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THE CELL

IN DEVELOPMENT AND INHERITANCE

C0Uimijia ^nibersitg Biological .Setieg.

EDITED BY
HENRY FAIRFIELD OSBORN

AND

EDMUND B. WILSON,

1. FROM THE GREEKS 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, LIVING 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 ZOOLOGYTCOLUMBIA UNIVERSITY

SECOND EDITION
REVISED AND ENLARGED

Natura nusquam magis est tota quam in minimis "

PLINY

THE MACMILLAN COMPANY

LONDON: MACMILLAN & CO., Ltd.
191I

All rights reserved

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 unci Gcwebe
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 Q,^%, — 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

vni PREFACE

part of the ground already so well covered by Hertwig.^ 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 Lemons sur la cellule is received, too late for further notice, as this volume
is going through the press.

2 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 limits, 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

Xii PREFACE TO THE SECOND 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 ^gg 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, ii,
12, and 1 88, from the Wood's Holl Biological Lectures for 1899.

Columbia University,
December 7, 1899.

Postscript. — Of especial importance for some of the discussions in Chapters I., V., and
VII. are Fischer'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 cf 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, 1 61

CHAPTER II

Cell-division

A. Outline of Indirect Division or Mitosis 65

B. Origin of the Mitotic Figure 72

C. Details of Mitosis , , , 77

1. Varieties of the Mitotic Figure 78

(a) The Achromatic Figure 78

(d) The Chromatic Figure 86

2. Bivalent and Plurivalent Chromosomes 87

3. Mitosis in the Unicellular Plants and Animals 88

4. Pathological Mitoses 97

xiii

XIV

TABLE OF CONTENTS

D. The Mechanism of Mitosis . . . .

1. Function of the Amphiaster

(ff) Theory of Fil)rillar Contractility
{J}) Other Facts and Theories

2. Division of the Chromosomes

E. Direct or Amitotic Division .....

1. General Sketch

2. Centrosome and Attraction-sphere in Amitosis

3. Biological Significance of Amitosis

F. Summary and Conclusion .....
Literature, II , ,

PAGE

100
100
100
106
"112
114
114

116
119

CHAFIER III
The Germ-cells

A. The Ovifm , , . , , 124

1. The Nucleus ..... , 125

2. The Cytoplasm .. ..o ...... , 130

3. The Egg-envelopes -132

B. The Spermatozoon . . . . . . . . . , .134

1. Th€ flagellate Spermatozoon < , , . . . . , .135

2. Other Forms of Spermatozoa . , , „ ,142

3. Paternal Germ-cells of Plants . .142

C. Origin of the Germ-cells , . . . , , ., . . 144

D. Growth and Differentiation of the Germ-cells . . . . . -150

1. The Ovum .... 150

(«) Growth and Nutrition . . . . . . . ., .150

{b) Differentiation of the Cytoplasni. Deposit of Deutoplasm . . 152

{c) Yolk-nucleus 155

2. Origin of the Spermatozoon . . . . . . . .160

3. Formation of the Spermatozoids in Plants . . . . .172

E. Staining-reactions of the Germ-nuclei ... . '175
Literature, III . . . . 177

CHAPTER IV
Fertilization of the Ovum

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 Fertilization ....

D. Fertilization in Plants

E. Conjugation in Unicellular Forms

F. Summary and Conclusion .....
Literature, IV. ..,.,..

180
181
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. Rciuction 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 . . . o . . 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 . , . 280

Appendix

1 . Accessory Cells of the Testis 284

2. Amitosis in the Early Sex-cells 285

H. Summary and Conclusion . . . , 285

i-iterature, V ..,,.,. 287

CHAPTER VI

Some Problems of Cell-organization

A. The Nature of Cell-organs ■» . 291

B. Structural Basis of the Cell » ... 293

C. Morphological Composition of the Nucleus ........ 294

I. The Chromatin . 294

{a) Hypothesis of the Individuality of the Chromosomes . . . 294

{b) Composition of the Chromosomes ...,„.. 301

D. Chromatin, Linin, and Cytoplasm , . . 302

E. The Centrosome 304

F. The Archoplasmic Structures . . . 316

1. Hypothesis of Fibrillar Persistence 316

2. The Archoplasm Hypothesis . -318

3. The Attraction-sphere .......... 323

G. Summary and Conclusion ......*>... 3^7

Literature, VI 328

CHAPTER VII
Some Aspects of C\ll-chemistry and Cell-physiology

A. Chemical Relations of Nucleus ana Cytoplasm ....... 330

1. The Proteids and their Allies 331

2. The Nuclein Series ZZ"^

3. Staining-reactions of the Nuclein Series ..••... 334

xvi TABLE OF CONTENTS

PAGE

B. Physiological Relations of Nucleus and Cytoplasm 341

1. Experiments on Unicellular Organisms 342

2. Position and Movements of the Nucleus 346

3. The Nucleus in Mitosis 351

4. The Nucleus in Fertilization 352

5. The Nucleus in Maturation 353

C. The Centrosome 354

D. Summary and Conclusion 358

Literature, VII 359

CHAPTER VIII

Cell-division and Development

A. Geometrical Relations of Cleavage-forms 362

B.' Promorphological Relations of Cleavage 378

1. Promorphology of the Ovum 378

(<2) Polarity and the Egg-axis 378

{F) Axial Relations of the Primary Cleavage- planes .... 379

(f) Other Promorphological Characters of the Ovum .... 382

2. Meaning of the Promorphology of the Ovum 384

C. Cell-division and Growth 388

Literature, VIII 394

CHAPTER IX
Theories of Inheritance and Development

A. The Theory of Germinal Localization 397

B. The Idioplasm Theory 401

C. Union of the Two Theories 403

D. The Roux-Weismann Theory of Development 404

E. Critique of the Roux-Weismann Theory 407

F. On the Nature and Causes of Differentiation 413

G. The Nucleus in Later Development 425

H. The External Conditions of Development 428

I. Development, Inheritance, and Metabolism 430

J. Preformation and Epigenesis. The Unknown Factor in Development . .431

Literature, IX 434

Glossary 437

General Literature-list 449

Index of Authors . 471

Index of Subjects 477

LIST OF FIGURES

INTRODUCTION

PAGE

1. Epidermis of larval salamander 3

2. Section of growing root-tip of the onion ........ 4

3. Amoeba Proteus 5

4. Cleavage of the ovum in Toxopneustes 1 1

5. Diagram of inheritance 13

CHAPTER I

6. Diagram of a cell 18

7. Spermatogonia of salamander 20

8. Group of cells, showing cytoplasm, nucleus, and centrosomes .... 21

9. Living cells of salamander larva, showing fibrillar structure 24

10. Alveolar or foam-structure of protoplasm, according to Biitschli .... 26

11. Structure of protoplasm in the echinoderm egg ....... 27

12. Aster-formation in alveolar protoplasm 28

13. Nuclei from the crypts of Lieberkiihn 32

14. Special forms of nuclei 35

15. Scattered nucleus in Trachelocerca 37

16. Scattered nucleus in Bacteria and Flagellata 39

17. Ciliated cells 43

18. Cells of amphibian pancreas 44

19. Nephridial cell of Clepsine 45

20. Nerve-cell of frog 47

21. Diagram of dividing cell • • . .49

22. Diagrams of cell-polarity 56

23. Centrosomes in epithelium and in blood-corpuscles 57

CHAPTER II

24. Remak's scheme of cell-division 64

25. Diagram of the prophases of mitosis 66

26. Diagram of later phases of mitosis 69

27. Prophases in salamander-cells 73

28. Metaphase and anaphases in salamander-cells 75

29. Telophases in salamander-cells 76

30. Mid-body and cell-pl^te in cells of Limax 79

31. Middle phases of mitosis in /^j^arzj-eggs 80

32. Mitosis in Stypocaulon 81

xrii

xviii LIST OF FIGURES

FIG. PAGE

i,T,. Mitosis in Erysiphe < . . 83

34. Mitosis in pollen- mother-cells of lily, according to Guignard 84

36. Mitosis in spore-cells of Equisetum . . . 85

37. Heterotypical mitosis 87

38. Mitosis in Infusoria 89

39. Mitosis in Euglypha 90

40. Mitosis in Euglena . . . , 91

41. Mitosis in Acanthocystis 92

42. Mitosis in Noctiluca . . . ' 93

43. Mitosis in Paramceba . . . v g^

44. Mitosis in Actinosphcerium . . . . .96

45. Mitosis in ActinosphcBrium , 97

46. Pathological mitoses in cancer-cells 98

47. Pathological mitosis caused by poisons ......... 99

48. Van Beneden's account of astral systems in /^5rarzj ...... 100

49. Leucocytes 102

50. Pigment-cells 103

51. Heidenhain's model of mitosis 104

52. Mitosis in the egg of Toxopneustes 107

53. Pathological mitoses in polyspermy , . . .109

54. Nuclei in the spireme-stage . .112

55. Early division of chromatin in ^^rrt-r/j . , „ 1 13

56. Amitotic division -115

CHAPTER III

57. Volvox 123

58. Ovum of Toxopneustes 126

59. Ovum of the cat 127

60. Ovwxa oi Nereis . . . . . 129

.61. Germinal vesicles of 6>^2o and i?/^z>« 130

62. Insect-egg 132

63. Micropyle in Argonauta 133

64. Germ-cells of Volvox 134

65. Diagram of the flagellate spermatozoon 135

66. Spermatozoa of fishes and amphibia . . . . . , , . .136

67. Spermatozoa of birds and other animals 138

68. Spermatozoa of mammals 140

69. Unusual forms of spermatozoa ..... o ... . 141

70. Spermatozoids of Char a . . . . . , . , . . .142

71. Spermatozoids of various plants 143

72. Germ-cells of Cladonema 146

73. Primordial germ-cells of Ascaris 147

74*. Primordial germ-cells of Cyclops 149

75. Ovarian ova and follicles of i%/?> 151

76. Egg and nurse-cells in Ophryotrocha 152

77. Ovarian eggs of insects . » . 153

78. Young' ovarian eggs of various animals . . . . . . . . .154

79. Young ovarian eggs of birds and mammals 155

80. Ovarian eggs of spider, earthworm, ascidian, showing yolk-nucleus , « • '57

LIST OF FIGURES

XIX

8 1. Ov3ina.n eggs of Limuhts a.nd Po/yzonium . , . „

82. Formation of the spermatozoon in Anasa

83. Transformation of the spermatids of the salamander .

84. Formation of the spermatozoon in Salamaiidra and Amphiuma

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 . „ „ „

159
162
164
166
168
169
173
174

CHAPTER IV

89. Fertilization of /^/^a . . . . . . . c » <. . ,180

90. Fertilization of /^i'far/j . . , . . „ . . o .. .183

91. Germ-nuclei of Nematodes , » . . c . 184

92. Fertilization of the mouse „ . , o . .185

93. Fertilization of Pterotrachea , . . 1 86

94. Entrance and rotation of sperm-head in Toxopneustes . ..... 187

95. Conjugation of the germ-nuclei in Toxopneustes ....... 189

96. Diagrams of fertilization 190

97. Fertilization of Nereis . . . o . . 191

98. Fertilization of Cyclops ............ 193

99. Fertilization and persistence of centrosomes in Thalassema . . . . 195

100. Entrance of spermatozoon into the egg , , . . . . , . 197

10 1. Pathological polyspermy . . . - . « . • . . . 199

102. Polar rings of Clepsine . . . . .. . . . . o . 20I

103. Paths of the germ-nuclei in Toxopneustes ........ 203

104. Fertilization of Myzostonia .... .0 .... . 209

105. Fertilization of Pilularia ........... 216

106. Penetration of the pollen-tube in angiosperms 217

107. Fertilization of the lily 219

108. Fertilization in Zatnia . . . 220

109. Diagram of conjugation in Infusoria » . . , . . . . . 223

1 10. Conjugation of Paramcecium « . . . 225

111. Conjugation of Vorticella 226

112. Conjugation of yV(9r/?7Mr<a! ........ .0 . 227

113. Conjugation of .S/^irf^jj/ra ...,..«'. ... . 228

CHAPTER V

114. Polar bodies in Toxopneustes

115. Genesis of the egg

116. Diagram of formation of polar bodies

117. Polar bodies in ^^m;-?\f

118. Genesis of the spermatozoon

119. Diagram of reduction in the male

120. Spermatogenesis of ^.yrr^r/^

121. Diagrams illustrating tetrad-formation

122. Tetrads of Gryllotalpa

123. Tetrads and polar bodies in Cyclops

234
235
237
239
240
242
244
247

249
250

XX LIST OF FIGURES

FIG. PAGB

124. Diagrams of tetrad-formation in arthropods 251

125. Germinal vesicles and tetrads 252

126. Maturation in Anasa ........,,., 254

127. Maturation in Anasa 255

128. Diagrams of reduction ' . 259

129. Maturation in llialassema 260

130. Maturation in Thalassema and Zirphcea 261

131. Maturation in Salamandra ......,,,.. 262

132. The maturation-divisions in angiosperms 264

133. Maturation in Lilium 266

134. Maturation in Lilium 268

135. Diagrams of reduction in the flowering plants 270

136. Ovary of Canthocamptus ........... 273

137. Polar spindles without centrosomes 276

138. Polar bodies in Actinophrys 278

139. Polar bodies in Actinospha7'ium 278

140. Conjugation and reduction in Closterium 279

141. First type of parthenogenetic maturation in ^r/^;;z?fl 282

142. Second type of parthenogenetic maturation in Artemia 283

CHAPTER VI

143. Abnormalities in the fertilization of Ascaris 295

144. Giant embryo of A scar is 296

145. Individuality of chromosomes in Ascaris 297

146. Independence of chromosomes in fertilization of Cyclops 298

147. Hybrid fertilization of Ascaris 300

148. Mitosis with intranuclear centrosome in Ascaris 305

149. Abnormal mitoses in Hemerocallis 306

150. Centrosomes in Chcetopterus and Cerebratulus 307

151. Artificially produced asters and centrosomes in echinoderms .... 308

152. Diagram of different types of centrosome and centrosphere .... 310

153. Polar mitoses in Diaulula 312

154. Astral systems in Unio 313

155. Astral systems in Cerebratulus and Thalassema 320

156. Structure of the aster in spermatogonium of salamander 326

CHAPTER VII

157. History of chromosomes in the germinal vesicle of sharks 339

158. Nucleated and enucleated fragments of ^/y^wjc/^/a 342

159. Regeneration in Stentor 343

160. Nucleated and enucleated fragments of ^w<^/5rt 344

161. Nucleated and enucleated fragments of plant-protoplasm 345

162. Position of nuclei in plant-cells 347

163. 0\2icy oi Forjicula 349

164. Normal and dwarf larvae of sea-urchins 352

165. Supernumerary centrosome in Asca>-is 355

166. Cleavage of dispermic egg of 7V;ir<?/«^«j/<?j 356

167. Centrosomes and cilia 357

LIST OF FIGURES XXI

CHAPTER VIII

FIG. PAGE

1 68. Geometrical relations of cleavage-planes in plants 363

169. Cleavage of Synapta 365

1 70. Cleavage of Polygordius ........... 367

171. Cleavage of A''<?r^?.s' 369

172. Variations in the third cleavage . . . 370

173. Meroblastic cleavage in the squid 372

174. Rudimentary cells in Aricia 373

175. Teloblasts of the earthworm . 374

176. Contradiction of Hertwig's rule in Ascaris 376

177. Bilateral cleavage in tunicates 380

178. Bilateral cleavage in Loligo 381

179- ^gg^oi Loligo 382

180. Eggs and embryos of Corixa . 383

181. Variations in axial relations of Cyclops 385

CHAPTER IX

182. Half-embryos of the frog 400

183. Half and whole cleavage in sea-urchins 407

184. Normal and dwarf gastrulas of ^w///z^;ir«5 408

185. Dwarf and double embryos of ^w/Zizc^rM^ 409

186. Cleavage of sea-urchin eggs under pressure 411

187. Cleavage of A^^r^/j-eggs under pressure 412

188. Diagrams of cleavage in mollusks and polyclades 414

189. Partial larvae of ctenophores 418

190. Partial cleavage in Ilyanassa 420

191. Double embryos of frog 421

192. Cleavage in Crepidula 424

193. Normal and modified larvae of sea-urchins 428

194. Regeneration in coelenterates . . . • 429

INTRODUCTION

" Jedes Thier erscheint ah eine Summe vitaler Einkeiten, von denen jede den voUen
Charakter des Lebens an sich tragt." ViRCHOW.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 reveahng 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 Strasburger
solved the long-standing riddle of the fertilization of the Qgg 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

^ Cellularpaihologie, p. 12, 1858.

2 INTRODUCTION

obvious characters of plants and animals — their forms, colours,
habits, distribution, their anatomy 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 plants 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 yet 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.^

1 Schleiden and Schwann are universally and justly recognized as the founders of the cell-
theory; but like every other great generalization the theory was based on along series of
earlier investigations beginning with the memorable microscopical researches of Leeuwen-
hoek, Malpighi, Hooke, and Grew in the second half of the seventeenth century.

Wolff, in the Theoria Generationis (1759), clearly recognized the "spheres" and "vesi-
cles " composing the embryonic parts both of animals and of plants, though without grasping
their real nature or mode of origin, and his conclusions were developed by the botanist
Mirbel at the beginning of the present 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 " Blaschen " can hardly be regarded as more than a lucky
guess. " A still closer approximation to the truth is found in the works of Turpin (1826),
Meyen (1830), Raspail (1831), and Dutrochet (1837); but these, like others of the same
period, only paved the way for the real founders of the cell-theory. Among other immedi-
ate predecessors or contemporaries of Schleiden and Schwann should be especially mentioned
Robert Brown, Dujardin, Johannes Miiller, Purkinje, Hugo von Mohl, Valentin, Unger,
Nageh, 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 Mikroskopische Untersuchungen 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
may 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

I '.^ I

> <

A

\

i ?■ ,■■

.".'•'r- '».,.

m-

-J^v M*^-; I: ^

&

I >-.;:> '^I^.'

/^<:

51^

X b

Fig. I. — A portion of the epidermis of a larval salamander {Ambly stoma) as seen in slightly
oblique horizontal section, enlarged 550 diameters. Most of the cells are polygonal in form, con-
tain large nuclei, and are connected by delicate protoplasmic bridges. Above ^ is a branched,
dark pigment-cell that has crept up from the deeper layers and lies between the epidermal cells.
Three of the latter are undergoing division, the earliest stage {spireme) at a, a later stage (mitotic
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 of plants and
animals are but varying 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-

4 INTR ODUC TION

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. I, 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
aggregate 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) ; <:. 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-

Fig. 3. — Amceba Proteus, an animal consisting of a single naked cell, x 280. (B'rom Sedgwick
and Wilson's Biology.)

n. The nucleus ; w.v. water-vacuoles ; c.v. contractile vacuole ; /.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-

^ 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 H. von Mohl (1846) to designate
the contents of the plant-cell. The full physiological significance 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, Unger,
Nageli, and Mohl, and that of Cohn, Huxley, Virchow, Leydig, Briicke, Kiihne, and Beale.

^ This comparison must be taken with some reservation, as will appear beyond.

6 INTRODUCTION

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 equally performed by all the cells, but
are in varying degree distributed among them, the cells thus falling
into physiological groups or tissues, each of 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. Each
bodily function, and even the hfe 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 KoUiker, 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 Biitschli, and
eagerly followed up by 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
regard the identification of the cell-nuclens as the vehicle of inheri-

^ 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 oi cell-physiology'''' (Verworn, All^emeine Physiologie, p. 53, 1895).

INTR OD UC TlOISr . J

tance, made independently and almost simultaneously in 1884-85 by
Oscar Hertwig, Strasburger, Kolliker, and Weismann/ 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 says, "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 Qgg, 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 germinal continuity 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 '^ calidiim
innatjim," out of not-living matter.* Neither could Harvey, great
physiologist and embryologist as he was, have had any adequate con*
ception of the real nature of the egg 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 rot obtainec*
until long afterward. "The internal nucleus provides for the transmission of hereditary-
characters, the external plasma on the other hand for accommodation or adaptation to the
external world" {Gen. Morph., pp. 287-289).

2 Evolution in Biology, 1878; Science and Culture, p. 291.

3 De Generatione, 1651; Trans., p. 271.

4 Whitman, too. in a brilliant essay, has shown how far Harvey was from any real grasp
of the law of genetic continuity, which is well characterized as the central fact of modern
biology. Evolution and Epigenesis, Wood's HoU Biological Lectures, 1894.

8 . INTRODUCTION

the body of which it forms a part, since the cellular structure of living
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 to 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 " praeformationists " the ^g^ 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 reproduction
of its own body. It was the service of Bonnet to push this concep-
tion to its logical consequence, the theory of emboitenient or encase-
ment, and thus to demonstrate the absurdity of its grosser forms,
pointing out that if the ^gg 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 emboitement. 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 ^gg 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 epigenesis 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 ^gg 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 the 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 g

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-
ing 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 ^gg, a single cell, pecuHarly 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 Q,gg 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 '' free
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
believed 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

1 The discovery of the spermatozoa is generally accredited to Ludwig Hamm, a pupil
of Leeuwenhoek (1677), though Hartsoeker afterward claimed the merit of having seen
them as early as 1674 (Dr. Allen Thomson).

lO INTRODUCTION'

Schwann's immediate followers threw doubts upon it,^ and as early
as 1855 Virchow positively maintained the universality of cell-divi-
sion, contending that every cell is the offspring of a preexisting
parent-cell, and summing lip in the since famous aphorism, " omnis
cellula e cellular ^ At the present day this conclusion rests upon a
foundation so firm that we are 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 ^g^ into two parts, each
of which is a cell, like the ^g^ 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 ^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 segmentation of the Qggy
was observed long before its meaning was understood. It seems to
have been first, definitely described in the case of the frog's ^gg, by
Prevost and Dumas (1824), though earlier observers had seen it; but
at this time neither the Qgg 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 generation ; for the
egg-cell, as well as the sperm-cell, arises by the division of a cell pre-
existing in the parent-body. It is tJierefore derived by direct descent
from an egg-cell of t/ie foregoing generation, and so on ad infinitum.
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 continuity in the series of cell-divisions by 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 mentioned Mohl, Unger, Nageli, Martin Barry, Goodsir,
and Remak.

2 Arch, fur Path. Anal., VIIL, p. 23, 1855.

^ See the quotation from the original edition of the Cellularpathologie at the head of
Chapter II., p. 63.

INTRODUCTION

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

Fig. 4. — Cleavage of the ovum of the sea-urchin Toxopneustes, X 330, from life. The suc-
cessive divisions up to the i6-cell stage {H) 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 inborn

12 INTR OD UC TION

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, chmate, 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, reviewmg the
facts of variation and inheritance with wonderful skill, and building
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 newperiod 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 a.void 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.

^ Ueber Vererhung, 1 883. See Essays upon Heredity, I., by A. Weismann, Clarendon
Press, Oxford, 1889.

INTRODUCTION

13

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 principle 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 1 It is
a physiological impossibility. If we turn to the facts, we find, Weis-
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-ceil, which by division gives rise to the body or soma (S) 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 from the
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

1 4 IN TR OD UCTION

evolution-theory into organic connection. It is from the point of view
thus suggested that the present vokime has been written. It has
accordingly not been my primary object to dwell on the mhmtice 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 profitably 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
chapter 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 ON THE CELL-THEORY i

Bergh, R. S. — Vodesungen liber die Zelle und die einfachen Gewebe : Wiesbaden^
1894.

Carnoy, J. B. — La Biologie Cellulaire : Lierre^ 1884.

Delage, Yves. — La Structure du Protoplasma et les Theories sur THdrddit^ et les
grands Problemes de la Biologie Generale : Paris, 1895.

Geddes & Thompson. — The Evolution of Sex : New York, 1890.

Hacker, V. — Praxis und Theorie der Zellen- und Befruchtungslehre : Jena, 1899.

Henneguy, L. F. — Lepons sur la Cellule : Paris, 1 896.

Hertwig, 0. — Die Zelle und die Gewebe: Fischer, Jena, L, 1893, IL, 1898. Trans-
lation, published by Macmillan, London and New York, 1895.

Hofmeister. Lehre von der Pflanzenzelle : Leipzig, i^6y .

Huxley, T. H. — Review of the Cell-theory : British and Foreign Medico-Chirurgical
Review, XH., 1853.

1 See also Literature, I., p. 61.

INTRODUCTION

IS

Minot, C. S. — Human Embryology: New York, 1892.

Remak, R. — Untersuchungen liber die Entwicklung der Wirbelthiere : Berlin^
1850-55.

Sachs, J. V. History of Botany. Translation: Oxford, \%c^o.

Schleiden, M. J. — Beitrage zur Phytogenesis : Milller's Archiv, 1838. Translation
in Sydenham Soc , XII. London, 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. Philadelphia, 1878.

Virchow, R. — Die Cellularpathologie in ihrer Begrlindung 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: New York, 1893.

CHAPTER I

GENERAL SKETCH OF THE CELL

" Wir haben gesehen, dass alle Organismen aus wesentlich gleichen Theilen, namlich aus
Zellen zusammengesetzt sind, dass diese Zellen nach wesentlich denselben Gesetzen sich
bilden und wachsen, dass also diese Prozesse iiberall auch durch dieselben Krafte hervorge-
bracht werden miissen." SCHWANN.^

In the passage quoted above Schwann expressed a truth which
subsequent research has established 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 soHd
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 part.
The living substance contained within the walls, to which Hugo von
Mohl gave the Vi2im^ protoplasm^ (1846), was at first overlooked or
was regarded as a waste-product, a view based upon the fact that in
many important plant-tissues such as cork or wood it may wholly
disappear, leaving only the lifeless walls. The researches of Berg-
mann, KolHker, Bischoff, Cohn, Max Schultze, and many others

1 Untei'suchungen, 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 described as made up of "little
boxes or cells distinct from one another " and separated by 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

i8

GENERAL SKETCH OF THE CELL

showed, however, that most living cells are not 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 protoplasm, must be the
seat of vital activity.

Within the protoplasm (Figs. 6-8) lies a body, usually of definite
rounded form, known as the nucleus} and this in turn often contains

Attractior)-cph°re enclosing two centrosomes.

r Plasmosome or
1 true

! nucleolus

Nucleus -I

Chromatin-

network

Linin-network

Karyosome,
net-knot, or
chromatin-
nucleolus

Plastids lying in the
cytoplasm

• V^ • • • •

dy^'c.

Vacuole

Passive bodies (meta-
plasm or paraplasm)
suspended in the cy-
toplasmic meshwork

Fig. 6. — Diagram of a cell. Its basis consists of a meshwork containing numerous minute
granules {microsomes) and traversing a transparent ground-substance.

one or more smaller bodies or nucleoli. By some of the earlier
workers the nucleus was supposed to be, Hke the cell-wall, of sec-
ondary importance, 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 1781, and recognized as a normal element of the cell by
Robert Brown in 1833.

GENERAL MORPHOLOGY OF THE CELL

19

We may therefore still accept as valid the definition given more than
thirty years ago by Leydig and Max Schultze, that a cell is a mass
of protoplasm containiiig a nucleus} to which we may add Schultze's
statement that both nucleus and protoplasm arise through 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 by 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 water, 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 may be collectively designated as metaplasm (Hanstein)
or/^r^/Z^i-;// (Kupffer), in contradistinction to the rcXxwo, protoplasm.

1 Leydig. Lehrbuch der Histologic, p. 9, 1857; Schultze, ^r<r/z. Anat.u. Phys.,^. ii, 1861.

2 Sachs has proposed the convenient word energid {Floj-a, '92, p. 57) to designate the
essential living part of the cell, i.e. the nucleus with that portion of the active cytoplasm
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. Kupffer
('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 fixed (coagulated)
material correspond with the conditions existing in life. See pp. 42-46.

20

GENERAL SKETCH OF 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 cell-wall or membrane by which the cell-body may

/

Fig. 7 — Spermatogonia of the salamander. [Meves.]
Above, two cells showing large nuclei, with linin-threads and scattered chromatin-granules; 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 Ley dig, Schultze, Briicke, 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 THE CELL

21

however, in 1882, extended the term so as to denote the entire
active cell-substance, including the nuclear material, suggesting that
the latter be called niicleoplasm, and that of the cell-body cytoplasin.

- g-^^a.^r

C D

Fig. S — Various cells showing tlie typical parts.

A. From peritoneal epithelium of the salamander-larva. Two centrosomes at the right
Nucleus showing net-knots. [Flemming.]

B. Spermatogonium of frog. Attraction-sphere (aster) containing a single centrosome
Nucleus with a single plasmosome. [Hermann.]

C. Spinal ganglion-cell of frog. Attraction-sphere near the centre, containing a single centro-
some with several centrioles. [Lenhossek.]

D. Spermatocyte of Proteus. Nucleus in the spireme-stage. Centrosome single; attraction-
sphere containing rod-shaped bodies. [HERMANN.]

These terms have been adopted by many, but not all, later writers,
the hybrid word nucleoplasm having, however,^at Flemming's sug-
gestion, been changed to karyoplasm. At the present time, there-
fore, the yNoxA protoplasm is used by some authors (Biitschli, Hertwig,

22 GENERAL SKETCH OF THE CELL

Kolliker, etc.) in its original 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 cytoplasm and karyoplasm 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
cytoplasm — 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 continuity. 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
the terms '^nucleus'' and ^^ cell-body'' sJiould probably be I'egarded as
only topographical expressions denoting tzvo 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 nnclein, 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. J^ 8). This is an extremely minute body which

^ Conklin ('97, i), Obst ('99), and some others have described a direct continuity in the
resting cell between the intranuclear and extranuclear meshworks.

STRUCTURAL BASIS OF PROTOPLASM 23

is concerned in the process of cell-division and in the fertilization of
the ^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 S2ib jndicc, 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 life and after fixation by reagents, the basis of the
structure is a more or less regular framework or mesJiwork, consisting
of at least two substances. One of these forms the substance of the
meshwork proper ; the other, often called the ground- siibstarice (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 meshwork 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 reticuhim 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. 2 cf. Glossary. » Cf. p. 29.

24

GENERAL SKETCH OF THE CELL

^'

V-^^MiJ-i

4.

^iy

\ /

' -<

£ \

W

h'
m

6^

,^'

^^

D

F

Fig. 9. — Living cells of sal^tfr^nder-larva. [FLEMMlNG.]

A. Group" of epidermal cells ai-different foci, sKowing 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-cells with cytoplasmic fibrillae (the latter somewhat exaggerated
in the plate). . •:

STRUCTURAL BASIS OF PROTOPLASM 2$

whether simple or branching ('* filar theory" of Flemming), 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^ thread-work^ spongioplasm, initome,
filar substance} all of which are still in use. Under this view the
''granules " described by Schultze, Virchow and still earlier 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 Biitschli,
which has won an increasing number of adherents. Biitschli regards
protoplasm as having a foam-like alveolar structure (''Waben-
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 interalveolar 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 interalveolar 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 opti-
cal sectior^ of the alveoH, the alveolar substance that fills them
corresponding to the ''ground substance." As explained beyond,^
Biitschli 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
alveoU (Fig. lo).

The two (three) general views above outlined may be designated
respectively as the fibrillar (reticular or filar) and alveolar theories
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,
the 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

^ See Glossary.

2 Measuring on an average about .001 mm. in diameter. 8 Qt; p jjq.

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.

Fig. 10. — Alveolar or foam-structure of protoplasm, according to Biitschli. [Butschli.J

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.i 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-

Fig. II. — (a) Protoplasm of the egg of the sea-urchin' {Toxopneustes) in section showing
meshwork of microsomes; [b) protoplasm from a living star-fish egg {Asterias) showing alveolar
spheres with microsomes scattered between them ; {c) the same in a dying condition after 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, i, '97, 2, and Butschli,
'92, 2, '99. 2 '^7^ i^ p^ 260. * '95, '97» 3. '<?8.

2S

GENERAL SKETCH OF THE CELL

given.^ In that classical object, the echinoderm-egg, for example,
it is easy to satisfy oneself, botJi in the living cell and in sections^
that the protoplasm has a beautiful alveolar structure, exactly as
described by Blitschli in the same object (Fig. ii). 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

Fig. 12. — Section of sea-urchin egg {Toxopneustes), \\ 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. Butschli's conclusions,
based on researches so thorough, prolonged, 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 ^vell 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 qiiasiA)ci^o-
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 lifeless, 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 futihty 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

^ 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 (r/ Wilson, '99). In the pancreas-cell granular and fibrillar
structures alternate with the varying phases of secretory activity {cf. Mathews, '99).

30 GENERAL SKETCH OF THE CELL

between ** protoplasmic " and " metaplasmic " 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, fibrillae, 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 metaboHsm 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), Kolliker ('97), Meyer ('96), and Kupffer ('96) on
energids. Sachs sharply distinguishes between the energid (nucleus and protoplasm) , which
forms a living unit, and the passive encxgi^-products, 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-elements into pj'oloplasmatic and alloplasmaiic organs,
,ne former (nucleus cytoplasm, chromatophores, and perhaps the centrosomes) arising only
by division, the latter (cilia, and according to Kolliker, also the muscle- and nerve-fibrillae)
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 are accepted 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

TT-

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 Structure

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, D).
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. yy). Perhaps the most remarkable deviations from the
usual type of nucleus occur among the unicellular forms. In the cili-
ate Infusoria the nuclei are massive bodies of two kinds, viz. a large
macronucleus and one or more smaller niicronuclei, both of which are
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 Biitschli in flagellates and Bacteria, by Gruber in certain
rhizopods and Infusoria, and by several authors in the Cyanophyceae
(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 judice
(p. 39). A transition from 'this condition to nuclei of the ordinary
type appears to be given in the nuclei of certain flagellates (e.g. CJii-
lomonas and Trachelmo7ias\ 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 composed 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):-

a. The nuclear membrane, a
^„ , . , , well-defined delicate wall which

Ihe character of the chromatin-network • i i

{basichromatin)\s2.ccma.XQ\y^'ho^vx. The upper glVCS the UUClCUS a Sharp COntOUr

nucleus contains three piasmosomes or true and differentiates it clcarly from

nucleoli; the lower, one. A few fine linin-threads ,, ,. ^ , -^ ^^, .

the surroundmg cytoplasm. This
wall sometimes stains but very
slightly, and can scarcely be dif-
ferentiated from the outlying
cytoplasm. In other and perhaps more frequent cases, it approaches
in staining capacity the chromatin.

b. The nuclear reticulum. This, the most essential part of the
nucleus, forms an irregular branching network or reticidum 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 linin

1 Calkins, '98, I.

Fig. 13. — Two nuclei from ihe crypts of
Lieberkiihn in the salamander. [Hkidenhain.]

{oxychrotnatin) are seen in the upper nucleus
running off from the chromatin-masses. The
clear spaces are occupied by the ground-sub-
stance.

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 chromatin
(Flemming), is the nuclear substance par 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-
nospJicerinin, Arcella) ; 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, chrom.atin is not
acted upon, or is but slowly affected, by peptic digestion. I: may thus
be easily isolated for chemical analysis, which shows it to consist
mainly of nnclein, i.e. a compound in varying proportions of a complex
phosphorus-containing acid known as nncleinic acid, with albumi-
nous bodies such as histon, protamin, or in some cases albumin itself.^
Upon this, as will be shown in Chapter VI., probably depends the pro-
nounced staining capacity when treated with the so-called " nuclear
stains " {e.g. haematoxylin, 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 often
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.

34

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 plasniosomes (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 like chromatin. NucleoH of a
quite different type are the "net-knots" (Netzknoten), chromatin-
nucleoli, or karyosomes, 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 (" nucleolo-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

1 Montgomery, '98, 2.

2 Flemming 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.

8 For very full review of the literature of the nucleoli see Montgomery ( '98, 2).
* Cf. p. 67.

THE NUCLEUS

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 "

C E

Fig. 14. — Special forms of nuclei.

A. Permanent spireme-nucleus, salivary gland of Chironomus larva. Chromatin in a single
thread, composed of chromatin-discs (chromomeres), terminating at each end in a true nucleolus
or plasmosome. [BalbianI.]

B. Permanent spireme-nuclei, intestinal epithelium of dipterous larva Ptychoptera. [VAN
Gehuchten.] C The same, side view.

D. Polymorphic ring-nucleus, giant-cell of bone-marrow of the rabbit ; c. a group of centro-
somes or centrioles. [Heidenhain.]

E. Branching nucleus, spinning gland of butterfiy-larva {Pieris) . [KORSCHELT.]

(p. 322), from which arises the achromatic part of the division-
figure (p. 82). On the other, hand, Hacker ( '95, '99) and other
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 nucleolus 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 applied to certain cases; but without
further evidence it must remain doubtful whether it appHes to
all.i

d. The ground-substance, nuclear sap, or karyolympJi, 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 "network," 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
granules of " lanthanin " 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 {cf. p. 65). The most striking case
of this kind occurs in the saHvary glands of dipterous larvae {Chirono-
mus), 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 {cf. p. 273). In certain gland-cells of the marine isopod
Anilocra it is arranged in regular rosettes (Vom Rath). Rabl, fol-
lowed by 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

^ Cf. pp. 126-130. 2 Qr jj^g polarity of the cell, p, 55.

THE NUCLEUS

37

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 Structure of the 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 chromosomes (Fig. 21), which split
lengthwise as the cell divides. These
bodies arise as aggregations of minute
rounded bodies or microsomes to which
various names h-ave been g\yQr\{chromo-
meres, 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 cJiromosornes (Figs. 8, D, 53,
B)\ but in many cases they are dis-
tinctly visible in the reticulum of the Tcatrered chrmna^n-gV7nir^^^

"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"). The
later studies of Meves ('97, i) make it probable that the latter are true nucleoli and the for-
mer chromatin-granules. Korschelt, however, regards the " macrosomes " as composed of
chromatin and the "microsomes" as representing the so-called "achromatic substance."

38 GENERAL SKETCH OF 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 Reinke,
Waldeyer, and others, has shown that the "■ achromatic " nuclear net-
work is likewise composed of granules, which he distinguishes as
lanthanin- or oxycJLro7natin-gX2iXiv\^'& from the basichromatm-gxdiXWxlQs
of the chromatic network. Like the latter, the oxychromatin-granules
are suspended in a non-staining clear substance, for which he reserves
the term linm. Both forms of granules occur in the chromatic
network, while the achromatic network contains only oxychromatin.
They 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
VII., 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 ('78) 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-
work. 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

\ ,.- .

B

I--/: i

n E

Fig. i6. — Forms of Cyanophyceae, Bacteria, and Flagellates showing the so-called scattered
or distributed nuclei. [^-C BiJTSCHLl ; Z)-/^. SCHEWIAKOFF; G-J. CALKINS.]

A. Oscillaria. B. Chroniatium. C. Bacterium lineola. D. Achromatimn. E. The same in
division. F, Fission of the granules. G. Tetramitus, with central sphere and scattered granules.
H. Aggregation of the granules. /. Division of the sphere. J. Fission of the cell.

Cyanophyceae and Bacteria among plants. In many of these forms
(Fig. 1 6) no distinct nucleus can be demonstrated, the cell consisting
of a mass of protoplasm in which are scattered numerous deeply
staining granules. Many of these granules stain intensely with
haematoxylin and other "nuclear" dyes; like chromatin, they resist
the action of peptic digestion, and in at least one case (the bacterium-
Uke Achroinatuim, according to Schewiakoff, '93) they have the power
of division like the chromatin-granules of higher forms. For these

40 GENERAL SKETCH OE THE CELL

reasons most observers (Biitschli, Gruber, Schewiakoff, Nadson, etc.)
regard them as true chromatin-granules which represent 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 Cyanophyceae 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
Achroinatiiini (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-^j//^, Trac/ielocerca,
Holes ticha, UroleptiLs\ as well as some rhizopods {Pelojnyxa), in
which the body contains very numerous minute chromatin-granules
of "nuclei" (Fig. 15), which Gruber {'^7) showed to multiply by
division. Balbiani ('61) long since showed that in Urostyla these
bodies become concentrated toward the centre of the ceil 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 Tetramitus 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 {Trachelomonas, Lagenella,
Chilomonas) or with {Eiigleria, Syntira) 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.
2 '84, p. 251.

THE CYTOPLASM ^ 4 1

house built to contain the chromatic elements, and its achromatic ele-
ments (linin, 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 " building
for itself a new dwelling."

3. Chemistry of the 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, though
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. Paralinin. The ground-substance.

4. Pyrenin or Parachromatin. The inner mass of true nucleoli.

5. Amphipyretiin. The substance of the nuclear membrane.

Chromatin is probably identical with nuclein (p. 332), which is a compound of
nudeinic 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. 333).
The linin 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. Hertwig calls it paranuclein. " Amphipyrenin "
has no very definite meaning; for the nuclear membrane sometimes appears to be of
the same nature as the linin, while 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 endoplasm 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

^ This fact was first pointed out in the tissue-cells of animals by Kupffer ('75), and its
importance has since been urged by Waldeyer, Reinke, and others. The cortical layer is
by Kupffer termed paraplasm, by Pfeffer hyaloplasm, by Pringsheim the LLautschicht. The
medullary zone is termed by Kupffer protoplasm, sensu strictu; by Strasburger, K'drner'
plasma; hy ^d^gtW, 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'developcd for the performance of particular functions
or a mere coagulation-product due to the action of fixatives. It has
been shown that structureless protcids, 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^^7\
and especially of Biitschli ('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 sub.stances, in various degrees of concentration, fixed and
stained by a great variety of the reagents ordinarily used for the
demonstration of celkstructures. The result was to produce a mar-
vellously close simulacrmn 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 Kupffer C90), 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-

wmrnmm f '■ '"J

■■v'oyov'.'o-

A CD

Fig. 17. — Ciliated cells, showing cytoplasmic fibrillcs terminating in a zone of peripheral
microsomes to which the cilia are attached. [EngelmanN.]

A. From intestinal epithelium of AnodoMta. 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 regular 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 protoplasm ; and Flem-
ming as well as many 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
granular threads in living plant-cells.

Almost equally conclusive is the beautifully regular arrangement
of the fibrillse in ciliated cells (Fig. 17, Engelmann), in muscle-fibres
and nerve-fibres, and especially in the mitotic figure of dividing cells

Fig. 18. — Cells of the pancreas in Amphibia. [Mathews.]

A-C. Necturus ; D. Rana. A and B represent two stages of the " loaded " cell, showing
zymogen-granules in the peripheral and fibrillar structures in the basal part of the cell. C shows
cells after discharge of the granule-material and invasion of the entire cell by fibrillae. In D por-
tions of the fibrillar material are coiled to form the mitosome (" paranucleus " or " Nebenkern ").

(Figs. 21, 31), where they are likewise more or less clearly visible
in life. A very convincing case is afforded by the pancreas-cells
of Necturus, which Mathews has carefully studied in my laboratory.
Here the thread-work consists of long, conspicuous, definite fibrillae,
some of which may under certain conditions be wound up more or
less closely in a spiral mass to form the so-called Nebenkeiii. 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 object stained
in thin sections after fixation by picro-acetic, sublimate-acetic, and

Fig. 19. — Section through a nephridial cell of the leech, Clepsine (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 liquid. 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 excretory
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, though the sec-
tions give at first sight an appearance somewhat different from that

46 GENERAL SKETCH 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, 12), 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 the form
and arrangement of all the elements is almost identically the 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, we
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 fibrillse or alveolar walls (as was long since suggested by Heitz-
mann and Biitschliy or to uniformity of refractive index in the
structural elements. It is highly probable that such is often the case ;
indeed, Biitschli 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 followed step by step in the living 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 intra
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 down to the
present limits of microscopical vision.

One of the must beautiful forms of cyto-reticulum with which I

1 Cf. Biitschli, '92, 2, p. 169.

THE CYTOPLASM

47

am acquainted has been described by Bolsius and Graf in the neph-
ridial cells of leeches as shown in Fig. 19 (from a preparation by
Dr. Arnold Graf). The meshwork is here of great distinctness and
regularity, and scattered microsomes are found along its threads. It

Fig. 20. — Spinal ganglion-cell of the frog. [LenhosseK.]
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.) The dark cyto-
plasmic masses are the deeply staining " chromophilic granules" (Nissl) 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 (Fig. 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
fibrillae corresponding in number with the cilia as if continuous with
their bases (Fig. 17V In nerve-fibres the threads form closely set
parallel fibrillae 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 Ballowitz
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 fibrillae 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 suppose,
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 lamellae which form
the walls of closed chambers filled by a more liquid substance.
Biitschli, followed by Reinke, 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 sul)Stance of the nerve-fibres.

THE CYTOPLASM

afforded by the appearance of the spindle and asters in cross-section.
In the early stages of the ^gg 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 oblique 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, though these are
connected by cross-branches (Fig. 28, F). Again, the crossing of

Centrosphere 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 meshuork.

the rays proceeding from the asters (Fig. 128), and their behaviour 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.

50

GENERAL SKETCH OF THE CELL

Space would not suffice 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 Hfe). When
the liquid drops are 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 Biitschli.^ 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

1 In the latter the alveolar spheres are, according to Biitschli, not more than one or two
microns in diameter.

2 This has been demonstrated in the cells of plants by Crato ('96), and more recently
by the writer ('99), in the case of echinoderm and other eggs.

3 Cf. 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

some other reagents, and surrounded by a cytoplasmic radiating aster
or by a rounded mass known as the attraction-sphere (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. 23).i

T-here 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 will 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 like 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 Q%^, 2, p. 68). Van Beneden's
and Boveri's independent identification of centrosome in Ascaris as a permanent cell-organ
('87) was quickly supported by numerous observations on other animals and on plants. In
rapid succession the centrosome and attraction-sphere were found to be present in pig-
ment-cells of fishes (Solger, '89, '90), in the spermatocytes of Amphibia (Hermann, '90), in
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 Bambeke, Van der
Stricht, '91), in the flagellate Noctihua (Ishikawa, '91), in the cells of marine algse (Stras-
burger, '92), in cartilage-cells (Van der Stricht, '92), in cells of cancerous growths (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-fibres (Lenhossek, '99),
and in embryonic cells of many kinds (Heidenhain, '97). Many others have confirmed
and extended this list. Guignard's identification of the centrosomes in higher plants is
open to grave doubt (^cf. p. 82). 1 0^ P- 57*

52

GENERAL SKETCH OF THE CELL

View it was regarded further as the especial fertihzing element in the
spermatozoon, which, when introduced into the egg, 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
cytology 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 de novo. Experimental researches, also, have giv^n 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 entirely 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 comparable with
that of the nucleus and centrosome. From a general point of view
the most interesting of these are the bodies known ^.splastids or proto-
plasts {¥\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 cJironiatopJwres or cJu'ouioplastids,
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

1 Cf. pp. Ill, 304. Eisen ('97) asserts that in the blood of a salamander, Batrachoseps,
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 pigment-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, Spongilia, some planarians) are in reality dis-
tinct Algae 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 application. 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 GENERAL SKETCH OF 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 sculpture. 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 cntiailar
in a purely topographical sense to denote membranes formed only
on one (the free) side of the cell,^ 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
agreed that the formation of new membranes on naked masses of

» Cf. '85, p. 12. 2 cf. 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 cellulose, a carbohydrate having the formula CgH^oOg ; but this sub-
stance is very frequently impregnated with other substances, such
as silica, lignin, 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 ^gg, which is so
important a factor in development, with that of the tissue-cells ; for
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 " i<inoplasm," or filar plasm of which he considers the " Haut-
schicht," as well as the spindle-fibres, to be largely composed. In support of 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 in
Erysiphe (Fig. 33), where the spore-membrane appears to arise directly from the astral
rays.

56

GENERAL SKETCH OF 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

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 und
Hatschek, B in a gland-cell, Cin 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 rhost 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.

.i(f/.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. 23)

irrTrunrmTTT

B

C

D

Fig. 23. — Centrosomes in epithelial and other cells. [^, Z), 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 monkey.
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 cilia 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 ^g'g is one of the

1 Zimmermann, '98; Heidenhain and Cohn, '97.

2 Cf. p. 356.

58 GENERAL SKETCH OF THE CELL

primary conditions of development, and we have here, as I beUeve,
a clue to its determination. ^

1. The Cell in Relation to the Multicellular Body

In analyzing the structm-e 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 unicellular plants 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 in this 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-

1 Cf. pp. 384, 424. We should remember that the germ-cells are themselves epithelial
products. 2 Untersuchungen^ Trans., p. 1 81.

THE CELL IN RELATION TO THE MULTICELLULAR BODY 59

cerned, it has now been clearly demonstrated that only in a limited
sense can the cells be regarded as cooperating units. They 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 equilibrium 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
remains sub jiidice, 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, Bizzozero, Retzius, 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 ( '^Z), 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, biU also
with the ovum ; 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 Volvox. 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
Peripatus 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 Amphioxus 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. 2 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 " delicate 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 epithelial 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. 1 2

Altmann, R. — Die Elementarorganismen und ihre Beziehungen zu den Zellen, 2d

ed. Leipzig, 1894.
L'Annee Biologique. — Paris, 1895-96. (Full Reviews and Literature-lists.)
Bohm and Davidoff. — Lehrbuch der Histologie des Menschen. Wiesbaden, 1895.
Boveri, Th. — (See Lists IV., V.)
Biitschli, 0. — Untersuchungen iiber mikroskopische Schaume und das Protoplasma.

Z(?/)^5'/^ (Engelmann), 1892.
Id. — Untersuchungen liber Struktur. Leipzig, 1898.
Carnoy, J. B. — La Biologie Cellulaire. Lierre, 1884.
Engelmann, T. W. — Zur Anatomic und Physiologie der Flimmerzellen : Arch. ges.

P/iys., XXIII. 1880.
Erlanger, R. v. Neuere Ansichten uber die Struktur des Protoplasmas : ZooL

CeiitralbL, III. 8, 9. 1896.
Fischer, A. Fixierung, Farbung und Bau des Protoplasmas. Jena, 1899.
Flemming, W. Zellsubstanz, Kern und Zelheilung. Leipzig, i^Z2.
Id. Z&Wt: Merkelimd Bontiefs Ergeb?iisse,\.-\ll\. 1891-97. (Admirable reviews

and literature-lists.)

1 '95, pp. lo-i I ; '97, p. 261. 2 See also Introductory list, p. 14.

62 GENERAL SKETCH OF THE CELL

Heidenhain, M. — (jber Kern und Protoplasma : Festschr. z. ^o-jdhr. Doctorjub. von

V. Kolliker. Leipzig^ 1893.
Klein, E. — Observations on the Structure of Cells and Nuclei : Quart. Journ. Mic.

Sc/., XVI II. 1878.
Kolliker, A. — Handbuch der Gewebelehre, 6th ed. Leipzig, 1889.
Leydig, Fr. — Zelle und Gevvebe. Bonn, 1885.
Schafer, E. A. — General Anatomy or Histology; in QtiairCs Anaiotny, I., 2, loth

ed. London, 1891.
Schiefferdecker & Kossel. — Die Gewebe des Menschlichen Kbrpers. Braunschweig^

1891.
Schwarz, Fr. — Die morphologische und chemische Zusammensetzung des Proto-

plasmas. Breslau, 1887.
Strasburger, E. — Zellbildung und Zellteilung, 3d ed. 1880.
Id. — Das Botanische Practicum, 3d ed. Jena, 1897.
Strasburger, Noll, Schenck, and Schimper. — Lehrbuch der Botanik, 3d ed. Jena',

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. Jena, 1889.
Waldeyer, W. — Die neueren Ansichten liber den Bau und das Wesen der Zelle :

Deu-sch. Med. Wochenschr ., Oct., Nov., 1895.
Wiesner, J. — Die Elementarstruktur u. das Wachstum der lebenden Substanz :

IVien, Holder. 1892.
Wilson, E. B. — The Structure of Protoplasm : Journ. Morph., XV. Suppl. ; also

Wood'^s Holl Biol. Lectures, 1899.
Zimmermann, A. — Beitrage zur Morphologie und Physiologic der Pflanzenzelle.

Tubingen, 1893.
Id. — Die Morphologie und Physiologic des Pflanzlichen Zellkernes. Jena, 1896.

CHAPTER II

CELL-DIVISION

" Wo eine Zelle entsteht, da muss eine Zelle vorausgegangen sein, ebenso wie das Thiet
nur aus dem 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 continuirlichen Entwicklung\it%\.t\)X.r ViRCHOW.^

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 ^s 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 (^ Introduction, p. lo.

3 For a full historical account of this period, see Remak, Untersuchungen uber die Ent-
wicklung derWirbelthiere, 1855, pp. 164-180. Also Tyson on the Cell-doctrine and Sachs's
Geschichte der Botanik.

6.^

64

CELL-DIVISION

body and membrane (Fig. 24). For many years this account was
accepted, and no essential advance beyond Remak'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 Baer, 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. In this year the discoveries
of Anton Schneider, quickly followed by others in the same direction
by BiitschH, 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
i^j'^) afterward gave the name
of karyokinesis. 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 fraginejitation,
corresponding nearly to the
simple process described by
Remak, and division, involving
the more complicated process
of karyokinesis. Three years
later Flemming ('79) proposed to substitute for these the terms direct
and indirect division, which are still used. Still later ('82) the same
author suggested the terms mitosis (indirect or karyokinetic division)
and amitosis (direct or akinetic division), which have rapidly made
their way 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

Fig. 24. — Direct division of blood-cells in
the embryo chick, illustrating Remak's scheme.
[Remak.]

a-e. Successive stages of division
dividing by mitosis.

/ cell

OUTLINE OF INDIRECT DIVISION 65

view be well founded or not, it is certain that in all the higher 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 ci
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 (Karyokinesis)

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 Prophases, or preparatory changes; (2) the Metaphase,
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 Nucleus. 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 intensefy
staining bodies, u.suaily_rQd-^shap_ed, known as chromosomes. 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 qrjess
conjvpluted thread, known as the £/fmg(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

F

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

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; above, the amphiaster (a). CD. 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). R. 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 type alone). F. The mitotic figure established;
e.p. the equatorial plate of chromosomes. {Cf. Figs. 21, 27, 32, etc.)

OUTLINE OF INDIRECT DIVISION 6/

its first appearance divided into a number of separate pieces or seg-
ments, forrning a segmented spireme. In either case it ultimately
breaks transversely to form the chromosomes, yN\{\Q\\ in most cases have
thejorm of rods, straight or curved, though they are sometimes spher-
ical or ovoidal, affd m certam cases may be joined together in the
fornLoFrings. The staining-power of the chromatin is now at a mg^i-
mum. As a rule the nuclear membrane meanwhile fades^awavand
finalTy 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
ol_the_jnucleuj becomes continuous with the surrounding cytoplasm
(Fig. 25, A E, F)} —

The remarkable fact has now been established with high probability
that every species of plant or animal has a fixed and characteristic nunz-
ber of chromosomes, which regularly recurs in the division' of all of its
cejlsj and in all forms arising by sexual reproduction the numha^Js
everi. 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 Hly, 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.

Tliejiucleoli 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 (Meunier^ 'Z6, and Moll, '93)or^^/^-
nosphcBrium (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 rm(-leoli or plasmo-
somes sooner or later disappear ; and thegreater number of observers
j^ree that they do not take part in the chrnmnsnme-formation. In a
considerable number of forms {eg. during the formation of the polar

1 The spireme-formation is by no means an invariable occurrence in mitosis. In a consid-
erable number of cases the chromatin-network resolves itself directly into the 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 Flemming believes the number in man to be considerably greater than 16.

3 For a more complete list see p. 206.

6S CELL-DIVISION

bodies in various eggs) the nucleolus 1> r^^^t nnt- infn jJTP^ryl-npTagrn^c

the spindle forms^to persist as a " metanucleus '^for some_jtime beforie
itS-Jinal disappearance (Fig. 104). More commonly the nucleolus
fades away in situ, 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 Strasburg-er ('95. '97), who suj^ests thqt the tnie_r>ucleoli are to be
reg^arded as storehouses^ of " kinoplasmic '^jp^j^nal^which is either
directly used in the formation of the spindle, or, upon being cast out
of the nucleus, adds to the cytoplasmic store of "■ kinoplasm " avail-
able for future mitosis.

{b) The Amphiaster, Meanwhile, more or less nearly parallel with
these changes in the chromatin, a complicated structure known as the
amphiaster {Yo\ 'yy) makes its appearance in the po'^i^'^'on fo^^^rly
occupied by the nucleus (Fig. 25, B-F). This structure consists of
a fibrous spjndle-shapeH b'^dy^ the spindje, 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 fiUngs in the field of a horseshoe magnet. The centre of each
aster is occupied by a minute body, known as the centrosojne (Boveri,
'88), which may be surrounded by ^ sphenV^I mat;^ known ^^__HTe
centrvsphere (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 ecjua torialpla te.

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 amphiaster 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 cen-
tral spindle (Figs. 25, D, F; 27). In other cases the originaLspllldJ^
disappears, and the two asters pass to opposite poles of the nucleus
(some plant mitoses and in many animal-cells). A spindle is now
formed from rays that growjnto the nucleus from each astex. 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 no
central spindle. In a few exceptional cases^ fijiallv. the amphiaster
x^^;^_^^^,jX^^^^'i!^^^vi^^s:^ (p- 304).

The entire structure, resulting from the foregoing changes, is

OUTLINE OF INDIRECT DIVISION

69

known as the karyokinetic ox mitotic figure. It may be. described as
consisting of two distinct parts; namely, i^he chromatic figure,
formed by the deeply staining chromosomes ; and, 2, the ackromdRc
^^jjire^ 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.). n. The cast-off nucleolus. H. Ana-
phase ; the daughter-chromosomes diverging, between them the interzonal-fibres {i./.), 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 begin-
ning reconstruction of the daughter-nuclei, y. Division completed.

ally known as ardinphum. (Boveri, '88), but this term is not applied
to the centrosome within the aster.

2. Metaphase. — The 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-DIVISION

finally gives .rise to a daughter-nucleus (Fig. 26). In some cases
the splitting of the chromosomes cannot be seen until they have
grouped themselves in the equatorial 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 chromatic network is converted into a thread ^ which, whether
rnrffifiunii^ nv rli ^rontiniious^ splUs tJiroii!3:hoiU its entire length jpj.n
two exoLtl^f eqnivoLmLJialm^- 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
('83), the entire substance of the chromatic network is precisely halved,
and the datigJiter-niiclei receive precisely equivalent portions of chro-
viatin from the mother-nucleus. It is very important to observe that
the nuclear division always shows this exact quaHty, 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 Qgg, 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~or~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 fromtHe~oth^rTpindle-fibres ; and they are believed
by many observers to have an entirely different origin and function.
A view now widely held is that of Plennann, who regards these fibres
as belonging to a central spindle, surrounded by a peripheral layer
of mantlefibres 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 {/jlItos, a thread).

2 This stage is termed by Flemming the dyaster, a term which should, however, be aban-
doned in order to avoid confusion with the earlier word amphiaster. The latter convenient
and appropriate term clearly has priority.

^ Verbitidungsfasern of German authors ; filanients reunissants of Van Beneden.
4 Cf. p. 105.

OUTLINE OF INDIRECT DIVISION 7I

fibres show during this period a series of deeply stainin^Jhickeni^
in_the_e^uatorial__pkne^ the cell-i)late 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 daiighter-spirenie, 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, corj:e^pondinglv unequal — a fact
which, as Conklin and others have pointed out, proves that the size
of the nucleus is controlled by that of the cvtoplasmic 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 {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. 1 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 prophases and meta-

1 Cf, Strasburger, '98. 2 cf. Hoffmann, '98,

J 2 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 ^g^. In a large number of
cases, however, the centrosome persists, lying either outside or mor*^
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 earliest observers ('73-75) supposed the achro-
matic figure to disappear entirely at the close of cell-division, and
most of them (Biitschli, 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-

^ 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

c

D

Fig. 27. — The prophases of mitosis (heterotypical form) in primary spermatocytes of
Salamandra. [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 amphiaster 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

74 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 Qgg 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 chromatin.
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-

Vv<>u«J^^

Fig. 28. — Metaphase and anaphases of mitosis in cells (spermatocytes) of the salamander,

[DRIJNER.]

E. Metaphase. The continuous central spindle-fibres pass from pole to pole of the spindle.
Outside them the thin layer of contractile mantle-fibres attached to the divided chromosomes, of
which only two are shown. Centrosomes and asters. F. Transverse section through the mitotic
figure showing the ring of chromosomes surrounding the central spindle, the cut fibres of the latter
appearing as dots. G. Anaphase ; divergence of the daughter-chromosomes, exposing the cen-
tral spindle as the interzonal fibres; contractile fibres (principal cones of Van Beneden) clearly
shown. H. Later anaphase (dyaster of Flemming) ; the central spindle fully exposed to view;
mantle-fibres attached to the chromosomes. Immediately afterward the cell divides (see Fig. 29) .

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 their 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 {^^J, 2,
p. 153), and the same general result was reached by Vejdovsky
nearly at the same time,^ though it was less clearly formulated than
by either Boveri or Van Beneden.

All these observers agreed, therefore, that the achromatic figure-
arose outside the nucleus, in the cj^toplasm ; 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 (telepliases) of mitosis in salamander cells. [FleMMING.]

/. Epithelial cell from the lung; chromosomes at the poles "of ihe spindle, the cell-body divid-
ing; granules of the "mid-body" or Z.wischenkorper ax the equator of the disappearing spindle.
J. Connective tissue-cell (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. TJie centrosome rep-
resents the dynamic centre of cell'' ^

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

'87, p. 279.

pp. 151, etc.

Boveri, '87, 2, p. 153.

ORIGIN OF THE MITOTIC FIGURE 'J'J

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 tjwo^ 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 7to 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 Beneden
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 point
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 may be 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. Varieties of the Mitotic Figure

{a) The 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, Driiner, 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 nuclqar 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 spindle
(Hern^ann, '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 (Hermann, Driiner,
MacFarland), or in part
cytoplasmic, in part dif-
ferentiated from the linin-
network (Flemming,
Meves). Druner ('95),
Braus ('95) (salamander),
and MacFarland {Pieui'o-
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 forms
may differ in the origin of the central spindle, since in Pleurophyllidia
it is of secondary origin, as described above, while in Diaiiliila 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

^ cf.v- 315-

Fig. 30. — Mid-body in embryonic ceiis oiLi/nax. [HOFF-
MANN.]

Earlier stage above, showing thickenings along the line
of cleavage. Later stage, below, showing spindle-plate and
cytoplasmic plate.

8o

CELL-DIVISION

an attraction-sphere surrounding the centrosomes, and is perhaps
comparable to Heidenhain's " centrodesmus."

In the second type, illustrated in the cleavage of echinoderm,
annelid, molluscan, and some other eggs, a central spindle 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-egg. [BOVERI.]
^.Closing prophase, the equatorial plate forming. B. Metaphase; equatorial plate estab-
lished and the chromosomes split; i. 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 Y\g. 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 8 1

origin, since the ends of the spindle begin to form before dissolution
of the^nuclear membxane. 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 Thalassevia, CJice-
toptenis\ and it is probable that the middle region oLlh£ spindle is
here formed from the linin-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

Fig. 32. — Mitosis in Sfypocaulon. [Swingle.]

A. Early prophase with single aster and centrosome. B. Initial formation of intranuclear
spindle. C. Divergence of the daughter-centrosomes. D. Early anaphase ; nuclear membrane
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 l.epidoptej'a, Watase ('91) on T olio-o, Eraus ('95) on
Triton, and (Iriffin ('96, '99) on IVialassetna. Erlanger ('97, 5) endeavours t<> show that in
the mitosis of embryonic cells in the cephalopods {Sepia), 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 earlieT in the
first type. When the nucleus lags behind the spindle-formation the
centrosomes take up their position prematurely, as it were, the cen-
tral spindle disappearing to make way 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
(Algae) 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 Stytopocaidon 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 /;/ the 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
prevailing 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 (3bservations,
begun by Farmer and Belajeff, and strongly sustained by the care-
ful studies of Osterhout, Mottier, Nemec, and others, the achromatic
figure is almost wholly of cytoplasmic origin, arising from a fibrillar
material (" kinoplasm " or " filar plasm," of Strasburger), which at the
beginning of mitosis forms a net-like mass surrounding the nucleus,
from which fibrillae radiate out into the cytoplasm. As the nuclear
membrane fades, these fibrillae, continually increasing, invade the
nuclear area, gather themselves into bundles, converging to a number

1 See especially Swingle ('97) on Sphacelariacece, Strasburger ('97) on Fucus, Mottier
('98) on Dictyota ; cf. also Harper ('97) on Erysiphe and Peziza.

2 Cf. Schaffner ('98), Fulmer ('98).

3 See Osterhout ('97) on Equiseiuin, Mottier ('97, I, '97, 2) on Lilium, Lawson ('98) on
CobcEu, Nemec ('99) on Allium, Debski ('97, '99) on Chara ; also Belajeff ('94) and
Farmer ('95).

DETAILS OF MITOSIS

83

'm.

of centres (without centrosomes), and thus give rise to an irregular
multipolar figure (Figs. 36, 133). This figure finally resolves itself
into a definite bipolar spindle which is devoid of centrosomes, and
in the earlier stages also of asters, though in the later phases some-
what irregular asters are formed. On the basis of these observations
Mottier ^ proposes to distinguish provisionally two well-defined types
of mitosis in plants which he designates as the "thallophyte " and the
" cormophyte " types. The latter seems wholly irreconcilable with
the process observed in animal-cells ; for the whole course of spindle-
formation seems diametrically opposed in the two cases, and should
the cormophyte-type be estabhshed it would, to say the least, greatly
restrict the application of the centrosome-theory of Van Beneden and
Boveri. Only future re-
search can definitely de-
termine the question.
There can be no doubt
that the descriptions of
Guignard and his follow-
ers do not rest upon pure
imagination ; for it is easy
to observe at the spindle-
poles in some prepara-
tions {e.g. sections of root-
tips of Allium, Lilinmy
etc.) deeply staining-
bodies such as these
authors describe. These
"centrosomes" seem,
however, to be of quite
inconstant occurrence ;
and the careful studies of
Osterhout, Mottier, and Nemec seem to give good ground for the
conclusion that they have no such significance as the centrosomes of
lower plants or of animals. It should nevertheless be borne in mind
that true centrosomes (" blepharoplasts ") have been demonstrated in
the spermatogenic divisions of some of the vascular cryptogams, and
that analogous bodies occur in the corresponding divisions of the
cycads (p. 175). We should therefore still hold open the possibility
that centrosomes may occur in the vegetative mitoses of the higher
plants, their apparent absence being possibly due to lack of staining-
capacity or similar conditions rendering their demonstration difficult.^

1 '97, 2, p. 183.

2 Mention may here be made of the barrel-shaped truncated spindles described in some
of the plants. In BasidioboluSy Fairchild ('97) finds spindles of this type, having no asters

Fig- 33. — Mitosis in ascus-nuclei of a fungus, Erysiphe,
[Harper.]

A. Resting nucleus with disc-shaped centrosome {c).
B. Early prophase with aster. C. Later prophase ; amphi-
aster; intranuclear spindle forming. D. Spindle estab-
lished. E. Daughter-nucleus after division ; spore-mem-
brane developing from astral rays.

84

CELL-DIVISION

A no less remarkable mode of spindle-formation, which is in a cer-
tain way intermediate between the cormophyte-type and the usual
animal type is described by Mead ('97, '98, i) in the first maturation-
division of Chcetopteriis. Here the completed amphiaster is of quite
typical form, and the centrosomes persist for the following mitosis ;
yet Mead is convinced that the amphiaster is synthetically formed by
the union of two separate asters and centrosomes (Fig. 150) which

B

Fig. 34. — Division of pollen-mother-cells in the lily as described by GUIGNARD.
A. Anaphase of the first division, showing the twelve daughter-chromosomes on each side, the
interzonal fibres stretching between them, and the centrosomes, already double, at the spindle-
poles. B. Later stage, showing the cell-plate at the equator of the spindle and the daughter-
spiremes (dispireme-stage of Flemming). C. Division completed; double centrosomes in the
resting cell. D. Ensuing division in progress ; the upper cell at the close of the prophases, the
chromosomes and centrosomes still undivided; lower cell in the late anaphase, cell-plate not yet
formed.

have no genetic connection, arising independently de novo in the
cytoplasm.^ Improbable as such a conclusion may seem on a priori
grounds, it is supported by very strong evidence,^ and, taken together

and nearly parallel fibres, each of which terminates in a deeply staining granule. Nearly
similar spindles have been described by Strasburger ('80) in Spirogyra, and in the embryo-
sac oi Monotropa. It is not impossit)le that such spindles may represent a type intermediate
between the "cormoptyte" and " thallophyte " types of Muttier.

1 Cf. p. 306. ,

2 I have had the privilege of examining some of Mead's beautiful preparations.

DETAILS OF MITOSIS

85

with the facts described in plants, it indicates that the forces involved
in spindle-formation are far more complex than Van Beneden's and
Boveri's hypothesis would lead one to suppose.^

The centrosome and centrosphere appear to present great varia-
tions that have not yet been thoroughly cleared up and will be more
critically discussed beyond.^ They are known to undergo extensive
changes in the cycle of cell-division and to vary greatly in different
forms (Fig. 152). In some cases the aster contains at its centre
nothing more than a minute deeply staining granule, which doubtless

C - - -^c-

Pig. 36- — Division of spore-mother-cells in ^'(^^/j-^/^;;/, showing spindle-formation. [OSTERHOUT.l
A. Early prophase, " kinoplasmic " fibrillas in the cytoplasm . B. Multipolar fibrillar figure invad-
ing the nuclear area, after disappearance of the nuclear membrane. C. Multipolar spindle.
D. Quadripolar spindle which finally condenses into a bipolar one.

represents the centrosome alone. In other cases the granule is sur-
rounded by a larger body, which in turn lies within the centrosphere
or attraction-sphere. In still other cases the centre of the aster is
occupied by a large reticular mass, within which no smaller body can
be distinguished {e.g. in pigment-cells) ; this mass is sometimes called
the centrosome, sometimes the centrosphere. Sometimes, again, the
spindle-fibres are not focussed at a single point, and the spindle

1 See p. 276 for the peculiar spindles, devoid of asters, observed during the maturation of
the egg in certain forms. Cf. also Morgan's experiments on the artificial production of asters
and centrosomes, p. 307. . 2 J^^_.e p_ ^q^_

86 CELL-DIVISION

appears truncated at the ends, its fibres terminating in a transverse
row of granules (maturation-spindles of Ascaris, and some plant-cells).
It is not entirely certain, however, that such spindles observed in
preparations represent the normal structure during life.

b. The Chrotnatic Figure. — The variations of the chromatic
figure must for the most part be considered in the more special
parts of this work. There seems to be no doubt that a single
continuous spireme-thread may be formed {cf. p. 113), but it is
equally certain that the thread may appear from the beginning
in a number of distinct segments, i.e. as a segmented spireme,
and there are some cases in which no distinct spireme can be
seen, the reticulum resolving itself directly into the chromosomes.
The chromosomes, when fully formed, vary greatly in appear-
ance. In many of the tissues of adult plants and animals they
are rod-shaped and are often bent in the middle Hke a V (Figs.
28, 131). They often have this form, too, in embryonic cells, as
in the segmentation-stages of the ^g^ in Ascaris (Fig. 31) and
other forms. The rods may, however, be short and straight (seg-
menting eggs of echinoderms, etc.), and may be reduced to spheres,
as in the maturation-stages of the germ-cells. In the equatorial plate
the V-shaped chromosomes are placed with the apex of the V turned
toward the spindle (Fig. 28), while the straight rods are placed
with one end toward the spindle. In either case the daughter-
chromosomes first begin to move apart at the point nearest the
spindle, the separation proceeding thence toward the free portion.
The V-shaped chromosomes, opening apart from the apex, thus give
rise in the early anaphase to < >-shaped figures ; while rod-shaped
chromosomes often produce a- and x-shaped figures (the stem of the
_L being double). The latter, opening farther apart, form straight
rods twice the length of the original chromosome (since each consists
of two daughter-chromosomes joined at one end). This rod finally
breaks across the middle, thus giving the deceptive appearance of a
transverse instead of a longitudinal division (Fig. 52). The <>-
shaped figures referred to above are nearly related to those that
occur in the so-called heterotypical mitosis. Under this name Flem-
ming i^'^y) first described a peculiar modification of the division of the
chromosomes that has since been shown to be of very great impor-
tance in the early history of the germ-cells, though it is not confined
to them. In this form the chromosomes split at an early period, but
the halves remain united by their ends. Each double chromosome
then opens out to form a closed ring (Fig. 37), which by its mode of
origin is shown to represent two daughter-chromosomes, each forming
half of the ring, united by their ends. The ring finally breaks in two
to form two U-shaped chromosomes which diverge to opposite poles

DETAILS OF MITOSIS

87

of the spindle as usual. As will be shown in Chapter V.,the divisions
by which the germ-cells are matured are in many cases of this type ;
but the primary rings here in many cases represent not two but four
chromosomes, into which they afterward break up.

Fig. 37. — Heterotypical mitosis in spermatocytes of the salamander. [Flemming.]
A. Prophase, chromosomes in the form of scattered rings, each of which represents two
daughter-chromosomes joined end to end. B. The rings ranged about the equator of the spindle
and dividing ; the swellings indicate the ends of the chromosomes. C. The same viewed from the
spindle-pole. D. Diagram (Hermann) showing the central spindle, asters, and centrosomes, and
the contractile mantle-fibres attached to the rings (one of the latter dividing).

2. Bivalent a7id Plurivalent Chromosomes

The last paragraph leads to the consideration of certain varia-
tions in the number of the chromosomes. Boveri discovered that the
species Ascaris megalocephala comprises two varieties which differ in
no visible respect save in the number of chromosomes, the germ-nuclei
of one form ('' variety bivalens " of Hertwig) having two chromosomes,

8 8 CELL-DI VISION

while in the other form (" variety univalens ") there is but one. Brauer
discovered a similar fact in the phyllopod Arteniia, the number of
somatic chromosomes being i68 in some individuals, in others only
84 (p. 281).

It will appear hert^after that in some cases the primordial germ-
cells show only half the usual number of chromosomes, and in
Cyclops the same is true, according to Hacker, of all the cells of
the early cleavage-stages.

In all cases where the number of chromosomes is apparently
reduced ("pseudo-reduction" of Riickert) it is highly probable that
each chromatin-rod represents not one but two or more chromosomes
united together, and Hacker has accordingly proposed the terms
bivalent and plurivalent for such chromatin-rods.^ The truth
of this view, which originated with Vom Rath, is, I think, conclusively
shown by the case of Artemia described at page 281, and by many facts
in the maturation of the germ-cells hereafter considered. In Ascarh
we may regard the chromosomes of Hertwig's ''variety univalens"
as really bivalent or double, i.e. equivalent to two such chromosomes
as appear in "variety bivalens." These latter, however, are probably
in their turn plurivalent, i.e. represent a number of units of a lower
order united together; for, as described at page 148, each of these
normally breaks up in the somatic cells into a large number of shorter
chromosomes closely similar to those of the related species Ascaris
himbricoidcs, where the normal number is 24.

Hacker has called attention to the striking fact that plurivalent
mitosis is very often of the heterotypical form, as is very common
in the maturation-mitoses of many animals (Chapter V.), and often
occurs in the early cleavages of Ascaris ; but it is doubtful whether
this is a universal rule.

3. Mitosis iji the Unicelhilar Plants and Animals

The process of mitosis in the one-celled plants and animals has a
peculiar interest, for it is here that we must look for indications of
its historical origin. But although traces of mitotic division were
seen in the Infusoria by Balbiani ('58-'6i), Stein ('59), and others
long before it was known in the higher forms, it has only recently
received adequate attention and is still imperfectly understood.

Mitotic division has now been observed in many of the main divi-
sions of Protozoa and unicellular plants ; but in the present state of

1 The words bivalent and univalent have been used in precisely the opposite sense
by Hertwig in the case of Ascaris., the former term being applied to that variety having two
chromosomes in the germ-cells, the latter to the variety with one. These terms certainly
have priority, but were applied only to a specific case. Hacker's use of the words, which is
strictly in accordance with tlieir etymology, is too valuable for general descriptive purposes to
be rejected.

DETAILS OF MITOSIS

89

the subject it must be left an open question whether it occurs in all.
In some of the gregarines and Heliozoa, the process is of nearly or
quite the same type as in the Metazoa. From such mitoses, how-
ever, various gradations may be traced toward a much simpler pro-
cess, such as occurs in Amoeba and the lower flagellates; and it is not
improbable that we have here representatives of more primitive con-
ditions. Among the more interesting of these modifications may be
mentioned : —

I. Even in forms that nearly approach the mitosis of higher types

D

Fig. 38. — Mitotic division in Infusoria. [R. Hertwig.]

A-C. Ma'cronucleus of Spirochona, showing pole-plates. D-H. Successive stao-es in the
division of the micronucleus of Paramcecium. D. The earliest stage, showing reticulum. G. Fol-
lowing stage (" sickle-form ") with nucleolus. E. Chromosomes and pole-plates. F. Late ana-
phase. H. B'inal phase.

the nuclear membrane may persist more or less completely through
every stage {Noctiluca, E ugly p ha, ActinospJicBrnini).

2. Asters may be present (Hehozoa, gregarines) or wanting (In-
fusoria, Radiolaria).

3. In one series of forms the centrosome or sphere is represented
by a persistent intranuclear body (nucleolo-centrosome) of consider-
able size, which divides to form a kind of central spmdle {Euglena
Amceba, Infusoria.?).

4. In a second series the centrosome or sphere is a persistent

90

CELL-DIVISION

extranuclear body, as in most Metazoa {Heliozoa, Noctiluca, Para-
moeba).

5. In a few forms having a scattered nucleus the chromatin-gran-
ules are only collected about the apparently persistent sphere or
centrosome at the time of its division, and afterward scatter through
the cell, leaving the sphere lying in the general cell-substance
( Tetramitus).

6. The arrangement of the chromatin-granules to form chromo-
somes appears to be of a secondary importance as compared with

A BCD

Fig. 39. — Mitosis in the rhizopod, Euglypha. [SCHEWIAKOFF.]
In this form the body is surrounded by a firm shell which prevents direct constriction of the
cell-body. The latter therefore divides by a process of budding from the opening of the shell
(the initial phase shown at A) ; the nucleus meanwhile divides, and one of the daughter-nuclei
afterward wanders out into the bud.

A. Early prophase ; nucleus near lower end containing a nucleolus and numerous chromo-
somes. B. Equatorial plate and spindle formed inside the nucleus ; pole-bodies or pole-plates
{i.e. attraction-spheres or centrosomes) at the spindle-poles. C. Metaphase. D. Late ana-
phase, spindle dividing; after division of the spindle the outer nucleus wanders out into the bud.

higher forms, and the essential feature in nuclear division appears to
be the fission of the individual granules.

We may first consider especially the achromatic figure. The basis
of our knowledge in this field was laid by Richard Hertwig through his
studies on an infusorian, SpirocJiona ( ''J'j\ and a rhizopod, Actino-
sphcerium ('84). In both these forms a typical spindle and equatorial
plate are formed inside the 7mclear membrane by a direct transfor-
mation of the nuclear substance. In Spirochona (Fig. 38, A-C^ a

DETAILS OF MITOSIS

91

hemispherical " end-plate " or ''pole-plate" is situated at either pole
of the spindle, and Hertwig's observations indicated, though they
did not prove, that these plates arose by the division of a large
'' nucleolus." Nearly similar pole-plates were somewhat described by
Schewiakoff {^Z%) in Eiiglypha (Fig. 39), and it seems clear that they
are the analogues of the centrosomes or attraction-spheres in higher
forms. In Euglena, as shown by Keuten, the pole-plates, or their
analogues, certainly arise by division of a distinct and persistent intra-
nuclear body (''nucleolus" or " nucleolo-centrosome ") which elon-

Fig. 40. — Mitosis in the flagellate, Lugleiia. [Keuti N.
A. Preparing for division; the nucleus contains a "nucleolus." or nucleolo-centrosome sur-
rounded by a group of chromosomes. B. Division of the "nucleolus" to form an intranucleai
spindle. C. Later stage. D. The nuclear division completed.

gates to form a kind of central spindle around which the chromatin
elements are grouped (Fig. 40); and Schaudinn ('95) described a
similar process in Aniceba. Richard Hertwig's latest work on
Infusoria ('95) indicates that a similar process occurs in the micro-
nuclei of Pararncecium^ which at first contain a large " nucleolus "
and afterward a conspicuous pole-plate at either end of the spindle
(Fig. 38, D-H). The origin of the pole-plates was not, however,
positively determined. A corresponding dividing body is found in
Ceratiuin (Lauterborn, '95), and as in the Infusoria the entire
nucleus transforms itself into a fibrillar spindle-like body.

92

CELL-DIVISION

Still simpler conditions are found in some of the flagellates.^ In
Chilomonas the sphere may still be regarded as intranuclear, since it
lies in the middle of an irregular mass of chromatin-granules, though
the latter are apparently not enclosed by a membrane. Nuclear
division is here accomphshed by fission of the sphere and the aggre-
gation of the chromatin-granules around the two products. In
Tetraniitus, finally (Fig. i6), the nucleus is represented by chromatin-
granules that are scattered irregularly through the cell and only at
the time of division collect about the dividing sphere.

r^z^

,>>4W<-.

^.

^

3,

.\y«^j

B

II

W'

'r

■f'^^

^5£^,

D

E F o

Fig. 41. — Mitosis in the Heliozoa. [SCHAUDINN.]
A SphcErastrum ; veq;etative cell showing nucleus, "central granule" (centrosome), and axial
rays. li-G. Acanthocystis. B-D, Prophases of mitosis. E. Budding to form swarm-spores.
F. Swarm-spores, devoid of centrosomes. G. Swarm-spores preparing for division ; intranuclear
origin of centrosome.

In a second series of forms, represented by Noctiluca (Ishikawa,
'94, '98), (Calkins, '98, 2), Paramaiba (Schaudinn, '96, i), Actinophrys
and Acanthocystis (Schaudinn, '96, 2), and the diatoms (Lauterborn,
'96), the sphere lies outside the nucleus in the cytoplasm and the
mitosis is closely similar to that observed in most Metazoa. This is
most striking in the Heliozoa, where the centrosome persists through
the vegetative condition of the cell as the " central granule," to which
the axial filaments of the pseudopodia converge. Schaudinn ('96, 2)
shows that by the division of this body a typical extranuclear amphi-
aster and central spindle are formed (Fig. 41), while the chromatin

1 Calkins, '98, i, '98, 2.

DETAILS OF MITOSIS

93

passes through a spireme-stage, breaks into very short rod-shaped
chromosomes which split lengthwise and arrange themselves in the
equator of the spindle, while the nuclear membrane fades away.
Noctiluca (Fig. 42), as shown by Ishikawa and Calkins, agrees with
this in the main points; but the nuclear membrane does not at any
period wholly disappear, and a distinct centrosome is found at the
centre of the sphere. The latter body, which is very large, gives

*•<**•>

.vr5«S^<v^i;;vj;.^.^

^v-^

D

Fig. 42. — Mitosis in Noctiluca. [Calkins.]
A. Prophase; division of the sphere to form the central spindle; chromosomes converging to
the nuclear pole. B. Late anaphase, in horizontal section, showing centrosomes; the central
spindle has sunk into the nucleus ; nuclear membrane still intact except at the poles. C. Early
anaphase ; mantle-fibres connected with the diverging chromosomes. D. Final anaphase (which
is also the initial prophase of the succeeding division of spore-forming mitosis) ; doubling of fcen-
trosome and splitting of chromosomes.

rise by a division to a fibrillated central spindle, about which the
nucleus wraps itself while mantle-fibres are developed from the
sphere-substance and become attached to the chromosomes, the nu-
clear membrane fading away along the surface of contact with the
central spindle (Calkins). Broadly speaking, the facts are similar in

94 CELL-DIVISION

the diatoms {Surirella, t. Lauterborn), where the central spindle,
arising by a peculiar process from an extranuclear centrosome,
(sphere ?) sinks into the nucleus in a manner strongly suggesting that
observed in Noctilnca.

In the interesting form Paramceba, as described by Schaudinn
('96, i), the sphere (*' Nebenkorper "), which is nearly as large as the
nucleus, divides to form a central spindle, about the equator of which
the chromatin-elements become arranged in a ring (Fig. 43); but no
centrosome has yet been demonstrated in the sphere. Paramceba
appears to differ from Euglena mainly in the fact that at the close of
division the sphere is in the former left outside the daughter-nucleus
and in the latter enclosed within it.^ The connecting link is perfectly
given by Tetraiuitus, where no morphological nucleus is formed, and
the sphere Ijes in the general cell-substance (p. 92); and we could
have no clearer demonstration that the extra- or intranuclear position
of sphere or centrosome is of quite secondary importance. As
regards the formation of the spheres (pole-plates) Actinosphcerium
(Figs. 44, 45) seems to show a simpler condition than any of the
above forms, since no permanent sphere exists, and Brauer ('94) and'
R. Hertwig ('98) agree that the pole-plates are formed by a gradual
accumulation of the achromatic substance of the nucleus at opposite
poles.

A distinct centrosome (centriole .'') in the interior of the sphere has
thus far only been observed in a few forms {Noctilnca, Actinosphce-
riimi), and neither its origin nor its relation to the sphere has yet
been sufficiently cleared up. Both Ishikawa ('94) and Calkins ('98, 2)
somewhat doubtfully concluded that in Noctiluca the centrosomes
arise within the nucleus, migrating thence out into the extranuclear
sphere. With this agree R. Hertwig's latest studies on Actinosphce-
riiim ('98), the spindle-poles being first formed from the pole-plates
(themselves of nuclear origin), and the centrosomes then passing into
them from the nucleus. Hertwig reaches the further remarkable
conclusion that the centrosomes arise as portions of the cJiroinatin-
netivork extruded at the nuclear poles (Fig. 45), first forming a
spongy irregular mass, but afterward condensing into a deeply
staining pair of granules which pass to the respective poles of the
spindle. It is a remarkable fact that these centrosomes are only
found in the two maturation-divisions, and are absent from the ordi-
nary vegetative mitoses where the spindle-poles are formed by two
cytoplasmic masses derived, as Hertwig believes, from the intra-
nuclear plates. Schaudinn ('96, 3) likewise describes and clearly
figures an intranuclear origin of the centrosome in buds of Acantho-
cystis (Fig. 41), which are derived by direct division of the mother-

IQC Calkins, '98, i, p. 388.

DETAILS OF MITOSIS

95

nucleus with no trace of a centrosome. In this same form, as
described above, the ordinary vegetative mitoses are quite of the
metazoan type, with a persistent extranuclear centrosome.

The hisiory of the chromatin in the mitosis of unicellular forms
shows some interesting modifications. In a considerable number of
forms a more or less clearly marked spireme-stage precedes the forma-
tion of chromosomes (diatoms, Infusoria, dinoflagellates, Eitglypha)\
in others, long chromosomes are formed without a distinct spireme-
stage {Noctihica). It has been clearly demonstrated that in some
cases these chromosomes split lengthwise, as in Metazoa {Noctiliica^

Fig. 43.— Mitosis in Paratnoeba. [SCHAUDINN.]
At the left, amoeboid phase, showing nucleus and " Nebenkorper." At the right, four stages
of division in the swarm-spores.

diatoms, Actinophrys, probably in Eiiglyphd) ; but in some cases they
are stated to divide transversely in the middle (Infusoria according
to Hertwig, Ceratiimt according to Lauterborn). These chromosomes
appear always to arise, as in Metazoa, through the linear arrangement
of chromatin-granules {N'octiluca, AcWiosphcBrium, Eiiglend), which
themselves in many cases arise by the preliminary fragmentation of
one or more large chromatin-masses {e.g. in Noctiluca or Actinosphce-
riuni). In other forms no such linear aggregates are formed, and
direct fission of the chromatin-granules appears to take place without
the formation of bodies morphologically comparable with the chromo-
somes of such forms as Noctiluca. This is apparently the case in
Tetramitus, and Achromatmm, other forms having a distributed

95

CELL-DIVISIOA

nucleus,^ and in such forms as CJiilomonas and 'TracJielomoiias^ where
the granules are permanently aggregated about a central body. Too
little is known of the facts to justify a very positive statement ; but
on the whole they point toward the conclusion that in the simplest

00

?oOo<

H

Fig. 44. — Mitosis in the rhizoped Acfmospkceriuni. [Brauer.]

A. Nucleus and surrounding structures in the early prophase ; above and below the reticular
nucleus lie the semilunar " pole-plates," and outside these the cytoplasmic masses in which the
asters afterward develop. B. Later stage of the nucleus. D. Mitotic figure in the metaphase,
showing equatorial plate, intra-nuclear spindle, and pole-plates {p.p.). C. Equatorial plate,
viewed ^«/<z^^, consisting of double chromatin-granules. E. Early anaphase. F. G. Later ana-
phases. //. Final anaphase. /. Telophase ; daughter-nucleus forming, chromatin in loop-shaped
threads ; outside the nuclear membrane the centrosome, already divided, and the aster, y. Later
stage; the daughter-nucleus established; divergence of the centrosomes. Beyond this point the
centrosomes have not been followed.

types of mitosis no true chromosome-formation occurs, thus sustaining
Brauer's conclusion that the essential fact in the history of the chro-
matin in mitosis is the fission of the individual jrranules.^

1 The fission of the individual granules is carefully described and figured by Schewiakoff
in Achromatium.

^ For speculations on the historical origin of the centrosome, etc., see p. 315.

DETAILS OF MITOSIS

97

4. Pathological Mitoses

Under certain circumstances the delicate mechanism of cell-division
may become deranged, and so give rise to various forms of patho-
logical mitoses. Such a miscarriage may be artificially produced, as
Hertwig, Galeotti, and others' have shown, by treating the dividing
cells with poisons and other chemical substances (quinine, chloral,
nicotine, potassic iodide, etc.). Pathological mitoses may, however,

B

Pig. 45. — Mitosis in Actinosphcerium. [R. Hertwig.]
A. Encysted form, with resting nucleus; chromatin aggregated into large nucleolus-like body.
B. prophase of division of the encysted form, showing chromosome-like bodies formed of granules,
and spindle without centrosomes. C. Earlier prophase of the first maturation division, showing
extrusion of chromatic substance to form the centrosome. D. Later stage, showing centrosome
and aster.

occur without discoverable external cause ; and it is a very interesting
fact, as Klebs, Hansemann, and Galeotti have especially pointed out,
that they are of frequent occurrence in abnormal growths such as
cancers and tumours.

The abnormal forms of mitoses are arranged by Hansemann in two
general groups, as follows: (i) asymmetrical mitoses, in which the
chromosomes are unequally distributed to the daughter-cells, and (2)
multipolar mitoses, in which the number of centrosomes is more than

98

CELL-DIVISION

two, and more than one spindle is formed. Under the first group are
included not only the cases of unequal distribution of the daughter-
chromosomes, but also those in which chromosomes fail to be drawn
into the equatorial plate and hence are lost in the cytoplasm.

Klebs first pointed out the occurrence of asymmetrical mitoses in
carcinoma-cells, where they have been carefully studied by Hanse-
mann and Galeotti. The inequality is here often extremely marked,
so that one of the daughter-cells may receive more than twice as
much chromatin as the other (Fig. 46). Hansemann, whose conclu-

D E F

Fig. 46. — Pathological mitoses in human cancer-cells. [Galeotti.]
A. Asymmetrical mitosis with unequal centrosomes. B. Later stage, showing unequal distri-
bution of the chromosomes. C. Quadripolar mitosis. D. Tripolar mitosis. E. Later stage.
F. Trinucleate cell resulting.

sions are accepted by Galeotti, believes that this asymmetry of mito-
sis gives an explanation of the familiar fact that in cancer-cells many
of the nuclei are especially rich in chromatin (hyperchromatic cells),
while others are abnormally poor (hypochromatic cells). Lustig and
Galeotti ('93) showed that the unequal distribution of chromatin is
correlated with and probably caused by a corresponding inequality in
the centrosomes which causes an asymmetrical development of the
amphiaster. A very interesting discovery made by Galeotti ('93) is
that asymmetrical mitoses, exactly like those seen in carcinoma, may
be artificially produced in the epithelial cells of salamanders (Fig. 47)
by treatment with dilute solutions of various drugs (antipyrin, cocaine,
quinine).

DETAILS OF MITOSIS

99

Normal multipolar mitoses, though rare, sometimes occur, as in the
division of the pollen-mother-cells and the endosperm-cells of flower-
ing plants (Strasburger) ; but such mitotic figures arise through the
union of two or more bipolar amphiasters in a syncytium and are
due to a rapid succession of the nuclear divisions unaccompanied by
fission of the cell-substance. These are not to be confounded with
pathological mitoses arising by premature or abnormal division of the
centrosome. If one centrosome divide, while the other does not,
triasters are produced, from which may arise three cells or a tri-
nucleated cell. If both centrosomes divide, tetrasters or polyasters
are formed. Here again the same result has been artificially attained
by chemical stimulus {cf. Schottlander, '88). Multipolar mitoses are.

B

Pig. 47. — Pathological mitoses in epidermal cells of salamander caused by poisons.
[Galeotti.]

A. Asymmetrical mitosis after treatment with 0.05% antipyrin solution. B, Tripolar mitosis
after treatment with 0,5 % potassic iodide solution.

also common in regenerating tissues after irritative stimulus (Strobe);
but it is uncertain whether such mitoses lead to the formation of
normal tissue.^

The frequency of abnormal mitoses in pathological growths is a
most suggestive fact, but it is still wholly undetermined whether the
abnormal mode of cell-division is the cause of the disease or the
reverse. The latter seems the more probable alternative, since nor-
mal mitosis is certainly the rule in abnormal growths ; and Galeotti's
experiments suggest that the pathological mitoses in such growths
may be caused by the presence of deleterious chemical products in
the diseased tissue, and perhaps point the way to their medical
treatment.

* The remarkable polyasters formed in polyspermic fertilization of the &^g are de«
scribed at page 198.

lOO

CELL-DIVISION

D. The Mechanism of Mitosis

We now pass to a consideration of the forces at work in mitotic
division, which leads us into one of the most debatable fields of

cytological inquiry.

I. Function of the Amphi-
aster

a.c

cz.

a.c

All observers agree that
the amphiaster is in some
manner an expression of
the forces by which cell-
division is caused, and
many accept, in one form
or another, the first view
clearly stated by Fol,^ that
the asters represent in
some manner centres of
attractive forces focussed
in the centrosome or dy-
namic centre of the cell.
Regarding the nature of
these forces, there is, how-
ever, so wide a divergence
of opinion as to compel the
admission that we have
thus far accomplished little
more than to clear the
ground for a precise in-
vestigation of the subject;
and the mechanism of mi-
tosis still lies before us as
one of the most fascinating

Fig. 48. — Slightly schematic figures of dividing eggs
of Ascaris, illustrating Van Beneden's theory of mitosis.
[Van Beneden and Julin,]

A. Early anaphase ; each chromosome has divided
into two. B. Later anaphase during divergence of the r . i

daughter-chromosomes, a.c. Antipodal cone of astral problems 01 CytOlOgy
rays ; c.z. cortical zone of tht- attraction-sphere ; /. inter-
zonnl fibres stretching between the daughter-chromo-
soms; 1/i.z. medullary zone of the attraction-sphere;
p.c. principal cone, forming one-half of the contractile
spindle (the action of these fibres is reenforced by that of
the antipodal cone) ; s.e.c. subequatorial circle, to which
the astral rays are attached.

{a) The Theory of Fi-
brillar Contractility. — The
view that has taken the
strongest hold on recent
research is the hypothesis
of fibrillar contractility.
First suggested by Klein in 1878, this hypothesis was independ-
ently put forward by Van Beneden in 1883, and fully outlined

73. P- 473.

THE MECHANISM OF MITOSIS lOI

by him four years later in the following words : " In our opinion
all the internal movements that accompany cell-division have their
immediate cause in the contractility of the protoplasmic fibrillae and
their arrangement in a kind of radial muscular system, composed of
antagonizing groups " {i.e. the asters with their rays). " In this sys-
tem the central corpuscle (centrosome) plays the part of an organ of
insertion. It is the first of all the various organs of the cells to divide,
and its division leads to the grouping of the contractile elements- in
two systems, each having its own centre. The presence of these two
systems brings about cell-division, and actively determines the paths
of the secondary chromatic asters" {i.e. the daughter-groups of chro-
mosomes) " in opposite directions. An important part of the phe-
nomena of (karyo-) kinesis has its efficient cause, not in the nucleus,
but in the protoplasmic body of the cell." ^ This beautiful hypothesis
was based on very convincing evidence derived from the study of the
Ascaris ^^^, and it was here that Van Beneden first demonstrated the
fact, already suspected by Flemming, that the daughter-chromosomes
move apart to the poles of .the spindle and give rise to the two
respective daughter-nuclei.^

Van Beneden's general hypothesis was accepted in the following
year by Boveri {^'^^^ 2), who contributed many important additional
facts in its support, though neither his observations nor those of later
investigators have sustained Van Beneden's account of the grouping
of the astral rays. Boveri showed in the clearest manner that, during
the fertilization of Ascaris, the astral rays become attached to the
chromosomes of the germ-nuclei; that each comes into connection
with rays from both the asters ; that the chromosomes, at first irregu-
larly scattered in the ^gg, are drawn into a position of equilibrium in
the equator of the spindle by the shortening of these rays (Figs. 90,
147); and that the rays tJiicken as tJiey shoj^ten. He showed that as
the chromosome splits, each half is connected only with rays (spindle-
fibres) from the aster on its own side ; and he followed, step by step,
the shortening and thickening of these rays as the daughter-chromo-
somes diverge. In all these operations the behaviour of the rays is

1 '87, p. 280.

2 '83, p. 544. Van Beneden describes the astral rays, both in Ascaris and in tunicates, as
differentiated into several groups. One set, forming the "principal cone," are attached to
the chromosomes and form one-half of the spindle, and, by the contractions of these fibres,
the chromosomes are passively dragged apart. An opposite group, forming the " antipodal
cone," extend from the centrosome to the cell-periphery, the base of the cone forming the
"polar circle." These rays, opposing the action of the principal cones, not only hold the
centrosomes in place, but, by their contractions, drag them apart, and thus cause an actual
divergence of the centres. The remaining astral rays are attached to .the cell-periphery and
are limited by a subequatorial circle (Fig. 48). Later observations indicate, however, that
this arrangement of the astral rays is not of general occurrence, and that the rays often do
not reach the periphery, but lose themselves in the general reticulum.

:o2

CELL-DIVISION

precisely like that of muscle-fibres ; and it is difficult to study Boveri's
beautiful figures and clear descriptions without sharing his conviction
that *'of the contractility of the fibrillae there can be no doubt." ^

Very convincing evidence in the same direction is afforded by
pigment-cells and leucocytes or wandering cells, in both of which
there is a very large permanent aster (attraction-sphere) even in the
resting cell. The structure of the aster in the leucocyte, where it
was first discovered by Flemming in 1891, has been studied very
carefully by Heidenhain in the salamander. The astral rays here
extend throughout nearly the whole cell (Fig. 49), and are believed

A

B

Fig. 49. — Leucocytes or wandering cells of the salamander. [HEIDENHAIN.]
A. Cell with a single nucleus containing a very coarse network of chromatin and two nucleoli
(plasmosomes) ; s. permanent aster, its centre occupied by a double centrosome surrounded by
an attraction-sphere. B. Similar cell, with double nucleus ; the smaller dark masses in the latter
are oxychromatin-granules (linin), the larger masses are basichromatin (chromatin proper).

by Heidenhain to represent the contractile elements by means of
which the cell changes its form and creeps about. A similar con-
clusion was reached by Solger ('91) and Zimmermann ('93, 2) in the
case of pigment-cells (chromatophores) in fishes. These cells have,
in an extraordinary degree, the power of changing their form and of
actively creeping about. Solger and Zimmermann have shown that
the pigment-cell contains an enormous aster, whose rays extend in
every direction through the pigment-mass, and it is almost impos-
sible to doubt that the aster is a contractile apparatus, like a radial
muscular system, by means of which the active changes of form are
produced (Fig. 50). This interpretation of the' aster receives addi-
tional support through Schaudinn's ('96, 3) highly interesting dis-

2, p. 99.

THE MECHANISM OF MITOSIS

103

covery that the '* central granule " of the Heliozoa is to be identified
with the centrosome and plays the same role in mitosis (Fig. 41).
In these animals the axial filaments of the radiating pseudopodia con-
verge to the central granule during the vegetative state of the cell,
thus forming a permanent aster which Schaudinn's observations prove
to be directly comparable to that of a leucocyte or of a mitotic figure.
There is in this case no doubt of the contractility of the rays, and a

Fig. 50. — Pigment-cells and asters from the epidermis of fishes. [ZiMMERMANN.]

A. Entire pigment-cell, from Blennius. The central clear space is the central mass of the aster
from which radiate the pigment-granules ; two nuclei below. B. Nucleus («) and aster after ex-
traction of the pigment, showing reticulated central mass. C. Two nuclei and aster with rod-
shaped central mass, from Sargus,

Strong, if indirect, argument is thus given in favour of contractility in
other forms of asters. ^ The contraction-hypothesis is beautifully
illustrated by means of a simple and easily constructed model, devised
by Heidenhain ('94, '96), which closely simulates some of the phenom-
ena of mitosis. In its simplest form the model consists of a circle,
marked on a flat surface, to the periphery of which are attached at equal

1 For an interesting discussion and development
Watase, '94.

of the contraction-hypothesis see

I04

CELL-DIVISION

Fig- 5^' — Heidenhain's model of mitosis (mainly from
Heidenhain).

A. Dotted lines show position of the rays upon sever-
ing connection between the small rings. B. Position upon
insertion of " nucleus." C D. Models with flexible hinged
hoops, showing division.

intervals a series of rub-
ber bands (astral rays).
At the other ends these
bands are attached to a
pair of small rings (cen-
trosomes) fastened to-
gether. In the position
of equilibrium, when the
rays are stretched at
equal tension, the rays
form a symmetrical aster
with the centrosomes at
the centre of the circle
(Fig. 51, A). If the con-
nection between the cen-
trosomes be severed, they
are immediately dragged
apart to a new position of
equilibrium with the rays
grouped in two asters, as
in the actual cell (dotted
lines in Fig. ^\, A). If
a round pasteboard box
of suitable size (nucleus)
be inserted between two
of the rays, it assumes
an eccentric position, the
cell-axis being formed by
a line passing through its
centre and 'that of the
pair of small rings {cf.
the epithelial cell, p. 57),
and upon division of the
aster it takes up a position
between the two asters.
In' a second form of the
models the circle is
formed of two half rings
of flexible steel, joined
by hinges; the diver-
gence of the small rings
is here accompanied by
an elongation and partial
constriction of the model

THE MECHANISAI OF MITOSIS IO5

in the equatorial plane ; and if, finally, the hinge-connection be re-
moved, each half of the ring closes to form a complete ring.^

Heidenhain has fully worked out a theory of mitosis based upon
the analogy of these pretty models. The astral rays of the cell
("organic radii") are assumed to be in like manner of equal length
and in a state of equal tonic contraction or tension, the centrosome
forming the common insertion-point of the rays, and equilibrium of
the system being maintained by turgor of the cell. Upon disappear-
ance of the nuclear membrane and division of this insertion-point, the
tension of the rays causes divergence of the centrosomes and forma-
tion of the spindle between them, and by further contraction of the
rays both the divergence of the daughter-chromosomes and the division
of the cell-body are caused. A new condition of equilibrium is thus
established in each daughter-cell until again disturbed by division of
the centrosome.^ In some cases (leucocytes) the organic radii are
visible at all periods. More commonly they are lost to view by
breaking up into the cell-reticulum, without, however, losing their
essential relations.

No one who witnesses the operation of Heidenhain's models can
fail to be impressed with its striking simulation of actual cell-division.
Closer study of the facts shows, however, that the contraction-hypothe-
sis must be considerably restricted, as has been done by the successive
modifications of Hermann ('91), Driiner ('95), and others. Hermann,
to whom the identification of the central spindle is due, pointed out
that there is no evidence of contractility in the central spindle-fibres,
which elongate instead of shorten during mitosis ; and he concluded
that these fibres are non-contractile supporting elements, which form
a basis on which the movements of the chromosomes take place. The
mantle-fibres are the only contractile elements in the spindle, and it
is by them that the chromosomes are brought into position about the
central spindle and the daughter-chromosomes are dragged apart.^
Druner ('95) still further restricts the hypothesis, maintaining that the
progressive divergence of the spindle-poles is caused not by contrac-
tion of the astral rays ("polar fibres"), as assumed by Heidenhain
(following Van Beneden and Boveri), but by an active growth or
elongation of the central spindle, which goes on throughout the
whole period from the earliest prophases until the close of the ana-
phases. This view is supported by the fact that the central spindle-

1 In a modification of the apparatus devised by Rhumbler ('97), the same effect is pro-
duced without the hinges.

^ QC p. 57. For critique of this hypothesis, see Fick ('97), Rhumbler ('96, '97), and
Meves ('97, 4).

3 Belajeff ('94) and Strasburger ('95) have accepted a similar view as applied to mitosis
in plant-cells.

I06 CELL-DIVISION

fibres are always contorted during the metaphases, as if pushing
against a resistance ; and it harmonizes with the facts observed in
the mitoses of infusorian nuclei, where no asters are present. This
view has been accepted, with slight modifications, by Flemming,
Boveri, Meves, Kostanecki, and also by Heidenhain. A nearly
decisive argument in its favour is given by such cases as the polar
bodies, or the mitosis of salamander spermatocytes as described by
Meves ('96, '97, 3), where the spindle-poles are pushed out to the
periphery of the cell, the polar astral rays meanwhile nearly or quite
disappearing (Fig. 130). This not only strongly indicates the push
of the central spindle, but also shows that the assumption of a pull
by the polar rays is superfluous. But beyond this both Driiner and
Meves have brought arguments against contractility in the other
astral rays, endeavouring to show that these, like the spindle-fibres,
are actively elongating elements, and that (Meves, '97, 3) the actual
grouping of the rays during the anaphases is such as to suggest that
even the division of the cell-body may be thus caused. A pushing
function of the astral rays is also indicated by infolding of the nuclear
membrane caused by the development of the aster as described by
Platner, Watase, Braus, Griffin, and others. ^ The contraction-hypothe-
sis is thus restricted by Driiner and Meves to the mantle-fibres
alone, though many others, among them Flemming and Kostanecki,
still accept the contractility of the astral rays.

{b) Other Facts and Theories. — Even in the restricted form indi-
cated above the contraction-hypothesis encounters serious difficulties,
one of which is the fact urged by me in an earlier paper ('95), and
subsequently by Richard Hertwig ('98), that in the eggs of echino-
derms and many other dividing cells the daughter-chromosome
plates, extending through the whole substance of the spindle,
wander to the extreme ends of the spindle — a process which
demands a contraction of the fibres almost to the vanishing point,
while in point of fact not even a shortening and thickening of
the fibres can be seen (Fig. 52). Moreover, in these cases, no
distinction can be seen between central spindle-fibres and mantle-
fibres, and we can only save the contraction-hypothesis by the
improbable assumption that fibres indistinguishably mingled, and
having the same mode of origin, structure, and staining-reaction, have
exactly opposite functions. The inadequacy of the general theory
is sufficiently apparent from the fact that in amitosis cells many

1 QC p. 68. It should be pointed out that the originator of the pushing theory was
Watase ('93), who ingeniously developed an hypothesis exactly the opposite of Van Bene-
den's, assuming both astral rays and spindle-fibres to be actively elongating fibres, dove-tailing
in the spindle-region, and pushing the chromosomes apart. This hypothesis is, I believe, in-
consistent with the phenomena observed in multiple asters and elsewhere, yet it probably
contains a nucleus of truth that forms the basis of Driiner's conception of the central spindle.

THE MECHANISM OF MITOSIS

107

divide without any amphiaster whatever. In Infusoria mitosis seems
to occur in the entire absence of asters, although the cells divide by
constriction, and the analogy with Heidenhain's model entirely fails.

\u!.r

\\\\l

'///// l'\V

^:^^'' % Mi-U/ '^ili^.SM^ku

Fig. 52. — The later stages of mitosis in the egg of the sea-urchin Toxopneustes (A-D, X 1000;
£-F, X 500).

A. Metaphase ; daughter-chromosomes drawing apart but still united at one end. B. Daugh-
ter-chromosomes separating. C Late anaphase; daughter-chromosomes lying near the spindle-
poles. D. Final anaphase; daughter chromosomes converted into vesicles. E. Immediately
after division, the asters undivided; the spindle has disappeared. F. Resting 2-cell stage, the
asters divided into two in anticipation of the next division.

In Figs. A and B the centrosome consists of a mass of intensely staining granules, which in
Cand Z) elongates at right angles to the spindie-axis. In Fthe centrosome appears as a single
or double granule, which in later stages gives rise to a pluricorpuscular centrum like that in A.
The connection between D and Fis not definitely determined.

In Eitglypha, according to Schewiakoff (Fig. 39), division of the cell-
body appears to take place quite independently of the mitotic figure.
Again, a considerable number of cases are now known in which dur-
ing the fertilization of the ^gg a large amphiaster is formed, with

1 08 CELL-DI VISION

astral rays sometimes extending throughout ahiiost the entire ^ggy
only to disappear or become greatly reduced without the occurrence
of division, the ensuing cleavage being effected by a new amphiaster
or by the recrudescence of the old.^ For these and other reasons we
must admit the probability that contractility of the astral fibrillae, if
it exists, is but the expression or consequence of a more deeply lying
phenomena of more general significance. The subtlety of the prob-
lem is strikingly shown by Boveri's remarkable observations on
abnormal sea-urchin eggs ('96), which show (i) that the periodic
division of the centrosome and formation of the amphiaster may take
place independently of the nucleus; (2) that the spindle, as well as
the asters, is concerned in division of the cell-body ; and (3) that an
amphiaster without chromosomes is unable to effect normal division
of the cell-body. The first and third of these facts are shown by eggs
in which during the first cleavage all of the chromatin passes to one
pole of the spindle, so that one of the resulting halves of the ^gg
receives no nucleus, but only a centrosome and aster. In this half
perfect amphiasters are formed simultaneously with each cleavage in
the other half, yet no divisioit of the pi'otoplasmic mass occurs? The
second fact is shown in polyspermic eggs, in which multipolar astral
systems are formed by union of the several sperm-asters (Figs. 53, loi).
In such eggs cleavages only occur between asters that are joined by a
spindle. Normal cleavage of the cell-body thus requires the complete
apparatus of mitosis, and even though the fibres be contractile they
cannot fully operate in the absence of chromatin.

We may now turn to theories based on the hypothesis, first sug-
gested by Fol in 1873, that the astral foci {i.e. centrosomes) represent
dynamic centres of attractive or other forces. It should be noted that
this hypothesis involves two distinct questions, one relating to the
origin of the amphiaster, the other to its mode of action ; and we have
seen that some of the foremost advocates of the contraction-hypothesis,
including Van Beneden and Boveri, have held the centrosomes to be
attractive centres. Apart from the movements of the chromosomes,
the most obvious indication that the centrosomes are dynamic centres
is the extraordinary resemblance of the amphiaster to the lines of force
in a magnetic field as shown by the arrangement of iron-filings about
the poles of a horseshoe magnet — a resemblance pointed out by Fol
himself, and urged by many later writers,^ especially Ziegler ('95)

1 Cf. p. 213.

2 This result is opposed to Boveri's earlier work on Ascaris (p. 355), and is modified by
Ziegler ('98), who observed in a single case that an irregular cleavage occurred in the
enucleated half after two or three divisions of the centrosome. On the other hand, it is sup-
ported by Morgan's convincing experiments on the eggs o{ Arbacia (p. 308).

3 Cf. the interesting photographic figures of Ziegler ('95). A still closer simulacrum of
the amphiaster is produced by fine crystals of sulphate of quinine (a semiconductor) sus-

THE MECHANISM OF MITOSIS

109

and Gallardo ('96, '97). It is impossible to regard this analogy as
exact ; first, because it is inconsistent with the occurrence of tripolar
astral figures ; second, as Meves has recently urged ^ the course of the
astral fibres does not really coincide with the lines of force, the most
important deviation being the crossing of the rays opposite the equa-
torial region of the spindle, which is impossible in the magnetic or
electric field. We must, however, remember that the amphiaster is
formed in a viscid medium, that it may perform various movements,
and that its fibres probably possess the power of active growth. The

Fig. 53. — Division of dispermic eggs in sea-urchin eggs, schematic. [BOVERI.]
A. C. E. Eggs before division, showing various connections of the asters. B. D. F. Result-
ing division in the three respective cases, showing cleavage only between centres connected by a
spindle.

physical or chemical effect of the centres, through which the amphias-
ter primarily arises, may thus be variously disturbed or modified in
later stages, and the crossing of the rays is therefore not necessarily
fatal to the assumption of dynamic centres. Biitschli ('92, '98) has,
moreover, recently shown that a close siniulacnim of the amphiaster,
showing a distinct crossing of the rays, may be produced in an arti-
ficial alveolar structure (coagulated gelatine) by tractive forces cen-

pended in spirits of turpentine (a poor conductor) between two electric poles. This experi-
ment, devised by Faraday, has recently been applied by Gallardo ('96, '97) to an analysis
of the mitotic figure. 1 '96, p. 371.

I lo CELL-DIVISION

tring in two adjacent points. This result is obtained by warming and
then cooKng a film of thick gelatine-solution, filled with air-bubbles,
and then coagulating the mass in chromic acid. Such a film shows a
fine alveolar structure, which assumes a radial arrangement about the
air-bubbles, owing to the traction exerted on the surrounding structure
by shrinkage of the bubbles on cooling. The amphiastral sin^ulacra
are produced about two adjacent bubbles, — a " spindle " being formed
between them, and the *' astral rays " sometimes showing a crossing
like that seen in the actual amphiaster (Biitschli is himself unable to
explain fully how the crossing arises). The protoplasmic asters are
maintained by Biitschli to be, in like manner, no more than a radial
configuration of the alveolar cell-substance caused by centripetal
diffusion-currents toward the astral centres.^ The most interesting
part of this view is the assumption that these currents are caused by
specific chemical changes taking place in the centrosome which causes
an absorption of liquid from the surrounding region. " The astral
bodies are structures which, under certain circumstances, function in
a measure as centres from which emanate chemical actions upon pro-
toplasm and nucleus ; and the astral phenomena which appear about
the centrosomes are only a result incidental to this action of the central
bodies upon the plasma." ^ Through centripetal currents thus caused
arise the asters, and they may even account, in a measure, for the move-
ments of the chromosomes.^ This latter part of BiitschH's conception
is, I believe, quite inadequate; but the hypothesis of definite chemical
activity in the centrosome is a highly important one, which is sustained
by the staining-reactions of the centrosome and by its definite morpho-
logical changes during the cycle of cell-division.

More or less similar chemical hypotheses have been suggested by
several other writers.* Of these perhaps the most interesting is
Strasburger's suggestion,^ that the movements of the chromosomes
may be of a chemotactic character, which I suspect may prove to have
been one of the most fruitful contributions to the subject. Beside this
may be placed Carnoy's still earlier hypothesis ('85), that the asters
are formed under the influence of specific ferments emanating from
the poles of the nucleus. Mathews ('99, 2) has recently pointed out
that there is a considerable analogy between the formation of the
astral rays and that of fibrin-fibrils under the influence of fibrin-fer-
ment, adding the suggestion that the centrosome may actually contain

1 Carnoy ('85) and Platner ('86) had previously held a similar view, suggesting that not
only the spindle-formation, but also the movements of the chromosomes, might be explained
as the result of protoplasmic currents.

2 '92, I, p. 538.

^ '92, 2, p. 160 ; '92, 3, p. 10.

* Cf. the first edition of this work, p. 77, also Ziegler ('95). ^ '93, 2.

THE MECHANISM OF MITOSIS III

fibrin-ferment. Attention may be called here to the fact, now definitely
determined by experiment/ that cell-division may be incited by chemi-
cal stimulus. In most of the cases thus far experimentally examined
the divisions so caused are pathological in character, but in others
they are quite normal, as shown in Loeb's remarkable results on the
production of parthenogenesis in sea-urchin eggs by chemical stimulus,
as described at pages 215 and 308. While these experiments by no
means show that division is itself merely a chemical process, they
strongly suggest that it cannot be adequately analyzed without reckon-
ing with the chemical changes involved in it.

RhiimL A review of the foregoing facts and theories shows how
far we still are from any real understanding of the process involved
either in the origin or in the mode of action of the mitotic figure. The
evidence seems well-nigh demonstrative, in case of the mantle-fibres
and the astral rays, that Van Beneden's hypothesis contains an element
of truth, but we must now recognize that it was formulated in too
simple a form for the solution of so complex a problem. No satisfac-
tory hypothesis can, I believe, be reached that does not reckon with
the chemical changes occurring at the spindle-poles and in the nucleus ;
and these changes are probably concerned not only with the origin of
the amphiaster, but also with the movements of the chromosomes. In
cases where the centrosome persists from cell to cell we may perhaps
regard it as the vehicle of specific substances (ferments .-*) which become
active at the onset of mitosis, and run through a definite cycle of
changes, to initiate a like cycle in the following generation ; and it is
quite conceivable that such substances may persist at the nuclear poles,
or may be re-formed there as an after-effect, even though the formed
centrosome disappears.^ In this consideration we may find a clue to
the strange fact — should it indeed prove to be a fact — that the cen-
trosome may divide, yet afterward disappear without discoverable
connection with the centrosomes of the succeeding mitosis, as several
recent observers have maintained. ^ When all is said, we must admit
that the mechanism of mitosis in every phase still awaits adequate
physiological analysis. The suggestive experiments of Biitschli and
Heidenhain lead us to hope that a partial solution of the problem may
be reached along the lines of physical and chemical experiment. At
present we can only admit that none of the conclusions thus far
reached, whether by observation or by experiment, are more than the
first naive attempts to analyze a group of most complex phenomena
of which we have little real understanding.

1 See pp. 306, 308. ^ ^ p. 215. ^Cf.^. 213.

112

CELL-DIVISION

2. Division of the Chromosomes

In developing his theory of fibrillar contractility, Van Beneden
expressed the view — only, however, as a possibility — that the
splitting of the chromosomes might be passively caused by the con-
tractions of the two sets of opposing spindle-fibres to which each is
attached.^ Later observations have demonstrated that this sugges-
tion cannot be sustained ; for in many cases the chromatin-thread
splits before division of the centrosome and the formation of the
achromatic figure — sometimes during the spireme-stage, or even in
the reticulum, while the nuclear membrane is still intact. Boveri
showed this to be the case in Ascaris, and a similar fact has been
observed by many observers since, both in plants and in animals.

Fig. 54. — Nuclei in the spireme-stage.

A. From the endosperm of the lily, showing true nucleoli. [Flemming.]

B. Spermatocyte of salamander. Segmented double spireme-thread composed of chromo-
meres and completely split. Two centrosomes and central spindle at s. [Hermann.]

C. Spireme-thread completely split, with six nucleoli. Endosperm of Fritillaria. [FLEM-
MING.]

The splitting of the chromosomes is therefore, in Boveri's words,
" an independent vital 7nanifestation, an act of reproduction on the part
of the chromosomes r ^

All of the recent researches in this field point to the conclusion
that this act of division must be referred to the fission of the
chromatin-granules or chromomeres of which the chromatin-thread
is built. These granules were first clearly described by Balbiani
i^J^) in the chromatin-network of epithelial cells in the insect-
ovary, and he found that the spireme-thread arose by the linear
arrangement of these granules in a single row like a chain of bacte-
ria.^ Six years later Pfitzner ('82) added the interesting discovery

1 '87, p. i79.

p. 113.

8 See '81, p. 638.

THE MECHANISM OF MITOSIS

113

that during the mitosis of various tissue-cells of the salamander, the
granules of the spireme-thread divide by fission and thus deternmie the
longitudinal splitting of the entire chromosome. This discovery was
confirmed by Flemming in the following year ('82, p. 219), and a simi-
lar result has been reached by many other observers (Fig. 54). The
division of the chromatin-granules may take place at a very early
period. Flemming observed as long ago as 1881 that the chromatin-

Fig. 55. — Formation of chromosomes and early splitting of the chiomatin-granules in sperma-
togonia oi As car is megalocephala, var. bivalens. [Brauer.]

A. Very early prophase; granules of the nuclear recticulum already divided. B. Spireme;
the continuous chromatin-thread split throughout. C. Later spireme. D. Shortening of the
thread. E, Spireme-thread divided into two parts. F. Spireme-thread segmented into four split
chromosomes.

thread might split in the spireme-stage (epithelial cells of the sala-
mander), and this has since been shown to occur in many other cases ;
for instance, by Guignard in the mother-cells of the pollen in the
lily ('91). Brauer's recent work on the spermatogenesis of Ascaris
shows that the fission of the chromatin-granules here takes place even
before the spireme-stage, when the chromatin is still in the form of a
reticulum, and long before the division of the centrosome (Fig. 55).
He therefore concludes : " With Boveri I regard the splitting as an

1 14 CELL-DIVISION

independent reproductive act of the chromatin. The reconstruction
of the nucleus, and in particular the breaking up of the chromosomes
after division into siiiall granules and their uniform distribution
through the nuclear cavity, is, in the first place, for the purpose of
allowing a uniform growth to take place ; and in the second place,
after the granules have grown to their normal size, to admit of their
precisely eqjial quantitative and qualitative division. I hold that all
the succeeding phenomena, such as the grouping of the granules
in threads, their union to form larger granules, the division of the
thread into segments and finally into chromosomes, are of secondary
importance ; all these are only for the purpose of bringing about in
the simplest and most certain manner the transmission of the daugh-
ter-granules (Spalthalften) to the daughter-cells." ^ " In my opinion
the chromosomes are not independent individuals, but only groups of
numberless minute chromatin-granules, which alone have the value
of individuals." 2

These observations certainly lend strong support to the view that
the chromatin is to be regarded as a morphological aggregate — as
a congeries or colony of self-propagating elementary organisms
capable of assimilation, growth, and division. They prove, more-
over, that mitosis involves two distinct though closely related factors,
one of which is the fission of the chromatic nuclear substance, while
the other is the distribution of that substance to the daughter-cells.
In the first of these it is the chromatin that takes the active part ;
in the second it would seem that the main role is played by the
amphiaster.

E. Direct or Amitotic Division

I. General Sketch

We turn now to the rarer and simpler mode of division known
as amitosis ; but as Flemming has well said, it is a somewhat trying
task to give an account of a subject of which the final outcome is
so unsatisfactory as this ; for in spite of extensive investigation, we
still have no very definite conclusion in regard either to the mechan-
ism of amitosis or. its biological meaning. Amitosis, or direct division,
differs in two essential respects from mitosis. First, the nucleus
remains in the resting state (reticulum), and there is no formation
of a spireme or of chromosomes. Second, division occurs without
the formation of an amphiaster ; hence the centrosome is not con-
cerned with the nuclear division, which takes place by a simple
constriction. The nuclear substance, accordingly, undergoes a divi-

* '93» PP- 203, 204. 2 ic., p. 205.

DIRECT OR AMITOTIC DIVISION

115

sion of its total mass, but not of its individual elements or chromatin-
granules (Fig. 56).

Before the discovery of mitosis, nuclear division was generally-
assumed to take place in accordance with Remak's scheme (p. 63).
The rapid extension of our knowledge of mitotic division between
the years 1875 and 1885 showed, however, that such a mode of
division was, to say the least, of rare occurrence, and led to doubts
as to whether it ever actually took place as a normal process. As
soon, however, as attention was especially directed to the subject,
many cases of amitotic division were accurately determined, though

Fig. 56. — Group of cells with amitotically dividing nuclei ; ovarian follicular epithelium of the
cockroach. [WHEELER.]

very few of them conformed precisely to Remak's scheme. One
such case is that described by Carnoy in the follicle-cells of the
^gg in the mole-cricket, where division begins in the fission of the
nucleolus, followed by that of the nucleus. Similar cases have
been since described, by Hoyer ('90) in the intestinal epithelium of
the nematode Rhabdonema, by Korschelt in the . intestine of the
annelid OpJnyoti'ocha, and in a few other cases. In many cases, how-
ever, no preliminary fission of the nucleolus occurs ; and Remak's
scheme must, therefore, be regarded as one of the rarest forms of
cell-division (!).

2. Centrosome and Attraction-sphere in Amitosis

The behaviour of the centrosome in amitosis forms an interesting question
on account of its bearing on the mechanics of cell-division. Flemming observed
('91) that the nucleus of leucocytes might in some cases divide directly without

Il6 CELL-DIVISION

the formation of an amphiaster, the attraction-sphere remaining undivided mean-
while. Heidenhain showed in the following year, however, that in some cases
leucocytes containing two nuclei (doubtless formed by amitotic division) might
also contain two asters connected by a spindle. Both Heidenhain and Flemming
drew from this the conclusion that direct division of the nucleus is in this case inde-
pendent, of the centrosome, but that the latter might be concerned in the division
of the cell-body, though no such process was observed. A little later, however,
Meves published remarkable observations that seem to indicate a functional activity
of the attraction-sphere during amitotic nuclear division in the '• spermatogonia "
of the salamander.^ Krause and Flemming observed that in the autumn many
of these cells show peculiarly lobed and irregular nuclei (the "polymorphic nuclei '' of
Bellonci). These were, and still are by some writers, regarded as degenerating
nuclei. Meves, however, asserts — and the accuracy of his observations is in the
main vouched for by Flemming — that in the ensuing spring these nuclei become
uniformly rounded, and may then divide amitotically. In the autumn the attraction-
sphere is represented by a diffused and irregular granular mass, which more or less
completely surrounds the nucleus. In the spring, as the nuclei become rounded,
the granular substance draws together to form a definite rounded sphere, in which
a distinct centrosome may sometimes be made out. Division takes place in the
following extraordinary manner : The nucleus assumes a dumb-bell shape, while
the attraction-sphere becomes drawn out into a band which surrounds the central
part of the nucleus, and finally forms a closed ring, encircling the nucleus. After
this the nucleus divides into two, while the ring-shaped attraction -sphere ("archo-
plasm ") is again condensed into a sphere. The appearances suggest that the ring-
shaped sphere actually compresses the nucleus and cuts it through. In a later
paper ('94) Meves shows that the diffused "archoplasm" of the autumn-stage
arises by the breaking down of a definite spherical attraction-sphere, w^hich is
re-formed again in the spring in the manner described, and in this condition the
cells may divide either niitotically or amitotically. He adds the interesting observa-
tion, since confirmed by Rawitz ('94), that in the spermatocytes of the salamander
the attraction-spheres of adjoining cells are often connected by intercellular bridges,
but the meaning of this has not yet been determined.

It is certain that the remarkable transformation of the sphere into a ring during
amitosis is not of universal, or even of general, occurrence, as shown by the later
studies of Vom Rath ('95, 3). In leucocytes, for example, the sphere persists in
its typical form, and contains a centrosome, during every stage of the division; but
it is an interesting fact that during all these stages the sphere lies on the concave
side of the nucleus in the bay which finally cuts through the entire nucleus. Again,
in the liver-cells of the isopod Porcellio, the nucleus divides, not by constriction, as
in the leucocyte, but by the appearance of a nuclear plate, in the formation of which
the attraction sphere is apparently not concerned.-^ The relations of the centro-
some and archoplasm in amitosis are, therefore, still in doubt ; but, on the whole,
the evidence goes to show that they take no essential part in the process.

3. Biological Significcince of Amitosis

A survey of the known cases of amitosis brings out the following
significant facts. It is of extreme rarity, if indeed it ever occurs in
embryonic cells or such as are in the course of rapid and continued

1 '91, p. 628.

2 Such a mode of amitotic division was first described by Sabatier in the Crustacea ('89),
and a similar mode has been observed by Carnoy and Van der Stricht.

DIRECT OR AMITOTIC DIVISION II7

multiplication. It is frequent in pathological growths and in cells
such as those of the vertebrate decidua, of the embryonic envelopes
of insects, or the yolk-nuclei (periblast, etc.), which are on the ivay
tozvard degeneration. In many cases, moreover, direct nuclear divi-
sion is not followed by fission of the cell-body, so that multinuclear
cells and polymorphic nuclei are thus often formed. These and
many similar facts led Flemming in 1891 to express the opinion that
so far as the higher plants and animals are concerned amitosis is "a
process which does not lead to a new production and multiplication
of cells, but wherever it occurs represents either a degeneration or an
aberration, or perhaps in many cases (as in the formation of multi-
nucleated cells by fragmentation) is tributary to metabolism through
the increase of nuclear surface." ^ In this direction Flemming
sought an explanation of the fact that leucocytes may divide either
mitotically or amitotically {t. Peremeschko, Lowit, Arnold, Flemming),
In the normal lymph-glands, where new leucocytes are continually
regenerated, mitosis is the prevalent mode. Elsewhere (wandering-
cells) both processes occur. '* Like the cells of other tissues the
leucocytes find their normal physiological origin (Neubildung) in
mitosis ; only those so produced have the power to live on and repro-
duce their kind through the same process."^ Those that divide ami-
totically are on the road to ruin. Amitosis in the higher forms is
thus conceived as a purely secondary process, not a survival of a
primitive process of direct division from the Protozoa, as Strasburger
('82) and Waldeyer (^SS) had conceived it.

This hypothesis has been carried still further by Ziegler and Vom
Rath ('91). In a paper on the origin of the blood in fishes, Ziegler
('87) showed that the periblast-nuclei in the egg of fishes divide ami-
totically, and he was thus led like Flemming to the view that amitosis
is connected with a high specialization of the cell and may be a fore-
runner of degeneration. In a second paper ('91), published shortly
after Flemming's, he points out the fact that amitotically dividing
nuclei are usually of large size and that the cells are in many cases
distinguished by a specially intense secretory or assimilative activity.
Thus, Riige ('90) showed that the absorption of degenerate eggs in
the Amphibia is effected by means of leucocytes which creep into the
egg-substance. The nuclei of these cells become enlarged, divide ami-
totically, and then frequently degenerate. Other observers ( Korschelt,
Carnoy) have noted the large size and amitotic division of the nuclei
in the ovarian follicle-cells and nutritive cells surrounding the ovum in
insects and Crustacea. Chun found in the entodermic cells of the
radial canals of siphonophores huge cells filled with nests of nuclei
amitotically produced, and suggested ('90) that the multiplication of

1 '91, 2, p. 291.

1 1 8 CELL-DI VISION

nuclei was for the purpose of increasing the nuclear surface as an aid
to metabolic interchanges between nucleus and cytoplasm. Amitotic
division leading to the formation of multinuclear cells is especially com-
mon in gland-cells. Thus, Klein has described such divisions in the
mucous skin-glands of Amphibia, and more recently Vom Rath has
carefully described it in the huge gland-cells (probably salivary) of the
isopod Anilocra ('95). Many other cases are known. Dogiel ('90)
has observed exceedingly significant facts in this field that place the
relations between mitosis and amitosis in a clear light. It is a well-
known fact that in stratified epitheUum new cells are continually
formed in the deeper layers to replace those cast off from the super-
ficial layers. Dogiel finds in the lining of the bladder of the mouse
that the nuclei of the superficial cells, which secrete the mucus cover-
ing the surface, regularly divide amitotically, giving rise to huge mul-
tinuclear cells, which finally degenerate and are cast off. The new
cells that take their place are formed in the deeper layers by mitosis
alone. Especially significant, again, is the case of the ciliate Infu-
soria, which possess two kinds of nuclei in the same cell, a macro-
nucleus and a micronucleus. The former is known to be intimately
concerned with the processes of metabolism {cf. p. 342). During con-
jugation the macronucleus degenerates and disappears and a new one
is formed from the micronucleus or one of its descendants. The macro-
nucleus is therefore essentially metabolic, the micronucleus genera-
tive in function. In view of this contrast it is a significant fact that
while both nuclei divide during the ordinary process of fission the
mitotic phenomena are as a rule less clearly marked in the macronu-
cleus than in the micronucleus, and in some cases the former appears to
divide directly while the latter always goes through a process of mitosis.
These conclusions received a very important support in the work of
Vom Rath on amitosis in the testis ('93). On the basis of a compara-
tive study of amitosis in the testis-cells of vertebrates, mollusks, and
arthropods he concludes that amitosis never occurs in the sperm-pro-
ducing cells (spermatogonia, etc.), but only in the supporting cells
(Randzellen, Stiitzzellen). The former multiply through mitosis alone.
The two kinds of cells have, it is true, a common origin in cells which
divide mitotically. When, however, they have once become differen-
tiated, they remain absolutely distinct ; amitosis never takes place in
the series which finally results in the formation of spermatozoa, and
the amitotically dividing " supporting-cells " sooner or later perish.
Vom Rath thus reached the remarkable conclusion that " when once
a cell has undergone amitotic division it has received its death-
warrant ; it may indeed continue for a time to divide by amitosis, but
inevitably perishes in the end." ^

^ '91, P- Zl^-

SUMMARY AND CONCLUSION II9

There is, however, strong evidence that this conclusion is too
extreme. Meves ('94) has given good reason for the conclusion that
in the salamander the nuclei of the sperm-producing cells (spermato-
gonia) may divide by amitosis yet afterward undergo normal mitotic
division, and Preusse ('95) has reached a similar result in the case of
insect-ovaries. Perhaps the most convincing evidence in this direc-
tion is afforded by Pfeffer's ('99) recent experiments on Spirogyra.
If this plant be placed in water containing 0.5 to 1.0% of ether, active
growth and division continue, but only by amitosis. If, however, the
same individuals be replaced in water, mitotic division is resumed and
entirely normal growth continues. This seems to show conclusively
that amitosis, in lower forms of life at least, does not necessarily mean
the approach of degeneration, but is a result of special conditions.
Nevertheless, there can be no doubt that Flemming's hypothesis in a
general way represents the truth, and that in the vast majority of cases
amitosis is a secondary process which does not fall in the generative
series of cell-divisions.

F. Summary and Conclusion

All cells arise by division from preexisting cells, cell-body from
cell-body, nucleus from nucleus, plastids (when these bodies are pres-
ent) from plastids, and in some cases centrosomes from centrosomes.
The law of genetic continuity thus applies not merely to the cell con-
sidered as a whole, but also to some of its structural constituents.

In mitosis, the usual and typical mode of division, the nucleus under-
goes a complicated transformation, and, together with some of the
cytoplasmic material, gives rise to the mitotic figure. Of this, the
most characteristic features are the chromatic figure, consisting of
chromosomes derived from the chromatin, and the achromatic figure,
derived from the cytoplasm, the nucleus, or from both, and consisting
of a spindle, at each pole of which, as a rule, is a centrosome and
aster. There is, however, strong evidence that both these latter struc-
tures may in some cases be wanting, and the spindle is therefore prob-
ably to be regarded as the most essential element.

The chromosomes, always of the same number in a given species
(with only apparent exceptions), arise by the transformation of the
chromatin-reticulum into a thread which breaks into segments and
splits lengthwise throughout its whole extent. The two halves are
thereupon transported in opposite directions along the spindle to
its respective poles and there enter into the formation of the two
corresponding daughter-nuclei. The spireme-thread, and hence the
chromosome, arises from a single series of chromatin-granules or
chromomeres which, by their fission, cause the splitting of the thread.

I20 CELL-DIVISION

Every individual chromatin-granule therefore contributes its quota
to each of the daughter-nuclei, but it is uncertain whether they are
persistent bodies or onjy temporary structures like the chromosomes
themselves.

The spindle may arise from the achromatic substance of the
nucleus, from the cytoplasmic substance, or from both. When cen-
trosomes are present it is they, as a rule, that lead the way in divi-
sion. About the daughter-centrosomes as foci are formed the asters
and between them stretches the spindle, forming an amphiaster
which is the most highly developed form of the achromatic figure.
When centrosomes are absent, as now appears to be the case in the
higher plants, the spindle is formed from fibrous protoplasmic ele-
ments that gradually group themselves into a spindle.

The mechanism of mitosis is imperfectly understood. Experi-
mental studies give ground for the conclusion that the changes
undergone by the chromatic and the achromatic figures respectively
are parallel but in a measure independent processes, which are how-
ever so correlated that both must cooperate for complete cell-division.
Thus there is strong evidence that the fission of the chromatin-gran-
ules, and the splitting of the thread, is not caused by division of the
centrosome or the formation of the spindle, but only accompanies it
as a parallel phenomenon. The divergence of the daughter-chromo-
somes, on the other hand, is in some manner determined by the
spindle-fibres. There are cogent reasons for the view that some of
these fibres are contractile elements which, like muscle-fibres, drag
the daughter-chromosomes asunder ; while other spindle-fibres act as
supporting and guiding .elements, and probably by their elongation
push the spindle-poles apart. The adequacy of this explanation is,
however, doubtful, and it is not improbable that the centrosome or
spindle-poles are centres of chemical or other physiological activities
that play an essential part in the process and are correlated with
those taking place in the chromatin. The functions of the astral
rays are likewise still involved in doubt, the rays being regarded by
some investigators as contractile elements like muscle-fibres, by others
as rigid supporting fibres, or even as actively pushing elements like
those of the central spindle. It is generally believed further that they
play a definite part in division of the cell-body — a conclusion sup-
ported by the fact that the size of the aster is directly related to that
of the resulting cell. On the other hand division of the cell-body
may apparently occur in the absence of asters (as in amitosis, or
among the Infusoria).

These facts show that mitosis is due to the coordinate play of an
extremely complex system of forces which are as yet scarcely com-
prehended. Its general significance is, however, obvious. The effect

LITERATURE 121

of mitosis is to produce a m eristic division, as opposed to a mere mass-
division, of the chromatin of the mother-cell, and its equal distribution
to the nuclei of the daugJiter-cells. To this result all the operations
of mitosis are tributary ; and it is a significant fact that this process
is characteristic of all embryonic and actively growing cells, while
mass-division, as shown in amitosis, is equally characteristic of highly
specialized or degenerating cells in which development is approaching
its end.

LITERATURE. II i

Auerbach, L. — Organologische Studien. Breslau, 1874.

Van Beneden, E. — Recherches sur la maturation de Toeuf, la fdcondation et la

division cellulaire : Arch, de Biol., IV. 1883.
Van Beneden and Neyt. — Nouvelles recherches sur la fecondation et la division

mitosque chez TAscaride megalocephale : Bidl. Acad. roy. de Belgique, III. 14,

No. 8. 1887.
Boveri, Th. — Zellenstudien: \. Jena. Zeitschr.,XX\. 1887; \\. Ibid.XXW. 1888;

III. Ibid. XXIV. 1890.
Driiner, L. — Studien liber den Mechanismus der Zelltheilung. Jena. Zeitschr.,

XXIX., II. 1894.
Erlanger, R. von. — Die neuesten Ansichten liber die Zelltheilung und ihre Mechanik :

Zo'dl. Cent r alb.. III. 2. 1896.
Id. — ijber die Befruchtung und erste Teilung des Ascariseies : Arch. mik. Anat.^

XLIX. 1897.
Flemming, W., '92. — Entwicklung und Stand der Kenntnisse liber Amitose :

Merkel und Bonnefs Ergebnisse, II. 1892.
Id. — Zelle. (See Introductory list. Also general list.)
Fol, H. — (See List IV.)

Heidenhain, M. — Cytomechanische Studien: Arch. f. Eniivickmech., I. 4. 1895.
Id. — Neue Erlauterungen zum Spannungsgesetz der centrirten Systeme : Morph.

Arh., VII. 1897.
Hermann, F. — Beitrag zur Lehre von der Entstehung der karyokinetischen Spindel :

Arch. mik. Anat., XXXVII. 1891.
Hertwig, R. — Uber Centrosoma und Centralspindel : Sits. -Berg. Ges. Morph. und

Phys. M'unchen, 1895, Heft I.
Kostanecki and Siedlecki. — Uber das Verhalten der Centrosomen zum Protoplasma ;

Arch. mik. Anal, XLVIII. ' 1896.
Mark, E. L. — (See List IV.)

Meves, Fr. — Zellteilung : Merkel und Bonnefs Ergebtiisse, VI. 1897.
Reinke, F. — Zellstudien : I. Arch. mik. Anat., XLIII. 1894 ; II. Ibid. XLIV. 1894.
Strasburger, E. — Karyokinetische Probleme : Jahrb.f. Wiss. Botan., XXVIII. 1895.
Strasburger, Osterhout, Mottier, and Others. — Cytologische Studien aus dem Bonner

Institut : Jahrb. wiss. Bot., XXX. 1897.
Waldeyer. W. — Uber Karyokinese und ihre Beziehungen zu den Befruchtungsvor-

gangen : Arch. mik. Anat., XXXII. 1888. Q.J. M.S., XXX. 1889-90.

1 See also Literature, IV., p. 231.

CHAPTER III

THE GERM-CELLS

" Not all the progeny of the primary impregnated germ-cells are required for the forma-
tion of the body in all animals; certain of the derivative germ-cells may remain unchanged
and become included in that body v^^hich has been composed of their metamorphosed and
diversely combined or confluent brethren; so included, any derivative germ-cell may com-
mence and repeat the same processes of growth by imbibition and of propagation by spon-
taneous fission as those to which itself owed its origin; followed by metamorphoses and
combinations of the germ-masses so produced, which concur to the development of another
individual." Richard Owen.^

" Es theilt sich demgemass das befruchtete Ei in das Zellenmaterial des Individuums und
in die Zellen fiir die Erhaltung der Art." M. Nussbaum.-^

The germ from which every living form arises is a single cell, de-
rived by the division of a parent-cell of the preceding generation.
In the unicellular plants and animals this fact appears in its simplest
form as the fission of the entire parent-body to form two new and
separate individuals like itself. In all the multicellular types the
cells of the body sooner or later become differentiated into two groups,
which as a matter of practical convenience may be sharply distin-
guished from one another. These are, to use Weismann's terms : (i)
the somatic cells, which are differentiated into various tissues by
which the functions of individual life are performed and which col-
lectively form the "■ body," and (2) the germ-cells, which are of minor
significance for the individual life and are destined to give rise to
new individuals by detachment from the body. It must, however, be
borne in mind that the distinction between germ-cells and somatic
cells is not absolute, as some naturalists have maintained, but only
relative. The cells of both groups have a common origin in the
parent germ-cell ; both arise through mitotic cell-division during the
cleavage of the ovum or in the later stages of development ; both have
essentially the same structure and both 7nay have the same power of
development, for there are many cases in which a small fragment
of the body consisting of only a few somatic cells, perhaps only of
one, may give rise by regeneration to a complete body. The dis-
tinction between somatic and germ-cells is an expression of the

1 Parthenogenesis, p. 3, 1849.

2 Arch. Mik. Anat., XVIIL, p. 112, 1880.

THE GERM-CELLS

123

physiological division of labour; and while it is no doubt the most
fundamental and important differentiation in the multicellular body,
it is nevertheless to be regarded as differing only in degree, not in
kind, from the distinctions between the various kinds of somatic cells.
In the lowest multicellular forms, such as Volvox (Fig. 57), the
differentiation appears in a very clear form. Here the body consists
of a hollow sphere, the walls of which consist of two kinds of cells.
The very numerous smaller cells are devoted to the functions of nutri-

i^j3t

Fig. 57. — Volvox, showing the small ciliated somatic cells and eight large germ-cells (drawn
from life by J. H. Emerton).

tion and locomotion, and sooner or later die. A number, usually eight,
of larger cells are set aside as germ-cells, each of which by progressive
fission may form a new individual like the parent. In this case the
germ-cells are simply scattered about among the somatic cells, and no
special sexual organs exist. In all the higher types the germ-cells
are more or less definitely aggregated in groups, supported and nour-
ished by somatic cells specially set apart for that purpose and forming
distinct sexual organs, the ovaries and sperniaries or their equivalents.
Within these organs the germ-cells are carried, protected, and nour-
ished ; and here they undergo various differentiations to prepare
them for their future functions.

In the earlier stages of embryological development the progenitors
of the germ-cells are exactly alike in the two sexes and are indistin-

124 THE GERM-CELLS

guishable from the surrounding somatic cells. As development pro-
ceeds, they are first differentiated from the somatic cells and then
diverge very widely in the two sexes, undergoing remarkable trans-
formations of structure to fit them for their specific functions. The
structural difference thus brought about between the germ-cells is,
however, only the result of physiological division of labour. The
female germ-cell, or ovum, supplies most of the material for the
body of the embryo and stores the food by which it is nourished. It
is therefore very large, contains a great amount of cytoplasm more or
less laden with food-matter {yolk or deutoplasni), and in many cases
becomes surrounded by membranes or other envelopes for the pro-
tection of the developing embryo. On the whole, therefore, the early
life of the ovum is devoted to the accumulation of cytoplasm and the
storage of potential energy, and its nutritive processes are largely
constructive or anabolic. On. the other hand, the male germ-cell or
spermatozoon contributes to the mass of the embryo only a very
small amount of substance, comprising as a rule only a single nucleus
and a very small quantity of cytoplasm. It is thus relieved from the
drudgery of making and storing food and providing protection for
the embryo, and is provided with only sufficient cytoplasm to form a
locomotor apparatus, usually in the form of one or more cilia, by
which it seeks the ovum. It is therefore very small, performs active
movements, and its metabolism is characterized by the predominance
of the destructive or katabolic processes by which the energy neces-
sary for these movements is set free.^ When finally matured, there-
fore, the ovum and spermatozoon have no external resemblance ; and
while Schwann recognized, though somewhat doubtfully, the fact
that the ovum is a cell, it was not until many years afterward that
the spermatozoon was proved to be of the same nature.

A. The Ovum

The animal ^^g (Figs. 58-59) is a huge spheroidal cell, sometimes
naked, but more commonly surrounded by one or more membranes
which may be perforated by a minute opening, the micropyle, through
which the spermatozoon enters (Fig. 63). It contains an enormous
nucleus known as the geiiniiial vesicle, within which is a very con-
spicuous nucleolus known to the earlier observers as the germinal
spot. In many eggs the latter is single, but in other forms many

1 The metabolic contrast between the germ-cells has been fully discussed in a most sug-
gestive manner by Geddes and Thompson in their work on the Evolution of Sex ; and these
authors regard this contrast as but a particular manifestation of a metabolic contrast charac-
teristic of the sexes in general.

THE OVUM 125

nucleoli are present, and they are sometimes of more than one kind^
as in tissue-cells.^ In many forms no centrosome or attraction-sphere
is found in the ^gg until the initial stages in the formation of the
polar bodies, though Mertens ('93) describes a centrosome and attrac-
tion-sphere in the young ovarian eggs of a number of vertebrates
(Fig. 79), while Platner ('89) and Stauffacher ('93) find what they
believe to be centrosomes in much later stages of Aulostomum and
Cyclas, lying outside the nuclear membrane. Beside these cases
should be placed those described by Balbiani, Munson, Nemec, and
others in which a body closely resembling an attraction-sphere is
identified as a "yolk-nucleus" or ** vitelline body," as described at
page 158. In none of these cases is the identification of this
body wholly satisfactory, nor is it known to have any connection with
the polar mitoses. Most observers find no centrosome until the
prophases of the first polar mitosis. Its origin is still problematical,
some observers beheving it to arise de novo in the cytoplasm (Mead),
others concluding that it is of nuclear origin (Mathews, Van der
Stricht, Riickert), still others that it persists in the cytoplasm hidden
among the granules. In any case it is again lost to view after forma-
tion of the polar bodies, to be replaced by the cleavage-centrosomes
which arise in connection with the spermatozoon (p. 187).

The egg-cytoplasm almost always contains a certain amount of
nutritive matter, the yolk or deittoplasm, in the form of liquid drops,
solid spheres or other bodies suspended in the meshwork and varying
greatly in different cases in respect to amount, distribution, form, and
chemical composition.

I. The Nucleus

The nucleus or germinal vesicle occupies at first a central or nearly
central position, though it shows in some cases a distinct eccentricity
even in its earliest stages. As the growth of the ^gg proceeds, the
eccentricity often becomes more marked, and the nucleus may thus
come to lie very near the periphery. In some cases, however, the
peripheral movement of the germinal vesicle occurs only a very short
time before the final stages of maturation, which may coincide with
the time of fertilization. Its form is typically that of a spherical sac,
surrounded by a very distinct membrane (Fig. 58); but during the
growth of the ^g% it may become irregular or even amoeboid (Fig. jy),
and, as Korschelt has shown in the case of insect-eggs, may move
through the cytoplasm toward the source of food. Its structure is

^ Hacker ('95, p. 249) has called attention to the fact that the nucleolus is as a rule
single in small eggs containing relatively little deutoplasm (coelenterates, echinoderms,
many annelids, and some copepods), while it is multiple in large eggs heavily laden with
deutoplasm (lower vertebrates, insects, many Crustacea).

136

TMtE C£JtM-4:£LLS

tlttt of a IjpicadodDkMidoK, but is subject to Toy great
Mit OD^'m different amimaK but also in different stages of
as in tbe ecbinodeim OTmn, the chro>
a iweauauim and r^nlar leticnlam consisting of numer-
so^pended in a network ^ finin (Fig. 58).
no trae retknalar stage exists^ the nndeos ccHitaining
tike wbofe period of its gnyvtii tbe separate dangbter-cfaro-
: tbe preceding dMsion (copqpods» sebrbians Ampbilnay

at Ae

cf &n.ba&.
It is a ^cij

Ibe most; eztraiMi
[-reacfiaii dnrii^ tiie gnnrtb of flie ^g.'
and important fact that dming tbe growth
of tbe cmnn a bnge part of tbe cbrofnatin of the
waj be bist, citber bj passing out bodily into the
cytoplisB, by cumeiMun into Mipeinumeiaiy or accessoiy nucleoli
nli^ dcgcncAat^ or by being cast oat and d^enerating at
tbe polar bodies are fonned (F%Su 97, 128).
Mlnihw of tihe egg-cell is; as elsewbere, a variable quantity
Is sliD iaqieffectiy nndecstood. It often attains an enormous
tbe ''Keiniflecfc'' or '^germinal spot'' of tbe

^F-^T^

F-33*-

^'44^ 3^fJ'JJK

BS^

rTf^

128 THE GERM-CELLS

to the formation of chromosomes. It cannot therefore be directly com
pared to the net-knots or karyosomes of tissue-cells. This nucleolus is
often vacuolated and sometimes assumes the form of a hollow vesicle.
It is rarely double or multiple. The accessory nucleoli, on the other
hand, are in general coloured by plasma-stains, thus resembling the
plasmosomes of tissue-cells ; they are often multiple, and as a rule
they arise secondarily during the growth of the ^gg (Fig. 6i). The
accessory nucleoli often have no connection with the principal ; but
in some mollusks and annelids an accessory and a principal nucleolus
are closely united to form a single compound body (Figs. 60, 61).
The numerous nucleoli of the amphibian or reptihan ^gg appear to be
of the "accessory" type. The singular inconstancy of the nucleolus
is evidenced by the fact that even closely related species may differ
in this regard. Thus, in Cyclops brevicoimis, according to Hacker, the
very young ovum contains a single intensely chromatic nucleolus ; at
a later period a number of paler accessory nucleoU appear ; and still
later the principal nucleolus disappears, leaving only the accessory
ones. In C. strenmcs, on the other hand, there is throughout but a
single nucleolus.

The physiological meaning of the nucleoli is still involved in doubt.
Many cases are, however, certainly known in which the nucleolus
plays no part in the later development of the nucleus, being cast out
or degenerating in situ at the time the polar bodies are formed. It
is, for example, cast out bodily in the medusa y^qtiorea (Hacker) and
in various annelids and echinoderms, afterward lying for some time,
as a '' metanucleus " in the egg-cytoplasm before degenerating. In
these cases the chromosomes are formed in the germinal vesicle inde-
pendently of the nucleoU (Fig. 125), which degenerate in situ when
the membrane of the germinal vesicle disappears. In such cases it
seems quite certain that the nucleoli do not contribute to the forma-
tion of the chromosomes, and that their substance represents passive
material which is of no further direct use. Hence we can hardly
doubt the conclusion of Hacker, that the nucleoli of the germ-cells
are, in some cases at least, accumulations of by-products of the nuclear
action, derived from the chromatin either by direct transformation of
its substance, or as chemical cleavage-products or secretions. It will
be shown in Chapter V. that in some cases a large part of the
chromatic reticulum is cast out, and degenerates at the time the polar
bodies are formed. The immense growth of the chromatin during
the ovarian development is probably correlated in some way with the
intense constructive activity of the cytoplasm (p. 339); and when this
latter process has ceased a large part of the chromatin-substance,
having fulfilled its functions, is cast aside. It seems not improbable
that the nucleoli are tributary to the same general process, perhaps

THE OVUM

129

Fig. 60. — Eggs of the annelid iV^rm, before and after fertilization, X 400 (for intermediate
stages see Fig. 95),

A. Before fertilization. The large germinal vesicle occupies a nearly central position. It con-
tains a network of chromatin in which are seen five small darker bodies ; these are the quadruple
chromosome-groups, or tetrads, in process of formation (not all of them are shown) ; these alone
persist in later stages, the principal mass of the network being lost ; g.s. double germinal spot,
consisting of a chromatic and an achromatic sphere. This ^gg is heavily laden with yolk, in the
form of clear deutoplasm-spheres {d) and fat-drops (/), uniformly distributed through the cyto-
plasm. The peripheral layer of cytoplasm (peri-vitelline layer) is free from deutoplasm. Outside
this the membrane. B. The &gg some time after fertilization and about to divide. The deuto-
plasm is now concentrated in the lower hemisphere, and the peri-vitelline layer has disappeared.
Above are the two polar bodies {p.b.). Below them lies the mitotic figure, the chromosomes
dividing.

I30

THE GERM-CELLS

serving as storehouses of material formed incidentally to the general
nuclear activity, but not of further direct use.

Carnoy and Le Brun ('97, '99) reach, however, the conclusion that
in the germinal vesicle of Amphibia the chromosomes are derived
not from the chromatin-network, but solely from the nucleoli. The
apparent contradiction of this result with that of other observers is,

B

k?.- ■■ « -"i

Fig. 61. — Germinal vesicles of growing ovarian eggs of the lamellibranch, Unio {A-D), and
the spider, Epeira {E-F). [Obst.]

A. Youngest stage with single (principal) nucleolus. B. Older egg, showing accessory nucle-
olus attached to the principal. C. The two nucleoli separated. D. Much older stage, showing
the two nucleoli united. E. Germinal vesicle of Epeira, showing one accessory nucleolus at-
tached to the principal, and one free. F. Later stage ; several accessory nucleoli attached to the
principal.

perhaps, only a verbal one; for the '* nucleoli" are here evidently
chromatin-masses, and the disappearance of the chromatic network is
comparable with what occurs at a later period in the annelid egg
(Figs. 97, 128).

2. The Cytoplasm

The egg-cytoplasm varies greatly in appearance with the varia-
tions of the deutoplasm. In such eggs as those of the echinoderm

THE OVUM 131

(Fig. 58), which have little or no deutoplasm, the cytoplasm forms a
regular meshwork, which is in this case an undoubted alveolar struc-
ture, the structure of which has already been described at p. 28. In
eggs containing yolk the deutoplasm-spheres or granules are laid
down in the spaces of the meshwork and appear to correspond to the
alveolar spheres of the echinoderm ^gg (p. 50). If they are of large
size the cytoplasm assumes a "pseudo-alveolar" structure (Fig. 60),
much as in plant-cells laden with reserve starch ; but reasons have
already been given (p. 50) for regarding this as only a modification
of the "primary" alveolar structure of Biitschli. There is good
reason to believe, however, that the egg-cytoplasm may in some cases
form a true reticular structure with the yolk-granules lying in its
interstices, as many observers have described. In many cases a pe-
ripheral layer of the ovum, known as the cortical or peri-vitelline layer,
is free from deutoplasm-spheres, though it is continuous with the
protoplasmic meshwork in which the latter lie (Fig. 60). Upon
fertilization, or sometimes before, this layer may disappear by a
peripheral movement of the yolk, as appears to be the case in
Nereis. In other cases the peri-vitelline substance rapidly flows
toward the point at which the spermatozoon enters, where a proto-
plasmic germinal disc is then formed ; for example, in many fish-eggs.

The character of the yolk varies so widely that it can here be con-
sidered only in very general terms. The deutoplasm-bodies are com-
monly spherical, but often show a more or less distinctly rhomboidal
or crystalloid form as in Amphibia and some fishes, and in such cases
they may sometimes be split up into parallel lamellae known as yolk-
plates. Their chemical composition varies widely, judging by the
staining-reactions; but we have very little definite knowledge on this
subject, and have to rely mainly on the results of analysis of the total
yolk, which in the hen's Q.gg is thus shown to consist largely of pro-
teids, nucleo-albumins, and a variety of related substances which are
often associated with fatty substances and small quantities of car-
bohydrates (glucose, etc.). In some cases the deutoplasm-spheres
stain intensely with nuclear dyes, such as haematoxylin ; e.g. in many
worms and mollusks; in other cases they show a greater affinity for
plasma-stains, as in many fishes and Amphibia and annelids (Fig. 60).
Often, associated with the proper deutoplasm-spheres are drops of oil,
either scattered through the yolk (Fig. 60) or united to form a single
large drop, as in many pelagic fish-eggs.

The deutoplasm is as a rule heavier than the protoplasm ; and in
such cases, if the yolk is accumulated in one hemisphere, the ^gg
assumes a constant position with respect to gravity, the egg-axis
standing vertically with the animal pole turned upward, as in the
frog, the bird, and many other cases. There are, however, many

132

THE GERM-CELLS

cases in which the ^g'g may lie in any position. When fat-drops are
present they usually lie in the vegetative hemisphere, and since they

are lighter than the other constituents
they usually cause the ^^^g to lie with
the animal pole turned downwards, as
is the case with some annelids {Nereis)
and many pelagic fish-eggs.

en

Fig. 62. — Schematic figure of a
median longitudinal section of the egg
of a fiy {Musca), showing axes of the
bilateral <i.g'g and the membranes.
[From KORSCHELT and Heider,
after Henking and Blochmann.]

e.n. The germ-nuclei uniting;
m. micropyle; p.b. the polar bodies.
The flat side of the egg is the dorsal,
the convex side the ventral, and the
micropyle is at the anterior end.
The deutoplasm (small circles) lies
in the centre surrounded by a periph-
eral or peri-vitelline layer of proto-
plasm. The outer heavy line is the
chorion, the inner lighter line the
vitelline membrane, both being per-
forated by the micropyle, from which
exudes a mass of jelly-like substance.

3. The Egg-envelopes

The egg-envelopes fall under three
categories. These are : —

i^a) The vitelline membrane , secreted

by the ovum itself.
{b) The chorion, formed outside the
ovum by the activity of the
maternal follicle-cells.
{c) Accessory envelopes, secreted by
the walls of the oviduct or other
maternal structures after the
ovum has left the ovary.
Only the first of these properly be-
longs to the ovum, the second and third
being purely maternal products. There
are some eggs, such as those of certain
coelenterates {e.g. Rejiilla), that are
naked throughout their whole develop-
ment. In many others, of which the
sea-urchin is a type, the fresh-laid Qgg is
naked but forms a vitelline membrane
almost instantaneously after the sperma-
tozoon touches it.^ In other forms (in-
sects, birds) the vitelline membrane may
be present before fertilization, and in
such cases the &gg is often surrounded
by a chorion as well. The latter is.
usually very thick and firm and may
have a shell-Hke consistency, its surface
sometimes showing various peculiar
markings, prominences, or sculptured
patterns characteristic of the species
(insects).^

1 That the vitelline membrane does not preexist seems to be established by the fact that
egg-fragments likewise surround themselves with a membrane when fertilized. [Hertwig.]

2 In some cases, according to Wheeler, the insect-egg has only a chorion, the vitelline
membrane being absent.

THE OVUM 133

The accessory envelopes are too varied to be more than touched
upon here. They inchide not only the products of the oviduct or
uterus, such as the albumin, shell-membrane, and shell of birds and
reptiles, the gelatinous mass investing amphibian ova, the capsules
of molluscan ova and the like, but also nutritive fluids. and capsules
secreted by the external surface of the body, as in leeches and earth-
worms.

When the ^gg is surrounded by a membrane before fertilization it
is often perforated by one or more openings known as micropyles,
through which the spermatozoa make their entrance (Figs. 62, 63).
Where there is but one micropyle, ^

it is usually situated very near the
upper or anterior pole (fishes,
many insects), but it may be at the
opposite pole (some insects and
mollusks), or even on the side
(insects). In many insects there
is a group of half a dozen or more ' ^.^ 03.- Upper pole of the egg of Argo-
micropyles near the upper pole of nauta. [Ussow.]

the ^gg, and perhaps correlated The egg is surrounded by a. very thick

with this is the fact that several membrane, perforated at m by the funnel-

1. 1 shaped micropyle ; below the latter lies the

spermatozoa enter the ^gg, though egg-nucleus in the peri-vitelline layer of pro-

only one is concerned with the topiasm ; /.^j. the polar bodies,
actual process of fertilization.

The plant-ovum, which is usually known as the oospJiere (Figs. 64,
107), shows the same general features as that of animals, being a
relatively large, quiescent, rounded cell containing a large nucleus.
It never, however, attains the dimensions or the complexity of struc-
ture shown in many animal eggs, since it always remains attached to
the maternal structures, by which it is provided with food and invested
with protective envelopes. It is therefore naked, as a rule, and is
not heavily laden with reserve food-matters such as the deutoplasm
of animal ova. A vitelline membrane is, however, often formed soon
after fertilization, as in echinoderms. The most interesting feature,
of the plant-ovum is the fact that it often contains plastids (leuco-
plasts or chromatophores) which, by their division, give rise to those
of the embryonic cells. These sometimes have the form of typical
chromatophores containing pyrenoids, as in Volvox and many other
Algae (Fig. 64). In the higher forms (archegoniate plants), according
to the researches of Schmitz and Schimper, the Qgg contains numer-
ous minute colourless *' leucoplasts," which afterward develop into
green chromatophores or into the starch-building amyloplasts. This
is a point of great theoretical interest ; for the researches of Schmitz,
Schimper, and others have rendered it highly probable that these

134

THE GERM-CELLS

plastids are persistent morphological bodies that arise only by the
division of preexisting bodies of the same kind, and hence may be
traced continuously from one generation to another through the

A C

Fig. 64. — Germ-cells of Volvox. [OvERTON.]
A. Ovum (oosphere) containing a large central nucleus and a peripherallayer of chromato-
phores ; p. pyrenoid. B. Spermatozoid ; c.v. contractile vacuoles ; e. " eye-spot " (chromoplastid) ;
/. pyrenoid. C. Spermatozoid stained to show the nucleus («).

germ-cells. In the lower plants (Algae) they may occur in both germ-
cells ; in the higher forms they are found in the female alone, and in
such cases the plastids of the embryonic body are of purely maternal
origin.

B. The Spermatozoon

Although spermatozoa were among the first of animal cells ob-
served by the microscope, their real nature was not determined for
more than two hundred years after their discovery. Our modern
knowledge of the subject may be dated from the year 1841, when
Kolliker proved that they were not parasitic animalcules, as the early
observers supposed, but the products of cells preexisting in the
parent body. Kolliker, however, did not identify them as cells, but
beUeved them to be of purely nuclear origin. We owe to Schweigger-
Seidel and La Valette St. George the proof, simultaneously brought
forward by these authors in 1865,^ that the spermatozoon is a com-
plete cell, consisting of nucleus and cytoplasm, and hence of the same
morphological nature as the ovum. It is of extraordinary minute-
ness, being in many cases less than yooVoo" ^^^ ^^^^ ^^ ^^^ ovum.^

1 Arr/r. Mik. Anat., I. '65.

'^ In the sea-urchin, Toxopneustes, I estimate its bulk as being between ^o oVo 0 ^^^ 3o~ffV(ro
the volume of the ovum. The inequality is in many cases very much greater.

THE SPERMATOZOON

135

Its precise study is therefore difficult, and it is not surprising that our
knowledge of its structure and origin is still far from complete.

Apical body or acrosome.

Nucleus.

End-knob.

Middle-piece.

Envelope of the tail.

Axial filament.

I. Flagellate Spermatozoa

In its more usual form the animal spermatozoon resembles a
minute, elongated tadpole, which swims very actively about by the
vibrations of a long, slender tail morpho-
logically comparable with a single cilium
or flagellum. Such a spermatozoon con-
sists typically of four parts, as shown in
Fig. 65: —

1. The nucleus, which forms the main
portion of the '' head," and consists of a
very dense and usually homogeneous mass
of chromatin staining with great intensity
with the so-called ''nuclear dyes" {^e.g.
haematoxylin or the basic tar-colours such
as methyl-green). It is surrounded by a
very thin cytoplasmic envelope.

2. An apical body, or acrosome, lying at
the front end of the head, sometimes very
minute, sometimes almost as large as the
nucleus, and in some cases terminating in
a sharp spur by means of which the
spermatozoon bores its way into the ovum.

3. The middle-piece, or connecting
piece, a larger cytoplasmic body lying
behind the head and giving attachment to
the tail, from which it is not always dis-
tinctly marked off. This body shows the
same staining-reactions as the acrosome,
having an especial affinity for " plasma-
stains " (acid fuchsin, etc.). At its front
end it is in some forms (mammals) sepa-
rated from the nucleus by a short clear
region, the 7ieck. Like the acrosome, the
middle-piece is in some cases derived from an '' archoplasmic " mass,
representing an attraction-sphere {Ltimbricus) or a portion of the
Nebenkern (insects), and it contains, or according to some authors
actually arises from, the centrosome (salamander, mammals, insects,
etc.).

4. The tail, or flagelltmi, in part, at least, a cytoplasmic product
developed in connection with the centrosome and "archoplasm"

End-piece.

Fig. 65. — Diagram of
flagellate spermatozoon.

the

136

THE GERM- CELLS

(attraction-sphere or "Nebenkern") of the mother-cell. It consists
of a fibrillated axial filament surrounded by a cytoplasmic envelope,
and in certain cases (Amphibia) bears on one side a fin-like undulat-
ing membrane (Fig. 66). Toward the tip of the flagellum the enve-
lope suddenly disappears or becomes very thin, leaving a short
end-piece which by some authors is considered to consist of the naked
axial filament. The axial filament may be traced through the
middle-piece up to the head, at the base of which it usually termi-

I

f

Fig. 66, — Spermatozoa of fishes and Amphibia. [Ballowitz.]
A. Sturgeon. B. Pike. C. D. Leuciscus. E. Triton (anterior part). F. Triton (posterior
part of flagellum), G. Raja (anterior part), a. apical body; e. end-piece; / flagellum; k. end-
knob; w. middle-piece; «. nucleus; j. apical spur.

nates in a minute body, single or double, known as the end-kjwb.
Recent research has proved that the axial filament grows out from
the spermatid-centrosome, the latter in some cases persisting as the
end-knob (insects, mollusks, mammals), in other cases apparently
enlarging to form the main body of the middle-piece (salamander).
The tail-envelopes, on the other hand, arise either from the '*archo-
plasm " of the Nebenkern (insects) together with a small amount
of unmodified cytoplasm, or from the latter alone (salamander, rat).

THE SPERMATOZOON 1 37

From a physiological point of view we may arrange the parts of
the spermatozoon under two categories as follows : —

i . The essential structures which play a direct part in fertilization.

These are : —
{a) The nucleus, which contains the chromatin.
{b) The middle-piece, which either contains a formed centrosome
or pair of centrosomes (end-knob), or is itself a meta-
morphosed centrosome. This is probably to be regarded
as the fertilizing element par excellence, since there is reason
to believe that when introduced into the ^gg it gives the
stimulus to division.
2. The accessory structures, which play no direct part in fertilization,
viz. : —
(a) The apex or spur, by which the spermatozoon attaches itself
to the Q,gg or bores its way into it, and which also serves
for the attachment of the spermatozoon to the nurse-cells
or supporting cells of the testis.
{b) The tail, a locomotor organ which carries the nucleus and
centrosome, and, as it were, deposits them in the ^gg at
the time of fertilization. There can be little doubt that
the substance of the flagellum is contractile, and that its
movements are of the same nature as those of ordinary
cilia. Ballowitz's discovery of its fibrillated structure is
therefore of great interest, as indicating its structural as
well as physiological similarity to a muscle-fibre. The
outgrowth of the axial filament from the centrosome is
probably comparable to the formation of spindle-fibres or
astral rays, a conclusion of especial interest in its relation
to Van Beneden's theory of mitosis (p. 100).
Tailed spermatozoa conforming more or less nearly to the type
just described are with few exceptions found throughout the Metazoa
from the coelenterates up to man ; but they show a most surprising
diversity in form and structure in different groups of animals, and
the homologies between the different forms have not yet been fully
determined. The simpler forms, for example, those of echinoderms
and some of the fishes (Figs. 66 and 100), conform very nearly to the
foregoing description. Every part of the spermatozoon may, how-
ever, vary more or less widely from it (Figs. 66-68). The head
(nucleus) may be spherical, lance-shaped, rod-shaped, spirally twisted,
hook-shaped, hood-shaped, or drawn out into a long filament; and
it is often divided into an anterior and a posterior piece of different
staining-capacity, as is the case with many birds and mammals,
but it is probable that the anterior of these may represent the acro-
some. An interesting form of head is described by Wheeler ('97) in

138

THE GERM-CELLS

H

Fig. 67. — Spermatozoa of various animals. [A-I, L, from Ballowitz; J, K, from VON
Brunn.]

A (At the left). Beetle {Copris), partly macerated to show structure of fiagellum ; it con-
sists of a supporting fibre (j./) and a fin-like envelope (/) ; n. nucleus ; a. a. apical body divided
into two parts (the posterior of these is perhaps a part of the nucleus). B. Insect {Calathus),
with barbed head and fin-membrane. C. Bird {Phyllopneuste). D. Bird {Muscicapa) , '^o\N\x\g
spiral structure; nucleus divided into two parts {nX, tfi) ; no distinct middle-piece. E. Bulfinch;
spiral membrane of head. F. Gull {Lartis) with spiral middle-piece and apical knob. G. H. Giant
spermatozoon and ordinary form of Tadorna. /. Ordinary form of the same stained, showing
apex, nucleus, middle-piece and fiagellum. J. "Vermiform spermatozoon" and, K. ordinary
spermatozoon of the snail Paludliia. L. Snake {Coluber), showing apical body (a), nucleus,
greatly elongated middle-piece {m), and fiagellum. (/").

THE SPERMATOZOON 1 39

the spermatozoon of Myzostorna, where it is a greatly elongated
fusiform body, passing insensibly into the tail without distinct middle-
piece and containing a single series of chromatin-discs. The num-
ber of these in M. glabrum is 24, which is the somatic number
of chromosomes in this species. In M. ciiriferuiu the number of
chromatin-discs is more than 60. Somewhat similar spermatozoa
occur in the acoelous Turbellaria.^ The acrosome sometimes appears
to be wanting, e.g. in some fishes (Fig. 66). When present, it is
sometimes a minute rounded knob, sometimes a sharp stylet, and in
some cases terminates in a sharp barbed spur by which the sperma-
tozoon appears to penetrate the ovum {Triton). In the mammals it
is sometimes very small (rat), sometimes very large (guinea-pig), and
in some forms is surrounded by a cytoplasmic layer forming the
"head-cap" (Figs. 6^^ %6). It is sometimes divided into two distinct
parts, a longer posterior piece and a knob-like anterior piece (insects,
according to Ballowitz).

The middle-piece or connecting-piece shows a Uke diversity (Figs.
^-6%). In many cases it is sharply differentiated from the flagellum,
being sometimes nearly spherical, sometimes flattened like a cap
against the nucleus, and sometimes forming a short cylinder of the
same diameter as the nucleus, and hardly distinguishable from
the latter until after staining (newt, earthworm). In other cases it
is very long (reptiles, some mammals), and is scarcely distinguishable
from the flagellum. In still others (birds, some mammals) it passes
insensibly into the flagellum, and no sharply marked Hmit between
them can be seen. In many of the mammals the long connecting-
piece is separated from the head by a narrow " neck " in which the
end-knobs lie, as described below.

Internally, the middle-piece consists of an axial filament and an
envelope, both of which are continuous with those of the flagellum.
In some cases the envelope shows a distinctly spiral structure, Uke
that of the tail-envelope ; but this is not always visible. The most
interesting part of the middle-piece is the " end-knob " in which the
axial filament terminates, at the base of the nucleus. In some cases
this appears to be single. More commonly it consists of two or more
minute bodies lying side by side (Fig. 6%, B, D).

The flagellum or tail is merely a locomotor organ which plays no
part in fertiUzation. It is, however, the most complex part of the
spermatozoon, and shows a very great diversity in structure. Its
most characteristic feature is the axial filament, which, as Ballowitz
has shown, is composed of a large number of parallel fibrillse, like a
muscle-fibre. This is surrounded by a cytoplasmic envelope, which
sometimes shows a striated or spiral structure, and in which, or in

1 Cf. Wheeler, p. 7.

40

THE GERM-CELLS

connection with which, may be developed secondary or accessory fila-
ments and other structures. At the tip the axial filament may lose
its envelope and thus give rise to the so-called "end-piece " (Retzius).
In Triton, for example (Fig. 66, F), the envelope of the axial fila-
ment ("principal filament ") gives attachment to a remarkable fin-like

membrane, having a frilled or
undulating free margin along
which is developed a " mar-
ginal filament." Toward the
tip of the tail the fin, and
finally the entire envelope,
disappears, leaving only the
axial filament to form the end-
piece. After maceration the
envelope shows a conspicuous
cross-striation, which perhaps
indicates a spiral structure
such as occurs in the mam-
mals. The marginal filament,
on the other hand, breaks
up into numerous parallel
fibrillae, while the axial fila-
ment remains unaltered (Bal-
lowitz).

A fin-membrane has also
been observed in some insects
and fishes, and has been as-
serted to occur in mammals
(man included). Later ob-
servers have, however, failed
to find the fin in mammals,
and their observations indi-
cate that the axial filament is
merely surrounded by an
envelope which sometimes
shows traces of the same
spiral arrangement as that
which is so conspicuous in
the connecting-piece. In the
skate the tail has two fila-
ments, both composed of
parallel fibrillae, connected by
a membrane and spirally
twisted about each other; a

Fig. 63. — Spermatozoa of mammals. [A-Fixora
Ballovvitz.]

A. Badger (living). B. The same after staining.
C. Bat { Vesper ugd). D. The same, flagellum and
middle-piece or connecting-piece, showing end-knobs.
E. Head of the spermatozoon of the bat {Rhino-
lophus) showing details. F. Head of spermatozoon
of the pig. G. Opossum (after staining) . H. Double
spermatozoa from the vas deferens of the opossum.
/. Rat.

h.c. head-cap (acrosome) ; k. end-knob ; tn. mid-
dle-piece ; n. nucleus (in B, E, F, consisting of two
different parts).

THE SPERMATOZOON

141

somewhat similar structure occurs in the toad. In some beetles there
is a fin-membrane attached to a stiff axial "supporting fibre" (Fig.
Gy, A). The membrane itself is here composed of four parallel fibres,
which differ entirely from the supporting fibre in staining-capacity
and in the fact that each of them may be further resolved into a
large number of more elementary fibrillae.

Fig. 69. — Unusual forms of spermatozoa.

A. B. C. Living amoeboid spermatozoa of the crustacean Polyphemus. [Zacharias.]

D. E. Spermatozoa of crab, Drotnia. F. Of Ethusa, G. of A/aja, H. of liiachus. [Grobben.]

/. Spermatozoon of lobster, Homarus. [Herrick.]

y. Spermatozoon of crab, Porcellana. [Grobben.]

Many interesting details have necessarily been passed over in the foregoing
account. One of these is the occurrence, in soine mammals, birds, Amphibia (frog),
and mollusks, of two kinds of spermatozoa in the same animal. In the birds and
Amphibia the spermatozoa are of. two sizes, but of the same form, the larger being
known as "giant spermatozoa" (Fig. 67, G, H). In the gasteropod Paliidina the
two kinds differ entirely in structure, the smaller form being of the usual type and
not unlike those of birds, while the larger, or "vermiform,'' spermatozoa have a
worm-like shape and bear a tuft of cilia at one end, somewhat like the spermatozoids
of plants (Fig. 67, 7, A'). In this case only the smaller spermatozoa are functional
(von Brunn).

142

THE GERM-CELLS

No less remarkable is the, conjugation of spermatozoa in pairs (Fig. 68, H^, which
takes place in the vas deferens in the opossum (Selenka) and in some insects
(Ballowitz, Auerbach). Ballovvitz's researches ('95) on the double spermatozoa
of beetles {DytiscidcE) prove that the union is not primary, but is the result of an
actual conjugation of previously separate spermatozoa. Not merely two, but three
or more spermatozoa may thus unite to form a " spermatozeugma,"'' which swims like
a single spermatozoon. Whether the spermatozoa of such a group separate before
fertilization is unknown; but Ballowitz has found the groups, after copulation, in
the female receptaculum, and he believes that they may enter the egg in this form.
The physiological meaning of the process is unknown.

2. Other Forms of Spermatozoa

The principal deviations from the flagellate type of spermatozoon
occur among the arthropods and nematodes (Fig. 69). In many of
these forms the spermatozoa have no fiagellum, and in some cases they
are actively amoeboid ; for example, in the daphnid Polyphemus (Fig.
69, A, B, (7) as described by Leydig and Zacharias. More commonly
they are motionless like the ovum. In the chilognathous myriapods
the spermatozoon has sometimes the form of a bi-convex lens {Poly-
desmus\ sometimes the form of a hat or helmet having a double brim
(Jtilus). In the latter case the nucleus is a solid disc at the base of
the hat. In many decapod Crustacea the spermatozoon consists of a

cylindrical or conical body from
one end of which radiate a num-
ber of stiff spine-like processes.
The nucleus lies near the base.
In none of these cases has the
centrosome been identified.

3. Paternal Germ-cells of
Plants

In most of the flowering
plants the male germ-cells are
represented by two '* generative
nuclei," lying at the tip of the
pollen tube (Fig. 106). On the
other hand, in the cycads (Figs.
^J, 108) and in a large number
of the lower plants (pterido-
phytes, Muscineae, and many
others), the male germ-cell is a
minute actively swimming cell,,
known as the spermatozoid, which is closely analogous to the sper-
matozoon. The spermatozoids are in general less highly differenti-
ated than spermatozoa, and often show a distinct resemblance to the

Fig. 70. — Spermatozoids of Chara. [Bela-
JEFF.]

A. Me-tiier-cells with reticular nuclei. B. Later
stage, with spermatozoids forming. C. Mature sper-
matozoid (the elongate nucleus black).

THE SPERMATOZOON

143

asexual swarmers or zoospores so common in the lower plants (Figs.
70, 71). They differ in two respects from animal spermatozoa: first
in possessing not one but two or several flagella ; second, in the fact
that these are attached as a rule not to the end of the cell, but on
the side. In the lower forms plastids are present in the form of
chromatophores, one
of which may be dif-
ferentiated into a red
'' eye-spot," as in
Volvox and Fucus
(Figs. 57, 71, A\ and
they may even contain
contractile vacuoles
( Volvox) ; but both
these structures are
wanting in the higher
forms. These con-
sist only of a nucleus
with a very small
amount of cytoplasm,
and have typically a
spiral form. In Chara,
where their structure
and development
have recently been
carefully studied by
Belajeff, the sperma-
tozoids have an elon-
gated spiral form with
two long flagella at-
tached near the
pointed end, which is
directed forward in
swimming (Fig. 70).
The main body of the
spermatozoid is oc-
cupied by a dense,
apparently homoge-
neous nucleus sur-
rounded by a very delicate layer of cytoplasm. Behind the nucleus lies
a granular mass of cytoplasm, forming one end of the cell, while in
front is a slender cytoplasmic tip to which the flagella are attached.
Nearly similar spermatozoids occur in the liverworts and mosses. In
the ferns and other pteridophytes a somewhat different type occurs

Fig. 71.

GUIGNARD:

- Spprrnatozoids of plants. \_A, B, C, E, after
D, F, after Strasburger.]

A. Of an alga {Fucus) ; a red chromatophore at the right
of the nucleus. B. Liverwort {Pellia). C. Moss {Sphagnum) .
D. Marsilia. E. Fern, {Angiopteris). F. Fern, Phegopteris
(the nucleus dark). {Cf. Figs. 87, 88.)

144 ^^^ GERM-CELLS

(Figs. 71, 88). Here. the spermatozoid is twisted into a conical spiral
and bears numerous cilia attached along the upper turns of the spire.
The nucleus occupies the lower turns, and attached to them is a large
spheroidal cytoplasmic mass, which is cast off when the spermatozoid
is set free or at the time it enters the archegonium. This, according
to Strasburger, probably corresponds to the basal cytoplasmic mass of
Chara. The upper portion of the spire to which the cilia are attached
is composed of cytoplasm alone, as in Chara. Ciliated spermatozoids,
nearly similar in type to those of the higher cryptogams, have recently
been discovered in the cycads by Hirase {Gingko), Ikeno {Cycas), and
Webber {Zamia). They are here hemispherical or pear-shaped bodies
of relatively huge size (in Zamia upward of 250 /x in length), with a
large nucleus filhng most of the cell and a spiral band of cilia making
from two to six turns about the smaller end (Figs. Sy, 108).

As will be shown farther on (p. 173), the "anterior" cytoplasmic'
region of the spermatozoid, to which the cilia are attached, is probably
the analogue of the middle-piece of the animal spermatozoon ; and
the work of Belajeff, Strasburger, Ikeno, Hirase, Webber, and Shaw
gives good ground for the conclusion that it has an essentially simi-
lar mode of origin, though we are still unable to say exactly how far
the comparison can be carried. The " posterior " region of the sper-
matozoid appears to correspond, broadly speaking, to the acrosome.

C. Origin of the Germ-cells

Both ova and spermatozoa take their origin from cells known as
primordial germ-cells, which become clearly distinguishable from the
somatic cells at an early period of development, and are at first exactly
alike in the two sexes. What determines their subsequent sexual
differentiation is unknown save in a few special cases. From such
data as we possess, there is very strong reason to beHeve that, with
a few exceptions, the primordial germ-cells are sexually indifferent,
i.e. neither male nor female, and that their transformation into ova
or spermatozoa is not due to an inherent predisposition, but is a reac-
tion to external stimulus. Most of the observations thus far made
indicate that this stimulus is given by the character of the food, and
that the determination of sex is therefore in the last analysis a prob-
lem of nutrition. Thus Mrs. Treat ('73) found that if caterpillars
were starved before entering the chrysalis state they gave rise to a
preponderance of male imagoes, while conversely those of the same
brood that were highly fed produced an excess of females. Yung ('81)
reached the same result in the case of Amphibia, highly fed tadpoles
producing a great excess of females (in some cases as high as 92%)
and underfed ones an excess of males. The same result, again, is

ORIGIN OF THE GERM-CELLS 1 45

given b}' the interesting experiments of Nussbaum ('97) on the rotifer
Hydati7ia^ which is an especially favourable case since sex is here de-
termined at a very early period, before the egg is laid, the eggs being
of two sizes, of which the smaller give rise only to males, and the
larger only to females. The earlier experiments of Maupas ('91) on
this form seemed to show conclusively that the decisive factor was
temperature acting on the parent organism, since in a high tempera-
ture an excess of females produced male eggs, and in a low tempera-
ture the reverse result ensued. Nussbaum shows, however, that this
is not a direct effect of temperature, but an indirect one due to the
greater birth-rate and the greater ^tivity of the animals under a
higher temperature, which result in a speedier exhaustion of food.
Direct experiment shows that, under equal temperature-conditions,
well-fed females produce a preponderance of female offspring, and
vice versa, precisely as in the Lepidoptera and Amphibia. These cases
show that sex may be determined by conditions of nutrition either
affecting the embryo itself (Lepidoptera, Amphibia) long after the ^^^g
is laid, or by similar conditions affecting the parent-organism " and
through it the ovarian ^^g.

Nutrition is, however, not the only determining cause of sex, as is
shown by the long-known case of the honey-bee. Here sex is deter-
mined by fertilization, the males arising only from unfertilized eggs
by parthenogenesis, while the fertilized eggs give rise exclusively to
females, which develop into fertile forms (queens) or sterile forms
(workers), according to the nature of the food. This is a very excep-
tional case, yet here too it is the more highly fed larvae that produce
fertile females. It is interesting to compare with this case that of the
plant-lice or aphides. In these forms the summer broods, living
under favourable conditions of nutrition, produce only females the
eggs of which develop parthenogenetically. In the autumn, under
less favourable conditions, males as well as females are produced ; and
that this is due to the external conditions and not to a fixed cyclical
change of the organism is proved by the fact that in the favourable
environment of a greenhouse the production of females alone may
continue for years. ^

We are not yet able to state whether there is any one causal ele-
ment common to all known cases of sex-determination. The observa-
tions cited above, as well as a multitude of others that cannot here be
reviewed, render it certain, however, that sex as such is not inherited.
What is inherited is the capacity to develop into either male or
female, the actual result being determined by the combined effect of
conditions external to the primordial germ-cell.

1 See Geddes, Sex, in Encyclopadia Britannica ; Geddes and Thompson, The EvoluHon
of Sex, 1889; Brooks, The Law of Heredity, 1883; Watase ('92), 7he Phenomena of
Sex-differ entiatio7i.

146

THE GERM-CELLS

In the greater number of cases the primordial germ-cells arise in
a germinal epithelium which, in the coelenterates (Fig. 72), may be a
part of either the ectoderm or entoderm, and, in the higher types, is a
modified region of the peritoneal epithelium lining the body-cavity.
In such cases the primordial germ-cells may be scarcely distinguish-
able at first from the somatic cells of the epithelium. But in other
cases the germ-cells may be traced much farther back in the develop-
ment, and they or their progenitors may sometimes be identified in
the gastrula or blastula stage, or even in the early cleavage-stages.
Thus in the worm Sagitta, Hertwig has traced the germ-cells back to

Fig. 72. — Origin of the germ-cells in a hydro-medusa, Cladonema. [Weismann.]
A. Young stage; section through wall of manubrium of the medusa; ova developing in the
ectoderm {ec). B. Later stage, showing older ova {o) and "-nutritive cells" («). The ova
contain small nuclei probably derived from engulfed nutritive cells.

two primordial germ-cells lying at the apex of the archenteron. In
some of the insects they appear still earlier as the products of a large
"pole-cell" lying at one end of the segmenting ovum, which divides
into two and finally gives rise to two symmetrical groups of germ-
cells. Hacker has recently traced very carefully the origin of the
primordial germ-cells in Cyclops from a " stem-cell " (Fig. 74) clearly
distinguishable from surrounding cells in the early blastula stage, not
only by its size, but also by its large nuclei rich in chromatin, and by
its pecuhar mode of mitosis, as described beyond.

The most beautiful and remarkable known case of early differenti-
ation of the germ-cells is that of Ascaris, where Boveri was able to
trace them back continuously through all the cleavage-stages to the

ORIGIN OF THE GERM-CELLS

^47

two-cell stage ! Moreover, from the outset the progenitor of the germ-
cells differs from tJie sojnatic cells not only in the greater size and rich-
ness of chromatin of its nuclei ^ but also in its mode of mitosis; for in
all those blastomeres destined to produce somatic cells a portion of

Pig. 73. — Origin of the primordial germ-cells and casting out of chromatin in the somatic
cells of Ascaris. [BOVERI.]

A. Two-cell stage dividing; s. stem-cell, from which arise the germ-cells. B. The same from
the side, later in the second cleavage, showing the two types of mitosis and the casting out of
chromatin {c) in the somatic cell. C. Resulting 4-cell stage; the eliminated chromatin at c,
D. The third cleavage, repeating the foregoing process in the two upper cells.

the chromatin is cast out into the cytoplasm, where it degenerates, and
only in the germ-cells is the sum-total of the chromatiji retaifted. In
Ascaris megalocephala univalens the process is as follows (Fig. 73):
Each of the first two cells receives two elongated chromosomes. As

148 THE GERM-CELLS

the ovum prepares for the second cleavage, the two chromosomes
reappear in each, but differ in their behaviour (Fig. 73, A, B). In one
of them, which is destined to produce only somatic cells, the thickened
ends of each chromosome are cast off into the cytoplasm and degen-
erate. Only the thinner central part is retained and distributed to
the daughter-cells, breaking up meanwhile into a large number of
segments which split lengthwise in the usual manner. In the
other cell, which may be called the stem-cell (Fig. 73, s), all the
chromatin is preserved and the chromosomes do not segment into
smaller pieces. The results are plainly apparent in the four-cell stage,
the two somatic nuclei, which contain the reduced amount of chro-
matin, being small and pale, while those of the two stem-cells are far
larger and richer in chromatin (Fig. 73, C). At the ensuing division
(Fig. 73, D) the numerous minute segments reappear in the two
somatic cells, divide, and are distributed like ordinary chromosomes ;
and the same is true of all their descendants thenceforward. The
other two cells (containing the large nuclei) exactly repeat the
history of the two-cell stage, the two long chromosomes reappearing
in each of them, becoming segmented and casting off their ends
in one, but remaining intact in the other, which gives rise to two
cells with large nuclei as before. This process is repeated five
times (Boveri) or six (Zur Strassen), after which the chromatin-
elimination ceases, and the two stem-cells or primordial germ-cells
thenceforward give rise only to other germ-cells and the entire
chromatin is preserved. Through this remarkable process it comes
to pass that in this animal only .the germ-cells receive the sum-
total of the egg-chromatin handed dozvn from the parent. All of the
somatic cells contain only a portion of the original germ-siibstance.
*' The original nuclear constitution of the fertilized ^gg is transmitted,
as if by a law of primogeniture, only to one daughter-cell, and by
this again to one, and so on ; while in the other daughter-cells the
chromatin in part degenerates, in part is transformed, so that all of
the descendants of these side-branches receive small reduced nuclei." ^

It would be difficult to overestimate the importance of this dis-
covery ; for although it stands at present an almost isolated case, yet
it gives us, as I believe, the key to a true theory of differentiation
development,^ and may in the end prove the means of explaining
many phenomena that are now among the unsolved riddles of the cell.

Hacker ('95) has shown that the nuclear changes in the stem-
cells and primordial eggs of Cyclops show some analogy to those of
Ascaris, though no casting out of chromatin occurs. The nuclei are
very large and rich in chromatin as compared with the somatic cells,
and the number of chromosomes, though not precisely determined.

1 Boveri, '91, p. 437. ^ CJ- p. 426.

ORIGIN OF THE GERM- CELLS

149

is less than in the somatic cells (Fig. 74). Vom Rath, working
in the same direction, believes that in the salamander also the
number of chromosomes in the early progenitors of the germ-cells
is one-half that characteristic of the somatic cells. ^ In both these
cases, the chromosomes are doubtless bivalent, representing two

Fig. 74. — Priiiioidiai germ-cells in Cyclops, [HACKER.]

A. Young embryo, showing stem-cell {sf). B. The stem-cell has divided into two, giving
rise to the primordial germ-cell {g). C. Later stage, in section; the primordial germ-cell has
migrated into the interior and divided into two ; two groups of chromosomes in each.

chromosomes joined together. In Ascaris, in Hke manner, each of
the two chromosomes of the stem-cell or primordial germ-cells is
probably plurivalent, and represents a combination 01 several units
of a lower order which separate during the segmentation of the
thread when the somatic mitosis occurs.

Cf. p. 256, Chapter V.

150 THE GERM-CELLS

D. Growth and Differentiation of the Germ-cells

I, The Ovum

{a) Grozvt/i and Nutrition. — Aside from the transformations of
the nucleus, which are considered elsewhere, the story of "the ova-
rian history of the ^gg is largely a record of the changes involved in
nutrition and the storage of material. As the primordial germ-cells
enlarge to form the mother-cells of the eggs, they almost invariably
become intimately associated with neighbouring cells which not only
support and protect them, but also serve as a means for the elabora-
tion of food for the growing egg-cell. One of the simplest arrange-
ments is that occurring in coelenterates, where the ^gg lies loose
either in one of the general layers or in a mass of germinal tissue,
and may crawl actively about among the surrounding cells like an
Ainoeba. In such cases (hydroids) the ^gg may actually feed upon
the surrounding cells, taking them bodily into its substance or fusing
with them^ and assimilating their substance with its own. In such
cases {TubiilaiHaj Hydi'd) the nuclei of the food-cells long persist in
the egg-cytoplasm, forming the so-called " pseudo-cells," but finally
degenerate and are absorbed by the ^gg. It would here seem as
if a struggle for existence took place among the young ovarian cells,
the victorious individuals persisting as the eggs ; and this view is
probably applicable also to the more usual case where the ^%g is
only indirectly nourished by its brethren.

In most cases, as ovarian development proceeds, a definite associa-
tion is established between the ^gg and the surrounding cells. In
one of the most frequent arrangements the ovarian cells form a
regular layer or follicle about the ovum (Figs. 59, 79), and there is
very strong reason to believe that the follicle-cells are immediately
concerned with the conveyance of nutriment to the ovum. A num-
ber of observers have maintained that the follicle-cells may actually
migrate into the interior of the ^g'g, and this seems to be definitely
established in the case of the tunicates and mollusks (Fig. 75).^
Such cases are, however, extremely rare ; and, as a rule, the material
elaborated by the nutritive cells is passed into the ^gg either in solu-
tion or in the form of granular or protoplasmic substance.^ An
interesting case of this kind occurs in the cycads, where, according
to Ikeno ('98), the egg-cell is connected with the surrounding cells
by broad protoplasmic bridges through which cytoplasmic material
flows directly into the egg-cell.

Very curious and suggestive conditions occur among the annelids
and insects. In the annelids the nutritive cells often do not form

1 Cf. Doflein, '97. ^ See Floderus, '95, and Obst, '99. * ^ P- 349-

GROWTH AND DIFFERENTIATION OF THE GERM-CELLS

151

a follicle, but in some forms each ^^g is accompanied by a single
nurse-cell, attached to its side, with which it floats free in the body-
cavity. In Ophryotrocha, where it has been carefully described by
Korschelt, the nurse-cell is at first much larger than the ^gg itself,
and contains a large, irregular nucleus, rich in chromatin (Fig. jG).
The egg-cell rapidly grows, apparently at the expense of the nurse-,
cell, which becomes reduced to a mere rudiment attached to one side
of the ^gg and finally disappears. There can hardly be a doubt,
as Korschelt maintains, that the nurse-cell is in some manner con-
nected with the elaboration of food for the growing egg-cell; and
the intensely chromatic
character of the nucleus is
well worthy of note in this
connection. Still more in-
teresting are the conditions
observed by Wheeler ('96,
'97) in Myzostoma, where
the young Q,gg is accom-
panied by two nurse-cells,
one at either end. These
cells fuse bodily with the
^ggy one having " some-
thing to do in forming the
vacuolated cytoplasm at
the animal pole, . . . the
other in forming the granu-
lar cytoplasm at the vege-
tative pole " ('97, p. 42).
The polar axis thus deter-
mined persists as that of
the ripe ovum. This
seems one of the clearest

cases showing the establishment of the egg-polarity through the
relation of the ^gg to its environment.^

Somewhat similar nurse-cells occur in the insects, where they have
been carefully described by Korschelt. The eggs here he in a series
in the ovarian "egg-tubes" alternating with nutritive cells vari-
ously arranged in different cases. In the butterfly Vanessa, each
^gg is surrounded by a regular follicular layer of cells, a few of
which at one end are differentiated into nurse-cells. These cells
are very large and have huge amoeboid nuclei, rich in chromatin
(Fig. J J, A). In the ear-wig, Forfictda, the arrangement is still more
remarkable, and recalls that occurring in Ophryotrocha. Here each

1 Cf. p. 386.

Fig. 75. — Ovarian eggs of Helix. [Obst,

A. Earlier stage, surrounded by follicle. B. Later
stage, showing inward migration and absorption of fol-
licle-cells.

152

THE GERM-CELLS

^gg lies in the egg-tube just below a very large nurse-cell, which,
when fully developed, has an enormous branching nucleus as shown
in Fig. 163. In these two cases, again, the nurse-cell is character-
ized by the extraordinary development of its nucleus — a fact which
points to an intimate relation between the nucleus and the metaboHc
activity of the cell.^

In all these cases it is doubtful whether the nurse-cells are sister-
cells of the ^gg which have sacrificed their own development for the
sake of their companions, or whether they have had a distinct origin
from a very early period. That the former alternative is possible is
shown by the fact that such a sacrifice occurs in some animals after
the eggs have been laid. Thus in the earthworm, Ltimbricus terres-

Fig. 76. — Egg and nurse-cell in the annelid, Ophryotrocha. [KuKSCHELT.]
A. Young stage, the nurse-cell («) larger than the egg {0). B, Growth of the ovum. C. Late
Stage, the nurse-cell degenerating.

tris, several eggs are laid, but only one develops into an embryo, and
the latter devours the undeveloped eggs. A similar process occurs
in the marine gasteropods, where the eggs thus sacrificed may
undergo certain stages of development before their dissolution.^

(b) Differentiation of the Cytoplasm and Deposit of Deutoplasni. —
In the very young ovum the cytoplasm is small in amount and free
from deutoplasm. As the t,gg enlarges, the cytoplasm increases
enormously, a process which involves both the growth of the pro-
toplasm and the formation of passive deutoplasm-bodies suspended
in the protoplasmic network. During the growth-period a peculiar
body known as the yolk-nncleus appears in the cytoplasm of many
ova, and this is probably concerned in some manner with the growth

1 See p. 338. 2 See McMurrich, '96.

GROWTH AND DIFFERENTIATION OF THE GERM-CELLS

153

of the cytoplasm and the formation of the yolk. Both its origin and
its physiological role are, however, still involved in doubt.

The deutoplasm first appears, while the eggs are still very small,
in the form of granules which seem to have at first no constant posi-
tion with reference to the egg-nucleus, even in the same species.
Thus Jordan ('93) states that in the newt \Diemyctyliis) the yolk may
be first formed at one side of the ^g'g and afterward spread to other
parts, or it may appear in more or less irregular separate patches
which finally form an irregular ring about the nucleus, which at this
period has an approximately central position. In some Amphibia

Fig- 77- — Ovarian eggs of insects. [Korschelt.
A. Egg of the butterfly, Vanessa, surrounded by its follicle ; above, three nurse-cells («.<:.) with
branching nuclei ; g.v. germinal vesicle. B. Egg of water-beetle, Dytiscus, living ; the egg {o.v.)
lies between two groups of nutritive cells; the germinal vesicle sends amoeboid processes into the
dark mass of food-granules.

the deutoplasm appears near the periphery and advances inward
toward the nucleus. More commonly it first appears in a zone
surrounding the nucleus (Fig. yS, C, D) and advances thence toward
the periphery (trout, Henneguy ; cephalopods, Ussow). In still others
{e.g. in myriapods, Balbiani) it appears in irregular patches scattered
quite irregularly through the ovum (Fig. y^, A). In Branchiptis the
yolk is laid down at the centre of the ^gg, while the nucleus lies at
the extreme periphery (Brauer). These variations show in general
no definite relation to the ultimate arrangement — a fact which
proves that the eccentricity of the nucleus and the polarity of the

154

THE GERM-CELLS

^gg cannot be explained as the result of a simple mechanical dis-
placement of the germinal vesicle by the yolk, as some authors have
maintained.

The primary origin of the deutoplasm-grains is a question that
involves the whole theory of cell-action and the relation of nucleus

Fig. 78. — Young ovarian eggs, showing yolk-nuclei and deposit of deutoplasm.

A. Myriapod {Geophilus) with single "yolk-nucleus" (perhaps an attraction-sphere) and scat-
tered deutoplasm. [Balbiani.]

B. The same with several yolk-nuclei, and " attraction-sphere," s. [Balbiani.]

C. Fish {Scorpcena) , with deutoplasm forming a ring about the nucleus, and an irregular mass
of " eliminated chromatin" (? yolk-nucleus). [Van Bambeke.]

D. Ovarian ^gg of young duck (three months) surrounded by a follicle, and containing a " yolk-
nucleus," ^.«. [Mertens.]

and cytoplasm in metabolism. The evidence seems perfectly clear
that in many cases the deutoplasm arises in situ in the cytoplasm
like the zymogen-granules in gland-cells. But there is now also a
very considerable body of evidence indicating that a part of the
egg-cytoplasm is directly or indirectly derived from the nucleus
through the agency of the yolk-nucleus or otherwise; and the

GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 155

subject can best be considered after an account of that body. It
may be mentioned here, however, that a large number of observers
have maintained a giving off of nuclear substance to the cytoplasm,
in the form of actual buds from the nucleus (Blochmann, Scharff,
Balbiani, etc.) as separate chromatin-rods or portions of the chromatin
network (Fol, Blochmann, Van Bambeke, Erlanger, Mertens, Calkins,
Nemec, etc.) or as nucleolar substance (Leydig, Balbiani, Will, Ley-
dig, Henneguy), but nearly all of these cases demand reexamination.

B

Fig. 79. — Young ovarian eggs of birds and mammals. [Mertens.]
A. Egg of young magpie (eight days), surrounded by the follicle and containing germinal
vesicle and " attraction-sphere." B. Primordial egg (oogonium) of new-born cat, dividing. C. Egg
of new-born cat containing " attraction-sphere " {s) and centrosome. D. Of young thrush sur-
rounded by follicle and containing besides the nucleus an attraction-sphere and centrosome {s),
and a yolk-nucleus (/.«.)• ^- O^ young chick containing nucleus, attraction-sphere, and fatty
deutoplasm-spheres (black). F. Egg of new-born child, surrounded by follicle and containing
nucleus and attraction-sphere.

{c) Yolk-niicleus. — The term yolk-miclejis or vitelline body {Dotter-
kern, corps vitellin) has " been appHed to various bodies or masses
that appear in the cytoplasm of the growing ovarian ^gg ; and it
must be said that the word has at present no well-defined mean-
ing. As originally described by von Wittich ('45) in the eggs of
spiders, and later by Balbiani ('93) in those of certain myriapods,
the yolk-nucleus has the form of a single well-defined spheroidal

156 ■ THE GERM-CELLS

mass which appears at a very early period and persists throughout
the later ovarian history. In other forms there are several so-called
" yolk-nuclei," sometimes of fairly definite form as described in the
Amphibia by Jordan ('93) and in some of the myriapods by Balbiani
('93). In some forms the numerous "yolk-nuclei" are irregular, ill-
defined granular masses scattered through the cytoplasm, as described
by Stuhlman (^^6) in the eggs of insects. In still others the ** yolk-
nucleus " or "vitelline body" closely simulates an attraction-sphere,
being surrounded by distinct astral radiations and enclosing one or
more central granules like centrosomes {GeophiluSj Balbiani, '93, and
Limtilus, Munson, '98). Balbiani is thus led to regard the yolk-
nucleus in general as being a metamorphosed attraction-sphere.
Miss Foot ('96) has brought forward evidence to show that the polar
rings, observed in the eggs of certain leeches and earthworms, are
also to be regarded as "yolk-nuclei" (Fig. 102). Henneguy ('93,
'96) finally compares the yolk-nucleus to the macronucleus of the
Infusoria (!).

In the present state of the subject it is quite impossible to reconcile
the discordant accounts that have been given regarding the structure,
origin, and fate of the " yolk-nuclei " , and from the facts thus far
determined we can only conclude that the various forms of " yolk-
nuclei" have little more in common than the name. It is, in the
first place, doubtful whether the " yolk-nuclei " simulating an attrac-
tion-sphere have anything in common with the other forms ; and
Mertens ('93), Munson ('98), have shown that the young ovarian ova
of various birds and mammals (including man) and of Limiilus
contain one or more "yolk-nuclei" in addition to the "attraction-
sphere " ("vitelline body" of Munson). In the second place there
seem to be two well-defined modes of origin of the yolk-nucleus. In
one type, illustrated by Jordan's observations on the newt ('93), the
" yolk-nuclei " arise separately m situ in the cytoplasm without direct
relation to the nucleus. The same is true of the small peripheral
" yolk-nuclei " of Lhnulus (Munson). In a second and more frequent
type the "yolk-nucleus" first appears very near to or in contact with
the nucleus, suggesting that the latter is directly concerned in its
formation. The latter is the case, for example, in the eggs of Cyma-
togaster (Hubbard, '94) SyngnatJius (Henneguy, '96), the earthworm
(Calkins, '95, Foot, '96), Polyzonium and other myriapods (Nemec,
'97, Van Bambeke, '98), Lhmtliis (Munson, '98), Cypris (Woltereck,
'98), and Molgida (Crampton, '99). In nearly all of these forms the
yolk-nucleus first appears in the form of a cap closely applied to one
side of the nucleus (Figs. 80, 81), sometimes so closely united to the
latter that it is difficult to trace a boundary between them. At a
later period the yolk-nucleus moves away from the nucleus and in

GROWTH AND DIFFERENTIATION OF THE GERM-CELLS

157

most, if not in all, cases breaks up into smaller and smaller fragments
which contribute, directly or indirectly, to the cytoplasmic growth.
In all these cases the history of the yolk-nucleus is such as to indi-
cate the participation of the nucleus in its formation. Calkins ('95)
endeavours to show that the yolk-nucleus in Lwnbricus is directly
derived from the nucleus by a casting out of a portion of the chro-

Fig. 80. —Yolk-nucleus in earthworm, spider, and ascidian. \^A, B, CALKINS; C-E, VAN
Bambeke; /^-/, Crampton.]

A. Early ovarian egg of Z,«Wi5r/V/«. B. Later stage; fragmentation of yolk-nucleus. C. Ova-
rian Ggg of Pholcus. D. Later stage; disintegration of yolk-nucleus. E. Remains of the yolk-
nucleus scattered through the cytoplasm. F. Early stage of yolk-nucleus in Molgula. G-I. Dis-
integration of the yolk-nucleus and enlargement of the products to form deutoplasm-spheres.

matin-reticulum — a result agreeing in principle with earlier obser-
vations on other eggs by Balbiani, Henneguy, Leydig, Will, and
other observers. This conclusion rests partly on the apparent direct
continuity of yolk-nucleus and chromatin, partly on the staining-
reactions. Thus when treated with the Biondi-Ehrlich mixture (basic
methyl-green, acid red fuchsin), the yolk-nucleus at first stains green
like the chromatin, while the cytoplasm is red, and this is the case

158 THE GERM-CELLS

even after the yolk-nucleus has quite separated from the nuclear
membrane. Later, however, as the yolk-nucleus breaks up, it changes
its staining power, and stains red like the cytoplasm. The later
observations of Miss Foot ('96) give ground to doubt the conclusion
that the yolk-nucleus is here actually metamorphosed chromatin,
for by the combined action of lithium carmine and Lyons blue its
substance is sharply differentiated from the chromatin. Still later
studies by Crampton ('99) on Molgtila demonstrate that in this case
the yolk-nucleus is not directly derived from chromatin, but they
nevertheless indicate clearly the formation of the yolk-nucleus by or
under the immediate influence of the nucleus — a conclusion also
reached on less satisfactory evidence by Hubbard, Van Bambeke,
Woltereck, and Nemec. The general morphological history of the
yolk-nucleus is here closely similar to that of Ltimbricns (Fig. 80),
except that no direct continuity between it and the nuclear substance
was observed. Stained with methyl-green-fuchsin the yolk-nucleus
and major part of the nuclear substance stain red, while the scattered
nuclear chromatin-granules and the cytoplasm stain green. Millon's
test, combined with digestion-experiments and the foregoing staining-
reactions, proves that the yolk-nucleus and the red staining nuclear
substance consist of albuminous substance and differ widely from
the general cytoplasm, which probably consists largely of nucleo-
albumins {cf. p. 331). These reactions give strong ground for the
conclusion that the substance of the yolk-nucleus, which progressively
accumulates just outside the egg-nucleus, is formed through the direct
activity of the latter, perhaps arising within the nucleus and passing
out into the cytoplasm. It is possible, further, that even the scattered
" yolk-nuclei " that seem to be of purely cytoplasmic origin may arise
in a similar manner, either, as Crampton suggests, through the early
formation and breaking up of a single yolk-nucleus, or in some less
obvious way.

Interesting questions are suggested by those *' yolk-nuclei," such
as occur in Geophiliis and Lhmibis, that so closely simulate an
attraction-sphere. Munson's observations show that this body
("vitelline body") first appears in the very young ova as a crescent
applied to the nucleus precisely as in Molgiila or Litmbriciis, but
containing one or more central granules (Fig. 81). In later stages
it becomes spherical, moves away from the nucleus, and assumes the
form of a typical radial attraction-sphere with concentric microsome-
circles and astral rays. It is hardly possible to doubt that this body
in Limulus is of the same general nature as the yolk-nucleus of
Lumbriais, Molgula, Cypris, Cymatogaster, or Pholciis ; and if it be
a true attraction-sphere in the one case we must probably so regard
it in all. This identification is, however, by no means complete ;

GROWTH AND DIFFERENTIATION OF THE GERM-CELIS

159

and even Munson's careful studies do not seem definitely to establish
its connection with the attraction-sphere or centrosome of the last
oogonium-division. That a body simulating an attraction-sphere and
containing a central granule may arise de novo in the cytoplasm
is shown by Lenhossek's observations on the spermatids of the
rat (p. 170); and the central granule is in this case certainly not
a centrosome, since the true centrosomes are found in another
part of the cell. It is quite possible that the "vitelline body" of
Limidus may have a similar origin. Nemec ('97) finds in Polyzonium
in the earliest stages a single body applied to the nucleus and
later two bodies, one of which enlarges to form a cap-shaped yolk-

Fig. 81. — Forms of yolk-nuclei in Limulus and Polyzonium. [A-C, MUNSON; D-F, Nemec]
A. Very young ovarian eggs oi Limulus ; at the left "vitelline body" {v) in the form of a cap
on the nucleus; at the right older egg showing astral formation. B. Older stage of the same;
"vitelline body" in the form of an attraction-sphere with central granule. C. Peripheral "yolk-
nuclei" {y.n^ in Limulus. D. Very early ovarian egg of a myriapod, Polyzonium, \i\ih yolk-
nucleus. E. Older egg with yolk-nucleus and astral body ya). F. Still later stage, beginning
disintegration of the yolk-nucleus.

nucleus like those described above, while the other assumes the
structure of a radiating attraction-sphere containing a central
granule (centrosome }), and his observations suggest that the two
bodies in question may have a common origin (Fig. 81). In none
of these cases do the astral radiations, surrounding this body, seem
to have any connection with cell-division, and it is probable that
a careful comparison of their physiological significance here, in
leucocytes, and in mitotic division, may give us a better understand-
ing of the general significance of astral formations in protoplasm.

The fate and physiological significance of the yolk-nucleus are
still to a considerable extent involved in doubt. In many cases it

l60 THE GERM-CELLS

breaks up into smaller and smaller granules {Lumbric?ts, Molgida^
PholcuSy some myriapods, Ajitedo7i), which scatter through the cyto-
plasm and are believed by many observers (Balbiani, Mertens, Will,
Calkins, Crampton, Nemec), following the earlier views of Allen
Thomson, to become directly converted into deutoplasm-spheres
(Fig. 80). Other observers (Van Bambeke, Foot, Stuhlman, and
others) adopt the original view of Siebold, that the fragments of
the yolk-nucleus are absorbed or converted into protoplasmic
elements and thus only " indirectly contribute to the yolk. In still
other cases {e.g. the "vitelline body" of Limiilus) the yolk-nucleus
does not fragment, but seems to serve as a centre about which new
deutoplasmic material is formed. A review of the general subject
shows that we are justified only in the somewhat vague conclusion
that the yolk-nucleus is probably involved in some manner in the
general cytoplasmic growth ; and that the facts strongly suggest,
though they hardly yet prove, that at least some forms of yolk-nuclei
are products of the nuclear activity and form a connecting link
between that activity and the constructive processes of the cyto-
plasm. That the yolk-nuclei have no very definite morphological
value, and that they are not necessary to growth, seems to be shown
by Henneguy's observation, that in the eggs of vertebrates it is in
some forms invariably present, in others only rarely, and in still
others is quite wanting ('96, p. 162). If this be the case, we must
conclude that the yolk-nucleus consists of material that contributes to
the constructive process, but is not necessarily localized in a definite
body. As to its exact role we are, as Henneguy has said, reduced
to mere hypotheses.^ The facts indicate that this material is a prod-
uct of the nuclear activity, and that it may in some cases contribute
directly to formed elements of the cytoplasm. It is probable, how-
ever, that beyond this the yolk-nucleus may supply materials, perhaps
ferments, that play a more subtle part in the constructive process,
and of whose physiological significance we are quite ignorant. The
whole subject seems a most interesting and important one for further
study of the actions of the cell in constructive metabolism, and it is to
be hoped that further research will place the facts in a clearer light.

2. Origin of the Spermatozoon

(a) General. — The relation of the various parts of the sperma-
tozoon to the structures of the spermatid is one of the most
interesting questions in cytology, since it is here that we must
look for a basis of interpretation of the part played by the sperma-

^ '96, p. 170.

GROWTH AND DIFFERENTIATION OF THE GERM-CELLS l6l

tozoon in fertilization. Obviously the most important of the
questions, thus suggested, is the source of the sperm-nucleus and
centrosome, though the relation of the other parts to the spermatid-
cytoplasm involves some interesting problems.

Owing to the extreme minuteness of the spermatozoon, the
changes involved in the differentiation of its various parts have
always been, and in some respects still remain, among the most
vexed of cytological questions. The earlier observations of Kolliker,
Schweigger-Seidel, and La Valette St. George, already mentioned,
established the fact that the spermatozoon is a cell; but it required
a long series of subsequent researches by many observers, foremost
among them La Valette St. George himself, to make known the
general course of spermatogenesis. This is, briefly, as follows :
From the primordial germ-cells arise cells known as spermatogonia}
which at a certain period pause in their divisions and undergo a con-
siderable growth. Each spermatogonium is thus converted into a
spermatocyte, which by two rapidly succeeding divisions gives rise to
four spermatozoa, as follows.^ The primary spermatocyte first
divides to form two daughter-cells known as spermatocytes of the
second order or sperm-mother-cells. Each of these divides again —
as a rule, without pausing, and without the reconstruction of the
daughter-nuclei — to form two spermatids or sperm-cells. Each of
the four spermatids is then directly transformed into a single sperma-
tozoon, its nucleus becoming very small and compact, its cytoplasm
giving rise to the tail and to certain other structures. The number
of chromosomes entering into the nucleus of each spermatid and
spermatozoon is always one-half that characteristic of the tissue-cells,
and this reduction in number is in most, if not in all, cases effected
during the two divisions of the primary spermatocyte. The reduction
of the chromosomes, which is the most interesting and significant
feature of the process, will be considered in the following chapter,
and we are here only concerned with the transformation of the sper-
matid into the spermatozoon.

All observers are now agreed that the nucleus of the spermatid is
directly transformed into that of the spermatozoon, the chromatin
becoming extremely compact and losing, as a rule, all trace of its
reticular structure. It is further certain that in some cases at least
the spermatid-centrosome passes into, or gives rise to, a part of the
middle-piece, and that from it the axial filament grows out into the
tail. The remaining structures arise, as a rule, from the cytoplasm,
and both the acrosome and the envelope of the axial filament often
show a direct relation to the remains of the achromatic figure (" ar-

1 The terminology, now almost universally adopted, is due to La Valette St. George. Cf.
Fig. 1 1 8. 2 See Fig. 119.

1 62

THE GERM-CELLS

choplasm" or "kinoplasm") which is found in the spermatid in the
form of a sphere (sometimes an attraction-sphere) or " Nebenkern "
or both. Apart from the nuclear history, these facts have been
definitely determined in only a few cases, and much confusion still
exists in the accounts of different observers. Thus a number of
investigators {e.g. Platner, Field, Benda, Julin, Prenant, Niessing)
have asserted that the centrosome passes into the acrosome, instead of

Pig. 82. — Formation of the spermatozoon in an insect, Anasa. [Paulmier.]
A. Telophase of secondary spermatocyte-division, showing extra chromosome (small dyad of
Fig. 127) below. B. Reconstitution of the nuclei. C. Spermatid with Nebenkern (A^) and
acrosome (a). D. Nebenkern double, with centrosome between the two halves. E. F. G. Elon-
gation of the spermatid, outgrowth of axial filament, migration of acrosome. H. Giant spermatid
(double size) with two centrosomes and axial filaments. /. Giant spermatid (quadruple size)
with four centrosomes and axial filaments.

the middle-piece — a result which stands in contradiction with the fact
that during fertilization in a large number of accurately known cases
the centrosome arises from or in immediate relation to the middle-
piece (Amphibia, echinoderms, tunicates, annelids, mollusks, insects,
etc.; see p. 212). The clearest and most positive evidence on this
question, afforded by recent observations on the spermatogenesis of
insects, annelids, mollusks, Amphibia, and mammals, leaves, however,
little doubt that the former result was an error and that, as the facts

GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 1 63

of fertilization would lead us to expect, the centrosome of the sper-
matid passes into the middle-piece.

Accounts vary considerably regarding the origin of the acrosome,
which according to most authors is of cytoplasmic origin, while a few
describe it as arising inside or from the anterior part of the nucleus.

ib) Composition of the Spermatid. — The confusion that has arisen
in this difficult subject is owing to the fact that the spermatid may
contain, besides the nucleus and centrosome, no less than three addi-
tional bodies, which were endlessly confused in the earlier studies
on the subject. These are the Nebenkern} the attraction-sphere or
idiosome (Meves), and the chrouiatoid Nebenkoiper {^Qwddi).

The Nebenkern (Fig. 82), first described by Biitschli ('71) in the
spermatids of butterflies, was afterward shown by La Valette {'S6\
Platner (^^6, '89), and many later investigators to arise wholly or in
part from the remains of the spindle of the second spermatocyte
division. Its origin is thus related to that of an attraction-sphere.-
(which it often closely simulates), since the latter likewise arises-
from the achromatic figure. To the remains of the spindle, however,,
may be added granular elements, probably forming reserve-material
("centro-deutoplasm of Erlanger), that are scattered through the cyto-
plasm or aggregated about the equator of the spindle (Fig. 126)..
Thus the Nebenkern may have a double origin, though its basis is
formed by the spindle-remains. The Nebenkern sometimes takes a
definite part in the formation of the tail-envelopes and of the acro-
some (insects), but in many cases it seems to be wholly wanting.^
The idiozome is in some cases an undoubted attraction-sphere derived
from the aster of the last division and at first containing the centro-
some, e.g. in the earthworm as shown by Calkins ('95) and Er-
langer ('96, 4), in the salamander and guinea-pig, Meves ('96, '99),
and in Helix according to Korff ('99), though in later stages the
centrosomes usually pass out of the body of the idiozome. In some
cases, however (in the rat, according to Lenhossek, '99), the idiozome
seems to arise independently through condensation of the cytoplasmic
substance into a sphere having no relation to the centrosomes. In
some cases the idiozomes of adjoining cells remain for a time con-
nected by bridges of material (Fig. 7) representing the remains of
the spindle, and hence corresponding to a Nebenkern {e.g. salaman-
der, Meves, '96), and the distinction between Nebenkern and idio-
zome here fades away. The idiozome is usually concerned in the
formation of the acrosome (Amphibia, mammals), but sometimes seems

1 The English equivalent of this should be paranucleus^ but the latter word has already
been used in various other senses, and it seems preferable to retain Biitschli's original Ger-
man word.

2 For critical discussion, see Erlanger, '97, i.

164

THE GERM-CELLS

to degenerate without contributing directly to the sperm-formation
{Helix). The chromatoid Nebenkorper, finally, is a small rounded
body, staining with plasma-stains, which appear always to degenerate
without taking direct part in the formation of the spermatozoon. It
is possibly an extruded nucleolus (Lenhossek), but its origin and
meaning are not definitely known.

(c) Transformation of the Spei^matid into the Spermatozoon. — In
the works of earlier authors it is often impossible to distinguish

Fig. 83. — Formation of the spermatozoon from the spermatid in the salamander. [HER-
MANN.]

A. Young spermatid, showing the nucleus above, and below the colourless sphere, the ring,
and the chromatic sphere. B. Later stage, showing the chromatic sphere and ring at the base
of the nucleus. C. D. E. F. Later stages, showing the transformation of the chromatic sphere into
the middle-piece {m).

which of the various achromatic elements mentioned above have been
under observation. We may therefore confine ourselves mainly to
the latest works, in which these distinctions are clearly recognized.
Owing to their great size, the spermatozoa of Amphibia have been
the subject of most careful study; yet a clearer view of the subject

GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 165

may, perhaps, be obtained by taking the spermatogenesis of annelids
and insects as a basis of comparison. In the insects (butterflies),
Biitschli showed, in 1871, that the tail is formed by an elongation
of the cell-body, into which extends the elongated Nebenkern, now
divided into two longitudinal halves (Fig. 82). Platner ('89), confirm-
ing this observation, further showed that the Nebenkern (in Pygcera)
consisted of two parts, stating that one ("large mitosome") gives rise
to the investment of the axial filament, the other ('' smaH mitosome ")
to the middle-piece ; while a third still smaller body, described as a
** centrosome," passes to the apex. The later works of Henking ('91)
and Wilcox ('95, '96) render it nearly certain that Platner confused
the acrosome with the centrosome, the first-named observer finding in
Pyrrhocoris and the second in Caloptenus that Platner's "centrosome"
is derived from the Nebenkern, while Wilcox traced the centrosome
directly into the middle-piece. Paulmier, finally, has shown in Atiasa
that the axial filament grows out from the centrosome,^ proving that
such is the case by the highly interesting observation that in giant
spermatozoa, arising by the non-division of the primary or secondary
spermatocytes, either two or four centrosomes are present, each of
which gives rise to a single axial filament, though only one Nebenkern
is present (Fig. 82). (The bearing of this important fact on the
centrosome-question is indicated elsewhere.) These observations,
made on three widely different orders of insects, seem to leave no
doubt that in insects the centrosome lies in the middle-piece {i.e. at
the base of the nucleus), while both the acrosome and the inner tail-
envelopes are derived from the Nebenkern. The outer envelope of
the tail is derived from unmodified cytoplasm.

In the earthworm the phenomena are shghtly different, the middle-
piece arising from an idiozome or attraction-sphere (Calkins, '95), in
which lies the centrosome (Erlanger, '96), while the Nebenkern seems
to have no part in the formation of either acrosome or tail-envelopes.^

We turn now to the Amphibia, elasmobranchs, and mammals, in
which the same general result has been attained, though there is still
some divergence of opinion regarding the exact history of the centro-
some. Working on the basis laid by Flemming i^^y, '88), Hermann
('89) traced the middle-piece in the salamander to a " Nebenkorper,"
which he believed to be not a Nebenkern but an attraction-sphere,

^ Moore ('95) seems to have been the first actually to describe the outgrowth of the axial
filament from the centrosome, in' the elasmobranchs. It has since been described by Meves
('97, 2) and Hermann ('97) in the salamander, by Lenhossek ('97), Meves ('98, '99), and
Bardeleben ('97) in the rat, guinea-pig, and man; by Godlewski ('97) and Korff ('99) in
Helix, and by several others.

■^ Calkins's preparations, which I have carefully examined, seem to leave no doubt that the
middle-piece arises from a true attraction-sphere derived from the spindle-poles; but
Erlanger believes that the granular " centrodeutoplasm " also contributes to the sphere.

1 66

THE GERM-CELLS

consisting of three parts, lying side by side in the cytoplasm (Fig. 83).
These are {a) a colourless sphere, shown by Meves's later researches to
be probably an attraction-sphere ; (^) a minute, intensely staining cor-
puscle, and {c) a small, deeply staining ring. The concurrent results
of Hermann ('89, '92, '97), Benda ('93), and Meves ('96, '97, 2) have
shown that the small corpuscle {c) is one of the centi'osonies of the
spermatid, and all these observers agree that it passes into or gives

Fig. 84. — Formation of the spermatozoon in Amphibia. \A-E. Salamandra, MeveS;
F-K. Amphiuma, McGregor.]

A. Spermatid with peripheral pair of centrosomes lying outside the sphere, and axial filament.
B. Centrosomes near the nucleus, outer one ring-shaped. C. Inner centrosome inside the
nucleus, enlarging to form middle-piece. D. Portion of much older spermatid, showing divergence
of two halves of the ring (r). E. Portion of mature spermatozoon, showing upper half of ring at
r, and the axial filament proceeding from it.

F. Spermatid of Ainphiuma, showing sphere-bridges and ring-shaped mid-bodies. G. Later
stage ; outer centrosome ring-shaped, inner one double ; sphere {s) converted into the acrosome.
H. Migration of the centrosomes. /. Middle-piece at base of nucleus. 7- The inner centrosome
forms the end-knob within the middle-piece, which is now inside the nucleus. K. Enlargement of
middle-piece, end-knob within it; elongation of the ring.

GROWTH AND DIFFERENTIATION OF THE GERM-CEILS 16/

rise to the middle-piece. According to Meves, who has most thor-
oughly studied the entire formation of the spermatozoon, the history
of these parts is as follows : In .the young spjermatids the two centro-
somes lie quite at the periphery of the cell (Fig. 84),^ and from the
outer one grows out the axial filament. The two centrosomes, leav-
ing the idiozome by which they are first surrounded, now pass
inwards toward the nucleus, the outer one meanwhile becoming trans-
formed into the ring mentioned above, while the axial filament passes
through it to become attached to the inner centrosome. The latter
pushes into the base of the nucleus and enlarges enormously to form
a cylindrical body constituting the main body of the middle-piece.
The ring meanwhile divides into two parts, the anterior of which
gives rise to a small, deeply staining body at the posterior end of the
middle-piece identical with the "end-knob." The other half of the
ring wanders out along the tail, finally lying at the limit between
the main part of the latter and the end-piece. The envelope of the
axial filament, here confined to that side opposite the marginal fin
{i.e. the *' ventral " side of Czermak), is formed by an outgrowth of the
general cytoplasm along the axial filament. The fin and marginal
filament are believed by Meves, as I understand him, to be formed
from the axial filament ('97, 2, p. 127).^ The acrosome, finally, is
formed from the idiozome which wanders around the nucleus to its
anterior pole. McGregor's results on Amphiiima ('99) agree in their
broader features with those of Meves, but differ on two points, one of
which is of great importance. The acrosome here arises from only
a part of the sphere (idiozome), while a second smaller part passes to
the base of the nucleus and forms the main part of the middle-piece.
The inner centrosome passes into the middle-piece to persist as the end-
hiob from which the axial filament passes out into the tail (Fig. 84).
The history of the sphere thus recalls the phenomena seen in the Ne-
benkern of the insect-spermatid ; though the posterior moiety does not
contribute to the tail-envelope, while the history of the inner centrosome
is somewhat like that observed in the mammals, as described beyond.
In the elasmobranchs Moore ('95), Hermann ('98), Suzuki ('98), and
Benda ('98) likewise traced the spermatid-centrosome into the middle-
piece (Fig. 85), and Moore first showed that from it the axial filament
grows out.^ Moore derived, both middle-piece and acrosome from the

1 QC their position in epithelial cells, p. 57.

2 Hermann ('97) gives a somewhat different account of the process, believing that the
ring is derived from the mid-body of the last mitosis. Meves and McGregor have, however,
shown that the ring and mid-body coexist in the early spermatids (Fig. 84), which seems
decisive against Hermann's conclusion.

^ Heiuiann ;'n;ls also the ring observed in the salamander, and believes it to be the mid-
body. The middle-piece is regarded by him as a product of the spindle-remains, but on
both these points he is contradicted by Suzuki.

1 68

THE GERM-CELLS

" archoplasm " of the spermatid. Suzuki's studies clearly show, how-
ever, that the entire axial filament of the long middle-piece arises by
the elongation of the inner centrosome, while the outer centrosome,
from which the axial filament of the tail grows out, lies at the pos-
terior limit of the middle-piece (Fig. 85). A nearly similar result is
reached by Korff ('99) in the case of Helix. It was shown by God-
lewski ('97) that in this form the axial filament likewise grows out

Fig- 85. — Formation of the spermatozoon in elasmobranchs. [A-C, SUZUKI; D, MooRE;
and in Be/ix, E-G, KORFF.]

A-D. Outgrowth of axial filament from peripheral centrosome (<:!), which persists at the
posterior limit of the middle-piece or connecting-piece (wz). Elongation of inner centrosome (c^)
to form the axial filament of the latter. E-G show similar phenomena in Helix, with casting off
of the sphere {s) .

a. Acrosome; ^i. peripheral, and ^2. inner centrosome; /. flagellum ; k. end-knob, derived
from inner centrosome.

from the centrosome. Korff's later studies show that here, exactly
as in the elasmobranch, the axial filament grows out from the periph-
eral centrosome and is afterward transformed into a ring (Fig. 85).
The inner centrosome elongates to form a rod, which afterward
becomes a long filament traversing the elongated middle-piece and
terminating in front in an end-knob at the base of the nucleus, while
the ring lies at its posterior limit. The idiozome (a true attraction-
sphere) degenerates without taking part in the formation of an aero-

GROWTH AND DIFFERENTIATION OF THE GERM-CELLS

169

some. The envelope of the middle-piece is here formed out of the
general cytoplasm.

In the mammals the recent work of Lenhossek on the rat ('98) and
Meves on the rat, guinea-pig, and man ('98, '99) gives a result agree-
ing in its broader features with the forms already considered. In all
these mammals the young spermatids are closely similar to those of
the salamander, containing two peripherally placed centrosomes, from
the outer one of. which the axi^l filament grows out (Fig. %6). Meves

Fig. 86. — Formation of the spermatozoon in mammals. [MEVES.

A. Spermatid of man, showing centrosomes and axial filament. B. Spermatid of guinea-pig,
with acrosome. C. Nearly mature spermatozoon, showing backward migration of the ring.
D. Mature spermatozoon; r. final position of the ring.

a. Acrosome surrounded by cytoplasm of the cell-body, most of which is afterward thrown
off; c. centrosomes; c.p. connecting-piece; f. fiagellum ; k. neck, containing end-knobs;
s. remains of the sphere (idiozome).

and Lenhossek differ somewhat in their accounts of the later history
of these centrosomes, though agreeing that both contribute to the
formation of the middle-piece. Lenhossek states that in the rat both
centrosomes persist at the base of the nucleus to form the end-knob,
which, as Jensen showed ('87), is double in this animal. Meves finds
the process to be more complicated, agreeing in the main with that
observed by him in the salamander. In man and the rat the inner
centrosome passes to the base of the nucleus and flattens against it
to form a small disc-shaped body. The posterior centrosome divides

170 THE GERM-CELLS

into two parts, of which the anterior gives rise to the end-knob, while
the posterior is transformed into a ring, which wanders back to its
final position at the posterior end of the so-called "connecting-piece."
From this it follows that the latter body (Verbindungsstiick) does not
correspond to the middle-piece of the salamander (here represented
by the small disc-shaped body at the base of the nucleus), but belongs
to the flagellum proper. The origin of the axial filament and end-
knob is, however, nearly the same in the two cases. , In the guinea-
pig the process is somewhat more complicated and is not quite cleared
up by Meves ; but the origin and fate of the ring is the same, and the
end-knob passes into the neck of the spermatozoon as in the rat.
Taken together, these observations conclusively show that in mam-
mals and Amphibia the end-knob is a derivative of the centrosome,
thus sustaining, though with some modifications, Hermann's earlier
conjecture ('92) as to the nature of this body; and they overturn
Niessing's result ('96) that the centrosome passes into the acrosome.
As in the salamander, the acrosome is formed from an idiozome
derived in the guinea-pig from the remains of the attraction-sphere
(Meves), while in the rat, according to Lenhossek, it is independently
formed in the cytoplasm without relation to the preceding mitotic
figure or the centrosomes. Within the sphere appears a small, deeply
staining body, resembling a centrosome, yet staining differently from
the true centrosome, which enlarges to form the acrosome, while
about it is formed a clear substance forming the " head-cap " (p. 139).
In the rat the acrosome remains small (" Spitzenknopfchen " of Mer-
kel); in the guinea-pig it becomes nearly as large as the nucleus
itself (Fig. %6). An interesting feature in the formation of the
mammalian spermatozoon is the casting off of a portion of the
spermatid-cytoplasm in the form of a "cytoplasmic vesicle" or "tail-
vesicle," which degenerates without further use (Fig. '^6). This pro-
cess, described by Meves ('99) in the guinea-pig, is closely similar to
that which occurs in the spermatozoid-formation in ferns (p. 144).

Resume. In reviewing the foregoing facts we find, despite many
variations in detail, three points of fundamental agreement, namely :
(i)the origin of the sperm-nucleus from that of the spermatid ; (2) the
origin of a part at least of the "middle-piece" from the spermatid-
centrosomes; and (3) the outgrowth of the axial filament from one of
the spermatid-centrosomes. It is clear, however, that the term middle-
piece has been appHed to structures of quite different morphological
nature, which agree only in lying behind the nucleus. Thus in the
salamander the inner centrosome gives rise to the main body of the
middle-piece ; in the rat or in man it gives rise only to the small disc-
shaped body lying in the " neck " in front of the so-called middle-

GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 171

piece ; while in Helix or the elasmobranch it is transformed into a
long filament traversing a cytoplasmic " middle-piece " which forms
a considerable part of the flagellum. The term middle-piece has thus
become highly ambiguous and should only be employed, if at all, as
a convenient descriptive term which has no definite morphological
meaning.

A very striking fact in the origin of the spermatozoon is the promi-
nent part played by the " archoplasm," i.e. substance in the form
of idiozome or Nebenkern derived from the mitotic figure. Both the
source and the fate of this material seem, however, to vary in differ-
ent cases, the acrosome now arising from the Nebenkern (insects),
now from the idiozome (salamander), the envelope of the flagellum
being formed in some cases from the Nebenkern (insects), in others
from unmodified cytoplasm (salamander, snail), while the idiozome
may form the acrosome (salamander, mammal) or degenerate without
apparent use (snail). We find here, I think, additional reason for
regarding " archoplasm" not as a distinct and permanent form of
protoplasm, but only as a phase in the general metabolic transfor-
mation of the cell-substance, which may or may not persist and play a
definite morphological role in the cell according to circumstances.
The close relation of this substance to the motor phenomena of the
cell cannot, however, be overlooked. ^

The outgrowth of the axial filament from the centrosome is a highly
interesting fact, whether we compare it with the analogous phenomena
in plants (p. 172) or with the facts observed in ordinary ciliated cells.
In the latter case (Fig. 17), as has long been known, each cilium is
attached to a small, highly refracting body known as the ''basal
knob " lying near the cell-periphery. These bodies stain intensely
in iron haematoxylin, and it has been recently suggested by Henneguy
('98) and Lenhossek ('98) that they are of the same nature as centro-
somes. The truth of this surmise must be tested by further study ;
but it seems highly probable that they are at least analogous to the
spermatid-centrosome. Ishikawa ('99) has clearly shown that in the
formation of the swarm-spores of Noctiliica the flagellum grows out
from that end of the cell at which the centrosome lies, its substance
apparently arising from the central spindle, while the centrosome lies
at its base. A very interesting fact discovered by Moore ('95) in
elasmobranchs, and confirmed by Meves ('97, 5) and Henneguy ('98)
in the insects, is a more or less abortive attempt to form a flagellum
by the spermatocytes, i.e. one or two generations before the sper-
matozoon. In the insects (Fig. 166) Henneguy has found the cilia
actually attached to the centrosomes of the mitotic figure, thus remov-
ing every doubt as to their nature.^

1 Cf. 323. 2 (jj^ Paulmier on giant spermatozoa, p. 165.

1/2 THE GERM- CELLS

It is an important question whether the axial filament actually
arises from the substance of the centrosome or is formed by differ-
entiation from the cytoplasmic substance, after the fashion of an
astral ray or spindle-fibre. Meves ('97, p. 117) accepts the latter
alternative ; but the observations of Korff on Helix and of Suzuki
on elasmobranchs seem to show clearly that, in these cases at least,
the inner centrosome elongates bodily to form an extremely long fila-
ment traversing the greater part of the flagellum, and apparently of
the same nature as the true axial filament developed from the outer
or distal centrosome. This seems to establish a probability in favour
of the first of the above alternatives, and to show that contractile
elements may be directly derived from the centrosome-substance.
If this be true, this substance is itself nearly related with " archo-
plasm " ; and the origin of a centrosome de novo may be brought
under the same category with the formation of archoplasm.^

3. Formation of the Spermatozoids in Plants

While the origin of the spermatozoids has not yet been as fully
investigated as that of the spermatozoa, recent researches have given
good ground for the conclusion that essentially similar phenomena
are involved in the two cases. All recent observers are agreed that
the nucleus of the spermatozoid is directly derived from that of the
spermatid, while the cytoplasm of the latter giveg rise to the cilia and
to certain other structures. The principal interest of the subject now
lies in the origin of the cilia and their relation to the " archoplasmic "
or "kinoplasmic " structures of the mother-cell. Belajeff ('92, '94)
found that in Chara the cilia grow forth from a small, highly refract-
ing body, taking an intense plasma-stain, that lies in the cytoplasma
beside the nucleus. He afterward found the same body '* which
reminds one of a centrosome " in the developing spermatozoids of
ferns and Equisetaceae (Fig. 88), where it grows out into a band,
lying in the anterior part of the spermatozoid, frorn which the cilia
grow forth. Comparing these results with those of Hermann, Bela-
jeff concluded **that the deeply staining corpuscle" {i.e. the cen-
trosome) ** in the spermatids of the salamander and the mouse
corresponds completely to the deeply staining corpuscle in the sper-
matogenic cells of the Characese, ferns, and Equisetacese " ; that,
furthermore, '* the middle-piece of the spermatozoon represents the
band which bears the cilia of the plant spermatozoid, while the tail-
like flagella^ of the salamander or mouse represents the cilia." ^

1 QC p. 321. For the function of the centrosome in fertilization, see p. 208.

2 In the original " Faden " perhaps meant to designate the axial filament.
«'97.3.

GROWTH AND DIFFERENTIATION OF THE GERM-CELLS

173

This tallies with Strasburger's earlier conclusion that the cilia-bearing
region consists of *' kinoplasm " and corresponds to the middle-piece
('92, p. 139), but gives a still more definite basis of comparison.^

The history of the centrosome-like bodies {blepharoplasts of Web-
ber, '97, 3) has been carefully followed out in Zainia and Gingko by
Webber ('97), and in Cycas by Ikeno ('97, '98) with nearly similar
results. In all these forms (Fig. %7) the blepharoplasts appear in the

Fig. 87. — Formation of the spermatozoids in the cycads. [//, GiNGKO; B-D, Zamia,
Webber; E-I, Cycas, Ikeno.]

A. Developing pollen-tube, showing stalk-cell {s), vegetative cell {v) and generative cell {g),
the latter with two blepharoplasts. B. Generative cell, somewhat later, with blepharoplasts and
asters. C. The same in the prophases of division, showing breaking up of blepharoplasts.
D. The two spermatids formed by division of the generative cell; blepharoplasts fragmented ;
from these fragments arises the cilia-bearing band. E. Blepharoplast of Cycas, at a stage some-
what later than Fig. C; cilia developing. F. Later stage; ciliated band (derived from the last
stage) attached to a prolongation from the nucleus. G, Cilia-bearing band continuous. H. Nearly
ripe spermatozoid with nucleus in the centre; ciliated band, shown in section, forming a spiral.
/. Slightly later stage, viewed from above, showing the spiral course of the band (cilia omitted).

penultimate cell-generation lying one on either side the nucleus, and
in earlier stages surrounded by astral radiations very closely resem-
bling those of a typical mitotic aster, and they lie opposite the poles

1 The " anterior " region of the spermatozoid thus corresponds to the " posterior " region
of the spermatozoon, the confusion of terms having arisen from the fact that the former
swims with the cilia-bearing region in front, the latter with the flagellum directed backvirard.

174

THE GERM-CELLS

of the ensuing division-spindle. They seem, however, to have no
part in the formation of the mitotic figure or in division, and both

Fig. 88. — Formation of the spermatozoids in the vascular cryptogams, Marsilla {A, Z),.
£-G, Belajeff; B, C, O, Shaw), Gymnogramme {H-K, Belajeff), and Equisetutn {L-N,
Belajeff).

A. Primary spermatogonium (two generations before the primary spermatocytes) in division,,
showing centrosomes. B. Primary spermatocyte with pair of " blepharoplastoids " (centrosomes).
C. Spindle of primary spermatocyte (first maturation-division). D. Four of the eight secondary
spermatocytes with blepharoplast. E-G. Prophase of second maturation-division. H. Pair of
spermatids ( Gymnogramme) with blepharoplasts. I-J. Formation of the cihated band from the
blepharoplast. K. Nearly ripe spermatozoid, showing ciliated band {b), nucleus, and "cyto-
plasmic vesicle" (the latter is ultimately cast off). L. M. Spermatids of Equisetum. N. Ripe
spermatozoid from above, showing spiral ciliated band. O. Ripe spermatozoid oi Mars ilia with'
very long spiral ciliated band.

STAINING-REACTIONS OF THE GERM-NUCLEI 175

Webber and Ikeno have produced apparently strong evidence ^ that
they arise separately and de novo in the cytoplasm. After the ensu-
ing division (by which the two spermatids are formed) the astral rays
disappear, and the blepharoplast gives rise by a pecuHar process to a
long, spiral, deeply staining band, from which the cilia grow forth.
The later studies of Shaw ('98, i) and Belajeff ('99) on the blepharo-
plasts in Onoclea and Marsilia leave no doubt that these bodies are
to be identified with centrosomes. In Marsilia Shaw first found the
blepharoplasts lying at the poles of the spindle during the anaphase
of the first maturation-division and very closely resembHng centro-
somes. Each blepharoplast, at first single, divides into two during the
late telophase, and during the prophases of the second division the
halves diverge to opposite poles of the nucleus and lie at the respec-
tive spindle-poles. This account is confirmed by Belajeff, who shows
further that during the prophases astral rays surround the blepharo-
plasts, and a central spindle is formed between them (Fig. ^%\
Belajeff also finds centrosomes in all of the earlier spermatogenic
divisions. The blepharoplasts are thus proved to be, in one case at
least, dividing organs which in every way correspond to the centro-
somes of the animal spermatocytes ; and the justice of Belajeff's
comparison is demonstrated. Shaw believed that the primary blepha-
roplast, which by division gives rise to those of the two spermatids,
arose de novo. He made, however, the significant observation that
in Marsilia "blepharoplastoids," exactly like the blepharoplasts, ap-
pear at the spindle-poles of the preceding (antepenultimate) division,
and that each of these divides into two in the late telophase. These
are said to disappear, without relation to the blepharoplasts which at
a slightly later period are found at the spindle-poles of the first matu-
ration division ; but in view of the demonstrated continuity of the
blepharoplasts during the second division we may well hesitate to
accept this result, as well as Webber's conclusion regarding the
independent and separate origin of the blepharoplasts in Zamia. In
any case the facts give the strongest ground for the conclusion that
the formation of the spermatozoids agrees in its essential features
with that of the spermatozoa, and for the expectation that the history
of the achromatic structures in fertilization will yet be found to show
an essential agreement in plants and animals.

E. Staining-reactions of the Germ-nuclei

It was pointed out by Ryder in 1883 that in the oyster the germ-
nuclei stain differently in the two sexes ; for if the hermaphrodite

^ Dr. Webber has kindly given me an opportunity to look through his beautiful prepa-
rations.

1/6 THE GERM-CELLS

gland of this animal be treated with a mixture of saffranin and methyl-
green, the egg-nuclei are coloured red, the sperm-nuclei bluish green.
A similar difference was afterward observed by Auerbach ('91) in
the case of many vertebrate germ-cells, where the egg-nucleus was
shown to have a special affinity for various red and yellow dyes
(eosin, fuchsin, aurantia, carmine), while the sperm-nuclei were espe-
cially stained with blue and green dyes (methyl-green, aniline-blue,
haematoxylin). He was thus led to regard the chromatin of the ^gg
as especially " erythrophilous," and that of the sperm as " cyanophi-
lous." That the distinction as regards colour is of no value has been
shown by Zacharias, Heidenhain, and others ; for staining-agents can-
not be logically classed according to colour, but according to their
chemical composition ; and a red dye, such as saffranin, may in a
given cell show the same affinity for the chromatin as a green or blue
dye of different chemical nature, such as methyl-green or haema-
toxylin. Thus Field has shown that the sperm-nucleus of Asterias
may be stained green (methyl-green), blue (haematoxyhn, gentian
violet), red (saffranin), or yellow (iodine), and it is here a manifest
absurdity to speak of "cyanophilous " chromatin (^. p. 335). It is
certainly a very interesting fact that a difference of staining-reaction
exists between the two sexes, as indicating a corresponding difference
of chemical composition in the chromatin ; but even this has been
shown to be of a transitory character, for the staining-reactions of the
germ-nuclei vary at different periods and are exactly aHke at the time
of their union in fertilization. Thus Hermann has shown that when
the spermatids and immature spermatozoa of the salamander are
treated with saffranin (red) and gentian violet (blue),^ the chromatic
network is stained blue, the nucleoli and the middle-piece red ; while
in the mature spermatozoon the reverse effect is produced, the nuclei
being clear red, the middle-piece blue. A similar change of staining-
capacity occurs in the mammals. The great changes in the staining-
capacity of the egg-nucleus at different periods of its history are de-
scribed at pages 338-340. Again, Watase has observed in the newt
that the germ-nuclei, which stain differently throughout the whole
period of their maturation, and even during the earlier phases of
fertilization, become more and more alike in the later phases, and at
the time of their union show identical staining-reactions.^ A very
similar series of facts has been observed in the germ-nuclei of plants
by Strasburger (p. 220). These and many other facts of like import
demonstrate that the chemical differences between the germ-nuclei
are not of a fundamental but only of a secondary character. They
are doubtless connected with the very different character of the meta-
bolic processes that occur in the history of the two germ-cells ; and

1 By Flemming's triple method. ^ '92, p. 492.

LITERATURE 1/7

the difference of the staining-reaction is probably due to the fact
that the sperm-chromatin contains a higher percentage of nucleinic
acid, while the egg-chromatin is a nuclein containing a much higher
percentage of albumin.

LITERATURE. IIP

Ballowitz, E. — Untersuchungen iiber die Struktur der Spermatozoen : i. {birds)

Arch. mik. Anat., XXXII. 1888; 2. {insects) Zeitschr. wiss. Zool.^ L. 1890;

3. {fishes, amphibia, reptiles) Arch. viik. Anat., XXXVI. 1890; 4. {mam-

mats) Zeit. wiss. ZooL, LII. 1891.
Belajeff, W. - — tlber die Centrosomen in den spermatogenen ZeUen: Ber. d.

deutsch. bot. Ges., XVII., 6. 1899.
Boveri, Th. — Uber Diiferenzierung def Zellkerne wahrend der Furchung des Eies

von Ascaris meg.: Anat. Anz. 1887.
Id. — Die Entvvicklung von Ascaris megalocephala mit besonderer Riicksicht auf die

Kernverhaltnisse : Festschr. fi'ir C. v. Kiipffer. Jena, 1899.
Brunn, M. von. — Beitrage zur Kenntniss der Samenkorper und ihrer Entwickelung

bei Vogeln und Saugethieren : Arch. mik. Anat., XXXIII. 1889.
Hacker, V. — Die Eibildung bei Cyclops und Camptocanthus : Zool. Jahrb., V.

1892. (See also List V.)
Hermann, F. — Urogenitalsystem : Struktur und Histiogenese der Spermatozoen :

Merkel und Bonnefs Ergebnisse,\\. 1892.
Ikeno, S. — Untersuchungen liber die Entwickelung der Geschechtsorgane, .?/^., bei

Cycas : Jahrb. wiss. Bot., XXXIL, 4. 1898.
KoUiker, A. — Beitrage zur Kenntniss der Geschlechtsverhaltnisse und der Samen-

fllissigkeit wirbelloser Tiere. Berlin. 1841.
Leydig, Fr. — Beitrage zur Kenntniss des thierischen Eies im unbefruchteten Zu-

%X2iXidi&'. Zool. Jahrb. ,\\\. 1889.
Meves, F. — tJber die Entvvicklung der mannlichen Geschechtszellen von Salaman-

dra : Arch. mik. Anat., XLVIII. 1896.
Id. — Uber Struktur und Histogenese der Samenfaden des Meerschweinchens :

Arch. mik. Anat., LIV. 1899.
Schweigger-Seidel, F. — Uber die Samenkorperchen und ihre Entwicklung : Arch.

mik. Anat., I. 1865.
Strasburger, E. — Histologische Beitrage; Heft IV.: Das Verhalten des Pollens

und die Befruchtungsvorgange bei den Gymnospermen, Schwarmsporen, pflanz-

liche Spermatozoiden und das Wesen der Befruchtung. Fischer, Jena, 1892.
Thomson, Allen. — Article " Ovum," in Todd's Cyclopedia of Anatomy and Physi-
ology. 1859.
Van Beneden, E. — Recherches sur la composition et la signification de I'oeuf : Mem.

co7ir. de TAcad. roy. de Belgique. 1870.
Waldeyer, W. — Eierstock und Ei. Leipzig, 1870.
Id. — Bau und Entwickelung der Samenfaden : Verh. d. Anat. Ges. Leipzig, 1887.

N 1 See also Literature, V., p. 287.

CHAPTER IV

FERTILIZATION OF THE OVUM

" It is conceivable, and indeed probable, that every part of the adult contains molecules
derived both from the male and from the female parent; and that, regarded as a mass of
molecules, the entire organism may be compared to a web of which the warp is derived from
the female and the woof from the male." Huxley.^

In mitototic cell-division we have become acquainted with the means
by which, in all higher forms at least, not only the continuity of life,
but also the maintenance of the species, is effected ; for through this
beautiful mechanism the cell hands on to its descendants an exact dupli-
cate of the idioplasm by which its own organization is determined.
As far as we can see from an a priori point of view, there is no reason
why, barring accident, cell-division should not follow cell-division in
endless succession in the stream of life. It is possible, indeed prob-
able, that such may be the fact in some of the lower and simpler forms
of life where no form of sexual reproduction is known to occur. In
the vast majority of living forms, however, the series of cell-divisions
tends to run in cycles in each of which the energy of division finally
comes to an end and is only restored by an admixUire of living mat-
ter derived from another cell. This operation, known as fertiliza-
tion or fecundation, is the essence of sexual reproduction ; and in it we
behold a process by which on the one hand the energy of division is
restored, and by which on the other hand two independent lines of
descent are blended into one. Why this dual process should take
place we are as yet unable to say, nor do we know which of its two
elements is to be regarded as the primary and essential one.

Harvey and many other of the early embryologists regarded fer-
tilization as a stimulus, given by the spermatozoon, through which the
ovum was ''animated" and thus rendered capable of development.
In its modern form this conception appears in the " dynamic " theories
of Herbert Spencer, Biitschli, Hertwig, and others, which assume that
protoplasm tends gradually to pass into a state of increasingly sta-
ble equilibrium in which its activity diminishes, and that fertilization
restores it to a labile state, and hence to one of activity, through mix-
ture with protoplasm that has been subjected to different conditions.
Biitschli ('76) pointed out that the life-cycle of the metazoon is com-

1 Evolution, in Science and Culture, p, 296, from Enc. Brit., 1878.
178

FERTILIZATION OF THE OVUM 1 79

parable to that of a protozoan race, a long series of cell-divisions being
in each case followed by a mixture of protoplasms through conjuga-
tion ; and he assumed that, in both cases, conjugation results in reju-
venescence through which the energy of growth and division is
restored and a new cycle inaugurated. The same view has been
advocated by Minot, Engelman, Hensen, and many others. Mau-
pas {^%%, '89), in his celebrated researches in the conjugation of Infu-
soria, attempted to test this conclusion by following out continuously
the life-history of various species through the entire cycle of their exist-
ence. Though not yet adequately confirmed, and indeed opposed in
some particulars by more recent work,^ these researches have yielded
very strong evidence that in these unicellular animals, even under
normal conditions, the processes of growth and division sooner or
later come to an end, undergoing a process of natural " senescence,"
which can only be counteracted by conjugation. That fertilization in
higher plants and animals does in fact incite division and growth is a
matter of undisputed observation. We know, however, that in parthe-
nogenesis the ^gg may develop without fertilization, and we do not
know whether the tendency to " senescence " and the need for fer-
tilization are primary attributes of living matter.

The foregoing views may be classed together as the rejuvenescence
theory. Parallel to that theory, and not necessarily opposed to or
confirmatory of it, is the view that fertilization is in some way con-
cerned with the process of variation. Long since suggested by Tre-
viranus and more lately developed by Brooks ^ and Weismann ^ is the
hypothesis that fertihzation is a source of variation — a conclusion sug-
gested by the experience of practical breeders of plants and animals.
Weismann brings forward strong arguments against the rejuvenescence-
theory, and regards the need for fertilization as a secondary acquisi-
tion, the mixture of protoplasms to which it leads producing variations
— or rather insuring their "mingling and persistent renewal"* —
which form the material on which selection operates. On the other
hand, a considerable number of writers, including Darwin, Spencer,
O. Hertwig, Hatschek, and others, believe that although crossing may
lead to variability within certain limits, its effect in the long run tends
to neutralize indefinite variability and thus to hold the species true to
the type.

It is remarkable that we should still remain uncertain as to the physi-
ological meaning of a process so general and one that has been the
subject of such prolonged research. Both the foregoing general views
are in harmony with the results of Darwin's work on variation and
with the experience of practical breeders, which have shown that

1 Cf. Joukowsky, '99, ^ Amphimixis, 1891.

2 The Law of Heredity, 1883. * '99, p. 326.

i8o

FERTILIZATION OF THE OVUM

crossing produces both greater vigour and greater variability. In view
of all the facts, however, we are constrained to the admission that the
essential nature of sexual reproduction must remain undetermined until
the subject shall have been far more thoroughly investigated, espe-
cially in the unicellular forms, where the key to the ultimate problem
is undoubtedly to be sought.

A. Preliminary General Sketch

Among the unicellular plants and animals, fertilization is effected
by means of conjugation, a process in which two individuals either
fuse together permanently or unite temporarily and effect an exchange

Fig. 89. — Fertilization of the ^gg of the snail, Physa. [Kostanecki and WIERZEJSKI.]
A. The entire spermatozoon lies in the egg, its nucleus at the right, flagellum at the left, while
the minute sperm-amphiaster occupies the position of the middle-piece. The first polar body has
been formed, the second is forming. B. The enlarged sperm-nucleus and sperm-amphiaster lie
near the centre; second polar body forming and the first dividing. The egg-centrosomes and
asters afterward disappear, their place being taken by those of the spermatozoon.

of nuclear matter, after which they separate. /;/ all the Jiighei' forms
fertilization consists iii the permanent fusion of two germ-cells, one of
paternal and one of maternal origin. We may first consider the fer-
tilization of the animal ^gg, which appears to take place in essentially
the same manner throughout the animal kingdom, and to be closely
paralleled by the corresponding process in plants.

PRELIMINARY GENERAL SKETCH l8l

Leeuwenhoek, whose pupil Hamm discovered the spermatozoa
(1677), put forth the conjecture that the spermatozoon must pene-
trate into the ^g% ; and the classical experiments of Spallanzani on
the frog's ^^g (1786) proved that the fertihzing element must be the
spermatozoa and not the liquid in which they swim. The penetration
of the ovum was, however, not actually seen until 1854, when Newport
observed it in the case of the frog's ^gg\ and it was described by
Pringsheim a year later in one of the lower plants, Qldigonimn. The
first adequate description of the process was given by Hermann Fol,
in 1879,^ though many earlier observers, from the time of Martin
Barry ('43) onward, had seen the spermatozoon inside the egg-enve-
lopes, or asserted its entrance into the ^gg.

In many cases the entire spermatozoon enters the ^gg (moUusks,
insects, nematodes, some annelids, Petro7nyzoii, axolotl, etc.), and in
such cases the long flageilum may sometimes be seen coiled within
the Q.gg (Fig. 89). Only the nucleus and middle-piece, however, are
concerned in the actual fertihzation ; and there are some cases
(echinoderms) in which the tail is left outside the ^gg. At or near
the time of fertilization, the ^gg successively segments off at the upper
pole two minute cells, known as the polar bodies {¥\g^. 89, 90, 1 16) or
directive corpuscles, which degenerate and take no part in the subse-
quent development. This phenomenon takes place, as a rule, imme-
diately after entrance of the spermatozoon. It may, however, occur
before the spermatozoon enters, and it forms no part of 'the process
of fertilization proper. It is merely the final act in the process of
maturation, by which the ^gg is prepared for fertilization, and we
may defer its consideration to the following chapter.

I. The Gejin-Jiuclei in Fertilization

The modern era in the study of fertilization may be said to begin
with Oscar Hertwig s discovery, in 1875, of the fate of the sperma-
tozoon within the ^gg. Earlier observers had, it is true, paved the
way by showing that, at the time of fertilization, the ^gg contains
tivo nuclei that fuse together or become closely associated before
development begins. (Warneck, Butschli, Auerbach, Van Beneden,
Strasburger.) Hertwig discovered, in the ^gg of the sea-urchin
( Toxopneiistes lividius), that one of these nuclei belongs to the egg,
while the other is derived from the spermatozoon. This result was
speedily confirmed in a number of other animals, and has since been
extended to every species that has been carefully investigated. The
researches of Strasburger, De Bary, Schmitz, Guignard, and others
have shown that the same is true of plants. In every known case an

1 See Vllenogenie, pp. 124 ff., for a full historical account.

1 82 FERTILIZATION OF THE OVUM

essential phenomenon qf fertilization is the imion of a sperm-nucleus ^
of paternal origin, with an egg-nucleus^ of maternal origin, to form the
primary nucleus of the embryo. This nucleus, known as the cleavage-
or segmentation-nucleus, gives rise by division to all the nuclei of the
body, and hence every nucleus of the child may contain nuclear substance
derived from both parents. And thus Hertwig was led to the conclu-
sion ('84), independently reached at the same time by Strasburger,
KolUker, and Weismann, that the nucleus is the most essential ele-
ment concerned in hereditary transmission.

This conclusion received a strong support in the year 1883, through
the splendid discoveries of Van Beneden on the fertilization of the
thread-worm, Ascaris megalocepJiala, the ^gg of which has since ranked
with that of the echinoderm as a classical object for the study of cell-
problems. Van Beneden's researches especially elucidated the struc-
ture and transformations of the germ-nuclei, and carried the analysis
of fertilization far beyond that of Hertwig. In Ascaris, as in all
other animals, the sperm-nucleus is extremely minute, so that at first
sight a marked inequality between the two sexes appears to exist
in this respect. Van Beneden showed not only that the inequality in
size totally disappears during fertilization, but that the two nuclei
undergo a parallel series of structural changes which demonstrate
their precise morphological equivalence down to the minutest detail ;
and here, again, later researches, foremost among them those of
Boveri, Strasburger, and Guignard, have shown that, essentially, the
same is true of the germ-cells of other animals and of plants. The
facts in Ascaris (variety bivalens) are essentially as follows (Fig.
90) : After the entrance of the spermatozoon, and during the for-
mation of the polar bodies, the sperm-nucleus rapidly enlarges and
finally forms a typical nucleus exactly similar to the egg-nucleus.
The chromatin in each nucleus now resolves itself into two long,
worm-like chromosomes, which are exactly similar in form, size, and
staining-reaction in the two nuclei. Next, the nuclear membrane
fades away, and the four chromosomes lie naked in the egg-substance.
Every trace of sexual difference has now disappeared, and it is
impossible to distinguish the paternal from the maternal chromo-
somes (Fig. 90, D, E). Meanwhile an amphiaster has been devel-
oped which, with the four chromosomes, forms the mitotic figure for
the first cleavage of the ovum, the chromatic portion of which has
been synthetically formed by the union of two equal germ-imclei. The
later phases follow the usual course of mitosis. Each chromosome
splits lengthwise into equal halves, the daughter-chromosomes are
transported to the spindle-poles, and here they give rise, in the usual
manner, to the nuclei of the two-celled stage. Each of these nuclei,
therefore, receives exactly equal amounts of paternal and maternal
chromatin.

PRELIMINARY GENERAL SKETCH

183

E F

Fig. 90. —Fertilization of the egg of As car is megalocephala, var. bivalens. [BOVERI.] (For
later stages see Figs. 31, 145.)

A. The spermatozoon has entered the &^g, its nucleus is shown at cf ; beside it lies the granu-
lar mass of " archoplasm " (attraction-sphere) ; above are the closing phases in the formation
of the second polar body (two chromosomes in each nucleus). B. Germ-nuclei (?, cT) in the
reticular stage; the attraction-sphere {a) contains the dividing centrosome. C. Chromosomes
forming in the germ-nuclei ; the centrosome divided. D. Each germ-nucleus resolved into two
chromosomes; attraction-sphere {a) double. E. Mitotic figure forming for the first cleavage;
the chromosomes {c) already split. F. First cleavage in progress, showing divergence of the
daughter-chromosomes toward the spindle-poles (only three chromosomes shown).

1 84

FERTILIZATION OF THE OVUM

These discoveries were confirmed and extended in the case of
Ascaris by Boveri and by Van Beneden himself in 1887 and 1888
and in several other nematodes by Carnoy in 1887. Carnoy found
the number of chromosomes derived from each sex to be in Coronida
4, in Ophiostomuin 6, and in Filaroides 8. A little later Boveri
('90) showed that the law of numerical equality of the paternal and
maternal chromosomes held good for other groups of animals, being
in the sea-urchin Echinus 9, in the worm Sagitta 9, in th*e medusa
Tiara 14, and in the mollusk PterotracJiea 16 from each sex. Similar
results were obtained in other animals and in plants, as first shown by
Guignard in the lily ('91), where each sex contributes 12 chromosomes.

A B

Fig. 91. — Germ-nuclei and chromosomes in the eggs of nematodes. [Carnoy.]
A. Egg- of nematode parasitic in Scyllium ; the two germ-nuclei in apposition, each containing
four chromosomes; the two polar bodies above. B. Egg of Filaroides ; each germ-nucleus with
eight chromosomes ; polar bodies above, deutoplasm-spheres below.

In the onion the number is 8 (Strasburger); in the annelid Ophryo-
trocha it is only 2 from each sex (Korschelt). In all these cases the
number contributed by each is one-half the number characteristic of the
body-cells. The union of two germ-cells thus restores the normal
num.ber, and here we find the explanation of the remarkable fact
commented on at page 67 that the number of chromosomes in sexually
produced organisms is alzvays even}

These remarkable facts demonstrate the two germ-nuclei to be in
a morphological sense precisely equivalent, and they not only lend
very strong support to Hertwig's identification of the nucleus as the
bearer of hereditary qualities, but indicate further that these qualities

1 Cf. p. 67.

PRELIMINARY GENERAL SKETCH

185

must be carried by the chromosomes ; for their precise equivalence in
number, shape, and size is the physical correlative of the fact that
the two sexes play, on the whole, equal parts in hereditary transmis-
sion.

2. The AcJu'oniatic Strtictiires in Fertilization

It is generally agreed that the amphiaster of the primary mitotic
figure of the fertihzed ovum arises from the egg-substance precisely

D

Fig. 92. — Maturation and fertilization of the egg of the mouse. [SOBOTTA.]
A. The ovarian egg still surrounded by the follicle-cells and the membrane {z.p. zona pel-
lucida) ; the polar spindle formed. B. Egg immediately after entrance of the spermatozoon
(sperm-nucleus at cf). C. The two germ-nuclei (j", 9) still unequal; polar bodies above.
D. Germ-nuclei approaching, of equal size. E. The chromosomes forming, F. The minute
cleavage-spindle in the centre ; on either side the paternal and maternal groups of chromosomes.

as in the ordinary mitosis of tissue-cells, and its mode of origin there-
fore involves the same questions as those already discussed at page 72.
It is quite otherwise with the centrosomes at the astral centres, the

1 86

FERTILIZATION OF THE OVUM

origin of which still remains one of the most difficult, as it is one of
the most interesting, problems relating to fertilization.

After the formation of the polar bodies, the egg-nucleus is recon-
stituted near the upper pole of the ^gg, and the entire polar mitotic
apparatus disappears. In the meantime a new astral system (sperm-

^m.

y

B

Fig- 93- — Fertilization of the ^gg of the gasteropod, Pterotrachea. \ BOVERI.]
A. The egg-nucleus {E) and sperm-nucleus {S) approaching after formation of the polar
bodies; the latter shown above (/*. B.) ; each germ-nucleus contains sixteen chromosomes; the
sperm-amphiaster fully developed. B. The mitotic figure for the first cleavage nearly established;
the nuclear membranes have disappeared, leaving the maternal group of chromosomes above the
spindle, the paternal below it.

PRELIMINARY GENERAL SKETCH

187

aster or amphiaster) is developed in the neighbourhood of the sperm-
nucleus, and this in a large number of cases gives rise or is definitely
related to the cleavage-
amphiaster (coelente-
rates, flat-worms, echi-

noderms,

annelids,

mollusks

tebrates).

these

aster,

nematodes,

arthropods,

tunicates, ver-

In many of

cases the sperm-

which by divi-

.'"■•Xv.

%r

^v..

sion gives rise to the
amphiaster, has been
found to arise in inti-
mate relation with the
middle -piece of the
spermatozoon ; e.g. in
echinoderms(Flemming,
Hertwig, Boveri, Wil-
son, Mathews, Hill, etc.),
in the axolotl(Fick) and
salamander (Michaelis),
in the tunicates (Hill),
annelids (Foot, Vejdov-
sky), insects (Henking),
nematodes (Meyer, Er-
langer), and mollusks
(Henking, Kostanecki,
and Wierzejski). The
agreement between
forms so diverse is very
strong evidence that this
is a very general phe-
nomenon, and it is one of
great interest, owing to
the fact that the middle-
piece is itself derived
from or contains the
centrosome of the sper-
matid. ^

The facts may be il-
lustrated by a brief
description of the phe-

Fig. 94. — Entrance and rotation of the sperm-head and
formation of the sperm-aster in the sea-urchin, Toxopneustes
(A-F, X 1600 ; G, H,X 800).

A. Sperm-head before entrance; n. nucleus; m. mid-
dle-piece and part of the flagellum. B. C. Immediately
after entrance, showing entrance-cone. D. Rotation of the
sperm-head, formation of the sperm-aster about the middle-
piece. E. Casting off of middle-piece; centrosome at focus
of the rays {cf. Fig. 12). The changes figured occupy about
eight minutes. F. G. Approach of the germ-nuclei ; growth
of the aster.

1 Cf. p. 170.

1 88 FERTILIZATION OF THE OVUM

nomena in the sea-urchin Toxopneiistes (Fig. 94). As described at
page 197, the tail is in this case left outside, and only the head and
middle-piece enter the ^gg. Within a few minutes after its entrance,
and while still very near the periphery, the lance-shaped sperm-head,
carrying the middle-piece at its base, rotates through nearly or quite
180°, so that the pointed end is directed outward and the middle-
piece is turned inward (Fig. 94, A-F)} During or shortly after the
rotation appears a minute aster centring in or very near the middle-
piece. As it enlarges, the middle-piece itself is thrown to one side
(Fig. 12), where it soon degenerates, while in the centre of the aster
a minute intensely staining centrosome may be seen. Both sperm-
nucleus and aster now rapidly advance toward the centre of the Q,gg,
the aster leading the way and its rays extending far out into the
cytoplasm and finally traversing nearly an entire hemisphere. The
central mass of the aster comes in contact with the egg-nucleus,
divides into two, and the daughter-asters pass to opposite poles of
the egg-nucleus, while the sperm-nucleus flattens against the latter
and assumes the form of a biconvex lens (Fig. 95). The nuclei now
fuse to form the cleavage-nucleus. Shortly afterward the nuclear
membrane fades away, a spindle is developed between the asters, and
a group of chromosomes arises from the cleavage-nucleus. These
are 36 or 38 in number; and although their relation to the paternal
and maternal chromatin cannot in this case be accurately traced,
owing to the apparent fusion of the nuclei, there can be no doubt on
general grounds that one-half have been derived from each germ-
nucleus. The '^gg then divides into two, four, etc., by ordinary
mitosis (Figs. 4, 52).

In the type of fertiUzation just described, the polar bodies are
formed long before the entrance of the spermatozoon and the germ-
nuclei conjugate immediately upon entrance of the spermatozoon,
fusing to form a true cleavage-nucleus. In a second and more
frequent type (Ascaris, Fig. 90; Physa, Fig. 89; Nereis, Fig. 97;
Cyclops, Fig. 98) the sperm-nucleus penetrates for a certain distance,
often to the centre of the egg, and then pauses while the polar
bodies are formed. It then conjugates with the re-formed egg-
nucleus. In this case the sperm-aster always divides to form an
amphiaster before conjugation of the nuclei, while in the first case
the aster may be still undivided at the time of union. This
difference is doubtless due merely to a difference in the time
elapsing between entrance of the spermatozoon and conjugation
of the nuclei, the amphiaster having, in the second case, time to

1 The first, as far as I know, to observe the rotation of the sperm-head was Flemming
in the echinoderm-egg ('8i, pp. 17-19). It has since been clearly observed in several other
cases, and is probably a phenomenon of very general occurrence.

PRELIMINARY GENERAL SKETCH

189

form during extrusion of the polar bodies. The two types just
described (Fig. 96) are connected by various gradations. Thus,
in the lamprey, the frog, the rabbit, and in Ainphioxtis, one polar
body is expelled before, and one after, the entrance of the sper-
matozoon ; in the annelid OpJiryotrocha, entrance takes place when
the first polar spindle is in the stage of the equatorial plate ;

/

Zpli^./

^ig- 95' — Conjugation of the germ-nuclei and division of the sperm-aster in the sea-urchin
Toxopneustes, X looo. (For later stages see Fig. 52.)

A. Union of the nuclei ; extension of the aster. B. Flattening of the sperm-nucleus against the
egg-nucleus ; division of the aster.

I90

FERTILIZATION OF THE OVUM

while in Chcetopterus and Pieris the first polar spindle has advanced
into the anaphase.^

It is an interesting and significant fact that the aster or amphiaster
always leads the way in the march toward the egg-nucleus ; and in
many cases it may be far in advance of the sperm-nucleus.^ Boveri
i^'^Jy i) has observed in sea-urchins that the sperm-nucleus may indeed
be left entirely behind, the aster alone conjugating with the egg-

Fig. 90. — Diagrams of two principal types of ler.ilization. /. Polar bodies formed after the
entrance of the spermatozoa (annelids, mollusks, fiat-worms). //. Polar bodies formed before
entrance (echinoderms).

A. Sperm-nucleus and centrosome at d" ; first polar body forming at $ . B. Polar bodies
formed ; approach of the nuclei. C. Union of the nuclei. D. Approach of the nuclei. E. Union
of the nuclei. F. Cleavage-nucleus.

nucleus and causing division of the ^gg without union of the germ-
nuclei, though the sperm-nucleus afterward conjugates with one of
the nuclei of the two-cell stage. This process, known as " partial fer-
tilization," is. undoubtedly to be regarded as abnormal. It affords,
however, a beautiful illustration of the view that it is the centro-
some alone that incites division of the egg, and is therefore the fer-
tilizing element proper (Boveri, '^J, 2).

The foregoing facts lead us to a consideration of Boveri's theory
of fertilization, which has for several years formed a central point of
discussion. The ground for this theory had been prepared by Oscar

C/p.

* Cf. Kostanecki and Wierzejski, '96.

PRELIMINARY GENERAL SKETCH

191

Hertwig and Fol. The latter ('73) early reached the conclusion that
the asters represented ** centres of attraction " lying outside and
independent of the nucleus. Oscar Hertwig showed, in 1875, that

Fig. 97. — f'ertilization of the ^gg of Nereis, from sections. ( X 400.)
A. Soon after the entrance of the spermatozoon, showing the minute sperm-nucleus at cf , the
germinal vesicle disappearing, and the first polar mitotic figure forming. The empty spaces repre-
sent deutoplasm-spheres (slightly swollen by the reagents) , the firm circles oil-drops. B. Sperm-
nucleus (cf) advancing, a minute amphiaster in front of it; first polar mitotic figure established;
polar concentration of the protoplasm. C. Later stage; second polar body forming. D. The
polar bodies formed; conjugation of the germ-nuclei; the egg-centrosomes and asters have
disappeared, leaving only the sperm-amphiaster {cf. Fig. 60).

in the sea-urchin ^%g, the amphiaster arises by the division of a
single aster that first appears near the sperrn-nucleus and accompanies
it in its progress toward the egg-nucleus. A similar observation was
soon afterward made by Fol ('79) in the eggs of Asterias and
Sacritta, and in the latter case he determined the fact that the astral

192 FERTILIZATION OF THE OVUM

rays do not centre in the nucleus, as Hertwig described, but at a
point in advance of it — a fact afterward confirmed by Hertwig
himself and by Boveri {^d>^, i). Hertwig and Fol afterward found
that in cases of polyspermy, when several spermatozoa enter the Qgg,
each sperm-nucleus is accompanied by an aster, and Hertwig proved
that each of these might give rise to an amphiaster (Fig. loi). In
1886-87 Vejdovsky brought forward strong evidence to show that
in the fresh-water annelid Rhynchelmis the cleavage-amphiaster
arises directly from the sperm-amphiaster, itself derived by the
division of a " periplast " (attraction-sphere) imported into the ^gg by
the spermatozoon, while the polar amphiaster entirely disappears.
It was Boveri i^'^J, 2) who first carefully studied the facts with
reference to the centrosome, reaching the conclusion (in the case of
Ascaris and the sea-urchin) that a single centrosome is brought
in by the spermatozoon, and that it divides to form two centres about
which are developed the two asters of the cleavage-figure. He was
thus led to the following conclusion, which has received the sup-
port of many later investigators : The ripe egg possesses all of the
organs arid qualities necessary for division excepting the centrosome,
by which division is initiated. The spermatozoon, on the other hand,
is provided zvith a centrosome, but lacks the substance in which this
organ of division may exert its activity. ThrougJi the union of the
two cells in fertilization, all of the essential organs necessary for
division are brojight together ; the egg nozv contains a centrosome
zvhich by its own division leads the zvay in the embryonic develop-
ment} Very numerous observations, supporting this conclusion, have
been made by later observers. Bohm could find in Petroniyzon i^'^^)
and the trout ('91) no radiations near the egg-nucleus after the
formation of the polar-bodies, while a beautiful sperm-aster is devel-
oped near the sperm-nucleus and divides to form the amphiaster.
Platner {^"^6) had already made similar observations in the snail
Arion, and the same result was soon afterward reached by Brauer
('92) in the case of Branchipus, and by Julin ('93) in Styleopsis,
Fick's careful study of fertilization of the axolotl ('93) proved in
a very convincing manner not only that the amphiaster is a product
of the sperm-aster, but also that the latter is developed about the
middle-piece as a centre. The same result was indicated by Foot's
observations on the earthworm ('94), and it was soon afterward
conclusively demonstrated in echinoderms through the independent
and nearly simultaneous researches of myself on the ^gg of Toxo-
pneustes, of Mathews on Arbacia, and of Boveri on Echinus. Nearly
at the same time a careful study was made by Mead ('95, '98, i) of
the annelid Chcetopterus^ and of the starfish Asterias by Mathews,

1 '87, 2, p. 155.

PRELIMINARY GENERAL S FETCH

193

both observers independently showing that the polar spindle contains
distinct centrosomes, which, however, degenerate after the formation
of the polar bodies, their place being taken by the sperm-centrosome,
which divides to form an amphiaster before union of the nuclei, as
in Rhynchelrnis. Exactly the same result has since been reached by
Hill ('95) and Reinke ('95) in Sphcerechmiis, by Hill in the tunicate
Pkalltisia, by Kostanecki and Wierzejski ('96) in Physa (Fig. 89),
and by Van der Stricht ('98) in Thysanozoon ; and in all of these the
centrosome is likewise shown to arise from the middle-piece or in
its immediate neighbourhood. Among others who have produced

Fig. 98. — Fertilization of the egg in the copepod, Cyclops strenuus. [RiJCKERT,]
A. Sperm-nucleus soon after entrance, the sperm-aster dividing. B. The germ-nuclei ap-
proaching; cf, the enlarged sperm-nucleus with a large aster at each pole; 9, the egg-nucleus
re-formed after formation of the second polar body, shown at the right. C The apposed reticular
germ-nuclei, now of equal size; the spindle is immediately afterward developed between the two
enormous sperm-asters ; polar body at the left.

evidence that the cleavage-centrosome stands in definite relation to
the spermatozoon, may be mentioned Oppel ('92) in reptiles, Brauer
('92) in Bi-anchipiis, Henking ('92) in insects, Riickert ('95, 2) in
Cyclops, Sobotta ('95) in the mouse and ('98) AmpJiioxus, Ziegler ('95)
in Diplogaster and Rhabditis, Castle ('96) in Ciona, Korschelt
('95) in Ophryotrocha, Meyer ('95) in Stroiigylus, Griffin ('96, '99)
in TJialassema, and Coe ('98) in Cerebratiihis.

Beside the foregoing evidence may be placed the following addi-
tional data based on experiment and the study of pathological fer-
tilization, (i) In the case of sea-urchin eggs, Hertwig, Boveri, and

194 FERTILIZATION OF THE OVUM

several later observers have shown that egg-fragments, obtained by
shaking eggs to pieces, are readily penetrated by the spermatozoa,
and that such fragments, though containing no nuclear matter from
the ^gg. may segment and give rise to perfect larvae.^ (2) Boveri
('88) has observed that in ordinary fertilization the sperm-aster may
separate from the sperm-nucleus, travel through the cytoplasm to the
egg-nucleus and cause cleavage, the sperm-nucleus afterward fusing
with one of the nuclei of the two-cell stage (*' partial fertilization ").
(3) Most remarkable of all, Boveri, confirmed by Ziegler ('98), has
recently observed that during the first cleavage the whole of the
chromatin may pass to one pole, so that upon division one of the
halves of the ^%g receives only a centrosome without a nucleus. In
the nucleated half cleavage proceeds as usual. In the enucleated
half the centrosomes and asters continue for a considerable period
to multiply at the same rate as the cleavage of the nucleated half,
though the cell-body does not itself divide.^ Putting these facts
together we must conclude (i) that something is introduced into the
^ZZ by the middle-piece of each spermatozoon entering it that is
either a centrosome or has the power to incite the formation of one ;
(2) that the centrosome thus arising is structurally independent of
both nuclei and may divide independently of them ; (3) that indepen-
dently of the division of the nucleus or cell-body there is some kind
of historical continuity between the centrosomes of successive genera-
tions.

In the case of echinoderm-eggs this continuity is not yet known to
be effected by actual persistence of the centrosomes.^ There are,
however, a number of cases in which the division of the primary
cleavage-centrosomes and the persistence of their descendants as
those of the daughter-cells seem to have been conclusively shown —
for example on Ascaris (Van Beneden, Boveri, Kostanecki, and Sied-
lecki), in the trout (Henneguy, '96), in Thalassema (Griffin, '96, '99),
in Chcetopterns {MQ2id, '95, '98), in Physa (Kostanecki and Wierzejski,
'96), in Cerebratuliis (Coe, '98), and in Rhynchelmis (Vejdovsky and
Mrazek, '98). In Thalassema and Cerebratidns (Figs. 99, 155) the
centrosome is a minute granule at the focus of the sperm-aster,
which divides to form an amphiaster soon after the entrance of the
spermatozoon. During the early anaphase of the first cleavage, each
centrosome divides into two, passes to the outer periphery of the
centrosphere, and there forms a minute amphiaster for the second

1 Cf. p. 353. ' Cf. p. 108.

^ Erlanger's statement ('98) that the centrosomes persist through the first cleavage in
echinoderm-eggs is not supported by his figures ; and I am convinced from my own long-
continued studies of these eggs, as well as by an examination of Erlanger's preparations,
kindly placed in my hands by Professor Biitschli, that these difficult objects are very unfavour-
able for a decision of the question.

PRELIMINARY GENERAL SKETCH

195

cleavage before the first cleavage takes place. The minute centro-
somes of the second cleavage are therefore the direct descendants of
the sperm-centrosome ; and there is good reason to believe that the
continuity is not broken in later stages. The facts are nearly similar

^

I

• • ••

A ' :

• • .♦«

. - •

•3U'

♦•- ' •

/

C

• • • • r.

• • • 'X

H ^^

Fig. 99. — Fertilization in an annelid (armed Gephyrean), Thalassema. [Griffin.]

A. Second polar body forming ; sperm-nucleus and centrosome below. B. Approach of the
egg-nucleus and sperm-nucleus, the latter accompanied by the sperm-am phiaster. C. Union of
the nuclei. D. Later stage of last, E. Prophase of cleavage-spindle. /^ Anaphase of the same ;
centrosome divided. G. H. I. Successive stages in the nuclear reconstitution and formation of
the daughter-amphiasters for the second cleavage, jf. Two-cell stage.

in the trout, in Chcetopterus , and in Physa. In Ascaris division of
the centrosome first occurs at a somewhat later period (Figs. 90, 176).
If now the centrosomes were indeed permanent cell-organs, we
should thus reach the following result : During cleavage the cytoplasm
of the blastomeres is derived from that of the eggy the centrosom-es from

196 ■ FERTILIZATION OF THE OVUM

the Spermatozoon^ while the mtclei {chi'omatifi) are equally derived from
both germ-cells.

There is very strong reason to accept the first part of this con-
clusion (applying to nucleus and cytoplasm), but the question of
the centrosomes remains an open one. The array of evidence given
above, derived from the study of so many diverse groups, seems to
place Boveri's lucid and enticing hypothesis upon a strong foundation.
Two essential points still remain, however, to be determined : first,
whether the facts observed in Ascaris, Echinoderms, Physa, Thalas-
sema, and the like, are typical of all forms of fertilization ; and, second,
whether, if so, the primary cleavage-centrosome is actually imported
into the Qgg by the spermatozoon or is only formed under its influence
out of the egg-substance. Roth these questions have been raised by
recent investigators, apparently on good evidence, and some of this
evidence is directly opposed to both of the principal assumptions of
Boveri's theory. Thus, Wheeler ('97) has found that in Myzostouia
both centrosomes are derived from the Qgg', Carnoy and Le Brun
('97) maintain that in Ascaris one centrosome is derived from each of
the germ-nuclei; in some moUusks, according to MacFarland ('97)
and LiUie ('97), both egg-centrosomes and sperm-centrosomes dis-
appear, to be replaced by two centrosomes of unknown origin ; while
recent botanical workers are unable to find any centrosomes in fertili-
zation. These and other divergent results will be critically considered
beyond (p. 208) in connection with a more detailed examination of
the general subject. It may be pointed out here, however, that
recent researches on spermatogenesis (p. 170) render it nearly certain
that the centrosome of the sperm-aster cannot be the unmodified cen-
trosome of the spermatid, since the latter, in some cases, enlarges to
form a ** middle-piece " or analogous structure that is far larger than
the sperm-centrosome.

B. Union of the Germ-cells

It does not lie within the scope of this work to consider the
innumerable modes by which the germ-cells are brought together,
further than to recall the fact that their union may take place inside
the body of the mother or outside, and that in the latter case both
eggs and spermatozoa are as a rule discharged into the water, where
fertilization and development take place. The spermatozoa may
live for a long period, either before or after their discharge, without
losing their fertilizing power, and their movements may continue
throughout this period. In many cases they are motionless when
first discharged, and only begin their characteristic swimming move-
ments after coming in contact with the water. There is clear evi-

UNION OF THE GERM-CELLS

197

dence of a definite attraction between the germ-cells, which is
in some cases so marked (for example in the polyp Renilla) that
when spermatozoa and ova are mixed in a small vessel, each ovum
becomes in a few moments surrounded by a dense fringe of sperma-
tozoa attached to its periphery by their heads and by their move-
ments actually causing the ovum to move about. The nature of the
attraction is not positively known, but Pfeffer's researches on the
spermatozoids of plants leave little doubt that it is of a chemical
nature, since he found the spermatozoids of ferns and of Selaginella
to be as actively attracted by solutions of malic acid or malates (con-
tained in capillary tubes) as by the substance extruded from the

a

H

O

Fig. 100. — Entrance of the spermatozoon into the egg. A-G. In the sea-urchin, Toxopneustes.
H. In the medusa, Mitrocoma. [MetscHNIKOFF.] /. In the star-fish Asterias. [FOL.]

A. Spermatozoon of Toxopneustes, X 2000; a. the apical body, n. nucleus, m. middle-piece,
/ fiagellum. B. Contact with the egg-periphery. C. D. Entrance of the head, formation of the
entrance-cone and of the vitelline membrane \y), leaving the tail outside. E- F. Later stages.
G. Appearance of the sperm-aster {s) about 3-5 minutes after first contact; entrance-cone break-
ing up. H. Entrance of the spermatozoon into a preformed depression. /. Approach of the
spermatozoon, showing the preformed attraction-cone.

neck of the archegonium. Those of mosses, on the other hand, are
indifferent to malic acid, but are attracted by cane-sugar. These
experiments indicate that the specific attraction between the germ-
cells of the same species is owing to the presence of specific chemical
substances in each case. There is clear evidence, furthermore, that
the attractive force is not exerted by the egg-nucleus alone, but by
the egg-cytoplasm ; for, as the Hertwigs and others have shown,
spermatozoa will readily enter egg-fragments entirely devoid of a
nucleus.

In naked eggs, such as those of some echinoderms, and coelen-
terates, the spermatozoon may enter at apy point ; but there are
some cases in which the point of entrance is predetermined by the

198 FERTILIZATION OF THE OVUM

presence of special structures through which the spermatozoon
enters (Fig. 100). Thus, the starfish-egg, according to Fol, pos-
sesses before fertiUzation a peculiar protoplasmic '' attraction-cone "
to which the head of the spermatozoon becomes attached, and through
which it enters the ^gg. In some of the hydromedusae, on the other
hand, the entrance point is marked by a funnel-shaped depression at
the egg-periphery (Metschnikoff). When no preformed attraction-
cone is present, an " entrance-cone " is sometimes formed by a rush
of protoplasm toward the point at which the spermatozoon strikes
the ^g'g and there forming a conical elevation into which the sperm-
head passes. In the sea-urchin (Fig. 100) this structure persists
only a short time after the spermatozoon enters, soon assuming a
ragged flame-shape and breaking up into slender rays. In some
cases the ^g'g remains naked, even after fertilization, as appears to
be the case in many coelenterates. More commonly a vitelline mem-
brane is quickly formed after contact of the spermatozoon, — e.g.
in Amphioxus, in the echinoderms, and in many plants, — and by
means of this the entrance of other spermatozoa is prevented. In
eggs surrounded by a membrane before fertilization, the spermato-
zoon either bores its way through the membrane at any point, as is
probably the case with mammals and Amphibia, or may make its
entrance through a micropyle.

In some forms only one spermatozoon normally enters the ovum,
as in echinoderms, mammals, many annelids, etc., while in others
several may enter (insects, elasmobranchs, reptiles, the earthworm,
Petromyson, etc.). In the former case more than one spermatozoon
may accidentally enter (pathological polyspermy), but development
is then always abnormal. In such cases each sperm-centrosome
gives rise to an amphiaster, and the asters may then unite to form
the most complex polyasters, the nodes of which are formed by the
centrosomes (Fig. lOi). Such eggs either do not divide at all or
undergo an irregular multiple cleavage and soon perish. If, how-
ever, only two spermatozoa enter, the Qgg may develop for a time.
Thus Driesch has determined the interesting fact, which I have con-
firmed, that sea-urchin eggs into which two spermatozoa have acci-
dentally entered undergo a double cleavage, dividing into four at the
first cleavage, and forming eight instead of four micromeres at the
fourth cleavage. Such embryos develop as far as the blastula stage,
but never form a gastrula.^ In cases where several spermatozoa
normally enter the Qgg (physiological polyspermy), only one of the
sperm-nuclei normally unites with the egg-nucleus, the supernumer-
ary sperm-nuclei either degenerating, or in rare cases — e.g. in elas-
mobranchs and reptiles — living for a time and even dividing to form

1 For an account of the internal changes, see p. 355.

UNION OF THE GERM-CELLS

199

"merocytes" or accessory nuclei. The fate of the latter is still in
doubt ; but they certainly take no part in fertilization.

It is an interesting question how the entrance of supernumerary
spermatozoa is prevented in normal monospermic fertilization. In
the case of echinoderm-eggs Fol advanced the view that this is
mechanically effected by means of the vitelline membrane formed
instantly after the first spermatozoon touches the ^%^,. This is indi-
cated by the following facts. Immature eggs, before the formation

Fig. 101. — Pathological pwlyspermy.

A. Polyspermy in the egg of Ascaris ; below, the egg-nucleus ; above, three entire spermatozoa
writhin the egg. [Sala.]

B. Polyspermy in sea-urchin egg treated with 0.005% nicotine solution ; ten sperm-nuclei
shown, three of which have conjugated with the egg-nucleus. C. Later stage of an egg similarly
treated, showing polyasters formed by union of the sperm-amphiasters. [O. and R. Hertwig.]

of the polar bodies, have no power to form a vitelline membrane^
and the spermatozoa always enter them in considerable numbers.
Polyspermy also takes place, as O. and R. Hertwig's beautiful ex-
periments showed ('87), in ripe eggs whose vitality has been dimin-
ished by the action of dilute poisons, such as nicotine, strychnine,
and morphine, or by subjection to an abnormally high temperature

200 • FERTILIZATION OF THE OVUM

(31° C); and in these cases the vitelline membrane is only slowly
formed, so that several spermatozoa have time to enter.^ Similar
mechanical explanations have been given in various other cases.
Thus Hoffman believes that in teleosts the micropyle is blocked b/
the polar bodies after the entrance of the first spermatozoon ; ani
Calberla suggested {Petromyzofi) that the same result might be
caused by the tail of the entering spermatozoon. It is, however,
far from certain whether such rude mechanical explanations are
adequate ; and there is considerable reason to believe that the Q,gg
may possess a physiological power of exclusion called forth by the
first spermatozoon. Thus Driesch found that spermatozoa did not
enter fertilized sea-urchin eggs from which the membranes had been
removed by shaking.^ In some cases no membrane is formed (some
coelenterates), in others several spermatozoa are found inside the
membrane (nemertines), in others the spermatozoon may penetrate
the membrane at any point (mammals), yet monospermy is the rule.

I. Immediate Results of Union

The union of the germ-cells calls forth profound changes in both.

{a) The Spermatozoon. — Almost immediately after contact the tail
ceases "its movements. In some cases the tail is left outside, being
carried away on the outer side of the vitelline membrane, and only
the head and middle-piece enter the ^gg (echinoderms. Fig. 100).
In other cases the entire spermatozoon enters (amphibia, earthworm,
insects, etc.. Fig. 89), but the tail always degenerates within the
ovum and takes no part in fertilization. Within the ovum the
sperm-nucleus rapidly grows, and both its structure and staining-
capacity rapidly change {cf. p. 182). The most important and signifi-
cant result, however, is an immediate resumption by the spei'm-nucleus
and sperm-centrosome of the power of division, which has hitherto
been suspended. This is not due to the union of the germ-nuclei ;
for, as the Hertwigs and others have shown, the supernumerary
sperm-nuclei in polyspermic eggs may divide freely without copu-
lation with the egg-nucleus, and they divide as freely after entering
enucleated egg-fragments. The stimulus to division must therefore
be given by the egg-cytoplasm. It is a very interesting fact that
in some cases the cytoplasm has this effect on the sperm-nucleus

1 The Hertwigs attribute this to a diminished irritability on the part of the egg-substance.
Normally requiring the stimulus of only a single spermatozoon for the formation of the vitel-
line membrane, it here demands the more intense stimulus of two, three, or more before the
membrane is formed. That the membrane is not present before fertilization is admitted by
Hertwig on the ground stated at page 132.

2 On the other hand, Morgan states ('95, 5, p. 270) that one or more spermatozoa will
enter nucleated or enucleated egg-fragments whether obtained before or after fertilization.

UNION OF THE GERM-CELLS 201

only after formation of the polar bodies ; for when in sea-urchins the
spcmatozoa enter immature eggs, as they freely do, they penetrate
but a short distance, and no further change occurs.

[h) The Ovum. — The. entrance of the spermatozoon produces an
extraordinary effect on the egg, which extends to every part of its
organization. The rapid formation of the viteUine membrane, already
described, proves that the stimulus extends almost instantly through-
out the whole ovum.^ At the same time the physical consistency
of the cytoplasm may greatly alter, as for instance in echinoderm
eggs, where, as Morgan has observed, the cytoplasm assumes immedi-
ately after fertilization a peculiar
viscid character which it afterward
loses. In many cases the egg con-
tracts, performs amoeboid movements,
or shows wave-like changes of form.
Again, the egg-cytoplasm may show
active streaming movements, as in
the formation of the entrance-cone in
echinoderms, or in the flow of periph-
eral protoplasm toward the region
of entrance to form the germinal
disc, as in many pelagic fish-eggs.
An interesting phenomenon is the
formation, behind the advancing

sperm-nucleus, of a peculiar funnel- sm^ ^ h r

shaped mass of deeply staining pig. 102. - Egg of the leech a./.^^^
material extending outward to the during fertilization, [whitman.]
periphery. This has been carefully M poiar bodies; /.;. polar rings;

, -111 T- ^ /> \ • ^1 ^1 cleavage-nucleus near the centre.

described by Foot ( 94) m the earth-
worm, where it is very large and conspicuous, and I have since ob-
served it also in the sea-urchin (Fig. 94).

The most profound change in the ovum is, however, the migration
of the germinal vesicle to the periphery and the formation of the
polar bodies. In many cases either or both these processes may occur
before contact with the spermatozoon (echinoderms, some vertebrates).
In others, however, the egg awaits the entrance of the spermatozoon
(annelids, gasteropods, etc.), which gives it the necessary stimulus.
This is well illustrated by the egg of Nereis. In the newly dis-
charged egg the germinal vesicle occupies a central position, the
yolk, consisting of deutoplasm-spheres and oil-globules, is uniformly
distributed, and at the periphery of the egg is a zone of clear peri-
vitelline protoplasm (Fig. 60). Soon after entrance of the sperma-

1 I have often observed that the formation of the membrane, in Toxopnetistes, proceeds
like a wave from the entrance-point around the periphery, but this is often irregular.

202 FERTILIZATION OF THE OVUM

tozoon the germinal vesicle moves toward the periphery, its membrane
fades away, and a radially directed mitotic figure appears, by means
of which the first polar body is formed (Fig. 97). Meanwhile the
protoplasm flows toward the upper pole, the peri-vitelline zone disap-
pears, and the ^gg now shows a sharply marked polar differentiation.
A remarkable phenomenon, described by Whitman in the leech {^7'^\
and later by Foot in the earthworm ('94), is the formation of " polar
rings," a process which follows the entrance of the spermatozoon
and accompanies the formation of the polar bodies. These are two
ring-shaped cytoplasmic masses which form at the periphery of the
^gg near either pole and advance thence toward the poles, the upper
one surrounding the point at which the polar bodies are formed
(Fig. 102). Their meaning is unknown, but Foot ('96) has made
the interesting discovery that they are probably of the same nature
as the yolk-nuclei (p. 1 56).

2. Paths of the Germ-nuclei {Pro-nuclei) ^

After the entrance of the spermatozoon, both germ-nuclei move
through the egg-cytoplasm and finally meet one another. The paths
traversed by them vary widely in different forms. In general two
classes are to be distinguished, according as the polar bodies are
formed before or after entrance of the spermatozoon. In the former
case (echinoderms) the germ-nuclei unite at once. In the latter case
the sperm-nucleus advances a certain distance into the o^gg and then
pauses while the germinal vesicle moves toward the periphery, and
gives rise to the polar bodies {Ascaris, annelids, etc.). This signifi-
cant fact proves that the attractive force between the two nuclei is
only exerted after the formation of the polar bodies, and hence that
the entrance-path of the sperm-nucleus is not determined by such
attraction. A second important point, first pointed out by Roux, is
that the path of the sperm-nucleus is curved, its "entrance-path"
into the ^gg forming a considerable angle, with its " copulation-path "
toward the egg-nucleus.

These facts are well illustrated in the sea-urchin ^gg (Fig. 103),
where the egg-nucleus occupies an eccentric position near the point
at which the polar bodies are formed (before fertilization). Entering

1 The terms female pro-nucleus, male pro-nucleus (Van Beneden), are often applied
to the germ-nuclei before their union. These should, I think, be rejected in favour of
Hertwig's terms egg-nucleus and sper;n-nucleus, on two grounds: (i) The germ-nuclei are
true nuclei in every sense, differing from the somatic nuclei only in the reduced number of
chromosomes. As the latter character has recently been shown to be true also of the
somatic nuclei in the sexual generation of plants (p. 275), it cannot be made the ground for
a special designation of the germ-nuclei. (2) The germ-nuclei are not male and female
in any proper sense (p. 243).

UNION OF THE GERM-CELLS

203

the ^%% at any point, the sperm-nucleus first moves rapidly inward
along an entrance-path that shows no constant relation to the position
of the egg-nucleus and is approximately but never exactly radial,
i.e. toward a point near the centre of the ^gg. After penetrating a

Fig. 103. — Diagrams showing the paths of the germ-nuclei in four different eggs of the sea-
urchin, Toxopneustes. From camera drawings of the transparent living eggs.

In all the figures the original position of the egg-nucleus (reticulated) is shown at 9 ; the point
at which the spermatozoon enters at E (entrance-cone). Arrows indicate the paths traversed by
the nuclei. At the meeting-point {M^ the egg-nucleus is dotted. The cleavage-nucleus in its
final position is ruled in parallel lines, and through it is drawn the axis of the resulting cleavage-
figure. The axis of the egg is indicated by an arrow, the point of which is turned away from the
micromere-pole. Plane of first cleavage, passing near the entrance-point, shown by the curved
dotted line.

certain distance its direction changes sHghtly to that of the copula-
tion-path, which, again, is directed not precisely toward the egg-
nucleus, but toward a meeting-point where it comes in contact with
the egg-nucleus. The latter does not begin to move until the

204 FERTILIZATION OF THE OVUM

entrance-path of the sperm-nucleus changes to the copulation-path.
It then begins to move slowly in a somewhat curved path toward the
meeting-point, often showing slight amoeboid changes of form as it
forces its way through the cytoplasm. From the meeting-point the
apposed nuclei move slowly toward the point of final fusion, which
in this case is near, but never precisely at, the centre of the ^gg.

These facts indicate that the paths of the germ-nuclei are deter-
mined by at least two different factors, one of which is an attraction
or other dynamical relation between the nuclei and the cytoplasm,
the other an attraction between the nuclei. The former determines
the entrance-path of the sperm-nucleus, while both factors probably
operate in the determination of the copulation-path along which it
travels to meet the egg-nucleus. The real nature of neither factor
is known.

Hertwig first called attention to the fact — which is easy to observe in the living
sea-urchin ^gg — that the egg-nucleus does not begin to move until the sperm-
nucleus has penetrated some distance into the egg and the sperm-aster has attained
a considerable size ; and Conklin ('94) has suggested that the nuclei are passively
drawn together by the formation, attachment, and contraction of the astral rays.
While this view has some facts in its favour, it is, I believe, untenable, for many
reasons, among which may be mentioned the fact that neither the actual paths of
the pro-nuclei nor the arrangement of the rays support the hypothesis ; nor does
it account for the conjugation of nuclei when no astral rays are developed (as in
Protozoa or in plants). I have often observed in cases of dispermy in the sea-urchin,
that both sperm-nuclei move at an equal pace toward the egg-nucleus ; but if one of
them meets the egg-nucleus first, the movement of the other is immediately retarded,
and only conjugates with the egg-nucleus, if at all, after a considerable interval ; and
in polyspermy the egg-nucleus rarely conjugates with more than two sperm-nuclei.
Probably, therefore, the nuclei are drawn together by an actual attraction which is
neutralized by union, and their movements are not improbably of a chemotactic char-
acter. Conklin ('99) has recently suggested that the nuclei are drawn together by
the agency of protoplasmic currents in the egg-substance.

3. Union of the Germ-nuclei. The Chromosomes

The earlier observers of fertilization, such as Auerbach, Stras-
burger, and Hertwig, described the germ-nuclei as undergoing a com-
plete fusion to form the first embryonic nucleus, termed by Hertwig
the cleavage- or scgmcntation-nucleits. As early as 1881, however,
Mark clearly showed that in the slug Limax this is not the case, the
two nuclei merely becoming apposed without actual fusion. Two
years later appeared Van Beneden's epoch-making work on Ascaris,
in which it was shown not only that the nuclei do not fuse, but that
they give rise to two independent groups of chromosomes which
separately enter the equatorial plate and whose descendants pass
separately into the daughter-nuclei. Later observations have given
the strongest reason to believe that, as far as the chromatin is con-

UNION OF THE GERM-CELLS 20$

cerned, a true fusion of the nuclei never takes place during fertili-
zation, and that the paternal and maternal chromatin may remain
separate and distinct in the later stages of development — possibly
throughout life (p. 299). In this regard two general classes may be
distinguished. In one, exemplified by some echinoderms, hy Amp hi-
oxus, Phallus ia, and some other animals, the two nuclei meet each
other when in the reticular form, and apparently fuse in such a manner
that the chromatin of the resulting nucleus shows no visible distinc-
tion between the paternal and maternal moieties. In the other class,
which includes most accurately known cases, and is typically repre-
sented by Ascaris (Fig. 90) and other nematodes, by Cyclops (Fig. 98)^
and by PterotracJiea (Fig. 93), the two nuclei do not. fuse, but only
place themselves side by side, and in this position give rise each to
its own group of chromosomes. On general grounds we may confi-
dently maintain that the distinction between the two classes is only
apparent, and probably is due to corresponding differences in the rate
of development of the nuclei, or in the time that elapses before their
union. 1 If this time be very short, as in echinoderms, the nuclei
unite before the chromosomes are formed. If it be more prolonged,
as in Ascaris, the chromosome-formation takes place before union.

With a few exceptions, which are of such a character as not to
militate against the rule, tJie ntmiber of chromosomes arising from the
germ-nuclei is ahvays the same in both, and is one-half the number
characteristic of the tissue-cells of the species. By their union, there-
fore, the germ-nuclei give rise to an equatorial plate containing the
typical number of chromosomes. This remarkable discovery was first
made by Van Beneden in the case of Ascaris, where the number of
chromosomes derived from each sex is either one or two. It has
since been extended to a very large number of animals and plants, a
partial list of which follows.

i Indeed, Boveri has found that in Ascaris both modes occur, though the fusion of the
germ-nuclei is exceptional. (C/I P- 296.)

206

FERTILIZATION OF THE OVUM

A Partial List showing the Number of Chromosomes Char-
acteristic OF THE Germ-nuclei and Somatic Nuclei in
Various Plants and Animals ^

Germ-
Nuclei.

Somatic
Nuclei.

Name.

Group.

Authority.

I

2

Ascaris megalocephala,
var. univalens.

Nematodes.

Van Beneden,
Boveri.

2

4

Id., var. bivalens.

If

77

>y

??

Ophryotrocha.

Annelids.

Korschelt.

»

D,]

Styleopsis.

Tunicates.

Julin.

4

8

Coronilla.

Nematodes.

Carnoy.

»

??

Pallavicinia.

Hepaticae.

Farmer.

If

>?

Anthoceras.

7?

Davis.

6

12

Spiroptera.

Nematodes.

Carnoy.

»

??

Prostheceraeus.

Polyclades.

Klinckostrom,
Francotte.

>?

?>

Nais.

Phanerogams.

Guignard.

[„]

>?

Spirogyra.

Conjugatae.

Strasburger.

J?

D.]

Gryllotalpa.

Insects.

Vom Rath.

j>

?>

Caloptenus.

77

Wilcox.

DJ

?j

^quorea.

Hydromedusae.

Hacker.

7

14

Pentatoma.

Insects.

Montgomery.

8

i6

Filaroides.

Nematodes.

Carnoy.

?>

[.J

Prosthiostomum.

Polyclades.

Francotte.

)?

[.]

Leptoplana.

7?

7?

»>

D.]

Cycloporus.

7?

77

>j

??

Hydrophilus.

Insects.

Vom Rath.

J)

jj

Phallusia.

Tunicates.

Hill.

>>

7?

Limax.

Gasteropods.

Vom Rath.

»

DJ

Rat.

Mammals.

Moore.

>>

[..]

Ox, guinea-pig, man.

•77

Bardeleben.

>>

??

Ceratozamia.

Cyads.

Overton,
Guignard.

»

?>

Pinus.

Coniferae.

Dixon.

»>

??

Scilla, Triticum.

Angiosperms.

Overton.

»

??

Allium.

77

Strasburger,
Guignard.

»

??

Podophyllum.

77

Mottier.

9

i8

Echinus.

Echinoderms.

Boveri.

J)

??

Thysanozoon.

Polyclades.

Van der Stricht.

jj

??

Sagitta.

Chaetognaths.

Boveri.

»

?>

Chaetopterus.

Annelids.

Mead.

??

??

Ascidia.

Tunicates.

Boveri.

lO

20

Lasius.

Insects.

Henking.

II

[22]

Allolobophora.

Annelids.

Foot.

12

24

Myzostoma.

7?

Wheeler.

1 This table is compiled from papers both on fertilization and maturation. Numbers in
brackets are inferred.

UNION OF THE GERM-CELLS

207

Germ-
Nuclei.

Somatic

Nuclei.

Name.

Group.

Authority.

12

24

Thalassema.

Annelids.

Griffin.

II (12)

22 (24)

Cyclops strenuus.

Copepods.

Ruckert.

12

24

„ brevicornis.

??

Hacker.

>;

7?

Helix.

Gasteropods.

Platner,VomRath.

»>

?>

Branchipus.

Crustacea.

Brauer.

J)

?J

Pyrrhocoris.

Insects.

Henking.

>?

»

Salmo.

Teleosts.

Bohm.

„

JJ

Salamandra.

Amphibia.

Flemming.

»

?>

Rana.

?>

Vom Rath.

)?

?>

Mouse.

Mammals.

Sobotta.

»

>?

Osmunda.

Ferns.

Strasburger.

»

?7

Lilium.

Angiosperms.

Strasburger,
Guignard.

»

»

Helleborus.

If

Strasburger.

>^

>?

Leucojum, Paeonia,
Aconitum.

»

Overton.

14

28

Tiara.

Hydromedusae.

Boveri.

j»

I-!

Pieris.

Insects.

Henking.

16

32

Cerebratulus, Micrura.

Nemertines.

Coe.

5)

?>

Pterotrachea, Carinaria,

Phyllirhoe.

Gastropods.

Boveri.

)>

[.]

Diaptomus, Heterocope.

Copepods.

Ruckert.

»

[v]

Anomalocera, Euchaeta.

If

Vom Rath.

??

DJ

Lumbricus.

Annelids.

Calkins.

18

36

Torpedo, Pristiurus.

Elasmobranchs.

Ruckert.

[18(19)]

36(38)

Toxopneustes.

Echinoderms.

Wilson.

30

[60]

Crepidula.

Gasteropods.

Conklin.

84

168

Artemia.

Crustacea.

Brauer.

The above data are drawn from sources so diverse and show so
remarkable a uniformity as to establish the general law with a very-
high degree of probability. The few known exceptions are almost
certainly apparent only and are due to the occurrence of plurivalent
chromosomes. This is certainly the case with Ascaris {cf. p. Sy).
It is probably the case with the gasteropod Arion, where, as described
by Platner, the egg-nucleus gives rise to numerous chromosomes, the
sperm-nucleus to two only ; the latter are, however, plurivalent, for
Garnault showed that they break up into smaller chromatin-bodies,
and that the germ-nuclei are exactly ahke at the time of union. We
may here briefly refer to remarkable recent observations by Riickert
and others, which seem to show that not only the paternal and mater-
nal chromatin, but also the chromosomes, may retain their individu-
ality throughout development.^ Van Beneden, the pioneer observer

1 '89, pp. 10, zZ'

208 FERTILIZATION OF THE OVUM

in this direction, was unable to follow the paternal and maternal
chromatin beyond the first cleavage-nucleus, though he surmised that
they remained distinct in later stages as well ; but Rabl and Boveri
brought forward evidence that the chromosomes did not lose their
identity, even in the resting nucleus. Ruckert ('95, 3) and Hacker
('95, I) have recently shown that in Cyclops the paternal and mater-
nal chromatin-groups not only remain distinctly separated during the
anaphase, but give rise to double nuclei in the two-cell stage (Fig. 146).
Each half again gives rise to a separate group of chromosomes at
the second cleavage, and this is repeated at least as far as the blas-
tula stage. Herla and Zoja have shown furthermore that if in
Ascaris the ^g'g of variety bivalens, having two chromosomes, be
fertilized with the spermatozoon of variety univaletts having one
chromosome, the three chromosomes reappear at each cleavage, at
least as far as the twelve-cell stage (Fig. 145); and according to Zoja,
the paternal chromosome is distinguishable from the two maternal at
each step by its smaller size. We have thus what must be reckoned
as more than a possibility, that every cell in the body of the child may
receive from each parent not only half of its chromatin-substance,
but one-half of its chromosomes, as distinct and individual descendants
of those of the parents.

C. The Centrosome in Fertilization

In examining more critically the history of the centrosomes we may
conveniently take Boveri's hypothesis of fertilization as a point of
departure, since it has long formed the focus of discussion of the
entire subject. Before the hypothesis is more closely scrutinized we
may first eliminate two other views, both of which are irreconcilable
with it, though neither has stood the test of later research. The first
of these, doubtfully suggested by Van Beneden i^Sy) and definitely
maintained by Wheeler ('97) in the case of Myzostoma, is that the
cleavage-centrosomes have no definite relation to the spermatozoon,
but are derived from the ^gg — a conclusion that has the a priori
support of the fact that in parthenogenesis the centrosomes are cer
tainly of maternal origin.

Van Beneden's early statement may be passed by, since it was no
more than a surmise. Wheeler, after a careful research, found that no
sperm-aster accompanied the sperm-nucleus — a fact correlated with
the absence of a middle-piece in the spermatozoon, — and reached the
conclusion that after formation of the polar bodies, the egg-centro-
somes persisted to become directly converted into the cleavage-centro-
somes (Fig. 104). That the absence of a distinct middle-piece is not a
valid argument is shown by the insect-spermatozoon, where the region

THE CENTROSOME IN FERTILIZATION

209

of the middle-piece is likewise not marked off from the tail, yet as we
have seen (p. 165) the centrosome passes into this part of the sperma-
tozoon. Kostanecki's later examination of the fertilization of the

Fig. 104. — Fertilization of the egg of the parasitic annelid, Myzostoma. [WHEELER.
A. Soon after entrance of the spermatozoon ; the sperm-nucleus at cf ; at ? the germinal
vesicle ; at c the double centrosome. B. First polar body forming at 9 ; n. the cast-out nucle-
olus or germinal spot. C. The polar bodies formed {p.b^ ; germ-nuclei of equal size; at ^ the
centrosomes. D. Approach of the germ-nuclei ; the amphiaster formed.

same animal ('98), while inconclusive on the main point, leaves little
doubt that Wheeler's evidence was equally so ; for he has on the one
hand shown that the sperm-nucleus is often accompanied by a sperm-

210 FERTILIZATION OF THE OVUM

aster containing a pair of centrosomes, on the other hand that these,
like the egg-centrosomes, wholly disappear from view at a later period,
the cleavage-centrosomes having only a conjectural origin.

The second of the views in question is that the cleavage-centro-
somes are derived from both germ-cells ; and this in turn has in its
favour the a priori evidence that in the Infusoria conjugation takes
place between two mitotic figures (p. 224). It appears in two forms,
of which the iirst, though undoubtedly erroneous, has had so interest-
ing a history as to deserve a brief review. It was predicted by Rabl
in 1889 that if the centrosome be a permanent cell-organ, the con-
jugation of germ-cells and germ-nuclei would be found to involve
also a conjugation of centrosomes. Unusual interest was therefore
aroused when Fol, in 1891, under the somewhat dramatic title of the
" Quadrille of Centres," described precisely such a conjugation of
centrosomes as Rabl had predicted. The results of this veteran
observer were very positively and specifically set forth, and were of
so logical and consistent a character as to command instant accept-
ance on the part of many authorities. In the eggs of the sea-urchin
the sperm-centrosome and egg-centrosome were asserted to divide
each into two, the daughter-centrosomes then conjugating two and
two, paternal with maternal, to form the cleavage-centrosomes. The
same result was announced by Guignard ('91) in the lily, by Conklin
('93) in the gasteropod Crepidula, less definitely by Blanc ('93) in the
trout, and still later by Van der Stricht ('95) in A^nphioxiis. None
of these results have stood the test of later work. Fol's result was
opposed to the earlier conclusions of Boveri and Hertwig, and a careful
reexamination of the fertilization of the echinoderm ^gg, indepen-
dently made in 1 894-95 by Boveri (Echimis\ by myself ( Toxopneustes),
and Mathews {Arbacia, Asterias), and slightly later by Hill ('95) and
Reinke ('95) in Sphcerechiniis, demonstrated its erroneous character.
Various attempts have been made to explain Fol's results as based on
double-fertilized eggs, on imperfect method, on a misinterpretation of
the double centrosomes of the cleavage-spindle, yet they still remain
an inexplicable anomaly of scientific literature.

Serious doubt has also been thrown on Conkhn's conclusions by
subsequent research. Kostanecki and Wierzejski ('96) made a very
thorough study, by means of serial sections, of the fertiHzation of
the gasteropod Physa, and reached exactly the same result as that
obtained in the echinoderms. Here, also, the egg-centres degenerate,
their place being taken by a new pair, arising in intimate relation with
the middle-piece of the spermatozoon, about which forms a sperm-
amphiaster (Fig. 89). Conklin, after renewed research, himself
admitted that no quadrille occurs in Crepidula, though he still believes
that a union of paternal and maternal attraction-spheres takes place.

THE CENTROSOME IN FERTILIZATION 211

Guignard's results, too, have entirely failed of confirmation by later
observers (p. 221), and in his own latest contribution to the subject
('99) the centrosomes are conspicuous by their absence in both the
text and the figures. In like manner Van der Stricht's conclusions
have been shown by Sobotta ('97) to be without substantial founda-
tion, while Blanc's account, opposed to the earlier work of Bohm, is
too incomplete to carry any weight. The entire case for the " qua-
drille " has thus fallen to the ground. In its second form the supposed
double origin of the centrosomes rests upon a single research upon
Ascaris by Carnoy and Le Brun ('97, 2), who assert that the cleavage-
centrosomes arise de novo and separately, one inside of each of the
germ-nuclei, to migrate thence out into the cytoplasm. At the close
of mitosis they wholly disappear, to be replaced by a new pair, like-
wise of intranuclear origin. Since this result is totally opposed to
those of Van Beneden, Boveri, Erlanger, and Kostanecki and Sied-
lecki on the same object, and is contradicted in the most positive man-
ner by Fiirst,^ it may be received with some scepticism. The work of
Kostanecki and Siedlecki ('96) demonstrates the division of the sperm-
centrosome in Ascaris as described by Boveri; and while it still
remains possible that the daughter-centrosomes may for a very brief
period disappear (as in. some of the mollusks described beyond), no
ground is given for such a conclusion as Carnoy has drawn. No one
familiar with the object can repress the suspicion that Carnoy and
Le Brun have confused the centrosomes with the nucleoli ; but only
renewed research can determine the point.

The ground is now clear for a closer study of Boveri's hypothesis
in the light of more recent research. It should first be pointed out
that that hypothesis is based upon and forms a part of the more gen-
eral theory of the autonomy of the centrosome ; and if the latter
theory cannot be sustained, the a priori side of Boveri's hypothesis
assumes a different aspect. In point of fact the general outcome of
recent research on fertilization has been on the whole unfavourable to
the view that the cleavage-centrosomes must necessarily be individu-
ally identical with permanent preexisting centrosomes — indeed, it is
in this very field that some of the most convincing evidence against
the persistence of the centrosome has been produced. The mode of
origin of the cleavage-centrosomes is nevertheless a question of high
interest on account of the unmistakable genetic relations existing
between the centrosome of the spermatid and spermatozoon and those
of the sperm-amphiaster within the ^g^.

There are two points of capital importance to be determined before
a definite decision regarding the origin of the cleavage-centrosomes
can be reached. First, are the centrosomes of the sperm-aster within

1 '98, p. 105.

212 FERTILIZATION OF THE OVUM

the ^gg identical with, or the descendants of, a centrosome or pair
of centrosomes in the middle-piece of the spermatozoon ? Second,
do they actually persist to form those of the cleavage-amphiaster ?
In the present state of knowledge we are not in a position to give an
affirmative answer to the first of these questions. As has been shown
in Chapter III., it is no longer possible to doubt that the middle-
piece either contains or is itself a metamorphosed centrosome ; but, as
pointed out at page 196, it does not seem possible that the extremely
minute centrosome of the sperm-aster can represent the entire cen-
trosome of the middle-piece (however we conceive the origin of the
latter). At most we can only assume that a part of the latter per-
sists as the sperm-centrosome within the ^gg. The exact origin of
the latter still remains problematical. A large number of observers
are now agreed that the sperm-aster is formed about a focus that is
either in or very near the middle-piece ; ^ but no one, I believe, has
yet succeeded in showing that the centrosome actually is the meta-
morphosed middle-piece, or escapes from it.^ The possibility there-
fore remains that the centrosome of the sperm-aster is not actually
imported as such into the ^gg, but is either only a portion of the
original spermatid-centrosome, or, as was first suggested by Miss Foot
('97) and further discussed by Mead ('98, 2), is, like the aster, formed
anew in the egg-cytoplasm. If the latter alternative be the case, the
original form of Boveri's hypothesis would have to be abandoned;

1 For example, in echinoderms (Flemming, '8i, O. and R. Hertwig, '86, Boveri, '95,
Wilson and Mathews, '95, Hill, '95, Reinke, '95, R. Hertwig, '96, Doflein, '97, 2, Erlanger, '98),
in Pterotrachea and Pieris (Henking, '91, '92), in the axolotl (Fick, '93), and Triton
(Michaelis, '97), in Phallusia (Hill, '95), in Ophryotrocha (Korschelt, '95), in Physa
(Kostanecki and Wierzejski, '96), in Strongylus (Meyer, '95), in Thysanozoon (Van der
Stricht, '98), and Prosthiostomwn (Francotte, '98). In a large number of other cases the
sperm-aster is found near the sperm-nucleus, but its relation to the middle-piece has not
been demonstrated.

2 I myself formerly concluded ('95, 2) that the entire middle-piece of echinoderms is the
centrosome — a result apparently confirmed in a most positive manner by Erlanger ('98),
as well as by R. Hertwig ('96) and Doflein ('97, 2). I have, however, demonstrated this
to be an error, showing that the extremely minute centrosome is quite distinct from the
middle-piece, the latter being thrown aside and degenerating in the egg-cytoplasm outside
of the newly formed sperm-aster (Figs. 12, 94). This fact, of which the phenomena in
Toxopneustes leave no doubt (see Wilson, '97, '99), is, I think, fatal to Kostanecki's and
Wierzejski's theory of fertilization ('96, pp. 374-375), according to which the archoplasm of
the middle-piece gives rise to the new astral system and is thus the essential fertilizing sub-
stance (the centrosome being merely a mechanical centre for the attachment of the rays) ;
but the most careful examination has still failed to show whether the centrosome actually
escapes from the middle-piece, nor have other observers had better success with any animal.
Erlanger ('96, 2, '97, 4) believes he has seen the centrosome in the Ascaris spermatozoon
as a distinct body lying behind the nucleus, and that it can be traced continuously into the
egg and after its division into the two poles of the cleavage-figure. Neither the schematic
figures of his preliminary nor the photographic ones of his final paper seem sufficient to
establish either the identity or the subsequent history of the granule in question.

THE CENTROSOME IN FERTILIZATION' 21 3

though in substance it would still retain an element of truth, as pointed
out beyond.

We may now examine the question whether the sperm-centrosomes
are actually identical with the cleavage-centrosomes. That such is
the case is positively maintained in the case of Ascaris by Boveri,
Kostanecki, and Erlanger, in Physa by Kostanecki and Wierzejski
('96), in Thalasseina by Griffin ('96, '99), and in Chcetopterus by Mead
('95) '9^)- The two last-mentioned observers, who have followed
the phenomena with especial care, produce very strong evidence
that at no time do the sperm-centrosomes and asters disappear, and
that the former may be traced in unbroken continuity from the time
of their first appearance to the daughter-cells resulting from the first
cleavage (Figs. 99, 155). On the other hand, a considerable number
of observers, beginning with Htrtwig^Phyllirrhoe, Pterotrachea, '75),
have found that as the sperm-nucleus enlarges the sperm-asters di-
minish in size, until, in many cases, they nearly or quite disappear ; for
example, in ProsthecercBus {Y^\riQk.ov^^\x'6vs\, '97), in the mouse (Sobotta,.
'95), in Pleurophyllidia (MacFarland, '97), Physa (Kostanecki and:
Wierzejski, '96), Areiiicola (Child, '97), Unio (Lillie, '97), Myzostoma:
( Kostanecki, '98), and Cerebratultis (Coe, '98y Several of these
observers (Klinckowstrom, MacFarland, Lillie, Child) have found that
not only the asters btit also the centrosomes totally disappear about
the time the germ-nuclei come together, a new pair of cleavage-
centrosomes and asters being afterward developed at the poles of
the united nuclei. These conclusions, if correct, place in a new
light the disappearance of the egg-centrosomes ; for this process

1 Coe has pointed out that the eggs of various animals may be arranged in a series show-
ing successive graduations in the disappearance of the sperm-asters. " At the head of the
series we must place the eggs of Ascaris and Myzostoma (according to Kostanecki) and
similar ones in which the sperm-asters make their appearance only a short time before the
formation of the cleavage-spindle, and which, consequently, suffer no diminution in size.
Following these are the eggs of Chdtopterus (Mead) and Ophryotrocha (Korschelt) and
of some echinoderms in which the sperm-asters develop very early, but are not described
as decreasing in size before the formation of the cleavage-spindle. Then come the eggs of
Toxopneusies (Wilson) and Thalassenia (Griffin), where the sperm-asters appear early and
develop to a very considerable size, but nevertheless become very much smaller and less
conspicuous after the germ-nuclei have come together. After these we must place the eggs
of Physa (Kostanecki and Wierzejski), for here the sperm-asters, after becoming very large
and conspicuous, degenerate to such an extent that only a very few exceedingly delicate
fibres remain. Those of Cerehratulus follow next.

" Here the sperm-asters increase in size until they extend throughout the whole body
of the cell, but at the time of fusion of the germ-nuclei they degenerate completely. The
peripheral portions of their fibres, however, may be followed, as stated above of Pleuro-
phyllidia, Prosthecerccus, etc., where the sperm-asters degenerate soon after their forma-
tion, so that for a considerable period the egg is without trace of aster-fibres. Yet in all
of those cases where the sperm-asters disappear and their centrosomes become lost among
the other granules of the cell, we are justified in believing that the sperm-centrosomes
nevertheless retain their identity, and later reappear in the cleavage-asters " ('98, p. 455).

214 FERTILIZATION OF THE OVUM

would thus seem to be of the same nature as the disappearance of
the sperm-centrosomes, and both Boveri's theory of fertilization and
the general hypothesis of the permanence of the centrosomes would
receive a serious blow.

The investigators to whom these observations are due have ranged
themselves in two groups in the interpretation of the phenomena.
On the one hand, Lillie and Child do not hesitate to maintain that
the centrosomes actually go out of existence as such, to be re-formed
like the asters out of the egg-substance ; and that such a new forma-
tion of centrosomes is possible seems to be conclusively shown by the
experiments of Morgan and Loeb described at pages 215 and 307. On
the other hand, Sobotta, MacFarland, Kostanecki, and Coe, relying
partly on the analogy of other forms, partly on the occasional pres-
ence of the centrosomes during the critical stage, urge that the dis-
appearance of the sperm-centrosomes is only apparent, and is due to
the disappearance of the asters, which renders difficult or impossible
the identification of the centrosomes among the other protoplasmic
granules of the o.g'g. These authors accordingly still uphold Boveri's
theory.

It is difficult to sift the evidence at present, for it has now become
very important to reexamine, in the light of these facts, those cases
in which the absolute continuity of the centrosome has been main-
tained— for example, in Ascaris, Chcetopteriis, and Thalassema — in
order to determine whether there may not be here also a brief critical
period in which the centrosomes disappear. There are, however,
some facts which tend to sustain the conclusion that even though the
sperm-centrosomes disappear from view, there is some kind of genetic
continuity between them and the cleavage-centrosomes. First, both
Kostanecki and Wierzejski ('96) and Coe ('98) have found that there
is some variation in eggs apparently equally well preserved, a few
individuals showing the sperm-centrosomes at the poles of the united
nuclei at the same period when they are invisible in other individuals.
Second, both these observers, Coe most clearly, have shown that the
egg-centrosomes disappear considerably earlier than the sperm-cen-
trosomes, and Coe has traced the sperm-centrosomes continuously to
the exact points {the poles of the united nuclei) at which the cleavage-
centrosomes afterzuard appear (¥\g. 155). This important observation
leads to the suspicion that the apparent disappearance of the centro-
somes may be due to a loss of staining-capacity at the critical period,
or that even though the formed centrosome disappears its substance
reappears in its successor. Here again we come to the view sug-
gested at page 1 1 1, that the centrosome may be regarded as the vehicle
of a specific chemical substance which is transported to the nuclear
poles by its division, and may there persist even though the body of the

FERTILIZATION IN PLANTS 21 5

centrosome be no longer visible. On such a basis we may perhaps
find a reconciliation between these observations and Boveri's theory,
and may even bring the fertilization of plants into relation with it
(p. 221). Even in case of the nucleus, universally recognized as a
permanent cell-organ, it is not the whole structure that persists as
such during division, but only the chromatin-substance — in some
cases only a small fraction of that substance. The law of genetic
continuity therefore would not fail in case of the centrosome, though
only a portion of its substance were handed on by division ; and even
if we take the most extreme negative position, assuming that the
sperm-centrosome is wholly formed anew under the stimulus of the
spermatozoon, we should still not escape the causal nexus between
it and the centrosome of the spermatid.

Boveri himself has suggested^ that the Q,gg may be incited to
development by a specific chemical substance carried by the sperma-
tozoon, and the same view has been more recently urged by Mead,^
while Loeb's recent remarkable experiments on sea-urchins ('99) show
that the ^gg may in this case {Arbacid) undergo complete parthe-
nogenetic development as the result of artificial chemical stimulus.^
Assuming such a substance to exist, by what part of the spermato-
zoon is it carried } It is possible that the vehicle may be the nucleus,
which forms the main bulk of that which enters the ^gg ; and this
view seems to be supported by what is at present known of fertiliza-
tion in the plants (p. 221). Yet when we regard the facts of fertili-
zation in animals, taken in connection with the mode of formation of
the spermatozoon, we find it difificult to avoid the conclusion that the
substance by which the stimulus to development is normally given is
originally derived from the spermatid-centrosome, is conveyed into
the ^gg by the middle-piece, and is localized in the sperm-centro-
somes which are conveyed to the nuclear poles during the amphi-
aster-formation. Accepting such a view, we could gain an intelligible
view of the genetic relation between spermatid-centrosome, middle-
piece, sperm-centrosome, and cleavage-centrosomes, without commit-
ting ourselves to the morphological hypothesis of the persistence of
the centrosome as an individualized cell-organ. Such a conclusion,
I believe, would retain the substance of Boveri's theory while leaving
room for the abandonment of the too simple morphological form in
which it was originally cast.

D. Fertilization in Plants

The investigation of fertilization in the plants has always lagged
somewhat behind that of the animals, and even at the present time

i'9i, p. 431. 2 '98^ 2, p. 217. ^Cf,^^.\\\.

2l6 FERTILIZATION OF THE OVUM

our knowledge of it is rather incomplete. It is, however, sufficient
to show that the essential fact is everywhere a union of two germ-
nuclei — a process agreeing fundamentally with that observed in
animals. On the other hand, almost nothing is known regarding the
centrosome and the archoplasmic or kinoplasmic structures; and
most recent observations point to the conclusion that in the lowering
plants and pteridophytes no centrosomes are concerned in fertihzation.
Many early observers from the time of Pringsheim ('55) onward
described a conjugation of cells in the lower plants, but the union of
germ-nuclei, as far as I can find, was first clearly made out in the
flowering plants by Strasburger in xZyj-j^, and carefully described
by him in 1884. Schmitz observed a union of the nuclei of the

Fig. 105. — Fertilization in Pilularia. [CAMPBELL.]
A. B. Early stages in the formation of the spermatozoid. C. The mature spermatozoid ; the
nucleus lies above in the spiral turns; below is a cytoplasmic mass containing starch-grains (r/l
the spermatozoids of ferns and of Marsilia, Fig. 71). D. Archegonium during fertilization. In
the centre the ovum containing the apposed germ-nuclei (cf , ? ).

conjugating cells of Spirogyra in 1879, and made similar observations
on other algae in 1884. Among other forms in which the same
phenomenon has been described may be mentioned CEdigonium
(Klebahn, '92), Vaucheria (Oltmanns, '95), CystopiLS (Wager, '96),
Sphcerotheca ?ind E7yst/>/ie (Hd^rper, '96), /v/^^/j" (Farmer and Williams,
'96, Strasburger, '97), Basidioboltis (Fairchild, '97), Pilularia (Fig.
105, Campbell, '88), Onoclea (Shaw, '98, 2), Zamia (Webber, '97, 2),
and Liliuin (Guignard, '91, Mottier, '97), Ginkgo (Hirase, '97).^ In
all of these forms and many others fertilization is effected by the
union of a single paternal and a single maternal uninucleated cell,
such as occurs throughout the animal kingdom. There are, however,
some apparently well-determined exceptions to this rule occurring
in the ** compound " multinucleate oospheres of some of the lower

1 For unicellular forms see pp. 228, 280.

FERTILIZAriON IN PLANTS

217

plants. In Albugo bliti (one of the Peronosporese), for example, as
shown by the recent work of Stevens ('99), the mature ovum contains
about a hundred nuclei, and is fertilized by a multinucleate proto-
plasmic mass derived from the antheridium, each nucleus of the latter
conjugating with one of the egg-nuclei. But although the conjugat-
ing bodies are here multinucleate, the germ-nuclei conjugate two and
two (as is also the case in the multinucleate cysts of ActinospJicerinm,
p. 279); and the case therefore forms no real exception to the
general rule that one paternal nucleus unites with one maternal.

Fig. 106. — Formation of the ovum and penetration of the pollen-tube in flowering plants.
[Strasburger.]

A. Embryo-sac of Monotropa, showing the division that follows the two maturation-divisions
and produces the upper and lower " tetrads." B. The same, ready for fertilization, showing ovum
(tf), synergidae (5), upper and lower polar cells (/), and antipodal cells (a). C. Penetration of
the pollen-tube (/./.) in Orchis ; o. ovum, with synergidae at either side, g.n. generative nuclei in
the pollen-tube. D. Slightly later stage with generative nuclei entering the micropyle.

Whether a union of more than two germ-nuclei occurs in any of the
lower plants is a question still disputed by botanists.^ Such plural
fusion is rendered a priori improbable by the observations thus far
made upon the one-celled forms both in plants and in animals ; and
the known facts are sufficient to show that it must be, to say the
least, an exceptional process.

In cases where the paternal germ-cell is a ciliated spermatozoid, as
in F?icus, Pihilaria, and the ferns and cycads, the germ-nuclei differ

1 Cf. Hartog, '91, '96, Trow, '95, Stevens, '99, Zimmerman, '96, and literature there cited.

2l8 FERTILIZATION OF THE OVUM

more or less widely at the time of union, the sperm-nucleus being
smaller, more compact, and deeply staining (Figs. 105, 108), as is the
case in such forms of fertilization as the echinoderm-egg. In the
case of angiosperms all earlier observers, including Strasburger ('78,
'84), Guignard ('91, i), and Mottier ('97, i), found the conjugating
nuclei to be closely similar at the time of union. The recent obser-
vations of Guignard ('99) and Nawaschin ('99) show, however, that
even here the sperm-nucleus is smaller, more compact, and of differ-
ent form (spindle-shaped) from the egg-nucleus (Fig. 107).

The ovum or oosphere of the flowering plant is a large, rounded
cell containing a large nucleus and numerous minute colourless
plastids from which arise, by division, the plastids of the embryo
(chromatophores, amyloplasts). In the angiosperms the ovum forms
one of the eight cells constituting the embryo-sac which morphologi-
cally represents the female proth allium or sexual generation of the
pteridophyte and is itself embedded in the ovule within the ovary.^
The male germ-cells are represented in the cycads by two ciliated
spermatozoids (p. 175), in the angiosperms by two spindle-shaped
"generative nuclei" which are suspected by Guignard and Nawaschin
to be motile bodies, though no cilia were seen. These lie near the
tip of the pollen-tube (Fig. 107), which is developed as an outgrowth
from the pollen-grain and represents a rudimentary male prothallium
or sexual generation.^

The formation of the pollen-tube, and its growth down through
the tissue of the pistil to the ovule, was observed by Amici ('23),
Brongniart ('26), and Robert Brown ('31); and in 1833-34 Corda was
able to follow its tip through the micropyle into the ovule.^ Stras-
burger first demonstrated the fact that the generative nucleus, carried
at the tip of the pollen-tube, enters the ovum and unites with the egg-
nucleus, and the facts have been since carefully studied by himself,
by Guignard, Mottier, Webber, Ikeno, Hirase, and a number of others.
In the cycads, according to the last-named two observers, a single
spermatozoid enters the ^^g^ its nucleus soon fusing with that of the

1 The eight cells are at first arranged in an upper and a lower " tetrad " of four cells each,
the former including the ovum, two synergidae, and an " upper polar cell," the latter a
"lower polar cell" and three antipodal cells (Figs. 106, 107); cf. p. 263.

2 Cf. p. 264.

3 It is interesting to note that the botanists of the eighteenth century engaged in the same
fantastic controversy regarding the origin of the embryo as that of the zoologists of the
time. Moreland (1703), followed by Etienne Francois Geoff roy, Needham, and others,
placed himself on the side of Leeuwenhoek and the spermatists, maintaining that the pollen
supplied the embryo which entered the ovule through the micropyle (the latter had been
described by Grew in 1672); and even Schleiden adopted a similar view. On the other
hand, Adanson (1763) and others maintained that the ovule contained the germ which was
excited to development by an aura or vapour emanating from the pollen and entering through
the tracheae of the pistil.

FERTILIZATION IN PLANTS

219

egg (Fig. 108); and the earlier observers of the angiosperms, includ-
ing Strasburger ('84, '88) and Guignard ('91, i), likewise found that
only one of the generative nuclei entered the embryo-sac. Guignard

// •

Fig. 107. — Fertilization in the lily. \p from MOTTIER, the others from Guignard,]
A. Embryo-sac, ready for fertilization. B. Both generative nuclei have entered the embryo-
sac ; one is approaching the egg-nucleus, the other uniting with the upper polar nucleus. C. Union of
the germ-nuclei ; below, union of the second generative nucleus and the two polar nuclei. D. The
fertilized egg, showing fusion of the germ-nuclei. E. The fertilized egg dividing; below, division
of the endosperm-nuclei, a. antipodal cells; e. endosperm-nuclei; o. the oosphere or ovum;
p. polar nuclei ; /. t. pollen-tube.

and Nawaschin have, however, recently made the remarkable dis-
covery that in Liliiim and Fritillaria both generative nuclei enter
the embryo-sac. One of these conjugates with the egg-nucleus and

220

FERTILIZATION OF THE OVUM

thus effects fertilization (Fig. 107). The other conjugates with one of
the polarnuclei (usually the upper), which then unites with the other
polar nucleus {cf. p. 264). By division of the fertilized ^gg arises the
embryo ; while by division of the compound nucleus resulting from the

fusion of the polar nuclei
and the second sperm nu-
cleus are formed the endo-
sperm-cells, which serve
for the nourishment of the
embryo. This remarkable
double copulation within
the embryo-sac is without
a parallel and is of wholly
problematical meaning, but
in no way contradicts the
general rule regarding the
union of two germ-nuclei
to produce the embryo.^

1 As in the case of animals (p,
176), the germ-nuclei of phanero-
gams also show marked differ-
ences in structure and staining-reac-
tion before their union, though they
ultimately become exactly equiva-
lent. Thus, according to Rosen
('92, p. 443), on treatment by
fuchsin-methyl-blue the male germ-
nucleus is " cyanophilous," the
female " erythrophilous," as de-
scribed by Auerbach in animals.
Strasburger, while confirming this-
observation in some cases, finds the
reaction to be inconstant, though
the germ-nuclei usually show iriarked
differences in their staining-capac-
ity. These are ascribed by Stras-
burger ('92, '94) to differences in
the conditions of nutrition ; by
Zacharias and Schwarz to corre-
sponding differences in chemical
composition, the male nucleus being
in general richer in nuclein, and the
female nucleus poorer. This dis-
tinction disappears during fertiliza-
tion, and Strasburger has observed, in the case of gymnosperms (after treatment with a
mixture of fuchsin-iodine-green), that the paternal nucleus, which is at first " cyanophil-
ous," becomes " erythrophilous," like the egg-nucleus before the pollen-tube has reached
the egg. Within the egg both stain exactly alike. These facts indicate, as Strasburger
insists, that the differences between the germ-nuclei of plants are, as in animals, of a.
temporary and non-essential character.

Fig. 108. — Fertilization ina cycad, Zamia. [WEBBER.]

A. Spermatozoid. B. The same after entrance into

the egg, showing nucleus {tt) and cilia-bearing band {c).

C. The ovum shortly after entrance of the spermatozoid.

D. Union of the germ-nuclei, cilia-bearing band near
periphery (^r).

FERTILIZATION IN PLANTS 22 1

The nature and origin of the achromatic elements involved in the
fertilization of plants is still almost wholly in the dark. No observer
has yet succeeded in observing either centrosomes or asters in the
fertilization of the thallophytes, despite the fact that in some of these
forms mitosis takes place with both these structures in a manner
nearly analogous to that observed in animals.^ In the cycads Zamia
and Cycas, Webber and Ikeno ('98) agree that the entire spermato-
zoid enters, but only the nucleus appears to be concerned in fertiliza-
tion. The cilia-bearing band — a product of the blepharoplast, and,
as described at page 175, probably the analogue of the middle-piece
of the animal spermatozoon — remains near the egg-periphery, gives
rise to no astral or other fibrillar formations, and apparently remains
quite passive (Fig. 108).

In angiosperms, too, the evidence seems to show that no centro-
somes are concerned in fertilization. Guignard ('91, i), in a very
detailed and clearly illustrated paper, gave an account of the centro-
somes in the lily agreeing almost exactly with the " quadrille of
centres " as described by Fol,^ paternal and maternal centrosomes
conjugating two by two. The later and very careful studies of Mot-
tier and others have, however, entirely failed to confirm Guignard's
results, the germ-nuclei fusing without the participation of centro-
somes or astral formations, and after a time dividing, without centro-
somes, in the manner characteristic of the higher plants.^ Neither
in the cryptogams has any one thus far succeeded in finding fertiliza-
tion-centrosomes or asters at the time the germ-nuclei unite. Stras-
burger contributes, however, the interesting observation that in Fucus
the cleavage-centrosomes afterward appear on that side of the
cleavage-nucleus derived from the sperm-nucleus, which he believes
from analogy may indicate the importation of a "new dynamic
centre " into the ^gg by the spermatozoid.* Combining these facts
with the phenomena involved in the origin of the spermatozoids,
Strasburger suggests that the sperm-nucleus may import into the
^gg either a formed centrosome (probably thus in Fiictis) or a cer-
tain quantity of " kinoplasm," which incites the mitotic phenomena
in the absence of individualized centrosomes.^ This view harmo-
nizes with that suggested at pages 1 1 1 and 214, and we may perhaps
here in the end find a reconciliation between the various types, not
only of fertilization but also of mitosis, in plants and animals.

On their face the facts of fertilization in plants, especially in the
phanerogams, seem to indicate that the stimulus to development
is given by the paternal germ-nucleus. Nevertheless, the analogy of
animal fertilization would lead us to expect that the fertilizing sub-

1 (7: p. 82. ^ Cf. p. 82.

2 Cf. p. 210. 4 '97, p. 418.

222 FERTILIZATION OF THE OVUM

stance is contained not in the nucleus but in the cytoplasm — more
specifically, in the case of spermatozoids, in the cilia-bearing body
derived from the blepharoplast, which in its development so strongly
suggests a centrosome (p. 172). Webber's and Ikeno's observations
on the cycads are not necessarily fatal to this view; for, as I have
shown (p. 188), the middle-piece in the echinoderm is likewise cast
off and degenerates near the periphery of the ^gg, and the centro-
some is a body far more minute. The possibility has been admitted
that this centrosome may be formed de novo under the influence of
the middle-piece, which itself perishes. In like manner it may also
be possible that the primary stimulus in Zamia and like cases is given
by the cilia-bearing body, even though this body itself disappears and
the mitotic apparatus is not formed until long afterward.

E. Conjugation in Unicellular Forms

The conjugation of unicellular organisms possesses a peculiar inter-
est, since it is undoubtedly a prototype of the union of germ-cells
in the multicellular forms. Biitschli and Minot long ago maintained
that cell-divisions tend to run in cycles, each of which begins and
ends with an act of conjugation. In the higher forms the cells pro-
duced in each cycle cohere to form the multicellular body; in the
unicellular forms the cells separate as distinct individuals, but those
belonging to one cycle are collectively comparable with the multi-
cellular body. The validity of this comparison, in a morphological
sense, is generally admitted.^ No process of conjugation, it is true, is
known to occur in many unicellular and in some multicellular forms,
and the cyclical character of cell-division still remains sub jtidice?
It is none the less certain that a key to the fertilization of higher
forms must be sought in the conjugation of unicellular organisms.

The difficulties of observation are, however, so great that we are
as yet acquainted with only the outlines of the process, and have still
no very clear idea of its finer details or its physiological meaning.
The phenomena have been most closely followed in the Infusoria by
Biitschli, Engelmann, Maupas, and Richard Hertwig, though many
valuable observations on the conjugation of unicellular plants have
been made by De Bary, Schmitz, Klebahn, and Overton. All these
observers have reached the same general result as that attained
through study of the fertilization of the ^^^ ; namely, that an essen-
tial phenomenon of conjugation is a union of the 7tuclei of the conju-
gating cells. Among the unicellular plants both the cell-bodies and
the nuclei completely fuse. Among animals this may occur ; but in
c/ p. 58. 2 cf. p. 178.

CONJUGATION IN UNICELLULAR FORMS

223

many of the Infusoria union of the cell-bodies is only temporary, and
the conjugation consists of a mutual exchange and fusion of nuclei.
It is impossible within the limits of this work to attempt more than a
sketch of the process in a few forms.

We may first consider the conjugation of Infusoria. Maupas's
beautiful observations have shown that in this group the life-history

Second fission.

VI

> 4

\A

>\^

►' '0 /V ^

A^

Y

■^B

? ^

\ \ '

. ? % }

4

First fission, after separation.

DifTerentiation of micro- and
macronuclei.

Separation of the gametes.

> Division of the cleavage-nu-
cleus.

Cleavage-nucleus.
Exchange and fusion of the
germ-nuclei.

Germ-nuclei.

^ Formation of the polar bodies.

Union of the gametes.

Fig. 109. — Diagram showing the history of the micronuclei during the conjugation of Para-
mcecium, [Modified from Maupas.]

X and F represent the opposed macro- and micronuclei in the two respective gametes; circles
represent degenerating nuclei ; black dots, persisting nuclei.

of the species runs in cycles, a long period of multiplication by cell-
division being succeeded by an "epidemic of conjugation," which
inaugurates a new cycle, and is obviously comparable in its physio-
logical aspect with the period of sexual maturity in the Metazoa. If
conjugation does not occur, the race rapidly degenerates and dies out ;
and Maupas believes himself justified in the conclusion that conju-

224 FERTILIZATION OF THE OVUM

gation counteracts the tendency to senile degeneration and causes
rejuvenescence, as maintained by Biitschli and Minot.^

In Stylonychia pustidata, which Maupas followed continuously from the end of
February until July, the first conjugation occurred on April 29th, after 128 bi-parti-
tions; and the epidemic reached its height three weeks later, after 175 bi-partitions.
The descendants of individuals prevented from conjugation died out through '' senile
degeneracy," after 316 bi-partitions. Similar facts were .observed in many other
forms. The degeneracy is manifested by a very marked reduction in size, a partial
atrophy of the cilia, and especially by a more or less complete degradation of the
nuclear apparatus. In Stylonychia piistidata and Onychodronms grandis this process
especially affects the micronucleus, which atrophies, and finally disappears, though
the animals still actively swim, and for a time divide. Later, the macronucleus
becomes irregular, and sometimes breaks up into smaller bodies. In other cases,
the degeneration first affects the macronucleus, which may lose its chromatin,
undergo fatty degeneration, and may finally disappear altogether {Stylonychia
mytiliis), after which the micronucleus soon degenerates more or less completely,
and the race dies. It is a very significant fact that toward the end of the cycle, as
the nuclei degenerate, the animals become incapable of taking food and of growth ;
and it is probable, as Maupas points out. that the degeneration of the cytoplasmic
organs is due to disturbances in nutrition caused by the degeneration of the nucleus.

The more essential phenomena occurring during conjugation are
as follows. The Infusoria possess two kinds of nuclei, a large
macronucleus and one or more small micromiclei. During conjuga-
tion the macronucleus degenerates and disappears, and the micronu-
cleus alone is concerned in the essential part of the process. The
latter divides several times, one of the products, the germ-nucleus^
conjugating with a corresponding germ-nucleus from the other indi-
vidual, while the others degenerate as "corpuscules de rebut." The
dual nucleus thus formed, which corresponds with the cleavage-
nucleus of the ovum, then gives rise by division to both macronuclei
and micronuclei of the offspring of the conjugating animals (Fig. 109).

These facts may be illustrated by the conjugation of Paramoecmm
caudatum, which possesses a single macronucleus and micronucleus,
and in which conjugation is temporary and fertilization mutual. The
two animals become united by their ventral sides and the macronu-
cleus of each begins to degenerate, while the micronucleus divides
twice to form four spindle-shaped bodies (Fig. no, A, B). Three of
these degenerate, forming the "corpuscules de rebut," which play
no further part. The fourth divides into two, one of which, the
''female pronucleus," remains in the body, while the other, or "male
pronucleus," passes into the other animal and fuses with the female
pronucleus (Fig. no, C-H). Each animal now contains a cleavage-
nucleus equally derived from both the conjugating animals, and the
latter soon separate. The cleavage-nucleus in each divides three

1 67 p. 179.

Fig, no. — Conjugation of Paramoecium caudatum. [A-C, after R. HertwiG; Z)- A',- after
MaupaS.] (The niacronuclei dotted in all the figures.)

A. Micronuclei pre|jaring for their first division. B. Second division. C. Third division;
three polar bodies or " corpuscules de rebut," and one dividing germ-nucleus in each animal. D.
Exchange of the germ-nuclei. E. The same, enlarged. F. Fusion of the germ-nuclei. G. The
same, enlarged. //. Cleavage-nucleus, (c) preparing for the first division. /. The cleavage-
nucleus has divided twice. 7- After three divisions of the cleavage-nucleus; macronucleus
breaking up. K. Four of the nuclei enlarging to form new macronuclei. The first fission soon
takes place.

Q 225

226

FERTILIZATION OF THE OVUM

times successively, and of the eight resulting bodies four become
macronuclei and four micronuclei (Fig. no, H-K). By two suc-
ceeding fissions the four macronuclei are then distributed, one to each
of the four resulting individuals. In some other species the micro-
nuclei are equally distributed in like manner, but in P. catidatiim the
process is more complicated, since three of them degenerate, and
the fourth divides twice to produce four new micronuclei. In either
case at the close of the process each of the conjugating individuals

B

Fig. III. — Conjugation of Vorticellids. [Maupas.
A. Attachment of the small free-swimming microgamete to the large fixed macrogamete;
micronucleus dividing in each (Carchesmm). B. Microgamete containing eight micronuclei;
macrogamete four {Vorticella). C. All but one of the micronuclei have degenerated as polar
bodies or " corpuscules de rebut." D. Each of the micronuclei of the last stage has divided into
two to form the germ-nuclei ; two of these, one from each gamete, have conjugated to form the
cleavage-nucleus seen at the left ; the other two, at the right, are degenerating.

has given rise to four descendants, each containing a macronucleus
and micronucleus derived from the cleavage-nucleus. From this time
forward fission follows fission in the usual manner, both nuclei divid-
ing at each fission, until, after many generations, conjugation recurs.
Essentially similar facts have been observed by Richard Hertwig
and. Maupas in a large number of forms. In cases of permanent
conjugation, as in Vorticella, where a smaller microgamete unites with
a larger macrogamete, the process is essentially the same, though the
details are still more complex. Here the germ-nucleus derived from
each gamete is in the macrogamete one-fourth and in the microgamete

I

CONJUGATION IN UNICELLUIAR FORMS

227

one-eighth of the original micronucleus (Fig. in). Each germ-
nucleus divides into two, as usual, but one of the products of each
degenerates, and the two remaining pronuclei conjugate to form a
cleavage-nucleus.

The facts just described show a very close parallel to those observed
in the maturation and fertilization of the ^g'g. In both cases there
is a union of two similar nuclei to form a cleavage-nucleus or its
equivalent, equally derived from both gametes, and this is the pro-
genitor of all the nuclei of the daughter-cells arising by subsequent
divisions. In both cases, moreover (if we confine the comparison
to the ^gg), the original nucleus does not conjugate with its fellow
until it has by division produced a number of other nuclei all but
one of which degenerate. Maupas does not hesitate to compare

Fig. 112. — Conjugation oi Noctiluca. [ISHIKAWA.]
A. Union of the gametes, apposition of the nuclei. B. Complete fusion of the gametes.
Above and below the apposed nuclei are the centrosomes. C. Cleavage-spindle, consisting of
two separate halves.

these degenerating nuclei or "corpuscules de rebut" with the polar
bodies (p. 181), and it is a remarkable coincidence that their number,
like that of the polar bodies, is often three, though this is not always
the case.

A remarkable peculiarity in the conjugation of the Infusoria
is the fact that the germ-nuclei unite when in the form of spiiidles
or mitotic figures. These spindles consist of achromatic fibres, or
" archoplasm," and chromosomes, but no asters or undoubted cen-
trosomes have been thus far seen in them. During union the
spindles join side by side (Fig. no, G\ and this gives good reason
to believe that the chromatin of the two gametes is equally distrib-
uted to the daughter-nuclei as in Metazoa. In the conjugation of
some other Protozoa the nuclei unite while in the resting state; but
very little is known of the process save in the cystoflagellate Nocti-
luca, which has been studied with some care by Cienkowsky and
Ishikawa (Fig. 112). Here the conjugating animals completely fuse,
but the nuclei are merely apposed and give rise each to one-half of

228

FERTILIZATION OF THE OVUM

the mitotic figure. At either pole of the spindle is a centrosome, the
origin of which remains undetermined.

It is an interesting fact that in Noctihica, in the gregarines, and
probably in some other Protozoa, conjugation is followed by a very
rapid multiplication of the nucleus followed, by a corresponding divi-
sion of the cell-body to form '' spores," which remain for a time
closely aggregated before their liberation. The resemblance of this

D

Fig. 113. — Conjugation of Spirogyt a. [OVERTON.]
A, Union of the conjugating cells {S. communis), B. The typical, though not invariable,
mode of fusion in 6". Weberi ; the chromatophore 'of the "female" cell breaks in the middle,
while that of the " male " cell passes into the interval. C. The resulting zygospore filled with
pyrenoids, before union of the nuclei. D. Zygospore after fusion of the nuclei and formation
of the membrane.

process to the fertilization and subsequent cleavage of the ovum is
particularly striking.

The conjugation of unicellular plants shows some interesting
features. Here the conjugating cells completely fuse to form a
''zygospore " (Figs. 113, 140), which as a rule becomes surrounded
by a thick membrane, and, unlike the animal conjugate, may long
remain in a quiescent state before division. Not only do the nuclei

SUMMARY AND CONCLUSION 229

unite, but in many cases the plastids also (chromatophores). In
Spirogyra some interesting variations in this regard have been ob-
served. In some species De Bary has observed that the long band-
shaped chromatophores unite end to end so that in the zygote the
paternal and maternal chromatophores lie at opposite ends. In
5. Weberi, on the other hand, Overton has found that the single
maternal chromatophore breaks in two in the middle and the paternal
chromatophore is interpolated between the two halves, so as to lie
in the middle of the zygote (Fig. 113). It follows from this, as De
Vries has pointed out, that the origin of the chromatophores in the
daughter-cells differs in the two species, for in the former case one
receives a maternal, the other a paternal, chromatophore, while in
the latter, the chromatophore of each daughter-cell is equally derived
from those of the two gametes. The final result is, however, the
same; for, in both cases, the chromatophore of the zygote divides
in the middle at each ensuing division. In the first case, therefore,
the maternal chromatophore passes into one, the paternal into the
other, of the daughter-cells. In the second case the same result is
effected by two succeeding divisions, the two middle-cells of the four-
celled band receiving paternal, the two end-cells rnaternal, chro-
matophores. In the case of a Spirogyra filament having a single
chromatophore it is therefore "wholly immaterial whether the indi-
vidual cells receive the chlorophyll-band from the father or the
mother" (De Vries).i

F. Summary and Conclusion

All forms of fertihzation involve a conjugation of cells by a
process that is the exact converse of cell-division. In the lowest
forms, such as the unicellular algae, the conjugating cells are, in a
morphological sense, precisely equivalent, and conjugation takes
place between corresponding elements, nucleus uniting with nucleus,
cell-body with cell-body, and even, in some cases, plastid with plastid.
Whether this is true of the centrosomes is not known, but in the
Infusoria there is a conjugation of the achromatic spindles which
certainly points to a union of the centrosomes or their equivalents.
As we rise in the scale, the conjugating cells diverge more and more,
until in the higher plants and animals they differ widely not only
in form and size, but also in their internal structure, and to such an
extent that they are no longer equivalent either morphologically or
physiologically. Both in animals and in plants the paternal germ-

1 De Vries's conclusion is, however, not entirely certain; for it is impossible to deter-
mine, save by analogy, whether the chromatophores maintain their individuality in the
zygote.

.230 FERTILIZATION OF THE OVUM

cell loses most of its cytoplasm, the main bulk of which, and hence
the main body of the embryo, is now supplied by the ^^g ; and
in the higher plants, the ^gg alone retains the plastids which
are thus supplied by the mother alone. On the other hand, the
paternal germ-cell is the carrier of something which incites the ^^g
to development, and thus constitutes the fertilizing element in the
narrower sense. There is strong ground for the conclusion that in
the animal spermatozoon this element is, if not an actual centre-
some, a body or a substance directly derived from a centrosome of
the parent body and contained in the middle-piece. Boveri's theory,
according to which fertilization consists essentially of the replace-
ment of a missing or degenerating egg-centrosome by the importation
of a sperm-centrosome, was stated in too simple and mechanical a
form ; for the facts of spermatogenesis show conclusively that the
spermatid-centrosome is not simply handed on unmodified by the
spermatozoon to the ^gg, and the theory wholly breaks down in
the case of the higher plants. But although the theory probably
cannot be sustained in its morphological form, it may still contain
a large element of truth when recast in physiological terms. Like
mitosis, fertilization is perhaps at bottom a chemical process, the
stimulus to development being given by a specific chemical substance
carried in some cases by an individualized centrosome or one of its
morphological products, in other cases by less definitely formed
material. In the case of animals, we cannot ignore the historical
continuity shown in the origin of the spermatid-centrosomes, the
formation of the middle-piece, and the origin of the sperm-centro-
somes and sperm-amphiaster in the ^gg, even though we do not
yet know whether the sperm-centrosome is as such imported into
the ^gg. And this chain of phenomena suggests that even in the
higher plants, where no centrosomes seem to occur, the fertilizing
substance, even if brought into the ^gg in an unformed state, may
still be genetically related to the mitotic apparatus of the preceding
division.^

Through the differentiation between the paternal and germ-cells
in the higher forms indicated above, their original morphological
equivalence is lost and only the nuclei remain of exactly the same
value. This is shown by their history in fertilization, each giving
rise to the same number of chromosomes exactly similar in form,
size, and staining-reactions, equally distributed by cleavage to the
daughter-cells, and probably to all the cells of the body. We thus
find the essential fact of fertilization and sexual reproduction to be a
union of equivalent nuclei ; and to this all other processes are tributary.

As regards the most highly differentiated type of fertilization and

1 Cf. Strasburger's view, p. 221,

LITERATURE 231

development we reach therefore the following conception . From the
mother comes in the main the cytoplasm of the embryonic body which
is the principal substratum of growth and differentiation. From both
parents comes the hereditary basis or chromatin by which these pro-
cesses are controlled and from which they receive the specific stamp of
the race. From the father comes the stimulus inducing the organiza-
tion of the machinery of mitotic division by which the ^g% splits up
into the elements of the tissues, and by which each of these elements
receives its quota of the common heritage of chromatin. Huxley hit
the mark two score years ago when in the words that head this chap-
ter he compared the organism to a web of which the warp is derived
from the female and the woof from the male. Our principal advance
upon this view is the knowledge that this web is probably to be sought
in the chromatic substance of the nuclei ; and perhaps we shall not
push the figure too far if we compare the amphiaster to the loom on
which the fabric is woven.

LITERATURE. IV i

Van Beneden, E. — Recherches sur la maturation de I'oeuf, la f(^condation et la divi-
sion cellulaire : Arch. Biol., IV. 1883.
Van Beneden and Neyt. — Nouvelles recherches sur la f(fcondation et la division

mitosique chez TAscaride mdgalocephale : Bull. Acad. roy. de Belgique, III. 14,

No. 8. 1887.
Boveri, Th. — Uber den Anteil des Spermatozoon an der Teilung des Eies : Sitz.-

Ber. d. Ges.f. Morph. u. Phys. in MUnchen, B. III., Heft 3. 1887.
Id. — Zellenstudien, II. 1888.

Id. — Befruchtung: Merkel und Bonnefs Ergebnisse,\. 1891.
Id. — Uber das Verhalten der Centrosomen bei der Befruchtung des Seeigeleies, etc. :

Verhandl. Phys. Med. Ges. Wurzburg, XXIX. 1895.
Biitschli, 0. — Studien liber die ersten Entwicklungsvorgange der Eizelle, u. s. w. :

Abh. Senckenb. Ges., X. 1876.
Coe, W. R., 99. The Maturation and Fertilization of the Egg of Cerebratulus : Zool.

Jahrb., XII.
Fick, R. — Uber die Reifung und Befruchtung des Axolotleies : Zeitschr. Wiss. Zool..,

LVI.4. 1893.
Grif&n, B. B. — Studies on the Maturation, Fertilization, and Cleavage of Thalassema

and Zirphaea: Journ. Morph., XV. 1899.
Guignard, L. — Nouvelles Etudes sur la f^condation : Ann. d. Sciences nat. Bot.,

XIV. 1891.
Hartog, M. M. — Some Problems of Reproduction, etc. : Quart. Journ. Mic. Sci.,

XXXIII. 1891.
Hertwig, 0. — Beitrage zur Kenntniss der Bildung, Befruchtung und Teilung des

tierischen Eies, I. : Morph. Jahrb., I. 1875.
Hertwig, R. — Uber die Konjugation der Infusorien: Abh. d. bayr. Akad. d. Wiss.^

II. CI. XVII. 1888-89.
Id. — Uber Befruchtung und Konjugation: Verh. deutsch. Zool. Ges. Berlin, 1892.

1 See also Literature, V., p. 287.

232 FERTILIZATION OF THE OVUM

Kostanecki, K. v., and Wierzejski, A. — Uber das Verhalten der sogen. achromati-

schen Substanzen im befmchteten Ei : Arch. inik. AnaI,XLVll. 2. 1896.
Mark, E. L. — Maturation. Fecundation, and Segmentation of Liinax ca7npestris :

Bull. Mus. Camp. Zo'dl. Harvard College., Cambridge, Mass., VI. 1881.
Maupas. — Le rejeunissement karyogamique chez les Cilies : Arch. d. Zo'dl., 2"^

serie, VII. 1889.
Mead, A. D. — The Origin and Behaviour of the Centrosomes of the Annehd Egg:

Journ. Morph., XIV. 2. 1898.
Riickert, J. — Uber das Selbstandigbleiben der vaterHchen und miitterlichen Kern-

substanz wahrend der ersten Entwicklung des befruchteten Cyclops-Eies : Arch.

mik. Anat., XLV. 3. 1895.
Strasburger, E. — Neue Untersuchungen liber den Befruchtungsvorgang bei den

Phanerogamen, als Grundlage fur eine Theorie der Zeugung. Jena, 1884.
Id. — tJber Kern- und Zellteilung im Pflanzenreich, nebst einem Anhang liber

Befruchtung. Jena, 1888. (See Literature II.)
Vejdovsky, F. — EntwickelungsgeschichtUche Untersuchungen, Heft i, Reifung,

Befruchtung und Furchung des Rhynchelmis-Eies. Prag, 1888.
Waldeyer, W. — Befruchtung und Vererbung : Verh. Ges. detdsch. Natiirf. u. Aerzte,

LXIX. 1897.
Wilson, Edm. B. — Atlas of Fertilization and Karyokinesis. New York, 1895.
Zoja, R. — Stato Attuale degU Studi sulla Fecondazione : Boll. Scientif. di Pavia^

XVIII., XIX. 1896-97.

CHAPTER V

OOGENESIS AND SPERMATOGENESIS. REDUCTION OF THE
CHROMOSOMES

" Es kommt also in der Generationenreihe der Keimzelle irgendwo zu einer Reduktiot
der urspriinglich vorhandenen Chromosomenzahl auf die Halfte, und diese Zahlen-x^Avk.-
tion ist demnach nicht etwa nur ein theoretisches Postulat, sondern eine Thatsache."

BOVERI.I

Van Beneden's epoch-making discovery that the nuclei of the con-
jugating germ-cells contain each one-half the number of chromosomes
characteristic of the body-cells has now been extended to so many
plants and animals that it may probably be regarded as a universal
law of development. The process by which the reduction in number
is effected, forms the most essential part of the phenomena of matura-
tion by which the germ-cells are prepared for their union. No phe-
nomena of cell-life possess a higher theoretical interest than these.
For, on the one hand, nowhere in the history of the cell do we find so
unmistakable and striking an adaptation of means to ends or one of
so marked a prophetic character, since maturation looks not to the
present but to the future of the germ-cells. On the other hand, the
chromatin-reduction suggests questions relating to the morphological
constitution of nucleus and chromatin, which have an important
bearing on all theories of the ultimate structure of living matter and
now stand in the foreground of scientific discussion among the most
debatable and interesting of biological problems.

Two fundamentally different views have been held of the manner
in which the reduction is effected. The earher and simpler view,
which was suggested by Van Beneden and adopted in the earlier
works of Weismann, Boveri, and others, assumed an actual degenera-
tion or casting out of half of the chromosomes during the growth .
of the germ-cells — a simple and easily intelHgible process. Later
researches conclusively showed, however, that this view cannot be
sustained, and that reduction is effected by a rearrangement and redis-
tribution of the fiuclear substance without loss of any of its essential
constituents. It is true that a large amount of chromatin is lost dur-
ing the growth of the ^gg? It is nevertheless certain that this loss is
not directly connected with the process of reduction; for, as Hertwig

1 Zellenstudien, III., p. 62. 2 qt Figs. 97, n6.

233

234

REDUCTION OF THE CHROMOSOMES

and others have shown, no such loss occurs during spermatogenesis,
and even in the oogenesis the evidence is clear that an explanation
must be sought in another direction. The attempts to find such an
explanation have led to some of the most interesting researches of
modern cytology ; and though only partially successful, they have
raised many new questions which promise to give in the end a deeper
insight into some of the fundamental questions of cell-morphology.
For this reason they deserve careful consideration, despite the fact
that taken as a whole the subject still remains an unsolved riddle in
the face of which we can only return again and again to Boveri's
remark that whatever be its theoretical interpretation the numerical
reduction of the chromosomes is itself not a theory but a fact.

Fig. 114. — Formation of the polar bodies before entrance of the spermatozoon, as seen in the
living ovarian egg of the sea-urchin, Toxopneustes (x 365).

A. Preliminary change of form in the germinal vesicle. B. The first polar body formed, the
second forming. C. The ripe egg, ready for fertilization, after formation of the two polar bodies
(/. <J. I, 2) ; e. the egg-nucleus. In this animal the first polar body fails to divide. For its division
see Fig. 89.

A. General Outline

The general phenomena of maturation fall under two heads : viz.
oogenesis, which includes the formation and maturation of the ovum,
and spermatogenesis, comprising the corresponding phenomena in case
of the spermatozoon. Recent research has shown that maturation
conforms to the same type in both sexes, which show as close a paral-
lel in this regard as in the later history of the germ-nuclei. Stated in
the most general terms, this parallel is as follows : ^ In both sexes the
final reduction in the number of chromosomes is effected in the course
of the last two cell-divisions, or maturation-divisions, by which the
definitive germ-cells arise, each of the four cells thus formed having
but half the usual number of chromosomes. In the female but one

1 The parallel was first clearly pointed out by Plainer in il
strated by Oscar Hertwig in the following year.

and was brilliantly demon-

GENERAL OUTLINE

235

of the four cells forms the '' ovum " proper, while the other three,
known as the polar bodies, are minute, rudimentary, and incapable of
development (Figs. 89, 97, 114). In the male, on the other hand, all
four of the cells become functional spermatozoa. This difference
between the two sexes is probably due to the physiological division of
labour between the germ-cells, the spermatozoa being motile and very
small, while the ^gg contains a large amount of protoplasm and yolk,
out of which the main mass of the embryonic body is formed. In the
male, therefore, all of the four cells may become functional ; in the
female the functions of development have become restricted to but one

Primordial germ-cell.

Oogonia.

Primary oocyte or ovarian egg.

Secondary oocytes (egg and

first polar body).

Mature egg and three polar bodies

Division-period (the number of divi-
sions is much greater).

. Growth-period.

Maturation-period.

Fig. 115. — Diagram showing the genesis of the egg. [After BOVERI.]

of the four, while the others have become rudimentary {cf. p. 124).
The polar bodies are therefore not only rudimentary cells (Giard, '76),
but may further be regarded as abortive eggs — a view first put forward
by Mark in 1881, and ultimately adopted by nearly all investigators.^
The evidence is steadily accumulating that reduction is accomplished
by two maturation-divisions throughout the animal kingdom, even in
the unicellular forms ; though in certain Infusoria an additional divi-
sion occurs, while in some other Protozoa only one maturation-division
has thus far been made out. Among plants, also, two maturation-

1 A beautiful confirmation of this view is given by Francottes's ('97) observations on a
turbellarian, ProsthecercBus. The first polar body is here often abnormally large, all grada-
tions having been observed from the normal size up to cells nearly as large as the egg itself.
Such polar bodies are occasionally fertilized zxi^ develop into small gastrulas, first forming a
single polar body like the second polar body of the egg. Here, therefore, two of the four
cells are exceptionally capable of development. It may be added that Fol long ago observed
the penetration of the small polar bodies by spermatozoa in the echinoderms; and this has
been more recently observed by Kostanecki in mollusks.

236 REDUCTION OF THE CHROMOSOMES

divisions occur in all the higher forms (Muscineae, pteridophytes, and
phanerogams), and in some, at least, of the lower ones. Here, how-
ever, the phenomena are complicated by the fact that the two divi-
sions do not as a rule give rise directly to the four sexual germ-cells,
but to four asexual spores which undergo additional divisions before
the definitive germ-cells are produced. In the flowering plants there
are only a few such divisions, which give rise to structures within the
pollen-tube or embryo-sac. In the archegoniate cryptogams, on the
other hand, each spore gives rise, by repeated divisions, to a " sexual
generation " (prothallium, etc.) that intervenes between the process
of reduction and that of fertilization. The following account deals
primarily with reduction in animals, the plants being afterward con-
sidered.

I. Reduction in the Female. Formation of the Polar Bodies

As described in Chapter III., the ^gg arises by the division of cells
descended from the primordial egg-cells of the maternal organism,
and these may be differentiated from the somatic cells at a very early
period, sometimes even in the cleavage-stages. As development pro-
ceeds, each primordial cell gives rise, by division of the usual mitotic
type, to a number of descendants known as oogonia (Fig. 1 15), which
are the immediate predecessors of the ovarian Q,gg. At a certain
period these cease to divide. Each of them then grows to form an
ovarian ^gg, its nucleus enlarging to form the germinal vesicle, its
cytoplasm becoming more or less laden with food-matters (yolk or
deutoplasm), while egg-membranes may be formed around it. The
ovum may now be termed the oocyte (Boveri) or ovarian ^gg.

In this condition the egg-cell remains until near the time of fertili-
zation, when the process of maturation proper — i.e. the formation of
the polar bodies — takes place. In some cases, e.g. in the sea-urchin,
the polar bodies are formed before fertilization, while the ^gg is still
in the ovary. More commonly, as in annelids, gasteropods, nema-
todes, they are not formed until after the spermatozoon has made
its entrance ; while in a few cases one polar body may be formed
before fertilization and one afterward, as in the lamprey-eel, the frog,
and Amphioxns} In all these cases the essential phenomena are the
same. Two minute cells are formed, one after the other, near the
upper or animal pole of the ovum (Figs. 97, 1 16); and in many cases
the first of these divides into two as the second is formed (Fig. 89).

A group of four cells thus arises, namely, the mature ^gg, which
gives rise to the embryo, and three small cells or polar bodies which
take no part in the further development, are discarded, and soon die

1 Cf. p. 189.

GENERAL OUTLINE

237

without further change. The egg-nucleus is now ready for union
with the sperm-nucleus.

B H

Fig. 116. — Diagrams showing the essential facts in the maturation of the egg. The somatic
number of chromosomes is supposed to be four.

A. Initial phase; two tetrads have been formed in the germinal vesicle. B. The two tetrads
have been drawn up about the spindle to form the equatorial plate of the first polar mitotic
figure. C. The mitotic figure has rotated into position, leaving the remains of the germinal
vesicle at g.v. D. Formation of the first polar body; each tetrad divides into two dyads,
E. First polar body formed; two dyads in it and in the egg. F. Preparation for the second
division. G. Second polar body forming and the first dividing; each dyad divides into two
single chromosomes. H. Final result; three polar bodies and the egg-nucleus (?), each con-
taining two single chromosomes (half the somatic number) ; c. the egg-centrosome which now
degenerates and is lost. ,

238 REDUCTION OF THE CHROMOSOMES

A study of the nucleus during these changes brings out the follow-
ing facts. During the multiplication of the oogonia the number of
chromosomes is the same as that occurring in the division of the
somatic cells, and the same number enters into the formation of the
chromatic reticulum of the germinal vesicle. During the formation
of the polar bodies this number becomes reduced to one-half, the
nucleus of each polar body and the egg-nucleus receiving the reduced
number. In some manner, therefore, the formation of the polar
bodies is connected with the process by which the reduction is ef-
fected. The precise nature of this process is, however, a matter
which has been certainly determined in only a few cases.

We need not here consider the history of opinion on this subject
further than to point out that the early observers, such as Purkinie,
Von Baer, Bischoff, had no real understanding of the process and
believed the germinal vesicle to disappear at the time of fertilization.
To BiitschU {^'J^)y Hertwig, and Giard ('76, '77) we owe the discovery,
that the formation of the polar bodies is through mitotic division, the
chromosomes of the equatorial plate being derived from the chro-
matin of the germinal vesicle.^ In the formation of the first polar
body the group of chromosomes. splits into two daughter-groups, and
this process is immediately repeated in the formation of the second
without an ititervening reticular resting stage. The egg-nucleus
therefore receives, like each of the polar bodies, one-fourth of the
mass of chromatin derived from the germinal vesicle.

But although the formation of the polar bodies was thus shown to
be a process of true cell-division, the history of the chromosomes was
found to differ in some very important particulars from that of the
tissue-cells. The essential facts, which were first carefully studied
in Ascaris by Van Beneden ('83, '87), and especially by Boveri ('87, i),
are in a typical case as follows (Figs. 116, 117): As the Q,gg prepares
for the formation of the first polar body, the chromatin of the ger-
minal vesicle groups itself in a number of masses, each of which
splits up into a group of four bodies united by linin-threads to form a
" quadruple group" or tetrad (Vierergruppe). The number of tetrads
is always one-half the usual number of chromosomes. Thus in Ascaris
{megalocephala, bivalens) the germinal vesicle gives rise to two tetrads,
the normal number of chromosomes in the earlier divisions being
four ; in the mole-cricket there are six tetrads, the somatic number
of chromosomes being twelve ; in Cyclops the respective numbers are
twelve and twenty-four (one of the most frequent cases); while in
Artemia there are eighty-four tetrads and one hundred and sixty-

1 The early accounts asserting the disappearance of the germinal vesicle were based on
the fact that in many cases only a small fraction of the chromatic network gives rise to
chromosomes, the remainder disintegrating and being scattered through the yolk.

GENERAL OUTLINE

239

eight somatic chromosomes — the highest number thus far accurately
counted. As the first polar body forms, each of the tetrads is halved
to form two double groups, or dyads, one of which remains in the ^gg

K

Fig. 117. — Furmation of the polar bodies in Ascaris megalocephala, var. bivalens. [BOVERI.]
A. The egg with the spermatozoon just entering at c? ; the germinal vesicle contains two rod-
shaped tetrads (only one clearly shown), the number of chromosomes in earlier divisions having
been four. B. The tetrads seen in profile. C. The same in end view. D. First spindle forming
(in this case inside the germinal vesicle). E. First polar spindle, F. The tetrads dividing.
G. First polar body foimed, containing, like the egg, two dyads. H. I. The dyads rotating into
position for the second division. J. The dyads dividing. K. Each dyad has divided into two
single chromosomes, completing the reduction. (For later stages see Fig. 90.)

240

REDUCTION OF THE CHROMOSOMES

while the other passes into the polar body. Both the ^^^ and the
first polar body therefore receive each a number of dyads equal to
one-half the usual number of chromosomes. The ^g^ now proceeds
at once to the formation of the second polar body without previous
reconstruction of the nucleus. Each dyad is halved to form two
single chromosomes, one of which, again, remains in the ^%g while
its sister passes into the polar body. Both the ^g^ and the second
polar body accordingly receive two single chromosomes (one-half the
usual number), each of which is one-fourth of an original tetrad
group. From the two remaining in the o^gg a reticular nucleus, much
smaller than the original germinal vesicle, is now formed.^

Primordial germ-cell.

Spermatogonia.

Primary spermatocyte.

Secondary spermatocytes.

Spermatids.
Spermatozoa.

Division -period (the number of divt
sions is much greater).

Growth-period.

M a t u ration-period.

Fig. Ii8. — Diagram showing the genesis of tne spermatozoon, [After BOVERI.]

Essentially similar facts have now been determined in a consider-
able number of animals, though, as we shall presently see, tetrad-
formation is not of universal occurrence, nor is it always of the same
type. For the moment we need only point out that the numerical
reduction of Q)cixovi\'dX\n-masses takes place before the polar bodies
are actually formed, through processes which determine the number
of tetrads within the germinal vesicle. The numerical reduction is
therefore determined in the grandmother-cell of the ^gg. The actual
divisions by which the polar bodies are formed merely distribute the
elements of the tetrads.

1 It is nearly certain that the division of the first polar body (which, however, may be
omitted) is analogous to that by which the second is formed, i.e. each of the dyads is
siniilarly halved. Cf. Griffin, '99.

GENERAL OUTLINE 24 1

2. Reductiofi in the Male. Spermatogenesis

The researches of Plainer ('89), Boveri, and especially of Oscar
Hertwig ('90, i) have demonstrated that reduction takes place in the
male in a manner almost precisely parallel to that occurring in the
female. Platner first suggested ('89) that the formation of the polar
bodies is directly comparable to the last two divisions of the sperm
mother-cells (spermatocytes). In the following year Boveri reached
the same result in Ascaris, stating his conclusion that reduction in
the male must take place in the "grandmother-cell of the sperma-
tozoon, just as in the female it takes place in the grandmother-cell
of the Qgg,'' and that the egg-formation and sperm-formation really
agree down to the smallest detail ('90, p. 64). Later in the same
year appeared Oscar Hertwig's splendid work on the spermato-
genesis of Ascaris, which established this conclusion in the most
striking manner. Like the ova, the spermatozoa are descended from
primordial germ-cells which by mitotic division give rise to the
spermatogonia from which the spermatozoa are ultimately formed
(Fig. 118). Like the oogonia, the spermatogonia continue for a time
to divide with the usual (somatic) number of chromosomes, i.e. four
in Ascaris megalocephala bivalens. Ceasing for a time to divide, they
now enlarge considerably to form spermatocytes, each of which is
morphologically equivalent to an unripe ovarian ovum, or oocyte.
Each spermatocyte finally divides twice in rapid succession, giving
rise first to two daughter-spermatocytes and then to four spermatids,
each of which is directly converted into a single spermatozoon. The
history of the chromatin in these two divisions is exactly pai'allel to
that in the formation of the polar bodies (Figs. 1 19, 120). From the
chromatin of the spermatocyte are formed a number of tetrads equal
to one-half the usual number of chromosomes. Each tetrad is halved
at the first division to form two dyads which pass into the respective
daughter-spermatocytes. At the ensuing division, which occurs with-
out the previous formation of a resting reticular nucleus, each dyad
is halved to form two single chromosomes which enter the respec-
tive spermatids (ultimately spermatozoa). From each spermatocyte,
therefore, arise four spermatozoa, and each sperm-nucleus receives
half the usual number of single chromosomes. The parallel with the
egg-reduction is complete.

These facts leave no doubt that the spermatocyte is the morpho-
logical equivalent of the oocyte or immature ovarian Q,gg, and that
the group of four spermatozoa to which it gives rise is equivalent
to the ripe Qgg plus the three polar bodies. Hertwig was thus led to
the following beautifully clear and simple conclusion : " The polar
bodies are abortive eggs which are formed by a final process of

242

REDUCTION OF THE CHRaMOSOMES

division from the egg-mother-cell (oocyte) in the same manner as the
spermatozoa are formed from the sperm-mother-cell (spermatocyte).
But while in the latter case the products of the division are all used
as functional spermatozoa, in the former case one of the products

Fig. 119. — Diagrams showing the essential facts of reduction in the male. The somatic num-
ber of chromosomes is supposed to be four.

A. /?. Division of one of the spermatogonia, showing the full number (four) of chromosomes.
C. Primary spermatocyte preparing for division ; the chromatin forms two tetrads. D. E. F. First
division to form two secondary spermatocytes each of which receives two dyads. G. H. Division
of the two secondary spermatocytes to form four spermatids. Each of the latter receives two
single chromosomes and a centrosome which passes into the middle-piece of the spermatozoon.

of the egg-mother-cell becomes the ^^g, appropriating to itself the
entire mass of the yolk at the cost of the others which persist in
rudimentary form as the polar bodies." ^

'90,

126.

GENERAL OUTLINE 243

3 . WeismanrC s Interpretation of Reduction

Up to this point the facts are clear and intelligible. Before com-
ing to closer quarters with them it will be useful to make a digression
in order to consider some of the theoretical aspects of reduction ;
though the reader must be warned that this will lead us into very
uncertain ground traversed by a labyrinth of conflicting hypotheses
from which no exit has yet been discovered.

The process of reduction is very obviously a provision to hold con-
stant the number of chromosomes characteristic of the species ; for
if it did not occur, the number would be doubled in each succeeding
generation through union of the germ-cells.^ A number of writers
have contented themselves with this simple interpretation, Oscar
Hertwig, for example, regarding reduction as "merely a process to
prevent a summation through fertilization of the nuclear mass and of
the chromatic elements." ^ A moment's reflection reveals the entire
inadequacy of such an explanation. As far as the chromatin-mass is
concerned, it does not agree with the facts ; for in reduction with
tetrad-formation the chromatin-mass is reduced not to one-half, but to
one-fourth. That reduction must mean more than mere mass-reduc-
tion is moreover proved by the fact that the bulk of the nucleus may
enormously increase or decrease at different periods in the same cell,
irrespective of the number of chromosomes. The real problem is
why the number of chromosomes should be held constant. The

1 Of the many earlier attempts to interpret the meaning of the polar bodies, we need only
consider at this point the very interesting suggestion of Minot ('77), afterward adopted by
Van Beneden ('83), that the ordinary cell is hermaphrodite, and that maturation is for the
purpose of producing a unisexual germ-cell by dividing the mother-cell into its sexual con-
stituents, or "genoblasts." Thus, the male element is removed from the egg in the polar
bodies, leaving the mature egg a female. In like manner he believed the female element to
be cast out during spermatogenesis (in the " Sertoli cells "), thus rendering the spermatozoa
male. By the union of the germ-cells in fertihzation, the male and female elements are
brought together so that the fertilized egg or oosperm is again hermaphrodite or neuter.
This ingenious view was independently advocated by Van Beneden in his great work on
Ascaris ('83). A fatal objection to it, on which both Strasburger and Weismann have
insisted, lies in the fact that male as well as female qualities are transmitted by the egg-cell,
while the sperm-cell also transmits female qualities. The germ-cells are therefore non-sexual.
The researches of many observers show, moreover, that all of the four spermatids derived
from a spermatocyte become functional spermatozoa. Minot's hypothesis must, therefore, in
my opinion, be abandoned.

Balfour doubtless approximated more nearly to the truth when he said, " In the formation
of the polar cells part of the constituents of the germinal vesicle, which are requisite for its
functions as a complete and independent nucleus, is removed to make room for the supply
of the necessary parts to it again by the spermatic nucleus" ('80, p. 62). He fell, however,
into the same error as Minot and Van Beneden in characterizing the germ-nuclei as " male "
and "female"; and, as shown at pages 194, 353, it has been found that a single germ-
nucleus is able to carry out development of an embryo without union with another.

2 '90, I, p. 112. Cf. Hartog, '91, p. 57.

244

REDUCTION OF THE CHROMOSOMES

deeper meaning of the phenomena was first seriously considered by
Weismann in his essays of 1885 and 1887 ; and, -although his conclu-
sions were of a highly speculative character, they nevertheless gave so

D

w3

\

/

5- 5^

«>^ii'

IJ

\%

K

Fig. 120. — Reduction in the spermatogenesis oiAscarts megalocephala^-VAX. bivalens. [Brauer.] I
A-G. Successive stages in the division of the primary spermatocyte. The original reticulum
undergoes a very early division of the chromatin-granules which then form a doubly split spireme-
thread, 5. This shortens (C), and breaks in two to form the two tetrads (Z) in profile, £■ viewed
endwise). F. G. H. First division to form two secondary spermatocytes, each receiving two dyads.
/. Secondary spermatocyte. % K. The same dividing. Z. Two resulting spermatids, each with
two single chromosomes and a centrosome.

great a stimulus to the study of the entire problem that his views
deserve special attention. Weismann's interpretation was based on a
remarkable paper published by Wilhelm Roux in 1883,^ in which are

1 For division of the spermatogonia see Fig. 55; for the corresponding phenomena in var.
univalens see Fig. 148.

^ Vber die Bedeutung der Kerntheilungsjiguren,

GENERAL OUTLINE 245

developed certain ideas which afterward formed the foundation of
Weismann's whole theory of inheritance and development. Roux
argued that the facts of mitosis are only explicable under the assump-
tion that chromatin is not a uniform and homogeneous substance, but
differs qualitatively in different regions of the nucleus ; that the col-
lection of the chromatin into a thread and its accurate division into
two halves is meaningless unless the chromatin in different regions of
the thread represents different qualities which are to be divided and
distributed to the daughter-cells according to some definite law. He
urged that if the chromatin were qualitatively the same throughout
the nucleus, direct division would be as efficacious as indirect, and the
complicated apparatus of mitosis would be superfluous. Roux and
Weismann, each in his own way, subsequently elaborated this con-
ception to a complete theory of inheritance and development, but at
this point we may confine our attention to the views of Weismann.
The starting-point of his theory is the hypothesis of De Vries that the
chromatin is a congeries or colony of invisible self-propagating vital
units or biophores somewhat Hke Darwin's "gemmules" (p. 12), each
of which has the power of determining the development of a particu-
lar quality. Weismann conceives these units as aggregated to form
units of a higher order known as " determinants," which in turn are
grouped to form ** ids," each of which, for reasons that need not here
be specified,^ is assumed to possess the complete architecture of the
germ-plasm characteristic of the species. The "ids" finally, which
are identified with the visible chromatin-granules, are arranged in
linear series to form " idants " or chromosomes. It is assumed further
that the " ids " differ slightly in a manner corresponding with the indi-
vidual variations of the species, each chromosome therefore being a
particular group of slightly different germ-plasms and differing quali-
tatively from all the others.

We come now to the essence of Weismann's interpretation. The
end of fertilization is to produce new combinations of variations by
the mixture of different ids. Sihce, however, their number, like that
of the chromosomes which they form, is doubled by the union of two
germ-nuclei, an infinite complexity of the chromatin would soon arise
did not a periodic reduction occur. Assuming, then, that the '* ances-
tral germ-plasms" (ids) are arranged in a linear series in the spireme-
thread or the chromosomes derived from it, Weismann ventured the
prediction i^^j) that two kinds of mitosis would be found to occur.
The first of these is characterized by a longitudinal splitting of the
thread, as in ordinary cell-division, *' by means of which all the ances-
tral germ-plasms are equally distributed in each of the daughter-nuclei
after having been divided into halves." This form of division, which

1 Cf. the Germ-plasm, p. 60.

24& REDUCTION OF THE CHROMOSOMES

he called equal divisiort (Aequationstheilung), was then a known fact.
The second form, at that time a purely theoretical postulate, he as-
sumed to be of such a character that each daughter-nucleus should
receive only half the number of ancestral germ-plasms possessed
by the mother-nucleus. This he termed a reducing division (Re-
duktionstheilung), and suggested that this might be effected either
by a transverse division of the chromosomes, or by the elimination of
entire chromosomes without division.^ By either method the number
of " ids " would be reduced ; and Weismann argued that such reduc-
ing divisions must be involved in the formation of the polar bodies,
and in the parallel phenomena of spermatogenesis.

The fulfilment of Weismann's prediction is one of the most inter-
esting results of recent cytological research. It has been demon-
strated, in a manner which seems to be incontrovertible, that the
reducing divisions postulated by Weismann actually occur, though
not precisely in the manner conceived by him. Unfortunately for
the general theory, however, transverse divisions have been cer-
tainly determined in only a few types, while in others, of which
Ascaris is the best-known example, the facts thus far known seem
clearly opposed to the assumption. On the whole, the evidence of
reducing divisions, i.e. such as involve a transverse and not a longi-
tudinal division of the chromatin-thread, has steadily increased ; but
it remains quite an open question whether they have the significance
attributed to them by Weismann.

» B. Origin of the Tetrads

I. General Sketch

In considering the origin of the tetrads or their equivalents, it
should be borne in mind that true tetrad-formation, as described
above, has only been certainly observed in a few groups (most
clearly in the nematodes and arthropods). But even in cases where
the chromatin does not condense into actual tetrads these bodies
are represented by chromosomes in the form of rings, crosses, and
the like, which are closely similar, and doubtless equivalent, to those
from which actual tetrads arise, and present us with the same prob-
lems. With a few apparent exceptions, described hereafter, the
tetrads of their equivalents always arise by a double division of a
single primary chromatin-rod or mass. Nearly all observers agree
further that the number of primary rods at their first appearance in
the germinal vesicle or in the spermatocyte-nucleus is one-half the
usual number of chromosomes^ and that this numerical reduction is
due to the fact that the spireme-thread segments into one-half the

1 Essay VI., p. 375.

ORIGIN OF THE TETRADS

247

usual number of pieces. Apparently, however, there are two radi-
cally different types of tetrad-formation as follows.

In the first type the tetrad arises by one longitudinal and one trans-
verse division of each primary chromatin-rod, the latter effecting the
reduction demanded by

A B

D

I

» / /(

H 1

> 2. 4

y-n

>^ <

.0

)4 i

H

► 5 <

M

s6 t

►H

•7 <

h

i^8 8i

H

< ti i

a

i y\ >

•-#

a

a

IH^ ^

<^4

<H>

<3^

z;^

}

<\4

<»7
8

(H>

• / ^fSf

^

Ct

Weismann's hypothesis(Fig.
121, I). To give the usual
graphic representation, let
us, for the sake of discus-
sion, assume the somatic
number of chromosomes to
be four, designating the
spireme-thread as a b c d,
each letter representing a
chromosome, each of which
we may in turn assume to
consist of a series of four
granules or " ids " (Fig. 121).
In ordinary mitosis the spi-
reme would segment into
a — b — c — d, which then
would divide lengthwise to
form pairs of identical sister

, abed

chromosomes

abed

To form the tetrad, on the
other hand, the spireme first
segments into two rods ab
and cd, each of which, in
view of its subsequent his-
tory, may be regarded as
bivalent, representing two
chromosomes united end to
end (Vom Rath, Riickert,
Hacker). Each of these
divides once longitudinally,
giving the identical pairs or

dyads — • , and once transversely, giving the tetrads

ab cd

Inspection of Fig. 121, 1, shows that through the second or transverse
division, each member of the tetrad receives only half the number of
ids contained in the original segment. This number, four, is the same
as that assumed for a single chromosome ; and, since each of the two
tetrads contributes one chromosome to the germ-cell, the latter receives

8 4 4 8

8

lis

8

ab ab

ab ab

Fig. 121. — Diagrams of tetrad-formation; I, v, »/a
one transverse and one longitudinal division (copepod
type) ; II, with two longitudinal divisions {Ascaris type).

A-D, successive stages; chromatin-granules num-
bered from I to 8. The two types diverge at C. In D
the granules of each constituent of the tetrad fuse to form
a homogeneous sphere.

a
a

b c
b c

d
d

ab cd ■, c- n ab

— , and nnally — -

ab cd ab

— -, by two longitudinal divisions. In
cd

248 REDUCTION OF THE CHROMOSOMES

but half the usual number both of chromosomes and of ids. This mode
of tetrad-formation has been most clearly demonstrated in insects
and copepods, and an equivalent process occurs also in mollusks,
annelids, turbellarians, and some other animals, as described beyond.
In the second type, illustrated especially by Ascaris, the tetrad is
apparently formed by tivo longitudinal divisiojis of each primary
chromatin-rod, and no reducing division occurs. If, therefore, we
adopt the same terminology as before, we have first ab and cd, then

ab cd

ab cd

this case, according to Brauer's careful studies, each chromatin-granule
(" id ") divides at each longitudinal division of the primary rod. The
four chromosomes of the tetrad are therefore exactly equivalent, being
derived from the same region of the spireme-thread, and containing
the undiminished number of ** ids " (Fig. 121, II).

The contradiction may be stated in a different way. In the first
type of tetrad formation, the number both of granules and of chro-
mosomes is first doubled {i.e. in the assumed case, through the forma-
tion of two tetrads, each consisting of four chromosomes, or eight in
all), and then reduced to half that number by the two successive matu-
ration-divisions. In the second type, on the other hand, the number
of chromosomes is likewise doubled, but that of the granules is qzcad-
rupled, so that, although in both types the two maturation-divisions
reduce the number of chromosomes to one-half, only in the first type
do they reduce the number of granules or "ids," as Weismann's
hypothesis demands. We must therefore distinguish sharply between
the reduction of the chromosomes and that of the "ids." The former
is primarily effected by the segmentation of the primary spireme-
thread, or the resolution of the nuclear reticulum, into one-half the
usual number of segments (?>. the "pseudo-reduction" of Riickert);
and here the real secret of the reduction of the chromosomes lies. The
reduction of the "ids," if they have any real existence, is a distinct,
and as yet unsolved, question.

2. Detailed Evidence

We may now consider some of the phenomena in detail, though the
limits of this work will only allow the consideration of a few typical
cases.

(a) Tctradformation with one Longitudinal and 07ie Transverse
Division. — In many of the cases of this type the tetrads arise from
ring-shaped bodies which are analogous to the ring-shaped chromo-
somes occurring in heterotypical mitosis (p. 86). First observed by
HenkingCgi) in /^j/r///<?7?r/j', tetrad-origin of this type has since been
found in other insects by Vom Rath, Toyama, Paulmier, and others,

ORIGIN OF THE TETRADS

249

in copepods by Riickert, Hacker, and Vom Rath, in pteridophytes by
Calkins and Osterhout, in the onion, Allium, by Ishikawa, and in
various other forms where their history has been less clearly made
out. The genesis of the ring was first determined by Vom Rath in
the mole cricket {Gjyllotalpa, '92), and has been thoroughly elucidated
by the later work of Riickert ('94), Hacker ('95, i), and Paulmier
('99). All these observers have reached the same conclusion ;
namely, that the ring arises by the longitudinal splitting of a primary
chromatin-rod, the two halves remaining united by their ends, and
opening out to form a ring. The ring-formation is, in fact, a form of

D E F

Fig. 122. — Origin of the tetrads by ring-formation in the spermatogenesis of the mole-cricket
Gryllotalpa. [VOM Rath.]

A. Primary spermatocyte, containing six double rods, each of which represents two chromo-
somes united end to end and longitudinally split except at the free ends. B. C. Opening out of
the double rods to form rings. D. Concentration of the rings. E. The rings broken up into
tetrads. F. First division-figure established.

heterotypical mitosis (p. ^6). The breaking of the ring into four
parts involves, first, the separation of these two halves (corresponding
with the original longitudinal split), and second, the transverse division
of each half, the latter being the reducing division of Weismann.
The number of primary rods, from which the rings arise, is one-half
the somatic number. Hence each of them is conceived by Vom Rath,
Hacker, and Riickert as bivalent or double ; i.e. as representing two
chromosomes united end to end. This appears with the greatest
clearness in the spermatogenesis of Gryllotalpa (Fig. 122). Here

250

REDUCTION OF THE CHROMOSOMES

the spireme-thread splits lengthwise before its segmentation into rods.
It then divides transversely to form six double rods (half the usual
number of chromosomes), which open out to form six closed rings.
These become small and thick, break each into four parts, and thus

Fig. 123. — Formation of the tetrads and polar bodies in Cyclops, slightly schematic. (The
full number of tetrads is not shown.) [RiJCKERT.]

A. Germinal vesicle containing eight longitudinally split chromatin-rods (half the somatic
number). B. Shortening of the rods; transverse division (to form the tetrads) in progress.
C. Position of the tetrads in the first polar spindle, the longitudinal split horizontal. D. Ana-
phase ; longitudinal divisions of the tetrads. E. The first polar body formed ; second polar
spindle with the eight dyads in position for the ensuing division, which will be a transverse or
reducing division.

give rise to six typical tetrads. An essentially similar account of the
ring-formation is given by Vom Rath in Etichceta and Calanus, and
by Riickert in Heterocope and Diaptomtis.

That the foregoing interpretation of the rings is correct, is beauti-
fully demonstrated by the observations of Hacker, and especially of

ORIGIN OF THE TETRADS

251

Riickert, on a number of other Q.o'^^^Q^^\Cy clops, Canthocamptus),
in which rings are not formed, since the splitting of the primary
chromatin-rods is complete. The origin of the tetrads has here been
traced with especial care in Cyclops strenuus, by Riickert ('94), whose
observations, confirmed by Hacker, are quite as convincing as those

a

Pig, 124. — Diagrams of various modes of tetrad-formation. [HACKER.]
a. Common starting-point, a double spireme-thread in the germinal vesicle ; d. common re-
sult, the typical tetrads ; b. c. intermediate stages : at the left the ring-formation (as in Diaptomus,
Gryllotalpa, Heterocope) ; middle series, complete splitting of the rods (as in Cyclops according to
Riickert, and in Canthocamptus) ; at the right by breaking of the V-shaped rods (as in Cyclops
strenuus, according to Hacker.

of Brauer on Ascaris, though they led to a diametrically opposite
result.

The normal number of chromosomes is here twenty-two. In the
germinal vesicle arise eleven threads, which split lengthwise (Fig. 123),
and finally shorten to form double rods, manifestly equivalent to the
closed rings of Diaptomus. Each of these now segments transversely

52

REDUCTION OF THE CHROMOSOMES

to form a tetrad group, and the eleven tetrads then place themselves
in the equator of the spindle for the first polar body (Fig. 123, C\ in
such a manner that the loiigitudmal split is transverse to the axis of
the spindle. As the polar body is formed, the longitudinal halves
of the tetrad separate, and the formation of the first polar body is
thus demonstrated to be an ** equal division " in Weismann's sense.
The eleven dyads remaining in the eggs now rotate (as in Ascaris\

Fig. 125. — Germinal vesicles of various eggs, showing chromosomes, tetrads, and nucleoli.

A. A copepod {Heterocope) showing eight of the sixteen ring-shaped tetrads and the nucleo-
lus. [RiJCKERT.]

B. Later stage of the same, condensation and segmentation of the rings. [RiJCKERT.]

C. " Cyclops strenuus" illustrating Hacker's account of the tetrad-formation from elongate
double rods ; a group of " accessory nucleoli." [Hacker.]

D. Germinal vesicle of an annelid {Ophryotrocha) showing nucleolus and four chromosomes.
[KORSCHKLT.]

SO that the transverse division lies in the equatorial plane, and are
halved during the formation of the second polar body. The division
is accordingly a "reducing division," which leaves eleven single chro-
mosomes in the Qgg. Paulmier's work on Anasa and other Hemip-
tera ('99) gives the same result as the above in regard to the origin
of the tetrads (Figs. 126, 127). The process is, however, slightly
complicated by the fact that no continuous spireme-thread is formed,
while the rings are often bent or twisted and never open out to a

ORIGIN OF THE TETRADS 253

circular form. They finally condense into true tetrads which are
successively divided into dyads and monads by the two divisions ;
but it is an interesting fact that the order of division occurring in the
copepods appears here to be reversed, the first division being the
transverse and the second the longitudinal one — a result agreeing
with Henking's earlier conclusion in the case of Pyrrochoris. Oster-
hout ('97) and Calkins ('97) independently discovered tetrads in the
vascular cryptogams {Equisetum, Pteris), and the last-named observer
finds that in Pteris they may arise either from rings, as in Gryllotalpa
or Heterocope, or from double rods as in Cyclops, the halves in the
latter case being either parallel or forming a cross. This longitu-
dinal split, occurring in the spireme, is followed by a transverse
division by which the tetrad is formed. Tetrads having an essentially
similar mode of origin are also described by Atkinson ('99) in Ari-
scema, and tetrad-formation is nearly approached in Allium according
to Ishikawa ('99).^ These cases are considered at page 263.

Resume. In all the foregoing cases the tetrads arise from a spi-
reme which splits lengthwise, segments into one-half the somatic
number of rods (each longitudinally divided) and each of the latter
divides transversely to form the tetrad. When the ends of the
daughter-chromosomes resulting from the longitudinal split remain
united (as in insects) ring-forms result, and the earlier phases of tetrad-
formation are thus identical with those of heterotypical mitosis.
When the split is complete, so that the ends remain free, double
rods result; while, if the daughter-chromosomes remain temporarily
united at the middle or at the end, X-, Y-, and V-shaped figures may
arise. In all these forms tetrad-formation is completed by the com-
plete separation of the daughter-rods, the transverse division of
each in the middle, and the condensation of the four resulting bodies
into a quadruple mass. As will be shown in Section C (p. 258)
the transverse division is in many forms delayed until after sepa-
ration of the longitudinal halves. In such cases no actual tetrads
are formed, though the result is the same.

{b) Second Type. Tetrad-formation with two Longitudinal Divi-
sions. — The only accurately known case of this type is Ascaris, the
object in which tetrads were first discovered by Van Beneden in
1883. Carnoy ('86, 2) reached the conclusion that the tetrads in
some other nematodes {OpJiiostomum, Ascaris clavata, A. lumbricoides)
arose by a double longitudinal splitting of the primary chromatin-rods.

1 Vom Rath ('93, '59) has endeavoured to show that a process involving the formation of
true tetrads occurs in the salamander and the frog, but the later and more accurate studies
of Meves ('96) seem to leave little doubt that this was an error, and that the tetrads observed
in these forms are not of normal occurrence, as Flemming ('87) had earlier concluded.
Cf. p. 259.

254

REDUCTION OF THE CHROMOSOMES

In the first of his classical cell-studies Boveri i^^J, i) reached the
same result through a careful study of Ascaris megalocephala, showing
that each tetrad appears in the germinal vesicle in the form of four
parallel rods, each consisting of a row of chromatin-granules (Fig. 1 17,
A-C). He believed these rods to arise by the double longitudinal
splitting of a single primary chromatin-rod, each cleavage being a

Fig. 126. — Tetrad-formation in an insect, ^«ajfl. [Paulmier.] .
A. Resting spermatogonium with single plasmosome and two chromatin-nucleoli. B. Equa-
torial plate of dividing spermatogonium ; twenty large and two small chromosomes. C. Final
spermatogonium-division. D-I. Prophases of first maturation-division. D. E. Synapsis, with
single chromatin-nucleolus. F. Segmented split spireme. G. H. Formation of the tetrad-rings.
H. I. Concentration of the rings to form tetrads.

preparation for one of the polar bodies. In his opinion, therefore,
the formation of the polar bodies differs from ordinary mitosis only
in the fact that the chromosomes split very early, and not once, but
twice, in preparation for two rapidly succeeding divisions without an
intervening resting period. He supported this view by further obser-
vations in 1890 on the polar bodies of Sagitta and several gastero-
pods, in which he again determined, as he believed, that the tetrads

ORIGIN OF THE TETRADS

255

arose by double longitudinal splitting. An essentially similar view
of the tetrads was taken by Hertwig in 1890, in the spermatogenesis
of Ascaris, though he could not support this conclusion by very con-
vincing evidence. In 1893, finally, Brauer made a most thorough
and apparently exhaustive study of their origin in the spermatogene-
sis of Ascaris, which seemed to leave no doubt of the correctness of
Boveri's result. Every step in the origin of the tetrads from the retic-
ulum of the resting spermatocytes was traced with the most pains-
taking care. In the early prophases of the first division the nuclear
reticulum breaks up more or less completely into granules, which

Pig. 127. — Maturation-divisions in an insect, Anasa. [Paulmier.]
A. Primary spermatocyte in metaphase. B. Equatorial plate, showing ten large tetrads and
one small one; "odd chromosome" at o. C. Separation of the dyads. D. Telophase, which is
also a prophase of the second division. E. Secondary spermatocyte; division of the dyads;
small dyad shown undivided. F. Final anaphase ; small dyad near the lower chromosome-group.
(The figures are numbered from left to right. For later states, see Fig. 82.)

become in part aggregated in a mass at one side of the nucleus
(''synapsis," p. 276), from which delicate threads extend through the
remaining nuclear space (Fig. 120, A). Even at this period the
granules of the threads are divided into four parts. As the process
proceeds the chromatin resolves itself into a single spireme-thread,
consisting of four parallel rows of granules, which break in two to
form the two tetrads (var. bivalens), or is directly converted into a
single tetrad (var. iinivaletis) (Fig. 120). From these observations
Brauer concludes that each tetrad arises from a rod, doubly split
lengthwise by a process initiated at a very early period through the

256 REDUCTION OF THE CHROMOSOMES

double fission of the chromatin-granules. If this be correct, there
can be no reduction in Weismann's sense ; for the four products of
each primary chromatin-granule are equally distributed among the
four daughter-cells. A similar conclusion, based on much more
incomplete evidence, was reached by Brauer ('92) in the phyllopod
Branchipus.

Brauer's evidently conscientious figures very strongly sustain his
conclusion, which, reinforced by the earlier work of Hertwig and
Boveri, has until now seemed to rest upon an unassailable basis.
The recent work of Sabaschnikoff ('97) nevertheless raises the possi-
bility of a different interpretation. Brauer himself justly urges that
the essence of the process lies in the double fission of the chromatin-
granules to which the formation of chromosomes is secondary.^
Everything, therefore, turns on the manner in which the quadruple
granules arise ; and Sabaschnikoff's work gives some ground for the
view that they may arise, not by a double fission, but in some other
way.

According to this author there is a period (in the oogenesis) at which the nuclear
threads wholly disappear, the entire chromatin being broken up into granules.
From this state the granules emerge in quadruple form to arrange themselves in the
doubly split spireme exactly as Brauer describes ; and a few observations are given
(regarding the size and arrangement of the granules) which suggest the possibility
that the quadruple granules may arise by the conjugation either of four separate
granules or of two pairs of double granules. Since there is ground for the view that
tetrads may arise by the conjugation of chromosomes (see following section), there
is no a priori objection to such a conclusion. Could it be sustained, the maturation-
divisions of Ascaris would in fact involve a true reduction in Weismann's sense ;
for despite the fact that the chromosomes are only longitudinally divided, the four
longitudinal constituents of each tetrad would not be equivalent with respect to the
granules, and it is the reduction of the latter (" ids") that forms the essence of Weis-
mann's hypothesis (p. 245). Another consideration, suggested to me by Professor
T. H. Morgan, opens still another possibility, which seems well worthy of test by
further research. As already stated (p. 88). the long chromosomes oi Ascaj'is are
plurivalent, since in all but the germ-cells each breaks up into a much larger number
of smaller chromosomes (Fig. 73, p. 148). If, therefore, the latter correspond to
the chromosomes of other forms in which tetrads occur (e.g. Cyclops or Artemia)^
the so-called " tetrad " of Ascaris is a compound body ; and the true process of
reduction must be sought in the origin of the smaller elements of which it is com-
posed, which are, perhaps, directly comparable with Sabaschnikoff's '-granules."
Until the questions thus opened have been further studied, the case for Ascaris must
remain open ; and it is perhaps worth suggesting that a new point of view may here
be found for further study also of reduction in the vertebrates.'-^

iC/p. 113.

2 Bodies closely resembling tetrads are sometimes formed in mitosis, where no reduction
should occur. Thus, R. Hertwig ('95) has observed tetrads in the first cleavage-spindle of
echinoderm-eggs after treatment with dilute poisons (p. 306). Klinckowstrom figures them
in the second T^oXdiX: spindle of Prosthecerceus eggs, while Moore ('95) describes in the elasmo-
branchs small ring-shaped chromosomes, not only in the first but also in the second sperma
tocyte-divisions, concluding that no reduction occurs in either division.

ORIGIN OF THE TETRADS 2 $7

(c) The Formation of Tetrads by Conjugatio7i. — A considerable
number of observers have maintained that reduction may be effected
by the union or conjugation of chromosomes that were previously
separate. This view agrees in principle with that of Riickert,
Hacker, and Vom Rath ; for the bivalent chromosomes assumed by
these authors may be conceived as two conjugated chromosomes. It
seems to be confirmed by the observations of Born and Fick on
Amphibia and those of Riickert on selachians {Pristiurus)\ for in all
these cases the number of chromatin-masses at the time the first
polar body is formed is but half the number observed in younger
stages of the germinal vesicle. In Pristiiinis there are at first thirty-
six double segments in the germinal vesicle. At a later period these
give rise to a close spireme, which 'then becomes more open, and is
found to form a double thread segmented into eighteen double seg-
ments ; i.e. the reduced number. In this case, therefore, the prelimi-
nary pseudo-reduction is almost certainly effected by the union of
the original thirty-six double chromosomes, two by two. The most
specific accounts of such a mode of origin have, however, been given
by Calkins (earthworm) and Wilcox (grasshopper). The latter
author asserts ('95) that in Caloptemcs the spireme of the first sperma-
tocyte gives rise without longitudinal division to twenty-four chromo-
somes (double the somatic number). These then become associated
in pairs, and still later the twelve pairs conjugate two and two to form
six tetrads. There is, therefore, no longitudinal splitting of the chro-
mosomes. The a priori improbability of such a conclusion is in-
creased by the studies of Paulmier on the Hemiptera, which demon-
strate the occurrence of a longitudinal division in a number of these
forms and confirm the original studies of Vom Rath on Gryllotalpa}

The second case, which is perhaps better founded, is that of the
earthworm {Ltimbriais terrestris\ as described by Calkins ('95, 2),
whose work was done under my own direction. Calkins finds that
the spireme splits longitudinally and then divides transversely-
into 32 double segments. These then unite, two by two, to form
16 tetrads. The 32 primary double segments therefore represent
chromosomes of the normal number that have split longitudinally,

b a

i.e. -r, etc., and the formula for a tetrad is --

a o a

or -
b a

Such

a tetrad, therefore, agrees as to its composition with the formulas
of Hacker, Vom Rath, and Riickert, and agrees in mode of origin
with the process described by Riickert in the eggs of Pristiurus.
While these observations are not absolutely conclusive, they never-

^ Montgomery, who has denied the occurrence of a longitudinal division in Pentatoma
('98, i), has subsequently found such a division in the nearly related if not identical genus
EuchisHs ('99).

s

I

258 REDUCTION OF THE CHROMOSOMES

theless rest on strong evidence, and they do not stand in actual con-
tradiction of what is known in the copepods and vertebrates. The
possibility of such a mode of origin in other forms must, I think, be
held open.

Under the same category must be placed Korschelt's unique
results in the egg-reduction of the annelid Ophryotrocha ('95), which
are very difficult to reconcile with anything known in other forms.
The typical somatic number of chromosomes is here four. The same
number of chromosomes appear in the germinal vesicle (Fig. 125, D\
They are at first single, then double by a longitudinal split, but after-
ward single again by a reunion of the halves. The four chromo-
somes group themselves in a single tetrad, two passing into the first
polar body, while two remain in the ^^g, but meanwhile each of them
again splits into two. Of the four chromosomes thus left in the ^gg
two are passed out into the second polar body, while the two remain-
ing in the Qgg give rise to the germ-nucleus. From this it follows
that the formation of the first polar body is a reducing division — a
result which agrees with the earlier conclusions of Henking on
Pyrrochoris, and with those of Paulmier on the Hemiptera.

C. Reduction without Tetrad-formation

As already stated (p. 246), the formation of actual tetrads is of
relatively rare occurrence, being thus far certainly known only in the
arthropods, nematodes, and some annelids. In the greater number of
cases the two divisions of the primary chromatin-masses {i.e. of the
primary oocyte or spermatocyte) are separated by a considerable inter-
val, during which the first maturation cell-division takes place or is ini-
tiated, and hence no actual tetrads are formed. This obviously differs
only in degree from tetrad-formation, the latter occurring only when
the two divisions are simultaneous or occur in rapid succession.

In the cases now to be considered the length of the pause between
the maturation-divisions varies considerably, and in some forms (verte-
brates, flowering plants) it is so prolonged that the nucleus is partially
reconstructed. In all, or nearly all, these cases the first maturation-
division is of the heterotypical form, the chromosomes having the form
of rings and arising by a process that agrees in most of its features
with that leading to tetrad-formation. There is here, however, exactly
the same contradiction of results as in the case of tetrad-formation
described at page 247, and a bewildering confusion of the subject still
exists. In brief, it may be stated that most observers of reduction of
this type in the lower animals (flat-worms, annelids, mollusks) have
found one transverse and one longitudinal division ; most of those

REDUCTION WITHOUT TETRAD-FORMATION

259

who have studied the vertebrates find two longitudinal divisions;
while opinion regarding the plants is still divided.

{a) Animals. — In the gephyrean Thalassema and the mollusk
Zirphcea (Figs. 128-130) Griffin ('99) finds that the rings, arising as
described above, place themselves in the equator of the spindle with
the longitudinal division in the equatorial plane. They are then
drawn out toward the spindle-poles from the middle point, first
assuming the form of a double cross, then of elongated ellipses, and
finally break into two daughter- U's or -Vs. The first division is
therefore longitudinal. During the late anaphase the V's break at
the apex, the two Hmbs come close tog^ether, so as to give the decep-

A

B

Fig. 128. — Diagrams of reduction in the types represented by Thalassema (A) and Sa/a-
mandra (B). In both the first division is heterotypical. The second division (6) is transverse in
the first and longitudinal in the second.

tive appearance of a longitudinal split, and are separated by the
second division (following immediately upon the first without inter-
vening resting stage). The latter is therefore a transverse division
(Fig. 130). An essentially similar result, though less completely
worked out, is independently reached by Bolles Lee ('97) in Helix ;
by Klinckowstrom ('97) in the turbellarian Prosthecercejis ; and by
Francotte ('97) and Van der Stricht ('98, i) in Thysanzoon. Klinckow-
strom shows that there is much variation in the way in which the
rings open out and break apart, though the result is the same in all.
In case of the vertebrates, Flemming {"^f) long since described
and figured typical tetrads in the salamander, but regarded them as
"anomalies." Vom Rath's later conclusion ('93, '95) that they are

I

26o REDUCTION OF THE CHROMOSOMES

normal tetrads has not been sustained by the still more recent work
of Meves ('96), whose careful studies, together with those of Moore,
Lenhossek, and others, thus far give no evidence of tetrad-formation,
and seem opposed to the occurrence of reducing divisions in the
vertebrates. Meves's work in the main confirms the earlier results
of Flemming, except that he shows that, as in so many other animals,
only two generations of spermatocytes exist. At the first division
the nuclear reticulum resolves itself into twelve (the reduced num-
ber) segments, which split lengthwise, the halves remaining united to
form elongated rings (Figs. 27, 37). These do not, ^however, con-

■•.:■»<

B

.•V •

• * .

• • •

"•

•••5.

• .

*. . .

• • • ,•

*• •• .

•

•?.-.,:,
•^•:

•

-•.:>.•:?•

.:»•

0^

F

Fig. 129. — Maturation and fertilization in an annelid (armed gephyrean) Thalassema.
[Griffin.]

A. A few moments after entrance of the spermatozoon, showing accessory asters; tetrads
forming. B. Early prophase of first polar mitosis with centrosomes. C. In-pushing of nuclear
wall. Z). Central spindle established; elimination of nucleolus aind nuclear reticulum. E. Slightly
later stage viewed from above. F. First polar spindte established, cross-shaped tetrads, crossing
of astral rays; sperm-head at j".

dense into tetrads, but break apart during the first division at the
points corresponding with the ends of the united halves. The first
division is therefore an equation-division. As the V-shaped halves sep-
arate they again split lengthwise (Fig. 131), each of the secondary sper-
matocytes receiving twelve double V's or dyads. In the telophases
and ensuing resting stage, however, all traces of this splitting are
lost, the nuclei partially returning to the resting stage, but retaining
traces of a spireme-like arrangement (Fig. 131). In the second
division twelve double V's reappear, showing a longitudinal division
which Flemming and Meves believe to be directly related to that

REDUCTION WITHOUT TETRAD-FORMATION 26 1

seen during the foregoing anaphases. There is therefore no evi-
dence of a transverse division. McGregor ('99) describes a nearly
similar process in Amphmma, where the longitudinal division of the

4

B ' C

♦ «♦♦♦

D

F -

'/V

^t***"^**

H

J

\^

I

Fig. 130. — Maturation in the lamellibranch Zirphcea and in Ihalassetna. [Griffin.]

A-E, y/irphcEa ; F-I, Thalassema.

A. Unfertilized o^gg, ring-sliaped and cross-shaped chromosomes. B. Prophase of first polar
mitosis. C. First polar spindle ; double crosses. Z). Slightly later stage. ^. The double crosses
have broken apart (equation-division). G. Ensuing stage: daughter-V's broken apart at the
apex. H. Telophase of first, early prophase of second, division ; limbs of the V's separate but
closely opposed. F. Later prophase of second division. /. Second polar spindle in metaphase.

daughter-V's is seen with the greatest clearness throughout the
anaphases.

The weak point in both the foregoing cases is the fact that all
traces of the second longitudinal division are lost during the ensuing

262

REDUCTION OF THE CHROMOSOMES

resting period ; and I do not think that even the observations of
Flemming ('97), who has pubHshed the fullest evidence in the case,
completely establish the occurrence of a subsequent longitudinal divi-

Fig. 131. — (Compare Fig. 27). Maturation-divisions in Salamandra. [A' from FLEMMING,
the others from Meves.]

A. First division in metaphase, showing heterotype rings. B. Anaphase; longitudinal split-
ting of the daughter-loops. C. Telophase. D. Ensuing pause. E. Early prophase of second
division with longitudinally divided segmented spireme. F. Later prophase. G. Metaphase of
second division.

sion of the chromosomes in the second mitosis. In DesmognathuSy
however, where the resting stage is less complete, Kingsbury ('99)
finds the longitudinal split in the persistent chromosomes of the

REDUCTION WITHOUT TETRAD-FORMATION 263

pause following the first division ; and he believes this to be the
same division as that seen during the anaphase. Carnoy and Le Brun
('99) reach the same result in the formation of the polar bodies in
Triton, though their general account of the heterotypical mitosis
differs very considerably from that of other authors, the rings being
stated to arise by a double instead of a single longitudinal split.
These observers describe the rings of the early anaphase as having
almost exactly the same double cross-form as those in Thalassema or
Zirphcea (Griffin, '99), but believe them to arise in a manner nearly
in accordance with Strasburger's abandoned view of 1895,^ and with
Guignard's ('98, 2) and Gregoire's ('99) latest results on the flowering
plants, the ring being stated to arise by a double longitudinal split-
ting, as explained at page 265.

In the elasmobranch Scyllium Moore ('95) finds twelve (the re-
duced number) ring-shaped chromosomes at the first division. These
closely resemble tetrads ; but a resting stage follows, and the second
division is likewise stated to be of the heterotypical form. Both divi-
sions are stated to be equation-divisions — a conclusion well sup-
ported in case of the first, but so far from clear in the second that a
careful reexamination of the matter is highly desirable.

In mammals the first division is of the heterotypical form (Her-
mann, '89, Lenhossek, '98), though the rings are much smaller than
in the salamander, recalling those seen in arthropods. No true
tetrads are, however, formed, and the two divisions are separated by
a resting period. The character of the second division is undeter-
mined, though Lenhossek believes it to be heterotypical, like the first.

ib) Plants. — It is in the flowering plants, where reduction likewise
occurs, as a rule, without true tetrad-formation, that the contradiction
of results reaches its climax ; and it must be said that until further
research clears up the present confusion no definite result can be
stated. The earlier work of Strasburger and Guignard indicated that
no reducing division occurred, the numerical reduction being directly
effected by a segmentation of the spireme-thread into half the somatic
number of chromosomes. Thus these observers found in the male
that the chromosomes suddenly appeared in the reduced number
(twelve in the lily, eight in the onion) at the first division of the
pollen-mother-cell, and in the female at the first division of the
mother-cell of the embryo-sac. The subsequent phenomena differ
in a very interesting way from those in animals, owing to the fact
that the two maturation-divisions are followed in the female by one
and in the male by two or more additional divisions, in both of which
the reduced number of chromosomes persists. In the male the two
maturation-divisions give rise to four pollen-grains, in the female to

1 Cf. p. 269.

264

REDUCTION OF THE CHROMOSOMES

the four primary cells of the embryo-sac (Fig. 132); and these two
divisions undoubtedly correspond to the two maturation-divisions in
animals. In the female, as in the animals, only one of the four
resulting cells gives rise to the ^gg, the other three corresponding to
the polar bodies in the animal ^gg, though they here continue to
divide, and thus form a rudimentary prothallium.^ The first-m'en-

Fig. 132. — General view of the maturation-divisions in flowering plants. [MOITIER.]
A-C, in the male; D-F, in the female. 'A. The two secondary spermatocytes (pollen-mother-
cells) just after the first division {Lilimti). B. Final anaphase of second division {Podophyllum).

C. Resulting telophase, which by division of the cytoplasmic mass produces four pollen-grains.

D. Embryo-sac after completion of the first nuclear division {Lilium). E. The same after the
second division. F. The upper four cells resuUing from the third division {cf. Fig. 106) : o, ovum ;
p, upper polar cell ; s, synergidas. (For further details, see Figs. 133, 134.)

1 Of these three cells one divides to form the " synergidse," the other two divide to form
three "antipodal cells" (which like the synergidae finally degenerate) and a "lower polar
cell." The latter sooner or later conjugates with the " upper polar cell " (the sister-cell of
the egg) to form the " secondary embryo-sac-nucleus," by the division of which the endo-
sperm-cells arise. Of the whole group of eight cells thus arising only the egg contributes

REDUCTION WITHOUT TETRAD-FORMATION 265

tioned cell, however, does not directly become the Q,gg^ but divides
once, one of the products being the ^gg and the other the "■ upper
polar cell" (Fig. 132, F\ which contributes to the endosperm-forma-
tion (see footnote, and compare page 218).

In the male the two maturation-divisions are in the angiosperms
followed by two others, one of which separates a *' vegetative " from
a " generative " cell, while the second divides the generative nucleus
into two definite germ-nuclei. In the gymnosperms more than two
such additional divisions take place. In these later divisions, both in
the male and in the female (with the exception noted in the footnote
below), the reduced number persists, and the principal interest
centres in the first two or maturation-divisions. Strasburger and
Guignard found in Liliiim that while both these divisions differed in
many respects from the mitosis of ordinary vegetative cells, neither
involved a transverse or reducing division, the chromosomes under-
going a longitudinal splitting for each of the maturation-divisions.
Further investigations by Farmer ('93), Belajeff ('94), Dixon ('96),
Sargant ('96, '97), and others, showed that the first division is often
of the heterotypical form, the daughter-chromosomes in the late-meta-
phase having the form of two V's united by their bases (<>).
Despite the complication of these figures, due to torsion and other
modifications, their resemblance to the ring-shaped bodies observed
in the first maturation-division of so many animals is unmistakable,
as was first clearly pointed out by Farmer and Moore ('95).

Botanists have differed, and still differ, widely' in their interpreta-
tion both of the origin and subsequent history of these bodies upon
which the question of reduction turns. According to Strasburger's
('95) first account their origin has nothing in common with that of
the tetrad-rings, since they were described as arising by a double lon-
gitudinal splitting of a primary rod, the halves then separating first
from one end along one of the divisicn-planes, and then from the
other end along the other plane, meanwhile opening out to form a
ring such as is shown in Fig. 133. (This process, somewhat difficult
to understand from a description, will be understood from the dia-
gram, Fig. 135, E-I^ The four elements of the ring are then distrib-
uted without further division by the two ensuing maturation-divisions ;
and the process, except for the peculiar opening out of the ring, is

to the morphological formation of the embryo. It is a highly interesting fact that the num-
ber of chromosomes shown in the division of the lower of the two nuclei {i.e. the mother-
nucleus of the antipodal cells and lower polar-cell) formed at the first division of the
embryo-sac-nucleus is inconstant, varying in the lily from 12, 16, 20, to 24 (Guignard, '91, i),
in which respect they contrast with the descendants {^gg, synergidce) of the upper nucleus,
which always show the reduced number (Mottier, '97, i), i.e. in Lilium twelve. This
exception only emphasizes the rule of the constancy of the chromosome-number in general;
for these cells are destined to speedy degeneration.

266

REDUCTION OF THE CHROMOSOMES

essentially in agreement with the facts described in Ascaris, and
involves no reduction-division. Essentially the same result is reached
by Guignard ('98) in his latest paper on Naias, and by Gregoire ('99)
in the Liliaceae.

Strasburger twice shifted ground in rapid succession. First ('97, 2),
with Mottier ('97, i), he somewhat doubtfully adopted a view agreeing

^ n^flilta^

^frX)^

Fig. 133. — The first maturation-division in flowering plants. [F, STRASBURGER and MOT-
TiER; the others from Mottier.]

A. Mother-cell of the embryo-sac in Liliufn ; early prophase of first division ; chroinatin-
threads already longitudinally divided. B. Slightly later stage (split spireme) in the nucleus of
the pollen-mother-cell. C. A slightly later prophase (pollen-mother-cell, Podophyllum) with
twisted split spireme. D. Earlier prophase {Lilium, female) ; split twisted chromosomes. E.
Equatorial plate {^Lilium, male). F. First maturation-spindle {Fritillaria, male). G. Diver-
gence of the daughter-chromosomes {Lilium, male).

essentially with the interpretation of Vom Rath, Riickert, etc. (p. 247).
The primary rods split once, and bend into a V, the branches of
which often come close together, and may be twisted on themselves,
thus giving the appearance of the second longitudinal split described
in Strasburger's paper of 1895. The two halves of the split U then
separate, opening out from the apex, to form the 0 -figure. In the

JiEDUCTION WITHOUT TETRAD-FORMATION 26/

second division the limbs of the daughter-V's again come close
together, remaining, however, united at one end, where they were
believed finally to break apart during the second division. The latter
was, therefore, regarded as a true reduction-division, the apparent
longitudinal split being merely the plane along which the halves of
the V come into contact (Fig. 134, C, D).

The two accounts just given represent two extremes, the first
agreeing essentially with Ascaris, the second with the copepods or
insects. When we compare them with others, we encounter a truly
bewildering confusion. Strasburger and Mottier ('97) themselves
soon abandoned their acceptance of the reducing division, returning
to the conclusion that in both sexes {Lilium, Podophyllum) both divi-
sions involve a longitudinal splitting of the chromosomes (Figs. 133,
134). In the first division the longitudinally split spireme segments
into twelve double rods, which bend at the middle to form double V's,
with closely approximated halves. Becoming attached to the spindle
by the apex, the limbs of each separate to form a 0 -figure. At
telophase the daughter-V's shorten, thicken, and join together to form
a da.ughter-spireme consisting of a single contorted thread. This
splits lengthwise throughout its whole extent^ and then segments into
double chromosomes, the halves of which separate at the second
division (Fig. 135, L-M). The latter, therefore, Hke the first,
involves no reducing division. This result agrees in substance with
the slightly earlier work of Dixon ('96) and of Miss Sargant ('96,
'97), whose account of the origin of the 0 -figure of the first division
differs, however, in some interesting details. It is also in harmony
with the general results of Farmer and Moore ('95), of Gregoire ('99),
and of Guignard ('98), who, however, describes the first division nearly
in accordance with Strasburger's account of 1895, as stated above.
On the other hand, Ishikawa (pollen-mother-cells of Allium, '97) and
especially Belajeff (pollen-mother-cells of Iris, '98) conclude that the
second division is a true transverse or reducing division.^ Ishikawa
described the first division as being nearly similar to the ring-forma-
tion in copepods, the four elements of the ring being often so
condensed as nearly to resemble an actual tetrad. In the early ana-
phases the daughter-V's break at the apex ; and, although in the later
anaphases the limbs reunite, Ishikawa is inclined to regard the trans-
verse division as being a preparation for the second mitosis. Bela-
jeff' s earlier work ('94) on Liliunt gave an indecisive result, though
one on the whole favourable to a reducing division. In his latest
paper, however ('98, i), Belajeff takes more positive ground, stating
that after the examination of a large number of forms he has found

1 Schaffner ('97, 2) reaches exactly the reverse result in Lilium philadelpkicum, i.e. the
first division is transverse, the second longitudinal.

268

REDUCTION OF THE CHROMOSOMES

in the pollen-mother-cells of Iris a much more favourable object of
investigation than Lilhim, Fritillaria, and the other forms on which
most of the work thus far has been done, and one in which the sec-
ond division takes place with " admirable clearness " ; he also gives
interesting additional details of the first division in this and other
forms. In the first division the spireme splits lengthwise, and then
breaks into chromosomes, which assume the shape of a V, Y, or X
(Fig. 135, N-Q). The two limbs of these bodies do not, as might be

Fig. 134. — The second maturation-division in flowering plants. ^B. Strasburger and
MOTTlER; the others from MOTTIER.]

A. Nucleus of secondary spermatocyte {Podophyllum). B. Prophase of second division
{Lilium, male) with longitudinally divided chromatin-threads. E. Corresponding stage in the
female. F. Metaphase of second division {Podophyllnm, male). G. Initial anaphase {Lilium,
female). CD. illustrate Mottier's earlier conclusions, C. Second division {Lilium, male), with
chromosomes bent together so as to simulate a split. D. Slightly later stage {Fritillm-ia, male),
showing stage supposed to result from breaking apart of the limbs of the U at point of flexure.

supposed, represent sister-chromosomes (resulting from the longitu-
dinal division of the spireme) attached by one end or at the middle,
since each X, Y, or V is double, consisting of two similar superim-
posed halves. Belajeff, therefore, regards these figures as longitu-
dinally divided bivalent chromosomes, having the value of tetrads,
each limb being a longitudinally split single chromosome. The
double V's, Y's, and X's take up a position with the apex (or one end
of the X) attached to the spindle, and the longitudinal division in the
equatorial plane. The halves then progressively diverge from the

REDUCTION WITHOUT TETRAD-FORMATION 269

point of attachment, thus giving rise to o -shaped, -<>- -shaped, or
XX -shaped figures, all of which in the end assume the o -shape.
This part of the process is in the main similar to that described by
Strasburger and Mottier, and the daughter-V's diverge in the same
way as these authors describe. The second division, however, differs
radically from their account, since no splitting of the spireme-thread
occurs. The chromosomes reappear in the V-, Y-, and X-forms, but are
undivided, and only half as thick as in the first division. Passing to
the equator of the spindle, the V- and Y-forms break apart at the
apex, while the X-forms separate into the two branches of the X, the
daughter-chromosomes having the form of rods shghtly bent at the
outer end to form a J-figure (Fig. 135, R-T\ This division is,
accordingly, a transverse or reducing one, which ** corresponds com-
pletely to the reduction-division in the animal organism" ('98,2,
p. 33.) Atkinson ('99) reaches the same general result in Trillium^
stating very positively that no longitudinal division occurs in the
second mitosis, and believing that the daughter-V's of the first (hete-
rotypical) mitosis retain their individuality throughout the ensuing
pause, and break apart at the apex (reducing division) in the second
mitosis. This observer finds further that in Ariscmna the heterotypi-
cal rings of the first mitosis condense into true tetrads, by one longi-
tudinal and one transverse division, but believes that in this case it
is \ki^ first division that effects the reduction, as in the insects.

Such confusion in the results of the most competent observers of
reduction in the flowering plants is itself a sufficient commentary on
the very great difificulty and uncertainty of the subject; and it would
be obviously premature to draw any positive conclusions until further
research shall have cleared up the matter.^

1 Strasburger's new book, entitled IJber Reduktiotistheihing, Spindelbildung, Ceniroso-
men tind Cilienbildner im Pftanzenreich (Jena, 1900), is received while this work is in
press, too late for analysis in the text. In this treatise the author gives an exhaustive review
of the entire subject, contributing also many new and important observations on Liliutn,
Iris, Podophyllum, Tradescantia, Allium, larix, and several other forms. The general
result of these renewed researches leads Strasburger to return, in the main, to his conclu-
sions of 1895, with which agree, as stated above, the results of Guignard and Gregoire; and,
in a careful critique of Belajeff's work, he shows how the results of this observer may be
reconciled with his own. The essence of Strasburger's interpretation is as follows. In the
prophases of the first division the chromosomes first undergo a longitudinal division, shorten
to form double rods, and then again split lengthwise in a plane at right angles to the first.
The following stages vary even in the same species {liliujn) ; and here lies the explanation
of much of the divergence between the accounts of different observers, (i) In the typical
case, the chromosomes are placed radially, with one end next the spindle; and, during the
metaphase, they open apart along the first division-plane, from the spindle outwards, to form
I — shaped figures. These figures meanwhile open apart from the free end inwards along the
second division-plane. Thus arise the characteristic <:> -shaped figures, the daughter-V's
having separated along the first (equatorial) division-plane, while the two limbs of each V
have resulted, not through bending, but from a second (axial) split (Fig. 135, E-II). The

^r

R S T

Fig. 135. — Diagrams illustrating different accounts of reduction in the flowering plants.

A-D. Vegetative mitoses (heterotypical form) in Picea. [Belajeff.]

E-I. Illustrate Strasburger's earlier account (95) and the later one of Guignard, of the first
maturation-division. E. Doubly split rod. F. Metaphase, in profile. G. The same e7i face,
show^ing the heterotype ring. H. I. Opening out and breaking apart of the ring.

y-M. Later account of Strasburger and Mottier {cf. Figs. 133, 134). J. Longitudinally split,
V-shaped chromosome of first division. K. Opening out of the ring. L. Prophase of second
division, showing longitudinally split segmented spireme. M. Initial anaphase of second division.

N-Q. First division. [BELAJEFF.] N. Longitudinally split chromosomes, viewed in the equa-
torial plane. O. The same viewed in the axis of the spindle. P. Separation of the daughter-
chromosomes. Q. Anaphase, all the chromosomes assuming the V-form.

i?- 7". Second division in /Wj. [Belajeff.] R. Equatorial plate, limbs of X's and V's break-
ing apart (reducing division). S. Slightly later stage, with daughter-chromosomes still united at
one end. T. Anaphase.

270

PECULIARITIES OF REDUCTION IN THE INSECTS 2/1

Rhume. In reduction without tetrad-formation the spireme seg-
ments into half the somatic number of chromosomes, which split
lengthwise and open out to form rings for the first (heterotypical)
mitosis. According to one set of observers, including Flemming,
Meves, McGregor, Kingsbury, Moore, KHnckowstrom, Van der Stricht,
Francotte, Griffin, Belajeff, Farmer, Dixon, Strasburger, Sargant,
Mottier, Ishikawa, and Atkinson, the ring arises by a single longi-
tudinal division. According to another group, including Carnoy,
Le Brun, Guignard, and Gregoire, the ring arises through a double
longitudinal division, one representing the axial and the other the
equatorial plane of the o -figure. The second group of observers
regard both maturation-divisions as longitudinal. Among the first
group, Flemming, Meves, McGregor, Kingsbury, Moore, Farmer,
Dixon, Strasburger, Sargant, and Mottier likewise beHeve both divi-
sions to be longitudinal, the daughter-V's or their products again
splitting lengthwise for the second division ; while KHnckowstrom,
Van der Stricht, Francotte, Griffin, Belajeff, Ishikawa, and Atkinson
believe one of them to be transverse, the daughter-V's breaking apart
at the apex, and thus giving the reducing division of Weismann.^

D. Some Peculiarities of Reduction in the Insects

We may here briefly consider some interesting observations which show that in
some cases the nuclear substance may be unequally distributed to the germ-nuclei.
Henking ('90) discovered that in the second spermatocyte-division of Pyrrocho-
ris one of the *' chromosomes '' passes undivided into one of the daughter-cells
(spermatids) which receives twelve chromatin-elements while its sister receives but
eleven. (The number of chromosomes in the spermatogonia, and of rings in the
first spermatocyte-division is twenty-four). This anomalous process is confirmed
with interesting additional details by Paulmier ('99) in Anasa, and obviously related
phenomena are described by Montgomery ('99, i ) in Pentatoma^ and by McClung ('99)
in Xiphidiunf.

breaking apart of the V's at the apex, as described by Belajeff, is, therefore, not a transverse
division, but merely the completion of the second longitudinal division. (2) In a second
and exceptional type, the chromosomes are placed tangentially to the spindle, and the
halves separate from the middle, again producing <:> -shaped figures. These, however, are
not of the same nature as those arising in the first case, since they are formed by a bending
out of each daughter-chromosome at the middle to form the V, and not by the second longi-
tudinal split. The effect of the latter is in this case to render each daughter-V in itself
double, precisely as in the salamander. The difference between the two types results
merely from the difference of position of the chromosome with respect to the spindle, and
the final result is the same in both, i.e. two longitudinal divisions and no reducing one.

This highly important work brings very strong evidence against the occurrence of trans-
verse or reducing divisions in the higher plants, and seems to explain satisfactorily most of
the differences of interpretation given by other observers. It will be interesting to see
whether a similar interpretation is possible in the case of moUusks, annelids, and arthropods,
where the early stages, in many cases, so strikingly resemble those occurring in the plants.

1 Cf. footnote on page 269.

272 REDUCTION OP THE CHROMOSOMES

In Pentat07na the number of chromosomes in the spermatocyte is fourteen.
During the final anaphases of the last division, one of the fourteen daughter-chromo-
somes assumes a different staining-capacity from the others, and becomes a '' chro-
matin-nucleolus " which fragments into several smaller bodies during the ensuing
resting-stage. During each of the succeeding spermatocyte-divisions appear seven
chromosomes and a single small chromatin-nucleolus, and both of these kinds of
bodies are halved at each division, so that each spermatid receives seven chromo-
somes and a single chromatin-nucleolus. ^ In XipJiidium a body called by McClung
the ''accessory chromosome,'' and believed by him to correspond to the ''chromatin-
nucleolus " of Pentatoj/ia, appears in the early prophases of the last spermatogonium-
division while the remaining chromatin still forms a reticulum. In the equatorial
plate this lies outside the ring of chromosomes, but divides like the latter. The
same body appears in the ensuing resting-stage, and during both of the spermatocyte-
divisions. In these it lies, as before, outside the chromosome-ring, and differs
markedly from the other chromosomes, but divides like the latter, each of the halves
passing into one of the spermatids, where it appears to form an important part of the
sperm-nucleus.

Despite the peculiarities described above, the chromatin, as a whole, seems to be
equally distributed in both Pentatoma and Xiphidhun. In Atiasa, however, Paul-
miers studies ('98, '99), made in my laboratory, give a result agreeing with that of
Henking, and suggest some very interesting further questions. The spermatogonia-
nuclei contain two nucleolus-like bodies, and give rise to twenty-two chromosomes,
of which two are smaller than the others (Fig. 126). In the first spermatocyte-divi-
sion appear eleven tetrads. Ten of these arise from rings like those of Gryllotalpa^
etc. The eleventh, which is much smaller than the others, seems to arise from a
single nucleolus-like body of the spermatocyte-nucleus, and by a process differing
considerably from the others. All of these bodies are halved to form dyads at the
first division. In the second spermatocyte-division (Fig. 127) the larger dyads
divide to form single chromosomes in the usual manner. The small dyad, however,
fails to divide, passing over bodily i?ito one of the spermatids. In this case, there-
fore, half of the spermatids receive ten single chromosomes, while the remainder
receive in addition a small dyad.

A comparison of the foregoing results indicates that the small tetrad (dyad) corre-
sponds to the extra chromosome observed by Henking in Pyrrochoris, and perhaps
also to the "accessory chromosome" oi Xiphidium. Whether it corresponds to the
'' chromatin-nucleolus " of Pentatoma is not yet clear. The most remarkable of
these strange phenomena is the formation of the small tetrad, which seems to be a
non-essential element, since it does not contribute to all the spermatozoa. Paulmier
is inclined to ascribe to it a vestigial significance, regarding it as a "-degenerating"
chromosome which has lost its functional value, though still undergoing in some
measure its original morphological transformation ; in this connection it should be
pointed out that the spermatocyte-nucleolus, from which it seems to be derived, is
represented in the spermatogonia by two such nucleoli, just as the single small tetrad
is represented by two small chromosomes in the spermatogonia-mitoses. The real
meaning of the phenomenon is, however, wholly conjectural.

E. The Early History of the Germ-nuclei

There are many peculiarities in the early history of the germ-
nuclei, both in plants and animals, that have a special interest in con-

^ On this latter point Montgomery's observations do not seem quite decisive.

\

EARLY HISTORY OF THE GERM-NUCLEI

27Z

nection with the reduction-problem ; and some of these have raised
some remarkable questions regarding the origin of reduction. A
large number of observers are now agreed that during the growth-
period preceding the maturation-division (p. 236), in both sexes, the
nucleus of the mother-cell (spermatogonium, oogonium), both in
plants and in animals, passes through some of the changes prepara-
tory to reduction at a very early period. Thus, in the Qgg the pri-
mary chromatin-rods are often present in the very young ovarian
eggs, and from their first appearance are already split longitudinally. ^
Hacker ('92, 2) made the interesting discovery that in some of the
copepods {CantJiocampttis, Cyclops) these double rods could be traced

og^

Fig. 136. — Longitudinal section through the ovary of the copepod Canthocarnptus. [Hacker.]
og. The youngest germ-cells or oogonia (dividing at og.'^) ; a. upper part of the growth-zone;
oc. oocyte, or growing ovarian egg; ov. fully formed egg, with double chromatin-rods.

back continuously to a double spireme-thread, following immediately
upon the division of the last generation of oogonia, and that at no
period is a true reticulum formed in the genninal vesicle (Fig. 136).
In the following year Ruckert('93, 2) made a precisely similar discov-
ery in the case of selachians. After division of the last generation
of oogonia the daughter-chromosomes do not give rise to a reticu-
lum, but split lengthwise, and persist in this condition throughout
the entire growth-period of the ^gg. Riickert therefore concluded
that the germinal vesicle of the selachians is to be regarded as a
" daughter-spireme of the oogonium ( Ur-ei) grown to enormous
dimensions, the chromosomes of which are doubled and arranged in

? ^ Hacker, Vom Rath, Riickert, in copepods; Riickert in selachians; Born and Fick in
Amphibia; HoU in the chick; Riickert in the rabbit.

274 REDUCTION OF THE CI/IWMOSOMES

pairs." ^ In this case their number seems to be at first the somatic
number (thirty-six), which is afterward halved by conjugation of the
elements two and two (Riickert), as in Liimbriats (Calkins). It is,
however, certain that in many cases (insects, copepods) the double
rods first appear in the reduced number, and the observations of Vom
Rath ('93) and Hacker ('95, 3) give some reason to believe that the
reduced number may in some forms be present in the earlier progeni-
tors of the germ-cells, the former author having found but half the
normal number in some of the embryonic cells of the salamander, while
Hacker ('95, 3) finds that in Cyclops brevicorjiis the reduced number
of chromosomes (twelve) appears in the primordial germ-cells which
are differentiated in the blastula-stage (Fig. 74). He adds the inter-
esting discovery that in this form the somatic nuclei of the cleavage-
stages show the same number, and hence concludes that all the
chromosomes of these stages are bivalent. As development proceeds,
the germ-cells retain this character, while the somatic cells acquire
the usual number (twenty-four) — a process which, if the conception
of bivalent chromosomes be valid, must consist in the division of each
bivalent rod into its two elements. We have here a wholly new light
on the historical origin of reduction ; for the pseudo-reduction of the
germ-nuclei seems to be in this case a persistence of the embryonic
condition, and we may therefore hope for a future explanation of the
process by which it has in other cases been deferred until the penul-
timate cell-generation, as is certainly the fact in Ascaris?

This leads to the consideration of some very interesting recent dis-
coveries regarding the relation of reduction to the alternation of gen-
erations in the higher plants. As already stated (p. 263), Strasburger,
Guignard, and other observers have found that in the angiosperms
the two maturation-divisions are in both sexes followed by one or
more divisions in which the reduced number persists. The cells thus
formed are generally recognized as belonging to the vestiges of the
sexual generation (prothallium) of the higher cryptogams, the pollen-
grains (or their analogues in the female) corresponding to the asexual
spores of the archegoniate cryptogams. We should, therefore, expect
to find reduction in the latter forms occurring in the two correspond-
ing divisions, by which the ''tetrad" of spores is formed (as was first
pointed out by Hartog, '91). Botanists were thus led to the surmise,
first expressed by Overton in 1892, that the reduced number would
be found to occur in the prothallium-cells derived from those spores.

1 '92, 2, p. 51.

2 It may be recalled that in Ascaris Boveri proved that the primordial germ-cells have
the full number of chromosomes, and Hertwig clearly showed that this number is retained
up to the last division of the spermatogonia. Ishikawa ('97) finds that in Allium the
reduced number (eight) appears in the mitosis of the " UrpoUenzellen " preceding the
pollen-mother-cells. This is, however, contradicted by Mottier ('97, 2).

EARLY HISTORY OF THE GERM-NUCLEI 2/5

This surmise quickly became a certainty. Overton himself dis-
covered ('93) that the cells of the endosperm in the gymnosperm
Ceratozamia divide with the reduced number, namely eight ; and
Dixon observed the same fact in Pimis at the same time. In the
following year Strasburger brought the matter to a definite conclusion
in the case of a fern {Osmimda), showing that all the cells of the
prothallmm, from the original spore-mothei'-cell onwards to the for-
mation of the germ-cells, have one-half the number of chromosomes
found in the asexual generation, namely twelve instead of twenty-
four ; in other words, the reduction takes place in the formation of
the spore from which the sexual generation arises, many cell-genera-
tions before the germ-cells are formed, indeed before the formation of
the body from which these cells arise. Similar facts were determined
by Farmer in Pallavicinia, one of the Hepaticae, where all of the
nuclei of the asexual generation (sporogonium) show eight chromo-
somes during division, those of the sexual generation (thallus) four.
It now seems highly probable that this will be found a general rule.

The striking point in these, as in Hacker's observations, is that the
numerical reduction takes place so long before the fertilization for
which it is the obvious preparation. Speculating on the meaning of
this remarkable fact, Strasburger advances the hypothesis that the
reduced number is the ancestral number inherited from the ancestral
type. The normal, i.e. somatic, number arose through conjugation
by which the chromosomes of two germ-cells were brought together.
Strasburger does not hesitate to apply the same conception to ani-
mals, and suggests that the four cells arising by the division of the
oogonium (^^gg plus three polar bodies) represent the remains of a
separate generation, now a mere remnant included in the body in
somewhat the same manner that the rudimentary prothallium of angi-
osperms is included in the embryo-sac. This may seem a highly
improbable conclusion, but it must not be forgotten that so able a
zoologist as Whitman expressed a nearly related thought, as long ago
as 1878 : " I interpret the formation of polar globules as a r^lic of the
primitive mode of asexual reproduction.'' ^ Strasburger's view is
exactly the reverse of this in identifying the polar bodies as the
remains of a sexual generation; and as Hacker has pointed out ('98,
p. 102), it is difficult to reconcile with the fact that true reduction
appears to occur already in the unicellular organisms (p. 2^/). The
hypothesis is nevertheless highly suggestive and one which suggests
a quite new point of view for the study not only of maturation but
also of the whole problem of sexuality.

We may now return to the consideration of some details. In a
considerable number of forms, though not in all, the early prophase is

1 '78, p. 262.

2/6

REDUCTION OF THE CHROMOSOMES

characterized, especially in the male, by a more or less complete
concentration of the chromatin-substance at one side of the nucleus.
This stage, to which Moore has given the name synapsis {Y\g. 120, A)^
sometimes occurs when the spireme thread is already split {Ascaris,
Lilimn), sometimes before the division is visible (insects). In either
case the cJiro7natin-scgincnts emerge from the synapsis stage longitudi-
nally divided, and in the reduced 7inmber, a fact which gives ground
for the conclusion that the synapsis is in some way concerned with
the rearrangement of the chromatin-substance involved in the numer-
ical reduction. During the synapsis the nucleolus remains quite
distinct from the chromatin, and in many cases it afterward persists
beside the tetrads, in the formation of which it takes no part, to be
cast out into the cytoplasm (Fig. 124) or to degenerate in situ during
the first maturation-division.

A suggestive phenomena, described by several observers,^ is the
casting out of a large part of the nuclear reticulum of the germinal

B c

Fig. 137. — Types ot maturation-spindles in the female.

A. First polar spindle with tetrads, in Heterocope. [HacKER]. B. Second polar spindle
in Triton. [Carnoy and LeBrun.] C. First polar spindle of Ascaris. [FiJRST.]

vesicle at the time the polar bodies are formed (Figs. 97, 128). In
these cases {Asterias, Polychoerns, Thalassema, Nereis) only a small
fraction of the chromatin-substance is preserved to form the chromo-
somes, the remainder degenerating in the cytoplasm.^

As a final point we must briefly consider the varying accounts of
the achromatic maturation-figures in the female already briefly referred
to at page 85. In many forms {e.g. in turbellarians, nemertines, anne-
lids, mollusks, echinoderms) the polar amphiasters are of quite typical
form, with large asters and distinct centrosomes nearly similar to those
of the cleavage-figures. In others, however (nematodes, arthropods,
tunicates, vertebrates), the polar spindles differ markedly from those
of the cleavage-figures, being described by many authors as entirely
devoid of asters and even in some cases of centrosomes (Fig. 137).

1 Cf. Mathews (Wilson and Mathews, '95), Gardiner ('98), Griffin ('99).

2 Cf. the enormous reduction of the chromatin-substance in the elasmobranch egg, p. 338.

REDUCTION IN UNICEILULAR FORMS 2//

There can be no doubt that these polar spindles differ from the usual
type, and that they approach those recently described in the mitosis of
the higher plants, but it is doubtful whether the apparent absence of
asters and centrosomes is normal. In Ascaris, the first polar spindle
arising by a direct transformation of the germinal vesicle (Fig. 117)
has a barrel-shape, with no trace of asters. At the poles of the
spindle, however, are one or two deeply staining granules (Fig. 137),
which have been identified as centrosomes by Hacker ('94) and
Erlanger ('97, 4), but by Fiirst ('98) are regarded as central granules,
the whole spindle being conceived as an enlarged centrosome.^ For
the reasons stated at page 314, I believe the former to be the correct
interpretation.^ Spindles without centrosomes have been described in
the eggs of tunicates (Julin, Hill, Crampton), in Amp hiox n s {Sohott?i\
in some species of copepods (Hacker), and in some vertebrates {Die-
myctylus, Jordan ; mouse, Sobotta). In Amphioxiis (Sobotta) and
Triton (Carnoy and LeBrun) complete asters are not formed, but
fibrillae apparently corresponding to astral rays and converging to.
the spindle-poles are found outside the limits of the spindle (Fig. 137)..
In the guinea-pig, according to Montgomery ('98), centrosomes and
asters are present in the first polar spindle, but absent in the second.
The evidence is on the whole rather strong that the achromatic figure
in these cases approaches in form that seen in the higher plants ;
but it is an open question whether the appearances described may not
be a result of imperfect fixation.

F. Reduction in Unicellular Forms

Although the one-celled and. other lower f^rms have not yet been
sufficiently investigated, we have already good ground for the conclu-
sion that a process analogous to the reduction of higher types regularly
recurs in them. In the conjugation of Infusoria, as already described
(p. 223), the original nucleus divides several times before union, and
only one of the resulting nuclei becomes the conjugating germ-nucleus,
while the others perish, like the polar bodies. The numerical corre-
spondence between the rejected nuclei or " corpuscules de rebut" has
already been pointed out (p. 227). Hertwig could not count the chro-
mosomes with absolute certainty, yet he states ('89) that in Paramce-
cinm caicdatiim, during the final division, the number of spindle-fibres
and of the corresponding chromatic elements is but 4-6, while in the

iC/p. 312.

2 Sala ('94) and Fiirst have shown that occasionally the polar spindles of Ascaris are
provided with large typical asters, and thus resemble those of annelids or mollusks. Sala
believed this to be an effect of lowered temperature, but Fiirst's observations are unfavour-
able to this conclusion.

2/8

REDUCTION OF THE CHROMOSOMES

earlier divisions the number is approximately double this (8-9). This
observation makes it nearly certain that a numerical reduction of
chromosomes occurs in the Protozoa in a manner similar to that of
the higher forms ; but the reduction here appears to be deferred until

/

A

Fig. 138. — Conjugation and formation of the polar bodies in Actinophrys. [SCHAUDINN,]
A. Union of the gametes; first polar spindle. B, Fusion of the cell-bodies; a single polar
body near the periphery of each. C. Fusion of the nuclei.

the final division. In the gregarines Wolters ('91) has observed the
formation of an actual polar body as a small cell segmented off from
each of the two conjugating animals soon after their union ; but the

number of chromo-
somes was not deter-
mined. Schaudinn
('96, 2) has observed a
like process in Acti-
nophiys, each of the
gametes segmenting
off a single polar body,
after which the germ-
nuclei fuse (Fig. 138).
It is possible, as R.
Hertwig ('98) points
out, that in both these
forms a second polar
body may have been
overlooked, owing per-
haps to its rapid dis-

Fig. 139. — Formation of polar bodies and conjugation in integration. \v\. ActtnO-

Actmosphcerium. [R. HERTWIG.] sphcerimu, according to

A. Two gametes ("secondary cysts"), resulting from the ■„ HertwiV ('c\R\ thf^

division of a "primary cyst"; second maturation-spindle in ^' J^*-^ ^wi^ \^^h ^^^^

each; first polar body shown in the right gamete, at/. B. Both nUClcUS of Cach gamete

polar bodies {p^^p"^) formed in the right gamete, the second pliviplpjj twirp in r-jni'H

one forming in the left gamete. C Subsequent fusion of the '^^v lucb LWiuc in i dpiu

gametes; nuclei uniting, two polar bodies (probably the second, SUCCCSSion tO form tWO

the first having been absorbed) at/. D.^Yli^yonn^ Actinosphce- pQJg^^ bodicS Tnuclei)

rium escaping from the cyst-wall ; the cleavage-nucleus has , . , ,

divided. which degenerate, after

REDUCTION IN UNICELLULAR FORMS

279

which the germ-nuclei unite (Fig. 1 39). Whether a reduction in the
number of chromosomes occurs in these cases was not determined.^

B

D

G

Fig. 140. — Conjugation of Clostermm. [Klebahn.]
A. Soon after union, four chromatophores. B., Chromatophores reduced to two, nuclei
distinct. C. Fusion of the nuclei. D. First cleavage of the zygote. E. Resulting 2-cell stage.
F. Second cleavage. G. Resulting stage, each cell bi-nucleate. H. Separation of the cells;
one of the nuclei in each enlarging to form the permanent nucleus, the other (probably repre-
senting a polar body) degenerating.

1 Actinosphcerium forms one of the most extreme knovi^n cases of in-breeding; for the
gametes are sister-cells which immediately reunite after forming the polar bodies. The
general facts are as follows : The mother animal, containing very numerous nuclei, becomes
encysted, and a very large number of the nuclei degenerate. The body then segments into

28o REDUCTION OF THE CHROMOSOMES

Adelea (one of the.Coccidiae) is a very interesting case, for accord-
ing to Siedlecki ('99) polar bodies or their analogues are formed in
both sexes. The gametes are here of very unequal size. Upon their
union the smaller male cell divides twice to form apparently equiva-
lent spermatozoids, of which, however, only one enters the ovum, while
three degenerate as polar bodies; These two divisions are of different
type ; the first resembles true mitosis, while the second is of simpler
character and is believed by Siedlecki to effect a reduction in the
num'ber of chromosomes. In the meantime the nucleus of the macro-
gamete moves to the surface and there expels a portion of its chro-
matin, after which union of the nuclei takes place. Interesting facts
have been observed in unicellular plants which indicate that the
reduction may here occur either before (diatoms) or after (desmids)
fusion of the conjugating nuclei. In the former {Rhopalodiiia) Klebahn
('96) finds that each nucleus divides twice, as in many Infusoria, giving
rise to two large and two small nuclei. Each of the conjugates then
divides, each daughter-cell receiving one large and one small nucleus.
The four resulting individuals then conjugate, two and two, the large
nuclei fusing while the small (polar bodies) degenerate. The com-
parison of this case with that of the Infusoria is highly interesting. In
the desmids on the o\.\\e,r \\2ind{Closte7'i?tm and Cosmarium, Fig. 140),
according to Klebahn ('92), the nuclei first unite to form a cleavage-
nucleus, after which the zygote divides into two. Each of the new
nuclei now divides, one of the products persisting as the perma-
nent nucleus, while the other degenerates and disappears. Chmie-
lewski asserts that a similar process occurs in Spirogyra. Although
the numerical relations of the chromosomes have not been determined
in these cases, it appears probable that the elimination of a nucleus in
each cell is a process of reduction occurring after fertilization.

G. Maturation of Parthenogenetic Eggs

The maturation of eggs that develop without fertilization is a sub-
ject of special interest, partly because of its bearing on the general
theory of fertilization, partly because it is here, as I believe, that one
of the strongest supports is found for the hypothesis of the individ-
uality of chromosomes. In an early article by Minot ('//) on the

a number (five to twelve) of " primary cysts," each containing one of the remaining nuclei.
Each primary cyst divides by mitosis to form two gametes (" secondary cysts "), which, after
forming the polar bodies, reunite, their nuclei fusing to form a single one. The resulting
cell soon creeps out of the cyst-wall and assumes the active life, its nucleus meanwhile mul-
tiplying to produce the multinuclear condition characteristic of the adult animal. What is
here the physiological motive for the formation of the polar bodies, and how shall it be
explained under the Wcismann hypothesis?

MATURATION OF PARTHENOGENETIC EGGS 28 1

theoretical meaning of maturation, the suggestion is made that
parthenogenesis may be due to failure on the part of the Qgg to
form the polar bodies, the egg-nucleus thus remaining hermaphrodite,
and hence capable of development without fertilization. This sug-
gestion forms the germ of all later theories of parthenogenesis. Bal-
four ('80) suggested that the function of forming polar cells has been
acquired by the ovum for the express purpose of preventing parthe-
nogenesis, and a nearly similar view was afterward maintained by
Van Beneden.^ These authors assumed accordingly that in par-
thenogenetic eggs no polar bodies are formed. Weismann {'^6) soon
discovered, however, that the parthenogenetic eggs of PolypJiemiis
(one of the Daphnidae) produce a single polar body. This observa-
tion was quickly followed by the still more significant discovery by
Blochmann {'SS) that /;/ Aphis the parthenogenetic eggs produce a single
polar body y while the fertilized eggs produce tzvo. Weismann was able
to determine the same fact in ostracodes and Rotifera, and was thus
led to the view ^ which later researches have entirely confirmed, that
it is the second polar body that is of special significance in partheno-
genesis. Blochmann observed that in insects the polar bodies were
not actually thrown out of the (tgg, but remained embedded in its
substance near the periphery. At the same time Boveri {'^7, i) dis-
covered that in Ascaris the second polar body might in exceptional
cases remain in the Qgg and there give rise to a resting-nucleus indis-
tinguishable from the egg-nucleus or sperm-nucleus. He was thus
led to the interesting suggestion that parthenogenesis might be due
to the retention of the second polar body in the ^gg and its union
with the egg-nucleus, " The second polar body would thus, in a
certain sense, assume the role of the spermatozoon, and it might not
without reason be said : "Parthenogenesis is the result of fertilization
by the second polar body.'' ^

This conclusion received a brilliant confirmation through the obser-
vations of Brauer ('93) on the parthenogenetic ^gg of Arte7nia,
though it appeared that Boveri arrived at only a part of the truth.
Blochmann ('88-'89) had found that in the parthenogenetic eggs
of the honey-bee two polar bodies are formed, and Platner discov-
ered the same fact in the -butterfly Liparis ('89) — a fact which
seemed to contradict Boveri's hypothesis. Brauer's beautiful re-
searches resolved the contradiction by showing that there are two
types oi parthenogenesis which may occur in the same animal. In the
one case Boveri's conception is exactly realized, while the other is
easily brought into relation with it.

(/z) In both modes typical tetrads are formed in the germ-nucleus
to the number of eighty-four. In the first and more frequent case

^ '83, p. 622. 2 Essay VI., p. 359. 8 Lc, p. 73.

■

282

REDUCTION OF THE CHROMOSOMES

(Fig. 141) but one polar body is formed, which removes eighty-four
dyads, leaving eighty-four in the ^gg. There may be an abortive
attempt to form a second polar spindle, but no division results, and
the eighty-four dyads give rise to a reticular cleavage-nucleus. From

OO A

o

-V-, 'v:;^.-

D

<.'.

Wh^

D

O"C^0boCJ

7

Pig. 141. — First type of maturation in the parthenogenetic &gg of Artemia. [BraUER.]
A. The first polar spindle ; the equatorial plate contains 84 tetrads. B. C. Formation of the
first polar body; 84 dyads remain in the egg, and these give rise to the egg-nucleus, shown in D.
F. Appearance of the egg-centrosome and aster, E. G. Division of the aster and formation
of the cleavage-figure ; the equatorial plate consists of 84 apparently single but in reality bivalent
chromosomes.

this arise eighty- four thread-like chromosomes, and the same number
appears in later cleavage-stages.

{b) It is the second and rarer mode that reahzes Boveri's concep-
tion (Fig. 142). Both polar bodies are formed, the first removing
eighty-four dyads and leaving the same number in the Qgg. In the
formation of the second, the eighty-four dyads are halved to form

MATURATION OF PARTFIENOGENETIC EGGS

283

two daughter-groups, each containing eighty-four single chromosomes.
BotJi these groups remain in the egg, and each gives rise to a single
7'eticular nucleus, as described by Boveri in Ascaris. These two nuclei
place themselves side by side in the cleavage-figure, and give rise each
to eighty-four chromosomes^ precisely like two gerjn-nnclei in ordinary
fertilization. The one hundred and sixty-eight chromosomes spHt

"h}

8

(Sli -,Bf

^oi

iO

D E

Pig. 142. — Second tj'pe of maturation in the parthenogenetic egg of Artemia. [Brauer.]
A. Formation of second polar body. B. Return of the second polar nucleus (/>. ^.2) into the
egg; development of the egg-amphiaster. C. Union of the egg-nucleus (?) with the second
polar nucleus {p. bP-). D. Cleavage-nucleus and amphiaster. E. First cleavage-figure with
equatorial plate containing 168 chromosomes in two groups of 84 each.

lengthwise, and are distributed in the usual manner, and reappear
in the sajne number in later stages. In other words, the second polar
body here plays the part of a sperm-nucleus precisely as maintained
by Boveri.

In all individuals arising from eggs of the first type, therefore, the
somatic number of chromosomes is eighty-four ; in all those arising
from eggs of the second type, it is one hundred and sixty-eight. This

284 REDUCTION OF THE CHROMOSOMES

difference is clearly due to the fact that in the latter case the chromo-
somes are single or univalent, while in the former they are bivalent
(actually arising from dyads or double chromosomes). The remark-
able feature, on which too much emphasis cannot be laid, is that the
numerical difference should persist despite the fact that the mass, and,
as far as we can see, the quality, of the chromatin is the same in both
cases. In this fact we must recognize a strong support, not only of
Hacker's and Vom Rath's conception of bivalent chromosomes, but
also of the more general hypothesis of the individuality of chromo-
somes (Chapter VI.).

I . Accessory Cells of the Testis

It is necessary to touch here on the nature of the so-called " Sertoli-cells," or sup-
porting cells of the testis in mammals, partly because of the theoretical significance
attached to them by Minot, partly because of their relations to the question of amito-
sis in the testis. In the seminiferous tubules of the mammalian testis, the parent-
cells of the spermatozoa develop from the periphery inwards toward the lumen, where
the spermatozoa are finally formed and set free. At the periphery is a layer of cells
next the basement-membrane, having flat, oval nuclei. Within this, the cells are
arranged in columns alternating more or less regularly with long, clear cells, con-
taining large nuclei. The latter are the Sertoli-cells, or supporting cells ; they extend
nearly through from the basement-membrane to the lumen, and to their inner ends
the young spermatozoa are attached by their heads, and there complete their growth.
The spermatozoa are developed from cells which lie in columns between the Sertoli-
cells, and which undoubtedly represent spermatogonia, spermatocytes, and sperma-
tids, though their precise relationship is, to some extent, in doubt. The innermost
of these cells, next the iumen, are spermatids, which, after their formation, are found
attached to the Sertoli-cells, and are there converted into spermatozoa without further
division. The deeper cells from which they arise are spermatocytes, and the sper-
matogonia lie deeper still, being probably represented by the large, rounded cells.

Two entirely different interpretations of the Sertoli-cells were advanced as long
ago as 1 87 1, and both views still have their adherents. Von Ebner ('7O ^^ first
regarded the Sertoli-cell as the parent-cell of the group of spermatozoa attached to it^
and the same view was afterward especially advocated by Biondi ('85) and by Minot
('92), the latter of whom regarded the nucleus of the Sertoli-cell as the physiological
analogue of the polar bodies, i.e. as containing the female nuclear substance ('92,
p. Tj). According to the opposing view, first suggested by Merkel ('71), the Sertoli-
cell is not the parent-cell, but a nurse-cell, the spermatozoa developing from the
columns of rounded cells, and becoming secondarily attached to the Sertoli-cell,
which serves merely as a support and a means of conveying nourishment to the
growing spermatozoa. This view was advocated by Brown ('85), and especially by
Benda ('87). In the following year ('88), von Ebner himself abandoned his early
hypothesis and strongly advocated Benda's views, adding the very significant result
\.\\2i\.four spermatids arise from each sper^natocyie., precisely as was afterward shown
to be the case in Ascaris, etc. The very careful and thorough work of Benda and
von Ebner, confirmed by that of Lenhossek ('98, 2), leaves no doubt that mamma-
lian spermatogenesis conforms, in its main outlines, with that of Ascaris^ the sala-
mander, and other forms, and that Biondi's account is untenable. Minors theoretical
interpretation of the Sertoli-cell, as the physiological equivalent of the polar bodies,,
therefore collapses.

SUMMARY AND CONCLUSION 285

2. A mitosis in the Early Sex-cells

Whether the progenitors of the germ-cells ever divide amitotically is a question of
high theoretical interest. Numerous observers have described amitotic division in
testis-cells. and a few also in those of the ovary. The recent observations of Meves
('91), Vom Rath ('93), and others leave no doubt whatever that such divisions
occur in the testis of many animals. Vom Rath maintains, after an extended inves-
tigation, that all cells so dividing do not belong in the cycle of development of the
germ-cells ('93, p. 164) ; that amitosis occurs only in the supporting or nutritive cells
(Sertoli-cells, etc.), or in such as are destined to degenerate, like the ''residual
bodies" of Van Beneden. Meves has, however, produced strong evidence ('94) that
in the salamander the spermatogonia may, in the autumn, divide by amitosis, and in
the ensuing spring may again resume the process of mitotic division, and give rise to
functional spermatozoa. On the strength of thpsr- observations Flemming ('93) him-
self now admits the possibility that amitosis may form part of a normal cycle of devel-
opment.^

H. Summary and Conclusion

The one fact of maturation that stands out with perfect clearness
and certainty amid all the controversies surrounding it is a reduction
of the immber of chromosomes in the ultimate germ-cells to one-half the
number characteristic of the somatic cells. It is equally clear that this
reduction is a preparation of the germ-cells for their subsequent union,
and a means by which the number of chromosomes is held constant
in the species. With a few exceptions the first indication of the
numerical reduction appears through the segmentation of the spireme-
thread, or the resolution of the nuclear reticulum, into a number of
masses one-half that of the somatic chromosomes. In nearly all higher
animals this process first takes place two cell-generations before the
formation of the definitive germ-cells, and the process of reduction is
completed by two rapidly succeeding " maturation-divisions," giving
rise to four cells, all of which become functional in the male, while in
the female only one becomes the ^gg, while the other three — the
polar bodies or their analogues — are cast aside. During these two
divisions each of the original chromatin-masses gives rise to four
chromosomes, of which each of the four daughter-cells receives one ;
hence, each of the latter receives one-half the somatic number of
chromosomes. In the higher plants, however, the two maturation-
divisions are followed by a number of others, in which the reduced
number of chromosomes persists, a process most strikingly shown in
the pteridophytes, where a separate, sexual generation (prothallium)
thus arises, all the cells of which show the reduced number.

Two general types of maturation may be distinguished according
to the manner in which the primary chromatin-masses divide. In one,

1 For more recent literature on this subject see Meves, Zelltheilung, in Merkel and Bon-
net's Ergebnisse, VIII., 1898.

286 REDUCTION OF THE CHROMOSOMES

typically represented by Ascaris and the arthropods, each of these
masses divides into four to form a tetrad, thus preparing at once for
two rapidly succeeding divisions, which are not separated by a recon-
struction of the daughter-nuclei during an intervening resting period.
In the other, examples of which are given by the flowering plants and
the spermatogenesis of the Amphibia, no true tetrads are formed, the
primary chromatin-masses dividing separately for each of the matura-
tion-divisions, which are separated by a period in which the nuclei
regress toward the resting state, though often not completely return-
ing to the reticular condition. These two types differ, however, only
in degree, and with few exceptions they agree in the fact that during
the prophases of the first division the chromatin-bodies assume the
form of rings, the mitosis thus being of the heterotypical form, and
each ring having the prospective value of four chromosomes.

Thus far the phenomena present no difficulty, and they give us a
clear view of the process by which the numerical reduction of the
chromosomes is effected. The confusion of the subject arises, on the
one hand, from its complication with theories regarding the individu-
ality of the chromosomes and the functions of chromatin in inheri-
tance, on the other through conflicting results of observation on the
mode of tetrad-formation and the character of the maturation-divisions.
Regarding the latter question nearly all observers are now agreed that
one of these divisions, usually the first, is a longitudinal or equation-
division, essentially like that occurring in ordinary mitosis. The main
question turns upon the other division, which has been shown in some
cases to be transverse and not longitudinal, and thus separates what
were originally different regions of the spireme-thread or nuclear
substance. The evidence in favour of such a division seems at present
well-nigh demonstrative in the case of insects and copepods, and
hardly less convincing in the turbellarians, anneHds, and mollusks.
On the other hand, both divisions are regarded as longitudinal by most
of those who have investigated the phenomena in Ascaris and in the
vertebrates, and by some of the most competent investigators of the
flowering plants.

The evidence as it stands is so evenly balanced that the subject is
hardly yet ripe for discussion. The principle for which Weismann
contended in his theory of reducing division has received strong
support in fact; yet should it be finally estabHshed that numerical
reduction may be effected either with or without transverse division,
as now seems probable, not only will that theory have to be aban-
doned or wholly remodelled, but we shall have to seek a new basis
for the interpretation of mitosis in general. Weismann's theory is
no doubt of a highly artificial character ; but this should not close our
eyes to the great interest of the problem that it attempted to solve.

LITERATURE 287

The existing contradiction of results has led to the opinion, expressed
by a number of recent writers, that the difference between longitudinal
or transverse division is of minor importance, and that the entire
question of reduction is a barren one. This opinion fails to reckon
with the facts on which rests the hypothesis of the individuality of
chromosomes (Chap. VI.); but these facts cannot be left out of
account. We must find a common basis of interpretation for them
and for the phenomena of reduction ; yet how shall we reconcile
them with reduction by longitudinal division only .'* I cannot, there-
fore, share the opinion that we are deaUng with a barren problem.
The peculiarities of the maturation-mitoses are obviously correlated
in some way with the numerical reduction, and the fact that they
differ in so many ways from the characters of ordinary mitosis gives
ground to hope that their exhaustive study will throw further light
not only on the reduction-problem itself but also on mitosis in general
and on still wider problems relating to the individuality of the chromo-
somes and the morphological organization of the nucleus. It is indeed
very probable that Weismann's theory is but a rude attempt to attack
the problem, and one that may prove to have been futile. The prob-
lem itself cannot be ignored, nor can it be dissociated from the series
of kindred problems of which it forms a part.

LITERATURE. V^

Van Beneden, E. — Recherches sur la maturation de Toeuf, la fdcondation et la division

c^\\\x\?:\x^\ Arch. Biol, \M . 1883.
Boveri, Th. — Zellenstudien, I., III. Jena, 1887-90. See also " Befruchtung "

(List IV.).
Brauer, A. — Zur Kenntniss der Spermatogenese von Ascaris megalocephala : Arch.

mik. Anat., XLII. 1893.
Id. — Zur Kenntniss der Reifung der parthenogenetisch sich entwickelnden Eies von

Artemia Salina : Arch. mik. Anat., XLIII. 1894.
Guignard, L. — Le developpement du pollen et la reduction chromatique dans le

Naias : Arch. Anat. Mic, 11. 1899. (Full literature on reduction in plants.)
Griffin, B. B. — See Literature, IV.
Hacker, V. — Die Vorstadien der Eireifung (General Review) : Arch. mik. Anat.^

XLV. 2. 1895.
Id. — Uber weitere Ubereinstimmungen zwischen den Fertpflanzungsvorgangen der

Thiere und Pflanzen : Biol. Centralb., XVII. 1897.
Id. — iJber vorbereitende Theilungsvorgange bei Thieren und Pflanzen : Verh^

deiitsch. Zool. Ges., VIII. 1898.
Id. — Die Reifungserscheinungen : Merkel iind Bonne fs Ergebnisse, VIII. 1898.
Hertwig, 0. — Vergleich der Ei- und Samenbildung bei Nematoden. Eine Grund-

lage fiir cellulare Streitfragen : Arch. mik. Anat., XXXVI. 1890.
Mark, E. L. — (See List IV.)
Peter, K. — Die Bedeutung der Nahrzellen im Hoden : Arch. mik. Anat., LIII. 1898

1 See also Literature, IV,, p. 231.

288 REDUCTION OF THE CHROMOSOMES

Platner, G. — tJber die Bedeutung der Richtungskorperchen : Biol. Centralb., VIII

1889.
Vom Rath, 0. — Zur Kenntniss der Spermatogenese von G?yllotalpa vulgaris : Arch

mik. Anat., XL. 1892.
Id. — Neue Beitrage zur P>age der Chromatinreduktion in der Samen- und Eireife :

Arch. mik. Anat., XLVI. 1895.
Riickert, J. — Die Chromatinreduktion der Chromosomenzahl im Entwicklungsgang

der Organismen : Ergebn. d. Anat. tc. Entwick., III. 1893 (1894).
Strasburger, E. — tJber periodische Reduktion der Chromosomenzahl im Entwick*

lungsgang der Organismen : Biol. Centralb., XIV. 1894.
Id. — Reduktionstheilung, Spindelbildung, etc. : Jena, Fischer, 1900.

I

■

CHAPTER VI

SOME PROBLEMS OF CELL-ORGANIZATION

" Wir miissen deshalb den lebenden Zellen, abgesehen von der Molecularstructur der
organischen Verbindungen, welche sie enthalt, noch eine andere und in anderer Weise com-
plicirte Structur zuschreiben, und diese es ist, welche wir mit dem Namen Organization
bezeichnen." Brucke.^

" Was diese Zelle eigentlich ist, dariiber existieren sehr verschiedene Ansichten."

Hackel.2

The remarkable history of the chromatic substance in the matura-
tion of the germ-cells forces upon our attention the problem of the
ultimate morphological organization of the nucleus, and this in its
turn involves our whole conception of protoplasm and the cell. The
grosser and more obvious organization is revealed to us by the micro-
scope as a differentiation of its substance into nucleus, cytoplasm,
and the Hke. But, as Strasburger has well said, it would indeed be a
strange accident if the highest powers of our present microscopes had
laid bare the ultimate organization of the cell. Briicke insisted more
than thirty years ago that protoplasm must possess a far more com-
plicated morphological organization than is revealed to us in the
visible structure of the cell, repeating, though without accepting, an
earlier suggestion of Henle's ('41) that the cell might be composed of
more elementary vital units ranking between the molecule and the
cell. Many biological thinkers since Briicke's time have in one form
or other accepted this conception, which indeed lies at the root of
nearly all recent attempts to analyze exhaustively the phenomena of
cell-life. Without attempting to follow out the history of opinion in
detail or to give any extended review of the various theories,^ it may
be pointed out that this conception was based both on theoretical
a priori grounds and on the observed facts of cell-structure. On the
former basis it was developed by Herbert Spencer * in his theory of
** physiological units " by which he endeavoured to explain the phe-
nomena of regeneration, development, and heredity ; while Nageli
('84) developed on the same general lines his theory of micellce which

^ Elementarorgaiiismen, i86i, p. 386.
2 Antkropogenie, 189 1, p. 104.

^ For an exhaustive review see Yves Delage, La structure du protoplastna et les theories suf
Vheredite. Paris, 1895. ^ Principles of Biology, 1864.

u 289

290 SOME PROBLEMS OF CELL-ORGANIZATION

has been so widely accepted by botanists. In the meantime Darwin '
introduced a new element into the speculative edifice in his celebrated
hypothesis of pangenesis, where for the first time appear the two
assumptions of specific differences in the ultra-microscopic corpuscles
C'gemmules ") and the power of self-propagation by division. Dar-
win did not, however, definitely maintain that protoplasm was actually
built of such bodies. The latter hypothesis was added by De Vries
('89), who remodelled the theory of pangenesis on this assumption,
thus laying the basis for the theories of development which reached
their climax in the writings of Hertwig and Weismann.

The views of Spencer and Darwin were based on purely theoretical
grounds derived from the general phenomena of growth and inheri-
tance.^ Those of Nageli, De Vries, Wiesner, Altmann, and others
were more directly based on the results of microscopical investigation.
The view was first suggested by Henle ('41), and at a later period
developed by Bechamp and Estor, by Maggi and especially by Alt-
mann, that the protoplasmic granules might be actually organic units
or bioblasts, capable of assimilation, growth, and division, and hence
to be regarded as elementary units of structure standing between the
cell and the ultimate molecules of living matter. By Altmann, espe-
cially, this view was pushed to an extreme limit, which lay far beyond
anything justified by the known facts; and the theory of genetic con-
tinuity expressed by Redi in the aphorism '' omne viviim ex vivo,''
reduced by Virchow to '^ oninis celliila e celhda,'' finally appears in
the writings of Altmann as ^^ 07nne granuliLm e graniilo'' !^

Altmann's premature generalization rested upon a very insecure
foundation and was received with just scepticism. Except in the case
of plastids, the division of the cytoplasmic granules was and still
remains a pure assumption, and furthermore many of Altmann's
"granules" (zymogen-granules of gland-cells, etc.) are undoubtedly
metaplasmic bodies.* Yet the beautiful discoveries of Schimper ('85)
and others on the origin of plastids in plant-cells give evidence that
these cells do in fact contain large numbers of bodies, other than the
nuclei, that possess the power of growth and division. The division
of the chlorophyll-bodies, observed long ago by Mohl, was shown by
Schmitz and Schimper to be their usual if not their only mode of ori-
gin ; and Schimper was able to trace them back to minute colourless
plastids, scarcely larger than ** microsomes," that are present in large
numbers in the protoplasm of the embryonic cells and of the ^g^, and
give rise not only to chlorophyll-bodies but also to the amyloplasts or
starch-formers and the chromoplasts or pigment^bodies. While it still
remains doubtful whether the plastids arise solely by division or also

1 Variation of Animals and Plants, 1868. * Cf. Introduction, p. 12.

* Die Elemeniarorganismen, Leipsic, 1894, p. 155. * Cf. Lazarus, '98.

THE NATURE OF CELL-ORGANS 29 1

by new formation (as now seems to be the case with the centrosome),
the foregoing observations on the plastids give a substantial basis for
the hypothesis that protoplasm may be built of minute dividing bodies
which form its ultimate structural basis. It was these facts, taken in
connection with the phenomena of particulate inheritance and varia-
tion (Galton), that led De Vries and his followers to the fundamental
assumption of " pangens," "plasomes," " biophores," and the like as
final protoplasmic units ; ^ but these were conceived not as the visible
granules, plastids, etc., but as much smaller bodies, lying far beyond
the limits of present microscopical vision, through the growth or
aggregation of which the visible structures arise. This assumption
has been harshly criticised ; yet when we recall that in one form or
another it has been accepted by such men as Spencer, Darwin, Beale,
Hackel, Michael Foster, Nageli, De Vries,. Wiesner, Roux, Weis-
mann, Oscar Hertwig, Verworn, and Whitman, and on evidence drawn
from sources so diverse, we must admit that despite its highly specula-
tive character it is not to be lightly rejected. In the present chapter
we may inquire how far the known facts of cell-structure speak for or
against this hypothesis, incidentally considering a number of detailed
questions of cell-morphology which have not hitherto found a place.

A. The Nature of Cell-organs

The cell is, in Briicke's words, an elementary organism, which may
by itself perform all the characteristic operations of life, as is the case
with the unicellular organisms, and in a sense also with the germ-cells.
Even when the cell is but a constituent unit of a higher grade of
organization, as in multicellular forms, it is no less truly an organism,
and in a measure leads an independent life, even though its functions
be restricted and subordinated to the common life. It is true that the
earlier conception of the multicellular body as a colony of one-celled
forms cannot be accepted without certain reservations.^ Neverthe-
less, all the facts at our command indicate that the tissue-cell possesses
the same morphological organization as the egg-cell, or the protozoan,
and the same fundamental physiological properties as well. Like
these the tissue-cell has its differentiated structural parts or organs,
and we have now to inquire how these cell-organs are to be conceived.

1 The following list includes only some of the various names that have been given to
these hypothetical units by modern writers : Phyiiological units (Spencer) ; gemmules
(Darwin); pangens (De Vries); plasomes (Wiesner); micellcc (Nageli); plastidules
(Hackel and Elssberg) ; inotagmata (Engelmann); biophores (Weismann); bioblasts
(Beale); somacules {Yo%\.&x)\ idioblasts (Hertwig); idiosomes (Whitman); biogens (Ver-
worn); microzymas (Bechamp and Estor) ; getmncB (Haacke). These names are not
strictly synonymous, nor do all of the writers cited assume the power of division in the
units. 2 cf. p. 58.

292 SOME PROBLEMS OF CELL-ORGANIZATION

The visible organs of the cell fall under two categories, according as
they are merely temporary structures, formed anew in each successive
cell-generation out of the common structural basis, or permanent struc-
tures whose identity is never lost, since they are directly handed on by
divisioxi from cell to cell.^ To the former category belong, in general,
such structures as cilia, pseudopodia, and the Hke ; to the latter, the
nucleus, perhaps also the centrosomes, and the plastids of plant-cells.
A peculiar interest attaches to the permanent cell-organs. Closely
interrelated as these organs are, they nevertheless Have a remarkable
degree of morphological independence. They assimilate food, grow,
divide, and perform their own characteristic actions like coexistent but
independent organisms, of a lower grade than the cell, living together
in colonial or symbiotic association. So striking is this morphological
and physiological autonomy in the case of the green plastids or chro-
matophores that neither botanists nor zoologists are as yet able to dis-
tinguish with absolute certainty between those that form an integral
part of the cell, as in the higher green plants, and those that are
actually independent organisms living symbiotically within it, as is
probably the case with the yellow cells of Radiolaria. Even so
acute an investigator as Watase ('93, i) has seriously propounded the
view that the nucleus itself — or rather the chromosome — should be
regarded as a distinct organism living in symbiotic association with
the cytoplasm, but having had, in an historical sense, a different origin.
This rather fantastic view" has not found much favour, and even were
it true would teach us nothing of the origin of the power of division,
which must for the present be taken as an elementary process forming
one of the primary data of biology. Yet we may still inquire whether
the power of division shown by such protoplasmic masses as plastids,
chromosomes, centrosomes, nucleoli, and nuclei may not have its root
in a like power residing in ultimate protoplasmic units of which they
are made up. Could we accept such a view, we might much more
easily meet some puzzling cytological difficulties. For under this
assumption the difference between transient and permanent cell-
organs would become only one of degree, depending on the degree of
cohesion between their structural components ; and we could thus con-
ceive, for example, how such a body as a centrosome might form, per-
sist by division for a number of generations, and finally disintegrate.
In connection with this it may be pointed out that even such a typical
permanent organ as the nucleus does not persist as such during the
ordinary form of division ; for it loses its boundary and many of its
other structural characters, becoming resolved into a group of sepa-
rate chromosomes. What persists is here not the structural unit, but
the characteristic substance which forms its essential constituent, and

1 Cf. footnote, p. 30.

STRUCTURAL BASIS OF THE CELL 293

a large part even of this substance may be lost in the process. The
term "persistent organ" is therefore used in rather a figurative sense, ,
and if too literally understood may easily mislead us.

With the foregoing considerations in mind let us turn to the actual
structural relation of the cell-organs.

B. Structural Basis of the Cell

In Chapter I. some of the reasons have been given for the conclu-
sion that none of the obvious structural features of protoplasm (fibrillae,
alveoli, granules, and the like) can be regarded as necessary or uni-
versal ; and we may now inquire whether there is any evidence that
such structures may have such a common structural basis as De Vries's
theory assumes. I shall here take as a point of departure my observa-
tions on the structure of protoplasm in echinoderm-eggs, already briefly
reviewed at page 28. The beautiful alveolar structure of these eggs is
entirely of secondary origin, and all the visible structural elements
arise during the growth of the eggs by the deposit and subsequent
enlargement of minute spherical bodies, all apparently liquid drops,
in a homogeneous or finely granular basis which is itself a liquid.
Some of these spheres enlarge to form the alveolar spheres, while the
homogeneous basis or continuous substance remains as the interalve-
olar material. Others remain much smaller to constitute the " micro-
somes " scattered through the interalveolar walls ; and these bodies,
like the alveolar spheres, are perfectly visible in life, as well as in
section ; they are therefore not coagulation-products or artifacts. F'rom
these three elements arise all the other structures observed in these
eggs, deutoplasm-spheres iOphiiira) and pigment-bodies {Arbacia)
being formed by further enlargement and chemical alteration of the
alveolar spheres, while astral rays and spindle-fibres are differentiated
out of the inter-alveolar material and microsomes.^ These various
elements show a continuous gradation in size from the largest deuto-
plasm-spheres down to the smallest visible granules, the latter being
the source of all the larger elements, and in their turn emerging into
view from the " homogeneous " basis. Clearly, then, none of these
bodies can be regarded as the ultimate structural units ; for the latter,
if they exist, must lie in a region at present inaccessible to the micro-
scope. This fact, however, no more disproves their existence than it
does that of molecules and atoms. It only shows the futility of such
attempts as those of Altmann and his predecessors to identify *' gran-
ules " or "microsomes " as final morphological units, and compels us to
turn to indirect instead of direct evidence. It may, however, again be
pointed out that it would be quite irrational to conclude that the small-

1 Cf. Wilson, '99.

294 SOME PROBLEMS OF CELL-ORGANIZATION

est visible granules first come into existence when they first come
within view of the microscope. The ** homogeneous " substance must
itself contain or consist of granules still smaller. The real question
is not whether such ultra-microscopical bodies exist, but whether they
are permanent orga7iized\iO^\^% possessing besides the power of growth
also the power of division. This question can be only indirectly ap-
proached ; and we shall find it convenient to do so by beginning at
the opposite end of the series, through a reconsideration of the
phenomena of nuclear division.

C. Morphological Composition of the Nucleus
I. The Chromatin

(a) Hypothesis of the Individuality of the Chromosomes. — It may
now be taken as a well-established fact that the nucleus is never
formed de novo, but always arises by the division of a preexisting
nucleus. In the typical mode of division by mitosis the chromatic
substance is resolved into a group of chromosomes, always the same
in form and number in a given species of cell, and having the power
of assimilation, growth, and division, as if they were morphological
individuals of a lower order than the nucleus. That they are such
individuals or units has been maintained as a definite hypothesis, es-
pecially by Rabl and Boveri. As a result of careful study of mitosis
in epithelial cells of the salamander, Rabl ('85) concluded that the
chromosomes do not lose their individuality at the close of division, but
persist in the chromatic reticulum of the resting nucleus. The reticu-
lum arises through a transformation of the chromosomes, which give
off anastomosing branches, and thus give rise to the appearance of a
network. Their loss of identity is, however, only apparent. They
come into view again at the ensuing division, at the beginning of
which *' the chromatic substance flows back, through predetermined
paths, into the primary chromosome-bodies" (Kernfaden), which re-
appear in the ensuing spireme-stage in nearly or quite the same posi-
tion they occupied before. Even in the resting nucleus, Rabl believed
that he could discover traces of the chromosomes in the configuration
of the network, and he described the nucleus as showing a distinct
polarity having a " pole " corresponding with the point toward which
the apices of the chromosomes converge {i.e. toward the centrosome),
and an " antipole " (Gegenpol) at the opposite point {i.e. toward the
equator of the spindle) (Fig. 22). Rabl's hypothesis was precisely
formulated and ardently advocated by Boveri in 1887 and 1888, and
again in 1891, on the ground of his own studies and those of Van
Beneden on the early stages of Ascaris. The hypothesis was supported

MORPHOLOGICAL COMPOSITION OF THE NUCLEUS

295

by extremely strong evidence, derived especially from a study of ab-
normal variations in the early development of Ascaris, the force of
which has, I think, been underestimated by the critics of the hypothesis.
Some of this evidence may here be briefly reviewed. In some cases,
through a miscarriage of the mitotic mechanism, one or both of the
chromosomes destined for the second polar body are accidentally left

Fig. 143. — Evidence of the individuality of the chromosomes. Abnormalities in the fertiliza-
tion of ^Jtrar/j. [BOVERI.J

A. The two chromosomes of the egg-nucleus, accidentally separated, have given rise each to a
reticular nucleus (9, ?) ; the sperm-nucleus below (</). B. Later stage of the same, a single
chromosome in each egg-nucleus, two in the sperm-nucleus. C. An egg in which the second
polar body has been retained ; p.b:^ the two chromosomes arising from it ; 9 the egg-chromo-
somes; cf the sperm-chromosomes. D. Resulting equatorial plate with six chromosomes.

in the ^%^. These chromosomes give rise in the ^^g to a reticular nu-
cleus, indistinguishable from the egg-nucleus. At a later period this
nucleus gives rise to the same number of chromosomes as those that
entered into its formation, i.e. either one or two. These are drawn
into the equatorial plate along with those derived from the germ-
nuclei, and mitosis proceeds as usual, the number of chromosomes
being, however, abnormally increased from four to five or six (Fig. 143,

296

SOME PROBLEMS OF CELL-ORGANIZATION

C, D). Again, the two chromosomes left in the Qgg after removal of
the second polar body may accidentally become separated. In this
case each chromosome gives rise to a reticular nucleus of half the
usual size, and from each of these a single chromosome is afterward
formed (Fig. 143, A, B). Finally, it sometimes happens that the two
germ-nuclei completely fuse, while in the reticular state, as is normally
the case in sea-urchins and some other animals (p. 188). From the
cleavage-nucleus thus formed arise four chromosomes.

The same general result is given by the observations of Zur Strassen
('98) on the history of giant embryos in Ascaris. These embryos
arise by the fusion, either before or after the fertilization, of previ-
ously separate eggs, and have been shown to be capable of develop-
ment up to a late stage. Not only in the first but also in some, at
least, of the later mitoses, such eggs show an increased number of

chromosomes proportional to the number
of nuclei that have united. Thus in
monospermic double eggs (variety bi-
valens) the number is six instead of four ;
in dispermic double eggs the number is
increased to eight (Fig. 144).

These remarkable observations show
that whatever be the mmtber of chromo-
somes entering into the formation of a
reticular nnclens, the same number after-
ward issues from it — a result which de-
monstrates that the number of chromo-
somes is not due merely to the chemical
composition of the chromatin-substance,
but to a morphological organization of
Fig. 144.— Giant-embryo of ^j^arw, the nuclcus. A beautiful Confirmation
var bivaiens, arising from a double- ^f ^^is conclusion was afterward made

fertilized double egg, showing eight ,t^ ./, , s inx />

chromosomes {7,ur Strassen). by BoVCri ( 93, 95, l) and Morgan ( 95,

4), in the case of echinoderms, by rear-
ing larvae from enucleated egg-fragments, fertilized by a single sper-
matozoon (p. 194). All the nuclei of such larvae contain but half the
typical number of chromosomes, — i.e. in EcJiinns nine instead of
eighteen, — since all are descended from one germ-nucleus instead
of two !

Equally striking is the remarkable fact, described at page 275, that
all of the cells in the sexual generation (oophore) of the higher
cryptogams show half the number of chromosomes characteristic of
the sporophyte, the explanation being that while reduction occurs
at the time of spore-formation, the spores develop without fertilization,
the reduced chromosome-number persisting until fertilization occurs

MORPHOLOGICAL COMPOSITION OF THE NUCLEUS

297

long afterward. Attention may be again called to the surprising case
of Ariemia, described at page 281, which gives a strong argument in
favour of the hypothesis.

In addition to the foregoing evidence, Van Beneden and Boveri
were able to demonstrate in Ascaris that in the formation of the
spireme the chromosomes reappear in the same position as those
which entered into the formation of the reticulum, precisely as Rabl

Fig. 145. — Evidence of the individuality of the chromosomes in the egg of Ascaris. [BOVERI.]

E. Anaphase of the first cleavage. F. Two-cell stage with lobed nuclei, the lobes formed by
the ends of the chromosomes. G. Early prophase of the ensuing division ; chromosomes re-form-
ing, centrosomes dividing. H. Later prophase, the chromosomes lying with their ends in the
same position as before ; centrosomes divided.

maintained. As the long chromosomes diverge, their free ends are
always turned toward the middle plane (Fig. 31), and upon the re-
construction of the daughter-nuclei these ends give rise to correspond-
ing lobes of the nucleus, as in Fig. 145, which persist throughout the
resting state. At the vSucceeding division the chromosomes reappear
exactly in the same position, t/ieir ends lying' in the nnclear lobes as
before (Fig. 145, G, H). On the strength of these facts Boveri con-
cluded that the chromosomes must be regarded as " individuals " or
" elementary organisms," that have an independent existence in the

298

SOME PROBLEMS OF CELL-ORGANIZATION

cell. During the reconstruction of the nucleus they send forth pseu-
dopodia which anastomose to form a network in which their identity-
is lost to view. As the cell prepares for division, however, the chro-
mosomes contract, withdraw their processes, and return to their
"resting state," in which fission takes place. Applying this con-
clusion to the fertilization of the Q,gg, Boveri expressed his belief that

Fig. 146. — Independence of paternal and maternal chromatin in the segmenting eggs of
Cyclops. [A-C, from RiJCKKRT; D. from HaCKER.]

A. First cleavage-figure in C. strenuus ; complete independence of paternal and maternal
chromosomes. B. Resulting two-cell stage with double nuclei. C. Second cleavage ; chromosomes
still in double groups. D. Blastomeres with double nuclei from the eight-cell stage of C. brevicornis.

" we may identify every chromatic element arising from a resting

nucleus with a definite element that entered into the formation of

that nucleus, from which the remarkable conclusion follows that in

all cells derived in the regular course of division from the fertilized

egg, one-half of the chromosomes are of strictly paternal origin, the

other half of ^naternalT ^

1 '91, p. 410.

MORPHOLOGICAL COMPOSITION OF THE NUCLEUS 299

Boveri's hypothesis has been criticised by many writers, especially
by Hertwig, Guignard, and Brauer, and I myself have urged some
objections to it. Recently, however, it has received a support so
strong as to amount almost to a demonstration, through the remark-
able observations of Riickert, Hacker, Herla, and Zoja on the
independence of the paternal and maternal chromosomes. These
observations, already referred to at page 208, may be more fully re-
viewed at this point. Hacker ('92, 2) first showed that in Cyclops
stremiiis, as in Ascaris and other forms, the germ-nuclei do not fuse,
but give rise to two separate groups of chromosomes that lie side by
side near the equator of the cleavage-spindle. In the two-cell stage
(of Cyclops tenuicomis) each nucleus consists of two distinct though
closely united halves, which Hacker believed to be the derivatives of
the two respective germ-nuclei. The truth of this surmise was demon-
strated three years later by Riickert ('95, 3) in a species of Cyclops,
likewise identified as C streftuiis (Fig. 146). The number of chromo-
somes in each germ-nucleus is here twelve. Riickert was able to
trace the paternal and maternal groups of daughter-chromosomes not
only into the respective halves of the daughter-nuclei of the two-cell
stage, but into later cleavage -stages. From the bilobed nuclei of the
two-cell stage arise, in each cell, a double spireme and a double
group of chromosomes, from which are formed bilobed or double
nuclei in the four-cell stage. This process is repeated at the next
cleavage, and the double character of the nuclei was in many cases
distinctly recognizable at a late stage when the germ-layers were
being formed.

Finally Victor Herla's ('93) and Zoja's ('95, 2) remarkable obser-
vations on Ascaris showed that in Ascaris not only the chromatin of
the germ-i iclei, but also the paternal and maternal chromosomes,
remain perfectly distinct as far as the twelve-cell stage — certainly a
brilliant confirmation of Boveri's conclusion. Just how far the dis-
tinction is maintained is still uncertain, but Hacker's and Riickert's
observations give some ground to believe that it may persist through-
out the entire Hfe of the embryo. Both these observers have shown
that the chromosomes of the germinal vesicle appear in tzvo distinct
groups, and Riickert suggests that these may represent the paternal
and maternal elements that have remained distinct throughout the
entire cycle of development, even down to the formation of the ^gg !

Leaving aside all doubtful cases (such as the above suggestion of
Riickert's), the well-determined facts form an irresistible proof of the
general hypothesis ; and it is one with which every general analysis
of the cell has to reckon. I believe, however, that the hypothesis has
received an unfortunate name ; for, except in a few special cases,^

1 Cf. p. 273.

300

SOME PROBLEMS OE CELL-ORGANIZATION

almost no direct evidence exists to show that the chromosomes persist
as ** individuals " in the chromatin-reticulum of the resting cell. The
facts indicate, on the contrary, that in the vast majority of cases the
identity of the chromosomes is wholly lost in the resting nucleus, and
the attempts to identify them through the polarity or other morpho-
logical" features of the nuclear network have on the whole been futile.
It is therefore an abuse of language to speak of a persistent " individ-

C D

Fig. 147. — Hybrid fertilization of the %gg of Ascaris megalocephala, var. bivalens, by the sper-
matozoon of var. univalcns. [Herla.]

A. The germ-nuclei shortly before union. B. The cleavage-figure forming; the sperm-nucleus
has given rise to one chromosome (cf ), the egg-nucleus to two (9). C. Two-cell stage dividing,
showing the three chromosomes in each cell. D. Twelve-cell stage, with the three distinct chro-
mosomes still shown in the primordial germ-cell or stem-cell.

uality " of chromosomes. But this verbal difficulty should not blind
us to the extraordinary interest and significance of the facts. It is
difficult to suppose that the tendency of the chromatin to resolve
itself into a particular number of chromosomes is directly due to its
chemical or molecular structure, or is analogous to crystallization ; for
in the chromatin of the same species, or even in that of the same Qgg,
this tendency varies, not with chemical, but with purely morphological

i

I

MORPHOLOGICAL COMPOSITION OF THE NUCLEUS 30I

conditions, i.e. with the miniber of chromosomes that enter the
nucleus. Neither can we assume that it is due merely to the total
mass of the chromatin in each case ; for this varies in different nuclei
of the same species, or even in the nucleus of the same cell at dif-
ferent periods (as in the egg-cell), yet the same number of chromo-,
somes is characteristic of all. Indeed, we seek in vain for an analogy
to these phenomena and can only admit our entire inability to explain
them. No phenomena in the history of the cell more clearly indicate
the existence of a morphological organization which, though resting
upon, is not to be confounded with, the chemical and molecular
structure that underlies it ; and this remains true even though we are
wholly ignorant what that organization is.

(^) Composition of the CJiromosonies. — We owe to Roux ^ the first
clear formulation of the view that the chromosomes, or the chromatin-
thread, consist of successive regions or elements that are qualitatively
different (p. 244). This hypothesis, which has been accepted by
Weismann, Strasburger, and a number of others, lends a peculiar
interest to the morphological composition of the chromatic substance.
The facts are now well established (i) that in a large number of cases
the chromatin-thread consists of a series of. granules (chromomeres)
embedded in and held together by the linin-substance, (2) that the
splitting of the chromosomes is caused by the division of these more
elementary bodies, (3) that the chromatin-grains may divide at a time
when the spireme is only just beginning to emerge from the reticulum
of the resting nucleus. These facts point unmistakably to the conclu-
sion that these granules are perhaps to be regarded as independent
morphological elements of a lower grade than the chromosomes.
That they are not artifacts or coagulation-products is proved by their
uniform size and regular arrangement in the thread, especially when
the thread is split. A decisive test of their morphological nature is,
however, even more difficult than in the case of the chromosomes ;
for the chromatin-grains often become apparently fused together so
that the chromatin-thread appears perfectly homogeneous, and whether
they lose their individuality in this close union is undetermined.
Observations on their number are still very scanty, but they point to
some very interesting conclusions. In Boveri's figures of the egg-
maturation of Ascaris each element of the tetrad consists of six chro-
matin-discs arranged in a linear series (Van Beneden's figures of the
same object show at most five) which finally fuse to form an appar-
ently homogeneous body. In the chromosomes of the germ-nuclei
the number is at least double this (Van Beneden). Their number has
been more carefully followed out in the spermatogenesis of the same
animal (variety bivalens) by Brauer. At the time the chromatin-grains

^ Bedeutiing der Kerntheilungsfiguren, 1883, p. 15.

302 SOME PROBLEMS OF CELL-ORGANIZATIOISr

divide, in the reticulum of the spermatocyte-nucleus, they are very
numerous. His figures of the spireme-thread show at first nearly
forty granules in linear series (Fig. 120, B). Just before the breaking
of the thread into two the number is reduced to ten or twelve (Fig.
120, C). Just after the division to form the two tetrads the number
is four or five (Fig. 120, D), which finally fuse into a homogeneous
body.^

It is certain, therefore, that the number of chromomeres is not con-
stant in a given species, but it is a significant fact that in Ascaris the
final number, before fusion, appears to be nearly the same (four to
six) both in the oogenesis and the spermatogenesis. The facts re-
garding bivalent and plurivalent chromosomes (p. %j^ at once sug-
gest themselves, and one cannot avoid the thought that the smallest
chromatin-grains may successively group themselves in larger and
larger combinations of which the final term is the chromosome.
Whether these combinations are to be regarded as ** individuals " is a
question which can only lead to a barren play of words. The fact
that cannot be escaped is that the history of the chromatin-substance
reveals to us, not a homogeneous substance, but a definite morpho-
logical organization in which, as through an inverted telescope, we
behold a series of more and more elementary groups, the last visible
term of which is the smallest chromatin-granule, or nuclear microsome,
beyond which our present optical appliances do not allow us to see.
Are these the ultimate dividing units, as Brauer suggests (p. 113).'*
Here again we may well recall Strasburger's warning, and hesitate to
identify the end of the series with the limits reached by our best
lenses. Somewhere, however, the series must end in final chromatic
units which cannot be further subdivided without the decomposition
of chromatin into simpler chemical substances ; and these units must
be capable of assimilation, growth, and division without loss of their
specific character. It is in these ultimate units that we must seek the
"■ qualities," if they exist, postulated in Roux's hypothesis ; but the
existence of such qualitative differences is a physiological assumption
that in no manner prejudices our conclusion regarding the ultimate
morphological composition of the chromatin.

D. Chromatin, Linin, and Cytoplasm

What, now, is the relation of the chromatin-grains to the linin-net-
work and the cytoplasm.'' Van Beneden long ago maintained ^ that

1 Eisen ('99) finds that the chromosomes of the spermatogonia of Batrachoseps always
consist of six " chromomeres," each of which consists of three smaller granules or " chromi-
oles." The latter persist as the chromatin-granules of the resting nucleus; and it is through
their successive aggregation that the chromomeres and chromosomes are formed.

2 '83. pp. 580, 583.

CHROMATIN, LIN/N, AND CYTOPLASM 303

the achromatic network, the nuclear membrane, and the cell-mesh-
work have essentially the same structure, all consisting of microsomes
united by connective substance, and being only '' parts of one and the
same structure." But, more than this, he asserted that the chromatic
and achromatic microsomes might be trajtsformed into one another^ and
weir therefore of essentially the same morphological nature. ** They
pass successively, in the course of the nuclear evolution, through a
chromatic or an achromatic stage, according as they imbibe or give
off the chromophilous substance." ^ Both these conclusions are borne
out by recent researches. Heidenhain ('93, '94), confirmed by Reinke
and Schloter, finds that the nuclear network contains granules of two
kinds differing in their staining-capacity. The first are the basi-chro-
matin granules, which stain with the true nuclear dyes (basic tar-col-
ours, etc.), and are identical with the " chromatin-granules " of other
authors. The second are the oxychromatin-granules of the linin-net-
work, which stain with the plasma-stains (acid colours, etc.), and are
closely similar to those of the cytoreticulum. These two forms gradu-
ate into one another, and are conjectured to be different phases of the
same elemerits. This conception is furthermore supported by many
observations on the behaviour of the nuclear network as a whole.
The chromatic substance is known to undergo very great changes in
staining-capacity at different periods in the life of the nucleus (p. 338),
and is known to vary greatly in bulk. In certain cases a very large
amount of the original chromatic network is cast out of the nucleus
at the time of the division, and is converted into cytoplasm. And,
finally, in studying mitosis in sea-urchin eggs I found reason to con-
clude ('95, 2) that a considerable part of the linin-network, from which
the spindle-fibres are formed, is actually derived from the chromatin.

From the time of the earlier writings of Frommann ('65, '6^^
Arnold i^^y), Heitzmann ('73), and Klein i^'j'^\ down to the present,
an increasing number of observers have held that the nuclear reticu-
lum is to be conceived as a modification of the same structural basis
as that which forms the cytoplasm. The latest researches indicate,
indeed, that true chromatin (nuclein) is confined to the nucleus.^ But
the whole weight of the evidence now goes to show that the linin-
network is of the same nature as the cell-meshwork, and that the
achromatic nuclear membrane is formed as a condensation of the same
substance. Many investigators, among whom may be named From-
mann, Leydig, Klein, Van Beneden, Carnoy, and Reinke, have de-
scribed the fibres of both the intra- and extra-nuclear network as
terminating in the nuclear membrane ; and the membrane itself is
described by these and other observers as being itself reticular in
structure, and by some (Van Beneden) as consisting of closely crowded

1 U. p. 583. 2 cf. Hammarsten ('95).

304 SOME PROBLEMS OF CELL-ORGANIZATION

microsomes arranged in a network. The clearest evidence is, however,
afforded by the origin of the spindle-fibres in mitotic division ; for it
is now well established that these may be formed either inside or out-
side the nucleus, and at the close of mitosis the central portion of the
spindle appears always to give rise to a portion of the cytoplasm
lying between the daughter-nuclei. In such a case as that of the
sea-urchin (see above) we have, therefore, evidence of a direct trans-
formation of chromatin into linin-substance, of the latter into spindle-
fibres, and, finally, of these into cytoplasm.

When all these facts are placed in connection, we find it difficult to
escape the conclusion that no definite line can be drawn between the
cytoplasmic granules at one extreme and the chromatin-granules at
the other. And inasmuch as the latter are certainly capable of
growth and division, we cannot deny the possibility that the former
may themselves have, or arise from elements having like powers.
But while we may take this as a fair working hypothesis, we should
clearly recognize that the base of well-determined fact on which it
rests is approached by a circuitous route ; that in case of most of the
cytoplasmic granules there is not the sHghtest evidence that they
multiply by division ; and that even though some of them may have
such powers, we cannot regard them as the ultimate structural units,
for the latter must be bodies far more minute.

E. The Centrosome

From our present point of view the centrosome possesses a peculiar
interest as a cell-organ which may be scarcely larger than a cytomi-
crosome, yet possesses specific physiological properties, assimilates,
grows, divides, and may persist from cell to cell without loss of identity.
Nearly all observers of the centrosome have found it lying in the
cytoplasm, outside the nucleus; but apart from the Protozoa (p. 94)
there is at least one well-established case in which it lies within the
nucleus, namely, that of Ascaris, where Brauer made the interesting
discovery that in one variety {imivalens) the centrosome lies inside the
7iiicleus, in the other variety (bivalens) outside — a fact which proves
that its position is non-essential {cf. Figs. 120 and 148).

An intra-nuclear origin of the centrosome has also been asserted by
Julin ('93) in the primary spermatocytes of Styleopsis, by Riickert
('94) in the eggs of Cyclops, Mathews ('95) in those of Asterias, Car-
noy and Le Brun('97, 2) in Ascai'is, Van der Stricht ('98) in the eggs
of Thysanozoon, by R. Hertwig ('98) in Actinosphceriiim, Calkins
('98, i) in Noctihica, and Schaudinn ('96, 3) in spore-producing buds of
AcaritJiocystis, though in the last-named form the centrosome of the
vegetative forms is extra-nuclear (p. 92).

THE CENTROSOME

305

As already stated, ^ it is still undetermined whether a true centro-
some may ever arise de novo, but the evidence in favour of such a
possibility has of late rapidly increased. Carnoy ('86) long since
showed that the ^^g of Ascaris, during the formation of the polar
bodies, sometimes showed numerous accessory asters scattered
through the cytoplasm. Reinke ('94) described somewhat similar
asters in peritoneal cells of the salamander, distinguishing among
them three orders of magnitude, the largest containing distinct
centrosomes or "primary centres," while the smaller contained
*' secondary" and "tertiary" centres, the last named being single

it #

E

j^^

F . ^

Fig. 148. — Mitosis with intra-nuclear centrosome, in the spermatocytes of Ascaris megalo-
cephala, van univalens. [Brauer.]

A. Nucleus containing a quadruple group or tetrad of chromosomes (/), nucleolus («), and
centrosome (t). B. C. Division of the centrosome. D. E.F. G. Formation of the mitotic figure,
centrosomes escaping from the nucleus in G.

microsomes at the nodes of the cytoreticulum. By successive aggre-
gations of the tertiary and secondary centres arise true centrosomes
as new formations. Watase ('94-95) also finds in the Qgg of Macro-
bdella, besides the normal aster containing an undoubted centrosome,
numerous smaller asters graduating downwards to such " tertiary
asters " as Reinke describes with a microsome at the centre of each,
and on this basis concludes that the true centrosome differs from a
microsome only in degree and may arise de novo. Mottier ('97, 2)
finds in pollen-mother-cells numerous minute " cyto-asters " having
no direct relation to the spindle-formation (Fig. 133). Again Juel

1 Cf. pp. 52, 214.

3o6

SOME PROBLEMS OF CELL-ORGANIZATION

('97) finds that an isolated chromosome, accidentally separated from
the equatorial plate (pollen-mother-cells of Hcmerocallis), may give
rise to a small vesicular nucleus which may subsequently divide by
mitosis, though it is quite out of relation to the spindle-poles of the
preceding mitosis (Fig. 149). Strong evidence of the same character
as the last is given by the facts in the heliozoon Acanthocystis, as
shown by Schaudinn ('96, 3), the ordinary vegetative cells containing
a pejsistent extra-nuclear centrosome, while in the bud-formation of
the swarm-spores a centrosome is formed de novo, without relation to
that of the mother-cell, inside the nucleus of the bud (Fig. 41).

The strongest case in favour of the independent origin of centro-
somes is, however, given by the observations of Mead on Chcetopterus
('98) and the remarkable experiments of R. Hertwig ('95, '96) and

Fig. 149. — Abnormal mitosis in pollen-mother-cells of Hemerocallis, showing formation of
small nucleus from one or two stray chromosomes and its subsequent division. [JUEL.]

Morgan ('96, i ; '99, i) on the eggs of echinoderms and other animals.
When eggs of Chcetoptei'us are taken from the body-cavity and placed
in sea-water, a multitude of small asters appear in the cytoplasm, two
of which are believed to persist as those of the polar spindle, while
the others degenerate (Fig. 150). Mead is therefore convinced
that the polar centrosomes arise in this case separately and de novo}
R. Hertwig showed that when unfertilized eggs of sea-urchins
{Strongylocent)'ot?{s, Echinus) are kept for some time in sea-water or
treated with dilute solutions of strychnine the nuclei undergo some of

1 A number of other authors {e.g. Griffin, Thalassema, Coe, Cerebratulus) have likewise
found the first polar asters widely separated at their first appearance. On the other hand,
Mathews ('95), whose preparations I have seen, finds the polar centrosomes in Aste7'ias
close together, and Francotte ('97, '98) has demonstrated that in Cycloporus and Prosihece-
raus they arise by the division of a single primary centrosome. The same is stated by Gar-
diner ('98) to be the case in Polychoerus. It should be noted, further, that Mead could find
no undoubted centrosomes save in the " primary " or definitive polar asters.

A

THE CENTROSOME

307

the changes of mitosis, the chromatin-network giving rise to a group
of chromosomes and a spindle, or more frequently a fan-shaped
half-spindle, arising from the achromatic substance. In some cases
not only a complete spindle appeared but also asters at the poles,
though no centrosomes were observed (Fig. 151). Morgan's experi-
ments along the same lines were mainly performed upon the sea-
urchin Arhacia, but included also the eggs of Asterias, Sipuncuhis^
and Cerebratulus (Figs. 150, 151). In these eggs numerous asters
may arise in the cytoplasm, if they are allowed to lie some time in sea-

..v<S?^^fe>-.,

o^m

ti^kh^.

Fig. 150. — Formation de novo (?) of centrosomes. [A, B, Mead; C, Morgan.]

A. Unfertilized egg of Chcetopterus with " secondary asters " developed a few minutes after the

egg is placed in sea-water. B. Slightly later stage with two definitive polar asters and centrosomes.

C. Large " sun " (transformed polar aster) containing numerous small " secondary asters " and

centrosomes, from unfertilized egg of Cerebratulus after 22 hours in 1.5 % sodium chloride

solution.

water or treated by weak solutions of sodium or magnesium chloride.
These asters often contain deeply staining, central granules indistin-
guishable from the centrosomes of the normal asters ; and, what is of
high interest, such of them as lie near the nucleus take part in the
irregular nuclear division that ensues, forming centres toward which
the chromosomes pass. These divisions continue for some time, the
chromosomes being irregularly distributed through the ^^Z, and giving
rise to nuclei of various sizes apparently dependent upon the number
of chromosomes each receives. After a variable number of such

3o8

SOME PROBLEMS OF CELL-ORGANIZATION

divisions the asters disappear, yet the irregular nuclear divisions con-
tinue, nuclear spindles with distinct centrosomes being formed at each
division, but apparently without relation to the older asters, and they

Fig. 151. — Formation of centrosomes and asters in unfertilized echinoderm-eggs. \A, B,
Morgan ; C-E, R. Hertwig.]

A. Arbacia, after 4y2 hours in 1.5 % solution of sodium chloride, then 5 hours in sea-water;
scattered chromosomes and asters. B. Asters formed after bVi hours in NaCl. C-E. Echinus
after treatment with 0.5 % strychnine-solution, showing various forms of astral formations (fan-
shaped aster, half spindle, and complete mitotic figure).

are believed by Morgan to arise de novo from the Qgg substance.^
In the meantime irregular cleavage of the Qgg occurs, though no
embryo is produced.^ Loeb, however, in the remarkable experiments

^ '99, P- 479-

'■^ Morgan makes the important observation, which harmonizes with that of Boveri,
reported at page 108, that the divisions occur ivith respect to the number and position of the
nuclei, not of the asters, concluding that the former must therefore play an essential rble as
centres of division, and that the activity of the asters is in itself not sufficient to account
for division of the cytoplasm.

THE CENTROSOME 309

referred to at page 215, finds that after treatment with magnesium
chloride unfertilized sea-urchin eggs {Arbacia) may give rise to perfect
Pluteiis larvae — a result which if well founded seems to place the
new formation of true centrosomes beyond question.

Taken together, these researches give strong ground for the con-
clusion that true {i.e. physiological) centrosomes may arise de novo
from either the cytoplasmic or the nuclear substance and may play
the usual role (whatever that may be) in mitosis. If this conclusion
be sustained by future research, we shall no longer be able to accept
Van Beneden's and Boveri's conception of the centrosome as a per-
sistent organ in the same sense as the nucleus ; but on the other hand
we shall have gained important ground for further inquiry into the
nature and source of that power of division which is so characteristic,
of living things and upon which the law of genetic continuity rests.

Morphology of the Centrosome. — In its simplest form (Fig. 152, A)>
the centrosome appears under the highest powers as nothing more than
a single granule of extraordinary minuteness which stains intensely
with iron-haematoxylin, and can scarcely be distinguished from the
cyto-microsomes except for the fact that it Hes at the focus of the
astral rays. In this form it always appears at the centre of the very
young sperm-asters during fertilization (Figs. 97, 99), in the early
phases of ordinary mitosis (Figs. 27, 32), and in some cases also in the
resting cell, for example, in leucocytes and connective tissue corpuscles
(Figs. 8, 49), where, however, it is often triple or quadruple. In the
course of division the centrosome often increases in size and assumes
a more complex form, becoming also surrounded by various structures
involved in the aster-formation. The relation of these structures to
the centrosome itself has not yet been fully cleared up and there
is still much divergence of opinion regarding the cycle of changes
through which the centrosome passes. It is, therefore, not yet possi-
ble to give a very consistent account of the centrosome, still less to
frame a satisfactory morphological definition of it.

It is convenient to take up as a starting-point Boveri's ('88) account
of the centrosomes in the ^gg of Ascaris, supplemented by Brauer's
('93) description of those in the spermatocytes of the same animal.
During the early prophases of the first cleavage Boveri found the
centrosome as a minute granule which steadily enlarges as the spin-
dle forms, until shortly before the metaphase it becomes a rather large,
well-defined sphere in the centre of which a minute central granule or
centriole appears (Fig. 152, B, C). From this time onward the cen-
trosome decreases in size until in the daughter-cells it is again reduced
to a small granule which divides into two and goes through a similar
cycle during the second cleavage and so on. The centrosome is
at all stages surrounded by a clear zone (" Heller Hof ") in which

3IO

SOME PROBLEMS OF CELL-ORGANIZATION

the astral rays are thinner and stain less deeply than farther
out Brauer's account is substantially the same, though no definite
** Heller Hof " was found, and the astral rays were traced directly in
to the boundary of the centrosome. He added, however, two impor-
tant observations, viz. (i) that the central granule is visible at every
period ; and (2) division of the centrosome is preceded by division of
the central grannie {¥\^, 148) — an observation recently extended by
Boveri to the division of the egg-centrosome.^ Van Beneden and
Neyt {^'^J\ on the other hand, gave a quite different account of the

E F G H

Jbig. 152. — Diagicuiia illustrating various accounts of centrosome and aster.

A. Centrosome, a simple granule at the centre of the aster; ex. sperm-aster in various animals.
B. "Centrosome," a sphere enclosing a central granule or centriole; ex. Brauer's account of
spermatocytes oi Ascarls. C Like the last, but "centrosome" surrounded by a " Heller Hof";
ex. Boveri's account of the centrosome of the Ascarls egg. D. Central granule surrounded by a
radial sphere ("centrosome") bounded by a microsome-circle, and lying in a "Heller Hof";
ex. polar spindles of Thysanozoon, Van der Stricht. E. Central granule (" centrosome ") sur-
rounded by medullary and cortical radial zones, each bounded by a microsome-circle ; ex. polar
spindle of Unio, Lillie, F. Van Beneden's representation of aster of the Ascaris egg; like the last,
but the " corpuscula central " consisting of a group of granules. G. " Centrosome," a group of
granules surrounded by a "Heller Hof"; ex. the echinoderm-egg, ff. "Centrosome" (central
granule) surrounded by a vague larger body lying in a reticulated centrosphere ; ex. Thalassema.
[Griffin.]

Structures at the centre of the aster. The " corpuscule central "
(usually assumed by later writers to be the centrosome), described as
a "mass of granules," is surrounded by two well-defined astral zones,
formed as modifications of the inner part of the aster, and constitut-
ing the "attraction-sphere." These are an inner "medullary zone,"
and an outer "cortical zone," each bounded by a very distinct layer
of microsomes (Fig. 152, F\

1 Reported by Furst, '98, p. iii.

J

THE CENTKOSOME 3II

The discrepancy between these results on the part of the two
pioneer investigators of the centrosome has led to great confusion
in the terminology of the subject, which has not yet been fully
cleared away. Many of the observers who followed Boveri (Flem-
ming, Hermann, Van der Stricht, Heidenhain, etc.) found the centro-
some, in various cells, as a much smaller body than he had described,
often as a single or double minute granule, staining intensely with
iron-haematoxylin. Heidenhain ('93, '94) and Driiner ('94, '95) found
further that the asters in leucocytes and other forms often show
several concentric circles of microsomes, and that the sphere bounded
by the innermost circle often stains more deeply than the outer por-
tions and may appear nearly or quite homogeneous (Fig. 156). To
this sphere, with its contained central granule or granules Heidenhain
applies the term microcentrum ('94, p. 463), while Kostanecki and
Siedlecki suggest the term microsphere ('96, p. 217). Still later
Kostanecki and Siedlecki ('97) found that even in Ascaris, as in other
forms, sufficient extraction of the colour (iron-haematoxylin) reduces the
centrosome to a minute granule to which the astral rays converge,
and which is presumably identical with Boveri's "central granule."
Heidenhain ('93, '94) found that in leucocytes the central granule is
often double, triple, or even quadruple, while in giant-cells of certain
kinds there are numerous deeply staining granules (Fig. 14). He
therefore proposed to restrict the term centrosome to the individ-
ual granules, whatever be their number, applying the term microcen-
trum to the entire group ('94, p. 463).

With these facts in mind we can gain a clear view of the manner
in which both the confusion of terminology and the contradiction of
results has arisen. Brauer ('93) found in Ascaris (see above) that
division of the central granule precedes division of the ^^ centrosome^''
and therefore suggested that only the former is equivalent to Van
Beneden's " corpuscule central," while the body called "centrosome"
by Boveri is really the medullary astral zone, the " Heller Hof " being
the cortical zone. This is substantially the same conclusion reached
by Heidenhain, Rawitz, Lenhossek, Kostanecki and Siedlecki, Erlan-
ger. Van der Stricht, Lillie, and several others. The confusion of
the subject is owing, on the one hand, to the fact that those who
have accepted this conclusion continue to use the word centrosome
in two quite different senses, on the other hand to the fact that the
conclusion is itself repudiated by Boveri ('95), MacFarland ('97), and
Fiirst ('98).

As regards the terminology we find that most recent writers agree
with Heidenhain, Kostanecki and Siedlecki, in restricting the word
ce?itrosome to the minute, deeply staining granules, whether one or
more, at the centre of the aster. On the other hand, Brauer, Fran-

3,12

SOME PROBLEMS OF CELL-ORGANIZATION

cotte, Van der Stricht, Meves, and others apply the term to the central
granule or granules plus the surrounding sphere (" centrosome " of
Boveri), which they regard as equivalent to the medullary zone of
Van Beneden, the " corpuscule central " of the last-named author
being identified with the central granule or ''centriole" of Boveri,
though the latter structure is considerably smaller than the former
as described by Van Beneden.

The matter of fact turns largely on the question whether the astral
rays traverse the larger sphere to the central granule. That such is
the case in Ascaris is positively asserted by Kostanecki and Siedlecki,
('97) and as positively denied by Furst ('98) with whose observations

Fig. 153. — SiiLicture of the centrosome in tiic polar asters of a gasteropod, Diaulula. [Mac-
Farland.]

A. Mitotic figure, formation of first polar body. B. Inner aster at final anaphase; central
granule double within the " centrosome." C. Elongation of old " centrosome " to form second
polar spindle.

those of MacFarland ('97) on gasteropod-eggs agree. On the other
hand, in the turbellarians the observations of Francotte ('97, '98) and
Van der Stricht ('98, i) seem to leave no doubt that the larger sphere
(''centrosome"), here very sharply defined and staining deeply in
iron-haematoxylin, is traversed by well-defined astral rays converging
to the central corpuscle, and both these observers agree further that
both the corpuscle and the sphere divide to persist as the ^^ ceiitrosomes''
of the daughter-cells — a result in conformity with Van Beneden's con-
clusion in the case of Ascaris.

Lillie's valuable observations on the polar asters of Unio ('98) afford,
I believe, conclusive evidence as to the nature of the sphere. In the

THE CENTROSOME

313

earlier stages the aster has exactly the structure described by Van
Beneden in Ascaris, except that the innermost body {i.e. the *' cor-
puscule central") is a single minute granule. This is surrounded
by typical medullary and cortical zones, through both of which the

C D

Fig. 154. — Centrosome and aster in the polar mitoses of Unio. [LiLLlE.]

A. Aster of the first polar figure; central granule (centrosome) surrounded by medullary
(entosphere) and cortical (ectosphere) zones. B. Late anaphase of second polar mitosis; radial
entosphere bounded by continuous membrane. C. D. Prophases of second mitosis ; formation
of central spindle within and from the substance of the old entosphere.

rays pass (Fig. 152, E, Fig. 154). The inner sphere, consisting of
a dense and deeply staining substance, has at first a typical radiate
structure and is bounded by a microsome-circle. In later stages (late
anaphase) the central granule divides into two and afterward into
four or more granules, of which, however, only one or two actually

k

314 SOME PROBLEMS OF CELL-ORGANIZATION

persist. The inner sphere is now bounded by a definite membrane,
and its radiate structure becomes obscure, the astral rays extending
only to the boundary of the sphere, though a few rays persist within
it (Fig. 154, B). It is clear from this that the inner sphere and
central granule pass through phases that bridge the gap between
Van Beneden's and Boveri's descriptions. Lillie's observations fully
sustain the conclusion that the central gramile {^^ centriole'' of Boveri)
corresponds to the '^ corptiscule central' ' of Van Bene den, arid the inner
sphere {medullary zone) to Boveri's '' centrosoiney A comparison of
the polar aster of Unio with that of Thysanozoon, as described by
Van der Stricht ('98), leaves hardly room for doubt that the cortical
zone represents Boveri's " Heller Hof " ; for in both forms the rays
of the cortical zone are much thinner and lighter than the more
peripheral portions, thus giving a clear zone, which in Unio is bounded
by only a fairly definite microsome-circle and in Thysanozoon by none.
Lastly, we must recognize the justice of the view urged by Kos-
tanecki, Grififin, Mead, Lillie, Coe, and others, that the term centro-
some should be applied to the central granule and not to the sphere
surrounding it (medullary zone), despite the fact that historically the
word was first applied by Boveri to the latter structure. For in both
Diaulula (MacFarland) and Unio (Lillie) the second polar spindle
arises from the substance of the inner sphere, while the central
granule, becoming double, gives rise to the centrosomes at its poles.
By following Boveri's terminology, therefore, MacFarland is driven to
the strange conclusion that the second polar spindle is nothing other
than an enormously enlarged " centrosome " — a result little short of
a reductio ad absnrdiim when we consider that in Ascaris the polar
spindle arises by a direct transformation of the germinal vesicle
(p. 277). The obvious interpretation is that the central granule is
the only structure that should be called a centrosome, the surround-
ing sphere being a part of the aster, or rather of the attraction-sphere.
Thus regarded, the origin of the spindle in Dianlnla presents nothing
anomalous and a similar interpretation may be placed on the polar
spindles of Ascaris as described by Fiirst ('98).^

^ In echinoderms the concurrent results of Reinke ('95), Boveri ('95), myself ('96-'97),
show that the " centrosome " is a well-defined sphere containing a large group (ten to twenty)
of irregularly scattered, deeply staining granules. I have shown in this case that in the early
prophases there is but one such granule, which then becomes double and finally multiple,
forming a pluricorpuscular centrum (Fig, 52) not unlike that described by Heidenhain
in giant-cells. Kostanecki, who asserts that the centrosome of echinoderms is a single
granule ('96, i, '96, 2, p. 248), has not sufficiently studied the later phases of mitosis.
Cf. also Erlanger ('98). The centrosomes described in nerve cells by Lenhossek ('95) are
apparently of somewhat similar type. Until the facts are more fully known the exact nature
of these "centrosomes" remains an open question. Lillie's observations on Unio show
that here, too (first polar spindle), the centrosome divides to form a considerable number of

i

THE CENTROSOME 315

The genesis of the concentric spheres surrounding the centrosome
will be considered in the following section. We may here only
emphasize the remarkable fact that the centres of the dividing
system are bodies which are in many cases so small as to lie almost
at the limits of microscopical vision, and which in the absence of the
surrounding structures could not be distinguished from other proto-
plasmic granules. Full weight should be given to this fact in every
estimate of the centrosome theory, and it is no less interesting in its
bearing upon the corpuscular theory of protoplasm.

Watase ('93, '94) made the very interesting suggestion that the cen-
trosome is itself nothing other than a microsofne of the same morpho-
logical nature as those of the astral rays and the general meshwork,
differing from them only in size and in its peculiar powers.^ Despite
the vagueness of the word "microsome," which has no well-defined
meaning, Watase's suggestion is full of interest, indicating as it does
that the centrosome is morphologically comparable to other elemen-
tary bodies existing in the cytoplasmic sti-ucture, and which, minute
though they are, may have specific chemical and physiological prop-
erties.

An interesting hypothesis regarding the historical origin of centrosome is that of
Butschli ("91) and R. Hertwig ("92), who suggest that it may be a derivative of a
body comparable with the micro-nucleus of Infusoria, which has lost its chromatin
but retained the power of division ; and the last-named author has suggested further
that the so-called " archoplasmic loops " discovered by Platner in pulmonates may
be remnants of the chromatic elements. A similar view has been advocated by
Heidenhain ('93, '94) and Lauterborn ('96). Heidenhain regards central spindle
and centrosomes as forming essentially a unit ("microcentrum ") homologous with
the micro-nucleus of the Infusoria, the centrodesmus (p. 79) representing a part of
the original achromatic elements. The metazoan nucleus is compared to the proto-
zoan macro-nucleus. The improbability of a direct derivation of the Metazoa from
Infusoria, urged by Boveri C95) and Hertwig ('96), has led Lauterborn ('96) to the
view that the metazoan centrosome and nucleus are respectively derivatives of two
equivalent nuclei, such as Schaudinn ('95) describes in Amceba biniicleata, the
" Nebenkorper " of Paramoeba {cf. p. 94), being regarded as an intermediate step,
and the micro-nucleus of Infusoria a side-branch. R. Hertwig ('96), on the other
hand, regards the metazoon centrosome as a derivative of an intra-nuclear body such
as the '^ nucleolo-centrosome " of Euglena (p. 91), which has itself arisen through
a condensation of the general achromatic substance. With this view Calkins ('98),
on the whole, agrees; but he regards it as probable that the " nucleolo-centrosome "

gi-anules of which one or two remain as the persistent centrosome, whUe others are converted
into microsomes or other cytoplasmic structures. It is probable that something similar
occurs in the echinoderms.

^ The microsome is conceived, if I understand Watase rightly, not as a permanent mor-
phological body, but as a temporary varicosity of the thread, which may lose its identity in
the thread and reappear when the thread contracts. The centrosome is in like manner not
a permanent organ Hke the nucleus, but a temporary body formed at the focus of the astral
rays. Once formed, however, it may long persist even after disappearance of the aster, and
serve as a centre of formation for a new aster.

■

3l6 SOME PROBLEMS OF CELL-ORGANIZATION

of Euglena and A7noeba and the sphere of Noctiluca and Paramoeba are to be com-
pared with the attraction-sphere, while the centrosome may have had a different
origin.

It appears to me that none of these views rests upon a very substantial basis, and
they must be taken rather as suggestions for further work than as well-grounded
conclusions.

F. The Archoplasmic Structures
I. Hypothesis of Fibrillar Persistence

The asters and attraction-spheres have a special interest for the
study of cell-organs ; for they are structures that may divide and
persist from cell to cell or may lose their identity and re-form in suc-
cessive cell-generations, and we may here trace with the greatest
clearness the origin of a cell-organ by differentiation out of the struc-
tural basis. Two sharply opposing views of these structures have
been held, represented among the earlier observers on the one hand
by Boveri, on the other by Biitschli, Klein, Van Beneden, and Carnoy.
The latter observers held that the astral rays and spindle-fibres, and
hence the attraction-sphere, arise through a morphological rearrange-
ment of the preexisting protoplasmic meshwork, under the influence of
the centrosome. This view, which may be traced back to the early
work of Fol ('73) and Auerbach ('74)> was first clearly formulated
by Biitschli ('76), who regarded the aster as the optical expression of
a peculiar physico-chemical alteration of the protoplasm primarily
caused by diffusion-currents converging to the central area of the
aster.^ An essentially similar view is maintained in Biitschli's recent
great work on protoplasm,^ the astral " rays " being regarded as
nothing more than the meshes of an alveolar structure arranged
radially about the centrosomes (Fig. 10, E). The fibrous appearance
of the astral rays is an optical illusion, for they are not fibres, but flat
lamellae forming the walls of elongated closed chambers. This view
has recently been urged, especially by Erlanger ('97, 4, etc.), who
sees in all forms of asters and spindles nothing more than a modified
alveolar structure.

The same general conception of the aster is adopted by most of
those who accept the fibrillar or reticular theory of protoplasm, the
astral rays and spindle-fibres being regarded as actual fibres forming
part of the general network. One of the first to frame such a con-
ception was Klein {l7^)y who regarded the aster as due to "a radial
arrangement of what corresponds to the cell-substance," the latter

1 For a very careful review of the early views on this subject, see Mark, Limax^ 1881.
2 '92, 2, pp. 158-169.

i

THE ARCHOPLASMIC STRUCTURES 317

being described as having a fibrillar character.^ The same view is
advocated by Van Beneden in 1883. With Klein, Heitzman, and
Frommann he accepted the view that the intra-nuclear and extra-
nuclear networks were organically connected, and maintained that
the spindle-fibres arose from both.^ "The star-like rays of the asters
are nothing but local differentiations of the protoplasmic network. ^
. . . In my opinion the appearance of the attraction-spheres, the
polar corpuscle (centrosome), and the rays extending from it, includ-
ing the achromatic fibrils of the spindle, are the result of the appear-
ance in the egg-protoplasm of two centres of attraction comparable
to two magnetic poles. This appearance leads to a regular arrange-
ment of the reticulated protoplasmic fibrils and of the achromatic
nuclear substance with relation to the centres, in the same way that
a magnet produces the stellate arrangement of iron filings." *

This view is further developed in Van Beneden's second paper,
published jointly with Neyt i^^J^. " The spindle is nothing but a
differentiated portion of the asters." ^ The aster is a " radial structure
of the cell-protoplasm, whence results the image designated by the
name of aster." ^ The operations of cell-division are carried out
through the " contractility of the fibrillae of the cell-protoplasm and
their arrangement in a kind of radial muscular system composed of
antagonizing groups." ''

An essentially similar view of the achromatic figure has been
advocated by many later workers. Numerous observers, such as
Rabl, Flemming, Carnoy, Watase, Wilson, Reinke, etc., have ob-
served that the astral fibres branch out peripherally into the general
meshwork and become perfectly continuous with its meshes, and
tracing the development of the aster, step by step, have concluded
that the rays arise by a direct progressive modification of the pre-
existing structure. The most extreme development of this view is
contained in the works of Heidenhain ('93, '94), Biihler ('95), Kosta-
necki and Siedlecki ('97), which are, however, only a development of
the ideas suggested by Rabl in a brief paper pubHshed several years
before. Rabl ('89, 2) suggested that neither spindle-fibres nor astral
rays really lose their identity in the resting cell, being only modified
in form to constitute the mitome or filar substance (meshwork), but
still being centred in the centrosome. Fission of the centrosome is
followed by that of the latent spindle-fibres (forming the linin-
network); hence each chromosome is connected by pairs of daughter-

^ It is interesting to note that in the same place Klein anticipated the theory of fibrillar
contractility, both the nuclear and the cytoplasmic reticulum being regarded as contractile
{I.C., p. 417).

2 '83, p. 592. 4 'Zi„ p. 550. 6 ic,^ p. 275.

6 '87, p. 263. '' I.C., p. 280.

I

3l8 SOME PROBLEMS OF CELL-OKGANIZAIION

fibres with the respective centrosomes. Heidenhain, adopting the
first of these assumptions, builds upon it an elaborate theory of cell-
polarity and cell-division already considered in part at pages 103-105.
Sometimes the astral rays (" organic radii ") retain their radial arrange-
ment throughout the life of the cell (leucocytes, Fig. 49); more com-
monly they are disguised and lost to view in the cytoplasmic meshwork.
All, however, are equal in length and in tension — assumptions based
on the one hand on the occurrence of concentric circles of microsomes
in the aster, on the other hand on the analogy of the artificial model
described at page 104. Biihler ('95) and Kostanecki and Siedlecki
('97) likewise unreservedly accept the view that besides the centro-
some the entire system of " organic radii," including astral rays,
mantle-fibres, and central spindle-fibres, persists in the resting cell in
modified form, and is centred in the centrosome. Kostanecki finally
('97) takes the last step, logically necessitated by the foregoing con-
clusion, and apparently supported also by the crossing of the astral
rays opposite the equator of the spindle and the relations of their
peripheral ends, concluding that the monocentric astral system is con-
verted into the dicentric system (amphiaster) by longititdinal fissio7i
of the rays} Thus the entire mitome of the mother-cell divides into
equal halves for daughter-cells ; and since the radii consist of micro-
somes, each of these must likewise divide into two.^

Could this tempting hypothesis be established, Roux's interpretation
of nuclear division (p. 224) could be extended also to the cytoplasm ;
and the aster- and amphiaster-formation, with the spireme-forma-
tion, might be conceived as a device for the meristic division of the
entire cell-substance — a result which would place upon a substantial
basis the general corpuscular theory of protoplasm. Unfortunately,
however, the hypothesis rests upon a very insecure foundation : first,
because it is based solely upon the fibrillar theory of protoplasm ;
second, because of the very incomplete direct evidence of such a
splitting of the rays ; third, because there is very strong evidence that
in many cases the old astral rays wholly disappear, to be replaced by
new ones."^ We may best consider this adverse evidence in connec-
tion with a general account of the opposing archoplasm-hypothesis.

2. The Archoplasm Hypothesis

Entirely opposed to the foregoing conception are the views of
Boveri and his followers, the starting point of which is given by

1 '97, p. 680.

2 This view had been definitely stated also by O. Schultze in 1890.

^ There is, however, no doubt that the aster as a whole does, in some cases, divide into
two — for instance, in the echinoderm-egg, Fig. 95.

THE AKCHOPLASMIC STRUCTURES 319

Boveri's celebrated archoplasm-hypothesis. Boveri has from the first
maintained that the amphiastral fibres are quite distinct from the gen-
eral cell-meshwork. In his earlier papers he maintained i^ZZ, 2) that
the attraction-sphere of the resting cell is composed of a distinct sub-
stance, '' arcJioplasm,'' consisting of granules or microsomes aggre-
gated about the centrosome as the result of an attractive force exerted
by the latter. From the material of the attraction-sphere arises the
entire achromatic figure, including both the spindle-fibres and the
astral rays, and these have nothing to do with the general reticulum
of the cell. They grow out from the attraction-sphere into the reticu-
lum as the roots of a plant grow into the soil, and at the close of
mitosis are again withdrawn into the central mass, breaking up into
granules meanwhile, so that each daughter-cell receives one-half of
the entire archoplasmic material of the parent-cell. Boveri was
further inclined to believe that the individual granules or archoplas-
mic microsomes were '* independent structures, not the nodal points of a
general network," and that the archoplasmic rays arose by the arrange-
ment of these granules in rows without loss of their identity.^ In a
later paper on the sea-urchin this view underwent a considerable
modification through the admission that the archoplasm may not pre-
exist as formed material, but that the rays and fibres may be a new
formation, crystalHzing, as it were, out of the protoplasm about the
centrosome as a centre, but having no organic relation with the gen-
eral reticulum ; though Boveri still held open the possibility that the
archoplasm might preexist in the form of a specific homogeneous sub-
stance distributed through the cell, though not ordinarily demonstra-
ble by reagents.^ In this form the archoplasm-theory approaches
very nearly that of Strasburger, described below.

There are three orders of facts that tell in favour of Boveri's modi-
fied theory : first, the existence of persistent archoplasm-masses or
attraction-spheres from which the amphiasters arise ; second, the
origin of amphiasters in alveolar protoplasm ; and, third, the increas-
ing number of accounts asserting the replacement of the old asters
by others of quite new formation. In at least one case, namely, that
of Noctiluca, the entire achromatic figure is formed from a permanent
attraction-sphere lying outside the nucleus and perfectly distinct from
the general cell-meshwork.^ Other cases of this kind are very rare,
and in most cases the attraction-sphere sooner or later disintegrates,*
but in the formation of the spermatozoa we have many examples of
archoplasmic masses (Nebenkern, attraction-sphere, idiozome), which
apparently consist of a specific substance having a special relation to
the achromatic figure.

1 '88, 2, p. 80. « Ishikawa, '94, '98; Calkins, '98, 2.

2 '95, 2, p. 40. 4 Cf. p. 323.

320

SOME PROBLEMS OF CELL-ORGANIZATION

The amphiastral formation in alveolar protoplasm gives very clear
evidence against the theory of fibrillar persistence. Here the fibrillar
rays can be seen growing out through the walls of the alveoli ^ quite
distinct from, though embedded in, them. At the close of mitosis
every trace of the fibrillar formation may disappear, e.g. in echino-
derm-eggs after formation of the polar bodies, the protoplasm retain-
ing only a typical alveolar structure.

Fig. 155. — Stages in the first cleavage 01 nie &gg in Cerebratulus (A-C, COE) and Thalassema
{D-F, Griffin).

A. First appearance of the cleavage-centrosome at the poles of the fused germ-nuclei ; cleavage-
asters forming within the degenerating sperm-asters. B. Final anaphase of first cleavage, showing
persistent centrosomes and new asters forming. C. Immediately after division. D-F. Three
stages of the late anaphase in Thalassema, showing formation of new asters within the old. ( Cf.
Fig. 99.)

The strongest evidence against fibrillar persistence is, however,
given by recent studies on mitosis, showing on the one hand that the
new astral centres do not coincide with the old ones, on the other
that the old rays degenerate /;/ situ, to be replaced by new ones.
Aside from many earlier observers, who believed the entire aster to
disappear at the close of mitosis, the first to assert the wholly new

1 Cf. Reinke ('95), Wilson ('99).

THE ARCHOPLASMIC STRUCTURES 32 1

formation of the rays was Driiner, who maintained in the case of the
mitosis of salamander testis-cells, that " not a single fibre of the astral
system of the mother-cell is carried over unchanged into the organism
of the daughter-cell" ('95, p- 309). The same conclusion was soon
afterward supported by Braus ('95) in the case of the cleavage-
mitoses of Triton. The most convincing evidence of this fact has
been given by studies on the maturation and fertilization of the ^gg
by. Griffin ('96, '99), MacFarland ('97), Lillie ('99), and Coe ('99), all of
whom find that the new astral centres, arising by division of the cen-
trosome, move away from the old position, to whicJi, Jiozvever, the old
rays still converge while the new asters are independently forming
(Fig. 155). This is shown with especial clearness in the ^gg of Cere-
brattiliis (Coe), where the peripheral portions of the old asters persist
until the new amphiaster is completely formed. This observation
seems conclusively to overturn Kostanecki's hypothesis of the persist-
ence and division of the rays, and together with the work of MacFar-
land gives a very strong support to Boveri's later view.

It still remains an open question whether the rays actually arise
from the substance of the centrosome, from a specific surrounding
archoplasm, or by differentiation out of the general substance of the
meshwork. The first of these possibilities has been urged in a very
interesting way by Watase ('94), who believes that the centrosome
" spins out the cytoplasmic filaments " ^ of the spindle and aster, and
that ordinary microsomes may in like manner spin out the fibrillae of
ordinary cytoplasmic networks.^ This view is sustained by the mode
of origin of the axial filament in the spermatozoa and that of the cilia
in plant spermatozoids. It is, on the other hand, opposed by the
almost infinitesimal bulk of the centrosome as compared with that of
the aster that may form about it, and by the formation of the spindles
in higher plants in the apparent absence of centrosomes. On the
whole, the facts do not seem at present to warrant the acceptance of
Watase's ingenious hypothesis, and the most probable view is that
of Driiner and Boveri, that the rays are differentiated out of the walls
of the meshwork. In cases where the protoplasm is reticular or
fibrillar the differentiation of the rays may be indistinguishable from
a mere rearrangement of the thread-work ; in alveolar protoplasm
they may be seen as new formations, while in either case the
material of the old aster may be more or less directly utilized in the
building of the new. The feature common to all is the periodic
activity either of the centre itself or of the surrounding protoplasm,
and the coincidence or non-coincidence of the new aster with the old
is apparently a secondary matter.

1 Lc, p. 283.

2 See the same paper for a suggestive comparison of the astral fibrillae to muscle-fibres.

Y

322 SOME PROBLEMS OF CELL-ORGANIZATION

In its original form the archoplasm hypothesis, as stated by Boveri,
was developed with reference only to the material of the spindle-
fibres and astral rays. Later writers have greatly extended the con-
ception on the basis of Boveri's earlier view that archoplasm is a
specific form of protoplasm, possessing specially active properties.
Strasburger ('92-98), whose views have already been considered in
part, believes the protoplasm to consist of, or to show a tendency to
differentiate itself into, two distinct substances, namely, a specially
active fibrillar kinoplasm and a less active alveolar trophoplasm.
The former gives rise to the mitotic fibrillae, constitutes the periph-
eral cell layer, or Haiitschicht, from which the membrane arises,
forms the substance of the centrosomes, and gives origin to the con-
tractile substance of cilia and flagella. The kinoplasm is thus mainly
concerned with the motor phenomena of the cell, the trophoplasm
with those of nutrition ; and this physiological difference is morpho-
logically expressed in the fact that the former has in general a
fibrillar structure, the latter an alveolar. Beyond this the two forms
of protoplasm show a difference of staining-reaction, the kinoplasmic
fibrillae staining deeply with gentian-violet and iron-hsematoxylin,
while the trophoplasm is but slightly stained.

Prenant ('98, '99) still further extends the hypothesis, adopting the
view that the " ergastoplasmic " (Garnier) fibrillae of gland-cells ^ are
equivalent to the kinoplasmic or archoplasmic fibrillae of the mitotic
figure, and to the fibrillae of nerve- and muscle-fibres as well. He is
thus led to the conception of a dominating or *' superior " cytoplasm
(including "archoplasm," "kinoplasm," "ergastoplasm"), which arises
by differentiation out of the general cytoplasm, plays the leading rdle
in the elaboration of active cell-elements (" cytosomes "), such as
mitotic, neural, and glandular fibrillae, and finally, its role accom-
plished, may disappear. Under the same category with the foregoing
structures are placed the centrosome, attraction-sphere, mid-body,
idiozome, Nebenkern, and yolk-nucleus.

Such a generous expansion of the archoplasm-hypothesis brings it
perilously near to a reductio ad absiirdiim ; for the step is not a great
one to the identification of the " superior protoplasm " with the active
cell-substance in general, which would render the whole hypothesis
superfluous. Physiologically, we can draw no definite line of demar-
cation between the more and the less active protoplasmic elements,
and it may further be doubted whether such a boundary exists even
between the latter and the metaplasmic substances.^ It is further
quite unjustifiable to infer physiological likeness from similarity in
staining-reaction^ or in fibrillar structure. For these reasons the
hypothesis of " superior protoplasm " seems one of doubtful utility.
1 Q; the pancreas, p. 44. 2 cf. p, 29. ^ <^ P- 335-

J

THE ARCHOPLASMIC STRUCTURES 323

In its more restricted form, however, the archoplasm or kinoplasra
hypothesis is of high interest as indicating a common element in the
origin and function of the mitotic fibrillae, the centrosome and mid-
body, and the contractile substances of cilia, flagella, and muscle-
fibres. The main interest of the hypothesis seems to me to lie in the
definite genetic relations that have been traced between the archo-
plasmic structures of successive cell-generations (as is most clearly
shown in the phenomena of maturation and fertilization). It has
been pointed out at various places in the preceding chapters ^ how
many apparently contradictory phenomena in cell-division, fertiliza-
tion, and related processes can be brought into relation with one
another under the assumption of a specific substance, carried by the
centrosome or less definitely localized, which gives the stimulus to
division, which is concerned in the formation of the mitotic figure
and of contractile elements, and which may be transmitted from cell
to cell without loss of its specific character. There seems, however,
to be clear evidence that such substance (or substances), if it exists,
is not to be regarded as being necessarily a permanent constituent of
the cell, but only as a phase, more or less persistent, in the general
metabolic transformation of the cell-substance.^

3. The Attraction-sphere

As originally used by Van Beneden ^ the term attraction-sphere was
appUed (in Ascaris) to the central mass of the aster surrounding the
" corpuscule central " and consisting of medullary and cortical zones,
as already described (p. 310). The cortical zone is bounded by a dis-
tinct circle of microsomes from which the astral rays proceed ; and at
the close of cell-division, the rays were stated to fade away, leaving
only the attraction-sphere, which, like the centrosome, was regarded
as a permanent cell-organ. Later researches have conclusively shown
that the attraction-sphere cannot be regarded as a permanent organ,
since in many cases it disintegrates and disappears. This occurs, for
example, in the early prophases of mitosis in the testis-cells of the sala-
mander,* where the sphere breaks up and scatters through the cell as
the new amphiaster forms (Fig. 27). A very interesting case of this
kind occurs in the cleavage of the ovum in Crepidula^ as described by
Conklin ('99). The spheres here persist for a considerable period
after division (Fig. 192), but have no direct relation to those of the
ensuing division, finally disappearing in situ. The new spheres are
formed about the centrosomes, which Conklin believes to migrate
out of the old spheres (somewhat as occurs in the spermatid, p. 167) to
their new position. The interesting point here is that the old sphere

1 Cf, pp. Ill, 215. 2 cf. p. 171. 3 ^zz, p. 548.

* Driiner, '95, Rawitz, '96, Meves, '96.

324 SOME PROBLEMS OE' CELL-ORGANIZATION

takes up such a position as to pass entirely into one of the grand-
daughter-cells, while the new sphere-substance is equally distributed
between them and in its turn passes into one of the cells of the en-
suing division.^

In Crepidida, as in Ascaris, the attraction-sphere represents only
the central part (centrosphere) of the aster. In some cases, however,
e.g. in leucocytes, the entire aster may persist, and the term attrac-
tion-sphere has by some authors been applied to the whole structure.
Later workers have proposed different terminologies, which are at
present in a state of complete confusion. Fol ('91) proposed to call
the centrosome the astrocentre, and the spherical mass surrounding it
(attraction-sphere of Van Beneden) the astrospJiere. Strasburger
accepted the latter term but proposed the new word centrosphere
for the astrosphere and the centrosome .taken together.^ A new
complication was introduced by Boveri ('95), who applied the word
" astrosphere " to the entire <7i"/^r exclusive of the centrosome, in which
sense the phrase " astral sphere " had been employed by Mark in 1881.
The word '' astrosphere " has therefore a double meaning and would
better be abandoned in favour of Strasburger's convenient term centro-
sphere, which may be understood as equivalent to the ** astrosphere "
of Fol.

Besides these terms we have Heidenhain's microcentrum (p. 311),
equivalent to the centrosome or group of centrosomes at the centre of
the aster, with its surrounding sphere ; ^ Kostanecki's and Siedlecki's
microsphere, applied to the central region of the aster surrounding the
centrosome whether bounded by a distinct microsome-circle or not ; *
Erlanger's centroplasm, equivalent to microsphere ; ^ Ziegler's ecto-
sphere and entosphere, applied to the cortical and medullary zones
respectively ; and Meves's idiozome, applied to the ''attraction-sphere "
of the spermatids.^ This profusion of technical terms has arisen
through the desire to avoid ambiguity in the use of the term " attrac-
tion-sphere," which, like the word " Nebenkern " (p. 163), has been
applied to bodies of quite different origin and fate. If we adhere to
Van Beneden's original use of the term it must be confined to the body
surrounding the centrosome, forming a part of, or directly derived
from, an aster, and giving rise wholly or in part to the succeeding aster.
Meves ('96), Rawitz ('96), Erlanger ('97, 2), and others have, however,
clearly shown that the ''attraction-sphere" surrounding the centro-
some (in testis-cells) may not only contain other material derived from
the cytoplasm, e.g. the " centrodeutoplasm " of Erlanger, but may
take no direct part in the succeeding aster-formation, disintegrating
and scattering through the cell as the new aster forms (Fig. 27). In

1 Cf. p. 424. 8 '94. p- 463- ^ '96, z, p. 8.

2 '92, p. 5. * '96, p. 217. 6 '97, 4, p. 315-

THE ARCHOPLASMIC STRUCTURES 325

Other cases a sphere closely simulating an attraction-sphere may
arise in the cytoplasm without apparent relation to the centrosomes
or to the preceding aster, e.g. the yolk-nucleus or the sphere from
which the acrosome arises in mammalian spermatogenesis.^ To call
such structures " attraction-spheres " or " archoplasm-masses " is to
beg an important question ; and in all such doubtful cases the simple
word sphere should be used.^ In case of the aster itself we may, for
descriptive purposes, employ Strasburger's convenient and non-com-
mittal term centrosphere, to designate in a somewhat vague and general
way the central mass of the aster surrounding the centrosome, leaving
its exact relation to Van Beneden's attraction-sphere to be determined
in each individual case. Where the centrosphere shows two concen-
tric zones (medullary and cortical), they may be well designated with
Ziegler as ^72/^i;^//^r^(" centrosome " of Boveri) and ectosphere.

As regards the structure of the centrosphere, two well-marked types
have been described. In one of these, described by Van Beneden in
Ascaris, by Heidenhain in leucocytes, by Driiner and Braus in divid-
ing cells of Amphibia, and by Francotte, Van der Stricht, Lillie, Kos-
tanecki, and others, in various segmenting eggs, the centrosphere has
a radiate structure, being traversed by rays which stretch between the
centrosome and the peripheral microsome-circle (Fig. 152, D, E, F\
when the latter exists. In the other form, described by Vejdovsky in
the eggs of Rhyfichelmis, by Solger and Zimmermann in pigment-cells,
by myself in Nereis, by Riickert in Cyclops, by Mead in Choetopterus ,
Griffin in Thalassema, Coe in Cerebratidiis, Gardiner in Polychoerus,
and many others, the centrosphere has a non-radial reticular or vesicu-
lar structure, in which the centrosomes lie (Figs. 152, H, 155). Kos-
tanecki and others have endeavoured to show that such structures are
artifacts, insisting that in perfectly fixed material the astral rays always
traverse the centrosphere to the centrosome. This interpretation is,
however, contradicted by the fact that the new asters developing in
the centrospheres during the anaphases and telophases of such forms
as Thalassema or Cerebratiihis (Figs. 99, 155) show perfect fixation of
the rays. The reticular centrosphere almost certainly arises as a nor-
mal differentiation of the interior of the aster, which, as Griffin ('96)
has suggested, probably marks the beginning of the degeneration of
the whole astral apparatus, to make way for the newly developing
system.

The radial centrosphere is in Ascaris divided into cortical and medul-
lary zones, as already described (p. 310), the aster being bounded by
a distinct circle of microsomes. The true interpretation of these zones
was given through Heidenhain's beautiful studies on the asters in leu-
cocytes, and the still more thorough later work of Driiner on the sper-

1 C/: p. 1 70. 2 c/. Lenhossek, '98.

326

SOME PROBLEMS OF CELL-ORGANIZATION

matocyte-divisions of the salamander. In leucocytes (Fig. 49) the
large persistent aster has at its centre a well-marked radial sphere
bounded by a circle of microsomes, as described by Van Beneden, but
without division into cortical and medullary zones. The astral rays,

however, show indications of other
circles of microsomes lying outside
the centrosphere. Driiner found
that a whole series of such concen-
tric circles might exist (in the cell
shown in Fig. 1 56 no less than nine),
but that the innermost two are often
especially distinct, so as to mark off
a centrosphere composed of a medul-
lary and a cortical zone precisely as
described by Van Beneden. These
observations show conclusively that
the centrosphere of the radial type
is merely the innermost portion of
the aster, which acquires a boundary
through the especial development
of a ring of microsomes, or other-
wise, and which often further
acquires an intense staining-capac-
ity so as to appear like a centrosome
(P- 313)- Iri T/iysa7iozodn{Y2.ndQr
Stricht) only a single ring of micro-
somes exists, and this lies at the
boundary between the medullary
and cortical zones (Fig. 152, D), the latter differing from the outer
region only in the greater delicacy of the rays and their lack of
staining-capacity, thus producing a " Heller Hof." In other cases, no
" microsome-circles " exist; but even here a clear zone often surrounds
the centrosome {e.g. in Physa, t. Kostanecki and Wierzejski), like that
seen in the cortical zone of Tkysanozoon.

There are some observations indicating that the entosphere (medul-
lary zone) may be directly derived from the centrosome (central
granule). This is the conclusion reached by Lillie in the case of Unto
referred to above, where, during the prophases of the second polar
spindle, the central granule enlarges and breaks up into a group of
granules from which the new entosphere is formed. Van der Stricht
('98) reaches a similar conclusion in case of the first polar spindle of
Thysanozoofi. We may perhaps give the same interpretation to the
large pluricorpuscular centrum of echinoderms (p. 314). This obser-
vation may be used in support of the probability that the astral rays

Fig. 156. — Spermatogonium of salaman-
der. [Druner.]

The nucleus lies below. Above is the enor-
mous aster, the centrosome at its centre, its
rays showing indications of nine concentric
circles of microsomes. The area within the
second circle probably represents the " attrac-
tion-sphere " of Van Beneden.

SUMMARY AND CONCLUSION 327

may be actually derived from the centrosome (p. 321) ; but Lillie finds
in some cases that in the same mitosis the entosphere is formed by a
different process, arising by a differentiation of the cytoplasm around
the central granule. The former case, therefore, may be interpreted
to mean simply that the centrosome may give rise to other cytoplasmic
elements (as has already been shown in the formation of the sperma-
tozoon, p. 172), the material of which may then contribute either
directly or indirectly to the building of the aster ; and the facts do
not come into collision with the view that the astral rays are in gen-
eral formed from the cytoplasmic substance.

G. Summary and Conclusion

A minute analysis of the various parts of the cell leads to the con-
clusion that all cell-organs, whether temporary or " permanent," are
local differentiations of a common structural basis. Temporary organs,
such as cilia or pseudopodia, are formed out of this basis, persist for a
time, and finally merge their identity in the common basis again. Per-
manent organs, such as the nucleus or plastids, are constant areas in
the same basis, which never are formed de novo, but arise by the divi-
sion of preexisting areas of the same kind. These two extremes are,
however, connected by various intermediate gradations, examples of
which are the contractile vacuoles of Protozoa, which belong to the
category of temporary organs, yet in many cases are handed on from
one cell to another by fission, and the attraction-spheres and asters,
which may either persist from cell to cell or disappear and re-form
about the centrosome. There is now considerable evidence that the
centrosome itself may in some cases have the character of a perma-
nent organ, in others may disappear and re-form like the asters.

The facts point toward the conclusion, which has been especially
urged by De Vries and Wiesner, that the power of division, not only
of the cell-organs, but also of the cell as a whole, may have its root in
a like power on the part of more elementary masses or units of which
the structural basis is itself builf , the degree of permanence in the cell-
organs depending on the degree of cohesion manifested by these elemen-
tary bodies. If such bodies exist, they must, however, in their primary
form, lie beyond the present limits of the microscope, the visible struc-
tures arising by their enlargement or aggregation. The cell, therefore,
cannot be regarded as a colony of "granules " or other gross morpho-
logical elements. The phenomena of cell-division show, however, that
the dividing substance tends to differentiate itself into several orders
of visible morphological aggregates, as is most clearly shown in the
nuclear substance. Here the highest term is the plurivalent chromo-
some, the lowest the smallest visible dividing basichromatin-grains,

328 SOME PROBLEMS OF CELL-ORGANIZATION

while the intermediate terms are formed by the successive aggrega-
tion of these to form the chromatin-granules of which the dividing
chromosomes consist. Whether any or all of these bodies are " indi-
viduals " is a question of words. The facts point, however, to the
conclusion that at the bottom of the series there must be masses that
cannot be further split up without loss of their characteristic proper-
ties, and which form the elementary morphological units of the nucleus.
In case of the cytoplasm the evidence is far less satisfactory.
Could Rabl's theory of fibrillar persistence, as developed by Heiden-
hain and Kostanecki, be estabhshed, we should indeed have almost a
demonstration of panmeristic division in the cytoplasm. At present,
however, the facts do not admit the acceptance of that theory, and
the division of the visible cytoplasmic granules must remain a quite
open question. Yet we should remember that the dividing plastids
of plant-cells are often very minute, and that in the centrosome we
have a body, no larger in many cases than a *' microsome," which is
positively known to be in some cases a persistent morphological ele-
ment, having the power of growth, division, and persistence in the
daughter-cells. Probably these powers of the centrosome would
never have been discovered were it not that its staining-capacity ren-
ders it conspicuous and its position at the focus of the astral rays
isolates it for observation. When we consider the analogy between
the centrosome and the basichromatin-grains, when we recall the
evidence that the latter graduate into the oxychromatin-granules, and
these in turn into the cytomicrosomes, we must admit that Briicke's
cautious suggestion that the whole cell might be a congeries of self-
propagating units of a lower order is sufficiently supported by fact
to constitute a legitimate working hypothesis.

LITERATURE. VI i

Van Beneden, E. — (See List IV.)

Van Beneden and Julin. — La segmentation chez les Ascidiens et ses rapports avec

Torganisation de la larve : Arch. Biol.., V. 1884. '
Boveri, Th. — Zellenstudien. (See List IV.)

Briicke, C. — Die Elementarorganismen : Wiejier Sits.-Ber.,X\AY. 1861.
Biitschli, 0- — Protoplasma. (See List I.)
Delage, Yves. — La structure du protoplasma, et les theories sur I'heredite. Paris,

1895. „
Hacker, V. — Uber den heutigen Stand der Centrosomenfrage : Verh. d. deutsch.

Zool. Ges. 1894.
Heidenhain, M. — (See List I.)
Herla, V. — Etude des variations de la mitose chez Tascaride megalocephale : Arch.

Biol.,X\\l. 1893.

1 See also Literature, I., XL, IV., V.

LITERATURE 329

Morgan, To H. — The Action of Salt-solutions on the Fertilized and Unfertilized

Eggs of Arbacia and Other Animals. Arch. Etitw., VIII. 3. 1898.
Kostanecki, K. — Ueber die Bedeutung der Polstrahung wahrend der Mitose. Arch.

mik. Aiiat.^ XLIX. 1897.
Nussbaum, M. — Uber die Teilbarkeit der lebendigen Materie : Arch. mik. Anat.,

XXVI. 1886.
Prenant, A. — Sur le protoplasma superieure (archiplasme, kinoplasme, ergastro-

plasme) : Jotirn. Anat. et Fhys., XXIV.-V. 1898-99. (Full Literature-lists.)
Rabl, C. — iJber Zellteilung : J/^r//^. /«/zr^., X. 1885. Anat. Anzeiger^YV. 1889.
Ruckert, J. — (See List IV.)

De Vries, H. — I ntracellulare Pangenesis : y^;/^, 1889.

Watas§, S. — Homology of the Centrosome : Journ. Morph., VIII. 2. 1893.
Id. — On the Nature of Cell-organization : . Woods Holl Biol. Lectures. 1893.
Wiesner, J. — Die Elementarstruktur und das Wachstum der lebenden Substanz :

Wien^ 1892.
Wilson, Edm. B. — Archoplasm, Centrosome, and Chromatin in the Sea-urchin Egg :

Journ. Morph., Vol. XI. 1895.

CHAPTER VII

SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY

" Les phenom^nes fonctionnels ou de depense vitale auraient done leur siege dans le proto-
flasme cellulaire.

" Le noyau est un appareil de synthese organique, Vinstrument de la production, le germe de la
cellule." Claude Bernard.i

A. Chemical Relations of Nucleus and Cytoplasm

It is no part of the purpose of this work to give even a sketch of
general cell-chemistry. I shall only attempt to consider certain ques-
tions that bear directly upon the functional relations of nucleus and
cytoplasm and are of especial interest in relation to the process of
nutrition and through it to the problems of development. It has
often been pointed out that we know little or nothing of the chemical
conditions existing in living protoplasm, since every attempt to examine
them by precise methods necessarily kills the protoplasm. We must,
therefore, in the main rest content with inferences based upon the
chemical behaviour of dead cells. But even here investigation is be-
set with difficulties, since it is in most cases impossible to isolate the
various parts of the cell for accurate chemical analysis, and we are
obliged to rely largely on the less precise method of observing with
the microscope the visible effects of dyes and other reagents. This
difficulty is increased by the fact that both cytoplasm and karyoplasm
are not simple chemical compounds, but mixtures of many complex
substances ; and both, moreover, undergo periodic changes of a com-
plicated character which differ very widely in different kinds of cells.
Our knowledge is, therefore, still fragmentary, and we have as yet
scarcely passed the threshold of a subject which belongs largely to
the cytology of the future.

It has been shown in the foregoing chapter that all the parts of the
cell arise as local differentiations of a general protoplasmic basis.
Despite the difficulties of chemical analysis referred to above, it has
been determined with certainty that some at least of these organs are
the seat of specific chemical change ; just as is the case in the various
organs and tissues of the organism at large. Thus, the nucleus is

1 T e(;ons stir les phenonienes de la vie, I., 1878, p. 198.

k

CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM 33 1

characterized by the presence of nuclein (chromatin) which has been
proved by chemical analysis to differ widely from the cytoplasmic
substances,^ while the various forms of plastids are centres for the
formation of chlorophyll, starch, or pigment. These facts give ground
for the conclusion that the morphological differentiation of cell-organs
is in general accompanied by underlying chemical specializations
which are themselves the expression of differences of metabolic ac-
tivity ; and these relations, imperfectly comprehended as they are, are
of fundamental importance to the student of development.

I. The Proteids and their Allies

The most important chemical compounds found in the cell are the
group oi protein substances^ and there is every reason to believe that
these form the principal basis of living protoplasm in all of its forms.
These substances are complex compounds of carbon, hydrogen, nitro-
gen, and oxygen, often containing a small percentage of sulphur, and
in some cases also phosphorus and iron. They form a very extensive
group of which the different members differ considerably in physical
and chemical properties, though all have certain common traits and
are closely related. They are variously classified even by the latest
writers. By many authors (for example Halliburton, '93) the word
^'proteids " is used in a broad sense as synonymous with albuminous
substances, including under them the various forms of albumin (egg-
albumin, cell-albumin, muscle-albumin, vegetable-albumins), globulin
(fibrinogin vitellin, etc.), and the /^//^/^^^ (diffusible hydrated proteids).
Another series of nearly related substances are the albuminoids
(reckoned by some chemists among the " proteids "), examples of
which are gelatin, mucin, and, according to some authors also,
nuclein, and the nucleo-albumins. Some of the best authorities how-
ever, among them Kossel and Hammarsten, follow the usage of
Hoppe-Seyler in restricting the word proteid to substances of greater
complexity than the albumins and globulins. Examples of these
are the nucleins and nucleo-proteids, which are compounds of nu-
cleinic acid with albumin, histon, or protamin. The nucleo-proteids,
found only in the nucleus, are not to be confounded with the nucleo-

1 It has long been known that a form of " nuclein " may also be obtained from the nucleo-
albumins of the cytoplasm, e.g. from the yolk of hens' eggs (vitellin). Such nucleins differ,
however, from those of nuclear origin in not yielding as cleavage-products the nuclein bases
(adenin, xanthin, etc.). The term " paranuclein " (Kossel) or " pseudo-nuclein " (Ham-
marsten) has therefore been suggested for this substance. True nucleins containing a large
percentage of albumin are distinguished as nucleo-proteids. They may be split into albumin
(or albumin radicals) and nucleinic acid, the latter yielding as cleavage-products the nuclein
bases. Pseudo-nucleins containing a large percentage of albumin are designated as nucleo-
albumins, which in like manner split into albumin and paranucleinic or pseudo-nucleinic acid,
which yields no nuclein bases. (See Hammarsten, '94.)

332

CELL-CHEMISTRY AND CELL-PHYSIOLOGY

albumins, which are compounds of pseudo-nucleinic acid with albumin
and yield no nuclein-bases (xanthin, hypoxanthin, adenin, guanin) as
decomposition products.

The distribution of these substances through the cell varies
greatly not only in different cells, but at different periods in the life
of the same cell. The cardinal fact always, however, remains, that
there is a definite and constant contrast between nncleus and cytoplasm.
The latter always contains large quantities of nucleo-albumins, certain
globulins, and sometimes small quantities of albumins and peptones ;
the former contains, in addition to these, miclein and niicleo-proteids,
which form its main bulk, and its most constant and characteristic
feature. It is the remarkable substance, nuclein, — which is almost
certainly identical with chromatin, — that chiefly claims our attention
here on account of the physiological role of the nucleus.

2. The Nuclein Series

Nuclein was first isolated and named by Miescher, in 1 871, by
subjecting cells to artificial gastric digestion. The cytoplasm is thus
digested, leaving only the nuclei ; and in some cases, for instance pus-
cells and spermatozoa, it is possible by this method to procure large
quantities of nuclear substance for accurate quantitative analysis.
The results of analysis show it to be a complex albuminoid substance,
rich in phosphorus, for which Miescher gave the chemical formula
Qg^ 49^9-^3022- The earlier analysis of this substance gave some-
what discordant results, as appears in the following table of per-
centage-compositions : ^ —

^

Pus-cells.

Spermatozoa of Salmon.

Human Brain.

(Hoppe-Seyler.)

(Miescher.)

(v. Jaksch.)

c

49-58

36.11

50.6

H

7.10

5-15

7.6

N

15.02

13.09

13.18

P

2.28

5-59 ■

1.89

These differences led to the opinion, first expressed by Hoppe-
Seyler, and confirmed by later investigations, that there are several
varieties of nuclein which form a group having certain characters
in common. Altmann ('89) opened the way to an understanding of
the matter by showing that "nuclein " may be split up into two sub-
stances ; namely, ( i ) an organic acid rich in phosphorus, to which he

iFrom Halliburton, '91, p. 203. [The oxygen-percentage is omitted in this table.]

CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM 333

gave the name jincleinic acid, and (2) a form of albumin. Moreover,
the nuclein may be synthetically formed by the re-combination of
these two substances. Pure nucleinic acid, for which Miescher ('96)
afterward gave the formula ^^^^^\^ fi^i} contains no sulphur, a
high percentage of phosphorus (above 9%), and no albumin. By
adding it to a solution of albumin a precipitate is formed which
contains sulphur, a lower percentage of phosphorus, and has the
chemical characters of " nuclein." This indicates that the discord-
ant results in the analyses of nuclein, referred to above, were
probably due to varying proportions of the two constituents ; and
Altmann suggested that the '' nuclein " of spermatozoa, which contains
no sulphur and a maximum of phosphorus, might be uncombined
nucleinic acid itself. Kossel accordingly drew the conclusion, based
on his own work as well as that of Liebermann, Altmann, Malfatti,
and others, that "what the histologists designate as chro^natin con-
sists essentially of combinations of nucleinic acid with more or less
albumin, and in some cases may even be free nucleinic acid. The
less the percentage of albumin in these compounds, the nearer do
their properties approach those of pure nucleinic acid, and we may
assume that the percentage of albumin in the chromatin of the same
nucleus may vary according to physiological conditions." ^ In the
same year Halliburton, following in part Hoppe-Seyler, stated the
same view as follows. The so-called " nucleins " form a series lead-
ing downward from nucleinic acid thus : —

(i) Those containing no albumin and a maximum (9-10%) of phos-
phorus (pure nucleinic acid). Nuclei of spermatozoa.

(2) Those containing little albumin and rich in phosphorus. Chro-

matin of ordinary nuclei.

(3) Those with a greater proportion of albumin — a series of sub-

stances in which may probably be 'nxoXyxA^^ pyrenin (nucleoli)
and plastin (linin). These graduate into

(4) Those containing a minimum (0.5 to 1%) of phosphorus —

the nucleo-albumins, which occur both in the nucleus and in
the cytoplasm (vitellin, caseinogen, etc.).

Finally, we reach the globulins and albumins, especially character-
istic of the cell-substance, and containing no nucleinic acid. "We thus
pass by a gradual transition (from the nucleo-albumins) to the other
proteid constituents of the cell, the cell-globulins, which contain no
phosphorus whatever, and to the products of cell-activity, such as
the proteids of serum and of egg-white, which are also principally

1 Derived from analysis of the salmon-sperm. '^''91, P- 158.

334 CELL-CHEMISTRY AND CELL-PIJYSWLOGY

phosphorus-free." ^ Further, ** in the processes of vital activity there
are changing relations between the phosphorized constituents of the
nucleus, just as in all metabolic processes there is a continual inter-
change, some constituents being elaborated, others breaking down
into simpler products." This latter conclusion has been well estab-
lished; the others, as stated by Halliburton, require some modification,
on the one hand, through the results of later analyses of chromatin,
on the other, because of the failure to distinguish .between the nucleo-
proteids and the nucleo-albumins. First, it has been shown by
Miescher ('96), Kossel ('96), and Mathews ('97, 2) that the chromatin
of the sperm-nuclei (in fish and sea-urchins) is not pure nucleinic acid,
as Altmann conjectured, but a salt of that acid, with histon, protamin,
or a related substance. Thus, in the spermatozoa of the salmon,
Miescher's analyses give 60.56% of nucleinic acid and 35.56% of
protamin (CigH28N902). In the herring the chromatin is a compound
of nucleinic acid (over 63%) and a form of protamin called by Kossel
" clupein " (CgoHgyNj^Og). In the sea-urchin Arbacia Mathews finds
the chromatin to be a compound of nucleinic acid and " arbacin," a
histon-like body. Kossel finds also that chromatin (nuclein^ derived
from the thymus gland, and from leucocytes, is largely 'a histon
salt of nucleinic acid, the proportion of the latter being, however,
much less than in the sperm-chromatin, while albumin is also present.
In these cases, therefore, the greater part of the nucleinic acid is com-
bined not with albumin but with a histon or protamin radical. Second,
the nucleo-albumins of the cytoplasm are in no sense transitional be-
tween the nucleins and the albumins, since they contain no true
nucleinic acid, but only pseudo-nucleinic acid.^ The fact nevertheless
remains that the nucleins and nucleo-proteids, though confined to the
nucleus, form a series descending from such highly phosphorized
bodies as the sperm-chromatin toward bodies such as the albumins,
which are especially characteristic of the cytoplasm ; and that they
vary in composition with varying physiological conditions. The way
is thus opened for a more precise investigation of the physiological
role of nucleus and cytoplasm in metaboUsm.

3. Staining-reaction of the Nuclein Series

In bringing these facts into relation with the staining-reactions of
the cell, it is necessary briefly to consider the nature of staining-
reactions in general, and especially to warn the reader that in the
whole field of "micro-chemistry" we are still on such uncertain
ground that all general conclusions must be taken with reserve.

First, it is still uncertain how far staining-reactions depend upon
chemical reaction and how far upon merely physical properties of

^'93»P. 574. '^p. 331- -

J

CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM 335

the bodies stained. The prevalent view that staining-reactions are
due to a chemical combination of the dye with the elements of the
cell has been attacked by Gierke ('85), Rawitz ('97), and Fischer
('97, '99), all of whom have endeavoured to show that these reactions
are of no value as a chemical test, being only a result of surface-
attraction and absorption due to purely physical qualities of the
bodies stained. On the other hand, a long series of experiments,
beginning with Miescher's discovery ('74) that isolated nucleinic acid
forms green insoluble salts with methyl-green, and continued by
Lilienfeld, Heidenhain, Paul Mayer, and others, gives strong reason
to believe that beyond the physical imbibition of colour a true chemical
union takes place, which, with due precautions, gives us at least a
rough test of the chemical conditions existing in the cell.^

Second, similarity of staining-reactio7i is by no means always indica-
tive of chemical similarity, as is shown, for example, by the fact that
in cartilage both nuclei and inter-cellular matrix are intensely stained
by methyl-green, though chemically they differ very widely.

Third, colour in itself gives no evidence of chemical nature ; for the
nucleus and other elements of the same cell may be stained red,
green, or blue, according to the dye employed, and to class them as
" erythrophilous," "cyanophilous," and the like, is therefore absurd.

Fourth, the character of the staining-re action is influenced and in
sofne cases determined by the fixation or other preliTninary treatment y
a principle made use of practically in the operations of mordaunting,
but one which may give very misleading results unless carefully con-
trolled. Thus Rawitz ('95) shows that certain colours which ordinarily
stain especially the nucleus (saffranin, gentian-violet), can be made to
stain only the cytoplasm through preliminary treatment of object
with solutions of tannin, followed by tartar-emetic. In Hke manner
Mathews ('98) shows that many of the '' nuclear " tar-colours (saffra-
nin, methyl-green, etc.) stain or do not stain the cytoplasm, according
as the material has been previously treated with alkaline or with acid
solutions.

The results with which we now have to deal are based mainly
upon experiments with tar-colours ("aniline dyes"). Ehrlich ('79)
long since characterized these dyes as **acid" or "basic," according
as the colouring matter plays the part of an acid or a base in the com-
pound employed, showing further that, other things equal, the basic
dyes (methyl-green, saffranin, etc.) are especially "nuclear stains"
and the acid (rubin, eosin, orange, etc.) "plasma stains." Malfatti
('91), and especially Lilienfeld ('^, '93), following out Miescher's
earlier work ('74), found that albumin stains preeminently in the
acid stains, nucleinic acid only in the basic ; and, further, that artifi-

^ Cf, Mayer, '91, '92, '97; Lilienfeld, '93; Mathews, '98.

336 CELL-CHEMISTRY AND CELL-PHYSIOLOGY

cial nucleins, prepared by combining egg-albumin with nucleinic acid
in various proportions, show a varying affinity for basic and acid
dyes according as the nucleinic acid is more or less completely
saturated with albumin. Lilienfeld's starting-point was given by the
results of Kossel's researches on the relations of the nuclein group,
which are expressed as follows : ^ —

Nucleo-proteid (i% of P or less),
by peptic digestion splits into

Peptone Nuclein (3-4% P),

by treatment with acid splits into

Albumin Niicleinic acid (9-10% P),

heated with mineral acids splits into

Phosphoric acid Nuclein bases (A carbohydrate.')

(adenin, guanin, etc.).

Now, according to Kossel and Lilienfeld, the principal nucleo-
proteid in the nucleus of leucocytes is micleo-histon, containing about
3% of phosphorus, which may be split into a form of nuclein playing
the part of an acid, and an albuminoid base, the histon of Kossel ;
the nuclein may in turn be split into albumin and nucleinic acid.
These four substances — albumin, nucleo-histon, nuclein, nucleinic
acid — thus form a series in which the proportion of phosphorus,
which is a measure of the nucleinic acid, successively increases from
zero to 9-10%. If the members of this series be treated with the
same mixture of red acid fuchsin and basic methyl-green, the result
is as follows. Albumin (egg-albumin) is stained red, nucleo-histon
greenish blue, nuclein bluish green, nucleinic acid intense green. "We
see, therefore, that the principle that determines the staining of the
nuclear substances is always the nucleinic acid. All the nuclear sub-
stances, from those richest in albumin to those poorest in it, or con-
taining none, assume the tone of the nuclear {i.e. basic) stain, but the
combined albumin modifies the green more or less toward blue." ^
Lilienfeld explains the fact that chromatin in the cell-nucleus seldom
appears pure green on the assumption, supported by many facts,
that the proportions of nucleinic acid and albumin vary with different
physiological conditions, and he suggests further that the intense
staining-power of the chromosomes during mitosis is probably due
to the fact that they contain a maximum of nucleinic acid. Very
interesting is a comparison of the foregoing staining-reactions with
those given by a mixture of a red basic dye (saffranin) and a green
acid one (" light green "). With this combination an effect is given
which reverses that of the Biondi-Ehrlich mixture ; i.e. the nuclein

I From Lilienfeld, after Kossel ('92, p. 129). 2 i^^., p. 394.

CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM 337

is coloured red, the albumin green, which is a beautiful demon-
stration of the fact that staining-reagents cannot be logically classified
according to colour, but only according to their chemical nature,
and gives additional ground for the view that staining-reactions of
this type are the result of a chemical rather than a merely physical
combination.

These results must be taken with some reserve for the following
reasons : Mathews ('98) has shown that methyl-green and other basic
dyes will energetically stain albumose, coagulated egg-albumin, and
the cell-cytoplasm in or after treatment by alkaline fluids ; while con-
versely the acid dyes do not stain, or only slightly stain, these sub-
stances under the same conditions. This probably does not affect
the validity of Heidenhain's results,^ since he worked with acid solu-
tions. What is more to the point is the fact that hyaline cartilage
and mucin, though containing no nucleinic acid, stain intensely with
basic dyes. Mathews probably gives the clue to this reaction, in
the suggestion that it is here probably due to the presence of other
acids (in the case of cartilage a salt of chondroitin-sulphuric acid,
according to Schmiedeberg) ; from which Mathews concludes that
the basic dyes will, in acid or neutral solutions, stain any element of
the tissues that contains an organic acid in a salt combination with a
strong base.2 Accepting this conclusion, we must therefore recognize
that, as far as the cytoplasm is concerned, the basic or " nuclear "
stains are in no sense a test for nuclein, but only for salts of organic
acids in general. In case of the nucleus, however, we know from
direct analysis that we are dealing with varying combinations of
nucleinic acid, and hence, with the precautions indicated above, may
draw provisional conditions from the staining-reactions.

Thus regarded, the changes of staining-reaction in the chromatin
are of high interest. Heidenhain ('93, '94), in his beautiful studies
on leucocytes, has correlated some of the foregoing results with the
staining-reactions of the cell as follows. Leucocytes stained with
the Biondi-Ehrlich mixture of acid fuchsin and methyl-green show
the following reactions. Cytoplasm, centrosome, attraction-sphere,
astral rays, and spindle-fibres are stained pure red. The nuclear sub-
stance shows a very sharp differentiation. The chromatic network
and the chromosomes of the mitotic figure are green. The linin-
substance and the true nucleoli or plasmosomes appear red, like the
cytoplasm. The linin-network of leucocytes is stated by Heidenhain
to consist of two elements, namely, of red granules or microsomes
suspended in a colourless network. The latter alone is called " linin "
by Heidenhain. To the red granules is applied the term " oxychro-
matin," while the green substance of the ordinary chromatic network,

1 See below. 2 '^g^ pp_ 4^1-4^2.

338 CELL-CHEMISTRY AND CELL-PLIYSIOLOGY

forming the *' chromatin " of Flemming, is called " basichromatin." ^
Morphologically, the granules of both kinds are exactly alike,^ and
in many cases the oxychromatin-granules are found not only in the
" achromatic " nuclear network, but also intermingled with the basi-
chromatin-granules of the chromatic network. Collating these results
with those of the physiological chemists, Heidenhain concludes that
basichromatin is a substance rich in phosphorus {i.e. nucleinic acid),
oxychromatin a substance poor in phosphorus, and that, further,
" basichromatin and oxychromatin are by no means to be regarded
as permanent unchangeable bodies but may change their colour-
reactions by combining with or giving off phosphorus." In other
words, *' the affinity of the chromatophilous microsomes of the nuclear
network for basic and acid aniline dyes is regulated by certain physio-
logical conditions of the nucleus or of the cell."^

This conclusion, which is entirely in harmony with the statements
of Kossel and Halliburton quoted above, opens up the most interest-
ing questions regarding the periodic changes in the nucleus. The
staining-power of chromatin is at a maximum when in the preparatory
stages of mitosis (spireme-thread, chromosomes). During the ensuing
growth of the nucleus it always diminishes, suggesting that a com-
bination with albumin has taken place. This is illustrated in a very
striking way by the history of the egg-nucleus or germinal vesicle,
which exhibits the nuclear changes on a large scale. It has long
been known that the chromatin of this nucleus undergoes great
changes during the growth of the ^g'g, and several observers have
maintained its entire disappearance at one period. Riickert first
carefully traced out the history of the chromatin in detail in the
eggs of sharks, and his general results have since been confirmed by
Born in the eggs of Triton. In the shark Pristinnis, Riickert ('92, i)
finds that the chromosomes, which persist throughout the entire
growth-period of the ^^g, undergo the following changes (Fig. 157):
At a very early stage they are small, and stain intensely with nuclear
dyes. During the growth of the ^g^ they undergo a great increase
in size, and progressively lose their staining-capacity. At the same
time their surface is enormously increased by the development of
long threads which grow out in every direction from the central axis
(Fig. isy, A). As the egg approaches its full size, the chromosomes
rapidly diminish in size, the radiating threads disappear, and the stain-
ing-capacity increases (Fig. 157, B). They are finally again reduced to
minute, intensely staining bodies which enter into the equatorial plate
of the first polar, mitotic figure (Fig. 157, C). How great the change
of volume is may be seen from the following figures. At the beginning
the chromosomes measure, at most, 12 /x (about ^^^ in.) in length and

^ '94, P- 543. ^ ^-^-^ P- 547- ^ ^■^■> P- 548.

CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM 339

\ fjL in diameter. At the height of their development they are almost
eight times their original length and twenty times their original
diameter. In the final period they are but 2 /x in length and i />t in di-
ameter. These measurements show a change of volume so enormous,
even after making due allowance for the loose structure of the large
chromosomes, that it cannot be accounted for by mere swelling or
shrinkage. The chromosomes evidently absorb a large amount of

:?tS5'

Fig. 157. — Chromosomes of the germinal vesicle in the shark Pristiurus, at different periods,
drawn to the same scale. [RUCKERT.]

A. At the period of maximal size and minimal staining-capacity (egg 3 mm. in diameter).
B. Later period (egg 13 mm. in diameter). C At the close of ovarian life, of minimal size and
maximal staining-power.

matter, combine with it to form a substance of diminished staining-
capacity, and finally give off matter, leaving an intensely staining
substance behind. As Riickert points out, the great increase of sur-
face in the chromosomes is adapted to facilitate an exchange of mate-
rial between the chromatin and the surrounding substance ; and he
concludes that the coincidence between the growth of the chromo-
somes and that of the tgg points to an intimate connection between
the nuclear activity and the formative energy of the cytoplasm.

340 CELL-CHEMISTRY AND CELL-PHYSIOLOGY

If these facts are considered in the light of the known staining-
reaction of the nuclein series, we must admit that the following con-
clusions are something more than mere possibilities. We may infer
that the original chromosomes contain a high percentage of nucleinic
acid ; that their growth and loss of staining-power is due to a combi-
nation with a large amount of albuminous substance to form a
lower member of the nuclein series, probably a nucleo-proteid ; that
their final diminution in size and resumption of staining-power is
caused by a giving up of the albumin constituent, restoring the
nuclein to its original state as a preparation for division. The growth
and diminished staining-capacity of the chromatin occurs during a
period of intense constructive activity in the cytoplasm ; its diminu-
tion in bulk and resumption of staining-capacity coincides with the
cessation of this activity. This result is in harmony with the obser-
vations of Schwarz and Zacharias on growing plant-cells, the per-
centage of nuclein in the nuclei of embryonic cells (meristem) being
at first relatively large and diminishing as the cells increase in size.
It agrees further with the fact that of all forms of nuclei those of the
spermatozoa, in which growth is suspended, are richest in nucleinic
acid, and in this respect stand at the opposite extreme from the nuclei
of the rapidly growing egg-cell.

Accurately determined facts in this direction are still too scanty to
admit of a safe generalization. They are, however, enough to indi-
cate the probability that chromatin passes through a certain cycle
in the life of the cell, the percentage of albumin or of albumin-radicals
increasing during the vegetative activity of the nucleus, decreasing in
its reproductive phase. In other words, a combination of albumin
with nuclein or nucleinic acid is an accompaniment of constructive
metabolism. As the cell prepares for division, the combination is
dissolved and the nuclein-radicle or nucleinic acid is handed on by
division to the daughter-cells. A tempting hypothesis, suggested
by Mathews on the basis of Kossel's work, is that nuclein, or one of
its constituent molecular groups, may in a chemical sense be regarded
as the formative centre of the cell which is directly involved in the
process by which food-matters are built up into the cell-substance.
Could this be established, we should have not only a clear light on
the changes of staining-reactions during the cycle of cell-life, but also
a clue to the nuclear "control" of the cell through the process of
synthetic metaboHsm. This hypothesis fits well with the conclusions
of other physiological chemists that the nucleus is especially con-
cerned in synthetic metabolism. Kossel concludes that the formation
of new organic matter is dependent on the nucleus,^ and that nuclein
in some manner plays a leading 7'dle in this process ; and he makes

1 Schiefferdecker and Kossel, Gexvehelehre,

P-57-

PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM 34 1

some interesting suggestions regarding the synthesis of complex
organic matters in the Hving cell with nuclein as a starting-point.
Chittenden, too, in a review of recent chemico-physiological dis-
coveries regarding the cell, concludes : " The cell-nucleus may be
looked upon as in some manner standing in close relation to those
processes which have to do with the formation of organic substances.
Whatever other functions it may possess, it evidently, through the
inherent quaUties of the bodies entering into its composition, has a
controlling power over the metabolic processes in the cell, modifying
and regulating the nutritional changes " ('94).

These conclusions, in their turn, are in harmony with the hypothesis
advanced twenty years ago by Claude Bernard i^7^\ who maintained
that the cytoplasm is the seat of destructive metabolism, the nucleus-
the organ of constructive metabolism and organic synthesis, and
insisted that the role of the nucleus in nutrition gives the key to its;
significance as the organ of development, regeneration, and inheri-
tance.^

B. Physiological Relations of Nucleus and Cytoplasm

How nearly the foregoing facts bear on the problem of the mor-
phological formative power of the cell is obvious ; and they have in a
measure anticipated certain conclusions regarding the role of nucleus
and cytoplasm, which we may now examine from a somewhat differ-
ent point of view.

Brucke long ago drew a clear distinction between the chemical and
molecular composition of organic substances, on the one hand, and,
on the other hand, their definite grouping in the cell by which arises
organization in a morphological sense. Claude Bernard, in like man-
ner, distinguished between chemical synthesis, through which organic
matters are formed, and morphological synthesis, by which they are
built into a specifically organized fabric ; but he insisted that these
two processes are but different phases or degrees of the same phe-
nomenon, and that both are expressions of the nuclear activity. We
have now to consider some of the evidence that the power of mor-
phological, as well as of chemical, synthesis centres in the nucleus,
and that this is therefore to be regarded as the especial organ of
inheritance. This evidence is mainly derived from the comparison
of nucleated and non-nucleated masses of protoplasm ; from the form,

1 " II semble done que la cellule qui a perdu son noyau soit sterilisee au point de vue de
la generation, c'est k dire de la synthese morphologique, et qu'elle le soit aussi au point de
vue de la synthese chimique, car elle cesse de produire des principes immediats, et ne peut
guere qu'oxyder et detruire ceux qui s'y etaient accumules par une elaboration anterieure du
noyau. II semble done que le noyau soit le germe de nutrition de la cellule ; il attire autour
de lui et elabore les materiaux nutritifs " ('78, p. 523).

342

CELL-CHEMISTRY AND CELL-PHYSIOLOGY

position, and movements of the nucleus in actively growing or metab-
olizing cells; and from the history of the nucleus in mitotic cell-
division, in fertilization, and in maturation.

I. Experiments on Unicellular Organisms

Brandt i^y"]) long since observed that enucleated fragments of Acti-
nosphcerium soon die, while nucleated fragments heal their wounds

and continue to live. The
first decisive comparison be-
tween nucleated and non-nu-
cleated masses of protoplasm
was, however, made by Moritz
Nussbaumin 1884 in the case
of an infusorian, Oxytricha.
If one of these animals be
cut into two pieces, the sub-
sequent behaviour of the two
fragments depends on the
presence or absence of the
nucleus or a nuclear frag-
ment. The nucleated frag-
ments quickly heal the wound,
regenerate the missing por-
tions, and thus produce a
perfect animal. On the other
hand, enucleated fragments,
consisting of cytoplasm only,
quickly perish. Nussbaum
therefore drew the conclusion
that the nucleus is indispens-
able for the formative energy
of the cell. The experiment
was soon after repeated by Gruber('85)in the case oi Stentor, another
infusorian, and with the same result (Fig. 159). Fragments possess-
ing a large fragment of the nucleus completely regenerated within
twenty-four hours. If the nuclear fragment were smaller, the re-
generation proceeded more slowly. If no nuclear substance were
present, no regeneration took place, though the wound closed and
the fragment lived for a considerable time. The only exception —
but it is a very significant one — was the case of individuals in which
the process of normal fission had begun ; in these a non-nucleated
fragment in which the formation of a new peristome had already been
initiated healed the wound and completed the formation of the peri-

Fig. i<,8. — Siyloftychia, and enucleated frag-
ments. [Verworn.]

At the left an entire animal, showing planes of
section. The middle piece, containing two nuclei,
regenerates a perfect animal. The enucleated pieces,
shown at the right, swim about for a time, but finally
perish.

PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM 343

stome. Lillie ('96) has recently found that Stentor may by shaking
be broken into fragments of all sizes, and that nucleated fragments
as small as 2V ^^^ volume of the entire animal are still capable of
complete regeneration. All non-nucleated fragments perish.

These studies of Nussbaum and Gruber formed a prelude to more
extended investigations in the same direction by Gruber, Balbiani,
Hofer, and especially Verworn Verworn ('88) proved that in Poly-
stomella, one of the Foraminifera, nucleated fragments are able to

Pig. 159. — Regeneration In the unicellular animal Stentor, [From Gruber after BALBIANI.]

A. Animal divided into three pieces, each containing a fragment of the nucleus. B. The three
fragments shortly afterward. C. The three fragments after twenty-four hours, each regenerated
to a perfect animal.

repair the shell, while non-nucleated fragments lack this power.
Balbiani ('89) showed that although non-nucleated fragments of
Infusoria had no power of regeneration, they might nevertheless
continue to live and swim actively about for many days after the
operation, the contractile vacuole pulsating as usual. Hofer ('89),
experimenting on Ainceba, found that non-nucleated fragments might
live as long as fourteen days after the operation (Fig. 160). Their
movements continued, but were somewhat modified, and little by
little ceased, but the pulsations of the contractile vacuole were but
slightly affected ; they lost more or less completely the capacity to

344

CELL-CHEMISTRY AND CELL-PHYSIOLOGY

digest food, and the power of adhering to the substratum. Nearly
at the same time Verworn ('89) pubHshed the results of an extended
comparative investigation of various Protozoa that placed the whole
matter in a very clear light. His experiments, while fully confirming
the accounts of his predecessors in regard to regeneration, added
many extremely important and significant results. Non-nucleated
fragments both of Infusoria {e.g. Lachrymaria) and rhizopods {Poly^

yi^-y^'^:^vi^;^^

im^

Fig. 160. — Nucleated and non-nucleated fragments of Amxba. [HOFER.]

A. B. An Amceba divided into nucleated and non-nucleated halves, five minutes after the opera-
tion. C. D. The two halves after eight days, each containing a contractile vacuole.

stomella^ Thalassicolla) not only live for a considerable period, but
perform perfectly normal and characteristic movements, show the
same susceptibility to stimulus, and have the same power of ingulf-
ing food, as the nucleated fragments. They lack, Jiozvever, the power
of digestion and secretion. Ingested food-matters may be slightly
altered, but are never completely digested. The non-nucleated frag-
ments are unable to secrete the material for a new shell (Polysto-

PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM 345

mella) or the slime by which the animals adhere to the substratum
(^Amoeba, Difflugia, Polystomella). Beside these results should be
placed the well-known fact that dissevered nerve-fibres in the higher
animals are only regenerated from that end which remains in con-
nection with the nerve-cell, while the remaining portion invariably
degenerates.

\ J

v^

A

D

Fig. 161. — Formation of membranes by protoplasmic fragments of plasmolyzed cells. [TOWN-

SEND.]

A. Plasmolyzed cell, leaf-hair of Cucurbita, showing protoplasmic balls connected by strands.

B. Calyx-hair of Gaillardia ; nucleated fragment with membrane, non-nucleated one naked.

C. Root-hair of Marchantia ; all the fragments, connected by protoplasmic strands, have formed
membranes. D. Leaf-hair of Cucurbita ; non-nucleated fragment, with membrane, connected
with nucleated fragment of adjoining cell.

These beautiful observations prove that destructive metabolism, as
manifested by coordinated forms of protoplasmic contractiHty, may
go on for some time undisturbed in a mass of cytoplasm deprived of
a nucleus. On the other hand, the building up of new chemical or
morphological products by the cytoplasm is only initiated in the
presence of a nucleus and soon ceases in its absence. These facts
form a complete demonstration that the nucleus plays an essential

346 CELL-CHEMISTRY AND CELL-PHYSIOLOGY

part not only in the operations of synthetic metabolism or chemical
synthesis, but also in the morphological determmatioti of these opera-
tions, i.e. the morphological synthesis of Bernard — a point of capital
importance for the theory of inheritance, as will appear beyond.

Convincing experiments of the same character and leading to the
same result have been made on the cells of plants. Francis Darwin
i^TJ^ observed more than twenty years ago that movements actively
continued in protoplasmic filaments, extruded from the leaf-hairs of
Dipsacus, that were completely severed from the body of the cell.
Conversely, Klebs ('79) soon afterward showed that naked proto-
plasmic fragments of Vaiicheria and other algae were incapable of
forming a new cellulose membrane if devoid of a nucleus; and he
afterward showed {'^7) that the same is true of Zygnema and CEdo-
gonium. By plasmolysis the cells of these forms may be broken up
into fragments, both nucleated and non-nucleated. The former sur-
round themselves with a new wall, grow, and develop into complete
plants ; the latter, while able to form starch by means of the chloro-
phyll they contain, are incapable of utilizing it, and are devoid of the
power of forming a new membrane, and of growth and regeneration.
A beautiful confirmation of this is given by Townsend ('97), who finds
in the case of root-hairs and pollen-tubes, that when the protoplasm is
thus broken up, a membrane may be formed by both nucleated and
non-nucleated fragments, by the latter however only when they re7nain
connected with the nucleated masses by protoplasmic strands, however
fine. If these strands be broken, the membrane-forming power is
lost. Of very great interest is the further observation (made on leaf-
hairs in Cuctirbita) that the influence of the nucleus may thus extend
from cell to cell, an enucleated fragment of one cell having the power
to form a membrane if connected by intercellular bridges with a
nucleated fragment of an adjoining cell (Fig. 161).

2. Position and Movements of the Nncleiis

Many observers have approached the same problem from a dif-
ferent direction by considering the position, movements, and changes
of form in the nucleus with regard to the formative activities in the
cytoplasm. To review these researches in full would be impossible,
and we must be content to consider only the well-known researches
of Haberlandt (^77) and Korschelt ('89), both of whom have given
extensive reviews of the entire subject in this regard. Haberlandt's
studies related to the position of the nucleus in plant-cells with
especial regard to the growth of the cellulose membrane. He deter-
mined the very significant fact that local growth of the cell-wall is
always preceded by a movement of the nucleus to the point of growth.
Thus, in the formation of epidermal cells, the nucleus lies at first near

PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM

347

the centre, but as the outer wall thickens, the nucleus moves toward
it, and remains closely applied to it throughout its growth, after which
the nucleus often moves into another part of the cell (Fig. 162, A, B).
That this is not due simply to a movement of the nucleus toward the
air and Hght is beautifully shown in the coats of certain seeds, where
the nucleus moves, not to the outer, but to the inner wall of the cell,
and here the thickening takes place (Fig. 162, C). The same position

Pig. 162. — Position of the nuclei in growing plant-cells. [Haberlandt.]
A. Young epidermal cell oi Luzula with central nucleus, before thickening of the membrane.
B. Three epidermal cells of Monstera, during the thickening of the outer wall. C. Cell from the
seed-coat of Scopulina, during the thickening of the inner wall. D. E. Position of the nuclei dur-
ing the formation of branches in the root-hairs of the pea.

of the nucleus is shown in the thickening of the walls of the guard-
cells of stomata, in the formation of the peristome of mosses, and in
many other cases. In the formation of root-hairs in the pea, the pri-
mary outgrowth always takes place from the immediate neighbourhood
of the nucleus, which is carried outward and remains near the tip of
the growing hair (Fig. 162, D, E). The same is true of the rhizoids
of fern-prothallia and liverworts. In the hairs of aerial plants this

348 CELL-CHEMISTRY AND CELL-PHYSIOLOGY

rule is reversed, the nucleus lying near the base of the hair ; but this
apparent exception proves the rule, for both Hunter and Haberlandt
show that in this case growth of the hair is not apical, but proceeds
from the base ! Very interesting is Haberlandt's observation that in
the regeneration of fragments of Vaucheria the growing region, where
a new membrane is formed, contains no chlorophyll, but numerous
nuclei. The general result, based on the study of a large number of
cases, is, in Haberlandt's words, that *'the nucleus is in most cases
placed in the neighbourhood, more or less immediate, of the points at
which growth is most active and continues longest." This fact points
to the conclusion that " its function is especially connected with the
developmental processes of the cell," ^ and that '*in the growth of the
cell, more especially in the growth of the cell-wall, the nucleus plays
a definite part."

Korschelt's work deals especially with the correlation between form
and position of the nucleus and the nutrition of the cell, and since it
bears more directly on chemical than on morphological synthesis, may
be only briefly reviewed at this point. His general conclusion is that
there is a definite correlation, on the one hand, between the position of
the nucleus and the source of food-supply, on the other hand, between
the size of the nucleus and the extent of its surface and the elabora-
tion of material by the cell. In support of the latter conclusion many
cases are brought forward of secreting cells in which the nucleus is of
enormous size and has a complex branching form. Such nuclei occur,
for example, in the silk-glands of various lepidopterous larvae (Meckel,
Zaddach, etc.), which are characterized by an intense secretory activity
concentrated into a very short period. Here the nucleus forms a
labyrinthine network (Fig. 14, E\ by which its surface is brought to a
maximum, pointing to an active exchange of material between nucleus
and cytoplasm. The same type of nucleus occurs in the Malpighian
tubules of insects (Leydig, R. Hertwig), in the spinning-glands of
amphipods (Mayer), and especially in the nutritive cells of the insect
ovary already referred to at page 151. Here the developing ovum is
accompanied and surrounded by cells, which there is good reason to
believe are concerned with the elaboration of food for the egg-cell.
In the earwig Forficula each ^gg is accompanied by a single large
nutritive cell (Fig. 163), which has a very large nucleus rich in chro-
matin (Korschelt). This cell increases in size as the ovum grows, and
its nucleus assumes the complex branching form shown in the figure.
In the butterfly Vanessa there is a group of such cells at one pole
of the ^gg, from which the latter is beheved to draw its nutriment
(Fig. Tjy A very interesting case is that of the annelid Ophryotrocha,
referred to at page 151. Here, as described by Korschelt, the o^gg floats

1 Lc, p. 99.

PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM 349

in the perivisceral fluid, accompanied by a nurse-cell having a very
large chromatic nucleus, while that of the tgg is smaller and poorer
in chromatin. As the egg
completes its growth, the
nurse-cell dwindles away
and finally perishes (Fig.
76). In all these cases
it is scarcely possible to
doubt that the Qgg is in a
measure relieved of the
task of elaborating cyto-
plasmic products by the
nurse-cell, and that the
great development of
the nucleus in the latter
is correlated with this
function.

Regarding the posi-
tion and movements of
the nucleus, Korschelt
reviews many facts
pointing toward the
same conclusion. Per-
haps the most sugges-
tive of these relate to
the nucleus of the &gg
during its ovarian his-
tory. In many of the „. . .. . . , . , • ^

•' ■ ' \^ \^ y, *^^S* ^"3- — Upper portion of the ovary in the earwig For-

msectS, as m both the Jicula, showing eggs and nurse-cells. [KORSCHELT.]
cases referred to above. Below, a portion of the nearly ripe egg (f), showing deuto-

thf^ pcro- nnrl^iic q1- fircf plasm-spheres and germinal vesicle (^.7^.) . Above it lies the

Liic c^g-iiuuicub dL lUbL nurse-cell («) with its enormous branching nucleus. Twosuc-

OCCUpieS a central posi- cessively younger stages of egg and nurse are shown above.

tion, but as the ^gg be-
gins to grow, it moves to the periphery on the side turned toward
the nutritive cells. The same is true in the ovarian eggs of some other
animals, good examples of which are afforded by various coelenterates,
e.g. in medusae (Claus, Hertwig) and actinians (Korschelt, Hertwig),
where the germinal vesicle is always near the point of attachment of
the Qgg. Most suggestive of all is the case of the water-beetle Dytis^
CHS, in which Korschelt was able to observe the movements and changes
of form in the living object. The eggs here lie in a single series alter-
nating with chambers of nutritive cells. The latter contain granules
which are believed by Korschelt to pass into the ^gg, perhaps bodily^
perhaps by dissolving and entering in a liquid form. At all events,

g 1'

350 CELL-CHEMISTRY AND CELL-PIIYSIOLOGY

the ^gg contains accumulations of similar granules, which extend
inward in dense masses from the nutritive cells to the germinal vesi-
cle, which they may more or less completely surround. The latter
meanwhile becomes amoeboid, sending out long pseudopodia, which
are always directed toward the principal mass of granules (Fig. jj^
The granules could not be traced into the nucleus, but the latter grows
rapidly during these changes, proving that matter must be absorbed
by it, probably in a liquid form.^

Among other facts pointing in the same direction may be mentioned
Miss Huie's ('97) observations on the gland-cells of Drosera, and those
of Mathews ('99) on the changes of the pancreas-cell in Necturiis.
Stimulus of the gland-cells in the leaf of Drosera causes a rapid
exhaustion and change of staining-capacity in the cytoplasm. During
the ensuing repose the cytoplasm is rebuilt out of material laid down
immediately around the nucleus, and agreeing closely in appearance
and staining-reaction with the achromatic nuclear constituents. The
chromatin increases in bulk during a period preceding the constructive
phase, but decreases (while the nucleolar material increases) as the
cytoplasm is restored. In the pancreas-cell, as has long been known,
the " loaded " cell (before secretion) is filled with metaplasmic zymo-
gen-granules, which disappear during secretion, the cell meanwhile
becoming filled with protoplasmic fibrils (Fig. 18). During the ensu-
ing period of " rest " the zymogen-granules are re-formed at the
expense of the fibrillar material, which is finally found only at the
base of the cell near the nucleus. Upon discharge of the secretion
(granule-material) the fibrillae again advance from the nucleus toward
the periphery. Mathews shows that many if not all of them may be
traced at one end actually into the nuclear wall, and concludes that
they are directly formed by the nucleus.

Beside the foregoing facts may be placed the strong evidence
reviewed at pages 156-158, indicating the formation of the yolk-nu-
cleus, and indirectly of the yolk-material, by the nucleus. All of these
and a large number of other observations in the same direction lead to
the conclusion that the cell-nucleus plays an active part in nutrition,
and that it is especially active during the constructive phases. On the
whole, therefore, the behaviour of the nucleus in this regard is in har-
mony with the result reached by experiment on the one-celled forms,
though it gives in itself a far less certain and convincing result.^

1 Mention may conveniently here be made of Richard Hertwig's interesting observation
that in starved individuals of Actinosphccrium the chromatin condenses into a single mass,
while in richly fed animals it is divided into fine granules scattered through the nucleus
('98, p. 8).

2 Loeb ('98, '99) makes the interesting suggestion that the nucleus is especially con-
cerned in the oxydative processes of the cell, and that this is the key to its rdle in the syn-
thetic process. It has been shown that oxydations in the living tissues are probably

PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM

We now turn to evidence which, though less direct than the above,
is scarcely less convincing. This evidence, which has been exhaus-
tively discussed by Hertwig, Weismann, and Strasburger, is drawn
from the history of the nucleus in mitosis, fertilization, and matura-
tion. It calls for only a brief review here, since the facts have been
fully described in earlier chapters.

3. The Nucleus in Mitosis

To Wilhelm Roux ('83) we owe the first clear recognition of the
fact that the transformation of the chromatic substance during mitotic
division is manifestly designed to effect a precise division of all its
parts, — i.e. a panmeristic division as opposed to a mere mass-division,
— and their definite distribution to the daughter-cells. ''The essential
operation of nuclear division is the division of the mother-granules "
{i.e. the individual chromatin-grains) ; " all the other phenomena are
for the purpose of transporting the daughter-granules derived from
the division of a mother-granule, one to the centre of one of the
daughter-cells, the other to the centre of the other." In this respect
the nucleus stands in marked contrast to the cytoplasm, which under-
goes on the whole a mass-division, although certain of its elements,
such as the plastids and the centrosome, may separately divide, like
the elements of the nucleus. From this fact Roux argued, first, that
different regions of the nuclear substance must represent different
qualities, and second, that the apparatus of mitosis is designed to
distribute these qualities, according to a definite law, to the daughter-
cells. The particular form in which Roux and Weismann developed
this conception has now been generally rejected, and in any form it
has some serious difficulties in its way. We cannot assume a precise
localization of chromatin-elements in all parts of the nucleus ; for on
the one hand a large part of the chromatin may degenerate or be cast
out (as in the maturation of the ^^z), and on the other hand in the
Protozoa a small fragment of the nucleus is able to regenerate the
whole. Nevertheless, the essential fact remains, as Hertwig, Kolliker,
Strasburger, De Vries, and many others have insisted, that in mitotic
cell-division the chromatin of the mother-cell is distributed with the
most scrupulous equality to the nuclei of the daughter-cells, and that
in this regard there is a most remarkable contrast between nucleus
and cytoplasm. This holds true with such wonderful constancy

dependent upon certain substances (oxydation ferments) that in some manner, not yet
clearly understood, facilitate the process; and the work of Spitzer ('97) has shown that
these substances (obtained from tissue-extracts) belong to the group of nucleo-proteids,
which are characteristic nuclear substances. The view thus suggested opens a further way
toward more exact inquiry into the nuclear functions, though it is not to be supposed that
the nucleus is the sole oxydative centre of the cell, as is obvious from the prolonged activity
of non-nucleated protoplasmic masses.

352

CELL-CHEMISTRY AND CELL-PHYSIOLOGY

throughout the series of living forms, from the lowest to the highest,
that it must have a deep significance. And while we are not yet in
a position to grasp its full meaning, this contrast points unmistakably
to the conclusion that the most essential material handed on by the
mother-cell to its progeny is the chromatin, and that this substance
therefore has a special significance in inheritance.

4. The Nucleus in Fertilization

The foregoing argument receives an overwhelming reenforcement
from the facts of fertilization. Although the ovum supplies nearly

all the cytoplasm for the embry-
onic body, and the spermatozoon
at most only a trace, the latter is
nevertheless as potent in its effect
on the offspring as the former. On
the other hand, the nuclei con-
tributed by the germ-cells, though
apparently different, become in
the end exactly equivalent in every
visible respect — in structure, in
staining-reactions, and in the num-
ber and form of the chromosomes
to which each gives rise. But
furthermore the substance of the
two germ-nuclei is distributed with
absolute equality, certainly to the
first two cells of the embryo, and
probably to all later-formed cells.
The latter conclusion, which long
remained a mere surmise, has been
rendered nearly a certainty by
the remarkable observations of
Riickert, Zoja, and Hacker, de-
scribed in Chapters IV. and VI.
We must therefore accept the high
probability of the conclusion that
the specific character of the cell is
in the last analysis determined by
that of the nucleus, that is by the
Fig. 164. -Normal and dwarf larvae of the chromatin, and that in the equal

sea-urchin. [Boverl] ..... ^ , ,

. T^ f Di . • ■ f 1 . J distribution 01 paternal and ma-

A. Dwarf Pluteus arising from an enucleated , ^

egg-fragment of Sphcerechinus granularis, fertil- tcmal chromatin tO all the Cclls of

ized with spermatozoon of Echinus microtuber- ^j^e offspring: WC find the phvsio-
culatus, a.x\6. ^iho'Amg purely paternal characters. . ii • ruri,

B. Normal Pluteus of Echinus miciotuberculatus. loglCal explanation of the f act that

PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM

every part of the latter may show the characteristics of either or both
parents.

Boveri ('89, '95, i) has attempted to test this conclusion by a most
ingenious and beautiful experiment ; and although his conclusions do
not rest on absolutely certain ground, they at least open the way to
a decisive test. The Hertwig brothers showed that the eggs of sea-
urchins might be broken into pieces by shaking, and that spermatozoa
would enter the enucleated fragments and cause them to segment.
Boveri proved that such a fragment, if fertilized by a spermatozoon,
would even give rise to a dwarf larva, indistinguishable from the nor-
mal in general appearance except in size. The nuclei of such larvae
are considerably smaller than those of the normal larvae, and were
shown by Morgan ('95, 4) to contain only half the number of chromo-
somes, thus demonstrating their origin from a single sperm-nucleus.
Now, by fertilizing enucleated egg-fragments of one species {Sphce-
rechinus granularis) with the spermatozoa of another {Echinus micro-
tuberculatiis), Boveri obtained in a few instances dwarf Plutei show-
ing except in size the pure paternal characters {i.e. those of Echinus,
Fig. 164). From this he concluded that the maternal cytoplasm has
no determining effect on the offspring, but supplies only the material
in which the sperm-nucleus operates. Inheritance is, therefore, ef-
fected by the nucleus alone.

The later studies of Seeliger ('94), Morgan ('95, 4), and Drisch
('98, 3) showed that this result is not entirely conclusive, since hybrid
larvae arising by the fertilization of an entire ovum of one species by
a spermatozoon of the other show a very considerable range of varia-
tion ; and while most such hybrids are intermediate in character
between the two species, some individuals may nearly approximate
to the characters of the father or the mother. Despite this fact
Boveri ('95, i) has strongly defended his conclusion, though admitting
that only further research can definitely decide the question. It is
to be hoped that this highly ingenious experiment may be repeated
on other forms which may afford a decisive result,

5. The Nucleus in Maturation

Scarcely less convincing, finally, is the contrast between nucleus
and cytoplasm in the maturation of the germ-cells. It is scarcely
an exaggeration to say that the whole process of maturation, in its
broadest sense, renders the cytoplasm of the germ-cells as unlike,
the nuclei as like, as possible. The latter undergo a series of com-
plicated changes which result in a perfect equivalence between them
at the time of their union, and, more remotely, a perfect equality of
distribution to the embryonic cells. The cytoplasm, on the other

354 CELL-CHEMISTRY AND CELL-PHYSIOLOGY

hand, undergoes a special differentiation in each to effect a second-
ary division of labour between the germ-cells. When this is corre-
lated with the fact that the germ-cells, on the whole, have an equal
effect on the specific character of the embryo, we are again forced
to the conclusion that this effect must primarily be sought in the
nucleus, and that the cytoplasm is in a sense only its agent.

C. The Centrosome

Existing views regarding the functions of the centrosome may con-
veniently be arranged in two general groups, the first including those
which regard this structure as a relatively passive body, the second
those which assume it to be an active organ. To the first belongs the
hypothesis of Heidenhain ('94), accepted by Kostanecki ('97, i) and
some others, that the centrosome serves essentially as an insertion-
point for the astral rays ("organic radii"), and plays a relatively
passive part in the phenomena of mitosis, the active functions being
mainly performed by the surrounding structures. To the same
category belongs the view of Miss Foot that the formation of the
centrosome is, as it were, incidental to that of the aster — *' the
expression, rather than the cause, of cell-activity " ('97, p. 810). To
the second group belong the views of Van Beneden, Boveri, Biitschli,
Carnoy, and others who regard the centrosome as playing a more
active role in the life of the cell. Both of the former authors have
assumed the centrosomes to be active centres by the action of which
the astral systems are organized ; and they are thus led to the concfu-
sion that the centrosome is essentially an organ for cell-division and
fertilization (Boveri), and in this sense is the "dynamic centre" of
the cell.^ To Carnoy and Biitschli is due the interesting suggestion ^
that the centrosomes are to be regarded further as centres of chemical
action to which their remarkable effect on the cytoplasm is due.
That the centrosome is an active centre, rather than a passive body
or one created by the aster-formation, is strongly indicated by its
behaviour both in mitosis and in fertilization. Griffin ('96, '99) points
out that at the close of division in Thalassetna the daughter-centro-
somes migrate away from the old astral centre and incite about
themselves in a different region the new astral systems for the
ensuing mitosis (Figs. 99, 155); and similar conditions are described
by Coe in Cerebratuhis ('98). In fertilization the aster-formation can-
not be regarded as a general action of the cytoplasm, but as a local
one due to a local stimulus given by something in the spermatozoon ;
for in polyspermy a sperm-aster is formed for every spermatozoon
(p. 198). This stimulus is given by something in the middle-

1 Cf. pp. 76, 192. 2 QT p. no.

THE CENTROSOME

piece (p. 212), which is itself genetically related to the centrosome of
the last cell-generation (p. 170). These facts seem explicable only
under the assumption that in these cases the centrosome, or a sub-
stance which it carries, gives an active stimulus to the cytoplasm
which incites the aster-formation about itself, and in the words of
Griffin " disengages the forces at work in mitosis " ('96, p. 174). For
these reasons I incline to the view that in the artificial aster-formation
described by Morgan ^ the centrosomes there observed should not be
regarded as the creations of the asters, but rather as local deposits
of material which incite the aster-formation around them. That the
centrosomes or astral centres are centres of division (whether active
or passive) is beautifully shown by Boveri's interesting observations
on ** partial fertilization " referred to at page 194.

Again, Boveri has observed that the segmenting ovum of Ascaris
sometimes contains a supernumerary centrosome that does not enter

Fig. 165. — Eggs oi Ascaru with supernumerary centrosome. [BoVERl.]
A. First cleavage-spindle above, isolated centrosome below. B. Result of the ensuing division.

into connection with the chromosomes, but lies alone in the cytoplasm
(Fig. 165). Such a centrosome forms an independent centre of divi-
sion, the cell dividing into three parts, two of which are normal
blastomeres, while the third contains only the centrosome and attrac-
tion-sphere. The fate of such eggs was not determined, but they
form a complete demonstration that it is in this case the centrosome
and not the nucleus that determines the centres of division in the
cell-body. Scarcely less conclusive is the case of dispermic eggs in
sea-urchins. In such eggs both sperm-nuclei conjugate with the egg-
nucleus, and both sperm-centrosomes divide (Fig. 166). The
cleavage-nucleus, therefore, arises by the union of three nuclei and
four centrosomes. Such eggs divide at the first cleavage into four
equal blastomeres, each of which receives one of the centrosomes.

1 Cf. p. 307.

356

CELL-CHEMISTRY AND CELL-PHYSIOLOGY

The latter must, therefore, be the centres of division ; ^ though it
must not be forgotten that, in some cases at any rate, normal division
requires the presence of nuclear matter (p. io8).

The centrosome must, however, be something more than a mere
division-centre ; for, on the one hand, in leucocytes and pigment-cells
the astral system formed about it is devoted, as there is good reason
to believe, not to cell-division, but to movements of the cell-body as a
whole; and, on the other hand, as we have seen (pp. 165, 172), it is
concerned in the formation of the flagella of the spermatozoa and
spermatozoids, and probably also in that of cilia in epithelial cells.
Strasburger ('97) was thus led to the conclusion that the centrosome
is essentially a mass of kiiioplasm, i.e. the active motor plasm,^ and
a nearly similar view has been adopted by several recent zoologists.

A B C

Fig. 166. — Cleavage of dispermic egg of Toxopneustes.

A. One sperm-nucleus has united with the egg-nucleus, shown at a. b. ; the other lies above.
Both sperm-asters have divided to form amphiasters {a. b. and c. d.). B. The cleavage-nucleus,
formed by union of the three germ-nuclei, is surrounded by the four asters. C. Result of the first
cleavage, the four blastomeres lettered to correspond with the four asters.

Henneguy concludes that the centrosomes are " motor centres of
the kinoplasm " both for external and for internal manifestations.^
Lenhossek regards them as " motors " for the control of ciliary action
as well as for that of the spermatozoon,* and perhaps also for that of
muscle-fibrillae.^ Zimmerman concludes that "the microcentrum is
the motor centre of the cell, that is, the * kinocentrum ' opposed to
the nucleus as the 'chemocentrum.' " ^ Regarding their control of
ciliary action, he makes the same suggestion as that of Henneguy and
Lenhossek cited above. He adds the further very interesting sug-
gestions that the centrosomes may be concerned with the pseudopodial
movements in the epithelial cells of the intestine, and that they may

1 This phenomenon was first observed by Hertwig, and afterward by Driesch. I have
repeatedly observed the internal changes in the living eggs of Toxopneustes.

2 Cf. p. 221. * '98, p. 107. 6 '98, p. 697.
^ '98, p. 495- ^ '99, P- 342.

THE CENTROSOME

357

also be concerned in the protoplasmic contraction of gland-cells by
which the excretion is expelled. [This is based on the fact that
the centrosomes are found in the free (pseudopodia-forming) ends of
the epithelial cells, and on the position of the centrosomes in goblet-
cells (Fig. 23) and in those of the lachrymal gland.] Peter ('99) has
attempted to test these conclusions experimentally by cutting or tear-
ing off cilia from the cell-body (gut-epithelium of Anodonta) and also
by isolating the tails of spermatozoa. In groups consisting of only a
few cilia, separated from the nucleus, the movements actively con-
tinue, while those that are separated from the basal bodies cease to
beat. Spermatozoon tails separated from the head also continue to

Fig. 167. — Centrosomes and cilia in spermatocytes of a butterfly. [Henneguy.]

move, but only if they remain connected with the middle-piece.
Peter, therefore, supports the above conclusions of Henneguy and
Lenhossek. On the other hand, Meves ('99) finds that movements
of the undulating membrane in the tails of salamander-spermatozoa
continue if the middle-piece be entirely removed; while a number
of earlier observers^ have observed in flagellates that a flagellum
separated from the body may actively continue its movements for a
considerable time.

Further research is therefore required to test these suggestions.
The intimate connection of the centrosomes with the formation, on the
one hand, of the astral rays, on the other of contractile organs, such

1 See Klebs, '83, Butschli, '85, Fischer, '94, 2.

3S8 CELL-CHEMISTRY AND CELL-PLIYSIOLOGY

as cilia, flagella, and pseudopodia,^ the centrosomes in ciliated cells
and spermatozoa, and in the swarm-spores of Noctihica, is, however, a
most striking fact, and is one of the strongest indirect arguments in
favour of the general theory of fibrillar contractility in mitosis.

D. Summary and Conclusion

The facts reviewed in the foregoing pages converge to the conclu-
sion that the differentiation of the cell-substance into nucleus and
cytoplasm is the expression of a fundamental physiological division
of labour in the cell. Experiments upon unicellular forms demonstrate
that, in the entire absence of a nucleus, protoplasm is able for a
considerable time to liberate energy and to manifest coordinated
activities dependent on destructive metabohsm. There is here sub-
stantial ground for the view that the cytoplasm is the principal
seat of these activities in the normal cell. On the other hand,
there is strong cumulative evidence that the nucleus is intimately
concerned in the constructive or synthetic processes, whether chemical
or morphological.

That the nucleus has such a significance in synthetic metabolism
is proved by the fact that digestion and absorption of food and
growth soon cease with its removal from the cytoplasm, while destruc-
tive metabolism may long continue as manifested by the phenomena
of irritability and contractility. It is indicated by the position and
movements of the nucleus in relation to the food-supply and to the
formation of specific cytoplasmic products. It harmonizes with the
fact, now universally admitted, that active exchanges of material
go on between nucleus and cytoplasm. The periodic changes of
staining-capacity undergone by the chromatin during the cycle of cell-
life, taken in connection with the researches of physiological chemists
on the chemical composition and staining-reactions of the nuclein
series, indicate that the phosphorus-rich substance known as nncleinic
acid plays a leading part in the constructive process. During the
vegetative phases of the cell this substance is combined with a large
amount of the albumin radicles histon, protamin, and related sub-
stances, and probably in part with albumin itself, to form nuclein.
During the mitotic or reproductive processes this combination appears
to be dissolved, the albuminous elements being in large part split
off, leaving the substance of the chromosomes with a high percentage
of nucleinic acid, as is shown by direct analysis of the sperm-nucleus
and is indicated by the staining-reactions of the chromosomes. There
is, therefore, considerable ground for the hypothesis that in a chemi-
cal sense this substance is the most essential nuclear element handed

^ Cf. pp. 92, 102, on the central granule of the Heliozoa.

LITERATURE 359

on from cell to cell, whether by cell-division or by fertilization ; and
that it may be a primary factor in the constructive processes of the
nucleus and through these be indirectly concerned with those of the
cytoplasm.

The role of the nucleus in constructive metabolism is intimately
related with its role in morphological synthesis, and thus in inheri-
tance ; for the recurrence of similar morphological characters must in
the last analysis be due to the recurrence of corresponding forms of
metaboUc action of which they are the outward expression. That the
nucleus is in fact a primary factor in morphological as well as chemi-
cal synthesis is demonstrated by experiments on unicellular plants and
animals, which prove that the power of regenerating lost parts disap-
pears with its removal, though the enucleated fragment may continue
to live and move for a considerable period. That the nuclear sub-
stance, and especially the chromatin, is a leading factor in inheritance
is powerfully supported by the facts of maturation, fertilization, and
cell-division. In maturation the germ-nuclei are by an elaborate
process prepared for the subsequent union of equivalent chromatic
elements from the two sexes. By fertilization these elements are
brought together, and by mitotic division distributed with exact equal-
ity to the embryonic cells. The result, which is especially striking in
the case of hybrid-fertilization, proves that the spermatozoon is as
potent in inheiitance as the ovum, though the latter contributes an
amount of cytoplasm which is but an infinitesimal fraction of that
supplied by the ovum.

It remains to be seen whether the chromatin can actually be re-
garded as the idioplasm or physical basis of inheritance, as maintained
by Hertwig and Strasburger. Verworn has justly urged that the
nucleus cannot be regarded as the sole vehicle of inheritance, since
the cooperation of both nucleus and cytoplasm is essential to com-
plete cell-life ; and, as will be shown in Chapter IX., the cytoplasmic
organization plays an important role in shaping the course of devel-
opment. Considered in all their bearings, however, the facts seem
to accord best with the hypothesis that the cytoplasmic organization
is itself determined, in the last analysis, by the nucleus ; ^ and the
principle for which Hertwig and Strasburger contended is thus sus-
tained.

LITERATURE. VII

Bernard, Claude. — Lemons sur les Phenomenes de la Vie : ist ed. 1878; 2d ed.

1885. Paris.
Chittenden, R. H. — Some Recent Chemico-physiological Discoveries regarding the

Cell: Atn. Nat., XXVIII., Feb., 1894.

1 Cf. p. 431.

36o CELL-CHEMISTRY AND CELL-PHYSIOLOGY

Fischer, A. — See Literature I.

Gniber, A. — Mikroskopische Vivisekton : Ber. d. Nattirf. Ges. Freiburg, VII., 1893.

Haberlandt, G. — Uber die Beziehungen zwischen Funktion und Lage des Zellkerns.

Fischer, 1887.
Id. — Physiologische Pflanzenatomie. Leipzig, 1896.
Halliburton, W. D. — A Text-book of Chemical Physiology and Pathology. London^

1891.
Id. — The Chemical Physiology of the Cell {Gouldstonian Lectures)-. Brit. Med.

Joiirn. 1893.
Hammarsten, 0. — Lehrbuch der physiologische Chemie. 3d ed. Wiesbade?i, 1895.
Hertwig, 0. and R. — Uber den Befruchtungs- und Teilungsvorgang des tierischen

Eies unter dem Einfluss ausserer Agentien. Jena, 1887.
KoUiker, A. — Das Karyoplasma und die Vererbung, eine Kritik der Weismann'schen

Theorie von der Kontinuitat des Keimplasmas : Zeitschr. wiss. ZooL, XLIV.

1886.
Korschelt, E. — Beitrage sur Morphologic und Physiologic des Zellkernes : ZooL

Jahrb. Anat. u. Ontog., IV. 1889.
Kossel, A. — tJber die chemische Zusammensetzungder Zelle: Arch. Anat. ti.Phys.

1891.
Id. — tlber die basischen Stoffe des Zellkernes: Zeit. Phys. Chem., XXII., 1896.
Lilienfeld, L. — Uber die Wahlverwandfschaft der Zellelemente zu Farbstoffen :

Arch. Anat. u. Phys. 1893.
Malfatti, H. — Beitrage zur Kenntniss der Nucleine : Zeitschr. Phys. Chem., XVI.

1891.
Mathews, A. P. — The Metabolism of the Pancreas CtW -.Joiirn. Morph.,X\ .SitppL

1899.
Miescher, F. — Physiologisch-chemische Untersuchungen Uber die Lachsmilch : Arch.

Exp. Path. u. Pharm., XXXVII., 1896.
Prenant, A. — See Literature VI.
Riickert, J. — Zur Entwicklungsgeschichte des Ovarialeies bei Selachiern : An. Anz.,

VII. 1892.
Sachs, J. — Vorlesungen liber Pflanzen-physiologie. Leipzig, 1882.
Id. — Stoff und Form der Pflanzen-organe : Gesa7n?nelte Abhandlungen, II. 1893.
Strasburger. — See footnote, p. 269.
Verworn, M. — Die Physiologische Bedeutung des Zellkerns : Arch, fur die Ges.

Phys.,XU.. 1892.
Id. — Allgemeine Physiologic. Jena, 1895.
Whitman, C. 0. — The Seat of Formative and Regenerative Energy : Journ. Merph.^

II. 1888.
Zacharias, E. — Uber des Verhalten des Zellkerns in vvachsenden Zellen : Flora, 81.

1895.

CHAPTER VIII

CELL-DIVISION AND DEVELOPMENT

" Wir konnen demnach endlich den Satz aufstellen, dass sammtliche im entwickelten
Zustande vorhandenen Zellen oder Aequivalente von Zellen durch eine fortschreitende
Gliederung der Eizelle in morphologisch ahnliche Elemente entstehen, und dass die in einer
embryonischen Organ-Anlage enthaltenden Zellen, so gering auch Hire Zahl sein mag,
dennoch die ausschliessliche ungegliederte Anlage fiir sammtliche Formbestandtheile der
spateren Organe enthalten." Remak.i

Since the early work of Kolliker and Remak it has been recog-
nized that the cleavage or segmentation of the ovum, with which
the development of all higher animals begins, is nothing other than
a rapid series of mitotic cell-divisions by which the egg splits up
into the elements of the tissues. This process is merely a contin-
uation of that by which the germ-cell arose in the parental body.
A long pause, however, intervenes during the latter period of its
ovarian life, during which no divisions take place. Throughout this
period the egg leads, on the whole, a somewhat passive existence,
devoting itself especially to the storage of potential energy to be used
during the intense activity that is to come. Its power of division
remains dormant until the period of full maturity approaches. The
entrance of the spermatozoon arouses in the egg a new phase of
activity. Its power of division, which may have lain dormant for
months or years, is suddenly raised to the highest pitch of intensity,
and in a very short time it gives rise by division to a myriad of de-
scendants which are ultimately differentiated into the elements of
the tissues.

The divisions of the egg during cleavage are exactly comparable
with those of tissue-cells, and all of the essential phenomena of
mitosis are of the same general character in both. But for two
reasons the cleavage of the egg possesses a higher interest than
any other case of cell-division. First, the egg-cell gives rise by divi-
sion not only to cells like itself, as is the case with most tissue-cells,
but also to many other kinds of cells. The operation of cleavage is'
therefore immediately connected with the process of differentiation,
which is the most fundamental phenomenon in development. Second,
definite relations may often be traced between the planes of division
and the structural axes of the adult body, and these relations are

1 Untersuchungen, 1855, p. 140.
.-.61

362 CELL-DIVISION AND DEVELOPMENT

sometimes so clearly marked and appear so early that with the very
first cleavage the position in which the embryo will finally appear in
the Qgg may be exactly predicted. Such " promorphological " rela-
tions of the segmenting ^gg possess a very high interest in their
bearing on the theory of germinal localization and on account of the
light which they throw on the conditions of the formative process.

The present chapter is in the main a prelude to that which
follows, its purpose being to sketch some of the external features
of early development regarded as particular expressions of the gen-
eral rules of cell-division. For this purpose we may consider the
cleavage of the ovum under two heads, namely : —

1. The Geometrical Relations of Cleavage-forms ^ vj'ith. reference
to the general rules of cell-division.

2. The Promorp ho logical Relations of the blastomeres and cleav-
age-planes to the parts of the adult body to which they give rise.

A. Geometrical Relations of Cleavage-forms

The geometrical relations of the cleavage-planes and the relative
size and position of the cells vary endlessly in detail, being modified
by innumerable mechanical and other conditions, such as the amount
and distribution of the inert yolk or deutoplasm, the shape of the
ovum as a whole, and the like. Yet all the forms of cleavage can
be referred to a single type which has been moulded this way or that
by special conditions, and which is itself an expression of two general
rules of cell-division, first formulated by Sachs in the case of plant-
cells. These are : —

1 . The cell typically tends to divide into equal parts.

2. Each new plane of division teiids to intersect the preceding plane
at a right angle.

In the simplest and least modified forms the direction of the
cleavage-planes, and hence the general configuration of the cell-
system, depends on the general form of the dividing mass ; for, as
Sachs has shown, the cleavage-planes tend to be either vertical to the
surface {anticlines^ or parallel to it (^peiHc lines). Ideal schemes of
division may thus be constructed for various geometrical figures. In
a flat circular disc, for example, the anticlinal planes pass through
the radii ; the periclines are circles concentric with the periphery. If
the disc be elongated to form an ellipse, the periclines also become
ellipses, while the anticlines are converted into hyperbolas confocal
with the periclines. If it have the form of a parabola, the periclines
and anticlines form two systems of confocal parabolas intersecting at
right angles. All these schemes are mutatis mutandis, directly con-
vertible into the corresponding solid forms in three dimensions.

GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS

363

Sachs has shown in the most beautiful manner that all the above
ideal types are closely approximated in nature, and Rauber has applied
the same principle to the cleavage of animal-cells. The discoid or
spheroid form is more or less nearly realized in the thalloid growths of

Fig. 168. — Geometrical relations of cleavage-planes in growing plant-tissues. [From Sachs,
after various authors.]

A. Flat ellipsoidal germ-disc of Melobesia (Rosanoft") ; nearly typical relation of elliptic
periclines and hyperbolic anticlines. B. C. Apical view of terminal knob on epidermal hair of
Pinguicola. B. shows the ellipsoid type, C. the circular (spherical type), somewhat modified
(only anticlines present). D. Growing point of Salvinia (Pringsheim), typical ellipsoid type;
the single pericline is, however, incomplete. E. Growing point of Azolla (Strasbur2;er) ; circular
or spheroidal type transitional to ellipsoidal. F. 'R.oo\-c?i\) oi F.quisetum (Nageli and Leitgeb) ;
modified circular type. G. Cross-section of leaf-vein, Trichovianes (Prantl) ; ellipsoidal type with
incomplete periclines. H. Embryo o{ Alisma ; typical ellipsoid type, pericline incomplete only
at lower side. /. Growing point of bud of the pine (Adies) ; typical paraboloid type, both anti-
clines and periclines having the form of parabolas (Sachs).

various lower plants, in the embryos of flowering plants, and else-
where (Fig. 168). The paraboloid form is according to Sachs charac-
teristic of the growing points of many higher plants ; and here, too,
the actual form is remarkably similar to the ideal scheme (Fig. 168, /).

364 CELL-DIVISION AND DEVELOPMENT

For our purpose the most important form is the sphere, which is
the typical shape of the egg-cell; and all forms of cleavage may
be related to the typical division of a sphere in accordance with Sachs's
rules. The ideal form of cleavage would here be a succession of
rectangular cleavages in the three dimensions of space, the anticlines
passing through the centre so as to split the ^gg in the initial stages
successively into halves, quadrants, and octants, the periclines being
parallel to the surface so as to separate the inner ends of these cells
from the outer. No case is known in which this order is accurately
followed throughout, and the periclinal cleavages are of compara-
tively rare occurrence, being found as a regular feature of the early
cleavage only in those cases where the primary germ-layers are sepa-
rated by delamination. The simplest and clearest form of egg-
cleavage occurs in eggs like those of echinoderms, which are of
spherical form, and in which the deutoplasm is small in amount and
equally distributed through its substance. Such a cleavage is beauti-
fully displayed in the ^gg of the holothurian Synapta, as shown in
the diagrams, Fig. 169, constructed from Selenka's drawings. The
first cleavage is vertical, or meridional, passing through the egg-axis
and dividing the ^gg into equal halves. The second, which is also
meridional, cuts the first plane at right angles and divides the ^gg
into quadrants. The third is horizontal, or equatorial, dividing the
tgg into equal octants. The order of division is thus far exactly
that demanded by Sachs's rule and agrees precisely with the cleavage
of various kinds of spherical plant-cells. The later cleavages depart
from the ideal type in the absence of periclinal divisions, the embryo
becoming hollow, and its walls consisting of a single layer of cells in
which anticlinal cleavages occur in regular rectangular 'succession.
The fourth cleavage is again meridional, giving two tiqrs of eight
cells each ; the fifth is horizontal, dividing each tier into an upper
and a lower layer. The regular alternation is continued up to the
ninth division (giving 512 cells), when the divisions pause while the
gastrulation begins. In later stages the regularity is lost.

Hertwigs Development of Sachs's Rules. — Beside Sachs's rules
may be placed two others formulated by Oscar Hertwig in 1884,
which bear directly on the facts just outlined and which lie behind
Sachs's principle of the rectangular intersection of successive division-
planes. These are : —

1 . The nucleus tends to take up a position at the centre of its sphere
of influence, i.e. of the protoplasmic mass in zvhich it lies.

2. The axis of the mitotic figures typically lies in the longest axis
of the protoplasmic mass, and division therefore tends to cut this axis
at a right angle.

The second rule explains the normal succession of the division-

GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS

365

planes according to Sachs's second rule. The first division of a homo-
geneous spherical ^gg, for example, is followed by a second division
at right angles to it, since each hemisphere is twice as long in the
plane of division as in any plane vertical to it The mitotic figure
of the second division Ues therefore parallel to the first plane, which
forms the base of the hemisphere, and the ensuing division is vertical
to it. The same applies to the third division, since each quadrant is
as long as the entire ^%g while at most only half its diameter. Divi-
sion is therefore transverse to the long axis and vertical to the first
two planes.

Taken together the rules of Sachs and Hertwig, applied to the
^ggj give us a kind of ideal type or model, well illustrated by the

Fig. 169. — Cleavage of the ovum in the holothurian 6y«a//a (slightly schematized). [After
Selenka.]

A-E. Successive cleavages to the 32-cell stage. F. Blastula of 128 cells.

cleavage of Synapta, described above, to which all the forms of cleav-
age may conveniently be referred as a basis of comparison. Numer-
ous exceptions to all four of these rules are, however, known, and they
are of little value save as a starting-point for a closer study of the
facts. Cleavage of such schematic regularity as that of Synapta is
extremely rare, both the form and the order of division being end-
lessly varied and in extreme cases showing scarcely a discoverable
connection with the *'type." We may conveniently consider these
modifications under the following three heads : —

366 CELL-DIVISION AND DEVELOPMENT

1. Variation in the rhythm of division.

2. Displacement of the cells {including variations in the direction
of cleavage^

3. Unequal division of the cells.

■ Nothing is more common than a departure from the regular
rhythm of division. The variations are sometimes quite irregular,
sometimes follow a definite rule, as, for instance, in the annelid Nereis
(Fig. 171), where the typical succession in the number of cells is with
great constancy 2, 4, 8, 16, 20, 23, 29, 32, 37, 38, 41, 42, after which
the order is more or less variable. The factors that determine such
variations in the rhythm of division are very little understood. Bal-
four, one of the first to consider the subject, sought an explanation in
the varying distribution of metaplasmic substances, maintaining ('75,
'80) that the rapidity of division in any part of the ovum is in general
inversely proportional to the amount of deutoplasm that it contains.
The entire inadequacy of this view has been demonstrated by a long
series of precise studies on cell-lineage, which show that while the
large deutoplasm-bearing cells often do divide more slowly than
the smaller protoplasmic ones the reverse is often the case, while
remarkable differences in the rhythm of division are often observed
in cells which do not perceptibly differ in metaplasmic content.^ All
the evidence indicates that the rhythm of division is at bottom deter-
mined by factors of a very complex character which cannot be
disentangled from those which control growth in general. Lillie
('95) '99) points out the very interesting fact, determined through an
analysis of the cell-lineage of mollusks and annelids, that the rate
of cleavage shows a direct relation to the period at which the prod-
ucts become functional. Thus in Unio the more rapid cleavage of a
certain large cell ("d. 2"), formed at the fourth cleavage, is obviously
correlated with the early formation of the shell-gland to which it gives
rise, w^hile the relatively slow rate of division in the first ectomere-
quartet is correlated with reduction of the prae-trochal region. The
prospective character shown here will be found to apply also to other
characters of cleavage, as described beyond.

When we turn to the factors that determine the direction of cleav-
age or the displacement of cells subsequent to division, we find, as
in the case of the division-rhythm, obvious mechanical factors com-
bined with others far more complex. The arrangement of tissue-cells
usually tends toward that of least resistance or greatest economy of
space ; and in this regard they have been shown to conform, broadly
speaking, with the behaviour of elastic spheres, such as soap-bubbles
when massed together and free to move. Such bodies, as Plateau

1 Cf. Wilson, '92, Kofoid, '94, Lillie, '95, Zur Strassen, '95, Ziegler, '95, and especially
Jennings, '97.

GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS

367

and Lamarle have shown, assume a polyhedral form and tend toward
such an arrangement that the area of surface-contact between ihem is
a rninirnum. Spheres in a mass thus tend to assume the form of
interlocking polyhedrons so arranged that three planes intersect in
a line, while four lines and six planes meet at a point. If arranged
in a single layer on an extended surface, they assume the form of

Fig. 170. — Cleavage of Polygordius, from life.
A. Four-cell stage, from above. B. Corresponding view of eight-cell stage,
the same (contrast Fig. 169, C) . D. Sixteen-cell stage from the side.

C. Side view ot

hexagonal prisms, three planes meeting along a line as before. Both
these forms are commonly shown in the arrangement of the cells of
plant and animal tissues ; and Berthold ('86) and Errera i^'^6, 'Sy\
carefully analyzing the phenomena, have endeavoured to show that
not only the form and relative position of cells, but also the direction
of cell-division, is, partially at least, thus determined.

It is through displacements of the cells of this type that many of

368 CELL-DIVISION AND DEVELOPMENT

the most frequent modifications of cleavage arise. Sometimes, as in
Synapta, the alternation of the cells is effected through displacement
of the blastomeres after their formation. More commonly it arises
during the division of the cells, and may even be predetermined by
the position of the mitotic figures before the slightest external sign
of division. Thus arises that form of cleavage known as the spiral,
oblique, or alternating type, where the blastomeres interlock during
their formation and lie in the position of least resistance from the
beginning. This form of cleavage, especially characteristic of many
worms and moUusks, is typically shown by the Qgg of Polygordiiis
(Fig. 170). The four-celled stage is nearly like that of Synapta^
though even here the cells slightly interlock. The third division is,
however, oblique, the four upper cells being virtually rotated to the
right (with the hands of a watch) so as to alternate with the four
lower ones. The fourth cleavage is likewise obHque, but at right
angles to the third, so that all of the cells interlock as shown in
Fig. 170, D. This alternation regularly recurs for a considerable
period.

In many worms and mollusks the obliquity of cleavage appears
still earlier, at the second cleavage, the four cells being so* arranged
that two of them meet along a "cross-furrow" at the lower pole of
the Q,gg, while the other two meet at the upper pole along a similar,
though often shorter, cross-furrow at right angles to the lower {e.g. in
Nereis, Fig. 171). It is a curious fact that the direction of the dis-
placement is quite constant, the first or upper quartet in the eight-
cell stage being rotated to the right, or with the hands of a watch,
the second quartet to the left, the third to the right, and so on.
Crampton ('94) has discovered the remarkable fact that in Physa, a
gasteropod having a reversed or sinistral shell, the whole order of
displacement is likewise reversed, and the same has recently been
shown by Holmes ('99) to be true of Ancylus.

The spiral or alternating type of cleavage beautifully illustrates
Sachs's second rule as affected by modifying conditions ; for, as may
be seen by an inspection of Figs. 170, 171, each division-plane is
approximately at right angles to the preceding and succeeding
(whence the " alternation of the spirals " described by students of
cell-lineage), while they are so directed that each cell as it is formed
is placed at once in the position of least resistance in the mass, i.e. in
the position of minimal surface-contact. It is impossible to resist
the conclusion that one of the factors by which the position of the
cells (and hence the direction of cell-division) is determined is a
purely mechanical one, identical with that which determines the
arrangement of soap-bubbles and the Hke.

Very little acquaintance with the facts of development is however

GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS

369

required to show that this purely mechanical factor, though doubtless
real, must be subordinate to some other. This is strikingly shown,
for example, in the development of annelids and mollusks, where the
spiral cleavage, strictly maintained during the earlier stages, finally
gives way more or less completely to a bilateral type of division in
which the rule of minimal surface-contact is often violated. We see
here a tendency operating directly against, and finally overcoming,

E F

Fig. 171. — Cleavage of Nereis. An example of a spiral cleavage, unequal from the beginning
and of a marked determinate character.

A Two-cell stage (the circles are oil-drops). B. Four-cell stage ; the second cleavage-plane
passes through the future median plane. C. The same from the right side. D. Eight-cell stage.
E. Sixteen cells; from the cells marked t arises the prototroch or larval ciliated belt, from X the
ventral nerve-cord and other structures, from D the mesoblast-bands, the germ-cells, and a part of
the alimentary canal. F. Twenty-nine-cell stage, from the right side ; /. girdle of prototrochal cells
which give rise to the ciliated belt.

the mechanical factor which predominates in the earlier stages ; and
in some cases, e.g. in the Qgg of Clavelina (Fig. 177) and other tuni-
cates, this tendency predominates from the beginning. In both
these cases this " tendency " is obviously related to the growth-process
to which the future bilateral embryo will owe its form ; ^ and every
attempt to explain the position of the cells and the direction of cleav-
age must reckon with the morphogenic process taken as a whole.
The blastomere is not merely a cell dividing under the stress of rude

1 Cf. Wilson ('92, p. 444).
2B

370

CELL-DIVISION AND DEVELOPMENT

mechanical conditions ; it is beyond this " a builder which lays one
stone here, another there, each of which is placed with reference to
future development."^

The third class of modifications, due to unequal division of the cells,
not only leads to the most extreme types of cleavage but also to its

C D

Fig. 172. — The eight-cell stage of four different animals showing gradations in the inequality of
the third cleavage.

A. The leech Clepsine (Whitman). B, The oh-c&Xo^oA Rhynchelmis (Vejdovsky). C. The
lamellibranch Unio (Lillie). D. Amphioxus.

most difficult problems. Unequal divisions appear sooner or later
in all forms of cleavage, the perfect equality so long maintained
in Synapta being a rare phenomenon. The period at which the in-
equality first appears varies greatly in different forms. In Polygordius
(Fig. 170) the first marked inequality appears at the fifth cleavage;

1 Lillie, '95, p. 46.

GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 371

in sea-urchins it appears at the fourth (Fig. 3) ; in Amphioxiis at the
third (Fig. 172); in the tunicate Clavelina at the second (Fig. 177);
in Nereis at the first division (Figs. 60, 171). The extent of the in-
equahty varies in Hke manner. Taking the third cleavage as a type,
we may trace every transition from an equal division (echinoderms,
Polygordius), through forms in which it is but slightly marked (^Am-
p/iioxHSy frog), those in which it is conspicuous {Nereis, Lymncea, poly-
clades, Petromyzon, etc.), to forms such as Clepsine, where the cells of
the upper quartet are so minute as to appear like mere buds from the
four large lower cells (Fig. 172). At the extreme of the series we
reach the partial or meroblastic cleavage, such as occurs in the ceph-
alopods, in many fishes, and in birds and reptiles. Here the lower
hemisphere of the ^gg does not divide at all, or only at a late period,
segmentation being confined to a disc-like region or blastoderm at one
pole of the ^gg (Fig. 173).

Very interesting is the case of the teloblasts or pole-cells character-
istic of the development of many annelids and mollusks and found in
some arthropods. These remarkable cells are large blastomeres, set
aside early in the development, which bud forth smaller cells in reg-
ular succession at a fixed point, thus giving rise to long cords of cells
(Fig. 175). The teloblasts are especially characteristic of apical
growth, such as occurs in the elongation of the body in annelids, and
they are closely analogous to the apical cells situated at the growing
point in many plants, such as the ferns and stoneworts.

Still more suggestive is the formation of rudimentary cells, arising
as minute buds from the larger blastomeres, and, in some cases, appar-
ently taking no part in the formation of the embryo (Fig. 1 74).^

We are as far removed from an explanation of unequal division as
from that of the rhythm and direction of division. Inequality of divi-
sion, like difference of rhythm, is often correlated with inequalities in
the distribution of metaplasmic substances — a fact generalized by
Balfour in the statement ('80) that the size of the cells formed in
cleavage varies inversely to the relative amount of protoplasm in the
region of the ^gg from which they arise. Thus, in all telolecithal
ova, where the deutoplasm is mainly stored in the lower or vegetative
hemisphere, as in many worms, mollusks, and vertebrates, the cells of
the upper or protoplasmic hemisphere are smaller than those of the
lower, and may be distinguished as micromeres from the larger macro-
meres of the lower hemisphere. The size-ratio between micromeres
and macromeres is on the whole directly proportional to the ratio
between protoplasm and deutoplasm. Partial or discoidal cleavage
occurs when the mass of deutoplasm is so great as entirely to prevent
cleavage in the lower hemisphere. This has been beautifully con-

1 See Wilson, '98, '99, 2.

372 CELL-DIVISION AND DEVELOPMENT

firmed by O. Hertwig ('98), who, by placing frogs' eggs in a centrifu-
gal machine, has caused them to undergo a meroblastic cleavage
through the artificial accumulation of yolk at the lower pole, due to
the centrifugal force.

While doubtless containing an element of truth, this explanation is,
however, no more adequate than Balfour's rule regarding the relation
between deutoplasm and rhythm (p. 366); for innumerable cases are
known in which no correlation can be made out between the distribu-
tion of inert substance and the inequality of division. This is the
case, for example, with the teloblasts mentioned above, which contain
no deutoplasm, yet regularly divide unequally. It seems to be inap-

A

Fig. 173. — Partial or meroblastic cleavage in the squid Loligo. yW .\\i\i>Y.?^

plicable to the inequalities of the first two divisions in annelids and
gasteropods. It is conspicuously inadequate in the history of indi-
vidual blastomeres, where the history of division has been accurately
determined. In Nereis^ for example, a large cell known as the first
somatoblast, formed at the fourth cleavage (X, Fig. 171, E\ under-
goes an invariable order of division, three unequal divisions being fol-
lowed by an equal one, then by three other unequal divisions, and
again by an equal. This cell contains httle or no deutoplasm and
undergoes no perceptible changes of substance.

The collapse of the rule is most complete in case of the rudi-
mentary cells referred to above. In some of the annelids, e.g. in
Aricia, where they were first observed,^ these cells are derived from
the very large primary mesoblast-cell, which first divides into equal
halves. Each of these then buds forth a cell so small as to be no
larger than a polar body, and then immediately proceeds to give rise

1 Cf. Wilson, '92, '98.

GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS

373

to the mesoblast-bands by continued divisions, always in the same
plane at right angles to that in which the rudimentary cells are
formed (Fig. 174). The cause of the definite succession of equal and
unequal divisions is here wholly unexplained. No less difficult is the
extreme inequality of division involved in the formation of the polar
bodies. We cannot explain this through the fact that deutoplasm is
collected in the lower hemisphere ; for, on the one hand, the succeed-
ing divisions (first cleavages) are often equal, while, on the other
hand, the inequality is no less pronounced in eggs having equally

A

B

Fig. i'74. — Rudimentary blastomeres in the embryo of an annelid, Aricia.

A. From lower pole; rudimentary cells at e. e; the heavy outline is the lip of the blastopore.
B. The same in sagittal optical- section, showing rudimentary cell {e), primary mesoblast {M)^
and mesoblast-band {m).

distributed deutoplasm, or in those, like echinoderm-eggs, which are
'' alecithal."

Such cases prove that Balfour's law is only a partial explanation,
being probably the expression of a more deeply lying cause, and
there is reason to believe that this cause lies outside the immediate
mechanism of mitosis. Conklin ('94) has called attention to the
fact^ that the immediate cause of the inequality probably does not
lie either in the nucleus or in the amphiaster ; for not only the
chromatin-halves, b?it also the asters, are exactly equal in the" early
prophases, and the inequality of the asters only appears as the
division proceeds. Probably, therefore, the cause lies in some rela-
tion between the mitotic figure and the cell-body in which it lies.

1 In the cleavage of gasteropod eggs.

374

CELL-DIVISION AND DEVELOPMENT

I believe there is reason to accept the conclusion that this relation
is one of position, however caused. A central position of the mitotic

^ B m

Pig. 175. — Embryos of the earthworm Allolobophora foetida, showing teloblasts or apical cells.

A. Gastrula from the ventral side. B. The same from the right side ; m. the terminal telo-
blasts or primary mesoblasts, vihxch bud forth the mesoblast-bands, cell by cell; t. lateral teloblasts,
comprising a neuroblast, nb, from which the ventral nerve-cord arises, and two nephroblasts, n, of
somewhat doubtful nature, but probably concerned in the formation of the nephridia. C. Lateral
group of teloblasts, more enlarged, the neuroblast, nb, in division ; n. the nephroblasts. D. The
primary mesoblasts enlarged ; one in division.

figure results in an equal division ; an eccentric position caused by a
radial movement of the mitotic figure, in the direction of its axis
toward the periphery, leads to unequal division, and the greater the

GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 375

eccentricity, the greater the inequality, an extreme form being beauti-
fully shown in the formation of the polar bodies. Here the original
amphiaster is perfectly symmetrical, with the asters of equal size
(Fig. 97, A). As the spindle rotates into its radial position and
approaches the periphery, the development of the outer aster be-
comes, as it were, suppressed, while the central aster becomes enor-
mously large. The size of the aster, in other words, depends upon the
extent of the cytoplasmic area that falls within the sphere of inflne^ice
of the ccntrosome ; and this area depends upon the position of the
centrosome. If, therefore, the polar amphiaster could be artificially
prevented from moving to its peripheral position, the Qgg would
probably divide equally.^

This leads us to a further consideration of the attempts that have
been made to explain the movements of the mitotic figure through
mechanical or other causes.^ Highly interesting experiments have
been made by Pfliiger ('84), Roux ('85), Driesch ('92), and a number
of later investigators which show that the direction of cleavage may
be determined, or at least modified, by such a purely mechanical
cause as pressure, through which the form of the dividing mass is
changed.

Thus, Driesch has shown that when the eggs of sea-urchins are
flattened by pressure, the amphiasters all assume the position of least
resistance, i.e. parallel to the flattened sides, so that the cleavages
are all vertical, and the Qgg segments as a flat plate of eight, sixteen,
or thirty-two cells (Fig. 186). This is fotally different from the nor-
mal form of cleavage ; yet such eggs, when released from pressure,
are capable of development and give rise to normal embryos. This
interesting experiment makes it highly probable that the disc-like
cleavage of meroblastic eggs, like that of the squid or bird, is in some
degree a mechanical result of the accumulation of yolk by which the
formative protoplasmic region of the ovum is reduced to a thin layer
at the upper pole ; and it indicates, further, that the unequal cleavage
of less modified telolecithal eggs, like those of the frog or snail, are
in like manner due to the displacement of the mitotic figures toward
the upper pole.

The results of Pfliiger's and Driesch's pressure experiments obvi-
ously harmonize with Hertwig's second rule, for the position of least
resistance for the spindle is obviously in the long axis of the proto-
plasmic mass which is here artificially modified ; and it harmonizes
further with Driiner's hypothesis of the active elongation of the
spindle in mitosis (p. 105). There are, however, a large number of
facts which show that neither the form of the protoplasmic mass nor

^ Cf. Francotte on the polar bodies of Turbellaria, p. 235.
2 For a good review and critique, see Jennings, '97 .

376

CELL-DIVISION AND DEVELOPMENT

the distribution of metaplasmic materials is sufficient to explain the
position of the spindle, whether with reference to the direction or the
inequality of the cleavage.

As regards the direction of the spindle, Berthold ('86) long since
clearly pointed out that prismatic or cylindrical vegetable cells, for
instance, those of the cambium, often divide lengthwise ; and numer-
ous contradictions of Hertwig's ** law " have since been observed by
students of cell-lineage with such accuracy that all attempts to explain
them away have failed.^ In some of these cases the position of the
spindle is not that of least but of greatest resistance,^ the spindle ac-

Fig. 176. — Segmenting eggs of ^j^raWjo [KOSTANECKI and SlEDLECKl.]
A. Early prophase of second division, showing double centrosomes. B. Second cleavage in
progress ; upper blastomere dividing parallel to long axis of the cell.

tually pushing away the adjoining cell to make way for itself. Simi-
lar difficulties, some of which have been already considered (p. 372),
stand in the way of the attempt to explain the eccentricity of the
spindle in unequal division. All these considerations drive us to the
view that the simpler mechanical factors, such as pressure, form, and
the Hke, are subordinate to far more subtle and complex operations
involved in the general development of the organism, a conclusion
strikingly illustrated by the phenomena of teloblastic division (p. 371),
where the constant succession of unequal divisions, always in the

1 Cf. Watase ('91), Mead ('94, '97, 2), Heidenhain ('95), Wheeler ('95), Castle ('96),
Jennings ('97).

2 See especially the case observed by Mead ('94, '97, 2), in the egg oi Amphitrite.

GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 2)77

same plane, is correlated with a deeply lying law of growth affecting
the entire formation of the body. We camtot comprehend the forms
of cleavage ivithoiit reference to the e?td-result ; and thus these phe-
nomena acquire a certain teleological character so happily expressed
by Lillie (p. 370). This has been clearly recognized in various ways
by a number of recent writers. Roux ('94), while seeking to explain
many of the operations of mitosis on a mechanical basis, holds that
the position of the spindle is partly determined by "immanent"
nuclear tendencies. Braem ('94) recognizes that the position of the
spindle is determined not merely as that of least resistance for the
mitotic figure, but also for that of the resulting products. I pointed
out ('92) that the bilateral form of cleavage in annelids must be
regarded as a " forerunner " of the adult bilaterality. Jennings ('97)
concludes that the form and direction of cleavage are related to the
later morphogenetic processes ; and many similar expressions occur
in the works of recent students of cell-lineage.^

The clearest and best expression of this view is, however, given by
LilHe ('95, '99), who not only correlates the direction and rate of
cleavage, but also the size-relations of the cleavage-cells with the
arrangement of the adult parts, pointing out that in general the size,
as well as the position, of the blastomeres is directly correlated with
that of the parts to which they give rise, and showing that on this
basis *' one can thus go over every detail of the cleavage, and know-
ing the fate of the cells, can explain all the irregularities and peculi-
arities exhibited." 2 Of the justice of this conclusion I think any one
must be thoroughly convinced who carefully examines the recent
literature of cell-Hneage. It gives no real explanation of the phenom-
ena, and is hardly more than a restatement of fact. Neither does it
in any way lessen the importance of studying fully the mechanical
conditions of cell-division. It does, however, show how inadequate
have been most of the attempts thus far to formulate the "■ laws " of
cell-division, and how superficially the subject has been considered by
some of those who have sought for such "laws."

We now pass naturally to the second or promorphological aspect
of cleavage, to a study of which we are driven by the foregoing con-
siderations.

1 Conklin ('99) believes that many of the peculiarities of cleavage may be explained by
the assumption of protoplasmic currents which '* carry the centrosomes where they will, and
control the direction of division and the relative size and quality of the daughter-cells,"
I.e., p. 90. He suggests that such currents are of a chemotropic character, but recognizes
that their causation and direction remain unexplained.

^ Cf ('95). p. 39.

378 CELL-DIVISION AND DEVELOPMENT

B. Promorphological Relations of Cleavage

The cleavage of the ovum has thus far been considered in the
main as a problem of cell-division. We have now to regard it in an
even more interesting and suggestive aspect ; namely, in its morpho-
logical relations to the body to which it gives rise. From what has
been said above it is evident that cleavage is not merely a process by
which the ^gg simply splits up into indifferent cells which, to use the
phrase of Pfliiger, have no more definite relation to the structure of
the adult body than have snowflakes to the avalanche to which they
contribute.^ It is a remarkable fact that in a very large number of
cases a precise relation exists between the cleavage-products and the
adult parts to which they give rise ; and this relation may often be
traced back to the beginning of development, so that from the first
division onward we are able to predict the exact future of every indi-
vidual cell. In this regard the cleavage of the ovum often goes for-
ward with a wonderful clocklike precision, giving the impression of a
strictly ordered series in which every division plays a definite role and
has a fixed relation to all that precedes and follows it.

But more than this, the apparent predetermination of the embryo
may often be traced still farther back to the regions of the undivided
and even unfertilized ovum. The ^gg^ therefore, may exhibit a
distinct promorphology ; and the morphological aspect of cleavage
must be considered in relation to the promorphology of the ovum of
which it is an expression.

I . Promorphology of the Ovum

(a) Polarity and the Egg-axis. — It was long ago recognized by
von Baer ('34) that the unsegmented ^gg of the frog has a definite
egg-axis connecting two differentiated poles, and that the position
of the embryo is definitely related to it. The great embryologist
pointed out, further, that the early cleavage-planes also are definitely
related to it, the first two passing through it in two meridians inter-
secting each other at a right angle, while the third is transverse to it,
and is hence equatorial.^ Remak afterward recognized the fact ^ that
the larger cells of the lower hemisphere represent, broadly speaking,
the "vegetative layer" of von Baer, ?>. the inner germ-layer or ento-
blast, from which the alimentary organs arise ; while the smaller cells

1 C83), p. 64.

2 The third plane is in this case not precisely at the equator, but considerably above it,
forming a " parallel " cleavage.

2 '55, p. 130. Among others who early laid stress on the importance of the egg-polarity
maybe mentioned Auerbach ('74), Hatschek ('77), Whitman ('78), and Van Beneden ('83).

PROMORPHOLOGICAL RELATIONS OF CLEAVAGE 379

of the upper hemisphere represent the " animal layer," outer germ-
layer or ectoblast from which arise the epidermis, the nervous system,
and the sense-organs. This fact, afterward confirmed in a very large
number of animals, led to the designation of the two poles as animal
and vegetative, formative and nutritive, or protoplasmic and deuto-
plasmic, the latter terms referring to the fact that the nutritive deuto-
plasm is mainly stored in the lower hemisphere, and that development
is therefore more active in the upper. The polarity of the ovum is
accentuated by other correlated phenomena. In every case where
an egg-axis can be determined by the accumulation of deutoplasm in
the lower hemisphere the egg-nucleus sooner or later lies eccentri-
cally in the upper hemisphere, and the polar bodies are formed at the
upper pole. Even in cases where the deutoplasm is equally distrib-
uted or is wanting — if there really be such cases — an egg-axis is
still determined by the eccentricity of the nucleus and the corre-
sponding point at which the polar bodies are formed.

In vastly the greater number of cases the polarity of the ovum has
a definite promorphological significance ; for the egg-axis shows a
definite and constant relation to the axes of the adult body. It
is a very general rule that the upper or ectodermic pole, as marked
by the position of the polar bodies, lies in the median plane at a point
which is afterward found to lie at or near the anterior end. Through-
out the annelids and mollusks, for example, the upper pole is the point
at which the cerebral ganglia are afterward formed; and these
organs lie in the adult on the dorsal side near the anterior extremity.
This relation holds true for many of the Bilateralia, though the
primitive relation is often disguised by asymmetrical growth in the
later stages, such as occur in echinoderms. There is, however, some
reason to believe that it is not a universal rule. The recent observa-
tions of Castle ('96), which are in accordance with the earlier work of
Seeliger, show that in the tunicate Ciona the usual relation is reversed,
the polar bodies being formed at the vegetative {i.e. deutoplasmic or
entodermic) pole, which afterward becomes the dorsal side of the
larva. My own observations ('95) on the echinoderm-egg indicate
that here the primitive egg-axis has an entirely inconstant and casual
relation to the gastrula-axis. It may, however, still be possible to
show that these exceptions are only apparent, and the principle in-
volved is too important to be accepted without further proof.

{b) Axial Relations of the Primary Cleavage- planes. — Since the
egg-axis is definitely related to the embryonic axes, and since the
first two cleavage-planes pass through it, we may naturally look for a
definite relation between these planes and the embryonic axes ; and
if such a relation exists, then the first two or four blastomeres must
likewise have a definite prospective value in the development. Such

38o

CELL-DIVISION AND DEVELOPMENT

relations have, in fact, been accurately determined in a large number
of cases. The first to call attention to such a relation seems to have
been Newport ('54), who discovered the remarkable fact that the first
cleavage-plane in the frog s egg coincides with the median plane of the
adult body ; that, in other words, one of the first two blastomeres
gives rise to the left side of the body, the other to the right. This
discovery, though long overlooked and, indeed, forgotten, was con-
firmed more than thirty years later by Pfl tiger and Roux i^'^j). It

Fig. 177. — Bilateral cleavage of the tunicate ^gg.
A. Four-celled stage of Clavelina, viewed from the ventral side. B. Sixteen-cell stage (VAN
Beneden and JULIN). C Cross-section through the gastrula stage (Castle) ; a. anterior;
p. posterior end; 7. left, r. right side. [Orientation according to Castle.]

was placed beyond all question by a remarkable experiment by Roux
(^S^), who succeeded in killing one of the blastomeres, by puncture
with a heated needle, whereupon the uninjured cell gave rise to a
half-body as if the embryo had been bisected down the middle line
(Fig. 182).

A similar result has been reached in a number of other animals by
following out the cell-lineage ; e.g. by Van Beneden and Julin ('84)

PROMOKPHOLOGICAL RELATIONS OF CLEAVAGE

3^r-

in the ^gg of the tunicate Clavelina (Fig. 177), and by Watase ('91)
in the eggs of cephalopods (Fig. 178). In both these cases all the
early stages of cleavage show a beautiful bilateral symmetry, and not
only can the right and left halves of the segmenting ^gg be distin-
guished with the greatest clearness, but also the anterior and poste-
rior regions, and the dorsal and ventral aspects. These discoveries
seemed, at first, to justify the hope that a fundamental law of develop-
ment had been discovered, and Van Beneden was thus led, as early
as 1883, to express the view that the development of all bilateral
animals would probably be found to agree with the frog and ascidian
in respect to the relations of the first cleavage.

This cleavage was soon proved to have been premature. In one
series of forms, not the first but the second cleavage-plants was found

A

Fig. 178. — Bilateral cleavage of the squid's &g%. [Watase.]
A. Eight-cell stage. B. The fifth cleavage in progress. The first cleavage {a-p) coincides
with the future median plane ; the second {l-r) is transverse.

to coincide with the future long axis {Nereis, and some other annelids ;
Crepidula, Umbrella, and other gasteropods). In another series of
forms neither of the first cleavages passes through the median plane,
but both form an angle of about 45° to it {Clepsine and other leeches ;
RhyncJielmis and other annelids ; Planorbis, Nassa, Unio, and other
mollusks ; Discoccelis and other platodes). In a few cases the first
cleavage departs entirely from the rule, and is equatorial, as in Ascaris
and some other nematodes. The whole subject was finally thrown
into apparent confusion, first by the discovery of Clapp ('91), Jordan,
and Eycleshymer ('94) that in some cases there seems to be no con-
stant relation whatever between the early cleavage-planes and the
adult axes, even in^the same species (teleosts, urodeles); and even in

382

CELL-DIVISION AND DEVELOPMENT

the frog Hertwig showed that the relation described by Newport and
Roux is not invariable. Driesch finally demonstrated that the direc-
tion of the early cleavage-planes might be artificially modified by
pressure without perceptibly affecting the end-result {cf. p. 375).

These facts prove that the promorphology of the early cleavage-
forms can have no fundamental significance. Nevertheless, they are
of the highest interest and importance ; for the fact that the forma-
tive forces by which development is determined may or may not
coincide with those controlling the cleavage, gives us some hope of

/

'V V

Fig. 179. — Outline of unsegmented squid's egg, to show bilaterality. [WATASfi.)
A. From right side. B. From posterior aspect.
a-p. antero-posterior axis ; d-v. dorso-ventral axis ; /. left side ; r. right side.

disentangling the complicated factors of development through a com-
parative study of the different forms.

(<:) Other Promorpho logical Characters of the Ovum. — Besides the
polarity of the ovum, which is the most constant and clearly marked
of its promorphological features, we are often able to discover other
characters that more or less clearly foreshadow the later develop-
ment. One of the most interesting and clearly marked of these is
the bilateral symmetry of the ovum in bilateral animals, which is
sometimes so clearly marked that the exact position of the embryo
may be predicted in the unfertilized ^gg, sometimes even before it is
laid. This is the case, for example, in the cephalopod ^gg, as shown
by Watas^ (Fig. 179). Here the form of the new-laid ^gg, before
cleavage begins, distinctly foreshadows that of the embryonic body,
and forms as it were a mould in which the whole development is cast.
Its general shape is that of a hen's ^gg sHghtly flattened on one side,

PROMORPHOLOGICAL RELATIONS OF CLEAVAGE

~^^

the narrow end, according to Watase, representing the dorsal aspect,
the broad end the ventral aspect, the flattened side the posterior
region, and the more convex side the anterior region. All the early
cleavage- furrows are bilaterally arranged with respect to the plane of

Of

Id*

Fig. 180. — Eggs of the insect Corixa. [Metschnikoff.]
A, Early stage before formation of the embryo, from one side. B. The same viewed in the
plane of symmetry. C. The embryo in its final position,

a. anterior end; p. posterior; /. left side, r. right; z*. ventral, d. dorsal aspect. (These letters
refer to the Jinal position of the embryo, which is nearly diametrically opposite to that in which it
first develops) ; m. micropyle; near/ is the pedicle by which the egg is attached.

symmetry in the undivided egg ; and the same is true of the later
development of all the bilateral parts.

Scarcely less striking is the case of the insect ^gg, as has been
pointed out especially by Hallez, Blochmann, and Wheeler (Figs.
62, 180). In a large number of cases the ^%% is elongated and

384 CELL-DIVISION AND DEVELOPMENT

bilaterally symmetrical, and, according to Blochmann and Wheeler,
may even show a bilateral distribution of the yolk corresponding
with the bilaterality of the ovum. Hallez asserts as the results of a
study of the cockroach {P eviplaneta\ the water-beetle {HydropJiilus\
and the locust {^Locusta) that " the egg-cell possesses the same orien-
tation as the maternal organism that produces it ; it has a cephalic
pole and a caudal pole, a right side and a left, a dorsal aspect and a
ventral ; and these different aspects of the egg-cell coincide with the
corresponding aspects of the embryo." ^ Wheeler ('93), after ex-
amining some thirty different species of insects, reached the same
result, and concluded that even when the ^^^ approaches the
spherical form the symmetry still exists, though obscured. More-
over, according to Hallez ('86) and later writers, the ^gg always lies
in the same position in the oviduct, its cephalic end being turned
forwards toward the upper end of the oviduct, and hence toward
the head-end of the mother.^

2. Meaning of the PromorpJiology of tJie Ovum

The interpretation of the promorphology of the ovum cannot be
adequately treated apart from the general discussion of development
given in the following chapter; nevertheless it may briefly be
considered at this point. Two widely different interpretations of
the facts have been given. On the one hand, it has been sug-
gested by Flemming and Van Beneden,^ and urged especially by
Whitman,* that the cytoplasm of the ovum possesses a definite
primordial organization which exists from the beginning of its exist-
ence even though invisible, and is revealed to observation thidugh
polar differentiation, bilateral symmetry, and other obvious characters
in the unsegmented Qg%. On the other hand, it has been maintained
by Pfliiger, Mark, Oscar Hertwig, Driesch, Watase, and the writer
that all the promorphological features of the ovum are of secondary
origin; that the egg-cytoplasm is at the beginning isotropous — i.e.
indifferent or homaxial — and gradually acquires its promorphological
features during its preembryonic history. Thus the ^gg of a bilateral
animal is at the beginning not actually, but only potentially, bilateral.
Bilaterality once established, however, it forms as it were the mould
in which the cleavage and other operations of development are cast

I believe that the evidence at our command weighs heavily on
the side of the second view, and that the first hypothesis fails to

1 See Wheeler, '93, p. 67.

2 The micropyle usually lies at or near the anterior end, but may be at the posterior.
It is a very important fact that the position of the polar bodies varies, being sometimes at
the anterior end, sometimes on the side, either dorsal or lateral (Heider, Blochmann).

3 See p. 298. * QC pp. 299, 300.

FROMORPHOLOGICAL RELATIONS OF CLEAVAGE

take sufficient account of the fact that development does not nec-
essarily begin with fertilization or cleavage, but may begin at a far
earlier period during ovarian life. As far as the visible promorpho-
logical features of the ovum are concerned, this conclusion is beyond
question. The only question that has any meaning is whether these
visible characters are merely the expression of a more subtle pre-

Fig. i8i. — Variations in the axial relations of the eggs of Cyclops. From sections of the eggs
as they lie in the oviduct. [Hacker.]

A. Group of eggs showing variations in relative position of the polar spindles and the sperm-
nucleus (the latter black) ; in a the sperm-nucleus is opposite to the polar spindle, in b, near it or
at the side. B. Group showing variations in the axis of first cleavage with reference to the polar
bodies (the latter black) ; a, b, and c show three different positions.

existing invisible organization of the same kind. I do not believe
that this question can be answered in the affirmative save by the
trite and, from this point of view, barren statement that every effect
must have its preexisting cause. That the ^gg possesses no fixed
and predetermined cytoplasmic localization with reference to the
adult parts, has, I think, been demonstrated through the remarkable

2C

386 CELL-DIVISION AND DEVELOPMENT

experiments of Driesch, Roux, and Boveri, which show that a frag,
ment of the Q,^<g may give rise to a complete larva (p. 353). There
is strong evidence, moreover, that the egg-axis is not primordial but
is established at a particular period ; and even after its establishment
it may be entirely altered by new conditions. This is proved, for
example, by the case of the frog's ^gg, in which, as Pfliiger ('84),
Born ('85), and Schultze ('94) have shown, the cytoplasmic materials
may be entirely rearranged under the influence of gravity, and a
new axis established. In sea-urchins, my own observations ('95)
render it probable that the egg-axis is not finally established until
after fertilization. These and other facts, to be more fully considered
in the following chapter, give strong ground for the conclusion that
the promorphological features of the ^gg are as truly a result of
development as the characters coming into view at later stages. They
are gradually established during the preembryonic stages, and the
^gg, when ready for fertilization, has already accompHshed part of
its task by laying the basis for what is to come.

Mark, who was one of the first to examine this subject carefully,
concluded that the ovum is at first an indifferent or homaxial cell
{i.e. isotropic), which afterward acquires polarity and other promor-
phological features.^ The same view was very precisely formulated
by Watase in 1891, in the following statement, which I believe to
express accurately the truth : '* It appears to me admissible to say
at present that the ovum, which may start out without any definite
axis at first, may acquire it later, and at the moment ready for its
cleavage the distribution of its protoplasmic substances may be such
as to exhibit a perfect symmetry, and the furrows of cleavage may
have a certain definite relation to the inherent arrangement of the
protoplasmic substances which constitute the ovum. Hence, in a
certain case, the plane of the first cleavage-furrow may coincide with
the plane of the median axis of the embryo, and the sundering of
the protoplasmic material may take place into right and left, accord-
ing to the preexisting organization of the ^gg at the time of cleav-
age ; and in another case the first cleavage may roughly correspond
to the differentiation of the ectoderm and the entoderm, also accord-
ing, to the preorganized constitution of the protoplasmic materials of
the ovum.

*' It does not appear strange, therefore, that we may detect a cer-
tain structural differentiation in the unsegmented ovum, with all the
axes foreshadowed in it, and the axial symmetry of the embryonic
organism identical with that of the adult." ^

This passage contains, I believe, the gist of the whole matter, as
far as the promorphological relations of the ovum and of cleavage-

i'8i, p. 512. 2'9i, p. 280.

PROiMORPHOLOGICAL RELATIONS OF CLEAVAGE -^7^

forms are concerned, though Watase does not enter into the question
as to how the arrangement of protoplasmic materials is effected. In
considering this question, we must hold fast to the fundamental fact
that the ^gg is a cell, like other cells, and that from an a priori point
of view there is every reason to believe that the cytoplasmic differ-
entiations that it undergoes must arise in essentially the same way as
in other cells. We know that such differentiations, whether in form
or in internal structure, show a definite relation to the environment
of the cell — to its fellows, to the source of food, and the like. We
know further, as Korschelt especially has pointed out, that the egg-
axis ^ as expressed by the eccentricity of the germinal vesicle^ often
shoivs a definite relation to the ovarian tissues, the germinal vesicle
lying near the point of attachment or of food-supply. Mark made
the pregnant suggestion, in 1 88 1 , that the primary polarity of the egg-
might be determined by " the topographical relation of the egg (whea
still in an indifferent state) to the remaining cells of the maternal tis-
sue from which it is differentiated,'' and added that this relation might
operate through the nutrition of the ovum. '' It would certainly be
interesting to know if that phase of polar differentiation which is
manifest in the position of the nutritive substance and of the germi-
nal vesicle bears a constant relation to the free surface of the epithe-
lium from which the ^g% takes its origin. If, in cases where the ^gg
is directly developed from epithelial cells, this relationship were
demonstrable, it would be fair to infer the existence of correspond-
ing, though obscured, relations in those cases where (as, for example,
in mammals) the origin of the ovum is less directly traceable to an
epithelial surface." ^ The polarity of the ^gg would therefore be
comparable to the polarity of epithelial or gland-cells, where, as
pointed out at page 57, the nucleus usually lies toward the base of the
cell, near the source of food, while the centrosomes, and often also
characteristic cytoplasmic products, such as zymogen granules and
other secretions, appear in the outer portion.^ The exact conditions
under which the ovarian tgg develops are still too little known to
allow of a positive conclusion regarding Mark's suggestion. More-
over, the force of Korschelt's observation is weakened by the fact that
in many eggs of the extreme telolecithal type, where the polarity is
very marked, the germinal vesicle occupies a central or sub-central
position during the period of yolk-formation and only moves toward
the periphery near the time of maturation.

Indeed, in mollusks, annelids, and many other cases, the germinal
vesicle remains in a central position, surrounded by yolk on all sides,
until the spermatozoon enters. Only then does the egg-nucleus move

1 '81, p. 515-

2 Hatschek has suggested the same comparison {Zoologie, p. 112).

388 CELL-DIVISION AND DEVELOPMENT

to the periphery, the deutoplasm become massed at one pole, and
the polarity of the ^gg come into view {^Nereis, Figs. 60 and 97).^ In
such cases the axis of the ^gg may perhaps be predetermined by
the position of the centrosome, and we have still to seek the causes
by which the position is established in the ovarian history of the ^gg.
These considerations show that this problem is a complex one, involv-
ing, as it does, the whole question of cell-polarity ; and I know of
no more promising field of investigation than the ovarian history of
the ovum with reference to this question. That Mark's view is cor-
rect in principle is indicated by a great array of general evidence
considered in the following chapter, where its bearing on the general
theory of development is more fully dealt with.

C. Cell-division and Growth

The general relations between cell-division and growth, which have
already been briefly considered at page 58 and in the course of this
chapter, may now be more critically examined, together with some
account of the causes that incite or inhibit division. It has been
shown above that every precise inquiry into the rate form, or direc-
tion of cell-division, inevitably merges into the larger problem of the
general determination of growth. We may conveniently approach
this subject by considering first the energy of division and the Hmita-
tion of growth.

All animals and plants have a limit of growth, which is, how-
ever, much more definite in some forms than in others, and differs
in different tissues. During the individual development the energy
of cell-division is most intense in the early stages (cleavage) and
diminishes more and more as the limit of growth is approached.
When the limit is attained a more or less definite equiUbrium is estab-
lished, some of the cells ceasing to divide and perhaps losing this
power altogether (nerve-cells), others dividing only under special con-
ditions (connective tissue-cells, gland-cells, muscle-cells), while others
continue to divide throughout life, and thus replace the worn-out cells
of the same tissue (Malpighian layer of the epidermis, etc.). The
limit of size at which this state of equilibrium is attained is an heredi-
tary character, which in many cases shows an obvious relation to the
environment, and has therefore probably been determined and is
maintained by natural selection. From the cytological point of view
the limit of body-size appears to be correlated with the total number
of cells formed rather than with their individual size. This relation
has been carefully studied by Conklin ('96) in the case of the gastero-

1 The immature egg of Nereis shows, however, a distinct polarity in the arrangement of
the fat-drops, which form a ring in the equatorial regions.

CELL-DIVISION AND GROWTH 389

pod Crepidula, an animal which varies greatly in size in the mature
condition, the dwarfs having in some cases not more than ^^ the vol-
ume of the giants. The eggs are, however, of the same size in all,
and their number is proportional to the size of the adult. The same
is true of the tissue-cells. Measurements of cells from the epidermis,
the kidney, the liver, the alimentary epithelium, and other tissues
show that they are on the whole as large in the dwarfs as in the
giants. The body-size therefore depends on the total number of cells
rather than on their size individually considered, and the same appears
to be the case in plants.^

A result which, broadly speaking, agrees with the foregoing, is
given through the interesting experimental studies of Morgan ('95, i,
'96), supplemented by those of Driesch ('98), in which the number of
cells in normal larvae of echinoderms, ascidians, and Aniphioxiis is
compared with those in dwarf larvae of the same species developed
from egg-fragments (Morgan) and isolated blastomeres (Driesch).
Unless otherwise specified, the following data are cited from Driesch.

The normal blastula of Sphcerechmus possesses about 500 cells
(Morgan), of which from 75 to 90 invaginate to form the archenteron
(Driesch). In half-gastrulas the Clumber varies from 35 to 45, occa-
sionally reaching 50. In the same species, the normal number of
mesenchyme-cells is 54 to 60, in the half-larvae 25 to 30. In Echinus
the corresponding numbers are 30 ± and 13 to 15. In the ascidian
larvae — a particularly favourable object — there are 29 to 35 (excep-
tionally as high as 40) chorda-cells ; in the half-larvae, 1 3 to 17. While
these comparisons are not mathematically precise, owing to the diffi-
culty of selecting exactly equivalent stages, they nevertheless show
that, on the whole, the size of the organ, as of the entire organism, is
directly proportional to the number and not to the size of the cells,
just as in the mature individuals of Crepidnla. The available data
are, however, too scanty to justify any very positive conclusions, and
it is probable that further experiment will disclose factors at present
unknown. It would be highly interesting to determine whether such
dwarf embryos could in the end restore the normal number of cells,
and, hence, the normal size of the body. In all the cases thus far
determined the dwarf gastrulas give rise to larvae {Plutei, etc.) corre-
spondingly dwarfed ; but their later history has not yet been suffi-
ciently followed out.

The gradual diminution of the energy of division during develop-
ment by no means proceeds at a uniform pace in all of the cells, and,
during the cleavage, the individual blastomeres are often found to
exhibit entirely different rhythms of division, periods of active division
being succeeded by long pauses, and sometimes by an entire cessa-

1 See Amelung ('93) and Strasburger ('93).

390 CELL-DIVISION AND DEVELOPMENT

tion of division even at a very early period. In the echinoderms, for
example, it is well established that division suddenly pauses, or changes
its rhythm, just before the gastrulation (in Synapta at the 512-cell
stage, according to Selenka), and the same is said to be the case in
Amphioxus (Hatschek, Lwoff). In Nereis, one of the blastomeres on
each side of the body in the forty-two-cell stage suddenly ceases to
divide, migrates into the interior of the body, and is converted into a
unicellular glandular organ. ^ In the same animal, the four lower cells
(macromeres) of the eight-cell stage divide in nearly regular succes-
sion up to the thirty-eight-cell stage, when a long pause takes place,
and when the divisions are resumed they are of a character totally
different from those of the earlier period. The cells of the ciliated
belt or prototroch in this and other annelids likewise cease to divide
at a certain period, their number remaining fixed thereafter.^ Again,
the number of cells produced for the foundation of particular struc-
tures is often definitely fixed, even when their number is afterward
increased by division. In annelids and gasteropods, for example, the
entire ectoblast arises from twelve micromeres segmented off in three
successive quartets of micromeres from the blastomeres of the four-
cell stage. Perhaps the most interesting numerical relations of this
kind are those recently discovered in the division of teloblasts, where
the number of divisions is directly correlated with the number of seg-
ments or somites. It is well known that this is the case in certain plants
{Characece), where the alternating nodes and intern odes of the stem
are derived from corresponding single cells successively segmented
off from the apical cell. Vejdovsky's observations on the annelid
Dejtdrobmia give strong ground to believe that the number of meta-
merically repeated parts of this animal, and probably of other anne-
lids, corresponds in like manner with that of the number of cells
segmented off from the teloblasts. The most remarkable and accu-
rately determined case of this kind is that of the isopod Crustacea,
where the number of somites is limited and perfectly constant. In
the embryos of these animals there are two groups of teloblasts near
the hinder end of the embryo, viz. an inner group of mesoblasts, from
which arise the mesoblast-bands, and an outer group of ectoblasts,
from which arise the neural plates and the ventral ectoblast. McMur-
rich ('95) has recently demonstrated that the mesoblasts always divide
exactly sixteen times, the ectoblasts thirty-two (or thirty-three) times,
before relinquishing their teleoblastic mode of division and breaking
up into smaller cells. Now the sixteen groups of cells thus formed
give rise to the sixteen respective somites of the post-naupUar region
of the embryo {i.e. from the second maxilla backward). In other

1 This organ, doubtfully identified by me as the head-kidney, is probably a mucus-gland
(Mead). 2(7: Fig. 171.

CELL-DIVISION AND GROWTH 39 1

words, each single division of the mesoblasts and each double division
of the ectoblasts splits off the material for a single somite ! The
number of these divisions, and hence of the corresponding somites,
is a fixed inheritance of the species.

The causes that determine the rhythm of division, and thus finally
establish the adult equilibrium, are but vaguely comprehended. The
ultimate causes must of course lie in the inherited constitution of the
organism, and are referable in the last analysis to the structure of
the germ-cells. Every division must, however, be the response of the
cell to a particular set of conditions or stimuli ; and it is through the
investigation of these stimuli that we may hope to penetrate farther
into the nature of development. The immediate, specific causes of
cell-division are still imperfectly known. In the adult, cells may be
stimulated to divide by the utmost variety of agencies — by chemical
stimulus, as in the formation of galls, or in hyperplasia induced by
the injection of foreign substances into the blood ; by mechanical
pressure, as in the formation of calluses ; by injury, as in the healing
of wounds and in the regeneration of lost parts ; and by a multitude
of more complex physiological and pathological conditions, — by any
agency, in short, that disturbs the normal equilibrium of the body.
In all these cases, however, it is difficult to determine the immediate
stimulus to division ; for a long chain of causes and effects may
intervene between the primary disturbance and the ultimate reaction
of the dividing cells. Thus there is reason to believe that the for-
mation of a callus is not directly caused by pressure or friction, but
through the determination of an increased blood-supply to the part
affected and a heightened nutrition of the cells. Cell-division is here
probably incited by local chemical changes ; and the opinion is gain-
ing ground that the immediate causes of division, whatever their
antecedents, are to be sought in this direction. That such is the
case is indicated by nothing more clearly than the recent experiments
on the Qgg by R. Hertwig, Mead, Morgan, and Loeb already referred
to in part at pages 1 1 1 and 215. The egg-cell is, in most cases, stimu-
lated to divide by the entrance of the spermatozoon, but in partheno-
genesis exactly the same result is produced by an apparently quite
different cause. The experiments in question give, however, ground
for the conclusion that the common element in the two cases is a
chemical stimulus. In the eggs of Ckcetopterus under normal condi-
tions the first polar mitosis pauses at the anaphase until the entrance
of the spermatozoon, when the mitotic activity is resumed and both
polar bodies are formed. Mead ('98) shows, however, that the same
effect may be produced without fertilization by placing the eggs for
a few minutes in a weak solution of potassium chloride. In like
manner R. Hertwig ('96) and Morgan ('99) show that unfertilized

392 CELL-DIVISION AND DEVELOPMENT

echinoderm-eggs may be stimulated to division by treatment with
weak solution of strychnine, sodium-chloride, and other reagents, the
result being here more striking than in the case of Chcetoptenis, since
the entire mitotic system is formed anew under the chemical stimulus.
The climax of these experiments is reached in Loeb's artificial pro-
duction of parthenogenesis in sea-urchin eggs by treatment with dilute
magnesium chloride. Beside these interesting results may be placed
the remarkable facts of gall-formation in plants, which seem to leave
no doubt that extremely complex and characteristic abnormal growths
may result from specific chemical stimuli, and many pathologists
have held that tumours and other pathological growths in the animal
body may be incited through disturbances of circulation or other
causes resulting in abnormal local chemical conditions.^

But while we have gained some light on the immediate causes of
division, we have still to inquire how those, causes are set in opera-
tion and are coordinated toward a typical end ; and we are thus
brought again to the general problem of growth.- A very interesting
suggestion is the resistance-theory of Thiersch and Boll, according
to which each tissue continues to grow up to the limit afforded by
the resistance of neighbouring tissues or organs. The removal or
lessening of this resistance through injury or disease causes a resump-
tion of growth and division, leading either to the regeneration of the
lost parts or to the formation of abnormal growths. Thus the
removal of a salamander's limb would seem to remove a barrier to
the proliferation and growth of the remaining cells. These processes
are therefore resumed, and continue until the normal barrier is re-
established by the regeneration. To speak of such a "barrier" or
''resistance" is, however, to use a highly figurative phrase which is
not to be construed in a rude mechanical sense. There is no doubt
that hypertrophy,- atrophy, or displacement of particular parts often
leads to compensatory changes in the neighbouring parts ; but it is
equally certain that such changes are not a direct mechanical effect
of the disturbance, but a highly complex physiological response to it.
How complex the problem is, is shown by the fact that even closely
related animals may differ widely in this respect. Thus Fraisse has
shown that the salamander may completely regenerate an amputated
limb, while the frog only heals the wound without further regenera-
tion.2 Again, in the case of coelenterates, Loeb and Bickford have
shown that the tubularian hydroids are able to regenerate the ten-
tacles at both ends of a segment of the stem, while the polyp Cerian-
thiis can regenerate them only at the distal end of a section (Fig. 194;.

1 QC p. 97. For a good discussion of this subject, see E. Ziegler, '89.

2 In salamanders regeneration only takes place when the bone is cut across, and does not
occur if the limb be exarticulated and removed at the joint.

CELL-DIVISION AND GROWTH 393

In the latter case, therefore, the body possesses an inherent polarity
which cannot be overturned by external conditions. A very curious
case is that of the earthworm, which has long been known to possess
a high regenerative capacity. If the posterior region of the worm
be cut off, a new tail is usually regenerated. If the same operation
be performed far forward in the anterior region, a new head is often
formed at the front end of the posterior piece. If, however, the sec-
tion be in the middle region the posterior piece sometimes regenerates
a head, but more usually a tail, as was long since shown by Spallanzani
and recently by Morgan ('99). Why such a blunder should be com-
mitted remains for the present quite unexplained.

It remains to inquire more critically' into the nature of the correla-
tion between growth and cell-division. In the growing tissues the
direction of the division-planes in the individual cells evidently stands
in a definite relation with the axes of growth in the body, as is espe-
cially clear in the case of rapidly elongating structures (apical buds,
teloblasts, and the like), where the division-planes are predominantly
transverse to the axis of elongation. Which of these is the primary
factor, the direction of general growth or the direction of the division-
planes } This question is a difficult one to answer, for the two phe-
nomena are often too closely related to be disentangled. As far as
the plants are concerned, however, it has been conclusively shown by
Hofmeister, De Bary, and Sachs that the growth of the mass is the
primary factor ; for the characteristic mode of growth is often shown
by the growing mass before it splits up into cells, and the form of
cell-division adapts itself to that of the mass : " Die Pfianze bildet
Zellen, nicht die Zelle bildet Pflanzen " (De Bary).

Much of the recent work in normal and experimental embryology,
as well as that on regeneration, indicates that the same is true in prin-
ciple of animal growth. Among recent writers who have urged this
view should be mentioned Rauber, Hertwig, Adam Sedgwick, and
especially Whitma'n, whose fine essay on the Inadequacy of the Cell-
theory of Development ('93) marks a distinct advance in our point of
view. Still more recently this view has been almost demonstrated
through some remarkable experiments on regeneration, which show
that definitely formed material, in some cases even the adult tissues,
may be directly moulded into new structures. Driesch has shown
('95> 2, '99) that if gastrulas of Sphcerechinus be bisected through the
equator so that each half contains both ectoderm and entoderm, the
wounds heal, each half forming a typical gastrula, in which the ente-
ron differentiates itself into the three typical regions (fore, middle,
and hind gut) correctly proportioned, though the whole structure is
but half the normal size. Here, therefore, the formative process is in
the main independent of cell-division or increase in size. Miss Bickford

394 CELL-DIVISION AND DEVELOPMENT

('94) found that in the regeneration of decapitated hydranths of tubu-
larians the new hydranth is primarily formed, not by new cell-formation
and growth from the cut end, but by direct transformation of the distal
portion of the stem.^ Morgan's remarkable observations on Planaria,
finally, show that here also, when the animal is cut into pieces, com-
plete animals are produced from these pieces, but only in small degree
through the formation of new tissue, and mainly by direct remould-
ing of the old material into a new body having the correct propor-
tions of the species. As Driesch has well said, it is as if a plan or
mould of the new little worm were first prepared and then the old
material were poured into it.^

Facts of this kind, of which a considerable store has been accumu-
lated, give strong ground for the view that cell-formation is subordi-
nate to growth, or rather to the general formative process of which
growth is an expression ; and they furnish a powerful argument against
Schwann's conception of the organism as a cell-composite (p. 58).
That conception is, however, not to be rejected in toto, but contains a
large element of truth ; for there are many cases in which cells pos-
sess so high a degree of independence that profound modifications
may occur in special regions through injury or disease, without affect-
ing the general equilibrium of the body. The most striking proof of
this lies in the fact that grafts or transplanted structures may perfectly
retain their specific character, though transferred to a different region
of the body, or even to another species. Nevertheless the facts of
regeneration prove that even in the adult the formative processes in
special parts are in many cases definitely correlated with the organi-
zation of the entire mass ; and there is reason to conclude that such
a correlation is a survival, in the adult, of a condition characteristic
of the embryonic stages, and that the independence of special parts
in the adult is a secondary result of development. The study of cell-
division thus brings us finally to a general consideration of develop-
ment which forms the subject of the following chapter.

LITERATURE. VIII

Berthold, G. — Studien liber Protoplasma-mechanik. Leipzig, 1886.

Boll, Fr. — Das Princip des Wachsthums. Berlin, 1876.

Bourne, G. C. — A Criticism of the Cell-theory ; being an answer to Mr. Sedgwick's

article on the Inadequacy of the Cellular Theory of Development : Quart. Journ.

Mic. Sci., XXXVIII. I. 1895.

1 Driesch suggests for such a process the term reparation in contradistinction to true
regeneration.

2 '99, p. 55. It is mainly on these considerations that Driesch ('99) has built his recent
theory of vitalism (r/C p. 417), the nature of the formative power being regarded as a
problem sui generis, and one which the "machine-theory of life " is powerless to solve. Cf.
also the views of Whitman, p. 416.

LITERA TURE 395

Castle, W. E. — The early Embryology of Ciona. Bull. Mus. Comp. ZooL, XXVII.

1896.
Conklin, E. G. — The Embryology of Crepidula: Journ. Morph., XIII. 1897.
Driesch, H. — (See Literature, IX.)
Errera, L. — Zellformen und Seifenblasen : Tagebl. der 60 Versaininlimg deutscher

Naturforscher und Aerzte zu Wiesbaden. 1887.
Hertwig, 0. — Das Problem der Befruchtung und der Isotropie des Eies, eine Theo-

rie der Vererbung. Jena., 1884.
Hofmeister. — Die Lehre von der Pflanzenzelle. Leipzig., 1867.
Jennings, H. S. — The Early Development of Asplanchna : Bull. Mus. Comp. Zo'dl.,

XXX. I. Cambridge, 1896.
Kofoid, C. A. — On the Early Development of Limax : Btill. Mus. Comp. Zo'dl.,

XXVII. 1895.
Lillie, F. R. —The Embryology of the Unionidae : Journ. Morph., X. 1895.
Id. — Adaptation in Cleavage : Wood's Holl Biol. Lectures. 1899.
McMurrich, J. P. — Embryology of the Isopod Crustacea: Journ. Morpk., X\. i.

1895.
Mark, E. L. —Limax. (See list IV.)
Morgan, T. H. — (See Literature, IX.)
Rauber, A. — Neue Grundlegungen zur Kenntniss der Zelle: Morph. Jahrb., VIII.

1883.
Rhumbler, L. — Allgemeine Zellmechanik : Merkel u. Botmet, Ergeb.,Wl\l. 1898.
Sachs, J. — Pflanzenphysiologie. (See list VII.)
Sedgwick, H. — On the Inadequacy of the Cellular Theory of Development, etc.:

Quart. Journ. Mic. Sci., XXXVII. i. 1894.
Strasburger, E. — tJber die Wirkungssphare der Kerne und die Zellgrosse : /^zV/^-

logische Beitrdge, V . 1 893 .
Zur Strassen, 0. — Embryonalentwickelung der Ascaris : Arch. Entom., III. 1896.
Watas6, S. — Studies on Cephalopods ; I., Cleavage of the Ovum : Journ. Morph.,

IV. 3. 1891.
Whitman, C. 0. — The Inadequacy of the Cell-theory of Development : Wood^s Holl

Biol. Lectures. 1893.
Wilson, Edm. B. — The Cell-lineage oi Nereis: Journ. Morph., VI. 3. 1892.*
Id. — Amphioxus and the Mosaic Theory of Development : Journ. Morph., VIII. 3.

1893.
Id. — Considerations on Cell-lineage and Ancestral Reminiscence: Ann. N. V.

Acad., XI. 1898; also Wood^s Holl Biol. Lectures, 1899.

CHAPTER IX

THEORIES OF INHERITANCE AND DEVELOPMENT

" It is certain that the germ is not merely a body in which life is dormant or potential,
but that it is itself simply a detached portion of the substance of a preexisting living body."

HUXLEY.I

" Inheritance must be looked at as merely a form of growth." Darwin.^

" Ich mochte daher wohl den Versuch wagen, durch eine Darstellung des Beobachteten
Sie zu einer tiefern Einsicht in die Zeugungs- und Entwickelungsgeschichte der organischen
Korper zu fiihren und zu zeigen, wie dieselben weder vorgebildet sind, noch auch, wie man
sich gewohnUch denkt, aus ungeformter Masse in einem bestimmten Momente plotzlich
ausschiessen." VON Baer.^

Every discussion of inheritance and development must take as its
point of departure the fact that the germ is a single cell similar in its
essential nature to any one of the tissue-cells of which the body is
composed. That a cell can carry with it the sum total of the heritage
of the species, that it can in the course of a few days or weeks give
rise to a mollusk or a man, is the greatest marvel of biological science.
In attempting to analyze the problems that it involves, we must from
the outset hold fast to the fact, on which Huxley insisted, that the
wonderful formative energy of the germ is not impressed upon it
from" without, but is inherent in the egg as a heritage from the paren-
tal life of which it was originally a part. The development of the
embryo is nothing new. It involves no breach of continuity, and is
but a continuation of the vital processes going on in the parental
body. What gives development its marvellous character is the rapid-
ity with which it proceeds and the diversity of the results attained in
a span so brief.

But when we have grasped this cardinal fact, we have but focussed
our instruments for a study of the real problem. How do the adult
characteristics lie latent in the germ-cell ; and how do they become
patent as development proceeds .'' This is the final question that looms
in the background of every investigation of the cell. In approaching
it we may well make a frank confession of ignorance ; for in spite of
all that the microscope has revealed, we have not yet penetrated the
mystery, and inheritance and development still remain in their fun-

1 Evolution, Science and Culture, p. 291.

2 Variation of Animals and Plants, II., p. 398,

3 Entwick, der Thiere, IL, 1837, p. 8.

396

THE THEORY OF GERMINAL LOCALIZATION

damental aspects as great a riddle as they were to the Greeks. What
we have gained is a tolerably precise acquaintance with the external
aspects of development. The gross errors of the early preformation-
ists have been dispelled.^ We know that the germ-cell contains no
predelineated embryo ; that development is manifested, on the one
hand, by the cleavage of the ^gg, on the other hand, by a process of
differentiation, through which the products of cleavage gradually
assume diverse forms and functions, and so accomplish a physiological
division of labour. We can clearly recognize the fact that these pro-
cesses fall in the same category as those that take place in the tissue-
cells ; for the cleavage of the ovum is a form of mitotic cell-division,
while, as many eminent naturalists have perceived, differentiation is
nearly related to growth and has its root in the phenomena of nutri-
tion and metabolism. The real problem of development is the orderly
sequence and correlation of these phe7iomena toward a typical result.
We cannot escape the conclusion that this is the outcome of the
organization of the germ-cells ; but the nature of that which, for lack
of a better term, we call " organization," is and doubtless long will
remain almost wholly in the dark.

In the following discussion, which is necessarily compressed within
narrow limits, we shall disregard the earlier baseless speculations,
such as those of the seventeenth and eighteenth centuries, which
attempted a merely formal solution of the problem, confining our-
selves to more recent discussions that have grown directly out of
modern research. An introduction to the general subject may be
given by a preliminary examination of two central hypotheses about
which most recent discussions have revolved. These are, first, the
theory of Germinal Localization ^ of Wilhelm His ('74), and, second,
the Idioplasm Hypothesis of Nageli ('84). The relation between these
two conceptions, close as it is, is not at first sight very apparent ;
and for the purpose of a preliminary sketch they may best be con-
sidered separately.

A. The Theory of Germinal Localization

Although the naive early theory of preformation and evolution was
long since abandoned, yet we find an after-image of it in the theory
of germinal localization which in one form or another has been advo-
cated by some of the foremost students of development. It is main-
tained that, although the embryo is not preformed in the germ, it must
nevertheless be ^^x^determined in the sense that the ^gg contains

1 Cf. Introduction, p. 8.

2 I venture to suggest this term as an English equivalent for the awkward exjKession
*' Organbildende Keimbezirke " of His.

398 INHERITANCE AND DEVELOPMENT

definite areas or definite substances predestined for the formation of
corresponding parts of the embryonic body. The first clear state-
ment of this conception is found in the interesting and suggestive
work of Wilhehn His ('74) entitled Unsere Korperfonn. Considering
the development of the chick, he says : ** It is clear, on the one hand,
that every point in the embryonic region of the blastoderm must rep-
resent a later organ or part of an organ, and, on the other hand, that
every organ developed from the blastoderm has its preformed germ
(vorgebildete Anlage) in a definitely located region of the flat germ-
disc, . . . The material of the germ is already present in the flat
germ-disc, but is not yet morphologically marked off and hence
not directly recognizable. But by following the development back-
wards we may determine the location of every such germ, even at a
period when the morphological differentiation is incomplete or before
it occurs ; logically, indeed, we must extend this process back to the
fertilized or even the unfertilized ^gg. According to this principle,
the germ-disc contains the organ-germs spread out in a flat plate, and,
conversely, every point 6i the germ-disc reappears in a later organ ;
I call this the principle of 07'gan- forming germ-regions.'' ^ His thus
conceived the embryo, not as ^rtf armed, but as having all of its parts
T^XQlocalized in the egg-protoplasm (cytoplasm).

A great impulse to this conception was given during the follow-
ing decade by discoveries relating, on the one hand, to protoplasmic
structure, on the other hand, to the promorphological relations of the
ovum. Ray Lankester writes, in 1877: ''Though the substance of a
celP may appear homogeneous under the most powerful microscope,
it is quite possible, indeed certain, that it may contain, already formed
and individualized, various kinds of physiological molecules. The
visible process of segregation is only the sequel of a differentiation
already established, and not visible."^ The egg-cytoplasm has a defi-
nite molecular organization directly handed down from the parent;,
cleavage sunders the various " physiological molecules " and iso-
lates them in particular cells. Whitman expresses a similar thought
in the following year : " While we cannot say that the embryo is pre-
delineated, we can say that it is predetermined. The ' histogenetic
sundering ' of embryonic elements begins with the cleavage, and every
step in the process bears a definite and invariable relation to antece-
dent and subsequent steps. ... It is, therefore, not surprising to
find certain important histological differentiations and fundamental
structural relations anticipated in the early phases of cleavage, and
foreshadowed even before cleavage begins." ^ It was, however, Flem-

1 /. f., p. 19.

2 It is clear from the context that by " substance " Lankester had in mind the cytoplasm,,
though this is not specifically stated. ^ '77, p. 14. * J^g^ p, ^g.

THE THEORY OF GERMINAL LOCALIZATION^^ 399

ming who gave the first specific statement of the matter from the cyto-
logical point of view : " But if the substance of the egg-cell has a
definite structure (Bau), and if this structure and the nature of the
network varies in different regions of the cell-body, we may seek in
it a basis for the predetermination of development wherein one ^g^
differs from another, and it will be possible to look for it with the
microscope. How far this search can be carried no one can say, but
its ultimate aim is nothing less than a true morphology of inheritance T ^
In the following year Van Beneden pointed out how nearly this con-
ception approaches to a theory of preformation : " If this were the
case {i.e. if the egg-axis coincided with the principal axis of the adult
body), the old theory of evolution would not be as baseless as we
think to-day. The fact that in the ascidians, and probably in other
bilateral animals, the median plane of the body of the future ani-
mal is marked out from the beginning of cleavage, fully justifies the
hypothesis that the materials destined to form the right side of the
body are situated in one of the lateral hemispheres of the ^gg, while
the left hemisphere gives rise to all of the organs of the left half."^

The hypothesis thus suggested seemed, for a time, to be placed on
a secure basis of fact through a remarkable experiment subsequently
performed by Roux {^%%) on the frog's ^gg. On killing one of the
blastomeres of the two-cell stage by means of a heated needle the un-
injured half developed in some cases into a well-formed half -larva
(Fig. 182), representing approximately the right or left half of the
body, containing one medullary fold, one auditory pit, etc.^ Analo-
gous, though less complete, results were obtained by operating with
the four-cell stage. Roux was thus led to the declaration (made with
certain subsequent reservations) that " the development of the frog-
gastrula and of the embryo formed from it is from the second cleavage
onward a mosaic-work, consisting of at least four vertical indepen-
dently developing pieces." * This conclusion seemed to form a very
strong support to His's theory of germinal localization, though, as
will appear beyond, Roux transferred this theory to the nucleus, and
thus developed it in a very different direction from Lankester or
Van Beneden. His's theory also received very strong apparent sup-
port through investigations on cell-lineage by Whitman, Rabl, and

1 Zellsubstanz, '82, p, 70 : the italics are in the original.

^ '83, p. 571.

* The accuracy of this result was disputed by Oscar Hertwig ('93, i), who found that the
uninjured blastomere gave rise to a defective larva, in which certain parts were missing, but
not to a true half-body. Later observers, especially Schultze, Endres, and Morgan, have,
however, shown that both Hertwig and Roux were right, proving that the uninjured blasto-
mere may give rise to a true half-larva, to a larva with irregular defects, or to a whole
larva of half-size, according to circumstances (p. 422).

4 I.e., p. 30.

400

INHERITANCE AND DEVELOPMENT

many later observers, which have shown that in the cleavage of anne^
lids, mollusks, platodes, tunicates, and many other animals, every cell
has a definite origin and fate, and plays a definite part in the building
of the body.i

Fig. 182. — Half-embryos of the frog (in transverse section) arising from a blastomere of the
two-cell stage after killing the other blastomere. [ROUX.]

A. Half-blastula (dead blastomere on the left). B. Later stage. C. Half-tadpole with one
medullary fold and one mesoblast plate ; regeneration of the missing (right) half in process.

ar. archenteric cavity ; c.c. cleavage-cavity ; ch. notochord ; m.f. medullary fold ; m,s. meso-
blast-plate.

In an able series of later works Whitman has followed out the sug-
gestion made in his paper of 1878, cited above, pointing out how
essential a part is playe9 in development by the cytoplasm and insist-
ing that cytoplasmic preorganization must be regarded as a leading
factor in the ontogeny. Whitman's interesting and suggestive views
are expressed with great caution and with a full recognition of the

1 Cf. p. 378.

THE IDIOPLASM THEORY

difficulty and complexity of the problem. From his latest essay, in-
deed ('94), it is not easy to gather his precise position regarding the
theory of cytoplasmic localization. Through all his writings, never-
theless, runs the leading idea that the germ is definitely organized
before development begins, and that cleavage only reveals an organi-
zation that exists from the beginning. " That organization precedes
cell-formation and regulates it, rather than the reverse, is a conclu-
sion that forces itself upon us from many sides." ^ " The organism
exists before cleavage sets in, and persists throughout every stage of
cell-multiplication." ^

All of these views, excepting those of Roux, lean more or less
distinctly toward the conclusion that the cytoplasm of the egg-cell
is from the first mapped out, as it were, into regions which corre-
spond with the parts of the future embryonic body. The cleavage
of the ovum does not create these regions, but only reveals them to
view by marking off their boundaries. Their topographical arrange-
ment in the t,gg does not necessarily coincide with that of the adult
parts, but only involves the latter as a necessary consequence — some-
what as a picture in the kaleidoscope gives rise to a succeeding pic-
ture composed of the same parts in a different arrangement. The
germinal localization may, however, in a greater or less degree, fore-
shadow the arrangement of adult parts — for instance, in the ^gg of
the tunicate or cephalopod, where the bilateral symmetry and antero-
posterior differentiation of the adult is foreshadowed not only in the
cleavage stages, but even in the un segmented ^gg.

By another set of writers, such as Roux, De Vries, Hertwig, and
Weismann, germinal localization is primarily sought not in the cyto-
plasm, but in the nucleus ; but these views can be best considered
after a review of the idioplasm hypothesis, to which we now proceed.

B. The Idioplasm Theory

We owe to Nageli the first systematic attempt to discuss heredity
regarded as inherent in a definite physical basis ;^ but it is hardly
necessary to point out his great debt to earlier writers, foremost
among them Darwin, Herbert Spencer, and Hackel. The essence of
Nageli's hypothesis was the assumption that inheritance is effected
by the transmission not of a cell, considered as a whole, but of a par-
ticular substance, the idioplasm, contained within a cell, and forming
the physical basis of heredity. The idioplasm, is to be sharply dis-
tinguished from the other constituents of the cell, which play no
direct part in inheritance and form a " nutritive plasma " or tropho-

1 '93, p. 115. "^Lc.y p. 112. 3 Xheorie der Abstammungslehre, 1884.

2D

409. INHERITANCE AND DEVELOPMENT

plasm. Hereditary traits are the outcome of a definite molecular
organization of the idioplasm. The hen's Q,gg differs from the frog's
because it contains a different idioplasm. The species is as com-
pletely contained in the one as in the other, and the hen's ^%g differs
from a frog's o^gg as widely as a hen from a frog.

The idioplasm was conceived as an extremely complex substance,
consisting of elementary complexes of molecules known as micellce.
These are variously grouped to form units of higher orders, which,
as development proceeds, determine the development of the adult
cells, tissues, and organs. The specific peculiarities of the idioplasm
are therefore due to the arrangement of the micellae ; and this, in its
turn, is owing to dynanjic properties of the micellae themselves.
During development the idioplasm undergoes a progressive trans-
formation of its substance, not through any material change, but
through dynamic alterations of the conditions of tension and move-
ment of the micellae. These changes in the idioplasm cause reactions
on the part of surrounding structures leading to definite chemical and
plastic changes, i.e. to differentiation and development.

Nageli made no attempt to locate the idioplasm precisely or to
identify it with any of the known morphological constituents of the
cell. It was somewhat vaguely conceived as a network extending
through both nucleus and cytoplasm, and from cell to cell through-
out the entire organism. Almost immediately after the publication
of his theory, however, several of the foremost leaders of biological
investigation were led to locate the idioplasm in the nucleus, and
concluded that it is to be identified with chromatin. The grounds
for this conclusion, which have already been stated in Chapter VII.,
may be here again briefly reviewed. The beautiful experiments
of Nussbaum, Gruber, and Verworn proved that the regeneration
of differentiated cytoplasmic structures in the Protozoa can only
take place when nuclear matter is present (cf. p. 342). The study of
fertilization by Hertwig, Strasburger, and Van Beneden proved that
in the sexual reproduction of both plants and animals the nucleus of
the germ is equally derived from both sexes, while the cytoplasm is
derived almost entirely from the female. The two germ-nuclei, which
by their union give rise to that of the germ, were shown by Van
Beneden to be of exactly the same morphological nature, since each
gives rise to chromosonies of the same number, form, and size. Van
Beneden and Boveri proved (p. 182) that the paternal and maternal
nuclear substances are equally distributed to each of the first two
cells, and the more recent work of Hacker, Riickert, Herla, and
Zoja establishes a strong probability that this equal distribution con-
tinues in the later divisions. Roux pointed out the telling fact that
the entire complicated mechanism of mitosis seems designed to affect

UNION OF THE TWO THE ORIRS _^403

the most accurate division of the entire nuclear substance in all of
its parts, while fission of the cytoplasmic cell-body is in the main
a mass-division, and not a meristic division of the individual parts.
Again, the complicated processes of maturation show the significant
fact that while the greatest pains is taken to prepare the germ-nuclei
for their coming union, by rendering them exactly equivalent, the
cytoplasm becomes widely different in the two germ-cells and is
devoted to entirely different functions.

It was in the main these considerations that led Hertwig, Stras-
burger, Kolliker, and Weismann independently and almost simultane-
ously to the conclusion that the nucleus contains the physical basis of
inheritance, and that chromatin, its essential constituent, is the idio-
plasm postulated in NdgeWs theory. This conclusion is now widely
accepted and rests upon a basis so firm that it must be regarded as a
working hypothesis of high value. To accept it is, however, to reject
the theory of germinal localization in so far as it assumes a prelocali-
zation of the egg-cytoplasm as a fundamental character of the ^gg.
For if the specific character of the organism be determined by an
idioplasm contained in the chromatin, then every characteristic of the
cytoplasm must in the long run be determined from the same source.
A striking illustration of this point is given by the phenomena of
colour-inheritance in plant-hybrids, as De Vries has pointed out.
Pigment is developed in the embryonic cytoplasm, which is derived
from the mother-cell ; yet in hybrids it may be inherited from the
male through the nucleus of the germ-cell. The specific form of
cytoplasmic metabolism by which the pigment is formed must there-
fore be determined by the paternal chromatin in the germ-nucleus,
and not by a predetermination of the egg-cytoplasm.

C. Union of the Two Theories

We have now to consider the attempts that have been made to
transfer the localization-theory from the cytoplasm to the nucleus,
and thus to bring it into harmony with the theory of nuclear idio-
plasm. These attempts are especially associated with the names of
Roux, De Vries, Weismann, and Hertwig ; but all of them may be
traced back to Darwin's celebrated hypothesis of pangenesis as a
prototype. This hypothesis is so well known as to require but a
brief review. Its fundamental postulate assumes that the germ-cells
contain innumerable ultra-microscopic organized bodies or gemmuleSy
each of which is the germ of a cell and determines the development
of a similar cell during the ontogeny. The germ-cell is, therefore,
in Darwin's words, a microcosm formed of a host of inconceivably
minute self-propagating organisms, every one of which predetermines

404

INHERITANCE AND DEVELOPMENT

the formation of one of the adult cells. De Vries ('89) brought this
conception into relation with the theory of nuclear idioplasm by
assuming that the gemmules of Darwin, which he called pangens, are
contained in the nucleus, migrating thence into the cytoplasm step
by step during ontogeny, and thus determining the successive stages
of development. The hypothesis is further modified by the assump-
tion that the pangens are not cell-germs, as Darwin assumed, but
ultimate protoplasmic units of which cells are built, and which are
the bearers of particular hereditary qualities. The same view was
afterward accepted by Hertwig and Weismann.^

The theory of germinal localization is thus transferred from the
cytoplasm to the nucleus. It is not denied that the egg-cytoplasm
may be more or less distinctly differentiated into regions that have a
constant relation to the parts of the embryo. This differentiation is,
however, conceived, not as a primordial characteristic of the ^gg, but
as one secondarily determined through the influence of the nucleus.
Both De Vries and Weismann assume, in fact, that the entire cyto-
plasm is a product of the nucleus, being composed of pangens that
migrate out from the latter, and by their active growth and multipli-
cation build up the cytoplasmic substance.^

D. The Roux-Weismann Theory of Development

We now proceed to an examination of two sharply opposing hy-
potheses of development based on the theory of nuclear idioplasm.
One of these originated with Roux ('83) and has been elaborated
especially by Weismann. The other was clearly outlined by De Vries
('89), and has been developed in various directions by Oscar Hertwig,
Driesch, and other writers. In discussing them, it should be borne
in mind that, although both have been especially developed by the
advocates of the pangen-hypothesis, neither necessarily involves that
hypothesis in its strict form, i.e. the postulate of discrete self-propa-
gating units in the idioplasm. This hypothesis may therefore be laid

1 QC p. 290.

2 The neo-pangenesis of De Vries dififers from Darwin's hypothesis in one very important
respect. Darwin assumed that the gemmules arose in the body, being thrown off as germs
by the individual tissue-cells, transported to the germ-cells, and there accumulated as in a
reservoir; and he thus endeavoured to explain the transmission of acquired characters. De
Vries, on the other hand, denies such a transportal from cell to cell, maintaining that the
pangens arise or preexist in the germ-cell, and those of the tissue-cells are derived from this
source by cell-division.

^ This conception obviously harmonizes with the role of the nucleus in the synthetic
process. In accepting the view that the nuclear control of the cell is effected by an emana-
tion of specihc substances from the nucleus, we need not, however, necessarily adopt the
pangen-hypothesis.

THE ROUX-WEISMANN THEORY OF DEVELOPMENT 405

aside as an open question/ and will be considered only in so far as it
is necessary to a presentation of the views of individual writers.

The Roux-Weismann hypothesis has already been touched on at
page 245. Roux conceived the idioplasm {i.e. the chromatin) not as a
single chemical compound or a homogeneous mass of molecules, but
as a highly complex mixture of different substances, representing
different qualities, and having their seat in the individual chromatin-
granules. In mitosis these become arranged in a linear series to
form the spireme-thread, and hence may be precisely divided by the
splitting of the thread. Roux assumes, as a fundamental postulate^
that division of the granules may be either quantitative or qualitative.
In the first mode the group of qualities represented in the mother-
granule is first doubled and then split into equivalent daughter-groups,
the daughter-cells therefore receiving the same qualities and remain-
ing of the same nature. In "qualitative division," on the other hand,,
the mother-group of qualities is split into dissimilar groups, which,
passing into the respective daughter-nuclei, lead to a corresponding
differentiation in the danghter-cells. By qualitative divisions, occur-
ring in a fixed and predetermined order, the idioplasm is thus split
up during ontogeny into its constituent qualities, which are, as it were,
sifted apart and distributed to the various nuclei of the embryo.
Every cell-nncleus, therefore , receives a specific form of chromatin which
determines the nature of the cell at a given period and its later his-
tory. Every cell is thus endowed with a power of self-determination^
which lies in the specific structure of its nucleus, and its course of
development is only in a minor degree capable of modification through
the relation of the cell to its fellows (''correlative differentiation ").

Roux's hypothesis, be it observed, does not commit him to the
theory of pangenesis. It was reserved for Weismann to develop the
hypothesis of qualitative division in terms of the pangen-hypothesis,
and to elaborate it as a complete theory of development. In his
first essay ('85), published before De Vries's paper, he went no far-
ther than Roux. " I believe that we must accept the hypothesis that
in indirect nuclear division, the formation of non-equivalent halves
may take place quite as readily as the formation of equivalent halves,
and that the equivalence or non-equivalence of the subsequently pro-
duced daughter-cells must depend upon that of the nuclei. Thus,
during ontogeny a gradual transformation of the nuclear substance
takes place, . necessarily imposed upon it, according to certain laws,
by its own nature, and such transformation is accompanied by a
gradual change in the character of the cell-bodies." ^ In later writ-
ings Weismann advanced far beyond this, building up an elaborate
artificial system, which appears in its final form in the remarkable

1 Cf. Chapter VI. 2 Essay IV., p. 193, 1885.

406 INHERITANCE AND DEVELOPMENT

book on the germ-plasm ('92). Accepting De Vries's conception of
the pangens, he assumes a definite grouping of these bodies in the
germ-plasm or idioplasm (chromatin), somewhat as in Nageli's concep-
tion. The pangens or biophores are conceived to be successively ag-
gregated in larger and larger groups; namely, {\) determinants, which
are still beyond the limits of microscopical vision ; (2) ids, which are
identified with the visible chromatin-granules ; and (3) idants, or
chromosomes. The chromatin has, therefore, a highly complex fixed
architecture, which is transmitted from generation to generation, and
determines the development of the embryo in a definite and specific
manner. Mitotic division is conceived as an apparatus which may
distribute the elements of the chromatin to the daughter-nuclei either
equally or unequally. In the former cdiSO, {'^ homceokinesis,'' integral
or quantitative division), the resulting nuclei remain precisely equiva-
lent. In the second case (" heterokinesis,'' qualitative or dijferential
division), the daughter-cells receive different groups of chromatin-
elements, and hence become differently modified. During ontogeny,
through successive qualitative divisions, the elements of the idioplasm
or germ-plas7n (chromatin) are gradually sifted apart, and distributed
in a definite and predetermined manner to the various parts of the
body. " Ontogeny depends on a gradual process of disintegration of
the id of germ-plasm, which splits into smaller and smaller groups of
determinants in the development of each individual. . . . Finally,
if we neglect possible complications, only one kind of determinant re-
mains in each cell, viz. that which has to control that particular cell or
group of cells. ... In this cell it breaks up into its constituent bio-
phores, and gives the cell its inherited specific character."^ Devel-
opment is, therefore, essentially evolutionary and not epigenetic ; ^ its
point of departure is a substance in which all of the adult characters
are represented by preformed, prearranged germs ; its course is the
result of a predetermined harmony in the succession of the qualitative
divisions by which the hereditary substance is progressively disinte-
grated. In order to account for heredity through successive genera-
tions, Weismann is obliged to assume that, by means of quantitative
or integral division, a certain part of the original germ-plasm is car-
ried on unchanged, and is finally delivered, with its original architecture
unaltered, to the germ-nuclei. The power of regeneration is explained,
in like manner, as the result of a transmission of unmodified or sHghtly
modified germ-plasm to those parts capable of regeneration.

1 Germ-plasm, pp. 76, 77. 2 /^^^ p^ j^^

CRITIQUE OF THE ROUX-WEISMANN THEORY

407

E. Critique of the Roux-Weismann Theory

It is impossible not to admire the thoroughness, candour, and logical
skill with which Weismann has developed his theory, or to deny that,
in its final form, it does afford up to a certain point a formal solution
of the problems with which it deals. Its fundamental weakness is its
^^^j^-metaphysical character, which, indeed, almost places it outside

C D

Fig, 183. — Half and whole cleavage in the eggs of sea-urchins.

A. Normal sixteen-cell stage, showing the four micromeres above (from Driesch, after Selenka).
B. Half sixteen-cell stage developed from one blastomere of the two-cell stage after killing the other
by shaking (Driesch). C. Half blastula resulting, the dead blastomere at the right (Driesch).
D. Half-sized sixteen-cell stage of Toxopneustes, viewed from the micromere-pole (the eight lower
not shown). This embryo, developed from an isolated blastomere of the two-cell stage, segmented
like an entire normal ovum.

the sphere of legitimate scientific hypothesis. Save in the maturation
of the germ-cells ("reducing divisions"), none of the visible phenom-
ena of cell-division give even a remote suggestion of qualitative divi-
sion. All the facts of ordinary mitosis, on the contrary, indicate that
the division of the chromatin is carried out with the most exact equality.

4o8

INHERITANCE AND DEVELOPMENT

The hypothesis mainly rests upon a quite different order of phenom-
ena, namely, on facts indicating that isolated blastomeres, or other
cells, have a certain power of self-determination, or " self-differentia-
tion" (Roux), peculiar to themselves, and which is assumed to be pri-
marily due to the specific quality of the nuclei. This assumption,
which may or may not be true,^ is itself based upon the further assump-
tion of qualitative nuclear division of which we actually know nothing
whatever. The fundamental hypothesis is thus of purely a priori
character; and every fact opposed to it has been met by subsidi-

A

Fig. 184. — Normal and dwarf gastrulas of.
A. Normal gastrula. B. Half-sized dwarf, from an isolated blastomere of the two-cell stage.
C. Quarter-sized dwarf, from an isolated blastomere of the four-cell stage.

ary hypotheses, which, like their principal, relate to matters beyond
the reach of observation.

Such an hypothesis cannot be actually overturned by a direct
appeal to fact. We can, however, make an indirect appeal, the
results of which show that the hypothesis of qualitative division is
not only so improbable as to lose all semblance of reality, but is in
fact quite superfluous. It is rather remarkable that Roux himself
led the way in this direction. In the course of his observations on
the development of a half-embryo from one of the blastomeres of
the two-cell stage of the frog's ^gg, he determined the significant
fact that the half -embryo in the end restores more or less completely

1 Cf. p. 426.

CRITIQUE OF THE ROUX-WEISMANN THEORY

409

the missing half by a peculiar process, related to regeneration, which
Roux designated as post-generation. Later studies showed that an
isolated blastomere is able to give rise to a complete embryo in many
other animals, sometimes developing in its earlier stages as though

Fig. 185. — Dwarf and double embryos oi Amphioxus.
A. Isolated blastomere of the two-cell stage segmenting like an entire ^%,g {cf. Fig. 183, D).
B. Twin gastrulas from a single egg. C Double cleavage resulting from the partial separation,
by shaking, of the blastomeres of the two-cell stage, D.E.F. Double gastrulas arising from such
terms as the last.

Still forming part of a complete embryo ("partial development"),
but in other cases developing directly into a complete dwarf embryo,
as if it were an egg of diminished size. In 1891 Driesch was able
to . follow out the development of isolated blastomeres of sea-urchin

410 INHERITANCE AND DEVELOPMENT

eggs separated by shaking to pieces the two-cell and four-cell stages.
Blastomeres thus isolated segment as if still forming part of an entire
larva, and give rise to a half- (or quarter-) blastula (Fig. 183). The
opening soon closes, however, to form a small complete blastula, and
the resulting gastrula and Pluteus larva is a perfectly formed dwarf
of only half (or quarter) the normal size. Incompletely separated
blastomeres give rise to double embryos like the Siamese twins.
Shortly afterward the writer obtained similar results in the case of
AmphioxuSy but here the isolated blastomere behaves from the begin-
ning like a complete ovum of half the tisual size, and gives rise to a
complete blastula, gastrula, and larva. Complete embryos have also
been obtained from a single blastomere in the teleost Fundtdus
(Morgan, '95, 2), in Triton (Herlitzka, '95), and in a number of
hydromedusae (Zoja, '95, Bunting, '99); and nearly complete em-
bryos in the tunicates Ascidiella (Chabry, '^J^ Phallusia (Driesch,
'94), and Molgula (Crampton, '98).^ Perhaps the most striking of
these cases is that of the hydroid Clytia, in which Zoja was able to
obtain perfect embryos, not only fronv the blastomeres of the two-
cell and four-cell stages, but from eight-cell and even from sixteen-
cell stages, the dwarfs in the last case being but one-sixteenth the
normal size.

These experiments render highly improbable the hypothesis of
qualitative division in its strict form, for they demonstrate that the
earlier cleavages, at least, do not in these cases sunder fundamentally
different materials, either nuclear or cytoplasmic, but only split the
Q,g<g up into a number of parts, each of which is capable of producing
an entire body of diminished size, and hence m.ust contain all of the
material essential to complete development. Both Roux and Weis-
mann endeavour to meet this adverse evidence with the assumption
of a "■ reserve idioplasm," containing all of the elements of the germ-
plasm which is in these cases distributed equally to all the cells in
addition to the specific chromatin conveyed to them by qualitative
division. This subsidiary hypothesis renders the principal one {i.e.
that of qualitative division) superfluous, and brings us back to the
same problems that arise when the assumption of qualitative division
is discarded.

The theory of qualitative nuclear division has been practically dis-
proved in another way by Driesch, through the pressure-experiments
already mentioned at page 375. Following the earlier experiments of
Pfliiger ('84) and Roux ('85) on the frog's ^gg, Driesch subjected
segmenting eggs of the sea-urchin to pressure, and thus obtained flat
plates of cells in which the arrangement of the nuclei differed totally

1 The " partial " development in the earlier stages of some of these forms is considered
at page 419.

CRITIQUE OF THE ROUX-WEISMANN THEORY

411

from the normal (Fig. 186); yet such eggs when released from press-
ure continue to segment, without rearr^angement of the nuclei, and
give rise to perfectly normal larvae. I have repeated these experi-
ments not only with sea-urchin eggs, but also with those of an annelid
{Nereis), which yield a very convincing result, since in this case the
histological differentiation of the cells appears very early. In the
normal development of this animal the archenteron arises from four
large cells or macromeres (entomeres), which remain after the suc-
cessive formation of three quartets of micromeres (ectomeres) and the
parent-cell of the mesoblast. After the primary differentiation of
the germ-layers the four entomeres do not divide again until a very
late period (free-swimming trochophore), and their substance always
retains a characteristic appearance, differing from that of the other

Fig. 186. — Modification of cleavage in sea-urchin eggs by pressure.

A. Normal eight-cell stage of Toxopneustes. B. Eight-cell stage of £^A?»«j segmenting under
pressure. Both forms produce normal Plutei. _^

blastomeres in its pale non-granular character and in the presence of
large oil-drops. If unsegmented eggs be subjected to pressure, as in
Driesch's echinoderm experiments, they segment in a flat plate, all
of the cleavages being vertical. In this way are formed eight-celled
plates in which all of the cells contain oil-drops (Fig. 187, D). If
they are now released from the pressure, each of the cells divides in
a plane approximately horizontal, a smaller granular micromere being
formed above, leaving below a larger clear macromere in which the
oil-drops remain. The sixteen-cell stage, therefore, consists of eight
deutoplasm-laden macromeres and eight protoplasmic micromeres
(instead of four macromeres and twelve micromeres, as in the usual
development). These embryos developed into free-swimming trocho-
phores containing eight instead of four macromeres, which have the
typical clear protoplasm containing oil-drops. In this case there can

412

INHERITANCE AND DEVEIOPMENT

be no doubt whatever that four of the entoblastic nuclei were nor-
mally destined for the first quartet of micromeres (Fig. 187, B\ from
which arise the apical ganglia and the prototroch. Under the condi-
tions of the experiment, however, they have given rise to the nuclei
of cells which differ in no wise from the other entoderm-cells. Even

Fig. 187. — Modifications of cleavage by pressure in Nereis.

A. B. Normal four- and eight-cell stages. C. Normal trochophore larva resulting, with four
entoderm-cells. D. Eight-cell stage arising from an egg flattened by pressure ; such eggs give rise
io trochophores with eight instead of four entoderm-cells. Numerals designate the successive
cleavages.

in a highly differentiated type of cleavage, therefore, the nuclei of the
segmenting ^^^ are not specifically different, as the Roux-Weismann
hypothesis demands, but contain the same materials even in the cells
that undergo the most diverse subsequent fate. But there is, further-
more, very strong reason for believing that this may be true in later

NATURE AND CAUSES OF DIFFERENTIATION 413

stages as well, as Koiliker insisted in opposition to Weismann as
early as 1886, and as has been urged by many subsequent writers.
The strongest evidence in this direction is afforded by the facts of
regeneration; and many cases are known — for instance, among the
hydroids and the plants — in which even a s^mall fragment of the
body is able to reproduce the whole. It is true that the power of
regeneration is always limited to a greater or less extent according
to the species. But there is no evidence whatever that such limita-
tion arises through specification of the nuclei by qualitative division,
and, as will appear beyond, its explanation is probably to be sought
in a very different direction.

F. On the Nature and Causes of Differentiation

We have now cleared the ground for a restatement of the problem
of development and an examination of the views opposed to the
Roux-Weismann theory. After discarding the hypothesis of quali-
tative division the problem confronts us in the following form. If
chromatin be the idioplasm in which inheres the sum total of heredi-
tary forces, and if it be equally distributed at every cell-division, how
can its mode of action so vary in different cells as to cause diversity
of structure, i.e. differentiation f It is perfectly certain that differen-
tiation is an actual progressive transformation of the egg-substance
involving both physical and chemical changes, occurring in a definite
order, and showing a definite distribution in the regions of the Qgg.
These changes are sooner or later accompanied by the cleavage
of the Qgg into cells whose boundaries may sharply mark the
areas of differentiation. What gives these cells their specific char-
acter? Why, in the four-cell stage of an annelid Qgg, should the
four cells contribute equally to the formation of the alimentary canal
and the cephalic nervous system, while only one of them (the left-
hand posterior) gives rise to the nervous system of the trunk-region
and to the muscles, connective tissues, and the germ-cells.'^ (Figs.
171, 188, ^.) There cannot be a fixed relation between the various
regions of the Qgg which these blastomeres represent and the adult
parts arising from them ; for in some eggs these relations may be
artificially changed. A portion of the Qgg which under normal con-
ditions would give rise to only a fragment of the body will, if split off
from the rest, give rise to an entire body of diminished size. What
then determines the history of such a portion .? What influence
moulds it now into an entire body, now into a part of a body }

De Vries, in his remarkable essay on Intracellular Pangenesis
('89), endeavoured to cut this Gordian knot by assuming that the
character of each cell is determined by pangens that migrate from

414

INHERITANCE AND DEVELOPMENT

the nucleus into the cytoplasm, and, there becoming active, set up
specific changes and determine the character of the cell, this way
or that, according to their nature. But what influence guides the
migrations of the pangens, and so correlates the operations of devel-
opment? Both Driesch and Oscar Hertwig have attempted to

Fig. i88. — Diagrams illustrating the value of the quartets in a polyclade {Leptoplana) , a lamel-
libranch {Unio), and a gasteropod {Crepiduld). A. Leptoplana, showing mesoblast-formation
in the second quartet. B. Crepidiila, showing source of ectomesoblast (from aP-, b'^, c^) and en-
tomesoblast (from quadrant B). C. Unio, ectomesoblast formed only from <

In all the figures the successive quartets are nun.bered with Arabic figures
mesoblest dotted, entoblast vertically lined.

ectoblast unshaded,

answer this question, though the first-named author does not commit
himself to the pangen-hypothesis. These writers have maintained
that the particular mode of development in a given region or blasto-
mere of the ^^g is a result of its relation to the remainder of the mass,
i.e. a product of what may be called the intra-embryonic environ-

NATURE AND CAUSES OF DIFFERENTIA 7V0N 415

ment. Hertwig insisted that the organism develops as a whole as
the result of a physiological interaction of equivalent blastomeres,
the transformation of each being due not to an inherent specific
power of self-differentiation, as Roux's mosaic-theory assumed, but
to the action upon it of the whole system of which it is a part.
" According to my conception," said Hertwig, " each of the first two
blastomeres contains the formative and differentiating forces not
simply for the production of a half-body, but for the entire organism ;
the left blastomere develops into the left half of the body only because
it is placed in relation to a right blastomere." ^ Again, in a later
paper : " The Qgg is a specifically organized elementary organism that
develops epigenetically by breaking up into cells and their subsequent
differentiation. Since every elementary part {i.e. cell) arises through
the division of the germ, or fertilized o^gg, it contains also the germ
of the whole, but during the process of development it becomes ever
more precisely differentiated and determined by the formation of
cytoplasmic products according to its position with reference to the
entire organism (blastula, gastrula, etc.)." ^

An essentially similar view was advocated by the writer ('93, '94)
nearly at the same time, and the same general conception was ex-
pressed with great clearness and precision by Driesch shortly after
Hertwig: "The fragments {i.e. cells) produced by cleavage are com-
pletely equivalent or indifferent." " The blastomeres of the sea-
urchin are to be regarded as forming a uniform material, and they
may be thrown about, like balls in a pile, without in the least degree
impairing thereby the normal power of development."^ '' The rela-
tive position of a blastomere in the whole determines in general what
develops from, it ; if its position be changed^ it gives rise to something
different; in other words, its prospective value is a function of its
position. "^

In this last aphorism the whole problem of development is brought
to a focus. It is clearly not a solution of the problem, but only a
highly suggestive restatement of it ; for everything turns upon how
the relation of the part to the whole is conceived. Very little con-
sideration is required to show that this relation cannot be a merely
geometrical or rudely mechanical one, for in the eggs of different

1 '92, I, p. 481.

^ '93» P- 793- It should be pointed out that Roux himself in several papers expressly
recognizes the fact that development cannot be regarded as a pure mosaic-work, and that
besides the power of self-differentiation postulated by his hypothesis we must assume a
•* correlative differentiation " or differentiating interaction of parts in the embryo. Cf. Roux,

*92, '93» I-

3 Studien IV., p. 25.

* Studien IV., p. 39. Cf. His, " Es muss die Wachsthumserregbarkeit des Eies eine
Function des Raumes sein." ('74, p. 153.)

41 6 INHERITANCE AND DEVELOPMENT

animals blastomeres may almost exactly correspond in origin and
relative position, yet differ widely in their relation to the resulting
embryo. Thus we find that the cleavage of polyclades, annelids, and
gasteropods (Fig. i88) shows a really wonderful agreement in form,
yet the individual cells differ markedly in prospective value. In all
of these forms three quartets of micromeres are successively formed
according to exactly the same remarkable law of the alternation of the
spirals ; ^ and, in all, the posterior cell of a fourth quartet lies at the
hinder end of the embryo in precisely the same geometrical relation
to the remainder of the embryo ; yet in the gasteropods and annelids
this cell gives rise to the mesoblast-bands and their products, in the
polyclade to a part of the archenteron, while important differences
also exist in the value of the other quartets. The relation of the
part to the whole is therefore of a highly subtle character, the pro-
spective value of a blastomere depending not merely upon its geomet-
rical position, but upon its relation to the whole complex inherited
organization of which it forms a part. The apparently simple con-
clusion stated in Driesch's clever aphorism thus leads to further prob-
lems of the highest complexity. It should be here pointed out that
Driesch does not accept Hertwig's theory of the interaction of blasto-
meres as such, but, like Whitman, Morgan, and others, has brought
forward effective arguments against that too simple and mechanical
conception. That theory is, in fact, merely Schwann's cell-composite
theory of the organism applied to the developing embryo, and the
general arguments against that theory find some of their strongest
support in the facts of growth and development.^ This has been
forcibly urged by Whitman ('93), who almost simultaneously with the
statements of Driesch and Hertwig, cited above, expressed the con-
viction that the morphogenic process cannot be conceived as merely
the sum total or resultant of the individual cell-activities, but operates
as a unit without respect to cell-boundaries, precisely as De Bary con-
cludes in the case of growing plant-tissues (p. 393), and the nature
of that process is due to the organization of the ^gg as a whole.

While recognizing fully the great value of the results attained
during the past few years in the field of experimental and specula-
tive embryology, we are constrained to admit that as far as the
essence of the problem is concerned we have not gone very far
beyond the conclusions stated above ; for beyond the fact that the
inherited organization is involved in that of the germ-cells we remain
quite ignorant of its essential nature. This has been recognized by
no one more clearly than by Driesch himself, to whose critical
researches we owe so much in this field. At the climax of a recent
elaborate analysis, the high interest of which is somewhat obscured by
1 Cf. p. 368. 2 cf. pp. 388-394.

NATURE AND CAUSES OF DIFFERENTIATION 417

its too abstruse form, Driesch can only reiterate his former aphorism/
finally taking refuge in an avowed theory of vitalism which assumes
the localization of morphogenic phenomena to be determined by " a
wholly unknown principle of correlation,"^ and forms a problem sui
ge7ieris? This conclusion recognizes the fact that the fundamental
problem of development remains wholly unsolved, thus confirming
from a new point of view a conclusion which it is only fair to point
out has been reached by many others.

But while the fundamental nature of the morphogenic process thus
remains unknown, we have learned some very interesting facts regard-
ing the conditions under which it takes place, and which show that
Driesch's aphorism loses its meaning unless carefully qualified. The
experiments referred to at pages 353, 410, show that up to a certain
stage of development the blastomeres of the early echinoderm, Amphi-
oxus or medusa-embryo, are ''totipotent" (Roux), or " equipotential"
(Driesch), i.e. capable of producing any or all parts of the body.
Even in these cases, however, we cannot accept the early conclusion
of Pfliiger ('83), applied by him to the frog's ^gg, and afterward
accepted by Hertwig, that the material of the ^z%, or of the blasto-
meres into which it splits up, is absolutely "isotropic," i.e. consists of
quite uniform indifferent material, devoid of preestablished axes.
Whitman and Morgan, and Driesch himself, showed that this cannot
be the case in the echinoderm ^gg', for the ovum possesses a polarity
predetermined before cleavage begins, as proved by the fact that at
the fourth cleavage a group of small cells or micromeres always arises
at a certain point, which may be precisely located before cleavage by
reference to the eccentricity of the first cleavage-nucleus,* and which,
as Morgan showed,^ is indicated before the third, and sometimes
before the second cleavage, by a migration of pigment away from the
micromere-pole. These observers are thus led to the assumption of
a primary polarity of the egg-protoplasm, to which Driesch, in the
course of further analysis of the phenomena, is compelled to add the
assumption of a secondary polarity at right angles to the first.^ These
polarities^ inherent not only in the entire ^gg, but also in each of the
blastomeres into which it divides, form the primary conditions under
which the bilaterally symmetrical organism develops by epigenesis.
To this extent, therefore, the material of the blastomeres, though
"totipotent," shows a certain predetermination with respect to the
adult body.

^ '99, pp. 86-87.

2 This phrase is cited by Driesch from an earlier work ('92, p. 596) as giving a correct
though " unanalytical " statement of his view. It may be questioned whether many readers
will regard as an improvement the " analytical " form it assumes in his last work.

3 Lc, p. 90. 4 Cf. Fig. 103. 5 »94^ p, J42.
^ See Driesch, '93, pp. 229, 241 ; '96, and '99, p. 44.

2 E

4i8

INHERITANCE AND DEVELOPMENT

We now proceed to the consideration of experiments which show
that in some animal eggs such predetermination may go much farther,
so that the development does, in fact, show many of the features of
a mosaic-work, as maintained by Roux. The best-determined of these
cases is that of the ctenophore-egg, as shown by the work of Chun,

Fig. 189. — Partial larvae of the ctenophore Beroe. [Driesch and Morgan.]

A. Half sixteen-cell stage, from an isolated blastomere. B. Resulting larva, with four rows of
swimming-plates and three gastric pouches. C. One-fourth sixteen-cell stage, from an isolated
blastomere, D. Resulting larva, with two rows of plates and two gastric pouches. E. Defective
larva, with six rows of plates and three gastric pouches, from a nucleated fragment of an unseg-
mented egg. F. Similar larva with five rows of plates, from above.

Driesch, and Morgan ('95), and Fischel ('98). These observers have
demonstrated that isolated blastomeres of the two-, four-, or eight-cell
stage undergo a cleavage which, through the earliest stages, is exactly
like that which it would have undergone if forming part of a com-

NATURE AND CAUSES OF DIFFERENTIATION

419

plete embryo, and gives rise to a defective larva, having only four,
two, or one row of swimming-plates (Fig. 189); and Fischel's obser-
vations give strong reason to believe that each of the eight micromeres
of the sixteen-cell stage is definitely specified for the formation of one
of the rows of plates. In like manner Crampton ('96) found that in
case of the marine gasteropod Ilyanassa isolated blastomeres of two-
cell or four-cell stages segmented exactly as if forming part of an
entire embryo, and gave rise to fragments of a larva, not to complete
dwarfs, as in the echinoderm (Fig. 190). Further, in embryos from
which the " yolk-lobe " (a region of that macromere from which the
primary mesoblast normally arises) had been removed, no mesoblast-
bands were formed. Most interesting of all, Driesch and Morgan
discovered that if a part of the cytoplasm of an unsegmented cteno-
phore-egg were removed, the remainder gave rise to an incomplete
larva, showing definite defects (Fig. 189, E, F).

In none of these cases is the embryo able to complete itself, though
it should be remarked that neither in the ctenophore nor in the snail
is the partial embryo identical with a fragment of a whole embryo,
since the micromeres finally enclose the macromeres, leaving no sur-
face of fracture. This extreme is, however, connected by a series of
forms with such cases as those of Amphioxus or the medusa, where
the fragment develops nearly or quite as if it were a whole. In the
tunicates the researches of Chabry i^^j), Driesch ('94), and Crampton
('97) show that an isolated blastomere of the two-cell stage undergoes
a typical half -cleavage (Crampton), but finally gives rise to a nearly
perfect tadpole larva lacking only one of the asymmetrically placed
sense-organs (Driesch). Next in the series may be placed the frog,
where, as Roux, Endres, and Walter have shown, a blastomere of the
two-cell stage may give rise to a typical half-morula, half-gastrula,
and half-embryo 1 (Fig. 182), yet finally produces a perfect larva. A
further stage is given by the echinoderm-egg, which, as Driesch
showed, undergoes a half-cleavage and produces a half-blastula, which,
however, closes to form a whole before the gastrula-stage (Fig. 183).
Perfectly formed though dwarf larvae result. Finally, we reach Amphi-
oxus and the hydromasae in which a perfect " whole development "
usually takes place from the beginning, though it is a very interest-
ing fact that the isolated blastomeres of Amphioxus sometimes show,
in the early stages of cleavage, peculiarities of development that recall
their behaviour when forming part of an entire embryo.^

We see throughout this series an effort, as it were, on the part of
the isolated blastomere to assume the mode of development character-
istic of a complete ^gg, but one that is striving against conditions that

1 This is not invariably the case, as described beyond.

2 Cf. Wilson, '93, pp. 590, 608.

420

INHERITANCE AND DEVELOPMENT

tend to confine its operations to the Tole it would have played if still
forming part of an entire developing ^^^g. In Amp/iioxus or Clytia
this tendency is successful almost from the beginning. In other
forms the limiting conditions are only overcome at a later period,
while in the ctenophore or snail they seem to afford an insurmount-

Fig. 190. — Partial development of isolated blastomeres of the gasteropod ^gg, Ilyanassa.
[Crampton.]

A. Normal eight-cell stage. B. Normal sixteen-cell stage. C. Half eight-cell stage, from
isolated blastomere of the two-cell stage. D. Half twelve-cell stage succeeding. E. Two stages
in the cleavage of an isolated blastomere of the four-cell stage ; above a one-fourth eight-cell stage,
below a one-fourth sixteen-cell stage.

able barrier to complete development. What determines the limita-
tions of development in these various cases } They cannot be due to
nuclear specification ; for in the ctenophore the frae^ment of an nnseg-
niented Q-gg, containing the normal egg-nucleus, gives rise to a defec-
tive larva ; and my experiments on Nereis show that even in a highly

NATURE AND CAUSES OF DIFFERENTIATION'

'421

determinate cleavage, essentially like that of the snail, the nuclei may
be shifted about by pressure without altering the end-result. Neither
can they lie in the form of the dividing mass as some authors have
assumed ; for in Crampton's experiments the half or quarter blasto-
mere does not retain the form of a half or quarter sphere, but rounds

Fig. 191. — Double embryos of frog developed from eggs inverted when in the two-cell stage.

[O. SCHULTZE.]

A. Twins with heads turned in opposite directions. B. Twins united back to back. C. Twins
united by their ventral sides. D. Double-headed tadpole.

off to a spheroid like the ^gg. But if the limiting conditions lie
neither in the nucleus nor in the form of the mass, we must seek them
in the cytoplasm ; and if we find here factors by which the tendency
of the part to develop into a whole may be, as it were, hemmed in, we
shall reach a proximate explanation of the mosaic-like character of
cleavage shown in the forms under consideration, and the mosaic

422 INHERITANCE AND DEVELOPMENT

theory of cytoplasmic localization will find a substantial if somewhat
restricted basis.

That we are here approaching the true explanation is indicated by
certain very remarkable and interesting experiments on the frog's ^g%^
which prove that each of the first two blastomeres may give rise either
to a half-embryo or to a whole embryo of half size, according to cir-
cumstances, and which indicate, furthermore, that these circumstances
lie in a measure in the arrangement of the cytoplasmic materials.
This most important result, which we owe especially to Morgan,^ was
reached in the following manner. Born had shown, in 1885, that if
frogs' eggs be fastened in an abnormal position, — e.g. upside down, or
on the side, — a rearrangement of the egg-material takes place, the
heavier deutoplasm sinking toward the lower side, while the nucleus
and protoplasm rise. A new axis is tJms establisJied in the egg, which
has the same relation to the body-axes as in the ordinary develop-
ment (though the pigment retains its original arrangement). This
proves that in eggs of this character (telolecithal) the distribution of
deutoplasm, or conversely of protoplasm, is one of the primary forma-
tive conditions of the cytoplasm ; and the significant fact is that by
artificially changing this distribution the axis of the embryo is shifted.
Oscar Schultze ('94) discovered that if the o.^^ be turned upside down
when in the two-cell stage, a whole embryo (or half of a double
embryo) may arise from each blastomere instead of a half-embryo
as in the normal development, and that the axes of these embryos
show no constant relation to one another (Fig. 191). Morgan ('95, 3)
added the important discovery that either a half-embryo or a whole
half- sized dwarf might be formed, according to the position of the bias-
tomere. If, after destruction of one blastomere, the other be allowed
to remain in its normal position, a half-embryo always results,^ pre-
cisely as described by Roux. If, on the other hand, the blastomere
be inverted, it may give rise either to a half-embryo ^ or to a whole
dwarf.* Morgan therefore concluded that the production of whole
embryos by the inverted blastomeres was, in part at least, due to a
rearrangement or rotation of the egg-materials under the influence of
gravity, the blastomere thus returning, as it were, to a state of equilib-
rium like that of an entire ovum.

This beautiful experiment gives most conclusive evidence that each
of tl^e two blastomeres contains all the materials, nuclear and cyto-
plasmic, necessary for the formation of a whole body ; and that these
materials may be used to build a whole body or half -body, according
to the grouping that they assume. After the first cleavage takes

i Anat. Anz.y X. 19, 1895. ' Three cases.

* Eleven cases observed. * Nine cases observed.

NATURE AND CAUSES OF DIFFERENTIATION 423

place, each blastomere is set^ as it were, for a half-development, but
not so firmly that a rearrangement is excluded.

I have reached a nearly related result in the case of both Amp hi-
oxtis and the echinoderms. In Amphioxus the isolated blastomere
usually segments like an entire ovum of diminished size. This is,
however, not invariable, for a certain number of such blastomeres
show a more or less marked tendency to divide as if still forming part
of an entire embryo. The sea-urchin Toxopneiistes reverses this rule,
for the isolated blastomere of the two-cell stage usually shows a per-
fectly typical half-cleavage, as described by Driesch, but in rare cases
it may segment like an entire ovum of half -size (Fig. 183, D) and give
rise to an entire blastula. We may interpret this to mean that in
Amphioxus the differentiation of the cytoplasmic substance is at first
very sHght, or readily alterable, so that the isolated blastomere, as a
rule, reverts at once to the condition of the entire ovum. In the sea-
urchin, the initial differentiations are more extensive or more firmly
established, so that only exceptionally can they be altered. In the
snail and ctenophore we have the opposite extreme to Amphioxus, the
cytoplasmic conditions having been so firmly established that they can-
not be readjusted, and the development must, from the outset, proceed
within the limits thus set up.

Through this conclusion we reconcile, as I believe, the theories of
cytoplasmic localization and mosaic development with the hypothesis
of cytoplasmic totipotence. Primarily the egg-cytoplasm is totipotent
in the sense that its various regions stand in no fixed relation with the
parts to which they respectively give rise, and the substance of each
of the blastomeres into which it splits up contains all of the materials
necessary to the formation of a complete body. Secondarily, how-
ever, development may assume more or less of a mosaic-like character
through differentiations of the cytoplasmic substance involving local
chemical and physical changes, deposits of metaplasmic material,
and doubtless many other unknown subtler processes. Both the ex-
tent and the rate of such differentiations seem to vary in different
cases; and here probably lies the explanation of the fact that the
isolated blastomeres of different eggs vary so widely in their mode
of development. When the initial differentiation is of small extent
or is of such a kind as to be readily modified, cleavage is indetermi-
nate in character and may easily be remodelled (as in Amphioxus).
When they are more extensive or more rigid, cleavage assumes a
mosaic-like or determinate character,^ and qualitative division, in a
certain sense, becomes a fact. Conklin's ('99) interesting observa-
tions on the highly determinate cleavage of gasteropods {Crepidiila)

^ The convenient terms indeterminate and determinate cleavage were suggested by
Conklin ('yS; ,

424

INHERITANCE AND DEVELOPMENT

show that here the substance of the attraction-spheres is unequally
distributed, in a quite definite way, among the cleavage-cells, each
sphere of a daughter-cell being carried over bodily into one of the
granddaughter-cells (Fig. 192). We have here a substantial basis for
the conclusion that in cleavage of this type qualitative division of the
cytoplasm may occur.

It is important not to lose sight of the fact that development and
differentiation do not in any proper sense first begin with the cleavage
of the ovum, but long before this, during its ovarian history. ^ The
primary differentiations thus established in the cytoplasm form the
immediate conditions to which the later development must conform ;
and the difference between Amphioxtis on the one hand, and the

A ^—^ B

Fig. 192. — Two successive stages in the third cleavage of the egg of Crepidida, seen from the
upper pole. [CONKLIN.]

In both figures the old spheres (dotted) lie at the upper pole of the embryo, and at the third
cleavage they pass into the four respective cells of the first quartet of micromeres. The centro-
somes are seen in the new spheres.

snail or ctenophore on the other, simply means, I think, that the
initial differentiation is less extensive or less firmly established in
the one than in the other.

The origin of the cytoplasmic differentiations existing at the be-
ginning of cleavage has already been considered (p. 386). If the
conclusions there reached be placed beside the above, we reach the
following conception. The primary determining cause of develop-
ment lies in the nucleus, which operates by setting up a continuous
series of specific metabolic changes in the cytoplasm. This process
begins during ovarian growth, establishing the external form of the
^gg, its primary polarity, and the distribution of substances within it.
The cytoplasmic differentiations thus set up form as it were a frame-

1 See Wilson ('96), Driesch ('98, i).

THE NUCLEUS IN LATER DEVELOPMENT 425

work within which the subsequent operations take place in a course
which is more or less firmly fixed in different cases. If the cyto-
plasmic conditions be artificially altered by isolation or other dis-
turbance of the blastomeres, a readjustment may take place and
development may be correspondingly altered. Whether such a read-
justment is possible depends on secondary factors — the extent of
the primary differentiations, the physical consistency of the egg-
substance, the susceptibility of the protoplasm to injury, and doubtless
a multitude of others. The same doubtless applies to the later stages
of development ; and we must here seek for some of the factors by
which the power of regeneration in the adult is determined and lim-
ited. It is, however, not improbable, as pointed out below, that in the
later stages differentiation may occur in the nuclear as well as in the
cytoplasmic substance.

G. The Nucleus in Later Development

The foregoing conception, as far as it goes, gives at least an in-
telligible view of the more general features of early development and
in a measure harmonizes the apparently conflicting results of experi-
ment on various forms. But there are a very large number of facts
relating especially to the later stages of differentiation, which it
seems to leave unexplained, and which indicate that the nucleus as
well as the cytoplasm may undergo progressive changes of its sub-
stance. It has been assumed by most critics of the Roux-Weismann
theory that all of the nuclei of the body contain the same idioplasm,
and that each therefore, in Hertwig's words, contains the germ of the
whole. It is, however, doubtful whether this assumption is well
founded. The power of a single cell to produce the entire body is in
general Hmited to the earliest stages of cleavage, rapidly diminishes,
and as a rule soon disappears entirely. When once the germ-layers
have been definitely separated, they lose entirely the power to regener-
ate one another save in a few exceptional cases. In asexual repro-
duction, in the regeneration of lost parts, in the formation of morbid
growths, each tissue is in general able to reproduce only a tissue of its
own or a nearly related kind. Transplanted or transposed groups of
cells (grafts and the like) retain more or less completely their autonomy
and vary only within certain well-defined limits, despite their change
of environment. All of these statements are, it is true, subject to
exception ; yet the facts afford an overwhelming demonstration that
differentiated cells possess a specific character, that their power of
development and adaptability to changed conditions becomes in a
greater or less degree limited with the progress of development.
As indicated above, this progressive specification of the tissue-cells

426 INHERITANCE AND DEVELOPMENT

is no doubt due in part to differentiation of the cytoplasm. There is,
however, reason to suspect that, beyond this, differentiation may sooner
or later involve a specification of the nnclear substance. When we
reflect on the general role of the nucleus in metabolism and its signifi-
cance as the especial seat of the formative power, we may well hesi-
tate to deny that this part of Roux's conception may be better founded
than his critics have admitted. Nageli insisted that the idioplasm
must undergo a progressive transformation during development, and
many subsequent writers, including such acute thinkers as Boveri and
Nussbaum, and many pathologists, have recognized the necessity for
such an assumption. Boveri's remarkable observations on the nuclei
of the primordial germ-cells in Ascaris demonstrate the truth of this
view in a particular case; for here all of the somatic nuclei lose a portion
of their chromatin, and only the progenitors of the germ-neclei retain the
e7ttire ancestral heritage. Boveri himself has in a measure pointed out
the significance of his discovery, insisting that the specific develop-
ment of the tissue-cells is conditioned by specific changes in the
chromatin that they receive,^ though he is careful not to commit him-
self to any definite theory. It hardly seems possible to doubt that in
Ascaris the limitation of the somatic cells in respect to the power of
development arises through a loss of particular portions of the
chromatin. One cannot avoid the thought that further and more
specific limitations in the various forms of somatic cells may arise
through an analogous process, and that we have here a key to the
origin of nuclear specification without recourse to the theojy of qualita-
tive division. We do not need to assume that the unused chromatin
is cast out bodily ; for it m.ay degenerate and dissolve, or may be
transformed into linin-substance or into nucleoli.

This suggestion is made only as a tentative hypothesis, but the
phenomena of mitosis seem well worthy of consideration from this
point of view. Its appHcation to the facts of development becomes
clearer when we consider the nature of the nuclear "control" of the
cell, i.e. the action of the nucleus upon the cytoplasm. Strasburger,
following in a measure the lines laid down by Nageli, regards the
action as essentially dynamic, i.e. as a propagation of molecular
movements from nucleus to cytoplasm in a manner which might be
compared to the transmission of a nervous impulse. When, however,
we consider the role of the nucleus in synthetic metabolism, and the
relation between this process and that of morphological synthesis,
we must regard the question in another light ; and opinion has of
late strongly tended to the conclusion that nuclear "control" can
only be explained as the result of active exchanges of material
between nucleus and cytoplasm. De Vries, followed by Hertwig,

^ '9i> p. 433.

THE NUCLEUS IN LATER DEVELOPMENT 427

assumes a migration of pangens from nucleus to cytoplasm, the
character of the cell being determined by the nature of the migrat-
ing pangens, and these being, as it were, selected by circumstances
(position of the cell, etc.). But, as already pointed out, the pangen-
hypothesis should be held quite distinct from the purely physiologi-
cal aspect of the question, and may be temporarily set aside ; for
specific nuclear substances may pass from the nucleus into the
cytoplasm in an unorganized form. Sachs, followed by Loeb, has
advanced the hypothesis that the development of particular organs
is determined by specific " formative substances " which incite cor-
responding forms of metabolic activity, growth, and differentiation.
It is but a step from this to the very interesting suggestion of
Driesch that the nucleus is a storehouse of ferments which pass
out into the cytoplasm and there set up specific activities. Under
the influence of these ferments the cytoplasmic organization is deter-
mined at every step of the development, and new conditions are
estabUshed for the ensuing change. This view is put forward only
tentatively as a " fiction " or working hypothesis ; but it is certainly
full of suggestion. Could we establish the fact that the number of
ferments or formative substances in the nucleus diminishes with the
progress of differentiation, we should have a comparatively simple
and intelligible explanation of the specification of nuclei and the
limitation of development. The power of regeneration might then
be conceived, somewhat as in the Roux-Weismann theory, as due to
a retention of idioplasm or germ-plasm — i.e. chromatin — in a less
highly modified condition, and the differences between the various
tissues in this regard, or between related organisms, would find a
natural explanation.

Development may thus be conceived as a progressive transforma-
tion of the egg-substance primarily incited by the nucleus, first mani-
festing itself by specific changes in the cytoplasm, but sooner or later
involving in some measure the nuclear substance itself. This process,
which one is tempted to compare to a complicated and progressive
form of crystallization, begins with the youngest ovarian ^gg and pro-
ceeds continuously until the cycle of individual life has run its course.
Cell-division is an accompaniment but not a direct cause of differen-
tiation. The cell is no more than a particular area of the germinal
substance comprising a certain quantity of cytoplasm and a mass of
idioplasm in 'its nucleus. Its character is primarily a manifestation
of the general formative energy acting at a particular point under
given conditions. When once such a circumscribed area has been
established, it may, however, emancipate itself in a greater or less
degree from the remainder of the mass, and acquire a specific char-
acter so fixed as to be incapable of further change save within the
limits imposed by its acquired character.

428

INHERITANCE AND DEVELOPMENT

H. The External Conditions of Development

We have thus far considered only the internal conditions of devel-
opment which are progressively created by the germ-cell itself. We
must now briefly glance at the external conditions afforded by the
environment of the embryo. That development is conditioned by
the external environment is obvious. But we have only recently

come to realize how intimate the rela-
tion is; and it has been especially the
service of Loeb, Herbst, and Driesch to
show how essential a part is played by
the environment in the development of
specific organic forms. The limits of
this work will not admit of any adequate
review of the vast array of known facts
in this field, for which the reader is re-
ferred to the works especially of Herbst.
I shall only consider one or two cases
which may serve to bring out the general
principle that they involve. Every liv-
ing organism at every stage of its exist-
ence reacts to its environment by physio-
logical and morphological changes. The
developing embryo, like the adult, is a
moving equilibrium — a product of the
response of the inherited organization to
the external stimuli working upon it. If
these stimuli be altered, development is
altered. This is beautifully shown by the
experiments of Herbst and others on the
development of sea-urchins. Pouchet
and Chabry showed that if the embryos
trotMs). B. Larva {Splicer echinus) at of thcsc auimals be made to dcvclop in
the same stage as the foregoing, devei- gga-water Containing no limc-salts, the

oped in sea-water containing a snght . . ^ '

excess of potassium chloride. larva fails to develop not only its calca-

- reous skeleton, but also its ciliated arms,
and a larva thus results that resembles in some particulars an entirely
different specific form ; namely, the Tomaria larva of Balanoglossns.
This result is not due simply to the lack of necessary material ; for
Herbst showed that the same result is attained if a shght excess of
potassium chloride be added to sea-water containing the normal
amount of lime (Fig. 193). In the latter case the specific metabolism
of the protoplasm is altered by a particular chemical stimulus, and a
new form results.

Fig. 193. — Normal and modified
larvae of sea-urchins. [HERBST.]

A. Normal Pluteus {Strotigylocen-

THE EXTERNAL CONDITIONS OF DEVELOPMENT

429

The changes thus caused by slight chemical alterations in the
water may be still more profound. Herbst ('92) observed, for
example, that when the water contains a very small percentage of
lithium chloride, the blastula of sea-urchins fails to invaginate to
form a typical gastrula, but evaginates to form an hour-glass-shaped

Fig. 194. — Regeneration in coelenterates {A, B, from LOEB; C, D, from BiCKFORD).

A. Polyp {Cerla/if/ius),\ixodncmg new tentacles from the aboral side of a lateral wound.
B. Hydroid (Tudu/aria) , generating a head at each end of a fragment of the stem suspended in
water, C D. Similar generation of heads at both ends of short pieces of the stem, in Tubularia.

larva, one half of which represents the archenteron, the other half
the ectoblast. Moreover, a much larger number of the blastu la-cells
undergo the differentiation into entoblast than in the normal de-
velopment, the ectoblast sometimes becoming greatly reduced and
occasionally disappearing altogether, so that the entire blastula is

430 INHERITANCE AND DEVELOPMENT

differentiated into cells having the histological character of the nor-
mal entoblast ! One of the most fundamental of embryonic differen-
tiations is thus shown to be intimately conditioned by the chemical
environment.

The observations of botanists on the production of roots and other
structures as the result of local stimuli are familiar to all. Loeb's
interesting experiments on hydroids give a similar result ('91). It
has long been known that Tubularia, like many other hydroids, has
the power to regenerate its *' head " — i.e. hypostome, mouth, and ten-
tacles— after decapitation. Loeb proved that in this case the power
to form a new head is conditioned by the environment. For if a
Tubularia stem be cut off at both ends and inserted in the sand
upside down, i.e. with the oral end buried, a new head is regen-
erated at the free (formerly aboral) end. Moreover, if such a piece
be suspended in the water by its middle point, a new head is produced
at each end (Fig. 194); while if both ends be buried in the sand,
neither end regenerates. This proves in the clearest manner that
in this case the power to form a definite compHcated structure is
called forth by the stimulus of the external environment.

These cases must suffice for our purpose. They prove incontesta-
bly that normal development is in a greater or less degree the response
of the developing organism to normal conditions ; and they show that
we cannot hope to solve the problems of development without reckon-
ing with these conditions. But neither can we regard specific forms
of development as directly caused by the external conditions ; for the
^gg of a fish and that of a polyp develop, side by side, in the same
drop of water, under identical conditions, each into its predestined
form. Every step of development is a physiological reaction, involv-
ing a long and complex chain of cause and effect between the stimu-
lus and the response. The character of the response is determined,
not by the stimulus, but by the inherited organization. While, there-
fore, the study of the external conditions is essential to the analysis-
of embryological phenomena, it serves only to reveal the mode of
action of the germ and gives but a dim insight into its ultimate
nature.

I. Development, Inheritance, and Metabolism

In bringing the foregoing discussion into more direct relation with
the general theory of cell-action, we may recall that the cell-nucleus
appears to us in two apparently different roles. On the one hand, it
is a primary factor in morphological synthesis and hence in inheri-
tance, on the other hand an organ of metabolism especially concerned
with the constructive process. These two functions we may with

DEVELOPMENT, INHERITANCE, AND METABOLISM 43 1

Claude Bernard regard as but different phases of one process. The
building of a definite cell-product, such as a muscle-fibre, a nerve-
process, a cilium, a pigment-granule, a zymogen-granule, is in the last
analysis the result of a specific form of metabolic activity, as we may
conclude from the fact that such products have not only a definite
physical and morphological character, but also a definite chemical
character. In its physiological aspect, therefore, inheritance is the
recurrence, in successive generations, of like forms of metabolism ;
and this is effected through the transmission from generation to gen-
eration of a specific substance or idioplasm which we have seen
reason to identify with chromatin. The validity of this conception
is not affected by the form in which we conceive the morphological
nature of the idioplasm — whether as simply a mixture of chemical
substances, as a microcosm of invisible germs or pangens, as assumed
by De Vries, Weismann, and Hertwig, as a storehouse of specific fer-
ments as Driesch suggests, or as a complex molecular substance grouped
in micellae as in Nageli's hypothesis. It is true, as Verworn insists,
that the cytoplasm is essential to inheritance ; for without a specifi-
cally organized cytoplasm the nucleus is unable to set up specific
forms of synthesis. This objection, which has already been con-
sidered from different points of view, by both De Vries and Driesch,
disappears as soon as we regard the egg-cytoplasm as itself a product
of the nuclear activity ; and it is just here that the general role of the
nucleus in metabolism is of such vital importance to the theory of
inheritance. If the nucleus be the formative centre of the cell, if
nutritive substances be elaborated by or under the influence of the
nucleus while they are built into the living fabric, then the specific
character of the cytoplasm is determined by that of the nucleus,
and the contradiction vanishes. In accepting this view we admit
that the cytoplasm of the ^gg is, in a measure, the substratum of
inheritance, but it is so only by virtue of its relation to the nucleus,
which is, so to speak, the ultimate court of appeal. The nucleus
cannot operate without a cytoplasmic field in which its peculiar
powers may come into play; but this field is created and moulded
by itself.

J. Preformation and Epigenesis. The Unknown Factor in

Development

We have now arrived at the farthest outposts of cell-research, and
here we find ourselves confronted with the same unsolved problems
before which the investigators of evolution have made a halt. For
we must now inquire what is the guiding principle of embryological
development that correlates its complex phenomena and directs them

432 INHERITANCE AND DEVELOPMENT

to a definite end. However we conceive the special mechanism of
development, we cannot escape the conclusion that the power behind
•it is involved in the structure of the germ-plasm inherited from fore-
going generations. What is the nature of this structure and how
has it been acquired t To the first of these questions we have as
yet no certain answer. The second question is merely the general
problem of evolution stated from the standpoint of the cell-theory. .
The first question raises once more the old puzzle of preformation
or epigenesis. The pangen-hypothesis of De Vries and Weismann
recognizes the fact that development is epigenetic in its external
features ; but like Darwin's hypothesis of pangenesis, it is at bottom
a theory of preformation, and Weismann expresses the conviction
that an epigenetic development is an impossibility. ^ He thus ex-
plicitly adopts the view, long since suggested by Huxley, that ''the
process which in its superficial aspect is epigenesis appears in es-
sence to be evolution in the modified sense adopted in Bonnet's later
writings ; and development is merely the expansion of a potential
organism or 'original preformation' according to fixed laws."^ Hert-
wig ('92, 2), while accepting the pangen-hypothesis, endeavours to
take a middle ground between preformation and epigenesis, by
assuming that the pangens (idioblasts) represent only cell-characters,
the traits of the multicellular body arising epigenetically by permu-
tations and combinations of these characters. This conception cer-
tainly tends to simplify our ideas of development in its outward
features, but it does not explain why cells of different characters
should be combined in a definite manner, and hence does not reach
the ultimate problem of inheritance.

What lies beyond our reach at present, as Driesch has very ably
urged, is to explain the orderly rhythm of development — the co-
ordinating power that guides development to its predestined end.
We are logically compelled to refer this power to the inherent
organization of the germ, but we neither know nor can we even
conceive what that organization is. The theory of Roux and Weis-
mann demands for the orderly distribution of the elements of the
germ-plasm a prearranged system of forces of absolutely incon-
ceivable complexity. Hertwig's and De Vries's theory, though ap-
parently simpler, makes no less a demand ; for how are we to
conceive the power which guides the countless hosts of migrating
pangens throughout all the long and complex events of development }
The same difficulty confronts us under any theory we can frame. If
with Herbert Spencer we assume the germ-plasm to be an aggrega-
tion of like units, molecular or supra-molecular, endowed with prede-
termined polarities which lead to their grouping in specific forms,

^ Germ-plasm, p. 14. 2 Evolution, Science, and Culture, p. 296.

PREFORMATION AND EPIGENESIS 433

^ve but throw the problem one stage farther back, and, as Weismani
himself has pointed out,^ substitute for one difficulty another of
exactly the same kind.

The truth is that an explanation of development is at present
beyond our reach. The controversy between preformation and
epigenesis has now arrived at a stage where it has little meaning
apart from the general problem of physical causality. What we
know is that a specific kind of living substance, derived from the
parent, tends to run through a specific cycle of changes during which
it transforms itself into a body like that of which it formed a part ;
and we are able to study with greater or less precision the mechanism
by which that transformation is effected and the conditions under
which it takes place. But despite all our theories we no more know
how the organization of the germ-cell involves the properties of the
adult body than we know how the properties of hydrogen and oxygen
involve those of water. So long as the chemist and physicist are
unable to solve so simple a problem of physical causaHty as this,
the embryologist may well be content to reserve his judgment on a
problem a hundred-fold more complex.

The second question, regarding the historical origin of the idio-
plasm, brings us to the side of the evolutionists. The idioplasm of
every species has been derived, as we must believe, by the modifica-
tion of a preexisting idioplasm through variation, and the survival
of the fittest. Whether these variations first arise in the idioplasm
of the germ-cells, as Weismann maintains, or whether they may arise
in the body-cells and then be reflected back upon the idioplasm, is
a question to which the study of the cell has thus far given
no certain answer. Whatever position we take on this question, ths
same difficulty is encountered ; namely, the origin of that coordi-
nated fitfiess, that power of active adjustment between internal and
external relations, which, as so many eminent biological thinkers
have insisted, overshadows every manifestation of life. The nature
and origin of this power is the fundamental problem of biology.
When, after removing the lens of the eye in the larval salamander,
we see it restored in perfect and typical form by regeneration from
the posterior layer of the iris,^ we behold an adaptive response to
changed conditions of which the organism can have had no antece-
dent experience either ontogenetic or phylogenetic, and one of so
marvellous a character that we are made to realize, as by a flash of
light, how far we still are from a solution of this problem. It may
be true, as Schwann himself urged, that the adaptive power of
living beings differs in degree only, not in kind, from that of unor-

1 Germinal Selection, January, 1896, p. 284.

2 See Wolff, '95, and MiiUer, '96.

434 INHERITANCE AND DEVELOPMENT

ganized bodies. It is true that we may trace in organic nature long
and finely graduated series leading upward from the lower to the
higher forms, and we must believe that the wonderful adaptive mani-
festations of the more complex forms have been derived from simpler
conditions through the progressive operation of natural causes. But
when all these admissions are made, and when the conserving
action of natural selection is in the fullest degree recognized, we can-
not close our eyes to two facts : first, that we are utterly ignorant of
the manner in which the idioplasm of the germ-cell can so respond
to the influence of the environment as to call forth an adaptive
variation ; and second, that the study of the cell has on the whole
seemed to widen rather than to narrow the enormous gap that sepa-
rates even the lowest forms of life from the inorganic world.

I am well aware that to many such a conclusion may appear reac-
tionary or even to involve a renunciation of what has been regarded
as the ultimate aim of biology. In reply to such a criticism I can
only express my conviction that the magnitude of the problem of
development, whether ontogenetic or phylogenetic, has been under-
estimated ; and that the progress of science is retarded rather than
advanced by a premature attack upon its ultimate problems. Yet
the splendid achievements of cell-research in the past twenty years
stand as the promise of its possibilities for the future, and we need
set no Hmit to its advance. To Schleiden and Schwann the present
standpoint of the cell-theory might well have seemed unattainable.
We cannot foretell its future triumphs, nor can we doubt that the
way has already been opened to better understanding of inheritance
and development.

LITERATURE. IX

Barfurth, D. — Regeneration und Involution: Merkel u. Bonnet, Ergeb., I.-VIII.

1891-99.
Boveri, Th. — Ein geschlechtlich erzeugter Organismus ohne miitterliche Eigen-

schaften: Sitz.-Ber. d. Ges.f. Morph. und Phys. in Munchen, V. 1889. See

2\so Arch. f. Entiv. 1895.
Brooks, W. K. — The Law of Heredity. Baltimore, 1883.
Id. — The Foundations of Zoology. New York, 1899.

Davenport, C. B. — Experimental Morphology: I., II. New York, 1897, 1899.
Driesch, H. — Analytische Theorie der organischen Entwicklung. Leipzig, 1894.
Id. — Die Localisation morphogenetischer Vorgange : Arch. Entw.,V\\. i. 1899.
Id. — Resultate und Probleme der Entwickelungs-physiologie der Tiere : Merkel u.

Bonnet, Ergeb., VIII., 1898. (Full literature.)
Herbst, C. — tJber die Bedeutung der Reizphysiologie fiir die kausale Auffassung

von Vorgangen in der tierischen Ontogenese : Biol. Centralb., XIV., XV.

1894-95.
Eertwig, 0. — Altere und neuere Entwicklungs-theorien. Berlin^ 1892.

LITERATURE 435

Hertwig, 0. — Urmund und Spina Bifida: Arch. tnik. Anat., XXXIX. 1892.
Id. — Uber den Werth der Ersten Furchungszellen fiir die Organbildung des Em-
bryo: Arch. mik. Anat., XLII. 1893.
Id. — Zeit und Streitfragen der Biologic. I. Berlin, 1894. II. Je7ia, 1897.
Id. — Die Zelle und die Gewebe, II. Jena, 1898.
His, W. — Unsere Korperform und das physiologische Problem ihrer Entstehung.

Leipzig, 1874.
Loeb, J. — Untersuchungen zur physiologischen Morphologic: I. Heteromorphosis.

JViirzdurg; iSgi. II. Organbildung und Wachsthum. IVurzdurg, iSg2.
Id. — Some Facts and Principles of Physiological Morphology: Wood's Holl Biol.

Lectures. 1893.
Morgan, T. H. — Experimental Studies of the Regeneration of Phanaria Maculata :

Arch. Entw., VII. 2, 3. 1898.
Id. — The Development of the Frog's Egg. New York, 1897.
Nageli, C. — Mechanisch-physiologische Theorie der Abstammungslehre. Miin-

chen u. Leipzig, 1884.
Roux, W. — Uber die Bedeutung der Kernteilungsfiguren. Leipzig, 1883.
Id. — Uber das kiinstliche Hervorbringen halber Embryonen durch Zerstorung einei

der beiden ersten Furchungskugeln, etc. : Virchow's Archiv, 114. 1888.
Id. — Fiir unsere Programme und seine Verwirklichung : Arch. Entw., V. 2. 1897.

(See also Gesammelte Abhandlungen uber Entwicklungsmechanik der Organ-

ismen, 1895.)
Sachs, J. — StofF und Form der Pflanzenorgane : Ges. Abhandlungen, II. 1893.
Weismann, A. — Essays upon Heredity, First Series. Oxford, 1891.
Id. — Essays upon Heredity, Second Series. Oxford, 1892.
Id. — Aussere Einfliisse als Entwicklungsreize. Jena, \%c)\.
Id. — The Germ-plasm. New York, 1893.

Whitman, CO. — Evolution and Epigenesis : Wood's Holl Biol. lectures. 1894.
Wilson, Edm. B. — On Cleavage and Mosaic-work: Arch, fiir Eniwicklungsm^

III. I. 1896. See also Literature, VIII., p. 394.)

GLOSSARY

[Obsolete terms are enclosed in brackets^ The name and date refer to the first use of the word-
subsequent changes of meaning are indicated in the definition.]

Achro'matin (see Chromatin), the non-staining substance of the nucleus, as
opposed to chromatin ; comprising the ground-substance and the linin-network..
(Flemming, 1879.)

A'crosome ( aKpov, apex, o-w/xa, body), the apical body situated at the anterior endl
of head of spermatozoon. (Lenhossek, 1897.)

[Akaryo'ta] (see Karyota), non-nucleated cells. (Flemming, 1882.)

Ale'cithal (d-priv. ; AcKt^os, the yolk of an egg), having little or no yolk (applied
to eggs). (Balfour, 1880.)

Alloplasma'tic (aAAos, different). Applied to active substances formed by differ-
entiation from the protoplasm proper, e.g. the substance of cilia, of nerve-fibrillae,
and of muscle-fibrillae. Alloplasmatic organs are opposed to " protoplasmatic,"
which arise only by division of preexisting bodies of the same kind. (A. Meyer,

Amito'sis (see Mitosis), direct or amitotic nuclear division ; mass-division of
the nuclear substance without the formation of chroinosomes and amphiaster.
(Flemming, 1882.)

Am'phiaster (d/x^:', on both sides ; axTTrjp, a star), the achromatic figure formed
in mitotic cell-division, consisting of two asters connected by a spindle. (FOL,

1877-)

Amphipy 'renin (see Pyrenin), the substance of the nuclear membrane.
(Schwarz, 1887.)

Amyloplasts (d/xvAoi/, starch ; TrAaa-ro?, irXacrcruv, form), the colourless starch-
forming plastids of plant-cells. (Errera, 1882.)

An'aphase (dvd, back or again), the later period of mitosis during the divergence
of the daughter-chromosomes. (Strasburger, 1884.)

Aniso'tropy (see Isotropy), having a predetermined axis or axes (as applied to
the egg). (Pfluger. 1883.)

Antiierozold. the same as Spermatozoid.

Anti'podal cone, the cone of astral rays opposite to the spindle-fibres. (Van
Beneden, 1883.)

Archiam'phiaster {apxi = first, -|- amphiaster), the amphiaster by which the first
or second polar body is formed. (Whitman, 1878.)

Ar'choplasma or Archoplasm {apxinv. 2. v\\\&x) (sometimes written archipiasni),
the substance from which the attraction-sphere, the astral rays, and the spindle-
fibres are developed, and of which they consist. (Boverl 1888.)

Arrhe'noid (apprjv. male). The sperm-aster or attraction-sphere formed during the
fertilization of the ovum. (Henking, 1890.)

As'ter (do-TT/p, a star), i. The star-shaped structure surrounding the centrosome.
(FoL, 1877.) [2. The star-shaped group of chromosomes during mitosis (see
Karyaster). (Flemming, 1892.)]

[Ai'trocoele] (aari^p, a star; koiAo?, hollow), a term somewhat vaguely applied to
the space in which the centrosome lies. (Fol, 1891.)

437

438 GLOSSARY

As'trosphere (see Centrosphere) . i» The central mass of the aster, exclusive
of the rays, in which the centrosome lies. Equivalent to the " attraction-sphere "
of Van Beneden. (Fol, 1891 ; Strasburger, 1892.) 2. The entire aster
exclusive of the centrosome. Equivalent to the "astral sphere" of Mark.
(BovERi, 1895.)

Attraction-sphere (see Centrosphere), the central mass of the aster from which
the rays proceed. Also the mass of " archoplasm," derived from the aster, by
■which the centrosome is surrounded in the resting cell. (Van Beneden, 1883.)

[Au'toblast] (avTos, self), applied by Altmann to bacteria and other minute organ-
isms, conceived as independent solitary "bioblasts." (1890.)

Axial filament, the central filament, probably contractile, of the spermatozoon-
flagellum. (Eimer, 1874.)

Basichro'matin (see Chromatin), the same as chromatin in the usual sense.
That portion of the nuclear network stained by basic tar-colours. (Heidenhain,
1894.)

Bi'oblast (^ios, life ; ^SAao-ro?, a germ), a term applied by Altmann to the hypo-
thetical ultimate vital unit (equivalent to plasome), and identified by him as
the "granulum."

Bf ogen (^ibs, life ; -ycio}?, producing), equivalent to plasome, etc. (Verworn,
1895.)

Bfophores {jiioq, life ; -<f)6pos, bearing), the ultimate supra-molecular vital units.
Equivalent to the pangens of De Vries, the p/asomes of Wiesner, etc. (Weismann,

Bi'oplasm (^lo<;, 7rXa9/xa). The active "living, forming germinal material," as
opposed to "formed material." Nearly equivalent to protoplasm in the wider
sense. (Beale, 1870.)

Bi'oplast, equivalent to cell. (Beale, 1870.)

Bi'valent, applied to chromatin-rods representing two chromosomes joined end to
end. (Hacker, 1892.)

Ble'pharoplast (l3Xecf)dpts, eye-lash or cilium). The centrosome-like bodies in
plant-spermatids in connection with which the cilia of the spermatozoids are
formed. (Webber, 1897.)

Cell-plate (see Mid-body), the equatorial thickening of the spindle-fibres from
which the partition-wall arises during the division of plant-cells. (Strasbur-
ger, 1875.)

Cell-sap, the more liquid ground-substance of the nucleus. [Kolliker, 1865;
more precisely defined by R. Hertwig, 1876.]

Central spindle, the primary spindle by which the centrosomes are connected, as
opposed to the contractile mantle-fibres surrounding it. (Hermann, 1891.)

Cen'triole, a term applied by Boveri to a minute body or bodies (" Central-korn ")
within the centrosome. In some cases not to be distinguished from the centro-
some. (Boveri, 1895.)

Centrodes'mus (KcVrpov, centre ; 8es/w,os, a band), the primary connection between
the centrosomes, formed by a substance from which arises the central spindle.
(Heidenhain, 1894.)

Centrodeu'toplasm, the granular material of the testis-cells which may contribute
to the formation of the Nebenkern or to that of the idiozome. (Erlanger,
1897.)

Centrole'cithal (Kivrpov, centre ; X€kl6o<s, yolk), that type of ovum in which the
deutoplasm is mainly accumulated in the centre. (Balfour, 1880.)

Cen'troplasm (Kevrpov, centre; Trkda-fxa), the protoplasm forming the attraction-
sphere or central region of the aster ; the substance of the centrosphere. (Er-
langer, 1895.)

GLOSSARY

439

Cen'trosome (Kevrpov, centre ; oru/xa, body), a body found at the centre of the aster
or attraction-sphere, regarded by some observers as the active centre of cell-
division and in this sense as the dynamic centre of the cell. Under its influence
arise the asters and spindle (amphiaster) _ of the mitotic figure. (Boveri,
1888.)

Cen'trosphere, used in this work as equivalent to the " astrosphere " of Stras-
burger ; the central mass of the aster from which the rays proceed and within
which lies the centrosome. The attraction-sphere. [Strasburger, 1892;
applied by him to the " astrosphere " and centrosome taken together.]

Chloroplas'tids (xAwpo?, green ; TrAao-ro?, form), the green plastids or chlorophyll-
bodies of plant and animal cells. (Schimper, 1883.)

Chro'matin (x/ow^ta, colour), the deeply staining substance of the nuclear network
and of the chromosomes, consisting of nuclein. (Flemming, 1879.)

Chro'matophore (xpw/wxx, colour; -^opos, bearing), a general term applied to the
coloured plastids of plant and animal cells, including chloroplastids and chromo-

plastids. (SCHAARSCHMIDT, 1880; SCHMITZ, 1882.)

Chro'matoplasm (xpw/txa, colour ; irXdafxa, anything formed or moulded), the sub-
stance of the chromoplastids and other plastids. (Strasburger, 1882.)
Chro'miole, the smallest chromatin-granules which by their aggregation form the

larger chromomeres of which the chromosomes are composed. (Eisen, 1899.)
Chro'momere (^/ow/xa, colour; fX€po<;, a part), one of the chromatin-granules of

which the chromosomes are made up. Identified by Weismann as the "id."

See Chromiole. (Fol, 1891.)
Chromoplas'tids (^puifjxi., colour ; TrAao-ros, form), the coloured plastids or pigment-
bodies other than the chloroplasts, in plant-cells. (Schimper, 1883.)
Chro'moplasts, net-knots or chromatin-nucleoli; also used by some authors as

equivalent to Chromoplastid. (Eisen, 1899.)
Chro'mosomes (^poj/xa, colour; crw/Aa, body), the deeply staining bodies into which

the chromatic nuclear network resolves itself during mitotic cell-division. (Wal-

DEYER, 1888.)
Cleavage-nucleus, the nucleus of the fertilized egg, resulting from the union of

egg-nucleus and sperm-nucleus. (O. Hertwig, 1875.)
Cortical zone, the outer zone of the centrosphere. (Van Beneden, 1887.)
Cyano'philous (/cvavos, blue ; ij^tAciv, to love), having an especial affinity for blue

or green dyes. (Auerbach.)
Cy'taster {Kvro'i, hollow (a cell) ; acrrrjp, star), the same as Aster, i. See Kary-

aster. (Flemming, 1882.)
[Cy'toblast] (^kvto<;, hollow (a cell); ^Aao-ros, germ). i. The cell-nucleus.

(Schleiden, 1838.) 2. One of the hypothetical ultimate vital units (bioblasts or

"granula") of which the cell is built up. (Altmann, 1890.) 3. A naked cell

or •• protoblast." (Kolliker.)
[Cytoblaste'ma] (see Cytoblast), the formative material from which cells were

supposed to arise by "free cell-formation.'" (Schleiden, 1838.)
[Cytochyle'ma] (kvtos, hollow (a cell) ; x^^o? juice), the ground-substance of the

cytoplasm as opposed to that of the nucleus. (Strasburger, 1882.)
Cy'tode (kvto<;, hollow (a cell) ; ctSo?, form), a non-nucleated cell. (Hackel, 1866.)
Cytodie'resis (kvto<s, hollow (a cell) ; Siatpeo-ts, division), the same as Mitosis.

(Henneguy, 188?,)
Cytohy'aloplasma (kvtos, hollow (a cell) ; vaAos, glass ; irXaapxi, anything formed),

the substance of the cytoreticulum in which are embedded the microsomes;

opposed to nucleohyaloplasma. (Strasburger, 1882.)
Cy'tolymph (kvtos, hollow (a cell) ; lympha, clear water), the cytoplasmic ground*

substance. (Hackel, 1891.)

440 GLOSSARY

Cytomi'crosomes (see Microsome), microsomes of the cytoplasm ; opposed to
nucleomicrosomes. (Strasburger, 1882.)

Cytomi'tome {kvtos, hollow (a cell) ; ixLTiD/jia, from jjllto^, thread), the cytoplasmic
as opposed to the nuclear thread-work. (Flemming, 1882.)

Cy'toplasm (kvtos, TrAttcr/xa). i. The protoplasmic ground-substance as opposed
to the granules. (Kolliker, 1863.) 2. Equivalent to protoplasm. (Kolliker,
1867.) 3. The substance of the cell-body as opposed to that of the nucleus.
(Strasburger, 1882.)

Cytoretic'ulum, the same as Cytomitome. (Strasburger, 1882.)

Cy'tosome (/ct'T09, hollow (a cell) ; croi/xa, body), i. The cell-body or cytoplasmic
mass as opposed to the nucleus. (Hackel, 1891.) 2. A term used as parallel to
chromosome to denote deeply staining definitely organized cytoplasmic filaments
or other cytoplasmic structures composed of "cytochromatin." (Prenant, 1898.)

Der^matoplasm (Bipfxa, skin), the living protoplasm asserted to form a part of the
cell-membrane in plants. (Wiesner, 1886.)

Der'matosomes (Bepim., skin ; aw/xa, body), the plasomes which form the cell-mem-
brane. (Wiesner, 1886.)

Determinant, a hypothetical unit formed as an aggregation of biophores, determin-
ing the development of a single cell or independently variable group of cells.
(Weismann, 1 89 1.)

[Deuthy'alosome] (8evT(epo<s), second ; see Hyalosome), the nucleus remaining
in the egg after formation of the first polar body. (Van Beneden. 1883.)

Deu'toplasm (Sevr (epos), second ; TrXacrfm, anything formed), 3701k, lifeless food-
matters deposited in the cytoplasm of the egg ; opposed to "protoplasm.'' (Van
Beneden, 1870.)

Diakine'sis (8ta, through), the segmented-spireme-stage, following the synapsis, in
the primary oocyte or spermatocyte, during which the chromosomes'persist for a
considerable period in the form of double rods. (Hacker, 1897.)

Directive bodies, the polar bodies. (Fr. Muller, 1848.)

Directive sphere, the attraction-sphere. (Guignard, 1891.)

Dispermy, the entrance of two spermatozoa into the egg.

Dispi'reme (see Spireme), that stage of mitosis in which each daughter-nucleus
has given rise to a spireme. (Flemming, 1882.)

Dy'aster (8ms, two; see Aster, 2), the double group of chromosomes during the
anaphases of cell-division. (Flemming, 1882.)

Ectosphere (cktos, outside), the outer or cortical zone of the attraction-sphere.
(Ziegler, 1899.)

Egg-nucleus, the nucleus of the egg after formation of the polar bodies and before
its union with the sperm-nucleus. Equivalent to the "female pronucleus" of Van
Beneden. (O. Hertwig, 1875.)

Enchyle'ma (ev, in; x^-^^'^? juice), i. The more fluid portion of protoplasm,
consisting of " hyaloplasma."" (Hanstein, 1880.) 2. The ground-substance
(cytolymph) of cytoplasm as opposed to the reticulum. (Carnoy, 1883.)

Endoplast, the cell-nucleus. (Huxley, 1853.)

Ener'gid, the cell-nucleus together with the cytoplasm lying within its sphere of
influence. (Sachs, 1892.)

Entosphere, (evros, inside), the inner or medullary zone of the attraction-sphere.
(Ziegler, 1899.)

Equatorial plate, the group of chromosomes lying at the equator of the spindle
during mitosis. (Van Beneden, 1875.)

Ergastic (cpyct^o/xat, to work). Applied to relatively passive substances "formed
anew through activity of the protoplasm." Equivalent to metaplasmic. Cf.
alloplasmatic. (A. Meyer, 1896.)

GLOSSARY 441

Ergastoplasm (epya^o/uai, to work). Nearly equivalent to the "kinoplasm" of

Strasburger and the " ergoplasm " of Davidoff. The more active protoplasmic

substance from which fibrillar formations arise. (Garnier, 1897.)
Ergoplasm {tpyov, work). The active protoplasm of the ^gg (in tunicates), mainly

derived from the achromatic part of the germinal vesicle, and giving rise in part

or wholly to the polar spindle. Analogous to archoplasm and kinoplasm.

(Davidoff, 1889.)
Erythro'philous (ipv0p6<s, red ; cfyiyeiv, to love), having an especial affinity for red

dyes. (AuERBACH.)
Ga'mete {yafxh-q, wife; ya/xc'rry?, husband), one of two conjugating cells. Usually

applied to the unicellular forms.
Gem'mule (see Pangen), one of the ultimate supra-molecular germs of the cell

assumed by Darwin. (Darwin, 1868.)
[Ge'noblasts] (yeVos, sex ; /^Aao-rd?, germ), a term applied by Minot to the mature

germ-cells. The female genoblast (egg or '' thelyblast ") unites with the male

(spermatozoon or " arsenoblast '') to form an hermaphrodite or indifferent cell.

(Minot, 1877.)
Germinal spot, the nucleolus of the germinal vesicle. (Wagner, 1836.)
Germinal vesicle, the nucleus of the egg before formation of the polar bodies.

(PuRKiNjE, 1825.)
Germ-plasm, the same as idioplasm. (Weismann.)
Heterokine'sis (ercpos, different), qualitative nuclear division ; a hypothetical mode

of mitosis assumed to separate chromatins of different quality ; opposed to

homobkinesis or equation-division. (Weismann, 1892.)
Heterole'cithal (erc/oos, different; Acki^os, yolk), having unequally distributed

deutoplasm (includes telolecithal and centrolecithal). (Mark, 1892.)
Heterotyp'ical mitosis (€Tepo<i, different ; see Mitosis), that mode of mitotic

division in which the daughter-chromosomes remain united by their ends to form

rings. (Flemming, 1887.)
[Holoschi'sis] (oAos, whole ; (rxt^«v, to split), direct nuclear division. Amitosis.

(Flemming, 1882.)
Homole'cithal (oyad?, the same, uniform ; XeVt^o?, yolk), equivalent to alecithal

Having little deutoplasm, equally distributed, or none. (Mark, 1892.)
Homookine'sis or Homaeokine'sis (o/ud?, the same), equation-division, separating

equivalent chromatins; opposed to heterokinesis. (Weismann, 1892.)
Homoeotyp'ical mitosis (d/xoio?, like ; see Mitosis), a form of mitosis occurring

in the secondary spermatocytes of the salamander, differing from the usual type

only in the shortness of the chromosomes and the irregular arrangement of

the daughter-chromosomes. (Flemming, 1887.)
Hy'aloplasma (vaXo<;, glass ; TrXdafxa, anything formed), i. The ground-substance

of the cell as distinguished from the granules or microsomes. [Hanstein, 1880.]

2. The achromatic substance of the nucleus in which the chromatin-particles are

embedded. (Strasburger, 1882.) 3. The ground-substance as distinguished

from the reticulum or " spongioplasm.". (Leydig, 1885.) 4. The exoplasm or

peripheral protoplasmic zone in plant-cells. (Pfeffer.)
Hy'alosomes (vaXo<i. glass ; orto/xa, body), nucleolar-Iike bodies but slightly stained

by either nuclear or plasma stains. (Lukjanow, 1888.)
[Hy'groplasma] (vypd?, wet; TrXdafxa, something formed), the more liquid part

of protoplasm as opposed- to the firmer stereoplasm. (Nageli, 1884.)
Id, the hypothetical structural unit resulti,ng from the successive aggregation of

biophores and determinants. Identified by Weismann as the chromomere, or

chromatin-granule. (Weismann, 1891.)
Idant, the hypothetical unit resulting from the successive aggregation of biophores,

442 GLOSSARY

determinants, and Ids. Identified by Weismann as the chromosome. (Weis-
MANN, 1891.)

Id'ioblasts (tStos, one's own ; /^Aao-ros, germ), the hypothetical ultimate units of
the cell; the same as biophores. (O. Hertwig, 1893.)

Idioplasm (18105, one's own ; TrAacr/ua, a thing formed), equivalent to the germ-
plasm of Weismann. The substance, now generally identified with chromatin,
which by its inherent organization involves the characteristics of the species.
The physical basis of inheritance. (Nageli, 1884.)

Idlosome {J.ho^, one's own; o-w/xa, body), the same as idioblast or plasome.
(Whitman, 1893.)

Idiozome (tStos, specially formed; ^w/^a, girdle). The sphere, often called attrac-
tion-sphere and usually enclosing the centrosomes, found in the spermatids of
animals. (Meves, 1897.)

Interfilar substance, the ground-substance of protoplasm as opposed to the thread-
work. (Flemming, 1882.)

Interzonal fibres (" Filaments reunissants " of Van Beneden. " Verbindungs-
fasern" of Flemming and others). Those spindle-fibres that stretch between
the two groups of daughter-chromosomes during the anaphase. Equivalent
in some cases to the central spindle. (Mark, 1881.)

Iso'tropy (t<ros, equal ; rpcmri, a turning), the absence of predetermined axes (as
applied to the ^gg)- (Pfluger, 1883.)

[Ka'ryaster] (xapvov, nut, nucleus ; see Aster, 2), the star-shaped group of chromo-
somes in mitosis. Opposed to cy taster. (Flemming, 1882.)

Karyenchy'ma {Kapvov, nut, nucleus; ev, in; x^/^09, juice), the "nuclear sap."
(Flemming, 1882,)

Karyokine'sis (Kapvov, nut, nucleus; Kcvr}(n<;, change, movement), the same as
mitosis. (Schleicher, 1878.)

[Karyoly'ma] , the "karyolytic" (mitotic) figure. (Auerbach, 1876.)

Ka'ryolymph, The nuclear sap. (Hackel, 1891.)

[Karyo'lysis] (Kapvov, nut, nucleus; Avo-ts, dissolution), the supposed dissolution
of the nucleus during cell-division. (Auerbach, 1874.)

[Karyoly'tic figure] (see Karyolysis), a term applied by Auerbach to the mitotic
figure in living cells. Believed by him to result from the dissolution of the
nucleus. (Auerbach, 1874.)

Karyomi'orosome (see Microsome), the same as nucleo-microsome.

Karyomi'tome (Kapvov, nut, nucleus ; /xtVw/xa, from /xiVos, a thread), the nuclear as
opposed to the cytoplasmic thread-work. (Flemming, 1882.)

Karyomito'sis (Kapvov, nut, nucleus ; see Mitosis), mitosis. (Flemming,
1882.)

Ka'ryon (Kapvov, nut, nucleus), the cell-nucleus. (Hackel, 1891.)

Ka'ryoplasm (Kapvov, nut, nucleus ; TrXda-fxa, a thing formed), nucleoplasm. The
nuclear as opposed to the cytoplasmic substance. (Flemming, 1882.)

Ka'ry osome (Kapvov, nut, nucleus ; a-oipua, body) . i . Nucleoli of the " net-knot "
type, staining with nuclear dyes, as opposed to plasmosomes or true nucleoli.
(Ogata, 1883.) 2. The same as chromosome. (Platner, 1886.) 3. Caryo-
some. The cell-nucleus. (Watase, 1894.)

[Karyo'ta] (Kapvov, nut, nucleus), nucleated cells. (Flemming, 1882.)

Karyothe'ca (Kapvov, nut, nucleus; OriKiq, case, box), the nuclear membrane.
(Hackel, 1891.)

Ki'noplasm (klv^Iv, to move ■ irXaapxi, a thing formed), nearly equivalent to
archoplasm, but used in a broader sense to denote in general the more
active elements of protoplasm from which arise fibrillae, the substance of cilia,
and (in plants) the peripheral " Hautschicht " from which the membrane is

GLOSSARY 443

formed ; opposed to tha " trophoplasm " or nutritive plasm. (Strasburger,
1892.)

[Lantbanin] (Aav^aveii/, to conceal), equivalent to oxychromatin. (Heidenhain,
1892.) ^ .

Leucoplas'tids {X^vKo^iy white ; TrAao-ro?, form), the colourless plastids of plant-
cells from which arise the starch-formers (amyloplastids), chloroplastids, and
chromoplastids. (Schimper, 1883.)

Li'nin (linum, a linen thread), the substance of the "achromatic" nuclear reticu-
lum. (SCHWARZ, 1887.)

Liniuoplast, the true nucleolus or plasmosome. (Eisen, 1899.)

Macrocentrosome, a term applied to the " centrosome " in Boveri's sense, i.e. to
the larger body in which lies the central granule. (Ziegler, 1898.) Probably
synonymous with entosphere.

Maturation, the final stages in the development of the germ-cells. More spe-
cifically, the process by which the reduction of the number of chromosomes
is effected.

Metakine'sis (see Metaphase) (/xera, beyond {i.e. further) ; KLwr](n<s, movement),
the middle stage of mitosis, when the chromosomes are grouped in the equatorial
plate. (Flemming, 1882.)

Metanu'cleus, a term applied to the nucleolus after its extrusion from the germi-
nal vesicle. (Hacker, 1892.)

Met'aphase, the middle stage of mitosis during which occurs the splitting of the
chromosomes in the equatorial plate. (Strasburger, 1884.)

Met'aplasm (/u-cra. after, beyond; TrXda-fm, a thing formed), a term collectively
applied to the lifeless inclusions (deutoplasm, starch, etc.) in protoplasm as op-
posed to the living substance. (Hanstein, 1868.)

Micella, one of the ultimate supra-molecular units of the cell. (Nageli, 1884.)

Microcentrosonie, equivalent to the central granule or centriole of Boveri.
(Ziegler, 1898.)

Microcen'trum, the centrosome or group of centrosomes united by a " primary
centrodesmus," forming the centre of the astral system. (Heidenhain, 1894.)

Mi'cropyle (/xtKpds, small ; ttvAt/, orifice), the aperture in the egg-membrane
through which the spermatozoon enters. [First applied by Turpin, in 1806,
to the opening through which the pollen-tube enters the ovule. /. Robert
Brown.]

Mi'crosome (/xiKpo?, small ; o-w/xo, body), the granules as opposed to the ground-
substance of protoplasm. (Hanstein, 1880.)

Microsphere, the central region of the aster (centrosphere) at the centre of which
He the centrosomes. (Kostanecki and Siedlecki, 1896.)

Middle-piece, that portion of the spermatozoon lying behind the nucleus at the
base of the flagellum. (Schweigger-Seidel, 1865.)

Mid-body ("Zwischenkorper"), a body or group of granules, probably comparable
with the cell-plate in plants, formed in the equatorial region of the spindle during
the anaphases of mitosis. (Flemming, 1890.)

Mi'tome (/xtrca^a, from /xtVo?, a thread), the reticulum or thread-work as opposed to
the ground-substance of protoplasm. (Flemming, 1882.)

[Mitoschi'sis (/xtVo?, thread; o-xi'^ctv, to split), indirect nuclear division; mitosis.
(Flemming, 1882.)

Mito'sis (/xtVos, a thread), indirect nuclear division typically involving: a, the
formation of an amphiaster; d, conversion of the chromatin into a thread
(spireme) ; c, segmentation of the thread into chromosomes ; d, splitting of the
chromosomes. (Flemming, 1882.)

Mi'tosome {ixLTo<iy a thread ; o-w/xa, body), a body derived from the spindle-fibres

444 GLOSSARY

of the secondary spermatocytes, giving rise, accorj;iing to Platner, to the mid-
dle-piece and the tdl-envelope of the spermatozoon. Equivalent to the Neben-
kern of La Valette St. George. (Platner, 1889.)

Nebenkern (Paranucleus), a name originally appliejd by Butschli (1871) to an
extranuclear body in the spermatid ; afterwards shown by La Valette St. George
and Platner to arise from the spindle-fibres of the secondary spermatocyte.
Since applied to many forms of cytoplasmic bodies (yolk-nucleus, etc.) of the
most diverse nature.

Nuclear plate, i. The equatorial plate. (Strasburger, 1875.) 2. The parti-
tion-wall which sometimes divides the nucleus in amitosis.

Nuclein, the chemical basis of chromatin ; a compound of nucleinic acid and albumin
or albumin radicles. (Miescher, 1871.)

Nucleinic or nucleic acid, a complex organic acid, rich in phosphorus, and an
essential. constituent of chromatin.

Nucleo-albumin, a nuclein having a relatively high percentage of albumin. Dis-
tinguished from nucleo-proteids by containing paranucleinic acid which yields no
xanthin-bodies.

[Nucleochyle'ma] (;(vAos, juice), the ground-substance of the nucleus as opposed
to that of the cytoplasm. (StrasburCtER, 1882.)

Nucleohy'aloplasma (see Hyaloplasm), the achromatic substance (linin) in which
the chromatin-granules are suspended. (Strasburger, 1882.)

Nucleomi'crosomes (see Microsome), the nuclear (chromatin) granules as
opposed to those of the cytoplasm. (Strasburger, 1882.)

Nu'cleoplasm. i. The reticular substance of the (egg-) nucleus. (Van Bene-
DEN, 1875.) 2. The substance of the nucleus as opposed to that of the cell-
body or cytoplasm. (Strasburger, 1882.)

Nucleo-pro'teid, a nuclein having a relatively high percentage of albumin. May
be split into albumin and true nucleinic acid, the latter yielding xanthin-bodies.

CEde'matin (ot8r;/xa, a swelling), the granules or microsomes of the nuclear ground-
substance. (Reinke, 1893.)

O'ocyte (Ovocyte) (woV, egg; kvtos, hollow (a cell)), the ultimate ovarian egg
before formation of the polar bodies. The primary oocyte divides to form the
first polar body and the secondary oocyte. The latter divides to form the second
polar body and the mature ^gg. (Boveri, 1891.)

Oogen'esis, Ovogenesis (woV, egg ; yeVco-i?, origin), the genesis of the egg after its
origin by division from the mother-cell. Often used more specifically to denote
the process of reduction in the female.

OSgo'nium, Ovogonium (a>di/, egg ; yoviy, generation), i. The primordial mother-
cell from which arises the ^gg and its follicle. (Pfluger.) 2. The descend-
ants of the primordial germ-cell which ultimately give rise to the oocytes or
ovarian eggs. (Boveri, 1891.)

Ookine'sis (woV, ^gg\ KiVr;o-t5, movement), the mitotic phenomena of the egg dur-
ing maturation and fertilization. (Whitman, 1887.)

O'vocentre, the egg-centrosome during fertilization. (FoL, 1891.)

Oxychro'matin (o^vs, acid ; see Chromatin), that portion of the nuclear substance
stained by acid tar-colours. Equivalent to 'Minin " in the usual sense.
(Heidenhain, 1894.)

Pangen'esis (ttSs (Trav-), all; ycVeo-ts, production), the theory of gemmules, accord-
ing to which hereditary traits are carried by invisible germs thrown off by the
individual cells of the body. (Darwin, 1868.)

Pangens (Tras (Trav-), all; -yevrjs, producing), the hypothetical ultimate supra-molec-
ular units of the idioplasm, and of the cell generally. Equivalent to gemmules,
micellae, idioblasts, biophores, etc. (De Vries, 1889.)

GLOSSARY 445

Par achro 'matin (see Chromatin), the achromatic nuclear substance (linin of
Schwarz) from which the spindle-fibres arise. (Pfitzner, 1883.)

Parali'nin (see Linin), the nuclear ground-substance or nuclear sap. (Schwarz,
1887.)

Parami'tome (see Mitome), the ground-substance or interfilar substance of proto-
plasm, opposed to mitome. (Flemming, 1892.)

Paranu'clein (see Nuclein), the substance of true nucleoli or plasmosomes.
Pyrenin of Schwarz. (O. Hertwig, 1878.) Applied by Kossel to " nucleins "
derived from the cytoplasm. These are compounds of albumin and paranucleic
acid which yields no xanthin-bodies.

Paranucleus (see Nebenkern).

Par'aplasm (Trapa, beside ; TrXdafxa, something formed), the less active portion of
the cell-substance. Originally applied by Kupffer to the cortical region of the
cell (exoplasm), but now often appHed to the ground-substance. (Kupffer,

1875.)

Periplast (-Trepi', around; TrXao-ro's, form), i. The peripheral part of the cell,
including those parts outside the nucleus or " endoplast.'' (Huxley, 1853.)
2. A term somewhat vaguely applied to the attraction-sphere. The term
daughter-peripiast is applied to the centrosome. (Vejdovsky, 1888.)

Perisphere (Trept, around), a term applied to the outer region of the attraction-
sphere in nerve-cells, and opposed to an inner "'centrosphere." (Lenhossek,
1895.)

Plasmocytes (7rAao-/>ia, kvto^^, colourless blood-corpuscles supposed to be free
attraction-spheres. (Eisen, 1897.)

Plasmosphere, the same as Perisphere.

Plas'mosome (irXda-fm, something formed (i.e. protoplasmic) ; crw/xa, body), the
true nucleus, distinguished by its affinity for acid tar-colours and other " plasma-
stains." (Ogata, 1883.)

Pla'some (TrXdafjui, a thing formed; o-a>p/i, body), the ultimate supra-molecular
vital unit. See Biophore, Pangen. (Wiesner, 1890.)

Plas'tid (TrAasTos, form), i. A cell, whether nucleated or non-nucleated. (Hackel,
1866.) 2. A general term applied to permanent cell-organs (chloroplasts, etc.)
other than the nucleus and centrosome. (Schimper, 1883.)

Plas'tidule, the ultimate supra-molecular vital unit. (Elssberg, 1874; Hackel,
1876.)

Plas'tin, a term of vague meaning applied to a substance related to the nucleo-
proteids and nucleo-albumins constituting the linin-network (Zacharias) and the
cytoreticulum (Carnoy). (Reinke and Rodewald, 1881.)

Pluri'valent (plus, more ; valere, to be worth), applied to chromatin-rods that
have the value of more than one chromosome sensu strictu. (Hacker, 1892.)

Polar bodies (Polar globules), two minute cells segmented off from the ovum
before union of the germ-nuclei. (Disc, by Carus, 1824; so named by Robin,
1862.)

Polar corpuscle, the centrosome. (Van Beneden, 1876.)

Polar rays (Polradien), a term sometimes applied to all of the astral rays as
opposed to the spindle-fibres, sometimes to the group of astral rays opposite to
the spindle-fibres.

Pole-plates (End-plates), the achromatic spheres or masses at the poles of the
spindle in the mitosis of Protozoa, probably representing the attraction-spheres.
(R. Hertwig, 1877.)

Polyspermy, the entrance into the ovum of more than one spermatozoon.

[Prochro'matin] (see Chromatin), the substance of true nucleoli, or plasmosomes.
Equivalent to paranuclein of O. Hertwig. (Pfitzner, 1883.)

446 GLOSSARY

Pronuclei, the germ-nuclei during fertilization; i.e. the egg-nucleus (female pro-
nucleus) after formation of the polar bodies, and the sperm-nucleus (male pro*
nucleus) after entrance of the spermatozoon into the Q.gg. (Van Beneden,
1875.)

[Prothy'alosome] (see Hyalosome), an area in the germinal vesicle (of Ascaris)
by which the germinal spot is surrounded, and which is concerned in formation
of the first polar body. (Van Beneden, 1883.)

Pro'toblast (Trpwro?, first; /^Aaorrd?, a germ), i. A naked cell, devoid of a mem-
brane. (Kolliker.) 2. A blastomere of the segmenting egg which- is the
parent-cell of a definite part or organ. (Wilson, 1892.)

Pro'toplasm (Trpwro?, first; TrAao-ytxa, a thing formed or moulded). The active
or " living " cell-substance. By all earlier and some present writers applied only
to the substance of the cell-body (equivalent to Strasburger's cytoplasm). By
many later writers applied to the entire active substance of the cell (karyoplasm
plus cytoplasm). (Purkinje, 1840; H. von Mohl, 1846.)

Pro'toplast (TrpwTo?, first; TrAao-ro?, formed), i. The protoplasmic body of the
cell, including nucleus and cytoplasm, regarded as a unit. Nearly equivalent to
the energid of Sachs. (Hanstein,"i88o.) 2. Used by some authors synony-
mously with plastid.

[Pseudochro'matin] (see Chromatin), the same as prochromatin. (Pfitzner,
1886.)

Pseudonu'clein (see Nuclein), the same as the paranuclein of Kossel. (Ham-

MARSTEN, 1894.)

Pseudo-reduction, the preliminary halving of the number of chromatin-rods as a

prelude to the formation of the tetrads and to the actual reduction in the number

of chromosomes in maturation. (Ruckert, 1894.)
Pyre'nin (iTvprjv, the stone of a fruit ; z'.e. relating to the nucleus), the substance of

true nucleoli. Equivalent to the paranuclein of Hertwig. (Schwarz, 1887.)
Pyre'noid (-n-vp-^v, the stone of a fruit; like a nucleus), colourless plastids (leuco-

plastids), occurring in the chromatophores of lower plants, forming centres for

the formation of starch. (Schmitz, 1883.)
Reduction, the halving of the number of chromosomes in the germ-nuclei during

maturation.
Sarcode ((rap$, flesh). The protoplasm of unicellular animals. (Du Jardin,

1835-)

Sertoli-cells, the large, digitate, supporting, and nutritive cells of the mammalian
testis to which the developing spermatozoa are attached. (Equivalent to " sper-
matoblast" as originally used by Von Ebner, 1871.)

Sper'matid ((nrep/xa, seed), the final cells which are converted without further
division into spermatozoa ; they arise by division of the secondary spermatocytes
or" Samenmiitterzellen." (La Valete St. George, 1886.)

Sper'matoblasts (o-Trep/xa, seed ; ^Aacrro?, germ), a word of vague meaning,
originally applied to the supporting cell or Sertoli-cell, from which a group of
spermatozoa was supposed to arise. By various later writers used synonymously
with spermatid. (Von Ebner, 1871.)

Sper'matocyst (o-Trep/xa, seed ; KvaTi<s, bladder), originally applied to a group of
sperm-producing cells (^'spermatocytes"), arising by division from an "Ursa-
menzelle" or "spermatogonium." (La Valette St. George, 1876.)

Sper'matocyte (o-Trep/xa, seed; kvtos, hollow (a cell)), the cells arising from the
spermatogonia. The primary spermatocyte arises by growth of one of the last
generation of spermatogonia. By its division are formed two secondary sper-
matocytes, each of which gives rise to two spermatids (ultimately spermatozoa).
(La Valette St. George, 1876.)

GLOSSARY 447

[Sperm atogem'ma] (cnrepfm, seed ; gemma, bud), nearly equivalent to spermato-

cyst. Differs in the absence of a surrounding membrane. [In mammals, La

Valette St. George, 1878.]
Spermatogen'esis (o-Trep/xa, seed; ycVco-ts, origin), the phenomena involved in

the formation of the spermatozoon. Often used more specifically to denote the

process of reduction in the male.
Spermatogo'nium ('' Ursamenzelle ") (cnrepfm, seed; yovrj, generation), the

descendants of the primordial germ-cells in the male. Each ultimate sper-
matogonium typically gives rise to four spermatozoa. (La Valette St.

George, 1876.)
Spermatome'rites (o-Trep/Aa, seed ; fxepos, a part), the chromatin-granules into

which the sperm-nucleus resolves itself after entrance of the spermatozoon. (In

Petromyzon, Bohm, 1887.)
Sper'matosome (crTrcp/xa, seed; aw/ixx, body), the same as spermatozoon. (La

Valette St. George, 1878.)
Spermatozold (see Spermatozoon), the ciliated paternal germ-cells in plants.

The word was first used by von Siebold as synonymous with spermatozoon.
Spermatozo'on (airepfxa, seed; ^aJoi/, animal), the paternal germ-cell of animals.

(Leeuwenhoek, 1677.)
Sperm-nucleus, the nucleus of the spermatozoon ; more especially applied to it after

entrance into the egg before its union with the egg-nucleus. In this sense

equivalent to the '' male pronucleus " of Van Beneden. (O. Hertwig,

1875.)
Sper'mocentre, the sperm-centrosome during fertilization. (Fol, 1891.)
Spi'reme {atrupy^pua., a thing wound or coiled ; a skein), the skein or " Knauel "

stage of the nucleus in mitosis, during which the chromatin appears in the form

of a thread, continuous or segmented. (Flemming, 1882.)
Spon'gioplasm (a-jroyyLOv, a sponge ; irXdapia, a thing formed), the cytoreticulum.

(Leydig, 1885.)
Ste'reoplasm (crripeoq, solid), the more solid part of protoplasm as opposed to the

more fluid " hygroplasm." (Nageli, 1884.)
Substantia hyalina, the protoplasmic ground-substance or '^ hyaloplasm."

(Leydig, 1885.)
Substantia opaca, the protoplasmic reticulum or ^'spongioplasm." (Leydig,

1885.)
Synap'sis (a-vvairTio, to fuse together) . A stage in the nucleus preceding the first

maturation-division, characterized by the massing of the chromatin at one side

of the nucleus. From it the chromatin-masses emerge in the reduced number.

(Moore, 1895.)
Te'loblast (xeAo?, end ; ^Aao-ro?, a germ), large cells situated at the growing end

of the embryo (in annelids, etc.), which bud forth rows of smaller cells. (Whit-
man, Wilson, 1887.)
Telole'cithal (TeXo<;, end ; Acki^o?. yolk), that- type of ovum in which the yolk is

mainly accumulated in one hemisphere. (Balfour, 1880.)
Telophases, Telokine'sis (xeAos, end), the closing phases of mitosis, during

which the daughter-nuclei are re-formed. (Heidenhain, 1894.)
To'noplasts (t6vo<^, tension ; TrAao-ros, form), plastids from which arise the vacuoles

in plant-cells. (De Vries, 1885.)
Trophoplasm (rpocfirj, nourishment ; TrXacrpa) . i . The nutritive or vegetative

substance of the cell, as distinguished from the idioplasm. (Nageli, 1884.)

2. The active substance of the cytoplasm other than the "kinoplasm " or archo-

plasm. (Strasburger, 1892.)
Tro'phoplasts (Tpo<t>ri, nourishment ; TrAao-ros, form), a general term, nearly equiv

448 GLOSSARY

alent to the "plastids" of Schimper, including "anaplasts" (amyloplasts),

"autoplasts" (chloroplasts), and chromoplasts. (A. iMeyer, 1882-83.)
Yolk-nucleus, a word of vague meaning applied to a cytoplasmic body, single or

multiple, that appears in the ovarian egg. [Named " Dotterkern " by Carus,

1850.)
Zy'gote or Zy'gospore (^ryov, a yoke), the cell produced by the fusion of two

conjugating cells or gametes in some of the lower plants.

GENERAL LITERATURE-LIST

The following list includes only the titles of works actually referred to in the text
and those immediately related to them. For more complete bibliography the reader
is referred to the literature-lists in the special works cited, especially the following.
For reviews of the early history of the cell-theory see Remak's Untersuchnngen
(■50-55), Huxley on the Cell-theory ('53), Sach's History of Botany and Tyson's
Cell-doctrine ('78)- An exhaustive review of the earlier literature on proto-
plasm, nucleus, and cell-division will be found in Flemming's Zellsnbstanz ('82),
and a later review of theories of protoplasmic structure in Butschli's Protoplasma
('92) and in Fischer's Fixierimg^ etc., des Frotoplasmas C99). The earlier work on
mitosis and fertilization is very thoroughly reviewed in Whitman's Clepsine ('78),
Fol's Henogenie ('79), and Mark's Liniax ('81). For more recent general litera-
ture-lists see especially Hertwig's Zelle und Gewebe ('93, '98), Yves Delage ('95),
Henneguy's Cellide C96), Hacker's Praxis und Theorie der Zelleti und Befruch-
tungslehre ('99). and the admirable reviews by Flemming, Boveri, RUckert, Meves,
Roux, and others in Merkel and Bonnet's Ergebnisse ('91-98).

The titles are arranged in alphabetical order, according to the system adopted in
Minofs Human Embryology . Each author's name is followed by the date of publi-
cation (the first two digits being omitted, except in case of works published before
the present century), and this by a single number to designate the paper, in case
two or more works were published in the same year. For example, Boveri, Th.,
'87, 2, denotes the second paper published by Boveri in 1887.

In order to economize space, the following abbreviations are used for the journals
most frequently referred to : —

ABBREVIATIONS

A. A. Anatomischer Anzeiger.

A. B. Archives de Biologic.

A. A. P. Archiv fUr Anatomic und Physiologie.

A. m. A. Archiv fiir mikroscopische Anatomic.

A. Entwm. Archiv fiir Entwicklungsmechanik.

B. C. Biologisches Centralblatt.

C. R. Comptes Rcndus.

/. M. Journal of Morphology.

J. w. Bot. Jahrbuch fur wissenschaftliche Botanik.

J. Z. Jenaische Zeitschrift.

M. A. Mviller's Archiv.

M. J. Morphologisches Jahrbuch.

Q.J. Quarterly Journal of Microscopical Science.

Z. A. Zoologischer Anzeiger.

Z. w. Z. Zeitschrift fiir wissenschaftliche Zoologie.

ALBRECHT, E., '98. Untersuchungen zur Structur des Seeigeleies : Sitzb. Ges.

Morph. Phys. Mi'mchen... 3. — Altman, R., '86. Studien iiber die Zelle, I. : Leipzig.

— Id., '87. Die Genese der Zellen : Leipzig. — Id., '89. Uber Nucleinsaure : A.

A. P., p. 524. — Id., '90, '94. Die Elementarorganismen und ihre Beziehung zu

2G 449

450 GENERAL LITERATURE-LIST

den Zellen : Leipzig. — Amelung, E., '93. Uber mittlere Zellgrosse : Flora, p. 176.

— Andrews, E. A., 9S, 1. Filose Activities in Metazoan Eggs: Zool. Bull.., 11., i.

— Id., '98, 2. Activities of Polar Bodies of Cerebratulus : Arch. Entwm., VI., 2. —
Andre-ws, G. F., "97. The Living Substance as Such and as Organism : J. M.y
XII., 2, Suppl. — Arnold, J., '79. Uber feinere Struktur der Zellen, etc. : Virchow's
Arch., 1879. (See earlier papers.) — Atkinson, G. F., '99. Studies on Reduction
in Plants : Bot. Gaz., XXVIII., 1,2. — Auerbach, L., '74. Organologische Studien :
Breslau. — Id., '91. Uber einen sexuellen Gegensatz in der Chromatophilie der
Keimsubstanzen : Sitzungsber. der K'dnigl. preuss. Akad. d. IViss. Berli?i, XXXV.

— Id. '96. Untersuchungen Uber die Spermatogenese von Paludina : /. Z., XXX.

VON BAER, C. E., '28, '37. ttber Entwickelungsgeschichte der Thiere. Beo-
bachtungund Reflexion: I. Konigsberg, 1828; II. 1837. — Id., '34. Die Metamor-
phose des Eies der Batrachier : Muller''s Arch. — Balbiani, B. G., '61. Recherches
sur les ph^nomenes sexuels des Inflisoires : Journ. de la Phys., IV. — Id., '64. Sur
la constitution du germe dans I'ceuf animal avant la fdcondation : C. R., LVIII. —
Id., '76. Sur les phenomenes de la division du noyau cellulaire : C. R., XXX.,
October, 1876. — Id., '81. Sur la structure du noyau des cellules salivares chez les
larves de Chironomus : Z. A., 1881, Nos. 99, 100. — Id., '89. Recherches expdri-
mentales sur la merotomie des Inflisoires cilies : Recueil Zool. Suisse, January, 1889.

— Id., '91, 1. Sur les regenerations successives du peristome chez les Stentors et
sur le rdle du noyau dans ce phdnomene : Z. A., 372, 373. — Id., '91, 2. Sur
la structure et division du noyau chez les Spirochona gemmipara : Ann. d,
Micrographie. — Id., '93. Centrosome et Dotterkern : Journ. de Vanat. et de la
physiol, XXIX.— Balfour, F. M., '80. Comparative Embryology: I. 1880.—
Ballo-witz, '88-'91. Untersuchungen uber die Struktur der Spermatozoen : i . (birds)
A. m. A.,y>.XX\\., 1888; 2. (insects) Z.w.Z., LX., 1890; 3. (fishes, amphibia,
reptiles) A. m. A., XXXVI., 1890; 4. (mammals) Z. w. Z., 1891. — Id., '89.
Fibrillare Struktur und Contractilitat : Arch. ges. Phys., XLVI. — Id., '91, 2.
Die innere Zusammensetzung des Spermatozoenkopfes der Saugetiere : Cetttralb.f.
Phys., V. — Id., '95. Die Doppelspermatozoa der Dytisciden : Z. w. Z., XLV., 3. —
Id., '97. tJber Sichtbarkeit und Aussehen der ungetarbten Centrosomen in ruhen-
den Gewebszellen : Z. w. Mic. XIV. — Id., '98. Zur Kenntniss der Zellsphare:
Arch. Anat. Phys., '98, II., III. — Van Bambeke, C, '93. Elimination d'ele'ments
nucleaires dans I'oeuf ovarien de Scorpaena scrofa: A. B., XIII., i. — Id., '96. De
I'emploi du terme Protoplasma : Bull. Soc. Beige. Mic, XXII. — Id., '97. A propos
de la delimitation cellulaire: Ibid., XXIII. — Id., '98. Recherches sur I'oocyte
de Pholcus phalangioides : A. B., XV. — De Bary, '58. Die Conjugaten. — Id., '62.
Uber den Bau und das Wesen der Zelle : Flora, 1862. — Id., '64. Die Mycetozoa :
2d Ed., Leipzig. — Barry, M. Spermatozoa observed within the Mammiferous
Ovum: Phil. Trans., 1843. — Beale, Lionel S., '61. On the Structure of Simple
Tissues of the Human Body : London. — B^champ and Ester, '82. De la consti-
tution elementaire des tissues: Montpellier. — Belajeff, W., '89. Mittheilung
iiber Bau und Entwicklung der Spermatozoiden : Ber. D. Bot. Ges. — Id., '92,1.
Uber den Bau und die Entwicklung der Antherozoiden, I., Characeen. — Id., '92, 2.
Uber die Karyokinesis in den Pollenmutterzellen bei Larix und Fritillaria : Sitzb.
Warsch. Naturf. Ges. — Id., '94, 1. Zur Kenntniss der Karyokinese bei den
Pflanzen: Flora, 1894, Erganzungsheft. — Id., '94, 2. Uber Bau und Entwicklung
der Spermatozoiden der Pflanzen: Flora^ LIV. — Id., '97, 1. tJber den Neben-
kern in Spermatogenen Zellen und die Spermatogenese bei den Farnkrauten : Ber.
D. Bot. Ges., XV. — Id., '97, 2. Uber die Spermatogenese bei den Schachtel-
halmen : Ibid. — Id., '97, 3. Uber die Aehnlichkeit einiger Erscheinungen in
der Spermatogenese bei Thieren und Pflanzen : Ibid. — Id., '97, 4. Einige Streit-

i

GENERAL LITERATURE-LIST 45 I

fragen in den Untersuchungen liber die Karyokinese: Ibid. — Id., '98, 1. Uber
die Reductionstheilung des Pflanzenkerns : Ibid.^ XVI. — Id., '98, 2. Uber
die Cilienbildner in den spermatogenen Zellen : Ibid. — Id., '99. Uber die
Centrosomen in den spermatogenen Zellen: Ibid., XVII., 6. — Benda, C, '87.
Untersuchungen uber den Bau des funktionirenden Samenkenkanalchens einiger
Saugethiere: A. m. A. — Id., '93. Zellstrukturen und Zelltheilungen des Sala-
manderhodens : Verh. d. Anat. Ges., 1893. — Van Beneden, E., '70. Recher-
ches sur la composition et la signification de Tceuf: Alem. cour. de VAc. roy.
d. S. de Belgigiie, 1870. — Id., '75. La maturation de I'oeuf, la f(^condation et les
premieres phases du developpement embryonnaire des mammiferes d'apres des
recherches faites chez le lapin : Bidl. Ac. roy. de Belgigue, XI. — Id., '76, 1.
Recherches sur les Dicyemides : Bidl. Ac. roy. Belgigue., XLI., XLII. — Id.,
'76,2. Contribution a I'histoire de la vesicule germinative et du premier noyau
embryonnaire : Ibid., XLI. ; also Q.J., XVI. — Id., '83. Recherches sur la matura-
tion de I'oeuf, la fecondation et la division cellulaire : A. B., IV. — Van Beneden
and Julin, '84, 1. La segmentation chez les Ascidiens et ses rapports avec I'organi-
sation de la larve : Ibid.,V . — Id., '84, 2. La spermatogenese chez I'Ascaride
megalocephale : BidL Ac. roy. Belgigue, 3me sen, VII. — Van Beneden, E., et
Neyt, A., '87. Nouvelles recherches sur la fecondation et la division mitosique
chez I'Ascaride megalocephale: Ibid., 1887. — Bergh, R. S., '89. Recherches sur
les noyaux de I'Urostyla : A. B. IX. — Id., '94. Vorlesungen iiber die Zelle und die
einfachen Gevvebe : Wiesbaden. — Id., '95. Uber die relativen Thdlungspotenzen
einiger Embryonalzellen : A. Entm., II., 2. — Bernard, Claude. Le9ons sur les
Phenomenes de la Vie: ist Ed. 1878, 2d Ed. 1885, /'^r/j. — Berthold, G, '86.
Studien iiber Protoplasma-mechanik : Leipzig. — Bickford, E. E., '94. Notes on
Regeneration and Heteromorphosis of Tubularian Hydroids : /. M., IX., 3. —
Biondi, D., '85. Die Entvvicklung der Spermatozoiden : A. m. A., XXV. — Blanc,
H., '93. Etude sur la fecondation de I'oeuf de la truite : Ber. Naturforsch. Ges.
zu Freiburg, VIII. — Blochmann, F.,'87, 2. Uber die Richtungskorper bei Insek-
teneiern : M. J., XII. — Id., '88. Uber die Richtungskorper bei unbefruchtet sich
entwickelnden Insekteneiern : Verh.naturh. med. Ver. Heidelberg, N. F., IV., 2. —
Id., '89. iiber die Zahl der Richtungskorper bei befruchteten und unbefruchteten
Bieneneiern : M.J. — Id., '94. tJber die Kerntheilung bei Euglena : B. C., XIV. —
B&hm, A., '88. Uber Reifung und Befruchtung des Eies von Petromyzon Planeri :
A. ifi. A., XXXII. — Id.. '91. Die Befruchtung des Forelleneies : Sitz.-Ber. d. Ges.
f. Morph. u. Phys. Muncheti, VII. — Boll, Fr., '76. Das Princip des Wachsthums :
Bet'lin. — Bonnet, C, 1762. Considerations sur les Corps organises : Amsterdam.
— Born, G., '85. Uber den Einfluss der Schwere auf das Froschei : A. m. A.,
XXIV. — Id., '94. Die Structur des Keimblaschens im Ovarialei von Triton
taeniatus : A. m. A., XLIII. — Bourne, G. C, '95. A Criticism of the Cell-theory ;
being an Answer to Mr. Sedgwick's Article on the Inadequacy of the Cellular
Theory of Development ; Q. J., XXXVIII., i.— Boveri, Th., '86. tlber die
Bedeutung der Richtungskorper: Sitz.-Ber. Ges. Morph. u. Phys. Munchen, II. —
Id., '87, 1. Zellenstudien, Heft I. ; /. Z., XXI. —Id., '87, 2. Uber die Befruch-
tung der Eier von Ascaris megalocephala : Sitz.-Ber. Ges. Morph. Phys. Mihtchen,
III. — Id., '87, 2. Uber den Anteil des Spermatozoon an der Teilung des Eies:
Sitz.-Ber. Ges. Morph. Phys. Miinchen, III., 3. — Id., '87, 3. tJber Differenzierung
der Zellkerne wah rend der Furchung des Eies von Ascaris meg.: A. A., 1887. —
Id., '88, 1. iiber partielle Befruchtung : Sitz.-Ber. Ges. Morph. Phys. Munchen, IV.,
2. — Id., '88, 2. Zellenstudien, II. : /. Z., XXII. — Id., '89. Ein geschlechtlich
erzeugter Organismus ohne miitterliche Eigenschaften : Sitz.-Ber. Ges. Morph. Phys.
Munchen, V. Trans, in A7n. N'at., March, '93. — Id., '90. Zellenstudien, Heft III. :
/. Z.y XXIV. — Id., '91. Befruchtung: Merkel und Bonnefs Ergebnisse, I. — Id.,

452 GENERAL LITERATURE-LIST

'95, 1. ttber die Befruchtungs- und Entwickelungsfahigkeit kernloser Seeigel-Eier,
etc. : A. EntwjH. II., 3. — Id., '95, 2. Uber das Verhalten der Centrosomen bei der
Befruchtung des Seeigeleies, nebst allgemeinen Bemerkungen liber . Centrosomen
und Vervvandtes : Verh. d. Physikal.-ined. Gesellschaft zji VVurzburg^ N. F.,
XXIX., I. — Id., '96. Zur Physiologie der Kern- und Zellteilung : Sitzb. Phys.-
Med. Ges. W'urzburg. — Braem, F., '93. Das Prinzip der organbildenden Keim-
bezirke und die entwicklungsmechanischen Studien von H. Driesch : B. C, XIII.,
4, 5, — Brandt, H., '77. Uber Actinosphaerium Eichhornii : Dissertation, Halle^
1877. — Brass, A., '83-4. Die Organisation der thierischen Zelle : Halle. —
Brauer, A., '92. Das Ei von Branchipus Grubii von der Bildung bis zur Ablage :
Abh. preuss. Akad. Wiss., '92. — Id., '93,1. Zur Kenntniss der Reifung des par-
thenogenetisch sich entwickelnden Eies von Artemia Salina: A. in. A., XLIII. —
Id., '93, 2. Zur Kenntniss der Spermatogenese von Ascaris megalocephala :
A. m. A., XLII. — Id,, '94. IJber die Encystierung von Actinosphaerium Eich-
hornii: Z. w. Z., LVIII., 2. — Braus, '95, Uber Zellteilung und Wachstum des
Tritoneies: /, Z., XXIX. — Brooks, W. K., '83. The Law of Heredity: Balti-
more. Bro-wn, H. H., '85. On Spermatogenesis in the Rat : Q. /., XXV. —
Brown, Robert, '33, Observations on the Organs and Mode of Fecundation in
Orchideae and Asclepiadeae : Trans. Linn. Soc, 1833. Briicke, C, '61. Die Ele-
mentarorganismen : Wiener Sitzber.,X.\A\ .,\Z6\. Brunn, M. von, '89. Beitrage
zur Kenntniss der Samenkorper und ihrer Entwickelung bei Vogein und Sauge-
thieren: A. m^ A., XXXIII. — De Bruyne, C, '95, La sphere attractive dans les
cellules fixes du tissu conjonctif: Bnll. Acad. Sc. de Belgique., XXX, — Burger, O.,
'91, Uber Attractionsspharen in den Zellkorpern einer Leibesfliissigkeit : A. A.y
VI, — Id., '92. Was sind die Attractionsspharen und ihre Centralkorper? A. A.,
1892. — Butschli, O., '73, Beitrage zur Kenntniss der freilebenden Nematoden :
Nova acta acad. Car. Leopold, XXXVL — Id., '75, Vorlaufige Mitteilungen liber
Untersuchungen betreffend die ersten Entwickelungsvorgange im befruchteten Ei
von Nematoden und Schnecken: Z, iv. Z., XXV. — Id,, '76. Studien liber die
ersten Entwickelungsvorgange der Eizelle, die Zellteilung und die Konjugation der
Infusorien : Abh. des Senckenb. Naturforscher-Ges., X. — Id,, '85. Organisations-
verhaltnisse der Sog. Cilioflagellaten und der Noctiluca: M. /., X. — Id., '90.
Uber den Bau der Bakterien, etc.: Leipzig. — Id., '91. Uber die sogenannten
Centralkorper der Zellen und ihre Bedeutung: Verh. Natiirhist. Med. Ver. Heidel-
berg, 1 891, — Id,, '92, 1, tlber die klinstHche Nachahmung der Karyokinetischen
Figuren : Ibid., N, F,, V, — Id,, '92, 2, Untersuchungen liber mikroskopische
Schaume und das Protoplasma (full review of literature on protoplasmic structure) :
Leipzig {Engelniajin) . — Id,, '94, Vorlaufige Berichte uber fortgesetzte Unter-
suchungen an Gerinnungsschaumen, etc, : Verh. Natiirhist. Ver. Heidelberg, V.
— Id,, '96, Weitere AusfUhrungen liber den Bau der Cyanophyceen und Bakterien :
Leipzig. — Id., '98, Untersuchungen liber Strukturen : Leipzig {Engehnami).

CALKINS, G, N,, '95, 1. Observations on the Yolk-nucleus in the Eggs of
Lumbricus : Trans. N.V. Acad. Sci., June, 1895, — Id,, '95, 2, The Spermato-
genesis of Lumbricus : /, M., XL, 2. — Id,, '97, Chromatin-reduction and Tetrad-
formation in Pteridophytes : Bull. Torrey Bot. Chib, XXIV, — Id,, '98, 1, The
Phylogenetic Significance of Certain Protozoan Nuclei: Ann. N. Y. Acad. Sci., XL,
16. —Id., '98, 2, Mitosis in Noctiluca: Ginn & Co., Boston, also/. M., XV., 3.—
Calberla, E., '78. Der Befruchtungsvorgang beim Ei von Petromyzon Planeri :
Z.W.Z., XXX. — Campbell, D. H,, '88-9, On the Development of Pilularia
globulifera : Ann. Bot., II, — Carnoy, J. B,, '84, La biologie cellulaire : Lierre. —
Id., '85. La cytodierese des Arthropodes : La Cellule. I, — Id,, '86. La cytodie-
rese de I'oeuf: La Cellule, III. — Id,, '86. La vesicule germinative et les globules

GENERAL LITERATURE-LIST 453

polaires chez quelqiies Nematodes: La Cellule, III. — Id., '86. La segmentation
de Toeuf chez les Nematodes: La Cellule, III., i. — Carnoy and Le Brun, '97, 1,
'b?8. "99. La v^sicule germinative et les globules polaires chez les Batraciens : La
Cellule, XII, XIV, XVI. — Id.-, "97, 2. La fecondation chez TAscaris megalo-
cephala : La Cellule, XIII. — Castle, W. E , '96. The Early Embryology of Ciona
intestinalis : Bull. Mus. Comp. Zo'dl., XXVTL, 7. — Chabry, L., '87. Contribu-
tions a Fembryologie normale et pathologique des ascidies simples: Paris., 1887.

— Child, C. M., '97. The Maturation and Fertilization of the Egg of Arenicola :
Trans. N. Y. Acad. Set., XVI. — Chittenden, R. H., '94. Some Recent Chemico-
physiological Discussions regarding the Cell: ^bn. Nat.., XXVIII. , Feb., 1894. —
Chun, C, '90. Uber die Bedeutung der direkten Zelltheilung : Sitzb. Schr. Physik-
Okon. Ges. Konigsberg, 1890. — Id., '92, 1. Die Dissogonie der Rippenquallen :
Festschr. f. Le7ickart, Leipzig^ 1892. — Id., '92,2. (In Roux, '92, p. 55): Verh d.
Anal. Ges.. VI., 1892. — Clapp, C. M., "91. Some Points in the Development of
the Toad-Fish : y. vJ/., V. — Clarke, J. Jackson, "95. Observations on various
Sporozoa: Q.J., XXXVIL, 3. — Coe, "W. R., '99. The Maturation and Fertiliza-
tion of the Egg of Cerebratulus : Zo'dl. Jahrb., XII. — Cohn, Ferd., '51. Nachtrage
zur Naturgeschichte des Protococcus pluvialis : Nova Acta, XXII. — Conklin, E. G.,
'94. The Fertilization of the Ovum : Biol. Led., Marine Biol. Lab., Wood's Holl,
Boston, 1894. — Id, '96. Cell-size and Body-size: Rept. of Am. Morph. Soc.
Science, III., Jan. 10, 1896. — Id,, '97, 1. Nuclei and Cytoplasm in the Intestinal
Cells of Land Isopods : Am. A^at., Jan. — Id., '97, 2. The Embryology of Crepidula :
/. J/., XIII., I. — Id., '98. Cleavage and Differentiation: Wood's Holl Biol. Lec-
tures. — Id., '99. Protoplasmic Movement as a Factor in Differentiation: Wood's
Holl Biol. Lectures. — Crampton, H. E., '94. Reversal of Cleavage in a Sinistral
Gasteropod: Ann. N. V. Acad. Sci., March, 1894. —Id., '97. The Ascidian Half-
Embryo: /bid., June 19. — Id., '99. The Ovarian History of the Egg of Molgula:
/. M., XV., Suppl. — Crampton and Wilson, '96. Experimental Studies on
Gasteropod Development (H. E. Crampton). Appendix on Cleavage and Mosaic-
Work (E. B. Wilson) : A. Entivm.. III., i. — Czermak, N., '99. tJber die Desin-
tegration und die Reintegration des Kernkorperchens, etc. : A. A., XV., 22.

DAR'WIN, F., '77. On the Protrusion of Protoplasmic Filaments, etc. : Q.J,
XVII.— Davis, B. M., '99. The Spore-mother-cell of Anthoceros : Bot. Gaz.^
XXVIII. , 2. — Debski, B., '97. Beobachtungen liber Kerntheilung bei Chara:
/. w. />'., XXX.— Id., '98. Weitere Beobachtungen an Chara: /bid., XXXII., 4.

— Delage, Yves, '95. La Structure du Protoplasma et les Theories sur I'heredite
et les grands Problemes de la Biologic Generale : Paris, 1895. — Id., '98. Embry-
ons sans noyau maternel : C. R., CXXVIL, 15. — Id., '99. La fecondation mero-
gonique et ses resultats : C. R., Oct. 23. — Demoor, J., '95. Contribution k
I'etude de la physiologic de la cellule (independance fonctionelle du protoplasme et
du noyau) : A. B., XIII. — Dendy, A., '88. Studies on the Comparative Anatomy
of Sponges: Q.J., Dec, 1888. — Dixon, H. H., '94. Fertilization of Pinus : Ann.
Bot., VIII. — Id., '96. On the Chromosomes of Lilium longliflorum : Proc. R. /r.
Ac, III. — Doflein, F. J., '97, 1. Die Eibildung bei Tubularia : Z. w. Z., LXII.,
I. — Id., '97, 2. Karyokinesis des Spermakerus : A. m. A., L, 2. — Dogiel, A. S.,
'90. Zur Frage liber das Epithel der Harnblase : A. m. A., XXXV. — Driesch,
H., '92, 1. Entwickelungsmechanisches : A. A., VII., 18. — Id. Entwicklungs-
mechanische Studien, I., II., 1892, Z. iv. Z., LIII. ; III. -VI., 1893, /bid., LV. ;
VII.-X., 1893 : Mitt. Zo'dl. St. Neapel, XL, 2. — Id., '94. Analytische Theorie der
organischen Entwicklung: Leipzig. — Id., '95, 1. Von der Entwickelung einzelner
Ascidienblastomeren : A. Entwm., I., 3. — Id., '95, 2. Zur Analysis der Potenzen
embryonaler Organzellen : • Lbid.^ II. — Id., '98,1. Uber den Organisation des

454 GENERAL LITERATURE-LIST

Eies: Entwm.y IV. — Id... '98, 2. Von der Beendigung morphogener Elemen-
tarprocesse: Arch. Entwm., VI. — Id., '98, 3. Ueber rein-miitterliche Charaktere
an Bastardlarven von Echiniden: Ibid., VII., i. — Id., '99. Die Localisation mor-
phogenetischer Vorgange : Ibid., VIII., i. — Driesch and Morgan, '95, 2. Zur
Analysis der ersten Entwickelungsstadien des Ctenophoreneies : Ibid., II., 2. —
Driiner, L., '94. Zur Morphologie der Centralspindel : /. Z., XXVIII. (XXI.).—
Id., '95. Studien liber den Mechanismus der Zelltheilung : Ibid., XXIX., 2. — Du-
sing, C, '84. Die Regulierung des Geschlechtsverhaltnisses : Jena, 1884.

VON EBNER, V., '71. Untersuchungen iiber den Bau der Samencanalchen und
die Entwicklung der Spermatozoiden bei den Saugethieren und beim Menschen :
Inst. Phys. u. Hist. Graz., 1871 (^Leipzig). — Id., '88. Zur Spermatogenese bei
den Saugethieren: A. m. A., XXXI. — Ehrlich, P., '79. Uber die specifischen
Granulationen des Blutes : A. A. P. {Phys.), i%7^, p. 573. — Eisen, G., '97.
Plasmocytes: Proc. Cat. Acad. Sci., I., i. — Id., '99. The Chromoplasts and the
Chromioles: B. C, XIX., 4. — Eismond, J., '95. Einige Beitrage zur Kenntniss
der Attraktionsspharen und der Centrosomen : A.A.,X. — Endres and "Walter,
'95. Anstichversuche an Eiern von Rana fusca : A. Entwm., II., i. — Engelmann,
T. W., '80. Zur Anatomic und Physiologic der Flimmerzellen : Arch. ges. Phys.,
XXIII. — von Erlanger, R., '96, 1. — Die neuesten Ansichten liber die Zelltheilung
und ihre Mechanik : Zo'dl. Centralb., III., 2. — Id., '96, 2. Zur Befruchtung des
Ascariseies nebst Bemerkungen liber die Struktur des Protoplasmas und des Centro-
somas: Z. A., XIX. — Id., '96, 3. Neuere Ansichten liber die Struktur des Proto-
plasmas: ZooL Centralb., Ill,, 8,9. — Id., '96, 4. Zur Kenntniss des feineren
Baues des Regenwurmhodens, etc. : A. m. A., XLVII. — Id., '96, 5. Die Versoni-
sche Zelle : Zo'dl. Centralb., III., 3. — Id., '96, 6. Die Entwicklung der mannlichen
Geschechtszellen : Ibid., III., 12. — Id., '97, 1. Uber Spindelreste und den echten
Nebenkern, etc.: Zo'dl. Centralb., IV., i. — Id., '97, 2. Uber die sogenannte
Sphare in den mannlichen Geschlechtszellen : Ibid., IV., S- — Id., '97, 3. Uber
die Chromatinreduktion in der Entwicklung der mannlichen Geschlechtszellen:
Ibid., IV., 8. — Id., '97, 4. Beitrage zur Kenntniss des Protoplasmas, etc. A. ni. A.y
XLIX. — Id., '97,5. tlber die Spindelbildung in den Zellen der Cephalopoden
Keimscheibe: B. C, XVII., 20. — Id., '98. Uber die Befruchtung, etc., des
Seeigeleies : B. C, XVIII., i. — Errera, '86. Eine fundamental Gleichgewichtsbe-
dingung organischen Zellen: Ber. Deiitsch. Bot. Ges., 1886. — Id., '87. Zellformen
und Seifenblasen : Tagebl. der 60 Versam?nlung dentscher Naturforscher und Aerzte
zu Wiesbaden, 1887.

FAIRCHILD, D. G., '97. Uber Kerntheilung und Befruchtung bei Basidio-
bolus : Jahrb. wiss. Bot., XXX. — Farmer, J. B., '93. On nuclear division of the
pollen-mother-cell of Lilium Martagon: Ann. Bot. VII., 27. — Id., '94. Studies in
Hepaticae: Ibid., VIII., 29. — Id., '95, 1. Uber Kernteilung in Lilium-Antheren,
besonders in Bezug auf die Centrosomenfrage : Flora, 1895, p. 57. — Id,, 95. 2. On
Spore-formation and Nuclear Division in the Hepaticae: Ami. Bot., IX. — Farmer
and Moore, '95. On the essential similarities existing between the heterotype
nuclear divisions in animals and plants : A. A., XI., 3. — Farmer and "Williams,
'96. On Fertilization, etc., in Fucus : A?in. Bot., X. — Fick, R., '93. Uber die
Reifung und Befruchtung des Axolotleies : Z.w.Z., LVL, 4. — Id., '97. Bemer-
kungen zu M. Heidenhain's Spannungsgesetz : Arch. Anat. u. Phys. (Anat.). —
Fiedler, C, '91» Entwickelungsmechanische Studien an Echinodermeneiern :
Festschr. N'dgeli u. Kblliker, Zurich, 1891. — Field, G. "W., '95. On the Mor-
phology and Physiology of the Echinoderm Spermatozoon: / M., XI. — Fischer,
A., '94, 1. Zur Kritik der Fixierungsmethoden der Granula: A. A., IX., 22. —

GENERAL LITERATURE-LIST 455

Id., 94, 2.— Uber die Geisseln einiger Flagellaten : /. w. B. XXVII. — Id.,
'95. Neue Beitrage zur Kritik der Fixierungsmethoden : A.A.,^. — Id., '97.
Untersuchungen iiber den Bau der Cyanophyceen und Bakterien : Jena, Fischer. —
Id., '99. Fixierung, Farbung und Bau des Protoplasmas : Ibid. — Flemming, W.,
'75. Studien in der Entwicklungsgeschichte der Najaden : Sitzb. d. k. k. Akad.
Wiss. Wieti, LXXI., 3. — Id., '79,1. Beitrage zur Kenntniss der Zelle und ihre
Lebenserscheinungen, I. : A. in. A.y XVI. — Id., '79, 2. Uber das Verhalten des
Kerns bei der Zelltheilung, etc.: Virchow's Arch.. LXXVII. — Id.. '80. Beitrage
zur Kenntniss der Zelle und ihrer Lebenserscheinungen. II. : A. m. A.. XIX. — Id.,
'81. Beitrage zur Kenntniss der Zelle und ihrer Lebenserscheinungen, III. : Ibid..,
XX.— Id., '82. Zellsubstanz, Kern und Zellteilung : Leipzig. 1882.— Id., "87.
Neue Beitrage zur Kenntniss der Zelle: A. m. A.. XXIX. — Id., '88. Weitere
Beobachtungen uber die Entwickelung der Spermatosomen bei Salamandra maculosa :
/^/^., XXXI. — Id., '91-'97. Zelle, I.-VI. : Ergebn. Anat. u. Entwicklmigsgesch.
{Merkel and Bonnet^. 1891-97. — Id., '91,1. Attraktionsspharen u. Centralkorper
in Gewebs- u. Wanderzellen : A. A. — Id., '91, 2. Neue Beitrage zur Kenntniss der
Zelle, II. TeU: A. m. A.. XXXVIL— Id., '95, 1. Uber die Struktur der Spinai-
ganglienzellen : VerhatidL der anat. Gesellschaft in Basel. 17 April. 1895, p. 19. —
Id., '95, 2. Zur Mechanik der Zelltheilung : A. m. A.. XLVI. — Id., '97. 2. Ueber
den Bau der Bindegewebszellen, etc.: Zeit. BioL.XXXW . — Floderus. M., '96.
tJber die Bildung der Follikelhiillen bei den Ascidien : Z. w. Z.. LXl.. 2. — Fol, H.,
'73. Die erste Entwickelung des Geryonideies : J. Z., VII. — Id., '75. Etudes sur
le developpement des Mollusques. — Id., '77. Sur le commencement de I'henogenie
chez divers animaux : Arch. Sci. Nat. et Phys. Geneve. LVIII. See also Arch. ZooL
Exp.., VI. — Id., '79. Recherches sur la fecondation et la commencement de Th^-
nogenie : Mem. de la Soc. de physique et d'hist. nat.^ Genh>e. XXVI. — Id., '91.
Le Quadrille des Centres. Un episode nouveau dans Thistoire de la fecondation :
Arch, des sci. phys. et tiat., 15 Avril, 1891 ; also, A. A.. 9-10. 1891. — Foot, K.,
'94. Preliminary Note on the Maturation and Fertilization of Allolobophora : /. M.y
IX., 3, '94. — Id., '96. Yolk-nucleus and Polar Rings: Ibid.. XII., i. — Id., "97.
The Origin of the Cleavage Centrosomes : /. M., XII., 3. — Fraucotte, P., '97.
Recherches sur la maturation, etc., chez les Polyclades : Mem. cour. Acad. Sci. Belg.

— Frenzel, J., '93. Die Mitteldarmdriise des Flusskrebses und die amitotische
Zelltheilung: A. in. A.. XLI. — Fromman, C, '65. Uber die Struktur der Binde-
substanzzellen des Ruckenmarks : Centrl. f. med. Wiss., III., 6. — Id., '75. Zur
Lehre von der Structur der Zellen : /. Z., IX. (earlier papers cited). — Id., '84.
Untersuchungen iiber Struktur, Lebenserscheinungen und Reactionen thierischer
und pflanzlicher Zellen: /. Z., XVII. — Fiirst, E., '98. Uber Centrosomen bei
Ascaris: A. ni. A., LII. — Fulmer, E. L., '98. Cell-division in Pine Seedlings:
Bot. Gaz., XXVI., 4.

GALEOTTI, GINO, '93. Uber expenmentelle Erzeugung von Unregelmassig-

keiten des karyokinetischen Processes: Bei. zur patholog. Anat. u. z. Allg. Pathol.,
XW .,1, Jena, Fischer, 1893. — Gallardo, Angel, '96. La Carioquinesis : Ann.
Soc. Cientif. Argentina. XLII. — Id., '97. Significado Dinamico de las* Figuras
Cariocineticas : Ibid., XLIV. — Gardiner, E. G., '98. The Growth of the Ovum,
etc., in Polychoerus: /. M., XV., i. — Gardiner, W., '83. Continuity of Proto-
plasm in Vegetable Cells: Phil. Trans., CLXXIV. — Garnault, '88, '89. Sur les
phenomenes de la fecondation chez Helix aspera et Arion empiricorum : Zo'dl. Anz.,
XL, XII. — Geddes and Thompson. The Evolution of Sex: London. 1899. —

— Gegenbaur, C, '54. Beitrage zur naheren Kenntniss der Schwimmpolypen :
Z. w. Z.. V. — Van Gehuchten, A., "90. Recherches histologiques sur I'appareil
digestif de la larve de la Ptychoptera contaminata : La Cellule, VI. — Giard, A., '77.

456 GENERAL LITERATURE-LIST

Sur la signification morphologique des globules polaires : Revue scientifique, XX.—
Id., '90. Sur les globules polaires et les homologues de ces elements chez les infu-
soires cilies : Bulletin scieiitifiqne de la France et de la Belgique. XXII. — God-
lewsky, E., '97, 1. tlber mehrfache bipolar Mitose bei der Spermatogenese von
Helix : Anz. Akad. Wiss. Krakau. — Id., '97, 2. Weitere Untersuchungen liber
die Umwandlung der Spermatiden, etc. : A?is. Akad. Wiss. Krakau., Nov., '97.

— Goroschanktin, J., '83. Zur Kenntniss der Corpuscula bei den Gymnosper-
men : Bot. Zeit., LXI. — Graf, A., '97. The Individuality of the Cell : N. V. State
Hosp. Bull., April. — Gr^goire, V., "99. Les cineses polliniques dans les Liliacees :
Bot. Cejttb.,XX., i; La Cellule, XVI., 2. — Griffin, B. B., ^6. The History of
the Achromatic Structures in the Maturation and Fertilization of Thalassema : Traus.
N. V. Acad. Sci. — Id., '99. Studies on the Maturation, Fertilization, and Cleavage
of Thalassema and Zirphaea : /. M., XV. — Gierke, H., '85. Farberei zu mikro-
skopischen Zwecken : Zeit. IViss. Mik., II. — Grobben, C, '78. Beitrage zur Kennt-
niss der mannlichen Geschlechtsorgane der Dekapoden : Arb. Zo'dl. Inst. Wien, I.

— Gruber, A., '84, Uber Kern und Kerntheilung bei den Protozoen : Z. w. Z.,
XL. — Id., '85. Uber kiinstliche Teilung bei Infusorien : B. C, IV., 23; V., 5. —
Id., '86. Beitrage zur Kenntniss der Physiologie und Biologie der Protozoen : Ber.
Naturf. Ges. Freiburg, I. — Id., '93. Mikroscopische Vivisektion : Ber. d. Naturf.
Ges. 2u Freiburg, Y\\., i. — Id., '97. Weitere Beobachtungen an vielkernigen
Infusorien: Ber. Natw^f. Ges. Freiburg, \\\. — Guignard, L., '89. Developpement
et constitution des Antherozoides : Rev. gen. Bot., I. — Id., '91, 1. Nouvelles etudes
sur la fecondation : An7i. d. Sciences Nat. Bot., XIV. — Id., '91, 2; Sur Texistence
des '' spheres attractives" dans les cellules vegetales : C.R., 9 Mars. — Id., '98, 1.
Les centres cinetiques chez les vegetaux : Aim. Sci. Nat. Bot., (VIII.) V. ; also, Bot.
Gaz., XXV. — Id., '98, 2. Le developpement du pollen et la reduction chromatique
dans le Nais i7tajor: Arch. Anal. Mik., II., 4. — Id., '99. Sur les antherozoides et
la double copulation sexuelle chez les vdgetaux angiospermes : C. R., CXXVIII., 14.

HABERLANDT, G., '87. Uber die Beziehungen zvvischen Funktion und Lage
des Zellkerns : Fischer, 1887. — Hackel, E., '66. Generelle Morphologie. —
Id., '91. Anthropogenie, 4th ed., Leipzig, 1891. — Hacker, V., '92. 1. Die Fur-
chung des Eies von ^quorea Forskalea: A. in. A., XL. — Id., '92, 2. Die Eibil-
dung bei Cyclops und Canthocamptus : Zool. Jahrb., V. — Id., "92, 3. Die
heterotypische Kerntheilung im Cyclus der generativen Zellen : Ber. naturf. Ges.
Freiburg, W. — Id., '93. Das Keimblaschen. seine Elemente und Lageverander-
ungen : A. vi. ^., XLI. — Id., "94. Uber den heutigen Stand der Centrosomen-
frage : Verhandl. d. deutschen Zool. Ges., 1894, p. 11. — Id., '95, 1. Die Vorstadien
der Eireifung : A. jn. A., XLV., 2. — Id., "95. 2. Zur frage nach dem Vorkommen
der Schein-Reduktion bei den Pflanzen : Ibid.. XLVI. Also Ann. Bot., IX. —
Id., '95, 3. liber die Selbstandigkeit der vaterhchen und miitterlichen Kernsbe-
standtheile wahrend der Embryonalentwicklung von Cyclops: A. ni. A., XLVI., 4.

— Id., '97, 1. Die Keimbahn von Cyclops : A. m. A., XLIX. — Id., "97, 2. Uber
weitere ^Ubereinstimmungen zwischen den Fortpflanzungsvorgangen der Thiere
und Pflanzen: B. C., XVII. — Id., "98. Uber vorbereitende Theilungsvorgange
bei Thieren und Pflanzen: Verh. d. Zool. Ges., VIII. — Id., "99. Praxis und
Theorie der Zellen und Befruchtungslehre : Jena, Fischer. — Hallez. P., "86. Sur
la loi de I'orientation de I'embryon chez les insectes : C. R., 103, 1886. — Hallibur-
ton, W. D., *91. A Text-book of Chemical Physiology and Pathology: London.

— Id., *93. The Chemical Physiology of the Cell: {Gouldstonian Lectures) Brit.
Med. Journ. — Hammar, J. A., '96. Uber einen primaren Zusammenhang zvvi-
schen den Furchungszellen des Seeigeleies : A. ?n. A., XLVII., i. — Id., '97. Uber
eine allgemein vorkommende primare Protoplasmaverbindung zwischen den Bias-

GENERAL LITERATURE-LIST 457

tomeren: A. m. A., XLIX, — Hammarsten. O., '94, Zur Kenntniss der Nucleo-
proteiden : Zcit. Phys. Chem.n XIX. — Id., "95. Lehrbuch der physiologischen
Chemie. 3e Ausgabe : VV'iesbadejt, 1895. — Hansemann, D., '91. Karyokinese und
Cellularpathologie : Berl. Klin. Wochenschrift, No. 42. — Id., '93. Spezilicitat;
Altmismus iind die Anaplasie der Zellen : Berlin^ 1893. — Hanstein, J., '80. Das
Protoplasma als Trager der pflanzlichen und thierischen Lebensverrichtungen.
Heidelberg. — Harper, R. A., "96. tJber das Verhalten der Kerne bei der
Fruchtentvvickelung einiger Ascomyceten : Jahrb. wiss. Hot., XXIX. — Id., '97.
Kernteilung und freie Zellbildung im Ascus : Ibid., XXX. — Hardy, W. B.,
'99. On the Structure of Ceil-protoplasm : Jour. Phys., XXIV., 2. — Harvey,
Wm., 1651. Exercitationes de Generatione Animalium : London. Trans, in
Sydenham Soc, X., 1847. — Hartog. M. M., '91. Some Problems of Reproduc-
tion, etc. : Q.J.J XXXIII. — Id.. "96. The Cytology of Saprolegnia : Ann. Bot.,
IX. — Id., '98. Nuclear Reduction and the Function of Chromatin : Nat. Sci.,
XII. — Hatschek, B., '87. tJber die Bedeutung der geschlechtlichen Fortpflan-
zung: Prager Med. IVochenschriJt, XLVI. — Id., "88. Lehrbuch der Zoologie. —
Heath, H., "99. The Development of Ischnochiton : Jena, Fischer. — Heiden-
hain, M., '93. tJber Kern und Protoplasma : Festchr. z. sojdhr. Doctorjub. von
V. Kolliker : Leipzig. — Id., '94. Neue Untersuchungen liber die Centralkorper und
ihre Beziehungen zum Kern und Zellenprotoplasma : A. m. A., XLIII. — Id., '95.
Cytomechanische Studien : A. Entwni.j I., 4. — Id., '96, 1. Ein neues Modell zum
Spannungsgesetz der centrirten Systeme : Verh. anat. Ges. — Id., "96, 2. iJber
die Mikrocentren mehrkerniger Ricsenzellen, etc.: Morph. Arb., VII., i. — Id., '99.
iJber eine eigenthlimliche Art Knospung an Epithelzellen, etc. : A. m. /i., LIV.,
I. — Heidenhain and Cohn,' 97. tJber die Mikrocentren in den Geweben des
Vogelembryos, etc.: Morph. Arb., VII. — Heitzmanii, J., '73. Untersuchungen
iiber das Protoplasma : Sitz. d. k. Acad. Wiss. li'ien.jLXVll. — Id., '83. Mikro-
scopische Morphologic des Thierkbrpers im gesunden und kranken Zustande : IVien,
1883. — Henking, H. Untersuchungen iiber die ersten Entvvicklungsvorgange in
den Eiern der Insekten, I., II., III.: Z. w. Z., XLIX., LI., LIV., 1890-92.—
Henle, J., *41. Allgemeine Anatomic: Leipzig. — Henneguy, L. F., '91. Nou-'
velles recherches sur la division cellulaire indirecte : Joiirn. Anat. et Physiol.,
XXVII. — Id., 93. Le Corps vitellin de Balbiani dans I'ceuf des Ve'rtebres : Lbid.,
XXIX. — Id., "96. Le9ons sur la cellule: Paris. — Id., '98. Sur les rapports des
cils vibratils avec les centrosomes : Arch. Anat. Mik., I. — Hensen, "V., '81. Phy-
siologic der Zeugung : Herniann''s Physiologie., VI. — Herbst, C. Experimentelle
Untersuchungen iiber den Einfluss der veranderten chemischen Zusammensetzung
des umgebenden Mediums auf die Entwicklung der Thiere, J. ; Z. w. Z., LV., 1892 ;
W.jMitt.Zool. St. Neapel^Xl., 1893 ; III.-VI., Arch. Entwm., II., 4, 1896. — Id., '94,
'95. iiber die Bedeutung der Reizphysiologie flir die Kausale AufFassung von Vor-
gangen in der tierischen Ontogenese : Bioi. Centralb., XIV., XV., 1894, 1895. —
Herla, "V., '93. Etude des variations de la mitose chez I'ascaride m^galocephale :
A. B., XIII. — Herlitzka, A., '95. Contributo alio studio della capacita evolutiva
dei due primi blastomeri nell' uove di Tritone : A. Entiuni., II., 3. — Hermann, F.,
'89. Beitrage zur Histologic des Hodens : A. in. A., XXXIV. — Id., '91. Beitrag
zur Lehre von der Entstehung der karyokinetischen Spindel : Ibid., XXXVII. —
Id., '92. Urogenitalsystem. Struktur und Histiogenese der Spermatozoen : Merkel
und Bonnefs Ergebtiisse, II. — Id., '97. Beitrage zur Kenntniss der Spermato-
genese: A. m. A., L. — Hertwig, O., *75. Beitrage zur Kenntniss der Bildung,
Befruchtung und Teilung des tierischen Eies, I. : M. /., I. — Id., '77. Beitrage, etc.,
II.; /^/V/., III. — Id. ,'78. Beitrage, etc.. III. ; /(^/^/., IV. — Id., '84. Das Problem
der Befruchtung und der Isotropic des Eies, eine Theorie der Vererbung : J. Z.,
XVIII. — Id., "90, 1. Vergleich der Ei- und Samenbildung bei Nematoden. Eine

458 GENERAL LITERATURE-LIST

Grundlage fiir cellulare Streitfragen : A. itt. A., XXXVI. — Id., '90, 2. Experi-
mentelle Studien am tierischen Ei vor, wahrend und nach der Befruchtung : /. Z.,
1890. — Id., '92, 1. — Urmund und Spina Bifida: A. jh. A., XXXIX. — Id., 92, 2.
Aeltere und neuere Entwicklungs-theorieen : Berlin. — Id., "93, 1. Uber den
Werth der ersten Furchungszellen fiir die Organbildung des Embryo: A. tn. A.,
XLIL— Id., 93, 2. Die Zelle und die Gewebe : Fischer, Jena, 1893, 1898.—
Id., '94. Zeit und Streitfragen der Biologic: Berlin. — Hertwig, O. and R., '86.
Experimentelle Untersuchungen liber die Bedingungen der Bastardbefruchtung :
/. Z., XIX, — Id., '87. liber den Befruchtungs- und Teilungsvorgang des tierischen
Eies unter dem Einfluss ausserer Agentien : Idid.,XX. — Hertwig, R., '77. tJber
den Bau und die Entwicklung der Spirochona gemmipara: Ibid.. XI. — Id., '84.
Die Kerntheilung bei Actinosphaerium Eichhorni : /<^z<^., XVII. — Id., '88. tJber
Kernstruktur und ihre Bedeutung fiir Zellteilung und Befruchtung: Ibid., IV., 1888.

— Id., '89. Uber die Konjugation der Infusorien : Abh. der bayr. Akad. d. IViss.,
II., CI., XVII. — Id., '92. Uber Befruchtung und Conjugation : Fer/i. deutsch. Zool.
Ges., Berlin. — Id., '95. Uber Centrosoma und Centralspindel : Sitz.-Ber. Ges.
Morph. und Phys., Miinchen, 1805, Heft I. — Id., '96. Uber die Entwicklung des
unbefruchteten Seeigeleies, etc. : Festchr. f. Gegenbaur. — Id., '97, 1. Uber die
Bedeutung der Nucleolen : Siizb. Ges. Morph. Fhys, Miinchen, 1898, I. — Id., '97, 2.

— Uber Karyokinese bei Actinosphaerium : Siizb. Ges. Morph. Fhys. Miinchen, Xlll.,

1. — Id., '98. Kerntheilung, Richtungskorperbildung und Befruchtung von Acti-
nosphaerium: Abh. K. buyer. Akad. IViss., XIX, 2. — Heuser, E., *84. Beobach-
tung liber Zelltheilung : Bot. Cent. — Hill, M. D., '95. Notes on the Fecundation
oiXh^^^goi SphcEr echinus graniilaris -aiXv^ on the Maturation and Fertilization of
the Egg of Fhallusia manwiillata : Q. /., XXXVIII. — Hirase, S., '97. Unter-
suchungen liber das erhalten des Pollens von Gingko biloba : .5^/. Cenib., LXIX.,

2, 3. — Id,, '98. Etudes sur la fecondation et I'embryogenie der Gingko : /^?/r.
Coll. Sci., Tokio, XII. — His, W., '74. Unsere Korperform und das physiologische
Problem ihrer Entstehung : Leipzig. — Hofer, B., '89. Experimentelle Untersuch-
ungen liber den Einfluss des Kerns auf das Protoplasma : /, Z., XXIV. — Hoff-
man, R. W., '98. Uber Zellplatten und Zellplattenrudimente : Z. w. Z., LXIII.

— Hofmeister, '67. Die Lehre von der Pflanzenzelle : Leipzig, 1867. — Holl,
M., '90. Uber die Reifung der Eizelle des Huhns : Sitzb. Acad. IViss. Wien,
XCIX., 3. — Hooke, Robt., 1665. Mikrographia, or some physiological Descrip-
tions of minute Bodies by magnifying Glasses : London. — Hoyer. H.. '90. Uber
ein fiir das Studium der " direkten '* Zelltheilung vorzliglich geeignetes Objekt : A.
A., v. — Hubbard, J. W., '94. The Yolk-Nucleus in Cymatogaster : Froc. Am.
Fhil. Soc, XXXIII. — Huie, L., '97. Changes in the Cell-organs oi Drosera
produced by Feeding with Egg-albumen: g. /., XXXIX. — Humphrey, J. E.,
'94. Nucleolen und Centrosomen : Ber. deutschen bot. Ges., XII., 5. — Id., '95.
On some Constituents of the Cell: Ann. Bot., IX. — Huxley, T. H., '53. Review
of the Cell-theory: Brit, and Foreign Med.-Chir. Review, XII. — Id., '78. Evo-
lution in Biology, Enc. Brit., 9th ed., 1878 ; Science and Culture, N. Y., 1882.

IKENO, S., '97. Vorlaufige Mitth. uber die Spermatozoiden bei Cycas : Bot.
Centb., LXIX., i. — Id., '98. 1. Zur Kenntniss des sogenannten centrosomahn-
lichen Kdrpers im Pollenschlauche der Cycaden : Flora, LXXXV., i. — Id., '98, 2.
Untersuchungen iiberdie Entwickelung der Geschlechtsorgane, etc., bei Cycas : Jahrb.
wiss. Bot., XXXII., 4. — Ishikawa, M., '91. Vorlaufige Mitteilungen uber die
Konjugationserscheinungen bei den Noctiluceen : Z. A., No. 353, 1891. — Id., '94.
Studies on Reproductive Elements: W., Noctiluca miliaris Sur., Its Division and
Spore-formation: Journ. College of Sc. Imp. Univ. Japan, VI. — Id., '97. Die

GENERAL LITERATURE-LIST 459

Entwickelung der Pollenkorner von Allium: Journ. Coll. Sci. Tokyo, X., 2. — Id.,
'99. Further Observations on the Nuclear Division of Noctiluca: Ibid., XII., 4.

JENNINGS, H. S., '96. The Early Development of Asplanchna: Bull. Mus.
Camp. Zo'dl., XXX. — Jensen, O. S., '83. Recherches sur la spermatogenese :
A. B., IV. — Johnson, H. P., '92. Amitosis in the embryonal envelopes of the
Scorpion: Bull. Mus. Cornp. Zo'dl., XXII., 3. — Jordan, E. O., '93. The Habits
and Development of the Newt: /. M., VIII., 2. — Jordan and Eycleshymer, '94.
On the Cleavage of Amphibian Ova: /. M., IX., 3, 1894. — Juel, H. O., '97. Die
Kerntheilungen in den Pollenmutterzellen, etc. : Jahrb. wiss. Bot., XXX. — Julin, J.,
'93, 1. Structure et developpement des glandes sexuelles, ovogen^se, spermatoge-
nese et fecondation chez Styleopsis grossularia : Btdl. Sc. de France et de Belgique,
XXIV. — Id., '93, 2. Le corps vitellin de Balbiani et les elements des Metazoaires
qui correspondent au Macronucleus des Infusoires cilies : Ibid., XXIV.

KARSTEN, G., '96. Untersuchungen iiber Diatomeen : Flora, LXXXII. —
Keuten, J., '95. Die Kerntheilung von Eji^lena viridis Ehr : Z.w.Z., LX. —
Kienitz-Gerloff, F.,'91. Review and Bibliography of Researches on Protoplasmic
Connection between adjacent Cells: in Bot. Zeitung., XLIX. — Kingsbury, B. F.,
'99. The Reducing Divisions in the Spermatogenesis of Desmognathus : Zo'dl.
Bull., II., 5. — Klebahn, '90. Die Keimung von Closterium und Cosmarium : Jahrb.
wiss. Bot., XXII. — Id., '92. Die Befruchtung von QEdigonium : Jahrb. f. wiss.
Bot., XXIV. — Id., '96. Beitrage zur Kenntniss der Auxosporenbildung, I., Rho-
palodia : Jahrb. wiss. Bot., XXIX. — Klebs, G., '83. Uber die Organisation einiger
Flagellaten-Gruppen, etc. : Bot. Inst. Tl'ibifigen, I., i. — Id., '84. Uber die neueren
Forschungen betreifs der Protoplasmaverbindungen benachbarter Zellen : Bot. Zeit.,
188.4 — Id., '8'7- tiber den Einfluss des Kerns in der Zelle : B. C, VII. — Klein,
E., '78-79. Observations on the Structure of Cells and Nuclei : Q.J., XVI 11., XIX.

— Klinckowstrom, A. v., '97. Beitrage zur Kenntniss der Eireife und Befruch-
tung bei Prostheceraeus : A. m. A., XLVIII. — von Kolliker, A., '41. Beitrage zur
Kenntniss der Geschlechtsverhaltnisse und der Samenfllissigkeit wirbelloser Tiere :
Berlin. — Id., '44. Entwicklungsgeschichte der Cephalopoden : ZUrich. — Id., '85.
Die Bedeutung der Zellkerne flir die Vorgange der Vererbung : Z. w. Z., XLII. —
Id., '86, Das Karyoplasma und die Vererbung, eine Kritik der Weismann'schen
Theorie von der Kontinuitat des Keimplasmas : Ibid., XLIII. — Id., '89. Handbuch
der Gewebelehre, 6th ed. : Leipzig. — Id., '97. Die Energiden von Sachs, etc.:
Verh. Phys. Med. Ges., Wurzburg, XXXI., 5. — Korff, '99. Zur Histogenese der
Spermien von Helix : A. m. A., LIV. Korschelt, E., "89. Beitrage zur Mor-
phologic und Physiologic des Zell-kernes : Zo'dl. Jahrb. Anat. u.pntog.. IV. — Id.,
'93. tJber Ophryotrocha puerilis : Z.w.Z.,\AV. — Id., '95. Uber Kerntheilung,
Eireifung und Befruchtung bei O/Zzry^/ri^^;-^^ puerilis: Ibid., LX. — Id. ,'96. Kern-
structuren und Zellmembranen in den Spinndrlisen der Raupen : A. in. A., XLVII.

— Id., '97. Uber den Bau der Kerne in den Spinndrlisen der Raupen r Ibid., XLIX.

— Kossel, A., '91. Uber die chemische Zusammensetzung der Zelle : Arch. Anat.
u. Phys. — Id., '93. Uber die Nucleinsaure : Ibid., 1893. — Id., '96. Uber die
basischen Stoffe des Zellkernes : Zeit. Phys. Chetn., XXII. — von Kostanecki, K.,
'91. tJber Centralspindelkorperchen bei karyokinetischer Zellteilung : Anat. Hefte,
1892, dat. 91. — Id., '96. Uber die Gestalt der Centrosomen im befruchteten See-
igelei: Ibid., VII., 2. — Id., '97, 1. Uber die Bedeutung der Polstrahlung, etc.:
A. m. A., LXIX. — Id., '98. Die Befruchtung des Eies won Myzostoma : Ibid.,
LI. — Kostanecki and Siedlecki, '96. Uber das Verhalten der Centrosomen
zum Protoplasma: Ibid., XLIX. — Kostanecki and "Wierzejski, '96. Uber das
Verhalten der sogenannten achromatischen Substanzen im befruchteten Ei : Ibid.,
XLII., 2. — KUhne, "W,, '64. Untersuchungen uber das Protoplasma und die Con-

460 GENERAL LITERATURE-LIST

tractilitat.— Kupffer, C, '75. tJber Differenzierung des Protoplasma an den
Zellen thierischer Gewebe : ScJir. jtaUir. Ver. Schles.-Holst., I., 3. — Id., '90. Die
Entwicklung von Petromyzon Planeri : A. m. A., XXXV. — Id., '96. tJber Ener-
giden und paraplastische Bildungen : Rektorat?'ede, Afiijichen, 1896.

LAMEERE, A., '90. Recherches sur la reduction karyogamique : Bruxelles. —
Lauterborii, R., '93. tlber Ban und Kerntheilung der Diatomeen : Verh. d.
Natiirh. Med. Ve?'. in Heidelberg. 1893. — Id., '95. Protozoenstudien, Kern- und
Zellteilung von Ceratium Iiirundinella O. F. M. : Z. w. Z., XLIX. — Id., '96. — La
Valette St. George, '65. Uber die Genese der Samenkorper: A. m. A., I. —
Id., '67. tJber die Genese der Samenkorper, II. (Terminology): Idid., III. —
Id., '76. Die Spermatogenese bei den Amphibien : /bid., XII. — Id., '78. Die
Spermatogenese bei den Saugethieren und dem Menschen : Ibid., XV. — Id.. '85-'87.
Spermatologische Beitrage, I.-V.: Ibid., XXV., XXVII., XXVIII., and XXX.

— Lankester, E. Ray, '77. Notes on Embryology and Classification: Loiidon. —
Lavdovsky, M., '94. Von der Entstehung der chromatischen und achromatischen
Substanzen in den tierischen und pflanzlichen Zellen : Merkel und Bonnefs Anat.
Hefte, IV., 13. — Lawson, A. A., '98. Some Observations on the Development
of the Karyokinetic Spindle, etc.: Proc. CaL Acad. Sci., I., 5. — Lazarus, A., '98.
Die Anaemie: Wien. — Lee, A. Bolles, '96. Sur le Nebenkern, etc., chez Helix:
La Cellule, XI. — Id., '97. Les cineses spermatogenetiques chez Helix: Ibid.,
XIII. — von Lenhossdk, M., *95. Centrosom und Sphare in den Spinalganglien
des Frosches: A. m. A., XLVI. — Id., '98, 1, tJber Flimmerzellen : Verh. An.
Ges., XII. — Id., '98, 2. Untersuchungen liber Spermatogenesis : A. m. A., LI. —
Id., '99. Das Mikrocentrum der glatten Muskelzellen : A. A., XVI., 13, 14. —
Leydig, Fr., '54. Lehrbuch der Histologie des Menschen und der Thiere : Frank-
furt.— Id., '85. Zelle und Gt\N&he, J3onn. — Id,, '89. Beitrage zur Kenntniss
des thierischen Eies im unbefruchteten Zustande : SpengeVs Jahrb. Anat. Ont.. III.

— Lilieufeld, L., '92, '93. Uber die Vervvandtschaft der Zellelemente zu gewissen
Farbstoffen : Verh. Phys. Ges., Berlin, 1892-93. — Id., '93. Uber die Wahlver-
w^andtschaft der Zellelemente zu Farbstoffen: A. A. P., 1893. — Lillie, F. R., '95.
The Embryology of the Unionidae : /. M., X. — Id., '96. On the Smallest Parts
of Stentor capable of Regeneration: /. i/., XII., i. — Id., '97. On the Origin of
the Centres of the First Cleavage-spindle in Unio : Scietice, V. — Id., '98. Centro-
some and Sphere in the Egg of Unio : Zo'dl. Bidl.. I., 6. — Id., '99. Adaptation in
Cleavage : Wood'^s Holl Biol. Led. — List, Th., '96. Beitrage zur Chemie der Zelle
und Gewebe, I. : Mitth. Zodl. St. Neap., XII., 3. — Loeb, J., '91-'92. Untersuch-
ungen zur physiologischen Morphologic. I. Heteromorphosis : Wurzburg, 1891.
II. Organbildung und Wachsthum : Ibid.^ 1892. — Id., '92. Experiments on Cleav-
age : J. M.,V\\. — Id., '93. Some Facts and Principles of Physiological Mor-
phology: Wood'' s Holl Biol. Lectures^ 1893. — Id., '94. Uber die Grenzen der
Theilbarkeit der Eisubstanz : A. ges. P., LIX., 6, 7. — Id., '95. tJber Kernthei-
lung ohne Zelltheilung : Arch. Entwm., II. — Id., '99, 1. Warum istdie Regenera- .
tion kernloser Protoplasmastucken unmoglich, etc.: Ibid., VIII., 4. — Id., '99, 2.
On the Nature of the Process of Fertilization and the Artificial Production of Nor-
mal Larvae, ^/^. ; Am. Jour n. Phys., III., 3. — LoT^rit, M.. '91. Uber amitotische
Kerntheilung: B. C, XI. — Lukjanow, '91. Grundziige einer allgemeinen Patho-
logic der Zelle: Leipzig. — Liistig and Galeotti, "93. Cytologische Studien iiber
pathologische menschliche Gewebe : Beitr. Path. Anat.., XIV.

MACALLUM, A. B., '91. Contribution to the Morphology and Physiology of
the Cell: Trans. Canad. Inst., I., 2. — McClung, C. E., '99. A Peculiar Nuclear
Element in the Male Reproductive Cells of Insects: Zo'dl. Bull., IL, 4. — MacFar-

GENERAL LITERATURE-LIST 46 1

land, F. M., '97. Cellulare Studien an Molluskeneiern : ZooL Jahrb. Anat., X. —
McGregor, J. H., '99. The Spermatogenesis of Amphiuma: /. J/., XV., Suppl.
— McMurrich, J. P., '86. A Contribution to the Embryology of the Prosobranch
Gasteropods : Studies Biol. Lab. Johns Hopkins Univ., III. — Id., '95. Embry-
ology of the Isopod Crustacea: /. M.., XL, i.— Id., '96. The Yolk-Lobe and the
Centrosome of Fulgur : A. A., XIL, 23. — Id., '97. The Epithelium of the Midgut
of the Terrestrial Isopods : /. M., XIV., i. — Maggi, L., '78. I plastiduli nei
ciliati ed i plastiduli liberamente viventi : Atti. Soc. Ital. Sc. Nat. Milano, XXI.
(also later papers). — Malfatti, H., '91. Beitrage zur Kenntniss der Nucleine :
Zeit. Phys. Chem., XVI. — Mark, E. L., '81. Maturation, Fecundation, and Seg-
mentation of Limax campestris : Bitll. Mns. Comp. Zool. Harvard College, VL —
Mathews, A. P., '97, 1. Internal Secretions considered in Relation to Variation
and Development: Science, V., 122. — Id., "97, 2. Zur Chemie der Spermatozoen :
Zeit. Phys. Chem., XXIIL, 4, 5. — Id., '98. A Contribution to the Chemistry of
Cytological Staining: A7n.Journ. Phys., I., 4. — Id., '99, 1. The Origin of Fibri-
nogen: Ibid., III. — Id., '99,2. The Metabolism of the Pancreas Cell: /. M.,
XV., Suppl. — Maupas, M., '88. Recherches exp^rimentales sur la multiplication
des Infusoires cilies : Arch. Zool. Exp., 2me serie, V^I. — Id., "89. Le rejeunisse-
ment karyogamique chez les Cilies : Ibid., 2me s^rie, VII. — Id., '91. Sur le de'ter-
minisme de la sexualite chez I'Hydatina senta : C. R., Paris. — Mayer, P., '91.
Uber das Farben mit Carmin, Cochenille und Hamatein-Thonerde: Mitth. Zool. St.
Neapol., X., 3. — Id., '97. Beruht die Farbung der Zellkerne auf einem chem-
ischen Vorgang oder nicht?: A. A., XIIL, 12 — Mead, A. D., '95. Some Obser-
vations on Maturation and Fecundation in Chaetopterus pergamentaceus Cuv. : /. M..,
X., I.— Id., '97, 1. The Origin of the Egg-centrosomes : Ibid., XIL— Id., '97, 2.
The early Development of marine Annelids: Ibid.. V. — Id., '98, 1. The Origin
and Behaviour of the Centrosomes in the Annelid Egg : Ibid., XIV., 2. — Id., '98, 2.
The Rate of Cell-division and the Function of the Centrosome: Wood's H oil Biol.
Lectures. — Merkel, F., '71. Die Stiitzzellen des menschlichen Hodens: Miiller'^s
Arch. — Mertens, H., '93. Recherches sur la signification du corps vitellin de
Balbiani dans I'ovule des Mammif^res et des Oiseaux : A. B.,X\\\. — Metschni-
koff, E., '66. Embryologische Studien an Insecten: Z.w.Z., XVI. — Meves,
P., '91. Dber amitotische Kernteilung in den Spermatogonien des Salamanders,
und das Verhalten der Attraktionsspharen bei derselben : A. A., 1891, No. 22. —
Id., '94. Uber eine Metamorphose der Attraktionssphare in den Spermatogonien
von Salamandra maculosa: A. in. A., XLIV. — Id., '95. Uber die Zellen des
Sesambeines der Achillessehne des Frosches {Ra?ia teniporaria) und liber ihre Cen-
tralkorper: Ibid.^XlN . — Id., '96. Uber die Entvvicklung der mannlichen Ge-
schlechtszellen von Salamandra: Ibid., XLVIII. — Id., '97, 1. Zur Struktur der
Kerne in den Spinndriisen der Raupen : Ibid., XLVIII. — Id., '97, 2. Uber
Struktur und Histiogenese der Samenfaden von Salamandra : Ibid., L. — Id., '97, 3.
Uber den Vorgang der Zelleinschniirung : Arch. Entwin., V., 2. — Id., '97, 4.
Zelltheilung: Merkel u. Bonnet, Erg., VL — Id., "97, 5. Uber Centralkorper in
mannlichen Geschlechtszellen von Schmetterlingen : A. A., XIV., i. — Id., '98.
liber das Verhalten der Centralkorper bei der Histogenese der Samenfaden vom
Mensch und Ratte : Verh. An. 6^^'^-., XIV. —Id., '99. Uber Struktur und Histo-
genesis der Samenfaden des Meerschweinschens : A. m. A., LIV. — Meyer, A., '96.
Die Plasmaverbindungen, etc.: Bot. Zeit., 11, 12. — Meyer, O., "95. Cellular-
Untersuehungen an Nematodeneiern : /. Z., XXIX. (XXIL). — Michaelis, L., '97.
Die Befruchtung des Tritoneies : A. m. A., XLV^III. — Miescher, P., '96.
Physiologisch-chemische Untersuchungen liber die Lachsmilch : Arch. Exp. Path,
u. Pharm., XXXVII. — Mikosch, '94. tlber Struktur im pflanzlichen Proto-
plasma : Verhandl. d. Ges. deittscher Naturf. und Arzte, 1 894 ; Abteil f. Pfianzen-

462 GENERAL LITERATURE-LIST

physiologie u. Pflanzenanatomie. — Minot, C. S., '77. Recent Investigations of
Embryologists : Proc. Bost. Soc. Nat. Hist., XIX. — Id., '79. Growth as a Function
of Cells : Ibid., XX. — Id., '82. Theorie der Genoblasten : B. C, II., 12. See also
Am. Nat., February, 1880, and JYoc. Bost. Soc. Nat. Hist., XIX., 1877. — Id., '91.
Senescence and Rejuvenation : /oi/r/i. Phys., XII., 2. — Id., '92, Human Embryol-
ogy : New York. — von Mohl Hugo, '46. tJber die Saftbewegung im Innern der
Zellen : Bat. Zeitung. — Moll, J. 'W., '93. Observations on Karyokinesis in Spiro-
gyra: Verh. Kon. Akad., Amsterdam, No. 9. — Montgomery, Th. H., '98, 1.
The Spermatogenesis of Pentaioma, etc. : Zo'dl. Jahrb. — Id., '98, 2. Comparative
Cytological Studies, with Especial Reference to the Morphology of the Nucleolus :
J. M., XV., 2. — Moore, J. E. S., '93. Mammalian Spermatogenesis: A. A.,
VIII. — Id., '95. On the Structural Changes in the Reproductive Cells during the
Spermatogenesis of Elasmobranchs : Q. J , XXXVIII. — Morgan, T. H., '93.
Experimental Studies on Echinoderm Eggs: A. A., IX., 5, 6. — Id., '95, 1.
Studies of the " Partial " Larvae of Sphaerechinus : A. Entwm., II., i. — Id., '95, 2.
Experimental Studies on Teleost-eggs : A. A., X., 19. — Id., '95, 3. Half-embryos
and Whole-embryos from one of the first two Blastomeres of the Frog's Egg :
Ibid., X., 19. — Id., '95, 4. The Fertilization of non-nucleated P^ragments of
Echinoderm-eggs : Arch. Entwm., II., 2. — Id., '95, 5. The Formation of the Fish-
embryo : /. M., X., 2. — Id., '96, 1. On the Production of artificial archoplasmic
Centres: Kept, of the Am. Morph. Soc, Science, III., January 10, 1896. — Id., '96,
2. The Number of Cells in Larvae from Isolated Blastomeres of Amphioxus :
Arch. Entwjn., III., 2. — Id., '96, 3. The Production of Artificial Astrosphaeres :
Arch. Entwm., III. — Id., '98, 1. Experimental Studies of the Regeneration
of Planaria maculata: Ibid., VII., 2, 3. — Id., '98, 2. Regeneration and Liability
to Injury: Zo'dl. BulL, I., 6. — Id., '99, 1. The Action of Salt-solutions on the
Unfertilized and Fertilized Eggs of Arbacia and other Animals : Arch. Entwm.,
VIII., 3. — Id., '99,2. A Confirmation of Spallanzani's Discovery, etc.: A. A.,
XV. 21 . — Mottier, D. M., '97, 1. Uber das Verhalten der Kerne bei der Entwick-
lung des Embryosacs, etc. : Jahrb. wiss. Bot., XXXI. — Id., '97, 2. Beitrage zur
Kenntniss der Kerntheilung in den Pollenmutterzellen, <?/^. : Ibid., XXX. — Id., '98.
Das Centrosoma bei Dictyota: Ber. D. Bot. Ges., XVI., 5. — Miiller, E., '96.
tJber die Regeneration der Augenlinse nach Exstirpation derselben bei Triton:
A. m. A., XLVIL, i. — Munson, J. P., '98. The Ovarian Egg of Limulus, etc. :
J. M., XV., 2. — Murray, J. A., '98. Contributions to a Knowledge of the Neben-
kern in the Spermatogenesis of Pulmonata: ZooL Jahrb., XL, 14.

NAD SON, G., '95. tJber den Bau des Cyanophyceen-Protoplastes : Script,
Botan. Horti. PetropoL, IV. — Nageli, C, '84. Mechanisch-physiologische Theorie
der Abstammungslehre : Munchen ti. Leipzig, 1884. — Nageli und Schwendener,
'67. Das Mikroskop. (See later editions.) Leipzig. — Nawasohin, '99. Neue
Beobachtungen iiber Befruchtung bei Fritillaria und Lilium: Bot. Centb.,
LXXVIL, 2.— Nemec, B., '97. tJber die Struktur der Diplopodeneier, A. A.y
XIII., 10, II. — Id., '99. liber die karyokinetische Kerntheilung in den Wiir-
zelspitzen von Allium : J. w. B., XXVIII, 2. — Newport, G. On the Impregnation
of the Ovum in the Amphibia : Phil. Trans., 1851, 1853, 1854. — Norman, "W. W.,
'96. Segmentation of the Nucleus without Segmentation of the Protoplasm : Arch.
Entwm., III. — Nussbaum, M., '80. Zur Differenzierung des Geschlechts im Tier-
reich : A. m. A., XVIII. — Id,, '84, 1. tJber Spontane und Kunstliche Theilung
von Infusorien : Verh. d. naturh. Ver. preus., Rheinland, 1884. — Id., '84, 2. tJber
die Veranderungen der Geschlechtsproducte bis zur Eifurchung: A. rn. A., XXIIL
— Id., '85. Uber die Teilbarkeit der lebendigen Materie, I. : A.m. A.,XXM\. —

GENERAL LITERATURE-LIST 463

Id., '94. Die mit der Entwickelung fortschreitende Differenzierung der Zellen :
Sitz.-Ber. d. niederrhein. Gesellschaft f. Natur- u. Heilktinde^ Bonn, 5 Nov., 1894;
also B. C, XVI., 2, 1896. —Id., *97. Die Entstehung des Geschlechts bei Hyda-
tina: A. m. A., XLIX.

OBST, P., '99. Untersuchungen liber das Verhalten der Nucleolen, «tc. : Z. w. Z.
LXVL, 2. — Ogata. '83. Die Veranderungen der Pancreaszellen bei der Secre-
tion: A. A. P. — Oppel, A., '92. Die Befruchtung des Reptilieneies : A. m. A.
XXXIX.— Osterhout, W. J. V., '97. tJber Entstehung der karyokinetischen
Spindel bei Equisetum : Jahrb. wiss. Bot., XXX. — Oltmanns, F., '95. Uber die
Entwickelung der Sexualorgane bei Vaucheria: Flora. — Overton, C. E., '88.
tJber den Conjugationsvorgang bei Spirogyra : Ber. deutsch. Bot. Ges., VI. — Id.,
'89. Beitrag zur Kenntniss der Gattung Volvox : Bot. Centralb., XXXIX. — Id., '93.
tlber die Reduktion der Chromosomen in den Kernen der Pflanzen : Vierteljahrschr.
naturf. Ges. Zurich, XXXVIII. Also Ann. Bot., VII., 25.

PALADINO, G., '90. I ponti intercellulari tra V novo ovarico e le cellule folli-
colari, etc. : A. A., V. — Paulmier, F. C, '98. Chromatin Reduction in the
Hemiptera: A. A., XIV. — Id., '99. The Spermatogenesis of Anasa tristis :
J. M., XV., Suppl. — Peter, K., '99. Das Centrum fur die Flimmer- und Geissel-
bewegung: A. ^., XV., 14, 15. — Pfeffer, "W., '99. tJber die Erzeugung und die
physiologische Bedeutung der Amitose : Ber. k'dnigl., sacks., Ges. Wiss. Leipzig.,
July 3. — Pfitzner, "W., '82. Uber den feineren Bau der bei der Zelltheilung
auffretenden fadenfdrmigen Differenzierungen des Zellkerns : M. J., VII. — Id., '83.
Beitrage zur Lehre vom Baue des Zellkerns und seinen Theilungserscheinungen :
A. m. A., XXII. — Pfluger, E., '83. Uber den Einfluss der Schwerkraft auf die
Theilung der Zellen : \., Arch. ges. Phys., XXXI.; II., Ibid., XXXII.; abstract in
Biol. Centb., III., 1884. — Id., '84. tJber die Einwirkung der Schwerkraft und
anderer Bedingungen auf die Richtung der Zelltheilung: Arch. ges. Phys., XXXIV.
— Id.. '89. Die allgemeinen Lebenserscheinungen : Bonn. — Platner, G., '86, 1.
Zur Bildungder Geschlechtsprodukte bei den Pulmonaten : A. in. A., XXVI. — Id.,
'86,2. — liber die Befruchtung von Arion empiricorian : A. m. A., XXVII. —
Id., '89, 1. Uber die Bedeutung der Richtungskbrperchen : B. C, VIII. — Id.,
'89, 2. Beitrage zur Kenntniss der Zelle und ihrer Teilungserscheinungen, I. -VI. :
A. 7n. A., XXXIII. — Poirault and Raciborski, '96. Uber konjugate Kerne und
die konjugate Kerntheilung : B. C, XVI., i. — Prenant, A., '94. Sur le corpus-
cule central: Btdl. Soc. Sci., Nancy, 1894. — Id., '98, '99. Sur le protoplasma
superieure (archoplasme, kinoplasme, ergastoplasme) : Jo7ir. Anat. Phys., XXXIV.,
XXXV. — Preusse, F., '95. Uber die amitotische Kerntheilung in den Ovarien
der Hemipteren : Z. w. Z., LIX., 2. — Provost and Dumas, '24. Nouvelle theorie
de la generation: Ann. Sci. Nat., I., II. — Pringsheim, N., '55. Uber die
Befruchtung der Algen : Monatsb. Berl. Akad., 1855-56.

RABL, C, '85. Uber Zellteilung: M. J., X. — Id., '89, 1. Uber Zelltheil-
ung: A. A., IV. — Id., '89, 2. tJber die Prinzipien der Histologie : Verh. Anat.
Ges., III. — vom Rath, O., '91. Uber die Bedeutung der amitotischen Kernteilung
im Hoden: Zo'dl. Anz., XIV. — Id., '92. Zur Kenntniss der Spermatogenese von
Gryllotalpa vulgaris: A. m. A., XL. — Id., '93. Beitrage zur Spermatogenese
von Salamandra: Z. w. Z., LVII. — Id., '94. Uber die Konstanz der Chromo-
somenzahl bei Tieren : B. C, XIV., 13. — Id., '95, 1. Neue Beitrage zur Frage
der Chromatinreduction in der Samen- und Eireife : A. m. A., XL VI. — Id., '95, 2.
Uber den feineren Bau der Driisenzellen des Kopfes von Anilocra, etc. : Z. w. Z.,
LX., I. — Rauber, A., '83. Neue Grundlegungen zur Kenntniss der Zelle : M./.^

464 GENERAL LITERATURE-LIST

VIII. — Rawitz. B.. '95. Centrosoma und Attraktionsphare in der ruhenden Zelle
des Salamanderhodens : A. m. A.. XLIV., 4. — Id., '97. Bemerkungen Uber
Mikrotomschneiden, etc.: A. A., XIII. — Reinke, Fr., '94. Zellstudien, I.,
A. m. A., XLIIL: III., Ibid., XLIV., 1894.— Id.. '95. Untersuchungen uber
Befruchtung und Furchung des Eies der Echinodermen : Sitz.-Ber. Akad. d. Wiss.
Berlin, 1895, June 20. — Reinke and Rodewald, '81. Studien liber das Proto-
plasma : Unterstich. mis. d. bot. Inst. G'dttingen, II. — Remak, R.. '41. Uber
Theilung rotlier Blutzellen beim Embryo: Med. Ver. Zeit., 1841. — Id., '50-55.,
Untersucliungen iiber die Entwicklung der Wirbelthiere : Berlin, 1850-55. — Id.,
'58. Uber die Theilung der Blutzellen beim Embryo: Milller''s Arch., 1858. — •
Retzius, G., '89. Die Intercellularbriicken des Eierstockeies und der FoUikelzellen :
Verh. Anat. Ges., 1889. — Rhumbler, L., '93. Uber Entstehung und Bedeutung
der in den Kernen vieler Protozoen und im Keimblaschen von Metazoen vorkom-
menden Binnenkorper (Nucleolen) : Z. w. Z., LVI. — Id., '96. Versuch einer
mechanischen Erklarung der indirekten Zell- und Kerntheilung: Arch. Entwm.y
III. — Id., '97, Stemmen die Strahlen der Astrosphare oder ziehen sie? Arch.
Entwin., IV. — Rompel, '94. Kentrochona Nebalias n. sp., ein neues Infusor
aus der Familie der Spirochoninen. Zugleich ein Beitrag zur Lehre von der Kern-
teilung und dem Centrosoma : Z. w. Z., LVIII., 4. — Rosen, '92. Uber tinctionelle
Unterscheidung verschiedener Kernbestandtheile und der Sexual-kerne : Cohn'^s
Beitr. z. Biol. d. Pflanzen,N . — Id., '94. Neueres liber die Chromatophilie der
Zellkerne : Schles. Ges. vdterl. Ktilt., 1894. — Roux, W., '83, 1. Uber die Bedeu-
tung der Kernteilungsfiguren : Leipzig. — Id., "83, 2. Uber die Zeit der Bestim-
mung der Hauptrichtungen des Froschembryo : Leipzig. — Id., '85. Uber die
Bestimmung der Hauptrichtungen des Froschembryos im Ei, und iiber die erste
Theilung des Froscheies : Breslaiier drtzl. Zeitg., 1885, — Id., '87. Bestimmung
der medianebene des Froschembryo durch die Kopulationsrichtung des Eikernes
und des Spermakernes : A. in. A., XXIX. — Id., "88. Uber das klinstliche Hervor-
bringen halber Embryonen durch Zerstorung einer der beiden ersten Furchungskugeln,
etc. : Virchow's Archiv, 114. — Id., '90. Die Entwickelungsmechanik der Organ-
ismen. lVien,i8go. — Id., '92, 1. Entwickelungsmechanik: Merkel and Bonnet,
Erg., II. — Id., '92, 2. Uber das entwickelungsmechanische Vermogen jeder der
beiden ersten Furchungszellen des Eies: Verh. Anat. Ges., VI. — Id., '93, 1.
tJber Mosaikarbeit und neuere Entvvickelungshypothesen : An. Hefte, Feb., 1893. —
Id., '93, 2. Uber die Spezifikation der Furchungzellen, etc.: B. C, XIII., 19-22

— Id., '94, 1. Uber den " Cytotropismus " der Furchungszellen des Grasfrosches :
Arch. Entwin., I., i, 2. — Id., '94, 2. Aufgabe der Entwickelungsmechanik, etc.:
Arch. Entwin., I., i. Trans, in Biol. Lectures, Wood''s Holl, 1894. — Riickert, J.,
'91. Zur Befruchtung des Selachiereies : A. A.,Y\. — Id., '92, 1. Zur Entwick-
lungsgeschichte des Ovarialeies bei Selachiern : A. A., VII. — Id., '92, 2. Uber die
Verdoppelung der Chromosomen im Keimblaschen des Selachiereies: Ibid., VIII.

— Id., '93, 2. Die Chromatinreduktion der Chromosomenzahl im Entwicklungs-
gang der Organismen : Merkel and Bonnet, Erg., III. — Id., '94. Zur Eireifung
bei Copepoden : An. Hefte. — Id., '95, 1. Zur Kenntniss des Befruchtungs-
vorganges : Sitsb. Bayer. Akad. ^/j-j-., XXVI., i. — Id., '95, 2. Zur Befruchtung
von Cyclops stremius : A. A., X., 22. — Id., '95, 3. Uber das Selbstandigbleiben
der vaterlichen und mlitterlichen Kernsubstanz wahrend der ersten Entwicklung des
befruchteten Cyclops-Eies : A. m. A., XLV., 3. — Riige, G., '89. Vorgange am
Eifollikel der Wirbelthiere: J/./., XV. — Ryder, J. A., '83. The Microscopic
Sexual Characteristics of the Oyster, etc.. Bull. U. S. Fish. Comm., March 14, 1883.
Also, Ann. Mag. Nat. Hist., XII., 1883.

GENERAL LITER ATUA'E-LIST 465

SABASCHNIKOFF, M., '97. Beitrage zur Kenntniss der Chromatinreduk-
tion in der Ovogenesis von Ascaris : Bull. Soc. Nat., Moscow, i. — Sabatier, A.^
'90. De la Spermatogenese chez les Locustides : Comptes Rend., CXI., 90. —
Sachs, J., '82. Vorlesungen iiber Pflanzen-physiologie : Leipzig. — Id. Uber die
Anordnung der Zellen in jiingsten Pflanzentheile : Arb. Bot. Inst. IVii^'zburg, II. —
Id., '92. Physiologische Notizen, II., Beitrage zur Zellentheorie : Flora, 1892,
Heft I. — Id., '93. Stoif und Form der Pflanzen-organe : Gesammelte Abhandliingen,
II., 1893. — Id., *95. Physiologische Notizen, IX., weitere Betrachtungen liber
Energiden und Zellen : Flora, LXXXL, 2. — Sala, L., '95. Experimentelle Unter-
suchungen iiber die Reifung und Befruchtung der Eier bei Ascaris megalocephala :
A. jn. A., XL. — Sargant, Ethel, '95. Some details of the first nuclear Division
in the Pollen-mother-cells oi Liliiwi ?nartagon: Journ. Roy. Mic. Soc, 1895, part 3.

— Id., '96. The Formation of the Sexual Nuclei in Lilium, I., Oogenesis: Ann.
Bot., X. — Id., '97. Same tide. II., Spermatogenesis : Ibid., XI. — Schafer, E. A.,
'91. General Anatomy or Histology: in Quain's Anatomy, I., 2, loth ed., London.

— Schaffner, J. H., '97, 1. The Life-history of Sagittaria : Bot. Gas., XXIII. , 4.
— Id., '97, 2. The Division of the Macrospore Nucleus (in Lilium) : Ibid., XXIII.j
6. — Id., '98. Karyokinesis in Root-tips of Allium : Ibid., XXVI., 4. — Schaudinn,
P., '95. Uber die Theilung von Amceba binncleata Gruber : Sitz.-Ber. Ges. Natur-
forsch. Freunde, Berlin, Jahrg. 1895, No. 6. — Id., '96,1. tJber den Zeugungs-
kreis von Par amoeba Eilhardi: Sitz.-Ber. Akad. IViss., Berlin, 1896, Jan. 16. —
Id., '96, 2. Uber die Copulation von Actinophrys Sol: Ibid.— Id., '96, 3.
Uber das Centralkorn der Helio'zoen : Fer/i. D. Zo'ol. Ges. — Schewiakoff, W.,
'88.' tJber die karyokinetische Kerntheilung der £'//jcr/j/^//rt: alveolata: J/./., XIII.

— Id., '93. Uber einen neuen Bakterienahnlichen Organismus : Hab. Schrift,
Heidelberg, Winter. — Schiefferdecker and Kossel, '91. Die Gewebe des,
Menschlichen Korpers : Braunschweig. — Schimper, '85. Untersuchungen iiber
die Chlorophyllkorper, etc. : Zeitsch. wiss. Bot., XVI. — Schleicher, W., '78. Die
Knorpelzelltheilung. Ein Beitrag zur Lehre der Theilung von Gewebezellen :
Centr. med. Wiss. Berlin, 1878. Also A. m. A., XVI., 1879. — Schleiden, M. J.,
'38. Beitrage zur Phytogenesis : Muller''s Archiv, 1838. [Trans, in Sydenham
Soc, XII.: London, 1847.] — Schloter, G., '94. Zur Morphologie der Zelle :
A. m. A., XLIV., 2. — Schmitz, '84. Die Chromatophoren der Algen. — Schnei-
der, A., '73. Untersuchungen iiber Plathelminthen : Jahrb. d. oberhess. Ges. f.
Natiir-Heilkunde, XIV., Giessen. — Schneider, C, '91. Untersuchungen iiber die
Zelle: Arb. Zo'ol. hist. Wien, IX., 2. — Schottlander, J., '88. Uber Kern und
Zelltheilungsvorgange in dem Endothel der entziindeten Hornhaut : A. ?n. A.,
XXXI. — Schottlander, P., '93. Beitrage zur Kenntniss des Zellkerns, etc:
Cohn's Beitrds;e, VI. — Schultze, Max, '61. Uber Muskelkorperchenund das was
man eine Zelle zu nennen hat: Arch. Atiat. Phys., 1861. — Schultze, O., '87.
Untersuchungen iiber die Reifung und Befruchtung des Amphibien-eies : Z. w. Z.,
XLV. — Id., '90. Uber Zelltheilung: Sit zb. phys. med. Ges. Wiirzburg. — Id.,
'94. Die kiinstliche Erzeugung von Doppelbildungen bei Froschlarven, etc. : Arch.
Entwm., I., 2. — Schwann, Th., '39. Mikroscopische Untersuchungen iiber die
Ubereinstimmung in der Structur und dem Wachsthum der Thiere und Pflanzen :
Berlin. [Trans, in Sydenham Soc, XII.: Lo7idon, 1847.] — Schwarz, Pr., '87.
Die Morphologische und chemische Zusammensetzung des Protoplasmas : Breslau.

— Schweigger-Seidel, O., '65. Uber die Samenkorperchen und ihre Entwick-
elung: A. m. A., I. — Sedgwick, A.. '85-88. The Development of the Cape
Species of Pcripatus, I.-VI. : Q. J., XXV.-XXVIII. — Id., '94. On the Inadequacy
of the Cellular Theory of Development, etc: Ibid., XXXVII., i. — Seeliger, O.,
'94. Giebt es geschlechtlicherzeugte Organismen ohne miitterliche Eigenschaften ? :
A. Ejtt., I., 2. — Selenka, E., '83. Die Keimblatter der Echinodermen : Studien

2H

466 GENERAL LITERATURE-LIST

iiber Entwick.^ II., Wiesbaden^ 1883. — Sertoli, E., '65. Dell' esistenza di parti-
colari cellule ramificate del canaliculi seminiferi del testicolo umano : // Morgagni.

— Shaw, W. R., '98, 1. Uber die Blepharoplasten bei Onoclea und Marsilia:
Ber. D. Bot. Ges., XVI., 7. —Id., '98, 2. The Fertilization of Onoclea : Ann. Bot.^
XII., 47. — Siedlecki, M., '95. tJber die Struktur und Kerntheilungsvorgange bei
den Leucocyten der Urodelen : Anz. Akad. IV/ss., Krakau^ 1895. — Id., '99. Stude
cytologique et cycle evolutif de Adelea: Ann. hist. Pasteur., XIII. — Sobotta, J.,
'95. Die Befruchtung und Furchung des Eies der Maus : A. m. A., XLV. — Id.,
'97. Die Reifung und Befruchtung des Eies von Amphioxus : Ibid., L. — Solger,
B., '91. Die radiaren Strukturen der Zellkorper im Zustand der Ruhe und bei der
Kerntheilung : Berl. Klin. PVoc/iensc/ir., XX., iSgi. — Spallanzani, 1786. Expe-
riences pour servir k Thistoire de la generation des animaux et des plantes : Geneva.

— Spitzer, '97. Die Bedeutung gewisser Nucleoproteide fiir die oxydative Leistung
der Zelle: Arch. ges. Phys., LXVII. — Stevens, 'W. C, '98. tJber Chromoso-
mentheilung bei der Sporenbildung der Fame: Ber. D. Bot. Ges., XVI., 8. — Ste-
vens, F. L., '99. The compound Oosphere of Albugo : Bot. Gaz., XXVIII., 3, 4.

— Strasburger, E., '75. Zellbildung und Zelltheilung : ist ed., Jena, 1875. — •^^•»
'77. iJber Befruchtung und Zelltheilung: /. Z., XL — Id., '80. Zellbildung und
Zellteilung: 3d ed. — Id., '82. Uber den Theilungsvorgang der Zellkerne und das
Verhaltniss der Kerntheilung zur Zelltheilung: A. m. A., XXI. — Id., '84, 1. Die
Controversen der indirecten Zelltheilung: Ibid., XXIII. — Id., '84, 2. Neue Unter-
suchungen liber den Befruchtungsvorgang bei den Phanerogamen, als Grundlage fiir
eine Theorie der Zeugung: Jena, 1884. — Id., '88. Uber Kern- und Zellteilung im
Pflanzenreich, nebst einem Anhang liber Befruchtung: Jena. — Id., '89. Uber das
Wachsthum vegetabilischer Zellhaute : Hist. Bei., II., Jena. — Id., '91. Das Proto-
plasma und die Reizbarkeit : Rektoratsrede, Bonn, Oct. 18, 1891. Jena, Fischer. —
Id., '92. Histologische Beitrage, Heft IV. : Das Verhalten des Pollens und die
Befruchtungsvorgange bei den Gymnospermen, Schwarmsporen, pflanzliche Sperma-
tozoiden und das Wesen der Befruchtung: Fischer, Jena, 1892. — Id., '93, 1. Uber
die Wirkungssphare der Kerne und die Zellengrosse : Hist. Beitr., V. — Id., '93, 2.
Zu dem jetzigen Stande der Kern- und Zelltheilungsfragen : A. A., VIII., p. 177. —
Id., '94. Uber periodische Reduktion der Chromosomenzahl im Entwicklungsgang
der Organismen : B. C, XIV. — Id., '95. Karyokinetische Probleme : Jahrb. f.
wiss. Botanik, XXVIII., i. — Id., '97, 1. Kerntheilung und Befruchtung bei Fucus :
Jahrb. wiss. Bot., XXX. —Id., '97, 2. Uber Befruchtung: Ibid.—ldi., '97, 3.
Uber Cytoplasmastrukturen, Kern- und Zelltheilung: Ibid. — Id., '98. Die Pflanz-
lichen Zellhaute: Ibid., XXXI. — Strasburger and Mottier, '97. Uber den
zweiten Theilungsschritt in Pollenmutterzellen : Ber. D. Bot. Ges., XV., 6. — Van
der Stricht, O., '92. Contribution a I'etude de la sphere attractive : A. B., XII.,
4. — Id., '95, 1. La maturation et la fdcondation de I'oeuf d'Amphioxus lanceoiatus :
Bull. Acad. Roy. Belgique, XXX., 2. — Id., '95, 2. De I'origine de la figure achro-
matique de I'ovule en mitose chez le Thysanozoon Brocchi : Verhandl. d. anat.
Versamml. in Strassburg, 1895, p. 223. — Id., '95, 3. Contributions a I'etude de la
forme, de la structure et de la division du noyau : Bull. Acad. Roy. Sc. Belgique,
XXIX. — Id., '98, 1. La formation des globules polaires, etc., chez Thysanozoon:
Arch. Biol., XV. — Id., '98, 2. Contribution a I'etude du noyau vitellin de Balbiani :
Verh. An. Ges., XII. — Strieker, S.. '71. Handbuch der Lehre von den Geweben :
Leipzig. — Stuhlmann, Fr., '86. Die Reifung des Arthropodeneies nach Beobach-
tungen an Insekten, Spinnen, Myriopoden und Peripatus : Ber. NattirJ. Ges. Frei-
burg, I. — Suzuki, B., '98. Notiz liber die Entstehung des Mittelstlickes von Sela-
chiern: A. A., XV., 8. — Swaen and Masquelin, '83. iStude sur la Spermato-
gdnese: A. B., IV. — Swingle, "W. T., '97. Zur Kenntniss der Kern- und
Zellteilungen bei den Sphacelariaceae : /. w. B., XXX.

GENERAL LITERATURE-LIST 467

THOMA, R., ^96. Text-book of General Pathology and Pathological Anatomy :
Trans, by A. Bruce, London. — Thomson, Allen. Article " Generation "' in Todd's
Cyclopaedia. — Id. Article "Ovum" in Todd's Cyclopaedia. — Townsend, C. O.,
'97. Der Einfluss des Zellkerns auf die Bildung der Zellhaut : Jahrb. wiss. Bot.y
XXX. — Treat, Mary, '73. Controlling Sex in Butterflies: Am, Nat., VII.—
Trow, A. H., '95. The Karyology of Saprolegnia: Ann. Bot., IX. — Tyson,
James, '78. The Cell-doctrine : 2d ed., Philadelphia.

UNNA, P., '95. tJber die neueren Protoplasmatheorien, und das Spongio-
plasma: Deutsche Med. Zeit., 1895, 98-100. — Ussow, M., '81. Untersuchungen
iiber die Entwickelung der Cephalopoden : Arch. Biol., II.

VEJDOVSKY, F., '88. Entwickelungsgeschichtliche Untersuchungen, Heft I. :
Reifung, Befruchtung und Furchung des Rhynchelmis-Eies : Prag, 1888. Vej-
dovsky and Mrdzek, '98. Centrosom und Periplast : Sitzber. b'ohm. Ges. Wiss. —
Verworn, M., '88. Biologische Protisten-studien : Z. w. Z., XLVI. — Id., '89.
Psychophysiologische Protisten-studien: Jena. — Id., '91. Die physiologische
Bedeutung des Zellkerns : Pfluger'^s Arch. f. d. ges. Physiol., II. — Id., '95. Allge-
meine Physiologic: /<?«^. — Vircho-w, R., '55. Cellular-Pathologie : Arch. Path.
Anat. Phys., VIII., i. — Id., '58. Die Cellularpathologie in ihrer Begriindung auf
physiologische und pathologische Gewebelehre : Berlin, 1858. — De Vries, H., *89.
Intracellulare Pangenesis : Jena.

WAGER, H., '96. On the Structure and Reproduction of Cystopus. Ann. Bot.,
X. — Waldeyer, W., '70. Eierstock und Ei : Leipzig. — Id., '87. Bau und Ent-
wickelung der Samenfaden : Verh. An. Ges. Leipzig, 1887. — Id., '88. tJber Karyo-
kinese und ihre Beziehungen zu den Befruchtungsvorgangen : A. m. .r4., XXXII.
[Trans, in Q.J.'] — Id., '95. Die neueren Ansichten iiber den Bau und das Wesen
der Zelle: Deutsch. Med. Woche7ischr ., No. 43, ff., Oct. ff., 1895. — "Warneck, N.
A., '50. liber die Bildung und Entwickelung des Embryos bei Gasteropoden :
Bidl. Soc. Imp. Nat. Moscou, XXIII., i. — Watas^, S., '91. Studies on Cephalo-
pods; I., Cleavage of the Ovum: /. M., IV., 3. — Id., '92. On the Phenomena of
Sex-differentiation: Ibid., VI., 2, 1892. — Id., '93, 1. On the Nature of Cell-
organization: Wood^s Holl Biol. Lectures, 1893. — Id., '93, 2. Homology of the
Centrosome : /. M., VIII., 2. — Id., '94. Origin of the Centrosome : Biological Lec-
tures, Wood'' s H oil, 1894. Webber, H. J., '97, 1. Peculiar Structures occurring
in the Pollen-tube of Zamia: Bot. Gazette, XXIII., 6. — Id., '97, 2. The Develop-
ment of the Antherozoids of Zamia: Ibid., XXIV., i. — Id., '97, 3. Notes on the
Fecundation of Zamia and the Pollen-tube Apparatus of Gingko: Ibid., XXIV., 4.
— Weismann, A., '83. Uber Vererbung: Jena. — Id., '85. Die Kontinuitat des
Keimplasmas als Grundlage einer Theorie der Vererbung: Jena. — Id., '86, 1.
Richtungskorper bei parthenogenetischen Eiern : Zool. Anz., No. 233. — Id., '86, 2.
Die Bedeutung der sexuellen Fortpflanzung fur die Selektionstheorie : Jena. —
Id., '87. tJber die Zahl der Richtungskorper und iiber ihre Bedeutung fiir die
Vererbung : /iS/m. — Id., '91, 1. Essays upon Heredity, First Series: Oxford. —
Id., '91, 2. Amphimixis, oder die Vermischung der Individuen: Jena, Fischer. —
Id., '92. Essays upon Heredity, Second Series: Oxford, 1892. — Id., '93. The
Germ-plasm: New York. — Id., '94. Aussere Einfliisse als Entwicklungsreize :
Jena. — IA.. "99. Regeneration: Nat. Sci., XIV., 6. [See also A. A., 1899.]
Wheeler, W. M., '89. The Embryology of Blatta Germanica and Doryphora
decetnlineata: J. M., III. — Id., '93. A Contribution to Insect-embryology: Ibid.,
VIII., I. — Id., '95. The Behaviour of the Centrosomes in the Fertilized Egg of
Myzostoma glabrum: Ibid., X. — Id., '96. The Sexual Phases of Myzostoma:

46S GENERAL LITERATURE-LIST

Mitth. Zool. St. Neapel,Xll.,2. — lA., '97. The Maturation, Fecundation, and
early Cleavage in Myzostoma : Arch. Biol., XV. — Whitman, C. O., "78. The
Embryology of Clepsine : Q. /., XVIII. — Id., '87. The Kinetic Phenomena of
the Egg during Maturation and Fecundation : /. M., I., 2. — Id., "88. The Seat of
Formative and Regenerative Energy : Ibid., II. — Id., '93. The Inadequacy of the
Cell-theory of Development : Wood'' s H oil Biol. Lectures, 1893. — Id., '94. Evolu-
tion and Epigenesis : Ibid., 1894. — "Wiesner, J., '92. Die Elementarstruktur und
das Wachstum der lebenden Substanz : Wien. — "Wilcox, E. V., "95, Spermato-
genesis of Caloptenus and Cicada : Bull, of the Museum of Comp. Zool., Harvard
College, \ o\.. XXVW., No. i. — Id., "96. Further Studies on the Spermatogenesis
of Caloptenus : Bull. Mus. Comp. Zool., XXIX.— Will, L., '86. Die Entstehung
des Eies von Colymbetes : Z. w. Z., X LI II. — Wilson, Edm. B., "92. The Cell-
lineage of Nereis: f. M., VI., 3. — Id., '93. Amphioxus and the Mosaic Theory
of Development : Ibid., VIII., 3. — Id., '94. The Mosaic Theory of Development:
IVood'^s Holl Biol. Led., 1894. — Id., '95, 1. Atlas of Fertilization and Karyo-
kinesis : Neiu York, Macinillan. — Id., '95, 2. Archoplasm, Centrosome, and
Chromatin in the Sea-urchin Egg: f. M., XI. — Id., '96. On Cleavage and
Mosaic-work. [Appendix to Crampton and Wilson, '96.] : A. Entwm., III., i. —
Id., '97. Centrosome and Middle-piece in the Fertilization of the Egg. Science-,
Vol. v.. No. 114. — Id.. '98. Considerations on Cell-lineage and ancestral Remi-
niscence: Ann. N. Y.Acad. Sci., XI. See also Wood'' s Holl Biol. Lectures, ''()<^.
— Id , '99. On protoplasmic Structure in the Eggs of Echinoderms and some
other Animals :/. J/., XV. Suppl. — Wilson and Mathevw^s. "95. Maturation,
Fertilization, and Polarity in the Echinoderm Egg: /. J/., X., i. — 'Wolff. Caspar
Friedrich, 1759. Theoria Generationis. — "Wolff, Gustav, "94. Bemerkungen
zum Darwinismus mit einem experimentellen Beitrag zur Physiologic der Entvvick-
lung : B. C, XIV., 17. — Id., '95. Die Regeneration der Urodelenlinse : Arch.
Entwm., I., 3. — 'Wolters, M., "91. Die Conjugation und Sporenbildung bei
Gregarinen : A. m. A., XXXVII. — 'Woltereck, R., '98. Zur Bildung und Ent-
wicklung des Ostrakoden-Eies : Z. w. Z., LXIV.

"yUNG, E., '81. De I'influence de la nature des aliments sur la sexualite :"
C. R., XCIII ; also Arch. Exp. Zool., 2d, I., 1883.

ZACHARIAS, O.. '85. tJber die amoboiden Bewegungen der Spermatozoen
von Polyphemus pediculus : Z. w. Z., XLl. — Zacharias. E.. '93, 1. Uber die
chemische Beschaifenheit von Cytoplasma und Zellkern : Ber. deutsch. Bot. Ges.,
II., 5. — Id.. '93, 2. Uber Chromatophilie : Ibid., 1893. — Id., '95. Uber das
Verhalten des Zellkerns in wachsenden Zellen : Flora, ^i, 1895. — Id., '94. Uber
Beziehungen des Zellenwachstums zur Beschaifenheit des Zellkerns : Bej'ichte der
deittschen botan. Gesellschaft, XII., 5. — Id.. "98. t'ber Nachweis tind Vorkommen
von Nuclein : Ber. d. Bot. Ges., XVI., 7. — Ziegler. E.. '88. Die neuesten Arbeiten
uber Vererbung und Abstammungslehre und ihre Bedeutung fur die Pathologic :
Beitr. zur path. Anat., IV. — Id., '89. Uber die Ursachen der pathologischen
Gewebsneubildungen : Itit. Beitr. zur. iviss. Med. Festschrift, R. Vircho^v, II. —
Id., '92, Lehrbuch der allgemeinen pathologischen Anatomic und Pathogenese, 7th
ed., fena. — Ziegler, H. E., '87. Die Entstehung des Blutes bei Knochenfischen-
embryonen:^. w. A. — Id., '91. Die biologische Bedeutung der amitotischen
Kerntheilung im Tierreich : B. C, XI. — Id., '94. Uber das Verhalten der Kerne
im Dotter der meroblastischen Wirbelthiere : Ber. Naturf. Ges. Freiburg, 1894. —
Id., '95. Untersuchungen liber die Zelltheilung: Verhandl. d. deutsch. Zool. Ges.,
1895. — Id., "96. Einige Betrachtungen zur Entwicklungsgeschichte der Echino-
dermen: Verh. d. Zool. Ges. — Id., '98. Experimentelle Studien iiber die Zellthei-

GENERAL LITERATURE-LIST 469

lung, I., II.: Arch. Entwfn., VI., 2. — Ziegler and vom Rath. Die amitotische
Kerntheilung bei den Arthropoden : B. C, XI. — Zimmermann, A., '93, 1. Bei-
trage zur Morphologic und Physiologic dcr Pflanzcnzclle : Tiibingen. — Id., '94.
Sammelreferate aus dem Gesammtgcbicte der Zellenlchre : Bot. Centb. Belhefte^
1894. Zimmermann, K. W., *93, 2. Studicn ubcr Pigmentzcllen, etc. : A. m. A.,
XLI. — Id., "98. Beitrage zur Kcnntniss ciniger Driiscn und Epithclzellen : A. m.
A.,IA\. — Zoja, R., ''95,1. Sullo sviluppo dei blastomcri isolati dalle uova di
alcune meduse : A. Entwm.,\., 4 ; II., i ; II., IV. — Id., '95, 2. Sulla indepcndcnza
della cromatina paferna e materna nel nucleo delle cellule embrionali : A. A., XL,
10. Id., '97. Stato attualc degli Studii sulla Fecondazione : Bo/L Sci. di Pavia^
XVIII., XIX. — Zar Strassen, O., '98. tJber die Riesenbildung bei Ascaris-
Eiern: Arch. E^itwm., VII., 4.

INDEX OF AUTHORS

Albrecht, nuclei, 32.

Altmann, granule -theory, 25, 27, 290; nu-

clein, 332.
Amici, pollen-tube, 218.
Andrews, spinning activities, 61.
Apathy, nerve-cells, 48.
Aristotle, epigenesis, 8.
Arnold, fibrillar theory of protoplasm, 23;

leucocytes, 117; nucleus and cytoplasm,

303.
Atkinson, reduction, 269.
Auerbach, 6; double spermatozoa, 142;

staining-reactions, 176; fertilization, iSl.

Von Baer, cleavage, 10; cell-division, 64;
egg-axis, 378; development, 396.

Balbiani, scattered nuclei, 40; spireme-
nuclei, 36; mitosis in Infusoria, 88; chro-
matin-granules, 112; yolk-nucleus, 155-
156; regeneration in Infusoria, 343.

Balfour, polar bodies, 243; rate of division,
366; unequal division, 371.

Ballowitz, structure of spermatozoa, 139,
140; double spermatozoa, 142.

Van Bambeke, deutoplasm and yolk -nucleus,
156-160; elimination of chromatin, 155.

Barry, fertilization, 181.

De Bary, protoplasm, 4, 5, 20; conjugation,
181; cell-division and growth, 393.

Beale, cell-organization, 291.

Bechamp and Estor, microsome-theory, 290,
291.

Belajeff, spermatozoids, 172-175; reduction
in plants, 267.

Benda, spermatogenesis, 163; Sertoli-cells,
284.

Van Beneden, cell-theory, i, 6, 7; proto-
plasm, 23; nuclear membrane, 38; cen-
trosome and attraction -sphere, 51, 74, 77,
310, 323; cell-polarity, 55; cell-division,
64, 74; origin of mitotic figure, 74-77;
theory of mitosis, 100 ; division of chromo-
somes, 112; fertilization oi Ascaris, 7, 182;
continuity of centrosomes, 75; germ-

nuclei, 205; centrosome in fertilization,
208; theory of sex, 243; parthenogenesis,
281; nucleus and cytoplasm, 303; nuclear
microsomes, 302 ; promorphology of cleav-
age, 381 ; germinal localization, 399.

Van Beneden and Julin, first cleavage-plane,
380.

Bergmann, cleavage, 10; cell, 17.

Bernard, Claude, nucleus and cytoplasm, 341 ;
organic synthesis, 431.

Berthold, protoplasm, 42 ; cell-division,
376.

Bickford, regeneration in coelenterates, 392,
429.

Biondi, Sertoli-cells, 284.

Biondi-Ehrlich, staining-fluid, 157.

Bischoff, cell, 17.

Bizzozero, cell-bridges, 60.

Blanc, fertilization of trout, 210.

Blochmann, insect-egg, 132; budding of nu-
cleus, 155; polar bodies, 281 ; bilateralitjr
of ovum, 383.

Bohm, fertilization in fishes, 192.

Bolsius, nephridial cells, 47.

Bonnet, theory of development, 8, 432.

Born, chromosomes in Tri/on-egg, 338;
gravitation-experiments, 386.

Boveri, centrosome, named, 51 ; a permanent
organ, 51, 74; in fertilization, 192, 211,
215, 230; structure, 309; functions, 354;
archoplasm, 69, 318; origin of mitotic
figure, 74, 77, 319; varieties of Ascaris,
87; theory of mitosis, loi, 108; division
of chromosomes, 112; origin of germ-cells,
147; fertilization of Ascaris, 182; of
Pterotrachea, 184; of Echinus, 192; the-
ory of fertilization, 190, 21 1; of partheno-
genesis, 281 ; partial fertilization, 190, 194;
reduction, 233; maturation in Ascaris^
238; tetrads, 238; centriole, 309; attrac-
tion-sphere, 324; egg-fragments, 353.

Braem, cell-division, 377.

Brandt, symbiosis, 53; regeneration in Pro-
tozoa, 342.

471

472

INDEX OF AUTHORS

Brauer, bivalent chromosomes, 82; mitosis
in rhizopod, 96; fission of chromatin-
granules, 113; deutoplasm, 153; fertiliza-
tion in Brajichipus, 192; parthenogenesis
in Artemia, 281 ; spermatogenesis in Asca-
ris, 255; intra-nuclear centrosome, 304.

Braus, 81.

Brogniard, pollen-tube, 218.

Brooks, heredity, 12; variation, 179.

Brown, Robert, cell-nucleus, 18; pollen-
tube, 218.

Briicke, cell-organization, 289.

Von Brunn, spermatozoon, 141.

Biihler, astral systems, 318.

Biitschli, 6; protoplasm, 25, 36, 50; diffused
nuclei, 40; artifacts, 42; asters, 48, 316;
cell-membrane, 54; mitosis, 109, iio;
centrosome in diatoms, 51; rejuvenes-
cence, 178; polar bodies, 238.

Calberla, micropyle, 2CX).

Calkins, nuclei of flagellates, 40; mitosis in
Noctiliica, 92; yolk-nucleus, 157; origin
of middle-piece, 165; reduction, 253, 257.

Campbell, fertilization in plants, 216.

Carnoy, nucleus, 40; muscle-fibre, 48; cen-
trosome, no; amitosis, 1 15, 1 1 7; germ-
nuclei, 184; asters, 305, 317.

Carnoy and Le Brun, nucleoli, 130; fertiliza-
tion, 211; reduction, 263.

Castle, egg-axis, 379; fertilization, 193.

Chittenden, organic synthesis, 341.

Chmielewski, reduction in Spirogyra, 280.

Chun, amitosis, 117; partial development of
ctenophores, 418.

Clapp, first cleavage-plane, 381.

Coe, fertilization, 194, 213; centrosome, 321.

Cohn, cell, 17.

Conklm, size of nuclei, 71 ; union of germ-
nuclei, 204; centrosome in fertilization,
210; centrosome and sphere, 323; un-
equal division, 373; protoplasmic cur-
rents, 377; cell-size and body-size, 388;
types of cleavage, 423.

Corda, pollen-tube, 218.

Crampton, yolk-nucleus, 158; reversal of
cleavage, 368; experiments on snail, 419,
421; on tunicates, 419.

Crato, protoplasm, 50.

Darwin, evolution, 2, 5 ; inheritance, 12, 396;
variation, 11 ; pangenesis, 12, 290; gem-
mules, 290.

Darwin, F., protoplasmic fragments, 346.

Dendy, cell-bridges, 60.

Dogiel, amitosis, 118.

Driesch, dispermy, 198; fertilization of egg-
fragments, 200, 353; pressure-experiments,
375,410; regeneration, 393; isolated blas-
tomeres, 409; theory of development, 394,
415; experiments on ctenophores, 418;
ferment-theory, 427.

Driiner, spindle-fibres, 79; central spindle,
105; aster, 321, 326.

Von Ebner, Sertoli-cells, 284.

Ehrlich, tar-colours, 335.

Eismond, structure of aster, 48.

Elssberg, plastidules, 291.

Endres, experiments on frog's egg, 399, 419.

Engelmann, ciliated cells, 44; rejuvenes-
cence, 179.

Von Erlanger, asters, 48, 316; spindle, 81;
elimination of chromatin, 155; Nebenkern,
163, 165; fertilization, 194, 212, 213; cen-
troplasm, 324.

Eycleshymer, first cleavage-plane, 381.

Farmer, reduction in plants, 275.

Fick, fertilization of axolotl, 192, 212.

P'ield, staining-reactions, 176.

Fischel, ctenophores, 419.

Fischer, nucleus, 40; artifacts, 42; staining-
reactions, 335.

Flemming, protoplasm, 25, 27 ; chromatin, ■},'}, ;
centrosome, 51 ; cell-bridges, 60, 61 ; cell-
division, 64, 70; splitting of chromosomes,
70; mitotic figure, 79; heterotypical mito-
sis, 86; leucocytes, 102; theory of mitosis,
106; division of chromatin, 113; amitosis,
117,285; nucleoli, 127; rotation of sperm-
head, 188; spermatogenesis, 259-262;
astral rays, 317; germinal localization,

399-

Floderus, follicle-cells, 150.

Fol, I, 6, 64; amphiaster, 68; theory of mi-
tosis, 108; sperm-centrosome, 191 ; poly-
spermy, 192; attraction-cone, 198; vitel-
line membrane, 199; asters, 316.

Foot, yolk-nucleus and polar rings, 156, 202;
fertilization in earthworm, 187; entrance-
funnel, 201; fertilization-centrosome, 212.

Foster, cell-organization, somacules, 291.

Francotte, polar bodies, 235; centrosome,
306; sphere, 312, 325.

Frommann, protoplasm, 23; nucleus and
cytoplasm, 303.

Galeotti, pathological mitoses, 97.
Gallardo, mitosis, 109.

INDEX OF AUTHORS

473

Gallon, inheritance, 9.

Gardiner, cell-bridges, 59; chromatin-elimi-

nation, 276; sphere, 325.
Garnault, fertilization in Avion, 207.
Geddes and Thompson, theory of sex, 124.
Van Gehuchten, spirenie-nuclei, 36; nuclear

polarity, 36; muscle-fibre, 48.
Giard, polar bodies, 235, 238.
Gierke, staining-reactions, 335.
Gilson, spireme-nuclei, 36.
Godlewski, spermatogenesis, 168.
Graf, nephridial cells, 47.
Gregoire, reduction, 267.
Griffin, fertilization, centrosomes in Thalas-

sema, 193, 194, 213; reduction, 259;

structure of centrosome, 314; aster- forma-
tion, 321.
Grobben, spermatozoa, 141.
Gruber, diffused nuclei, 40; regeneration in

Stentor, 342.
Guignard, mitosis in plants, 82; fertilization

in plants, 218, 221; reduction, 263, 267.

Haberlandt, position of nuclei, 346.

Hackel, inheritance, 7; epithelium, 56; cell-
state, 58.

Hacker, polar spindles, 276; bivalent chro-
mosomes, 88; nucleolus, 125, 128; primor-
dial germ-cells, 148; germ-nuclei, 208,
299; reduction in copepods, 249.

Hallez, promorphology of ovum, 384.

Halliburton, proteids, 331; nuclein, 333.

Hamm, discovery of spermatozoon, 9, 181.

Hammar, cell-bridges, 60.

Hammarsten, proteids, 331.

Hansemann, pathological mitoses, 97.

Hanstein, metaplasm, 19.

Hardy, artifacts, 42.

Harper, mitosis, 82.

Hartsoeker, spermatozoon, 9.

Harvey, inheritance, 7; epigenesis, 8.

Hatschek, cell-polarity, 56; fertilization, 179.

Heidenhain, nucleus, 36; basichromatin and
oxychromatin, 2>^, 337; cell-polarity, 55;
position of centrosome, 57; leucocytes,
102; theory of mitosis, 105; amitosis, 116;
staining-reactions, 337; nuclear micro-
somes, 303; microcentrum, 31 1 ; asters,
311,317; origin of centrosome, 315; po-
sition of spindle, 377.

Heider, insect-egg, 132.

Heitzmann, cell-bridges, 59; nucleus and
cytoplasm, 303,

Heiiking, fertihzation, 187; insect-egg, 96;
spermatogenesis, 165, 248, 253, 271.

Henle, granules, 289.

Henneguy, deutoplasm, 153; yolk-nucleus,
160; centrosome, 356.

Hensen, rejuvenescence, 179.

Herbst, development and environment, 428.

Herla, independence of chromosomes, 208,
299.

Hermann, central spindle, 78, 105; division
of chromatin, 112; spermatozoon, 165,
166; staining-reactions, 176.

Hertwig, O., i, 7, 9; bivalent chromosomes,
88; pathological mitoses, 97; rejuvenes-
cence, 178; fertilization, 181 ; middle-
piece, 187; polyspermy, 199; paths of
germ-nuclei, ^04; maturation, 241 ; polar
bodies, 238; inheritance, 182; laws of
cell-division, 364; theory of development,

415-

Hertwig, O. and R., 197; egg-fragments^
199; polyspermy, 199.

Hertwig, R., mitosis in Protozoa, 90; germ-
cells in Sagitta, 146; amphiasters in un-
fertilized eggs, 306; conjugation, 222;
reduction in Infusoria, 277; in Actino-
spharium, 278; origin of centrosome, 315;
cell-division, 391.

Hill, fertilization, 187, 193.

Hirase, spermatozoids, 144; fertilization,
218.

His, germinal localization, 398.

Hofer, regeneration in Amoeba, 343.

Hoffman, micropyle, 200.

Hofmeister, cell-division and growth, 393.

Holmes, cleavage, 368.

Hooke, R., cell, 17.

Hoyer, amitosis, 115.

Huie, Drosera, 350.

Huxley, protoplasm, 5; germ, 7, 396; fer-
tilization, 178, 231; evolution and epi-
genesis, 432.

Ikeno, cell-bridges, 150; blepharoplasts, 173;
fertilization, 221.

Ishikawa, Noctiluca, mitosis, 92; conjuga-
tion, 227; reduction, 267; flagellum, 171.

Jennings, cleavage, 377.

Jordan, deutoplasm and yolk-nucleus, 153,

156; first cleavage-plane, 381.
Julin, fertilization in Styleopsis, 192.

Keuten, mitosis in Euglena, 91.
Klebahn, conjugation and reduction in des-
mids and diatoms, 280.

474

INDEX OF AUTHORS

Klebs, pathological mitosis, 97, 98; cell-
membrane, 346.

Klein, nuclear membrane, 38; theory of
mitosis, 100; amitosis, 118; nucleus and
cytoplasm, 303; asters, 316.

Klinckowstrom, fertilization, 213; reduction,

259-

Von Kolliker, i, 6, 9, 10, 27; epithelium, 56;
cell-division, 63; spermatozoon, 9, 134;
inheritance, 182; development, 413.

Korff, spermatogenesis, 163, 168, 173.

Korschelt, nucleus, 37; amitosis, 115; move-
ments and position of nuclei, 125, 349,
387; nurse-cells, 151 ; fertilization, 193;
tetrads in Ophryotrocha, 258; physiology
of nucleus, 348 ; polarity of egg, 387.

Kossel, chromatin, 336; nuclein, 334; or-
ganic synthesis, 340.

Kostanecki, fertilization, 193 ; astral rays, 318.

Kostanecki and Wierzejski, fertilization of
Physa, 193, 210, 212; continuity of cen-
trosomes, 211.

Kupffer, energids, 30; cytoplasm, 41.

Lamarck, inheritance, 12.

Lamarle, minimal contact-areas, 361.

Lankester, germinal localization, 398.

Lauterborn, mitosis in diatoms, 95; origin
of centrosome, 315.

Leeuwenhoek, spermatozoon, 8; fertiliza-
tion, 181.

Von Lenhossek, nerve-cell, 21, 47; sperma-
togenesis, 169, 315; centrosome, 314, 356.

Leydig, cell, 19; protoplasm, 20; cell-mem-
brane, 54; spermatozoa, 142; elimination
of chromatin, 159.

Lilienfeld, staining-reactions of nucleins, 336.

Lillie, fertilization, 196, 213; centrosome
and aster, 312, 326, 327; regeneration in
Stentor, 343 ; cleavage, 360, 369, 377.

Loeb, chemical fertilization, 215, 392; re-
generation in coelenterates, 392; theory
of development, 427; environment and
development, 430.

Lustig and Galeotti, pathological mitoses,
98; centrosome, 51.

Maggi, granules, 290.

Malfatti, staining-reactions of nucleins, 335.

Mark, germ-nuclei, 204; polar bodies, 235;

polarity of ovum, 387.
Mathews, pancreas-cell, 44; aster-formation,

no; fertilization of echinoderms, 192,212;

origin of centrosome, 125; nucleic acid,

334; staining-reactions, 337.

Maupas, sex in Rotifers, 145; rejuvenes-
cence, 179; conjugation of Infusoria, 223.

Mayer, staining, 335.

McClung, spermatogenesis, 271.

MacFarland, spindle, 79; fertilization, 213,
214; centrosome and sphere, 312, 314,
321.

McGregor, spermatogenesis, 167; reduction,
261.

McMurrich, gasteropod development, 152;
metamerism in isopods, 390.

Mead, fertilization of Chatopterus, 192, 194,
215; sperm-centrosome, 215; centrosomes
de ttovo, 212, 306; cell-division, 391.

Merkel, Sertoli-cells, 284.

Mertens, yolk-nucleus and attraction-sphere,
156, 159.

Metschnikoff, insect-egg, 383.

Meves, amitosis, 119, 285; spermatogenesis,
167, 169; reduction, 260; cilia, 357.

Meyer, energids, 30; cell-bridges, 60.

Miescher, nuclein, 332.

Mikosch, protoplasm, 44.

Minot, rejuvenescence, 179; cyclical divi-
sion, 222; theory of sex, 243; Sertoli-
cells, 284; parthenogen sis, 280.

Von Mohl, cell-division, 9; protoplasm, 17.

Montgomery, nucleolus, 34; spermatogene-
sis, 257, 271.

Moore, spermatozoon, 167, 171 ; reduction,
263.

Morgan, centrosomes, 307; fertilization of
egg-fragments, 353; cell-division, 391;
effect of fertihzation, 201 ; numerical rela-
tions of cells, 389; regeneration, 393, 394;
isolated blastomeres, 410; polarity, 417;
experiments on ctenophores, 418; on
frog's egg, 422.

Mottier, mitosis, 83; fertilization, 221; re-
duction, 266; asters, 305.

Munson, yolk-nucleus, 156.

Nageli, development, i; cell-organization,
micellae, 289, 291; polioplasm, 41; idio-
plasm-theory, 401.

Nawaschin, fertihzation, 218.

Nemec, mitosis, 82; yolk-nucleus, 159.

Nevv'port, fertilization, 181 ; first cleavage-
plane, 380.

Nissl, chromophilic granules, 48.

Nussbaum, germ-cells, 122; sex, 145; re-
generation in Infusoria, 342; nucleus, 426.

Obst, nucleoli, 130; follicle-cells, 151.
Osterhout, spindle, 82; tetrads, 253.

INDEX OF AUTHORS

47S

Overton, germ-cells of Vohox, 134; conju-
gation of Spiro'gyra, 229; reduction, 274,

275-
Owen, germ-cells, 122.

Paladino, cell-bridges, 60.

Paulmier, spermatozoon, 165; reduction, 252,
271.

Peremeschko, leucocytes, 117.

Peter, cilia, 357.

Pfeffer, hyaloplasm, 41; amitosis, 1 19;
chemotaxis of germ-cells, 197.

Pfitzner, cell-bridges, 60; chromatin-gran-
ules, 112.

PflUger, position of spindle, 375 ; first cleav-
age-plane, 380; gravitation-experiments,
386; isotropy, 378.

Plateau, minimal contact-areas, 366.

Platner, mitosis, no; egg-centrosome, 125;
formation of spermatozoon, 163; fertiliza-
tion of .4;-?t7«, 207; maturation, 241.

Pouchet and Chabry, development and en-
vironment, 428.

Prenant, spermatozoon, 162; archoplasm,
322.

Preusse, amitosis, 119.

Prevost and Dumas, cleavage, lO.

Pringsheim, Hautschicht, 41 ; fertilization,
i8i.

Purkinje, protoplasm, 17.

Rabl, nuclear polarity, 36; cell-polarity, 56;
centrosome in fertilization, 210; individu-
ality of chromosomes, 294; astral systems,

Ranvier, blood-corpuscles, 54.

Vom Rath, bivalent chromosomes, 88; ami-
tosis, 118, 225; early germ-cells, 149;
reduction, 249.

Rauber, cell-division and growth, 393.

Rawitz, amitosis, 116; staining-reactions,

335-

Redi, genetic continuity, 290.

Reichert, cleavage, 10, 64.

Reinke, pseudo-alveolar structure, 50; nu-
cleus, 38, 303; oedematin, 36; asters, 305;
nucleus and cytoplasm, 303.

Remak, cleavage, i, 10, 361 ; cell-division,
64; egg-axis, 378.

Retzius, muscle-fibre, 48; cell-bridges, 60;
end-piece, 140.

Rhumbler, 105.

Robin, germinal vesicle, 64.

Rosen, staining-reactions, 220.

Roux, 245, 301, 351 ; meaning of mitosis, 244,

301, 351, 405; position of spindle, 377;
first cleavage-plane, 380; frog-experi-
ments, mosaic theory, 399; theory of de-
velopment, 405 ; post-generation, 408.

Riickert, pseudo-reduction, 248; fertilization
of Cyclops, 193; independence of germ-
nuclei, 208, 209; reduction in copepods,
249, 251; early history of germ-nuclei,
273; reduction in selachians, 257; history
of germinal vesicle, 338.

Riige, amitosis, 117. -

Ryder, staining-reactions, 175.

Sabaschnikoff, tetrads, 256.

Sabatier, amitosis, 116.

Sachs, energid, 19, 30; laws of cell-division,
362; cell-division and growth, 393; de-
velopment, 427.

St. George, La V'alette, spermatozoon, 10,
134; spermatogenesis (terminology), 161.

Sala, polyspermy, 199.

Sargant, reduction in plants, 267.

Schafer, protoplasm, 29.

Scharff, budding of nucleus, 155.

Schaudinn, mitosis in Protozoa, 92, 94, I02;
polar bodies, 278.

Schewiakoff, mitosis in Euglypha, 91.

Schimper, plastids, 290,

Schleicher, karyokinesis, 64.

Schleiden, cell-theory, i; cell-division, 9;
nature of cells, 17; fertilization, 218.

Schloter, granules, 38, 303.

Schmitz, plastids, 290; conjugation, 216.

Schneider, discovery of mitosis, 64.

Schottlander, multipolar mitosis, 99.

Schultze, M., cells, i, 19; protoplasm, 20.

Schultze, O., mitosis, 318; gravitation-ex-
periments, 422; double embryos, 422,

Schwann, cell-theory, i; the tgg a cell, 8;
origin of cells, 9; nature of cells, 17; or-
ganization, 58; adaptation, 433.

Schwarz, protoplasm, 42; linin, 33; chemis-
try of nucleus, 41 ; nuclei of growing cells,
340.

Schweigger-Seidel, spermatozoon, 9, 134.

Sedgwick, cell-bridges, 60.

Seeliger, egg-fragments, 353; egg-axis, 379.

Selenka, double spermatozoa, 142.

Shaw, spermatozoids, 175..

Siedlecki, polar bodies, 280.

Sobotta, fertilization, 185, 211.

Solger, pigment-cells, 102; attraction-sphere,

Spallanzani, spermatozoa, 9; regeneration,
393-

476

INDEX OF AUTHORS

Spencer, physiological units, 289; develop-
ment, /32.

Stauffacher, egg-centrosome, 125.

Stevens, fertilization, 217.

Strasburger, i, 7; cytoplasm, 20; Korner-
plasma, 41; centrosphere, 68, 356, 324;
membranes, 55; origin of amphiaster, 82;
multipolar mitoses, 99; theory of mitosis,
105, no; spermatozoids, 173; kinoplasm,
27, 82, 322; staining-reactions of germ-
nuclei, 220; fertilization in plants, 216,
219, 221; reduction, 265, 269; theory of
maturation, 275; organization, 289; in-
heritance, 7, 182, 351; action of nucleus,
426.

Zur Strassen, giant-embryos, 296; germ-
cells, 148.

Van der Stricht, spindle, 79; amitosis, 116;
fertilization, 210; reduction, 259; centro-
some and sphere, 312, 325.

Strobe, multipolar mitoses, 99.

Stuhlmann, yolk-nucleus, 156.

Suzuki, spermatogenesis, 168.

Swingle, mitosis, 82.

Tangl, cell-bridges, 59.

Thiersch and Boll, theory of growth, 392.

Townsend, cell-bridges, 61, 346.

Treat, sex, 145.

Treviranus, variation, 179.

Unna, protoplasm, 27.

Ussow, micropyle, 133; deutoplasm, 153.

Vejdovsky, centrosome, 76; fertilization in
Rhynchelmis, 192, 194; metamerism in
annelids, 390.

Verworh, cell-physiology, 6; regeneration in
Protozoa, 344; inheritance, 359, 431.

Virchow, I; cell-division, 10, 63; proto-
plasm, 25; cell-state, 58.

I)e Vries, organization, pangens, 291, 327,
406; tonoplasts, 53; plastids, 229; chro-
matin 43 1; development, 404.

Waldeyer, nucleus, 38; cytoplasm, 41 ; cell-
membrane, 54.
Walter, frog-experiments, 419.
Watase, theory of mitosis, 106; staining-

reactions of germ-nuclei, 176; nucleus and
cytoplasm, 292; asters, 305; theory of
centrosome, 315; astral rays, 321; cleav-
age of squid, 381 ; promorphology of ovum,
383, 386.

Webber, spermatozoids, 144, 173; fertiliza-
tion, 221.

Weismann, inheritance, 12; cell-organiza-
tion, biophores, 291; somatic and germ
cells, 122; amphimixis, 179; maturation,
243-246; constitution of the germ-plasm,
245; parthenogenesis, 281 ; theory of de-
velopment, 404, 407, 432.

Went, vacuoles, 53.

Wheeler, amitosis, 115; insect-egg, 132;
egg of Myzostoma, 151; fertilization in
Myzostotna, 208 ; bilaterality of ovum, 383.

Whitman, on Harvey, 7; polar rings, 202;
cell-division and growth, 393; polarity,
384; theory of development, 400, 416.

Wiesner, cell-organization, 290, 291.

Wilcox, sperm-centrosome, 165; reduction,

257-

Will, chromatin-elimination, 135.

Wilson, protoplasm, 27, 44; mitosis, 106;
fertilization in sea-urchin, 187, 212; paths
of germ-nuclei, 202; origin of linin, 303;
astral rays, 28; centrosphere and centro-
some, 314; dispermy, 355; rudimentary
cells, 372; pressure-experiments, 41 1;
experiments on Amphioxus, 410; theory
of development, 415.

Von Wittich, yolk-nucleus, 155.

Wolff, C. F., epigenesis, 8.

Wolff, G., regeneration of lens, 433.

Wolters, polar bodies in gregarines, 278.

Yung, sex, 144.

Zacharias, E,, nucleoli, 34; of meristem, 37;
staining-reactions, 176; nuclein in grow-
ing-cells, 340.

Zacharias, O., amoeboid spermatozoa, 142.

Ziegler, artificial mitotic figure, 108; amito-
sis, 117; sphere, 324.

Zimmerman, pigment-cells, 102; centrosome,

356.
Zoja, independence of chromosomes, 299;
isolated blastomeres, 410.

INDEX OF SUBJECTS

Acanthocystisy 94, 304, 306.

Achromatic figure (see Amphiaster), 69;

varieties of, 78; nature, 316.
Achromatium, 39.
Actinophrys, 92, 278.
Actinosph^erium, mitosis, 90, 94; reduction,

278; regeneration, 342.
yEquorea, metanucleus, 128.
Albugo, 217.
Albumin, 331.
Allium, 83, 253, 267,
Allolobophora, teloblasts, 374.
Alveoli, 25.
Amitosis, 114; biological significance, 116;

in sex-cells, 285.
Amoeba, 5; mitosis, 91; experiments on,

343-

Amphiaster, 68; asymmetry of, 70, 373;
origin, 72, 74, 316; in amitosis, 116; in
fertilization, 187, 213; nature, 316; posi-
tion, 375.

Amphibia, spermatozoa, 140; sex, 145.

Amphioxus, fertilizatit)n, 210; polar body,
236, 277; cleavage, 370; dwarf larvae,
389, 410; double embryos, 410.

Amphipyrenin, 41.

Amphiuma, 167, 261.

Amyloplasts, 53; in plant-ovum, 133.

Anaphases, 70; in sea-urchin egg, 106.

Anasa, sperm-formation, 165, 271; reduc-
tion, 272.

Ahcylus, 368.

Anilocra, gland-cells, nuclei, 36; amitosis,
116.

Anodonta, ciliated cells, 43, 357.

Antipodal cone, loi.

Aphis, 281.

Arbacia, 192, 215, 307.

Archoplasm, 69; in developing spermatozoa,
171; nature of, 318.

Archosome, 52.

Argonauta, micropyle, 133.

Aricia, rudimentary cells, 372.

Arion, spindle, 81 ; germ-nuclei, 207.

Ariscema, 269.

Artemia, chromosomes, 89; parthenogenetic
maturation, 281.

Artifacts, in protoplasm, 42.

Ascaris, chromosomes, 87, 301; mitosis, 80,
lOl ; primordial germ-cells, 146; fertiliza-
tion, 182, 211; polyspermy, 199; polar
bodies, 238; spermatogenesis, 241, 253;
individuality of chromosomes, 295; in-
tranuclear centrosome, 304; centrosome,
311; attraction-sphere, 323; supernumer-
ary centrosome, 355.

Aster, 68; asymmetry, 70; structure and
functions, loi; in amitosis, 116; in fertili-
zation, 187, 213; nature of, 316; finer
structure, 326; relative size, 70, 373.

Asterias, spermatozoa, 176; sperm-aster,
187; fertilization, 192, 210.

Astrocentre, 324.

Astrosphere, 324.

Attraction-cone, 198.

Attraction-sphere, 51, 72; in amitosis, 1 15;
of the ovum, 125; of the spermatid, 163;
in resting cells, 323; nature of, 323.

Axial filament, 136; origin of, 165.

Axis, of the cell, 55 ; of the nucleus, 36, 294;
of the ovum, 378, 386.

Axolotl, fertilization, 192.

Bacteria, nuclei, 31, 39.

Basichromatin, 38; staining-reactions, t^t^Z.

Biohlast, 290.

Biogen, 291.

Biophore, 245, 291.

Birds, blood-cells, 57; spermatozoa, 138;
young ova, 155.

Blastomeres, displacement of, 366; inli-
vidual history, 378; prospective value,
415; rhythm of division, 366, 389; de-
velopment of single, 409, 418; in normal
development, 423.

Blennius, pigment-cells, 103.

Blepharoplastoids, 175.

Blepharoplasts, 173, 221.

Branchipus, yolk, 153; sperm-aster, 192;
reduction, 256.

477

478

INDEX OF SUBJECTS

Calanus, tetrads, 250.

Caloptenus, 165, 257.

Cambium, 376.

Cancer-cells, mitosis, 98.

Canihocamptus, reduction, 251; ovarian
eggs, 273.

Cell, in general, 4; origin, 9; name, 17;
general sketch, 19; polarity of, 55; as a
structural unit, 58; structural basis, 23,
293; physiology and chemistry, 330; size
and numerical relations, 389; in inheri-
tance, 9, 430; differentiation of, 413, 426;
independence of, 427.

Cell-bridges, 59.

Cell-division (see Mitosis, Amitosis), general
significance, 10, 63; general account, 65;
types, 64; Remak's scheme, 63; indirect,
65; direct, 114; cyclical character, 178,
223; equal and reducing or qualitative,
405; relation to development, 388, 405,
410, 427; Sachs's laws, 362; rhythm, 366,
389; unequal, 370; of teloblasts, 371;
energy of, 388; relation to metamerism,
390; causes, 391 ; relation to growth,
388; and differentiation, 427.

Cell-membrane, 53.

Cell-organization, 289.

Cell-organs, 52; nature of, 291; temporary
and permanent, 292.

Cell-plate, 71.

Cell-state, 58.

Cell-theory, general sketch, I-14.

Central spindle, 70, 78.

Centrodesmus, 79, 315.

Centrodeutoplasm, 163, 324.

Centroplasm, 324.

Centrosome, 22; general sketch, 50, 304;
position, 55; in mitosis, 74; a permanent
organ, 74; dynamic centre, 76; historical
origin, 315; functions, loi, 354; in ami-
tosis, 115; of the ovum, 125; of the
spermatozoon, 137, 165-170; in fertiliza-
tion, 190, 208; degeneration of, 186, 213;
continuity, 74, 77, 194, 214, 321 ; nature,
304; intra-nuclear, 304; supernumerary,

355-

Centrosphere, 68, 85; nature of, 324.

Ceratium, 91.

Ceraiozamia, reduction, 275.

Cerebratulus, 193, 194,213, 306,307,321,325.

Cerianthus, regeneration in, 392.

Chatopterus, spindle, 81, 84; fertilization,
192; sperm-centrosome, 213; centrosomes
de novo, 306; cell-division, 391.

Chara, spermatozoids, 143.

Chilomonas, 32, 40, 192.

Chironomus, spireme-nuclei, 36.

Chorion, 132.

Chromatic figure, 69; origin, 72; varieties,
86; in fertilization, 181, 204.

Chromatin, 33; in meristem, 37; in mitosis,
65, 86; in cancer-cells, 98; of the egg-
nucleus, 126; elimination of, in cleavage,
147,426; in oogenesis, 233, 276; staining-
reactions, 334-340; morphological organi-
zation, 37, 245, 294; chemical nature, 332,
404; relations to linin, 302; physiological
changes, 338; as the idioplasm, 352; in
development, 405, 425, 431.

Chromatin-granules, 37; in mitosis, 112; in
reduction, 248; general significance, 301-
304; relations to linin, 302.

Chromatophore, 53; in the ovum, 133; in
fertilization, 229.

Chromiole, 302.

Chromomere (see Chromatin-granule), 37,
301.

Chromoplast, 53.

Chromosomes, 67, 70, 86, II2; number of,
67, 206; bivalent and plurivalent, 87;
division, 112; of the primordial germ-
cell, 148; in fertilization, 182, 204; inde-
pendence in fertilization, 204; reduction,
238, 243, 248; in early germ-nuclei, 273;
conjugation of, 257; in parthenogenesis,
281; individuality of, 294; composition of,
301; chemistry, 334, 336; history in ger-
minal vesicle, 338; in dwarf larvae, 296.

Ciliated cells, 44, 57.

Ciona, egg-axis, 379.

Clavelina, cleavage, 369, 381.

Cleavage, in general, 10 ; geometrical rela-
tions, 362; Sachs's rules, 362; Hertwig's
rules, 364; modifications of, 366; spiral,
368; reversal of, 368; unequal, 370; under
pressure, 375,41 1 ; promorphology of, 378;
bilateral, 381; rhythm, 366, 388; mosaic
theory, 399, 423; half cleavage, 410.

Cleavage-nucleus, 204.

Cleavage-planes, 362; axial relations, 378,

Clepsine, nephridial cell, 45; polar rings,
202 ; cleavage, 370.

Closterium, conjugation and reduction, 280.

Cockroach, amitosis, 115; orientation of egg,

384.
Coelenterates, germ-cells, 146; regeneration,

392, 393. 430-
Conjugation, in unicellular animals, 222;
unicellular plants, 228, 280; physiological
meaning, 178, 223.

INDEX OF SUBJECTS

479

Contractility, theory of mitosis, lOO; inade-
quacy, io6.

Copepods, reduction, 251.

Corixa, ovum, 383.

Corpuscule central, 310, 314.

Crepidula, fertilization, 210; dwarfs and
giants, 389; cleavage, 323, 423.

Cross-furrow, 368.

Crustacea, spermatozoa, 142.

Ctenophores, experiments on eggs, 418.

Cucurbita, 346.

Cuticular, 54.

Cyanophycese, nucleus, 31, 39,

Cycads, spermatozoids, 144, 173; fertiliza-
tion, 218, 221.

Cyclops, o\2i, 128; primordial germ-cells, 148;
fertilization, 188; reduction, 251; attrac-
tion-sphere, 325; axial relations, 385.

Cytoplasm, 21, 41, 293, 303; of the ovum,
130; of the spermatozoon, 134; morpho-
logical relations to nucleus, 302; to archo-
plasm, 316, 319; chemical relations to
nucleus, 333-341; physiological relations
to nucleus, 341; in inheritance, 352-354,
359; in development, 398,421; origin, 431.

Cytosome, 322.

DendrobcEna, metamerism, 390.

Determinants, 245.

Deutoplasm, 131 ; deposit, 153; effect on
cleavage, 366, 371; rearrangement by
gravity, 422.

Development, 1-12; and cell-division, 388;
mosaic theory, 399, 421; theory of Nageli,
402; Roux-yVeismann theory, 404; of
single blastomeres, 399, 409, 418; of egg-
fragments, 296, 353, 419; De Vries's the-
ory, 413; Hertwig's theory, 415, 432;
Driesch's theory, 394, 415; partial, 409,
419; half and whole, 419; nature of,4i3;
external conditions, 428; and metabolism,
430; unknown factor, 431 ; rhythm, 432;
adaptive character, 433.

Diaptomus, 250.

Diatoms, mitosis, 92; centrosome, 51.

Diatilula, 79, 314.

Diemyctylus, yolk, 153; yolk-nuclei, 156.

Differentiation, 361; theory of De Vries,
404; of Weismann, 405; nature and
causes, 413; of the nuclear substance,
425 ; and cell-division, 427.

Dipsacus, 346.

Dispermy, 355.

Double embryos, 410, 422.

Drosera, 350.

Dwarfs, formation of, 353, 410, 422; size of
cells, 389.

Dyads (Zweiergruppen), 239, 241; in par-
thenogenesis, 284.

Dyaster, 70.

Dycyemids, centrosome, 51.

Dytiscus, ovarian eggs, 153, 349.

Earthworm, ova, 152; spermatozoon, 165;
yolk-nucleus, 154; polar rings, 156, 202;
spermatogenesis, 257; teloblasts, 374.

Echinoderms, protoplasm, 28, 44, 293; sper-
matozoa, 137; fertilization, 188, 212;
polyspermy, 194, 198; dwarf larvae, 353,
410; half cleavage, 410; eggs under press-
ure, 41 1 ; modified larvae, 428.

Echinus, fertilization, 210; centrosome, 314;
dwarf larvae, 353; number of cells, 389.

Ectosphere, 324.

Egg-axis, 378; promorphological signifi-
cance, 379; determination, 386; alteration
of, 422.

Egg- fragments, fertilization, 194; develop-
ment, 352.

Elasmobranchs, spermatozoon, 140, 167, 169;
germinal vesicle, 245, 273; reduction, 257.

Embryo-sac, 218, 263.

Enchylema, 23.

End-knob, 136.

Endoplasm, 41.

End-piece, 140.

End-plate, 91.

Energid, 19, 30.

Entosphere, 324.

Envelopes, of the egg, 132.

Epigcnesis, 8, 432.

Equatorial plate, 68.

Equiseturti, mitosis, 85.

Ergastoplasm, 322.

Erysiphe, mitosis, 82.

Euchceta, tetrads, 250.

Euglena, mitosis, 91, 315.

Euglypha, mitpsis, 89, 95.

Evolution (preformation), 8, 399, 432.

Evolution, theory of, 2, 8.

Exoplasm, 41.

Fertilization, general aspect, 9; physiologi-
cal meaning, 180; general sketch, 180;
Ascaris, 182; mouse, 185; sea-urchin, 188;
Nereis, 188; Cyclops, 188; Thalassema,
ChcEtopterus, 193, 195; pathological, 198;
partial, 190, 194; of Myzosio?na, 196, 208;
in plants, 215; egg-fragments, 194; Bo-
veri's theory, 192, 211.

48o

INDEX OF SUBJECTS

Fishes, pigment-cells, 102 ; periblast-nuclei,
117; spermatozoa, 137; young ova, 116;
single blastomeres, 410.

Flagellates, diffused nuclei, 39.

Follicle, of the egg, 150.

Forficula, nurse-cells, 151.

Fragmentation, 64.

Fritillaria, spireme, II2; fertilization,
219.

Frog, tetrads, 259; egg-axis, 378; first cleav-
age-plane, 380; Roux's puncture experi-
ment, 399; post-generation, 409; pressure-
experiments, 410; effect of gravity on the
egg, 422; development of single blasto-
meres, 399, 408, 422; double embryos,
422.

Fucus, 143, 217, 221.

Ganglion-cell, 48; centrosome in, 51, 314.

Gemmae, 291.

Gemmules, 12, 291.

Genoblasts, 243.

Geophilus, deutoplasm, 154, 158; yolk-nu-
cleus, 156.

Germ, 7, 396.

Germ-cells, general, 8, 9; detailed account,
122; of plants, 133, 142; origin, 144;
growth and differentiation, 150; union,
196; results of union, 200; maturation,
233; early history of nuclei, 272.

Germinal localization, theory of, 397.

Germinal spot, 124.

Germinal vesicle, 124, 125; early history,
273; movements, 349; position, 387.

Germ-nuclei, of the ovum, 125; of the
spermatozoon, 135; of plants, 216; stain-
ing-reactions, 175; in fertilization, 182,
188; equivalence, 182, 205; paths, 202;
movements, 204; union, 204; indepen-
dence, 204, 299; in Infusoria, 224; early
history, 272.

Giant-cells, 31; microcentrum, 314.

Gingko, 173.

Globulin, 331, t,^Z'

Granules (see Microsomes), of Altmann, 290;
nuclear, 37, 303; chromophilic, 23, 48;
in general, 289.

Gravity, effect on the egg, 131, 422.

Gregarines, mitosis, 89; polar body, 278.

Ground-substance, of protoplasm, 23; of
nucleus, 36.

Growth, and cell-division, 58, 388.

Gryllotalpa, reduction, 249,

Guinea-pig, spermatogenesis, 170; matura-
tion, 277.

Heliozoa, 92, 103.

Helix, 163, 168, 259.

Hemerocallis, 306.

Heterocope, tetrads, 250.

Heterokinesis, 406.

Histon, 334, 336.

Homoeokinesis, 406.

Hydrophilus, orientation of egg, 384.

Id, in reduction, 245; in inheritance, 406.

Idant, 245.

Idioblast, 291.

Idioplasm, theory of, 401 ; as chromatin,
403; action of, 406, 414, 431, 432.

Idiosome, 291.

Idiozome, 163, 165, 324.

Ilyanassa, partial development, 419.

Infusoria, nuclei, 31, 224; mitosis, 90; con-
jugation, 223; reduction, 277.

Inheritance, of acquired characters, 12,
433 ; Weismann's theory, 12; through
the nucleus, 351-354 ; and metabolism,

430-
Inotagmata, 291.
Insect-eggs, 132, 386,
Interzonal fibres, 70.
Iris, 267.

Isopods, metamerism, 390.
Isotropy, of the egg, 384, 417.

Karyokinesis (see Mitosis), 64.
Karyokinetic figure (see Mitotic Figure),

69.
Karyolymph, 36.
Karyoplasm, 21.
Karyosome, 34.
Kinoplasm (archoplasm), 54, 77, 82, 173,

322.

Lanthanin, 38.

Lepidoptera, sex, 144.

Leucocytes, structure, 102; division, 117;
centrosome, 309; attraction-sphere, 326.

Leucoplasts, of plant-ovum, 133.

lilium, mitosis, 83; spireme, 112; fertiliza-
tion, 219; reduction, 265-271.

Umax, germ-nuclei, 204.

limulus, 158.

Linin, 32; relations to cytoreticulum and
chromatin, 302.

liparis, 281.

locusta, orientation of egg, 384.

Zo/?^tf, spindle, 81 ; cleavage, 381.

lumbricus, yolk-nucleus, 157; reduction,

257-

INDEX OF SUBJECTS

481

Macrobdella, 305.

Macrogamete, 226.

Macromeres, 371.

Mammals, spermatozoa, 139, 169; young
ova, 155.

Mantle-fibres, 78, 105.

Marsilia, 175.

Maturation (see Reduction), 234; theoreti-
cal significance, 243; of parthenogenetic
eggs, 280; nucleus in, 353.

Medusae, dwarf embryos, 410.

Meristem, nuclei of, 340.

Metamerism, 390.

Metanucleus, 128.

Metaphase, 69.

Metaplasm, 19.

Micellae, 291.

Microcentrum, 311, 315, 324.

Microgamete, 226.

Micromeres, 371.

Micropyle, 124, 133.

Microsomes, 23; of the egg-cytoplasm, 1 31;
nature of, 289, 290, 293 ; of the astral sys-
tems, 318, 326; of the nucleus, 301, 303;
relation to centrosome, 315; staining-
reactions, 337.

Microsphere, 324.

Microzyma, 291.

Mid-body, 71, 78.

Middle-piece, 135, 139; origin, 161, 165-
170; in fertilization, 187, 212.

Mitosis, 64; general outline, 65; modifica-
tions of, 77; heterotypical, 86; in unicellu-
lar forms, 87; pathological, 88; multipolar,
97; mechanism of, I CX); physiological sig-
nificance, 351; Roux-Weismann concep-
tion of, 245, 406.

Mitosome, 165.

Mitotic figure (see Mitosis, Spindle), 69;
origin, 72; varieties, 78.

Molgula, 158.

Mouse, fertilization, 185, 193.

Musca, ovum, 142.

Myriapods, spermatozoa, 142; yolk-nucleus,
156.

Myzostoma^ fertilization, 196, 208.

Naias, 266.

Nebenkern, pancreas-cells, 44; of spermatid,

163, 165.
Nebenkorper, 164, 165.
Neciurus, pancreas-cells, 44.
Nematodes, germ-nuclei, 184.
Nereis, asters, 49; perivitelline layer, 131;

ovum, 129; deutoplasm, 131 ; fertilization,

191; attraction-sphere and centrosome,
325; cleavage, 366, 369; pressure-experi-
ments on, 411.

Nerve-cell, 48.

Net-knot, 34.

iVbr/zVwc^?, mitosis, 93; flagellum, 171 ; con-
jugation, 227; sphere, 319.

Nuclear stains, 335.

Nuclein, t^t^, 332; staining-reactions, 334;
physiological significance, 340.

Nuclein-bases, 331.

Nucleinic acid, ■^t^, 332-334; staining-reac-
tions, 334 ; physiological significance,
340.

Nucleo-albumin, 331, 334.

Nucleo-proteid, 331, 334.

Nucleolus, 33 ; in mitosis, 67 ; of the ovum,
126; physiological meaning, 128.

Nucleoplasm, 21.

Nucleus, general structure and functions,

. 31; finer structure, 37; polarity, 36, 294;
chemistry, 41; in mitosis, 65; of the
ovum, 125; of the spermatozoon, 135, 137;
relation to cytoplasm, 302; morphological
composition, 294; in organic synthesis,
340, 430; physiology, 341; position and
movements, 346; in fertilization, 181, 352;
in maturation, 353; in later development,
425; in metabolism and inheritance, 430;
in inheritance and development, 341, 358,
405, 425, 431 ; control of the cell, 426.

Nurse-cells, 151."

CEdigonium, fertilization, 181; membrane,
346.

Onoclea, 175.

Oocyte, 236.

Oogenesis, 234, 236.

Oogonium, 236.

Oosphere, 133.

Ophryotrocha, amitosis, 115; nurse-cells,
151; fertilization, 189, 193; tetrads, 258.

Opossum, spermatozoa, 142.

Organization, 289, 291 ; of the nucleus, 294,
301; of the egg, 397, 433.

Origin of species, 3.

Osmunda, reduction, 275.

Ovary, 123; of Canthocampius, 273.

Ovum, in general, 8, 9; detailed account,
124; nucleus, 125; cytoplasm, 130; en-
velopes, 132; of plants, 133; origin and
growth, 150; fertilization, 178; effects of
spermatozoon upon, 201 ; maturation, 236;
parthenogenetic, 280; promorphology^
378; bilaterality, 382.

482

INDEX OF SUBJECTS

Oxychromatin, 38, 303; staining-reactions,

337-
Oxydation-ferments, 351.
Oxytricha, 342.
Oyster, germ-nuclei, staining-reactions, 175.

Fallavicinia, reduction, 275.

Paludina, dimorphic spermatozoa, 141.

Pangenesis, 12, 290, 431.

Pangens, 291.

Parachromatin, 41.

Paralinin, 41.

Paramoeba, mitosis, 94, 315.

/V2raw(K«Mw, mitosis, 91 ; conjugation, 224;
reduction, 277.

Paranucleus, 163.

Parthenogenesis, theories of, 281; polar
bodies in, 280.

Pellicle, 54.

Pentatoma^ 271.

Petromyzon, fertilization, 192, 212.

Phallusia, fertilization, 193, 212.

Physa, fertilization, 193, 210, 212; reversed
cleavage, 368.

Physiological units, 289.

Piei'is, spinning-gland, 37.

Pigment-cells, 102.

Filularia, fertilization, 216.

Pinus, reduction, 275.

Planaria, regeneration, 394.

Plant-cells, plastids, 52; membranes, 54;
mitosis, 82; cleavage-planes, 363.

Plasma-stains, 335.

Plasmocyte, 52.

Plasmosome, 34.

Plasome, 291.

Plastids, 52; of the ovum, 133; of the sper-
matozoid, 143; conjugation of, 229.

Plastidule, 291.

Plastin, 41, 331.

Pleiirophyllidia, 78, 94.

Podophyllmn, 267.

Polar bodies, 181 ; nature and mode of for-
mation, 235-240; division, 236; in par-
thenogenesis, 281.

Polar rings, 156, 202.

Polarity, of the nucleus, 36; of the cell, 55;
of the ovum, 378; determination of, 382.

Pole -plates, 91.

Pollen-grains, formation, 263-265.

Pollen-tube, 218.

Polyclades, cleavage, 416.

Polychcerus, 276, 325.

Polygordius, cleavage, 368.

Polyspermy, 198; prevention of, 199.

Polystomella, regeneration, 344.

Polyzonium^ 159.

Porcellio, amitosis, 1 1 6.

Predelineation, 398.

Preformation (see Evolution).

Pressure, experiments, 375, 410.

Principal cone, loi.

Pristiurus, 338.

Promorphology (see Cleavage, Ovum).

Pronuclei, 202.

Prophase, 65.

ProsthecercBus, 213, 235, 256, 259, 306.

Prosth iostovi um, 212.

Protamin, 334.

Proteids, 331.

Prothalliuni, 264; chromosomes in, 275.

Protoplasm, 4, 5, 17, 19; structure, 23,42,

293; chemistry, 331.
Protoplast (see Plastid).
Pseudo-alveolar structure, 50.
Pseudo-reduction, 248.
Pieris, 253.

Pieroirachea, germ-nuclei, 186, 205.
Ptychoptera, spireme-nuclei, 35.
Pygcera, 165.
Pyrenin, 4J.
Pyrenoid, 133.
Pyrrhocoris, 165, 248.

Quadrille of centres, 210.

Rat, spermatogenesis, 170.

Reduction, general outline, 234; parallel
between the two sexes, 241 ; theoretical
significance, 243; detailed account, 246;
in plants, 263; Strasburger's theory of,
275; in unicellular forms, 277; by conju
gation, 257; modes contrasted, 247.

Regeneration, Weismann's theory, 406; in
frog-embryo, 409; nature of, 425, 427;
in coelenterates, 430; of lens, 433.

Rejuvenescence, 179, 224.

Renilla, ovum, 132.

Rhabdonema, amitosis, 115.

Rhynchelmis, fertiUzation, 192, 193, 212;
cleavage, 370.

Rotifers, sex, 145.

Sagitta, number of chromosomes, 184; pri-
mordial germ-cells, 146; germ-nuclei, 184;
spermaster, 191.

Salamander, epidermis, 3; spermatogonia,
20; mitosis in, 71, 78; pathological mito-
sis, 98; leucocytes, 102; spermatozoa,
140; maturation, 259.

INDEX OF SUBJECTS

483

Sargus, pigment-cells, 103.
Scylliuni, 263.

Segmentation (see Cleavage).
Selaginella, spermatozoids, 197.
Senescence, 179.
Sepia, spindle, 81.
Sertoli-cells, 284.

Sex, 9; determination of, 144; Minot's the-
ory of, 243.
Siphonophores, amitosis, 1 1 7.
Soma, 13.
Somacule, 291.
Somatic cells, 122; number of chromosomes,

233.

Spermary, 123.

Spermatid, 161, 163; development into sper-
matozoon, 164.

Spermatocyte, 161, 241.

Spermatogenesis (see Reduction), 234; gen-
eral outline, parallel with oogenesis, 241.

Spermatogonium, 161, 241.

Spermatozeugma, 142.

Spermatozoid, structure and origin, 142,
172; in fertilization, 217, 221.

Spermatozoon, discovery, 9;- structure, 134;
essential parts, 135; giant, 141; double,
142; unusual forms, 142; of plants, 142;
formation, 160; in fertilization, 181, 192;
entrance into ovum, 197.

Sperm-centrosome, 135, 164-171; in fertili-
zation, 192, 211-215, 221.

Sperm-nucleus, 135; origin, 164-171; in
fertilization, 182, 190; rotation, 188; path
in the egg, 202; in inheritance, 353;
chemistry, 334.

Sphar echinus, fertilization, 193, 210; num-
ber of cells, 389; hybrids, 353; regenera-
tion, 393.

Spindle (see Amphiaster, Central Spindle) , 68 ;
origin, 72, 79,82; in Protozoa, 90; conjuga-
tion of, 227; nature of, 316; position, 375.

Spireme, 65.

Spirochona, mitosis, 90.

Spirogyra, nucleolus, 67; amitosis, 1 19;
conjugation, 229; reduction, 280.

Spongioplasm, 25.

Spontaneous generation, 7.

Stem-cells, 148.

Stentor, regeneration, 342.

Stylonychia, senescence, 224.

Stypocaulon, mitosis, 82.

Surirella, 94.

Symbiosis, 53, 292.

Synapta, cleavage, 364.

Syncytium, 59.

Teloblasts, 371, 390.
Telophase, 71.

Tetrads (Vierergruppen), 238; origin, 246;
in Ascaris, 241, 253; in arthropods, 248;
ring-shaped, 248; in amphibia, 259; ori-
gin by conjugation, 257; formulas for,
247.

Tetramitus, 40, 92.

Thalassema, spindle, 81 ; fertihzation, 193,
194, 213; reduction, 259, 263; centro-
some, 321; attraction-sphere, 325.

Thalassicolla, experiments on, 344.

Thysanozoon, 212, 259, 326.

Tonoplast, 53.

Toxopneustes, cleavage, 10; mitosis, 107;
ovum, 126; spermatozoon, 134; fertiliza-
tion, 188; paths of germ-nuclei, 202;
polar bodies, 114; double cleavage, 355.

Trachelocerca, diffused nuclei, 40.

Trillium, 269.

Triton, 140, 212, 263, 277.

Trophoplasm, 322, 401.

Tubularia, regeneration, 430.

Tunicates, egg-axis, 379; cleavage, 381.

Unicellular organisms, 5; mitosis, 88; con-
jugation, 222; reduction, 277; experi-
ments on, 342.

Unio, centrosome and aster, 314; cleavage.

381.

Urostyla, 40.

Vacuole, 50, 53.
Vanessa, ovarian egg, 153.
Variations, 1 1 ; origin of, 433.
Vaucheria, membrane, 348.
Vitalism, 394, 417.
Vitelline membrane, 132; of egg-fragments,

132; formation of, 198; function, 199.
Volvox, germ-cells, 133.
Vorticella, conjugation, 226.

Xiphidium, 271.

Yellow cells (of Radiolaria), 53.
Yolk (see Deutoplasm), 152.
Yolk-nucleus, 155.
Yolk-plates, 131.

Zamia, 173, 221.
Zirphtea, 259, 263.
Zwischenkorper (mid-body), 71.
Zygnema, membrane, 346.
' Zygospore, 228.

Columbia University Biological Series.

EDITED BY
HENRY FAIRFIELD OSBORN,

Da CoHta Profesmr of Zoology in Columbia Uiviversiti;,

AND

EDMUND B. WILSON,

Professor of Zoology in Columbia University.

This series is founded upon a course of popular University
lectures given during the winter of 1892-3, in connection with
the opening of the new department of Biology in Columbia
College. The lectures are in a measure consecutive in charac-
ter, illustrating phases in the discovery and application of the
theory of Evolution. Thus the first course outlined the de-
velopment of the Descent theory; the second, the application
of this theory to the problem of the ancestry of the Vertebrates,
largely based upon embryological data; the third, the applica-
tion of the Descent theory to the interpretation of the structure
and phylogeny of the Fishes or lowest Vertebrates, chiefly based
upon comparative anatomy ; the fourth, upon the problems of
individual development and Inheritance, chiefly based upon the
structure and functions of the cell.

Since their original delivery the lectures have been carefully
rewritten and illustrated so as to adapt them to the use of Col-
lege and University students and of general readers. The vol-
umes as at present arranged for include:

I. From the Greeks to Darwin. By Henry Fairfield

OSBORN".

II. Amphioxus and the Ancestry of the Vertebrates.

By Arthur Willey.
III. Fishes, Living and Fossil. By Bashford Dean.

IV. The Cell in Development and Inheritance. By

Edmund B. Wilson.

V. The Foundations of Zoology. By William Keith
Brooks.

THE MACMILLAN COMPANY,

66 FIFTH AVENUE, NEW YORK.

I. FROM THE GREEKS TO DARWIN.

THE DEVELOPMENT OF THE EVOLUTION IDEA.

BY

HENRY FAIRFIELD OSBORN, Sc.D., Princeton.

Da Costa Professor of Zoology in Columbia University.
8vo. Cloth. $2.00, net.

This opening volume, " From the Greeks to Darwin," is an
outline of the development from the earliest times of the idea of
the origin of life by evolution. It brings together in a continu-
ous treatment the progress of this idea from the Greek philoso-
pher Thales (640 B.C.) to Darwin and Wallace. It is based
partly upon critical studies of the original authorities, partly
upon the studies of Zeller, Perrier, Quatrefages, Martin, and
other writers less known to English readers.

This history differs from the outlines which have been pre-
viously published, in attempting to establish a complete conti-
nuity of thought in the growth of the various elements in the
Evolution idea, and especially in the more critical and exact
study of the pre-Darwinian writers, such as Buffon, Goethe,
Erasmus Darwin, Treviranus, Lamarck, and St. Hilaire, about
whose actual share in the establishment of the Evolution theory
vague ideas are still current.

TABLE OF CONTENTS.
I. The Akticipation^ ai^d Interpretation^ of Nature.
XL Amokg the Greeks.

III. The Theologians and Natural Philosophers.

IV. The Evolutionists of the Eighteenth Century.
V. From Lamarck to St. Hilaire.

VI. The First Half-century and Darwin.
In the opening chapter the elements and environment of the
Evolution idea are discussed, and in the second chapter the re-
markable parallelism between the growth of this idea in Greece
and in modern times is pointed out. In the succeeding chap-
ters the various periods of European thought on the subject are
covered, concluding with the first half of the present century,
especially with the development of the Evolution idea in the
mind of Darwin.

II. AMPHIOXUS AND THE ANCESTRY
OF THE VERTEBRATES.

BY

ARTHUR WILLEY, B.Sc. LOND.,

Thiior in Biology, Columbia University ; Balfour Student of the
University of Cambridge.

8vo. Cloth. $2.50, net.

The purpose of this volume is to consider the problem of the
ancestry of the Vertebrates from the standpoint of the anat-
omy and development of Amphioxus and other members of the
group Protochordata. The work opens with an Introduction,
in which is given a brief historical sketch of the speculations
of the celebrated anatomists and embryologists, from Etienne
Geoftroy St. Hilaire down to our own day, upon this problem.
The remainder of the first and the whole of the second chapter
is devoted to a detailed account of the anatomy of Amphioxus
as compared with that of higher Vertebrates. The third chapter
deals with the embryonic and larval development of Amphioxus,
while the fourth deals more briefly with the anatomy, embryology,
and relationships of the Ascidians; then the other allied forms,
Balanoglossus, Cephalodiscus, are described.

The work concludes with a series of discussions touch-
ing the problem proposed in the Introduction, in which it is
attempted to define certain general principles of Evolution by
which the descent of the Vertebrates from Invertebrate ancestors
may be supposed to have taken place.

The work contains an extensive bibliography, full notes, and
135 illustrations.

TABLE OF CONTENTS.

Introduction.

Chapter I. Anatomy of Amphioxus.
II. Ditto.

ni. Development of Amphioxus.
IV. The Ascidians.

V. The Protochordata in their Relation to
the Problem of Vertebrate Descent.

III. FISHES, LIVING AND FOSSIL,

AN INTBODUCrORY STUDY.

BY

BASHFORD DEAN, Ph.D., Columbia,

Instructor in Biology. Columbia University.
8vo. Cloth, $2.50, net.

This work has been prepared to meet the needs of the gen-
eral student for a concise knowledge of the Fishes. It contains
a review of the four larger groups of the strictly fishlike forms.
Sharks, Chimaeroids, Teleostomes, and the Dipnoaus, and adds
to this a chapter on the Lampreys. It presents in figures the
prominent members, living and fossil, of each group; illustrates
characteristic structures; adds notes upon the important phases
of development, and formulates the views of investigators as to
relationships and descent.

The recent contributions to the knowledge of extinct Fishes
are taken into special account in the treatment of the entire
subject, and restorations have been attempted, as of Dinichthys,
Ctenodus, and Cladoselache.

The writer has also indicated diagram matically, as far as
generally accepted, the genetic relationships of fossil and living
forms.

The aim of the book has been mainly to furnish the student
with a well-marked ground-plan of Ichthyology, to enable him to
better understand special works, such as those of Smith Wood-
ward and Gi'inther. The work is fully illustrated, mainly from
the writer's original pen-drawings.

TABLE OF CONTENTS.

CHAPTER

I. Fishes. Their Essential Characters. Sharks, Chimaeroids, Teleo-
stomes, aud Luug-tishes. Their Appearance in Time and their
Distribution.
II. The Lampreys. Their Position with Reference to Fishes. Bdel-
lostonia, Myxine, Petromyzon, Palaeospoudylus.

III. The Shark Group. Anatomical Characters. Its Extinct Members,

Acaiithodian, Cladoselachid, Xenacanthid, Cestracionts.

IV. Chimaeroids. Structures of Callorhynchus and Chimaera. Squalo-

raja and Myriacanthus. Life-habits aud Probable Relationships.
V. Teleostomes. The Forms of Recent " Ganoids." Habits and Dis-
tribution. The Relations of Prominent Extinct Forms. Crosso-
pterygians. Typical " Bony Fishes. "

VI. The Evolution of the Groups of Fishes. Aquatic Metameiism.

Numerical Lines. Evolution of Gill-cleft Characters, Paired and
Unpaired Fins, Aquatic Sense-organs.

VII. The Development of Fishes. Prominent Features in Embryonic

and Larval Development of Members of each Group. Summarie*.

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