an earlier period in the spireme-stage, or even, in a few cases, in the reticulum of the mother-nucleus (Figs. 54. 55). Such variations do not, however, affect the essential fact that the clnvDiatic nctivork is converted into a thread^ which, ivhether continuous or discontinuous, splits tJiroughojit its entire lengtJi into two exactlj equivalent halves. The splitting of the chromosomes, discovered by Flemming in 1880, is the most significant and funda- mental operation' of cell-division; for by it, as Roux first pointed out {^^}i\ the entire substance of the chromatic network is precisely halved, and the daughter-nuclei receive precisely equivalent portions oj chro- matin from the mother-nucleus. It is very important to observe that the nuclear division always shows this exact quality, whether division of the cell-body be equal or unequal. The minute polar body, for example (p. 238), receives exactly the same amount of chromatin as the Q^^g, though the latter is of gigantic size as compared with the former. On the other hand, the size of the asters varies with that of the daughter-cells (Figs. 58, 175), though not in strict ratio. The fact is one of great significance for the general theory of mitosis, as will appear beyond.
3. Anaphases. — After splitting of the chromosomes, the daughter- chromosomes, arranged in two corresponding groups,^ diverge to oppo- site poles of the spindle, where they become closely crowded in a mass near the centre of the aster. As they diverge, the two groups of daughter-chromosomes are connected by a bundle of achromatic fibres, stretching across the interval between them, and known as the interzonal fibres or connecting fibres? In some cases these differ in a marked degree from the other spindle-fibres ; and they are believed by many observers to have an entirely different origin and function. A view now widely held is that of Hermann, who regards these fibres as belonging to a central spindle, surrounded by a peripheral layer of mantle-fibres to which the chromosomes are attached, and only exposed to view as the chromosomes separate.^ Almost invariably in the division of plant-cells and often in that of animal cells these
1 It was this fact that led Flemming to employ the word mitosis (fiiros, a thread).
2 This stage is termed by Flemming the dyasier, a term which should, however, be aban- doned in order to avoid confusion with the earlier word amphiafter. The latter convenient and appropriate term clearly has priority.
3 Verlnndutigsfasern of German authors ; filaments reunissants of Van Beneden.
* Cf. p. 105.
OUTLINE OF INDIRECT DIVISION
71
fibres show during this period a series of deeply staining thickenings in the equatorial plane forming the cell-plate or mid-body. In plant- mitoses this is a very conspicuous structure ( Fig. 34). In animal cells the mid-body is usually less developed and sometimes rudimentary, being represented by only a few granules or even a single one (Fig. 29). Its later history is described below.
4. Telophases. — In the final phases of mitosis, the entire cell divides in two in a plane passing through the equator of the spindle, each of the daughter-cells receiving a group of chromosomes, half of the spindle, and one of the asters with its centrosome. Meanwhile, a daughter-nucleus is reconstructed in each cell from the group of chromosomes it contains. The nature of this process- differs greatly in different kinds of cells. Sometimes, as in the epithelial cells of Amphibia, especially studied by Flemming and Rabl, and in many plant-cells, the daughter-chromosomes become thickened, contorted, and closely crowded to form a daugJiter-sph'enie , closely similar to that of the mother-nucleus (Fig. 29); this becomes surrounded by a mem- brane, the threads give forth branches, and -thus produce a reticular nucleus. A somewhat similar set of changes takes place in the seg- menting eggs of Ascaris (Van Beneden, Boveri). In other cases, as in many segmenting ova, each chromosome gives rise to a hollow vesicle, after which the vesicles fuse together to produce a single nucleus (Fig. 52). When first formed, the daughter-nuclei are of equal size. If, however, division of the cell-body has been unequal, the nuclei become, in the end, correspondingly unequal — a fact which, as Conklin and others have pointed out, proves that the size of the nucleus is controlled by that of the cytoplasmic mass in which it lies.
The fate of the achromatic structures varies considerably, and has been accurately determined in only a few cases. As a rule, the spindle-fibres disappear more or less completely, but a portion of their substance sometimes persists in a modified form i^e.g. the Nebenkern, p. 163). In dividing plant-cells, the cell-plate finally extends across the entire cell and splits into two layers, between which appears the membrane by which the daughter-cells are cut apart.i A nearly similar process occurs in a few animal cells,- but as a rule the cell-plate is here greatly reduced and forms no mem- brane, the cell dividing by constriction through the equatorial plane. Even in this case, however, the division-plane is often indicated before division takes place by a peculiar modification of the cyto- plasm in the equatorial plane outside the spindle (Fig. 30)- This region is sometimes called the cytoplasmic plate, in contradistinction to the spindle-plate, or mid-body proper. In the proi)hases and meta- 1 Cf. Strasburger, '98. 2 Cf. Hoffmann, '98.
7^
CELL-DIVISION
phases the astral rays often cross one another in the equatorial region outside the spindle. During the anaphases, however, this crossing disappears, the rays from the two asters now meeting at an angle along the cytoplasmic plate (Fig. 31). Constriction and division of the cell then occur.^
The aster may in some cases entirely disappear, together with the centrosome (as occurs in the mature dgg). In a large number of cases, however, the centrosome persists, lying either outside or more rarely inside the nucleus and dividing into two at a very early period. This'division is clearly a precocious preparation for the ensuing divi- sion of the daughter-cell, and it is a remarkable fact that it occurs as a rule during the early anaphase, before the mother-cell itself has divided. There are apparently, however, some cases in which the centrosome remains undivided during the resting stage and only divides as the process of mitosis begins.
Like the centrosome, the aster or its central portion may persist in a more or less modified form throughout the resting state of the cell, forming a structure generally known as the attraction-sphere. This body often shows a true astral structure with radiating fibres (Figs. 8, 49); but it is sometimes reduced to a regular spherical mass which may represent only a portion of the original aster (Fig. 7).
B. Origin of the Mitotic Figure
The nature and source of the material from which the mitotic figure arises form a problem that has been almost continuously under discussion since the first discovery of mitosis, and is even now but partially solved. The discussion relates, however, almost solely to the achromatic figure (centrosome, spindle, and asters) ; for every one is agreed that the chromatic figure (chromosomes) is directly derived from the chromatin-network, as described above, so that there is no breach in the continuity of the chromatin from one cell-generation to another. With the achromatic figure the case is widely different. The material of the spindle and asters must be derived from the nucleus, from the cytoplasm, or from both ; and most of the earlier research was devoted to an endeavour to decide between these possibilities. The earhest observers {'71-7S) supposed the achro- matic figure to disappear entirely at the close of cell-division, and most of them (Butschli, Strasburger, Van Beneden, '75) believed it to be re-formed at each succeeding division out of the nuclear substance. The entire mitotic figure was thus conceived as a metamorphosed nucleus. Later researches ('75- 85) gave contradic-
i See p. 318. Cf. Kostanecki, '97, and Hoffmann, '98.
ORIGIN OF THE MITOTIC FIGURE
73
tory and apparently irreconcilable results. Fol ('79) derived the spindle from the nuclear material, the asters from the cytoplasm. Strasburger ('80) asserted that the entire achromatic figure arose
A
c
%-!# »^
D
Fig. 27. — The prophases of mitosis (heterotypical form) in primary spermatocytes of Salama)idra. [Meves.]
A. Early segmented spireme ; two centrosomes outside the nucleus in the remains of the attraction-sphere. B. Longitudinal splitting of the spireme, appearance of the astral rays, disin- tegration of the sphere. C. Early amphiastcr and central spindle. D. Chromosomes in the form of rings, nuclear membrane disappeared, amphiaster enlarging, mantle-fibres developing.
from the cytoplasm, and to that view, in a modified form, he still adheres. Flemming ('82), on the whole, inclined to the opinion that the achromatic figure arose inside the nucleus, yet expressed the
y^ CELL-DIVISION
opinion that the question of nuclear or cytoplasmic origin was one of minor importance. A long series of later researches on both plants and animals has fully sustained this opinion, showing that the origin of the achromatic figure does in fact differ in different cases. Thus in Infusoria the entire mitotic figure is of intranuclear origin (there are, however, no asters); in echinoderm eggs the spindle is of nuclear, the asters of cytoplasmic, origin ; in the testis-cells and some tissue- cells of the salamander, a complete amphiaster is first formed in the cytoplasm, but to this are afterward added elements probably derived from the linin-network; while in higher plants there is some reason to believe that the entire achromatic figure may be of cytoplasmic origin. Such differences need not surprise us when we reflect that the achromatic part of the nucleus (linin-network, etc.) is probably of the same general nature as the cytoplasm. ^
Many observers have maintained that the material of the astral rays and spindle-fibres is directly derived from the substance of the protoplasmic meshwork, whether nuclear, cytoplasmic, or both ; but its precise origin has long been a subject of debate. This question, critically considered in Chapter VI., will be here only briefly sketched. By Klein {;7%\ Van Beneden ('83), Carnoy ('84, '85), and a large num- ber of later observers, the achromatic fibres, both of spindles and of asters, are regarded as identical with those of a preexisting reticulum which have merely assumed a radiating arrangement about the cen- trosome. The amphiaster has, therefore, no independent existence, but is merely an image, as it were, somewhat like the bipolar figure arising when iron filings are strewn in the field of a horseshoe magnet. Boveri, on the other hand, who has a small but increasing following, maintains that the amphiastral fibres are not identical with those of the preexisting meshwork, but a new formation which, as it were, "crystallizes anew " out of the general protoplasmic substance. The amphiaster is therefore a new and independent structure, arising in, or indirectly from, the preexisting material, but not by a direct mor- phological transformation of that material. This view, which has been advocated by Druner ('94), Braus ('95), Meves ('97, 4, '98), and with which my own later observations ('99) also agree, is more fully discussed at page 318.
In 1887 an important forward step was taken through the inde- pendent discovery by Van Beneden and Boveri that in the ^gg of Ascaris the centrosome does not disappear at the close of mitosis, but remains as a distinct cell-organ lying beside the nucleus in the cyto-
1 In the case of echinoderm eggs, I have found reason ('95, 2) for the conclusion that the spindle- fibres are derived not merely from the linin-substance, but also from the chromatm. Despite some adverse criticism, I have found no reason to change my opinion on this point. The possible significance of such a derivation is indicated elsewhere (p. 302).
ORIGIN OF THE MITOTIC FIGURE
75
plasm. These investigators agreed that the amphiaster is formed under the influence of the centrosome, which by its division creates two new "centres of attraction" about which the astral systems arise, and which form the foci of the entire dividing system. In them are centred the fibrillae of the astral system, toward them the daughter-
F 0
Fig. 28. - Metaphase and anaphases of mitosis in cells (spermatocytes) of the salamander. [Druner.]
E Metaphase. Tlie continuous central spindle-fibres pass from pole to pole of the spmdle Outside them the thm layer of contractile mantle-fibres attached to the d.vKlcd ^^^'-:^'^^''^^'^} which only two are shown! Centrosomes and asters. F. Transverse section ^ -- ^ \ ^^ "^ !?^ J figure showing the ring of chromosomes surrounding the central spmdle. the cut ^^'^^/^^ ' rL'''!"^/ a;peanng as ''dots. G. Anaphase; divergence of the ^»-^g'^^--f-°"--"\^.^- ^j^J^^^ "S„f^,^^^ tral spin5leas the interzonal fibres; contractile fibres ^vn^^^'^-\^or..^-' \ '^Xj^^^^^^^ shown. H. Later anaphase (dyaster of Flemming) ; the central spmdle uly exposed to Mev. mantle-fibres attached to the chromosomes. Immediately aftenvard the cell d.vdes (see P .g. ^) .
chromosomes proceed, and within their respective spheres of influ- ence are formed the resulting daughter-cells. Both Van Beneden and Boveri fully recognized the importance of then" discovery. "We are justified," said Van Beneden, ^ in regarding the attraction-sphere with its central corpuscle as forming a permanent organ, not only of the early blastomeres, but of all cells, and as constituting a cell-organ equal
76
CELL-DIVISION
in rank to the nucleus itself ; and we may conclude that every central corpuscle is derived from a preexisting corpuscle, every attraction- sphere from a preexisting sphere, and that division of the sphere precedes that of the cell-nucleus." ^ Boveri expressed himself in similar terms regarding the centrosome in the same year {"^y, 2, p. 153), and the same general result was reached by Vejdovsky nearlv at the same time,- though it was less clearly formulated than bv either Boveri or Van Beneden.
All these observers agreed, therefore, that the achromatic figure arose outside the nucleus, in the cytoplasm ; that the primary impulse to cell-division was given, not by the nucleus, but by the centrosome, and that a new cell-organ had been discovered whose special office
Fig. 29. — Final phases (telephases) of mitosis in salamander cells. [Flemming.]
/. Epithelial cell from the lung; chromosomes at the poles of the spindle, the cell-body divid- ing; granules of the "mid-body" or '/Auischcnkorpcr ■M the equator of the disappearing spindle. y. Connective tissue-ceil (lung) immediately after division; daughter-nuclei reforming, the cen- trosome just outside of each ; mid-body a single granule in the middle of the remains of the spindle. •
was to preside over cell-division. **The centrosome is an indepen- dent permanent cell-organ, which, exactly like the chromatic elements, is transmitted by division to the daughter-cells. 77ic cciitrosojue rep- resents the dynamic cejitre of cell T "^
That the centrosome does in many cases, especially in embryonic cells, behave in the manner stated by Van Beneden and Boveri seems at present to admit of no doubt ; and it has been shown to occur in
1 '87, p. 279.
2 '88, pp. 151, etc.
3 Boveri, '87, 2, p. 153.
ORIGIN OF THE MITOTIC FIGURE yy
many kinds of adult tissue-cells during their resting state ; for example in pigment-cells, leucocytes, connective tissue-cells, epithelial and endothelial cells, in certain gland-cells and nerve-cells, in the cells of some plant-tissues, and in some of the unicellular plants and ani- mals, such as the diatoms and flagellates and rhizopods. On the other hand. Van Beneden's conception of the attraction-sphere has proved untenable ; for this structure has been clearly shown in some cases to disintegrate and disappear at the close or the beginning of mitosis^
(Fig- 27).
Whether the centrosome theory can be maintained is still in doubt ;
but evidence against it has of late rapidly accumulated.
In the first place, it has been shown that the primary impulse to cell-division cannot be given by fission of the centrosome, for there are several accurately determined cases in which the chromatin-elements divide independently of the centrosome, and it is now generally agreed that the division of chromatin and centrosome are two parallel events, the nexus between which still remains undetermined.^
Secondly, an increasing number of observers assert the total disap- pearance of the centrosome at the close of mitosis ; while some very convincing observations have been made favouring the view that cen- trosomes may be formed de novo without connection with preexisting ones (pp. 213, 305).
Thirdly, a large number of recent observers (including Strasburger and many of his pupils) of mitosis in the flowering plants and pteridophytes agree that in these forms no centrosome exists at any stage of mitosis, the centre of the aster being occupied by a vague reticular mass, and the entire achromatic figure arising by the gradual grouping of fibrous cytoplasmic elements (kinoplasm or filar plasm) about the nuclear elements.^ If we can assume the cor- rectness of these observations, the centrosome-theory must be greatly modified, and the origin of the amphiaster becomes a far more com- plex problem than it appeared under the hypothesis of Van Ik-neden and Boveri. That such is indeed the case is indicated by nothing more strongly than by Boveri's own remarkable recent experiments on cell-division (referred to at page 108).
C. Details of Mitosis
Comparative study has shown that almost every detail of the pro- cesses described above is subject to variation in different forms of cells. Before considering some of these modifications it may be well to pomt out what we are at present justified in regarding as its essential
1 Cf. p. 323. 2 cf. p. 108. « Cf. p. 82.
78
CELL-DIVISION
features. These are : (i) The formation of the chromatic and achro- matic figures; (2) the longitudinal splitting of the chromosomes or spireme-thread ; (3) the transportal of the chromatin-halves to the respective daughter-cells. Each of these three events is endlessly varied in detail ; yet the essential phenomena are everywhere the same, with one important exception relating to the division of the chromo- somes that occurs in the maturation of certain eggs and spermatozoa.^ It maybe stated further that the study of mitosis in some of the lower forms (Protozoa) gives reason to believe that the asters are of second- ary importance as compared with the spindle, and that the formation of spireme and chromosomes is but tributary to the division of the smaller chromatin-masses of which they are made up.
I. J'aricties of the Mitotic Figure
(a) TJic Achromatic Figure. The phenomena involved in the his- tory of the achromatic figure are in general most clearly displayed in embryonic or rapidly dividing cells, especially in egg-cells (Figs. 31, 60), where the asters attain an enormous development, and the centrosomes are especially distinct. In adult tissue-cells the asters are relatively small and difficult of demonstration, the spindle large and distinct ; and this is particularly striking in the cells of higher plants where the asters are but imperfectly developed. Plant-mitoses are characterized by the prominence of the cell-plate (Fig. 34), which is rudimentary or often wanting in animals, a fact correlated no doubt with the greater development of the cell-membrane in plants. With this again is correlated the fact that division of the cell-body in animal cells generally takes place by constriction in the equatorial plane of the spindle ; while in plant-cells the cell is usually cut in two by a cell-wall developed in the substance of the protoplasm and derived in large part from the cell-plate.
In animal cells we may distinguish two general types in the forma- tion of the amphiaster, which are, however, connected by interme- diate gradations. In the first of these, typically illustrated by the division of epithelial and testis-cells in the salamander (Flemming, Hermann, Drijner, Meves), a complete amphiaster is first formed in the cytoplasm outside the nucleus, while the nuclear membrane is still intact. As the latter fades away and the chromosomes appear, some of the astral rays grow into the nuclear space and become attached to the chromosomes, which finally arrange themselves in a ring about the original spindle (Figs. 27, 28). In the completed amphiaster, therefore, we may distinguish the original central spi7idle (Hermann, '91) from the surrounding mantle-fibres, the latter being
1 Cf. Chapter V.
DETAILS OF MITOSIS
79
attached to the chromosomes, and being, according to Hermann, the principal agents by which the daughter-chromosomes are dragged apart. The mantle-fibres thus form two hollow cones or half-spin- dles, separated at their bases by the chromosomes and completely surrounding the continuous fibres of the central spindle, which come into view as the ''interzonal fibres" during the anaphases (Fig. 28). There is still considerable uncertainty regarding the origin and relation of these two sets of fibres. It is now generally agreed with Van Beneden that the mantle-fibres are essentially a part of the asters, i.e. are simply those astral rays that come into connection with the chromosomes — wholly cytoplasmic in ori- gin (Herma.nn, Driiner, MacFarland), or in part cytoplasmic, in part dif- ferentiated from the linin- network (Flemming, Meves). Driiner ('95), Braus ('95) (salamander), and MacFarland (yPleicro- phyllidia, '97) believe the central spindle to arise secondarily through the union of two opposing groups of astral rays in the area between the centrosomes. On the other hand, Hermann ('91), Flemming ('91), Heidenhain ('94), Kos- tanecki ('97), Van der Stricht ('98), and others believe the central spindle to exist from the first in the form of fibres stretching between the diverging centrosomes ; and Heidenhain believes them to be developed from a special substance, forming a ''primary centrodesmus," which persists in the resting cell, and in which the centrosomes are embedded.^ MacFarland's observa- tions on gasteropod-eggs ('97) indicate that even nearly related torms may differ in the origin of the central spindle, since in Plcurophyllidia it is of secondary origin, as described above, while in Diaiilula it is a primary structure developed from what he describes as the " centro- some," but which, as shown at page 314, is probably to be regarded as
ir/p. 315-
Fig. 30. — Mid-body in embryornc cells oiUinax. [HOFF- MANN.]
Earlier stage above, showing thickenings along the line of cleavage. Later stage, below, showing spindle-plate and
cytoplasmic plate.
8o
CELL-DIVISIOy
an attraction-sphere surrounding; the centrosomes, and is perhaps comparable to Heidenhain's " centrodesmus."
In the second type, ilhistrated in the cleavaf]^e of echinoderm, annehd, niolhiscan, and some other egjrs, a central s])indle may be formed, — sometimes already during the anaphases of the preceding mitosis (Figs. 99, 155), — but afterward disappears, the asters moving
Fig. 31. — The middle phases ot mitosis in the first cleavage of the Ascaris-^gg. [BOVERI.]
y-f. Closing prophase, the equatorial plate forming, B. Metaphase; equatorial plate estab- lished and the chromosomes split; b. the equatorial plate, viewed en face, showing the four chro- mosomes, C. Early anaphase; divergence of the daughter-chromosomes (polar body at one side), D. Later anaphase; p.b. second polar body,
(For preceding stages see Fig. 90; for later stages Fig. 145.)
to opposite poles of the nucleus. Between these two poles a new spindle is then formed in the nuclear area, while astral rays grow out into the cytoplasm. There is strong evidence that in this case the entire spindle may arise inside the nucleus, i.e. from the sub- stance of the linin-network, as occurs, for example, in the eggs of echinoderms (Fig, 25, E), and in the testis-cells of arthropods. In other cases, however, a part at least of the spindle is of cytoplasmic
DETAILS OF MITOSIS
8l
origin, since the ends of the spindle begin to form before dissokition of the nuclear membrane, and the latter is pushed inwards in folds by the ingrowing fibres (Figs. 25, C, 99).! In some cases, however, it seems certain that the nuclear membrane fades away before com- pletion of the spindle (first maturation-division of TJialasscma, CJice^- toptcriis), and it is probable that the middle region of the spindle is here formed from the Hnin-network. In most, if not all, mitoses of the second type the chromosomes do not form a ring about the equator of the spindle, but extend in a flat plate completely through
u-'---«-
/ /
%l
\
N.
D
Fig. 32. — Mitosis in Sfypocaulon. [SWINGLE.]
A. Early prophase witli single aster and centrosonie. B. Initial formation of intraniK car spindle. C. Divergence of the daughter-centrosomes. D. Early anaphase ; nuclear nieniiji..ne still intact.
its substance. Here, therefore, it is impossible to speak of a " cen- tral spindle." It is nevertheless probable that the spindle-fibres are of two kinds, viz. continuous fibres, which form the interzonal fibres seen during the anaphases, and half-spindle fibres, extending only from the poles to the chromosomes. It is possible that these two kinds of fibres, though having the same origin, respectively corre-
1 Cf. Platner ('86) on Avion and Lepidoptera, Watase ('91) on Loligo, Braus ('95) on Triton, and Griffin ('96, '99) on Thalassema. Erlanger ('97, 5) endeavours to show that in the mitosis of embryonic cells in the cephalopods (^Scpia'), where the inpushing of the mem- brane was previously shown by Watase, the entire spindle arises from the nucleus. G
82 CELL-DIVISION
spond in function to those of the central spindle and to the mantle- fibres. It seems probable that the difference between the two types of spindle-formation may be due to, or is correlated with, the fact that the nuclear transformation takes place relatively earlier in the "first type. When the nucleus lags behind the spindle-formation the centrosomes take up their position prematurely, as it w^ere, the cen- tral spindle disappearing to make w^ay for the nucleus.
It is in the mitosis of plant-cells that the most remarkable type of achromatic figure has been observed. In some of the lower forms (Alga?) mitosis has been clearly shown to conform nearly to the process observed in animal cells, the amphiaster being provided with very large asters and distinct centrosomes, and its genesis corre- sponding broadly with the second type described above (Figs. 32, 33), though with some interesting modifications of detail.^ Swingle ('97) describes in Stytopocauloii a process closely similar to that seen in many animal cells, the minute but very distinct centrosomes being surrounded by quite typical cytoplasmic asters, passing to opposite poles of the nucleus, and a spindle then developing between them out of the achromatic nuclear substance (Fig. 32). In the flowering plants and pteridophytes, on the other hand, mitosis seems to be of a quite different type, apparently taking place in tJie entire absence of centrosomes. Guignard ('91, i, '92, 2) clearly described and figured typical centrosomes and attraction-spheres both in the ordinary mitosis (Fig. 34) and in the fertilization of the higher plants, giving an account of their behaviour nearly agreeing with the views then prevaiUng among zoologists. Although these accounts have been supported by some other workers,^ and have recently been in part reiterated by Guignard himself ('98, i), they have not been sustained by some of the best and most careful later observers, who describe a mode of spindle-formation differing radically from that seen in thal- lophytes and in animals generally.-^ According to these observations, begun by Farmer and Belajcff, and strongly sustained by the care- ful studies of Osterhout, Mottier, Nemec, and others, the achromatic figure is almost