Boston

Medical Library

8 The Fenway

r

HARRIS KENNEDY.

THE CELL

IN DEVELOPMENT AND INHERITANCE

Columbia SEntbcrsttg 33talogical SeriES.

EDITED BY
HENRY FAIRFIELD OSBORN.

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.

COLUMBIA UNIVERSITY BIOLOGICAL SERIES. IV.

THE CELL

Development and Inheritance

EDMUND B. WILSON, Ph.D.

PROFESSOR OF INVERTEBRATE ZOOLOGY, COLUMBIA UNIVERSITY

" Natura nusquam magis est tota quam in minimis "

PI.INY

THE MACMILLAN COMPANY

LONDON : MACMILLAN & CO., LTD.
1896

All rights reserved

By the MACMILLAN COMPANY.

J.^^.,2^-

Narfaool) '^xees

J. S. CushinK & Co. - Berwick & Smith
Norwood Mass. U.S.A.

THEODOR BOVERI

Digitized by the Internet Arciiive

in 2010 with funding from

Open Knowledge Commons and Harvard Medical School

http://www.archive.org/details/cellindevelopmenwils

PREFACE

This volume is the outcome of a course of lectures, delivered at
Columbia University in the winter of 1892-93, in which I endeavoured
to give to an audience of general university students some account
of recent advances in cellular biology, and more especially to trace
the steps by which the problems of evolution have been reduced to
problems of the cell. It was my first intention to publish these
lectures in a simple and general form, in the hope of showing to
wider circles how the varied and apparently heterogeneous cell-
researches of the past twenty years have grown together in a
coherent group, at the heart of which are a few elementary phe-
nomena, and how these phenomena, easily intelligible even to those
having no special knowledge of the subject, are related to the
problems of development. Such a treatment was facilitated by
the appearance, in 1893, of Oscar Hertwig's invaluable book on
the cell, which brought together, in a form well designed for the
use of special students, many of the more important results of
modern cell-research. I am glad to acknowledge my debt to Hert-
wig's book ; but it is proper to state that the present volume was
fully sketched in its main outlines at the time the Zelle mid Gczvebe
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 o.^^, — 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

viii 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

^ Henneguy's Le^ojis stir 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. 343). 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.

TABLE OF CONTENTS

INTRODUCTION

PAGE

List of Figures xv

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

Literatm-e . . . . . . . . . . . . . . ' . 12

CHAPTER I
General Sketch of the Cell

A. General Morphology of the Cell .......... 14

B. Structural Basis of Protoplasm . . . . . . . . . • 1 7

C. The Nucleus ............. 22-^

1. General Structure . . . . . . . . . . .23

2. Finer Structure of the Nucleus . . . . . . . . -27

3. Chemistry of the Nucleus .......... 28

D. The Cytoplasm ............. 29

E. The Centrosome . . . . . . ■ . . . . . . .36

F. Other Cell-organs . . . . ... . . . . . '37

G. The Cell-membrane . . . . . . . . . . . ■ . 38

H. Polarity of the Cell 38

I. The Cell in Relation to the Multicellular Body 41

Literature, I. .............. 43

D.

CHAPTER II

Cell-Division

Outline of Indirect Division or Mitosis .
Origin of the Mitotic Figure
Modifications of Mitosis

1. Varieties of the Mitotic Figure .

2. Heterotypical Mitosis

3. Bivalent and Plurivalent Chromosomes

4. Mitosis in the Unicellular Plants and Animals

5. Pathological Mitoses .....
The Mechanism of Mitosis .....

1. Function of the Amphiaster

(a) Theory of Fibrillar Contractility
(d) Other Theories ....

2. Division of the Chromosomes

47
53
57
57
60
61
62
67
70
70
70
75
77

xu

TABLE OF CONTENTS

E. Direct or Amitotic Division .....

1. General Sketcii ......

2. Centrosome and Attraction-spliere in Amitosis

3. Biological Significance of Amitosis

F. Summary and Conclusion

Literature, II. . . . . ....

PAGE
80
80
81
82

CHAPTER III
The Germ-Cells

A. The Ovum 90

1. The Nucleus 92

2. The Cytoplasm 94

3. The Egg-envelopes . . . . . . . . . . .96

B. The Spermatozoon ............ 98

1. The Flagellate Spermatozoon ......... 99

2. Other Forms of Spermatozoa ......... 106

3. Paternal Germ-cells of Plants ......... 106

C. Origin and Growth of the Germ-cells ......... 108

D. Growth and Differentiation of the Germ-cells . . . . ... • 113

1. The Ovum 113

{a) Growth and Nutrition 113

{J}) Differentiation of the Cytoplasm. Deposit of Deutoplasm . ■ "5

(c) Yolk-nucleus 118

2. Formation of the Spermatozoon ........ 122

E. Staining-reactions of the Germ-nuclei . . . . . . . . .127

Literature, III. . ............. 128

C.
D.
E.

F.

CHAPTER IV

Fertilization of the Ovum
General Sketch .....

1. The Germ-nuclei in Fertilization

2. The Centrosome in Fertilization
Union of the Germ-cells

1. Immediate Results of Union

2. Paths of the Germ-nuclei .

3. Union of the Germ-nuclei. The Chromosomes
Centrosome and Archoplasm in Fertilization
Fertilization in Plants . .....

Conjugation in Unicellular Forms

Summary and Conclusion .....

Literature, IV.

130
132
135
145
149

151
153
156
160
163
170
171

B.

CHAPTER V
Reduction of the Chromosomes, Oogenesis and Spermatogenesis

General Outline .......

1. Reduction in the Female. The Polar Bodies

2. Reduction in the Male. Spermatogenesis .

3. Theoretical Significance of Maturation
Origin of the Tetrads

1. General Sketch ......

2. Detailed Evidence .....
The Early History of the Germ-nuclei .
Reduction in the Plants .....

174
175
180
182
186
186
187
193
195

TABLE OF CONTENTS

XIU

E. Reduction in Unicellular Forms

F. Divergent Accounts of Reduction

I. Formation of Tetrads by Conjugation

G. Maturation of Parthenogenetic Eggs

H. Summary and Conclusion . . . .

Appendix .......

1. Accessory Cells of the Testis

2. Amitosis in the Early Sex-cells .
Literature, V. ..... .

PAGE
198
199
199
202
205
208
208
209
209

CHAPTER VI
Some Problems of Cell-Organization

A. The Nature of Cell-organs . . . . . . . . .' . .211

B. Structural Basis of the Cell ........... 212

I. Nucleus and Cytoplasm .......... 214

C. Morphological Composition of the Nucleus . . . . . . . -215

I. The Chromatin . . . . . . . . . . -215

(«) Hypothesis of the Individuality of the Chromosomes . . -215

(/;) Composition of the Chromosomes ....... 221

D. Chromatin, Linin, and the Cytoreticulum 223

E. The Centrosome ............. 224

F. The Archoplasmic Structures 229

1. Asters and Spindle 229

2. The Attraction-sphere .......... 232

G. Summary and Conclusion ........... 236

Literature, VI. .............. 237

CHAPTER VII

Some Aspects of Cell-Chemistry and Cell-Physiology

A. Chemical Relations of Nucleus and Cytoplasm

1. The Proteids and their Allies

2. The Nuclein Series ....

3. Staining-reactions of the Nuclein Series

B. Physiological Relations of Nucleus and Cytoplasm

1. Experiments on Unicellular Organisms

2. Position and Movements of the Nucleus

3. The Nucleus in Mitosis

4. The Nucleus in Fertilization

5. The Nucleus in Maturation

C. The Centrosome ....

D. Summary and Cbnclusion
Literature, VII. .....

CHAPTER VIII
Cell-Division and Development

A. Geometrical Relations of Cleavage -forms ....

B. Promorphological Relations of Cleavage ....

1. Promorphology of the Ovum .....

(«) Polarity and the Egg-axis ....
{b) Axial Relations of the Primary Cleavage-planes
{c) Other Promorphological Characters of the Ovum

2. Meaning of the Promorphology of the Ovum .

238

239
240
242
248
248
252
256
257
259
259
261
263

265
278
278
278
280
282
285

xiv TABLE OF CONTENTS

PAGE

C. The Energy of Division .... 289

D. Cell-division and Growthi .... 293

Literature, VIII 294

CHAPTER IX
Theories of Inheritance and Development

A.' The Theory of Germinal Localization ......... 296

B. The Idioplasm Theory 300

C. Union of the Two Theories . 302

D. The Roux-Weismann Theory of Development ....... 303

E. Critique of the Roux-Weismann Theory ........ 306

F. On the Nature and Causes of Differentiation . . . . . . • 311

G. The Nucleus in Later Development . ........ 321

H. The External Conditions of Development ........ 323

I. Development, Inheritance, and Metabolism ........ 326

J. Preformation and Epigenesis. The Unknown Factor in Development . . . 327

Literature, IX 330

Glossary 333

General Literature-List 343

Index of Authors 359

Index of Subjects 365

LIST OF FIGURES

PAGE

1. Epidermis of larval salamander 2

2. Amceba Proteus 4

3. Cleavage of the ovum in Toxopneustes 8

4. Diagram of inheritance 11

5. Diagram of a cell 14

6. Spermatogonium of salamander 15

7. Group of cells, showing cytoplasm, nu-

cleus, and centrosome 16

8. Alveolar or foam-structure of proto-

plasm, according to Biitschli 18

9. Living cells of salamander, showing

fibrillar structure. 20

10. Nuclei from the crypts of Lieberkiihn. 24

11. Special forms of nuclei 25

12. Diffused nucleus in Trachelocerca. . . . 26

13. Ciliated ceils 30

14. Nephridial cell of Clepsine 32

15. Nerve-cell of frog 33

16. Diagram of dividing cell 35

17. Diagrams of cell-polarity 39

18. Remak's scheme of cell-division 46

19. Diagram of the prophases of mitosis. . 48

20. Diagram of later phases of mitosis. .. . 50

21. Prophases in salamander cells 54

22. Metaphase and anaphases in salaman-

der cells 55

23. Telophases in salamander cells 56

24. Middle phases of mitosis in Ascaris-

eggs 58

25. Mitosis in pollen-mother-cells of lily. . 59

26. Heterotypical mitosis 60

27. Mitosis in Infusoria 62

28. Mitosis in Euglypha 63

29. Mitosis in Eiiglena . * 64

30. Mitosis in Noctiluca 65

31. Mitosis in ActlnosphcBrium 66

32. Pathological mitoses in cancer-cells... 68

33. Pathological mitosis caused by poisons 69

34. Mechanism of mitosis in .-^Jfarw 71

35. Leucocytes 72

36. Pigment-cells 73

37. Mitosis in the egg of Toxopneustes . ... 76

38. Nuclei in the spireme-stage 78

39. Early division of chromatin in Ascaris 79

40. Amitotic division 81

41. Volvox 89

42. Ovum of Toxopneustes 91

PAGE

Ovum of Nereis 95

Insect-egg 96

Micropyle in Argonauta 97

Germ-cells of Volvox 98

Diagram of the flagellate spermatozoon 99
Spermatozoa of fishes and amphibia. . 100
Spermatozoa of birds and other ani-
mals 102

Spermatozoa of mammals 104

Unusual forms of spermatozoa 105

Spermatozoids of Chara 106

Spermatozoids of various plants 107

Germ-cells of Hydractlnia 109

Primordial germ-cells of Ascaris no

Primordial germ-cells of Cyclops 112

Egg and nurse-cell in Ophryotrocha. . . 114

Ovarian eggs of insects 115

Young ovarian eggs of various animals 116
Young ovarian eggs of birds and mam-
mals 118

Young ovarian eggs of earthworm. . . . 120

Formation of the spermatozoon 124

Transformation of the spermatids of

the salamander 125

Fertilization of Physa 131

Fertilization of Ascaris 133

Germ-nuclei of nematodes 134

Fertilization of the mouse 136

Fertilization of Pterotrachea 137

Entrance and rotation of sperm-head

in Toxopneustes 138

Conjugation of the germ-nuclei in Tox-
opneustes 139

Fertilization of Nereis 141

Fertilization of Cyclops 142

Continuity of centrosomes in Thalas-

sema 144

Entrance of spermatozoon into the egg 146

Pathological polyspermy 147

Polar rings of Clepsine 150

Paths of the germ-nuclei in Toxo-
pneustes 152

Fertilization of Myzostonia 158

Fertilization of Pilularia 160

Fertilization of the lily 161

Diagram of conjugation in Infusoria. . 164
Conjugation of ParamcEcium 166

XVI

LIST OF FIGURES

PAGE

83. Conjugation of I'orticella 167

84. Conjugation of Noctiluca 168

85. Conjugation of Spirogyra 169

86. Polar bodies in Toxopneustes 174

87. Genesis of the egg 175

88. Diagram of formation of polar bodies 177
8g. Polar bodies in Astaris 178

90. Genesis of the spermatozoon 180

91. Diagram of reduction in the male.. . . 181

92. Spermatogenesis of Ascaris 184

93. Tetrads of Gryllotalpa 188

94. Tetrads and polar bodies in Cjij/o/j-.. 189

95. Diagrams of tetrad-formation in ar-

thropods 191

96. Germinal vesicles and tetrads 192

97. Ovary of Canthocamptus 194

98. Possible tetrad-formation in the lily. . 197

99. Conjugation and reduction in Closte-

rium 198

100. First type of parthenogenetic matura-

tion in Artemia 203

101. Second type of parthenogenetic mat-

uration in Artemia 204

102. Modes of tetrad-formation contrasted 206

103. Abnormalities in the fertilization of

Ascaris 216

104. Individuality of chromosomes in As-

caris 217

105. Independence of chromosomes in fer-

tilization of Cyclops 218

106. Hybrid fertilization of Ascaris 220

107. Mitosis with intra-nuclear centrosome

in Ascaris 225

108. Diagram of different types of centro-

some and centrosphere 233

109. Structure of the aster in spermatogo-

nium of salamander 234

no. History of chromosomes in the germi-
nal vesicle of sharlcs 245

III. Nucleated and enucleated fragments

of Stylonychia 249

P,\GE

112. Regeneration in Stefitor 250

113. Nucleated and enucleated fragments

of Amceba 251

114. Position of nuclei in plant-cells 253

115. Ovary of Foi-Jicula 255

116. Normal and dwarf larv« of sea-

urchins 258

117. Supernumerary centrosome in ^jiTfflrw 260

118. Cleavage of dispermic egg of Toxo-

pneustes 261

119. Geometrical relations of cleavage-

planes in plants 266

120. Cleavage of Syiiapta 268

121. Cleavage of Polygordius 269

122. Cleavage of Nereis 271

123. Variations in the third cleavage 272

124. Meroblastic cleavage in the squid. . . . 273

125. Teloblasts of the earthworm 274

126. Bilateral cleavage in tunicates 281

127. Bilateral cleavage in Loligo 282

128. Eggs of Loligo 283

129. Eggs and embryos of Corixa 284

130. Variations in axial relations of Cyclops 286

131. Half-embryos of the frog 299

132. Halfand whole cleavage in sea-urchins 306

133. Normal and dwarf gastrulas of Amphi-

oxus 307

134. Dwarf and double embryos of Ainphi-

oxus 308

135. Cleavage of sea-urchin eggs under

pressure 309

136. Cleavage of Nereis-&g^'s> under press-

ure 310

137. Diagrams of cleavage in annelids and

polyclades 313

138. Partial larvae of ctenophores 314

139. Partial cleavage in Ilyanassa 316

140. Double embryos of frog 318

141. Normal and modified larvae of sea-

urchins 324

142. Regeneration in coelenterates 325

INTRODUCTION

"yedes Thier erschehit ah eine Suni7ne vitaler Einheiten, 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 revealing their common plan of organization. It
was through the cell-theory that Kolliker and Remak opened the way
to an understanding of the nature of embryological development, and
the law of genetic continuity lying at the basis of inheritance. It
was the cell-theory again which, in the hands of 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-activi-
ties. And at a still later day it was through the cell-theory that Hert-
wig, Fol, Van Beneden, and Strasburger solved the long-standing
riddle of the fertilization of the &^z, and the mechanism of hereditary
transmission. No other biological generalization, save only the theory
of organic evolution, has brought so many apparently diverse phe-
nomena 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

^ Cellularpathologie, p. 12, 1858.

2 INTRODUCTION

of the Lamarckian period gave little heed to the finer details of
internal organization. They were concerned mainly with the more
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 afterwards it was, in the main, the study of like characters
with reference to their historical origin that led Darwin -to his splen-

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Fig. I. — A portion of the epidermis of a larval salamander {Amblystomd) 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 j; 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 i5, showing the chromosomes, and a final stage {telophase'), showing
fission of the cell-body, to the right.

did trium])hs. 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.

INTRODUCTION 3

indeed, has the ground been cleared for that close alliance of the
evolutionists and the cytologists which forms so striking a feature
of contemporary biology. We may best examine the steps by which
this alliance has been effected by an outline of the cell-theory, fol-
lowed by a brief statement of its historical connection with the evolu-
tion-theory.

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
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-
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 (Fig. i). 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. The microscope shows it to be an aggre-
gate composed of innumerable minute bodies, as if it were a colony
or congeries of organisms more elementary than itself. These elemen-
tary bodies, the cells, are essentially minute masses of living matter
or pj^o top lasrn, a substance characterized by Huxley many years ago
as the "physical basis of life" and now universally recognized as the
immediate substratum of all vital action. Endlessly diversified in the
details of their form and structure, cells nevertheless possess a charac-
teristic 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. In the lowest forms of life the
entire body consists of but a single cell (Fig. 2). In the higher multi-
cellular forms the body consists of a multitude of such cells asso-
ciated in 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. ^ From the physiological point of view a
like comparison may be drawn. In the one-celled forms all of the

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

4 INTRODUCTION

vital functions are performed by a single cell ; in the higher types they
are distributed by a physiological division of labour among different
groups of cells specially devoted to the performance of specific
functions. The cell is therefore not only a unit of structure, but
also a unit of function. " It is the cell to which the consideration
of every bodily function sooner or later drives us. In the muscle-
cell lies the riddle of the heart-beat, or of muscular contraction ; in the
gland-cell are 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 slumbering in the ganglion-cell. ... If
then physiology is not to rest content with the mere extension of our

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J.;*?;?/-?,.''-, k •■ ', '' -^/li" 0 ? « ■■'

^■;jV^L,-i> i'-'b"^. ^;'^^'f?o'>o>r- ,

:r^c<V%i^^?o,^;.-q|a:-;,?-'.'

''^M0p-^4$';SSl>y

■.:■-<?; j>cr,;f>»„o'ff;. ,;:.---. /■

mSSif

v;.;?v='loift^r::vi;r;i^

\:--^:^^^t'-^.:^^':-:

i Vv. r'o\^::l^to^.'/'']

/■;;..';'iy i'fe^s'^v''/

fv

■■^■•'•■/^::l;^-'"^■:

—J

cv

<ua^j

Fig. 2. — Ammba Proteus, an animal consisting of a single naked cell, X 280. (From Sedgwick
and Wilson's Biology.)

n. The nucleus ; w.v. Water-vacuoles ; c.v. Contractile vacuole ; f.v. Food-vacuole.

knowledge regarding the more obvious operations of the human
body, if it would seek a real explanation of the fundamental phe-
nomena of life, it can only attain its end through the study of cell-
physiologyT^

Great as was the impulse which the cell-theory gave to anatomical
and physiological investigation, it 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, if we except
the theory of pangenesis, did Darwin attempt at any later period to
bring it into any very definite relation to his views. The cell-theory
first came in contact with the evolution-theory nearly twenty years

^ Verworn, AUgenieiiie Physiologic, p. 53, 1895.

INTRODUCTION 5

later through researches on the early history of the germ-cells and the
fertilization of the ovum. Begun in 1873-74 by Auerbach, Fol, and
Butschli, and eagerly followed up 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. The identification of
the cell-mideiis as the vehicle of inheritance, made independently and
almost simultaneously in 1884-85 by Oscar Hertwig, Strasburger,
Kolliker, and Weismann, must be recognized as the first definite
advance ^ towards the internal problems of inheritance through the
cell-theory ; and the discussions to which it gave rise, in which Weis-
mann has taken the foremost place, must be reckoned as the most
interesting and significant of the post-Darwinian period.

These discussions have set forth in strong relief the truth that
the general problems of evolution and heredity are indissolubly
bound up with those of cell-structure and cell-action. This can best
be appreciated from an historical point of view. The views of the
early embryologists 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 egg," 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 Q.gg 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 ^gg, 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. To the modern student the germ is, in
Huxley's words, simply a detached living portion of the substance
of a pre-existing living body^ carrying with it a definite structural
organization characteristic of the species. Harvey's view is only
superficially similar to this; for, as Huxley pointed out, it was obscured
by his belief that the germ might arise "spontaneously," or through

1 It must not be forgotten that Haeckel expressed the same view in 1866 — -onh', how-
ever, as a speculation, since the data necessary to an inductive conclusion were not obtained
until long afterwards. "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. ATorph., p. 287-9).

"^ De Generatione, 165 1; Trans., p. 271.

^ Evohiiion in Biology, 1878; Science and Ctdiure, p. 291.

6 IXTRODUCTIOX

the influence of a mysterious " calidmn iiinatuvi," out of not-living
matter. Whitman, too, in a recent brilliant essay/ has shown how
far Har\-ev was from any real grasp of the law of genetic continuit}^
which is well characterized as the central fact of modem biolog\'.
Neither could the great physiologist of the seventeenth century have
had the remotest conception of the actual structure of the &gg. The
cellular structure of li\"ing 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 specu-
lation. For a hundred years after Harx^ey's time embrvologists
sought in vain to penetrate the mysteries enveloping the beginning
of the indi\"idual life, and despite their failure the controversial writ-
ings of this period form one of the most interesting chapters in the
history- of biolog}'. By the extreme "evolutionists" or "praeforma-
tionists" the Qgg was believed to contain an embr}o fully formed in
miniature, as the bud contains the flower or the chrysalis the butter-
flv. Development was to them merely the unfolding of that which
alreadv existed ; inheritance, the handing down from parent to child
of an infinitesimal reproduction of its own body. It was the senice
of Bonnet to push this conception to its logical consequence, the
theor}- of emboitemejit or encasement, and thus to demonstrate the
absurdit}- of its grosser forms ; for if the ^gg contains a complete
embr\-o, 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 Frederich Wolff (1759) led the way in a .return to the teach-
ings of Harvey, showing by precise actual observ^ation that the Q:gg
does not at first contain any formed embrvo whatever ; that the struct-
ure is wholly different from that of the adult ; that development is not
a mere process of unfolding, but a progressive process, involving the
continual formation, one after another, of new parts, pre\"iously non-
existent as such. This is somewhat as Harv^ey, himself following
Aristotle, had conceived it — a process of epige^iesis as opposed to
evolution. Later researches established this conclusion as the very
foundation of embr}-ological science.

But although the external nature of development was thus deter-
mined, the actual structure of the Q.gg and the mechanism of inheri-
tance remained for nearly a centur}^ in the dark. It was resented
for Schwann (^1839) and his immediate followers to recognize the
fact, conclusively demonstrated by all later researches, that tJie egg
is a cell having the same essential structure as other cells of the

^ Evolution and Epi^enens, Wood's Holl Biological Lectures, 1894.

INTRODUCTIOX 7

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 afterwards 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-
ment, and therefore regarded by the early obsen-ers 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. Xot
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 &^^, a single cell, peculiarly modified
in structure, it is true, and of extraordinaiy 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 ^^g is accomplished bv
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 plav, on
the whole, equal, though not identical parts m hereditarv trans-
mission. The ultimate problems of sex, fertilization, inheritance,
and development were thus shown to be cell-problems.

Meanwhile, during the years immediately follo^^Tng 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.-'
Schwann and Schleiden held that cells might arise in two different
ways ; viz. either by the division or fission of a pre-existing mother-
cell, or by " free cell-formation," new cells arising in the latter case
not from pre-existing cells, but by crvstallizing, as it were, out of
a formative or nutritive substance, termed the '' cytoblastema." It
was only after maiiy vears of painstaking research that "free cell-

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

8 INTRODUCTION

formation " was absolutely proved to be a myth, though many of
Schwann's immediate followers threw doubts upon it, and as early
as 1855 Virchow positively maintained the universality of cell-divis-
ion, contending that every cell is the offspring of a pre-existing
parent-cell, and summing up in the since famous aphorism, " omnis

B

Fig. 3. — Cleavage of the ovum of the sea-urchin Toxopneustes, X 330, from life. The suc-
cessive divisions up to the i6-cell stage (//) occupy about two hours. / is a section of the embryo
(blastula) of three hours, consisting of approximately 128 cells surrounding a central cavity or
blastocrel.

cellnla 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

1 Arch, fur Path. Anat., VIII., p. 23, 1855.

INTRODUCTION 9

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 Q.g^ into two parts, each
of which is a cell, like the Q.gg itself. The two then divide in turn to
form four, eight, sixteen, and so on in more or less regular progres-
sion (Fig. 3) 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 ^^^,
was observed long before its meaning was understood. It seems to
have been first definitely described in the case of the frog's Q^^g, 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. // is therefore derived by direct descent
from an egg-cell of the 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.

These facts clearly define the problems of heredity and variation
as they confront the investigator of the present day. All theories of
evolution take as fundamental postulates the facts of variation and
heredity ; 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
transformation of species. The first of these includes congenital or
inborn variations ; i.e. such as appear at birth or are developed
"spontaneously," without discoverable connection with the activities
of the organism itself or the direct effect of the environment upon it.
In a second class of variations are placed the so-called acquitted char-

1 See the quotation from the original edition of the Celliilarpatlwlogie at the head of
Chapter II., p. 45.

I O INTR OD UC TION

acters ; i.e. changes that arise in the course of the individual life as
the effect of use and disuse, or of food, climate, and the like. The
inheritajice of congenital 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 prob-
lem, 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 elab-
orated theory of pangenesis,^ Darwin framed a provisional physio-
logical hypothesis of inheritance in accordance 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 transmission both of acquired and of con-
genital variations, reviewing the facts of variation and inheritance
with wonderful skill, and building up a theory which, although it forms
the most speculative and hypothetical portion of his writings, must
always be reckoned one of his most interesting contributions to science.
The theory of pangenesis has been generally abandoned in spite
of the ingenious attempt to remodel it made by Brooks in 1883.^ In
the same year the whole aspect of the problem was changed, and a
new period of discussion inaugurated by Weismann, who put forth
a bold challenge of the entire Lamarckian principle.^ "I do not
propose to treat of the whole problem of heredity, but only of a
certain aspect of it, — the transmission of acquired characters, which
has been hitherto assumed to occur. In taking this course I may say
that it was impossible to avoid going back to the foundation of all
phenomena of heredity, and to determine the substance with which
they must be connected. In my opinion this can only be the sub-
stance of the germ-cells ; and this substance transfers 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 re-
quire thorough modification, for the whole principle of evolution by
means of exercise (use and disuse) as professed by Lamarck, and
accepted in some cases bv Darwin, entirely collapses " {I.e., p. 69).

1 Variation of Animals and Plants., Chapter XXVII.
^ The Law of Heredity, Baltimore, 1883.

3 Uebcr Vererbicng, 1S83. See Essays upon Heredity, I., by A. Weismann, Clarendon
Press, Oxford, 1889.

INTRODUCTION

I I

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 produce
a corresponding development in the hand of the child ? It is a physi-
ological impossibility. If we turn to the facts, we find, Weismann
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 geTin-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 pre-existing germ-cell of the same kind. Thus
the body is, as it were, an offshoot from the germ-cell (Fig. 4). As

■ o Line of succession.

(T) Line of inheritance.

G

Fig. 4. — Diagram illustrating Weismann's tlieory of inheritance.
G. The germ-cell, which by division gives rise to the body or soma (5) and to new germ-cells
(G) which separate from the soma and repeat the process in each successive generation.

far as inheritance is concerned, the body is merely the carrier of the
germ-cells, which are held in trust for coming generations.

Weismann's subsequent theories, built on this foundation, have
given rise to the most eagerly contested controversies of the post-
Darwinian period, and, whether they are to stand or fall, have played
a most important part in the progress of science. For aside 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
evolution-theory into organic connection. It is from this point of
view that the present volume has been written. It has been my
endeavour to treat the cell primarily as the organ of inheritance and
development; but, obviously, this aspect of the cell can only be
apprehended through a study of the general phenomena of cell-life.
The order of treatment, which is a convenient rather than a strictly
logical one, is as follows : —

The opening chapter is devoted to a general sketch of cell-struct-

12 INTRODUCTION

ure, and the second to the phenomena of cell-division. The follow-
ing three chapters deal with the germ-cells, — the third with their
structure and mode of origin, the fourth with their union in fertiliza-
tion, the fifth with the phenomena of maturation by which they are
prepared for their union. The sixth chapter contains a critical dis-
cussion 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 suc-
ceeding chapter approaches the objective point of the book by con-
sidering 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

Bergh, R. S. — Vorlesungen liber die Zelle unci die einfachen Gewebe : Wiesbaden,

1894.
Delage, Yves. — La Structure du Protoplasma et les Theories sur Pheredite et les

grands Problemes de la Biologie GeneraJe : Paris, 1895-
Geddes & Thompson. — The Evolution of Sex : New York, 1890.
Henneguy, L. F. — Legons sur la cellule : Paris, 1896.

Hertwig, 0. — Die Zelle und die Gewebe: Fischer, Jena, 1892. Translation, pub-
lished by Macinillan, London and New York, 1 895 .
Huxley, T. H. — Review of the Cell-theory : British and Foreign Medico-Chirurgical

Review, XIL 1853.
Minot, C. S. — Human Embryology: New York, 1892.
Remak, R. — Untersuchungen iiber die Entwicklung der Wirbelthiere : Berlin,

1850-55.
Schleiden, M. J. — Beitrage zur Phytogenesis : Muller^s Archiv, 1838. Translation

in Sydenham Soc, XIL London, 1847.
Schwann, Th. — Mikroscopische Untersuchungen liber die Uebereinstimmung in

der Structur und dem Wachsthum der Thiere und Pflanzen : Berlin, 1839.

Translation 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.^

The term "cell " is a biological misnomer; for whatever the living
cell is, it is not, as the word implies, a hollow chamber surrounded by
solid walls. The term is merely an historical survival of a word
casually employed by the botanists of the seventeenth century to
designate the cells of certain plant-tissues which, when viewed in
section, give somewhat the appearance of a honeycomb. ^ The cells
of these tissues are, in fact, separated by conspicuous solid walls
which were mistaken by Schleiden, unfortunately followed by
Schwann in this regard, for their essential part. The living sub-
stance contained within the walls, to which Hugo von Mohl gave
the nz-vcio, protoplasm^ (1846) was at first overlooked or was regarded
as a waste-product, a view based upon the fact that in many im-
portant plant-tissues such as cork or wood it may wholly disappear,
leaving only the lifeless walls. The researches of Bergmann,
Kolliker, Bischoff, Cohn, Max Schultze, and many others, showed,
however, that some kinds of cells, for example, the corpuscles of
the blood, are naked masses of living protoplasm not surrounded by
walls, — a fact which proves that not the wall, but the cell-contents,
is the essential part, and must therefore be the seat of life. It was
found further that with the possible exception of some of the lowest
forms of life, such as the bacteria, the protoplasm invariably contains
a definite rounded body, the mtcleits^^ which in turn may contain a still

^ Untersuchungen, 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.

^ The same word had been used by Purkyne some years before (1S40) to designate the
formative material of young animal embryos.

* First described by Robert Brown in 1833.

13

H

GENERAL SKETCH OE THE CELL

smaller body, the nucleolus. Thus the cell came to be defined by
Max Schultze and Leydig as a mass of protoplasm containing
a nucleus, a morphological definition which remains sufficiently satis-
factory even at the present day. 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.^

Attraction-sphere enclosing two centrosomes.

Plasmosome or
true nucleolus.

Chromatin-

network.

Linin-network. ,■

Karyosome or
net-knot.

Plastids lying in the
cytoplasm.

Lifeless bodies (meta-
plasm) suspended in
the cytoplasmic reticu-
lum.

Fig. 5. — Diagram of a cell. Its basis consists of a thread-work (mitome, or reticuluiii) com-
posed of minute granules (jnicrosomes) and traversing a transparent ground-substance.

A. General Morphology of the Cell

The cell is a rounded mass of protoplasm which in its simplest
form is approximately spherical. This form is, however, seldom
realized save in isolated cells such as the unicellular plants and
animals 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. The

1 Sachs has proposed the convenient word energid {Flora, '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 £ilso Flora, '95, p. 405.)

GENERAL MORPHOLOGY OF THE CELL

15

protoplasm which forms its living basis is a viscid, translucent,
granular substance, often forming a network or spo;ige-like structure
extending through the cell-body and showing various structural
modifications in different regions and under different physiological
states of the cell. Besides the living protoplasm the cell almost
invariably contains various lifeless bodies suspended in the meshes
of the network; examples of these are food-granules, pigment-bodies,
drops of oil or water, and excretory matters. These bodies play a
purely 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 protoplasm as waste
matters, or in order to play some role subsidiary to the actions of
the protoplasm itself. The lifeless inclusions in the protoplasm have
been collectively designated as inetaplasui (Hanstein) in contradis-
tinction to the living protoplasju ; but .^
this convenient term is not in general
use. Among the lifeless products of
the protoplasm must be reckoned
also the cell-zvall or menibnine by
which the cell-body may be sur-
rounded ; but it must be remembered
that the cell-wall in many cases arises
by a direct transformation of the
protoplasmic substance, and that it
often retains the power of growth by
intussusception like living matter.

In all save a few of the lowest and
simplest forms, perhaps even in them,
the protoplasmic substance is differ-
entiated into two very distinct parts,
viz., the cell-body, forming the princi-
pal mass of the cell, and a smaller
body, the micleus, which lies in its
interior (Fig. 5). Both structurally
and chemically tliese two parts show
differences of so marked and constant
a character that they must be re-
garded as the most important of all
protoplasmic differentiations. The
nuclear substance is therefore often designated as niLcleoplasm or
karyoplasin ; that of the cell-body as cytoplasm (Strasburger). Some
of the foremost authorities, however, among them Oscar Hertwig, re-
ject this terminology and use the word " protoplasm" in its historic
sense, applying it solely to the cytoplasm or substance of the cell-body.

Fig. 6. — A resting cell {spermatogo-
nium) from the testis of the salamander,
showing the typical parts. Above, the large
nucleus,, with scattered masses of chro-
matin, linin-network and membrane.
Around it, the cytoplasmic thread-work.
Below, the attraction-sphere (a) and cen-
trosome {c). [After Rawitz.]

i6

GENERAL SKETCH OF THE CELL

At a first examination the nucleus appears to be a perfectly dis-
tinct body suspended in the cytoplasm. Most of the latest researches
point, however, to the conclusion that nucleus and cytoplasm are
pervaded by a common structural basis, morphologically continuous

A

B

C D

Fig- 7- — Various cells showing the 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 smgle plasmosome. [HERMANN.]

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

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

under certain conditions from one to the other, and that both are to
be regarded as specially differentiated areas in that basis. ^ The terms

^ The fact that the nucleus may move actively through the cytoplasm, as occurs during
the fertilization of the egg and in some other cases, seems to show that the morphological
continuity may at times be interrupted.

STRUCTURAL BASIS OF PROTOPLASM \J

" nucleus " and " cell-body " are therefore only topographical expres-
sions, and in a measure the same is true of the terms "karyoplasm " and
" cytoplasm." The latter, however, acquire a special significance from
the fact that there is on the whole a definite chemical contrast be-
tween the nuclear substance and that of the cell-body, the former
being characterized by the abundance of a substance rich in phos-
phorus known as imclein, while the latter contains no true nuclein and
is especially rich in proteids and related substances (nucleo-albumins,
albumins, globulins, and others), which contain a much lower per-
centage of phosphorus.

The differentiation of the protoplasmic substance into nucleus and
cytoplasm is a fundamental character of the cell, both in a morpho-
logical and in a physiological sense ; and, as will appear hereafter,
there is reason to believe that it is in a measure the expression of
a corresponding localization of the operations of constructive and
destructive metabolism which lie at the basis of the individual cell-
life. A third element, the centrosome (Figs. 5-7), present in many
if not in all cells, is especially concerned with the process of division
and cell-reproduction. Recent research has rendered it probable that
in point of morphological persistency the centrosome is comparable
with the nucleus ; but this conclusion is not yet definitely established.

B. Structural Basis of Protoplasm

As ordinarily seen under moderate powers of the microscope proto-
plasm shows no definite structural organization. A more precise ex-
amination under high powers, especially after treatment with suitable
fixing and staining reagents, reveals the fact that both nucleus and
cytoplasm possess a complicated structure. Regarding the pre-
cise nature of this structure opinion still differs. According to the
view most widely held, one of its essential features is the presence
of two constituents, one of which, the groiind-sitbstance, cyto-
lymph, or encJiyleina, is more liquid, while the other, the spongio-
plasin or rcticubmi, is of firmer consistency, and forms a sponge-like
network or alveolar structure extending everywhere through the more
liquid portion. At the present time it seems probable that the
more solid portion is the more active and is perhaps to be identified
as the living substance proper, the ground-substance being passive ;
but the reverse of this view is maintained by Leydig, Schafer, and
some others. The most elaborate and painstaking investigation has
moreover failed to determine with absolute certainty even the physi-
cal configuration of the network.

Butschli and a considerable school of followers among both

GENERAL SKETCH OF THE CELL

zoologists and botanists regard protoplasm as essentially a liquid, or
rather a mixture of liquids, which forms a foam-like alveolar structure^
like an emulsion, in which the firmer portion forms the walls of sepa-
rate chambers, filled with the more liquid substance (Fig. 8). By

ff'

■4 V / *r*^

■-i

^

Fig. 8. — Alveolar or foam-structure of protoplasm, according to Biitschli. [BiJTSCHLl.]
A. Epidermal cell of the earthworm. B. Aster, attraction-sphere, and centrosome from sea-
urchin egg. C. Intra-capsular protoplasm of a radiolarian {T/ialassicolla) with vacuoles.
D. Peripheral cytoplasm of sea-urchin egg. E. Artificial emulsion of olive-oil, sodium chloride,
and water.

special local modifications of this structure all the parts of the cell are
formed. Biitschli has shown that artificial emulsions, variously pre-
pared, may show under the microscope a marvellously close resem-

^ " IVahenstriiktiir. "

STRUCTURAL BASIS OF PROTOPLASM IQ

blance to actual protoplasm, and that drops of oil-emulsions suspended
in water may even exhibit amoeboid changes of form.

Opposed to Biitschli's conception is the view, first clearly set forth
by Frommann and Arnold ('65—67), and now maintained by such
authorities as Flernming, Van Beneden, Strasburger, and perhaps the
greater number of contemporary investigators, that the more solid
portion consists of coherent threads which extend through the ground-
substance, either separately or connected by branches to form a mesh-
work like the fibres of a sponge (Figs. 7, 9).

In the present state of the subject it is difficult, indeed, impossible,
to decide which of these opposing views should be accepted ; for the
evidence is very strong that each expresses a part of the truth. It is
generally admitted that such an alveolar structure as Biitschli de-
scribes is characteristic of many unicellular forms, and occurs in
many higher forms where the cell-substance is filled with vacuoles or
with solid inclusions such as starch-grains or deutoplasm-spheres.
In the latter case the structure has been termed "pseudo-alveolar"
(Reinke); but it remains to be seen whether there is any real dis-
tinction between this and the true alveolar structure described by
Biitschli. On the other hand the evidence of true fibrillar or reticular
structure in many tissue-cells, especially during cell-division, is very
convincing; and my own observations have led me to regard this
structure as the more typical and characteristic. For descriptive pur-
poses I shall accordingly adopt the terms of the fibrillar or reticular
hypothesis, designating the more solid portion of protoplasm as the
thread-work or reticuhmi ("Geriistwerk," " Fadenwerk " of German
writers) in contradistinction to the more liquid ground'Snbstance. It
should be clearly understood, however, that these terms are used only
as a matter of convenience, and are not meant to exclude the possibility
that the "fibres" or the "reticulum" may in many cases be open to
Biitschli's interpretation.

From a theoretical point of view the finer structure of the network
is a question of very great interest and importance. The earlier
investigators, such as Virchow and Max Schultze, failed to observe
the thread-work, and described protoplasm as consisting of a clear
homogeneous basis in which were embedded numerous granules.
Even at the present time a similar view is held by a few investi-
gators, more especially among botanists {e.g., Berthold, Schwarz),
who regard the thread-work either as an artificial effect produced
by reagents, or, if normal, as an inconstant and hence unimportant
feature. The best and most careful recent studies on proto-
plasm have, however, yielded very convincing evidence that, what-
ever be the precise configuration of the protoplasmic reticulum,
it is not only a normal structure, but one of very wide occurrence.

20

GENERAL SKETCH OF THE CELL

Si /

)r

. S

/ 7

y r'

/

V ____^^

2-^-^

f .

A

B

\

D

E

F

Fig. 9. — Living cells of salamander-larva. [Flemming.]
A. Group of epidermal cells at different foci, showing protoplasmic bridges, nuclei, and cyto-
plasmic fibrillEe; the central cell with nucleus in the spireme-sfage. 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 fibrillas (the latter somewhat exaggerated
in the plate).

STRUCTURAL BASIS OF PROTOPLASM 21

These studies have raised interesting problems regarding the signifi-
cance of the granules described by the early observers. Many of the
granules, especially the larger and more obvious of them, are unques-
tionably inert bodies, such as reserve food-matters, suspended in the
meshwork. Others are nodes of the network or optical sections of
the threads. But there is some reason to believe that, apart from
these appearances, discrete living particles may form a constant
and essential structural feature of the protoplasmic thread. These
particles, now generally known as microsomes} are embedded in the
threads of the network, and are sometimes so closely and regularly set
as irresistibly to suggest the view that they are definite structural ele-
ments out of which the thread is built. More than this, their behaviour
is in some cases such as to have led to the hypothesis long since
suggested by Henle ('41) and at a later period developed by Bechamp
and Estor, by Maggi and especially by Altmann, that microsomes are
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.
And thus the theory of genetic continuity expressed by Redi in the
aphorism " ojJtJie vivum ex vivo,'" reduced by Virchow to '' 07nnis
celliUa e celhila," finally appears in the writings of Altmann as " oin7ie
gramdmn e gramdol " ^

Altmann's premature generalization rests upon a very insecure
foundation and has been received with just scepticism. That the cell
consists of more elementary units of organization is nevertheless in-
dicated hy a priori evidence so cogent as to have driven many of the
foremost leaders of biological thought into the belief that such units
must exist, whether or not the microscope reveals them to view.
Among those who have accepted this conception in one form or
another are numbered such men as Spencer, Darwin, Beale, Haeckel,
Michael Foster, Nageli, De Vries, Wiesner, Roux, Weismann, Oscar
Hertwig, Verworn, and Whitman. The modern conception of ultra-
cellular units, ranking between the molecule and the cell, was first
definitely suggested by Briicke ('61),^ only, however, to be rejected
as without the- support of facts, though this eminent physiologist
insisted that the cell must possess a more complicated organization
than that revealed by the best microscopes of his time. It was soon
afterwards taken up by Herbert Spencer, and elaborated into the
theory of physiological units by which he endeavoured to explain
the phenomena of regeneration, development, and heredity. Darwin

1 Hanstein ('82).

2 Die F.lementa7-organismen, Leipsic, 1894, p. 155.

^ For a review of speculations in the same direction by Buffon and other early writers see
Yves Delage ('95).

22 GENERAL SKETCH OE THE CELL

too, in his celebrated hypothesis of pangenesis, adopted a nearly
related conception, which as remodelled by De Vries twenty years
afterwards ('89) forms the basis of the theories of development
maintained by such leaders of biological research as Weismann and
Hertwig. The same view appears in a different form in the writ-
ings of Nageli, Wiesner, Foster, Verworn, and many other morpholo-
gists and physiologists. ^

An hypothesis backed by such authority and based on evidence
drawn from sources so diverse cannot be lightly rejected. We are
compelled by the most stringent evidence to admit that the ultimate
basis of living matter is not a single chemical substance, but a mixture
of many substances that arc self-perpetnating without loss of their
specific character. The open question is whether these substances
are localized in discrete morphological bodies aggregated to form the
cell somewhat as cells are aggregated to form tissues and organs,
and whether such bodies, if they exist, lie within the reach of the
microscope. Altmann's identification of the "granulum" as such a
body is undoubtedly premature ; it is certain that his description of
cell-structures from this point of view is often very inaccurate ; it
is extremely doubtful how far the granules or microsomes are normal
structures, and how far they are artefacts produced by the coagulat-
ing effect of the reagents. It is nevertheless certain, as will be
shown in Chapter VI., that at least one part of the cell, namely the
nucleus, actually consists of self-propagating units of a lower order
than itself, and there is some ground for regarding the cyto-micro-
somes in the same lisht.

C. The Nucleus

A fragment of a cell deprived of its nucleus may live for a con-
siderable time and manifest the power of co-ordinated movement
without perceptible impairment. Such a mass of protoplasm is, how-
ever, devoid of the powers of assimilation, growth, and repair, and
sooner or later dies. In other words, those functions that involve
destructive metabolism may continue for a time in the absence of
the nucleus ; those that involve constructive metabolism cease with
its removal. There is, therefore, strong reason to believe that the
nucleus plays an essential part in the constructive metabolism of the

1 The following list includes only some of the various names that have been given to
these hypothetical units by modern writers: Physiological units (Spencer); g-emmiiles
(Darwin); pangens (De Vries); plasonies (Wiesner); micella: (Nageli); plastidules
(Haeckel and Elssberg) ; inotaginata (Engelmann); biophores (Weismann); hioblasts
(Beale); soviacules (Foster); idioblasts (Hertwig); idiosoines (Whitman); hiogens (Ver-
worn); 7nicrozymas (Bechamp and Estor) ; gemm^ (Haacke).

THE NUCLEUS 23

eel], and through this is especially concerned with the formative proc-
esses involved in growth and development. For these and many
other reasons, to be discussed hereafter, the nucleus is generally re-
garded as a controlling centre of cell-activity, and hence a primary
factor in growth, development, and the transmission of specific quali-
ties 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 con-
dition, 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
conspicuous irregular network (Figs. 5, 7, 10). Its form, though
subject to variation, is on the whole singularly constant, and shows
no definite relation to that of the cell in which it lies. Typically
spherical, it may, in certain cases, assume an irregular or amoeboid
form, may break up into a group of more or less completely sepa-
rated lobes (polymorphic nuclei), or may be perforated to form an
irregular ring (Fig. 11, 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. 11, E). Interesting modi-
fications of the nucleus occur in the unicellular forms. In the
ciliate Infusoria the body contains nuclei of two kinds, viz. a large
macroniicleiis and one or more smaller micronuclei. The first of
these shows a remarkable diversity of structure in different forms,
being often greatly elongated and sometimes showing a moniliform
structure like a string of beads. In TracJielocerca and some other
Infusoria, according to Gruber ('84), the nucleus is not a single definite
body, but is represented by minute granules scattered throughout the
eell-substance (Fig. 12) ; Biitschli describes somewhat similar diffused
nuclei in some of the Flagellates, and in the Bacteria.

In the ordinary forms of nuclei in their resting state the following-
structural elements may as a rule be distinguished (Figs. 5,6, 7, 10, 11): —

a. The nuclear membrane, a well-defined delicate wall which gives
the nucleus a sharp contour and differentiates it clearly from the
surrounding cytoplasm.

b. The nuclear reticjilnm. This, the most essential part of the
nucleus, forms an irregular branching network or reticiUmn which
consists of two very different constituents. The first of these, the

24

GENERAL SKETCH OF THE CELL

nuclear substance par excellence, is known as chromatin (Flemming)
on account of its very marked staining capacity when treated with
various dyes. In some cases the chromatin forms a nearly continu-
ous network, but it often appears in the form of more or less detached
rounded granules or irregular bodies. The second constituent is a
transparent substance, invisible until after treatment by reagents,

known as li)iin (Schwarz). This
substance, which is probably of
the same nature as the cyto-
plasmic network outside the
nucleus, surrounds and supports
the chromatin, and thus forms
the basis of the nuclear net-
work.

c. The nucleoli, one or more
larger rounded or irregular
bodies, suspended in the net-
work, and staining intensely
with many dyes ; they may be
absent. The bodies known by
this name are of at least two
different kinds. The first of
these, the so-called true nucleoli
or plasmosomes (Figs. 5, 7, B,
10), are of spherical form, and
by treatment with differential
stains such as haematoxylin and
eosin are found to consist typi-
cally of a central mass staining
like the cytoplasm, surrounded
by a shell which stains like
chromatin. Those of the other
form, the "net-knots" (Netz-
knoten), or karyosovies, are either
spherical or irregular in form,
stain like the chromatin, and
appear to be no more than thickened portions of the chromatic
network (Figs. 5, 7, A, 10). Besides the nucleoli the nucleus may
in exceptional cases contain the centrosome (p. 225), which has
undoubtedly been confounded in some instances with a true nucleolus
or plasmosome.^ There is strong evidence that the true nucleoli are

1 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 afterwards
studied more closely the difference of staining-reaction in plant-cells, showing that the

Fig. 10. — Two nuclei from the crypts of
Lieberkiihn in the salamander. [Heidenhain.]

Tlie character of the chromatin-network
{basichromatin) is accurately shown. The iipper
nucleus contains three plasmosomes or true
nucleoli ; the lower, one. A few fine linin-threads
{oxychrojnatin) axe seen in the upper nucleus
running off from the chromatin-masses. The
clear spaces are occupied by the ground-sub-
stance.

THE NUCLEUS

25

relatively passive bodies that represent accumulations of reserve-
substance or by-products, and play no direct part in the nuclear
activity (p. 93).

d. The gj^oimd-siib stance , nuclear sap, or karyolyjnph, a clear sub-

C £

Fig. II. — Special forms of nuclei.

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

B. Permanent spire'ine-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 butterfly larva {Pieris). [KORSCHELT.]

stance occupying the interspaces of the network and left unstained
by many dyes which colour the chromatin intensely. Until recently

former are especially coloured by alkaline carmine solutions, the latter by acid solutions.
Still later studies by Zacharias, and especially by Heidenhain, show that the medullary
substance (pyrenin) of true nuclei is coloured by acid -anilines and other plasma stains,
while the chromatin has a special affinity for basic anilines. Cf. p. 242.

26

GENERAL SKETCH OF THE CELL

the ground-substance has been regarded as a fluid or semi-fluid, but
recent researches by Reinke and others have thrown doubt on this
view, as described at p. 28.

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.
10) ; 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 net-
work, but appears in the form of a
thread closely similar to the spireme-
stage of dividing nuclei (cf. p. 47).
The most striking case of this kind
occurs in the salivary glands of dip-
terous larvae {C/m'OJiomns), where, as
described by Balbiani, the chromatin
has the form of a single convoluted
thread, composed of transverse discs
and terminating at each end in a
large nucleolus (Fig. 11, A). Some-
what similar nuclei (Fig. 11, B) occur
in various glandular cells of other
insects (Van Gehuchten, Gilson), and
also in the young ovarian eggs of cer-
tain animals (cf. p. 193). In certain
gland-cells of the marine isopod Ani-
locj-a it is arranged in regular rosettes
(Vom Rath). Rabl, followed by Van
Gehuchten, Heidenhain, and others,
has endeavoured to show that the
nuclear network shows a distinct
polarity, the nucleus having a "pole"
towards 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

Fig. 12. — An infusorian, Trachelo-
cerca, with diffused nucleus consisting of
scattered chromatin-granules. [Gruber.]

Cf. the polarity of the cell, p. 38.

THE NUCLEUS 27

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.^

A description of the nucleus during division is deferred to the fol-
lowing chapter.

2. Fijier Structure of the Nucleus

Many recent researches indicate that some at least of the nuclear
structures are 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 revolves itself into a definite number of rod-shaped
bodies known as chromosomes (Fig. i6), which split lengthwise as
the cell divides. These bodies arise as aggregations of minute
rounded bodies or microsomes to which various names have been
given {chromo7neres, Fol ; ids, Weismann). They are as a rule
most clearly visible and most regularly arranged during cell-division,
when the chromatin is arranged in a thread {spireme), or in separate
chromosomes (Figs. 7, D, 3S, B) ; but in many cases they are dis-
tinctly visible in the reticulum of the "resting" nucleus (Fig. 39).
It is, however, an open question whether the chromatin-granules
of the reticulum are individually identical with those forming the
chromosomes or the spireme-thread. The larger masses of the
reticulum undoubtedly represent aggregations of such granules, but
whether the latter completely fuse or remain always distinct is
unknown. Even the chromosomes may appear perfectly homogene-
ous, and the same is sometimes true of the entire nucleus, as in the
spermatozoon. The opinion is nevertheless gaining ground that the
chromatin-granules have a persistent identity and are to be regarded
as morphological 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 oxychroinatin-gxd.x\.\:\&^ from the basicJiromatin-gxzx\yA^'s>
of the chromatic network. Like the latter, the oxychromatin-granules
are suspended in a non-staining clear substance, for which he reserves

1 See Chapter VII. - Cf. Chapter VI.

28 GENERAL SKETCH OF THE CELL

the term "linin." 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 anilines (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 anilines (rubin, eosin, etc.) and other "plasma
stains." This distinction, as will appear in Chapter VII., is probably
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 reticulum, and is part of
the same structure as the linin-network and the cyto-reticulum. Like
these, it is in some cases " achromatic," but in other cases it shows
the same staining reactions as chromatin, or may be double, con-
sisting of an outer achromatic and an inner chromatic layer. Ac-
cording to Reinke, it consists of oxychromatin-granules like those of
the linin-network.

3. die mis try of tJie Nucleus

The chemical nature of the various nuclear elements will be considered in
Chapter VII., and a brief statement will here suffice. The following classification
of the nuclear substances, proposed by Schwarz in 1887. has been widely accepted,
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. Liniii. The achromatic network and the spindle-fibres arising from it.

THE CYTOPLASM 29

3. Faralinhi . The ground-substance.

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

5. Aviphipyre7iin. The substance of the nuclear membrane.

Chromatin is probably identical with luidein (p. 240), which is a compound of
nucleic acid (a complex organic acid, rich in phosphorus) and albumin. In certain
cases (nuclei of spermatozoa, and probably also the chromosomes at the time of
mitosis), chromatin may be composed of nearly pure nucleic acid. The linin is
probably composed of "plastin," a substance similar to nuclein, but containing a
lower percentage of phosphorus, and either belonging to the nucleo-proteids or
approaching them. It is nearly related with the substance of the cyto-reticulum.
Pyrenin consists of a plastin-substance which stains like linin. Antphipyrenin is
probably identical with linin, since the nuclear membrane is probably a condensed
portion of the general reticulum which forms the boundary between the intra- and
extra- nuclear networks. It should be borne in mind, however, that the membrane
often has an inner chromatic layer composed of chromatin.

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 endoplasni 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 differ-
entiation 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
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 rea^son to conclude in Chapter VII., 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.

1 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 Haiitschicht. The
medullary zone is termed by Kupffer, protoplasm, sensu strictu ; by Strasburger A'oVwdT-
plasma, by Nageli polioplasm.

2 Cf. p. 38.

3 See Kupffer C'90), pp. 473-476.

30

GENERAL SKETCH OF THE CELL

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

A

IHiiiBi

B

C

D

Fig. 13. — Ciliated cells, showing cytoplasmic fibrillag terminating in a zone of peripheral
microsomes to which the cilia are attached. [Engelmann.]

A. From intestinal epithelium of .4?/ofi?(7«/a. B. Yxova g\\\ ol Ajwdonta. CD. Intestinal epi-
thelium of Cyclas.

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, a secondary differentiation of the cytoplasmic substance
specially developed for the performance of particular functions. It
has been shown, moreover, that structureless proteids, such as egg-

THE CYTOPLASM 3 I

albumin and other substances, when coagulated by various reagents,
often show a structure closely similar to that of protoplasm as ob-
served in microscopical sections. Biitschli has made careful studies
of such coagulation-phenomena which show that coagulated or dried
albumin, starch-solutions, gelatin, gum arable, and other substances
show a fine aveolar structure scarcely to be distinguished from that
which he believes to be the normal and typical structure of pro-
toplasm. Fischer ('94, '95) has made still more extensive tests of
solutions of albumin, peptone, and related substances, in various
degrees of concentration, fixed and stained by a great variety of the
reagents ordinarily used for the demonstration of cell-structures. The
result was to produce a marvellously close siinidacriivi of the appear-
ances observed in the cell, reticulated and fibrillar structures being-
produced that often consist of rows of granules closely similar in
every respect to those described by Altmann and other students of
the cell. After impregnating pith with peptone-solution and then
hardening, sectioning, and staining, 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-
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
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 struct-
ures 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 fibrillae in ciliated cells (Fig. 13, Engelmann), in muscle-fibres
and nerve fibres, and especially in the mitotic figure of dividing-cells
(Figs. 16, 24), where they are likewise more or less clearly visible
in life. A very convincing case is afforded by the pancreas-cells
of Nectitnts, 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 clearly in a spiral mass to form the so-called Nebenkern. In all
these cases it is impossible to regard the thread- work as an accidental

32

GENERAL SKETCH OF THE CELL

coagulation-product. On the whole, therefore, it is probable that
careful treatment by reagents gives at least an approximately true
picture of the normal thread-work, though we must always allow for
the possible occurrence of artificial products.

Fig. 14. — 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 chromatui-
granules, and a single large nucleolus within which is a vacuole. Above are two isolated nuclei
showing nucleoli and chromatin-granules suspended on the linin-threads.

One of the most beautiful forms of cyto-reticulum with which I
am acquainted has been described by Bolsius and Graf in the ne-

THE CYTOPLASM

33

phridial cells of leeches as shown in Fig. 14 (from a preparation by
Dr. Arnold Graf). The reticulum is here of great distinctness and
regularity, and scattered microsomes are found along its threads. It
appears with equal clearness, though in a somewhat different form,

Fig. 15. — Spinal ganglion-cell of the frog. [VON Lenhossek.]
The nucleus contains a single intensely chromatic nucleolus, and a paler linin-netvvork with
rounded chromatin-granules. The cytoplasmic fibrillee 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. 7, C.) At the left, two
connective tissue-cells.

in many eggs, where the meshes are rounded and often contain food-
matters or deutoplasm in the inter-spaces (Figs. 42, 43). In cartilage-
cells and connective tissue-cells, where the threads can be plainly seen
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-

34 GENERAL SKETCH OF THE CELL

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
librillse corresponding in number with the cilia as if continuous with
their bases (Fig. 13).-^ In nerve-iibres the threads form closely set
parallel fibrillas 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. 15). 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, however, is the thread-work shown with such beauty
as in dividing-cells, where (Figs. 16, 24) the fibrillas 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. 35) and pigment-
cells (Fig. 36), 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. 18, differ considerably from the foregoing, the fibrillas
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, interprets 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. 8, E\ Strong evidence against this view
is, I believe, afforded by the appearance of the spindle and asters
in cross-section. In the early stages of the egg of Nereis, for
example, the astral rays are coarse anastomosing fibres that stain
intensely and are therefore very favourable for observation (Fig. 43).
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

1 The structure of the ciliated cell, as described by Engelmann, may be beautifully
demonstrated in the funnel-cells of the nephridia and sperm-ducts of the earthworm.

THE CYTOPLASM

35

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 towards the centrosphere. Driiner, 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. 22, F). Again, the crossing of the rays pro-
ceeding from the asters (Fig. 69), 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 network varies greatly in

Centrosphere con-
taining the cen-
trosome.

Spindle.

Chromosomes forming the equatorial plate.

Fig. 16. — Diagram of the dividing cell, showing the mitotic figure and its relation to the cyto-
reticulum.

different 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 universal application. It is possible, nay probable, that
in one and the same cell a portion of the network 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 organ-
ization must be sought in a more subtle underlying structure.^

^ See Chapter VI.

2,6 GENERAL SKETCH OF THE CELL

E. The Centrosome

No element of the cell has aroused a wider interest of late than
the remarkable body known as the centrosome^ which is now gener-
ally regarded as the especial organ of cell-division, and in this sense
as the dynamic centre of the cell (Van Beneden, Boveri).^ In its
simplest form the centrosome is a body of extreme minuteness, often
indeed scarce larger than a microsome, which nevertheless exerts
an extraordinary influence on the cytoplasmic network during cell-
division and the fertilization of the Q-^^. As a rule it lies out-
side, though near, the nucleus, in the cyto-reticulum, surrounded
by a granular, reticular, or radiating area of the latter known
as the attraction-spJiere or centrospJiere (Figs. 5, 6, j)? It may,
however, lie within the nuclear membrane in the linin-network
(Fig. 107). In some cases the centrosome is a single body which
.divides into two as the cell prepares for division. More commonly,
it becomes double during the later phases of cell-division, in anticipa-
tion of the succeeding division, the two centrosomes thus formed
lying passively within the attraction-sphere during the ordinary life
of the cell. They only become active as the cell prepares for the
ensuing division, when they diverge from one another, and each
becomes the centre of one of the astral systems referred to at
p. 49. Each of the daughter-cells receives one of the centrosomes,
which meanwhile again divide into two. The centrosome seems,
therefore, to be in some cases a permanent cell-organ, like the
nucleus, being handed on by division from one cell to another.
There are, however, some cells, e.g. muscle-cells, most gland-cells,
and many unicellular organisms, in which no centrosome has thus
far been discovered in the resting-cell ; but it is uncertain whether
the centrosome is really absent in such cases, for it may be hidden
in the nucleus, or too small to be distinguished from other bodies
in the cytoplasm. There is, however, good reason to believe that
it degenerates and disappears in the mature eggs of many animals,
and this may likewise occur in other cells. At present, therefore,
we are not able to say whether the centrosome is of equal constancy
with the nucleus.^

1 The centrosome was discovered by Van Beneden in the cells of Dycyemids ('76), and
first carefully described by him in the egg of Ascaris seven years later. The name is due
to Boveri ('88, 2, p. 68).

^ Cf. p. 229.

2 Its nature is more fully discussed at p. 224.

OTHER ORGANS 37

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 as plastids ox proto-
plasts {Y\^. 5), 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 chromatophores or cJiromoplasts,
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 never arise spontaneously, so far as known, but
always by the division of pre-existing bodies of the same kind. They
possess in some cases a high degree of morphological independence,
and may even live for a time after removal from the remaining cell-
substance, as in the case of the "yellow cells" of Radiolaria. This
has led to the view, advocated by Brandt and others, that the
chlorophyll-bodies found in the cells of many Protozoa and a few
Metazoa {Hydra, Spongilla, some Planarians) are in reality distinct
Algae living symbiotically in the cell. This view is probably correct
in some cases, e.g. in the Radiolaria ; but it may well be doubted
whether it is of general application. In the plants the chlorophyll-
bodies and other chromoplasts are almost certainly to be regarded as
differentiations of the cytoplasmic substance. The same is true of
the aniyloplasts, which act as centres for the formation of starch.

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 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-
propagating units.

38 GENERAL SKETCH OE THE CELL

G. The Cell-membrane

From a general point of view the cell-membrane or intercellular
substance is of relatively minor importance, since it is not of constant
occurrence, belongs to the lifeless products of the cell, and hence
plays no direct part in the active cell-life. In plant-tissues the mem-
brane is almost invariably present and of firm consistency. Animal
tissues are in general characterized by the slight development or
absence of cell-walls. Many forms of cells, both among unicellular
and multicellular forms, are quite naked, for example Amoeba and the
leucocytes ; but in most, if not in all, such cases, the outer limit of
the cell-body is formed by a more resistant layer of protoplasm — the
"pellicle" of Biitschli — that may be so marked as to simulate a true
membrane, for example, in the red blood-corpuscles (Ranvier, Wal-
deyer) and in various naked animal eggs. Such a " pellicle " differs
from a true cell-membrane only in degree ; and it is now generally
agreed that the membranes of plant-cells, and of many animal-cells,
arise by a direct physical and chemical transformation of the periph-
eral layer of protoplasm. On the other hand, according to Leydig,
Waldeyer, and some others, the membrane of certain animal-cells may
be formed not by a direct transformation of the protoplasmic substance,
but as a secretion poured out by the protoplasm at its surface. Such
membranes, characterized as " cuticular," occur mainly or exclusively
on the free surfaces of cells (Waldeyer). It remains to be seen, how-
ever, how far this distinction can be maintained, and the greatest
diversity of opinion still exists regarding the origin of the different
forms of cell-membranes in animal-cells.

The chemical composition of the membrane or intercellular sub-
stance varies extremely. In plants 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

POLARITY OF THE CELL

39

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 very clearly 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. 17, A). With this view Rabl's theory ('85) of
nuclear polarity harmonizes, for the chromosome-loops converge tow-
ards the centrosome, and the nuclear axis coincides with the cell-axis.
Moreover, it identifies the polarity of the o.^^, which is so important
a factor in development, with that of the tissue-cells; for the ^^^-

7^

B C

Rabl, Hatschek.

Fig. 17. — Diagrams of cell-polarity.
A. Morphological polarity of Van Beneden. Axis passing through nucleus and centrosome.
Chromatin-threads converging towards the centrosome. B. C. Physiological polarity of Rabl
and Hatschek, ^ in a gland-cell, C in a ciliated cell.

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 struct-
ures 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
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. In the present state of the
subject, Heidenhain's theories must be regarded as somewhat trans-
cendental, though they give many suggestions for further investigation.

40 GENERAL SKETCH OF THE CELL

Hatschek ('88) 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. 17, B, C.) This is
typically shown in epithelium, which, as Kolliker and Hackel long
since pointed out, is to be regarded, both ontogenetically and phy-
logenetically, as the most primitive form of tissue. The free and
basal ends of the cells here differ widely in relation to the food-
supply, and show a corresponding structural differentiation. In such
cells the nucleus usually lies nearer the basal end, towards the source
of food, while differentiated products of the 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.

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 differentiations
are grouped. Moreover, the observations of Heidenhain, Lebrun, and
Kostanecki indicate that the same is true in epithelium ; for, accord-
ing to these authors, the centrosome is always situated on that side
of the nucleus turned towards the free end of the cell. How far this
law holds good remains to be seen, and, until the facts have been
further investigated, it is impossible to frame a consistent hypothesis
of cell-polarity. The facts observed in epithelial cells, are, however,
of great significance ; for the position of the centrosome, and hence
the direction of the axis, is here obviously related to the cell-environ-
ment, and it is difficult to avoid the conclusion that the latter must
be the determining condition to which the intracellular relations con-
form. When applied to the germ-cells, this conclusion becomes of
high interest ; for the polarity of the egg is one of the primary con-
ditions of development, and we have here, as I believe, a clue to its
origin.^

1 Cf. pp. 288, 320.

THE CELL IN RELATION TO THE MULTICELLULAR BODY 4 1

I. The Cell in Relation to the Multicellular Body

In analyzing the structure and functions of the individual cell we
are accustomed, as a matter of convenience, to regard it as an inde-
pendent elementary organism or organic unit. Actually, however,
it is such an organism only in the case of the 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 appa-
ratus 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 co-ordinated, 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
(1839), 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, afterwards elaborated by
Virchow and Hackel 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-
cerned, it has now been clearly demonstrated that only in a limited
sense can the cells be regarded as co-operating units. They are
rather local centres of a formative power pervading the growing

1 Untersuchtingen, p. 191.

42 GENERAL SKETCH OF THE CELL

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 becomes 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 in a
measure sustained by later researches, though it still remains sub
jiidice. Tangl, Gardiner, and many later observers have shown that
the cell-walls of many plant-tissues are traversed by delicate intercel-
lular bridges, and similar bridges have been conclusively demon-
strated by Bizzozero, Retzius, Flemming, Pfitzner, and many others
in the case of animal epithelial cells (Figs, i, 9). The same has
been asserted to be the case with the smooth muscle-fibres, with car-
tilage-cells and connective-tissue cells, and in a few cases with nerve-
cells. Paladino and Retzius ('89) have endeavoured to show, further,
that the follicle-cells of the ovary are connected by protoplasmic

iCf. Chapters VIII., IX.

^ For a fuller discussion see pp. 293 and 311.

THE CELL IN RELATION TO THE MULTLCELLULAR BODY 43

bridges not only with one another, bitt also with the ovum, a conclu-
sion which, if established by further research, will be of the greatest
interest.

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 the structure in which protoplasmic
continuity is to be expected, the latest researches are rendering it
more and more probable that no such continuity exists, and that
nerve-impulses are transmitted from cell to cell by contact-action.
When, however, we turn to the embryonic stages we find strong
reason for the belief that a material continuity between cells must
exist. This is certainly the case in the early stages of many arthro-
pods, 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 AmpJiioxns or the echinoderms, 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 mechanical displacement
of the blastomeres. This conclusion is supported by the recent work
of Hammar ('96), whose observations on sea-urchin eggs I can in the
main confirm.

As the subject now lies, however, the facts do not, I believe, jus-
tify any general statement regarding the occurrence, origin, or physi-
ological meaning of the protoplasmic continuity of cells ; and a most
important field here lies open for future investigation.

LITERATURE. P

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

ed. Leipzig, 1894.
Van Beneden, E. — (See Lists IL, IV.)
Boveri, Th. — (See Lists IV., V.)
BUtschli, 0. — Untersucliungen uber mikroskopische Schaume und das Protoplasma.

Leipzig (Engelmann), 1892.
Engelmann, T. W. — Zur Anatomie und Pliysiologie der Flimmerzellen : Arch. ges.

Phys., XXIII. 1880.
von Erlanger, R. — Neuere Ansichten iiber die Struktur des Protoplasmas : Zool.

Centralbl., III. 8, 9. 1896.

^ See also Introductory list, p. 12.

44 GENERAL SKETCH OF THE CELL

Flemming, W. — Zellsubstanz, Kern und Zellteilung. Leipzig, 1882.

Id. — Zelle : Merkel iind Bonnefs Ergebnisse, I. -IV. 1891-94. (Admirable reviews

and literature-lists.)
Heidenhain, M. — Uber Kern und Protoplasma : Festschr. 2. ^o-jdhr. Doctorjiib.

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

Sci., XVIII. 1878.
Kolliker, A. — Handbuch der Gewebelehre, 6th ed. Leipzig, 1889.
Leydig, Fr. — Zelle und Gewebe. Bonn, 1885.
Schafer, E. A. — General Anatomy or Histology; in Quain''s Anatomy, I. 2, loth

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

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

plasmas. Breslau, 1887.
Strasburger, E. — Zellbildung und Zellteilung, 3d ed. 1880.
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.
De Vries, H. — Intracellulare Pangenesis. Jena, 1889.
Waldeyer, W. — Die neueren Ansichten liber den Bau und das Wesen der Zelle :

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

Wien, LLolder. 1892.
Zimmerman, A. — Beitrage zur Morphologic und Physiologic der Pflanzenzelle.

Tubingen, 1893.

CHAPTER II

CELL-DIVISION

" Wo eine Zelle entsteht, da muss eine Zelle vorausgegangen sein, ebenso wie das Thier
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 strange Nachweis noch nicht gelie-
fert ist, doch das Princip gesichert, dass in der ganzen Reihe alles Lebendigen, dies niogen
nun ganze Pflanzen oder thierische Organismen oder integrirende Theile derselben sein, ein
ewiges Gesetz der continuirlichen Entivicklung besteht." VlRCHOW.^

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. The cell has no other mode of origin than by division of
a pre-existing cell. In the multicellular organism all the tissue-cells
have arisen by continued division from the original germ-cell, and
this in its turn arose by the division of a cell pre-existing 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 (1840-60) 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 afterwards, but the researches of Remak ('4O' 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

1 Cellularpathologie, p. 25, 1858.

2 Cf. Introduction, p. 9.

3 For a full historical account of this period, see Remak, Uniersuchiingen iiber die Ent-
ivicklung der WirbeWiiere, 1855, pp. 164-180.

* Uniersiichiingen, p. 175.

45

46

CELL-DIVISION

with the division of the nucleohis, is continued by simple constriction
and division of the nucleus, and is completed by division of the cell-
body and membrane (Fig. i8). 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 sim-
ple 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. i8, /). 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 Biitschli,
Fol, Strasburger, Van Beneden,
Flemming, and Hertwig, showed
cell-division to be a far more
elaborate process than had been
supposed, and to involve a com-
plicated transformation of the
nucleus to which Schleicher
('78) afterwards gave the name
of Karyokinesis. It soon ap-
peared, however, that this mode
of division was • not of univer-
sal occurrence ; and that cell-
division is of two widely different types, which Van Beneden {^7^)
distinguished as fragnientatioit, corresponding nearly to the simple
process described by Remak, and division, involving the more com-
plicated 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 employed.

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, in the cells of transitory em-

Fig. 18. — Direct division
the embryo chick, illustrating
[Remak.]

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

of blood-cells in
Remak's scheme.

/ Cell

OUTLINE OF INDIRECT DIVISION OR MITOSIS 4/

bryonic envelopes, and in tumours and other pathological forma-
tions, where it is of frequent occurrence. Whether this view be
well founded or not, it is certain that in all the higher and in many
of the lower forms of life, 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 matura-
tion, and by mitotic division the oosperm segments and gives rise
to the tissue-cells. It occurs not only in the highest forms of plants
and animals, but also in such simple forms as the Rhizopods, Flagel-
lates, and Diatoms. We may, therefore, justly regard it as the most
general expression of the " eternal law of continuous development "
on which Virchow insisted.

A. Outline of Indirect Division or Mitosis (Karyokinesis)

The process of mitosis 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, how-
ever, 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 Ajiaphases, 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 sub-
stance, involving both physical and chemical changes. The chroma-
tin resolves itself little by little into a more or less convoluted thread,
known as the skein (Knauel) or spireme, and its substance stains far
more intensely than that of the reticulum (Fig. 19). In some
cases there is but a single continuous thread ; in others, the thread
is from its first appearance divided into a number of separate pieces
or segments forming a segmented spireme. In either case it ulti-
mately breaks transversely into a definite number of distinct bodies,
known as chromosomes (Waldeyer, '88), which in most cases have
the form of rods, straight or curved, though they are sometimes
spherical or ovoidal, and in certain cases may be joined together
in the form of rings. The staining power of the chromatin is now
at a maximum. As a rule the nuclear membrane meanwhile fades
away and finally disappears. The chromosomes now lie naked in the
cell, and the ground-substance of the nucleus becomes continuous
with the surrounding cytoplasm (Fig. 19, D, E, F).

48

CELL- D I VISION

Every species of plant or animal has a fixed ajid characteristic mivi-
ber of chromosomes, zvJiicJi regularly recins in the division of all of its
cells; and in all forms arising by sexnal 7'eprodiiction the number is

Fig. 19. — Diagrams showing the prophases of mitosis.
A. Resting-cell with reticular nucleus and true nucleolus ; at c the attraction-sphere contain-
ing two centrosomes. B. Early prophase ; the chromatin forming a continuous spireme, nucleolus
still present; above, the amphiaster {a). C. D. Two different types of later prophases; C. Dis-
appearance of the primary spindle, divergence of the centrosomes to opposite poles of the nucleus
(examples, many 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 spirenr.e-thread to form the chromosomes (examples, epi-
dermal cells of salamander, formation of the polar bodies). E. Later prophase of type C\ fading
of the nuclear membrane at the poles, formation of a new spindle inside the nucleus; precocious
splitting of the chromosomes (the latter not characteristic of this type alone). F. The mitotic
figure established; e.p. The equatorial plate of chromosomes. (Cf. Figs. 16, 21, 24.)

OUTLINE OF INDIRECT DIVISION OR MITOSIS 49

even. Thus, in some of the sharks the number is 36; in certain
gasteropods 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
number is said to be 16, and the same number is characteristic of the
onion. In the grasshopper it is 12; in the \iQ.y^z!{\Q. Pallavicinia 2Ci\di
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. 61). The even number of chromosomes is a most interesting
fact, which, as will appear hereafter (p. 135), is due to the derivation
of one-half the number from each of the parents.

The nucleoli differ in their behaviour in different cases. Net-knots,
consisting of true chromatin, probably enter into the formation of the
spireme-thread. True nucleoli seem to dissolve and disappear, or in
some cases are cast out bodily into the cytoplasm, where they degen-
erate and have no further function. Whether they ever contribute
to the formation of chromosomes is uncertain.

(b) The Aviphiaster. Meanwhile, more or less nearly parallel with
these changes in the chromatin, a complicated structure known as the
anipJiiaster (Fol, 'j'j^ makes its appearance in the position formerly
occupied by the nucleus (Fig. 19, B-F). This structure consists
of a fibrous spindle-shaped body, the spindle, at either pole of which
is a star or aster formed of rays or astral fibres radiating into the sur-
rounding cytoplasm, the whole strongly suggesting the arrangement
of iron filings in the field of a horseshoe magnet. The centre of each
aster is occupied by a minute body,^known as the centrosonie (Boveri,
'^^), which may be surrounded by a spherical mass known as the
centrosphe7'e (Strasburger, '93). As the amphiaster forms, the chro-
mosomes group themselves in a plane passing through the equator of
the spindle, and thus form what is known as the equatorial plate.

The amphiaster arises under the influence of the centrosome of the
resting-cell, which divides into two similar halves, an aster being
developed around each while a spindle stretches between them (Fig.
19, A-D). 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. 19, D, F; 21). In other cases the original spindle

^ For a more complete list see p. 154-

50

CELL-DIVISION

disappears, and the two asters pass to opposite poles of the nucleus
(m^st pl^nt mitoses and in many animal cells). A spindle is now
formed from rays that grow into the nucleus from each aster, the
nuclear membrane fading away at the poles, though in some cases it
nffay be pushed in by the spindle-fibres for some distance before its

Fig. 20. — Diagrams of the later phases of mitosis.
G. Metaphase ; splitting of the chromosomes {e. /.) ; 7i. 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. J. Division completed.

disappearance (Fig. 19, C, £}. In this case there is apparently no
central spindle. In a few exceptional cases, finally, the amphiaster
may arise inside the nucleus (p. 225).

The entire structure, resulting from the foregoing changes, is
known as the karyokinetic or mitotic figure. It may be described as
consisting of two distinct parts; namely, i, the chromatic figure,
formed by the deeply staining chromosomes ; and, 2, the achromatic

OUTLINE OF INDIRECT DIVISION OR MITOSIS 5 I

figure, consisting of the spindle and asters which, in general, stain
but slightly. The fibrous substance of the achromatic figure is gener-
ally known as arcJioplasm (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 afterwards diverge to opposite
poles of the spindle, and here each group of daughter-chromosomes
finally gives rise to a daughter-nucleus (Fig. 20). 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.
38, 39). Such variations do not, however, affect the essential fact
that the chromatic netzvo7^k is converted into a thread^ zvJiicJi, tvJiether
continiioiis or discontinuous, splits throngJioiit its entire length into
tzvo exactly egnivalejit halves. The splitting of the chromosomes,
discovered by Flemming in 1880, is the most significant and funda-
mental operation of cell-division ; for by it, as Roux first pointed out
('83), the entire substance of the chromatic network is precisely halved,
and the danghter-nnclei receive precisely equivalent portions of chro-
matiji from tlie mother-nucleus. It is very important to observe that
the nuclear division always shows this exact equality, whether division
of the cell-body be equal or unequal. The minute polar body, for
example (p. 131), receives exactly the same amount of chromatin as
the &Z^, 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 (cf. Figs. 43, 71) though not in strict ratio.
The fact is one of great significance for the general theory of mitosis,
as will appear beyond.

3. Anaphases. — After splitting of the chromosomes, the daughter-
chromosomes, arranged in two corresponding groups,^ diverge to oppo-
site poles of the spindle, where they become closely crowded in a mass
near the centre of the aster. As they diverge, the two groups of
daughter-chromosomes are connected by a bundle of achromatic
fibres, stretching across the interval between them, and known as the
intersonal fibres ox connecting fibres? In some cases, these differ in a

^ It was this fact that led Flemming to employ the word "mitosis" (fjiiros, a thread).

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

3 Verbinditngsfasei'ji of German authors; filaments rennissaiits of Van Beneden.

52 CELL-DIVISION

marked degree from the other spindle-fibres ; and they are believed
by many observers to have an entirely different origin and function.
A view now widely held is that of Hermann, who regards these fibres
as belonging to a central spindle, surrounded by a peripheral layer
of mantle-fibres to which the chromosomes are attached, and only
exposed to view as the chromosomes separate.^ They are sometimes
thickened in the equatorial region to form a body known as the cell-
plate or mid-body, which, in the case of plant-cells, takes part in the
formation of the membrane by which the daughter-cells are separated.

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 danghter-spirenie, closely similar to that
of the mother-nucleus (Fig. 23); 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. 37). When first formed, the daughter-nuclei are of
equal size. If, however, division of the cell-body has been unequal,
the nuclei become, in the end, correspondingly unequal — a fact
which, as Conklin and others have pointed out, proves that the size
of the nucleus is controlled by that of the cytoplasmic mass in which
it lies.

The fate of the achromatic structures varies considerably, and has
been accurately determined in only a few cases. As a rule, the
spindle-fibres disappear more or less completely, but a portion of their
substance sometimes persists in a modified form. In dividing plant-
cells, the interzonal fibres become thickened at the equator of the
spindle and form a transverse plate of granules, known as the cell-
plate (Fig. 25), which gives rise to the membrane by which the two
daughter-cells are separated. The remainder of the spindle disap-
pears. A similar cell-plate occurs in some animal cells; but it is
often greatly reduced, and may form only a minute body known as
the mid-body (Zwischenkorper), which lies between the two cells after

1 Cf. p. 74.

ORIGIN OF THE MITOTIC FIGURE 53

their division (Fig. 23). In other cases, as in the cells of the testis,
the remains of the spindle in each cell sometimes gives rise to a more
or less definite body known as \}i\Q. paranitcleiis or Nebenkern (Fig. 62).

The aster may in some cases entirely disappear, together with the
centrosome (as occurs in the mature &gg). In a large number of
cases, however, the centrosome persists, lying either outside or more
rarely inside the nucleus and dividing into two at a very early period.
This division is clearly a precocious preparation for the ensuing divi-
sion of the daughter-cell, and it is a remarkable fact that it occurs as
a rule during the early anaphase, before the mother-cell itself has
divided. There are, however, some undoubted cases (cf. Figs. 6, 7) 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 att7'action-spJiere. This
body often shows a true astral structure with radiating fibres (Figs. 7,
35); but it is sometimes reduced to a regular spherical mass which
may represent only the centrosphere of the original aster (Fig. 6).

B. Origin of the Mitotic Figure

The chromatic figure (chromosomes) is derived directly from the
chromatic network of the resting-nucleus as described above. The
derivation of the achromatic figure (spindle and asters) is a far more
difficult question, which is still to some extent involved in doubt. By
the earlier observers (1873-75) the achromatic figure was supposed
to disappear entirely at the close of cell-division, and most of them
(Biitschli, Strasburger, Van Beneden, '75) believed it to be reformed
at each succeeding division out of the nuclear substance. Later re-
searches (1875-85) gave contradictory 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 from the cytoplasm. Flemming ('82)
was in doubt, and regarded the question of nuclear or cytoplasmic
origin as one of minor importance, yet on the whole inclined to the
opinion that the achromatic figure arose inside the nucleus.^ In 1887
a new face was put on the whole question through the independent
discovery by Van Beneden and Boveri that the centrosome does not
disappear at the close of mitosis, but remains as a distinct cell-organ
lying beside the nucleus in the cytoplasm. These investigators agreed
that the amphiaster is formed under the influence of the centrosome,

1 ZcUsiibstaiiz, p. 226.

54

CELL-DIVISION

which leads the way in cell-division by dividing into two similar
halves to form the centres of division. "Thus we are justified," said
Van Beneden, "in regarding the attraction-sphere with its central

D

Fig. 21. — The prophases in cells (spermatogonia and spermatocytes) of the salamander.
[DrOner.]

A. Spermatogonium in the spireme-stage ; the chromatin-thread lies in the linin-network, still
surrounded by the membrane; above, the two centrosomes, the central spindle not yet formed.
B. Later stage (spermatocyte) ; the nuclear membrane has disappeared, leaving the naked chro-
mosomes; above, the amphiaster, with centrosomes and central spindle; astral rays extending
towards the chromosomes. D. Following stage ; splitting of the chromosomes, growth of the
aster; mantle-fibres and central spindle clearly distinguished. C. The fully formed mitotic figure
(metaphase) ; the chromosomes, fully divided, grouped in the equatorial plate.

corpuscle as forming a permanent organ, not only of the early blas-
tomeres, but of all cells ; that it constitutes a cell-organ equal in rank
to the nucleus itself ; that every central corpuscle is derived from
a pre-existing corpuscle, every attraction-siohere from the pre-existing

ORIGIN OF THE MITOTIC FIGURE

55

sphere, and that division of the sphere precedes that of the cell-
nucleus."^ Boveri expressed himself in similar terms in the same
year i^^J, 2, p. 153), and the same general result was reached by
Vejdovsky nearly at the same time,^ though it was less clearly formu-
lated than by either Boveri or Van Beneden.

F G

Fig. 22. — Metaphase and anaphases of mitosis in cells (spermatocytes) of the salamander.
[Druner.]

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 afterwards the cell divides (see Fig. 23).

All these observers agreed, therefore, that the achromatic figure
arose outside the nucleus, in the cytoplasm ; that the primary impulse
to cell-division was given, not by the nucleus, but by the centrosome,
and that a new cell-organ had been discovered whose special office

'87, p. 279.

2 '88, pp. 151, etc.

56

CELL-DI VISION

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-^^/A-. The centroso^ne rep-
resents the dynainic centre of the cell." ^ This view has been widely
accepted by later investigators, and the centrosome has been shown
to occur in a large number 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 many plant-tissues, and in some of the unicellular

Fig. 23. — Final phases (telophases) of mitosis in salamander cells. [Flemming.]
/. Epithelial cell from the lung; chromosomes at the poles of the spindle, the cell-body divid-
ing; granules of the "mid-body" ox Zwischenkorper 2A. the equator of the disappearing spindle.
y. 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.

plants, and animals, such as the Diatoms and Flagellates. That
the centrosome gives the primary impulse to cell-division by its own
division has, however, been disproved ; for there are several accu-
rately determined cases in which the chromatin-elements divide
long before the centrosome, and it is now generally agreed that the
division of chromatin and centrosome are two parallel events, the
causal relation between which still remains undetermined. (Cf.

P- n-)

1 Boveri, '87, 2, p. 153.

MODIFICATIONS OF MITOSIS 57

C. Modifications of Mitosis

The evidence steadily accumulates that the essential phenomena
of mitosis are of the same general type in all forms of cells, both
in plants and in animals. Everywhere, with a single important
exception (maturation), the chromatin-thread splits lengthwise through-
out its whole extent, and everywhere an achromatic spindle is formed
that is in some manner an agent in the transportal of the chromatin-
halves to the respective daughter-cells. The exception to this general
law, which occurs during the preparation of the germ-cells for their
development and constitutes one of the most significant of all cyto-
logical phenomena, is considered in Chapter V. We have here only
to glance at a number of modifications that affect, not the essential
character, but only the details of the typical process.

I . Varieties of the Mitotic Figure

All of the mitotic phenomena, and especially those involved in the
history of the achromatic figure, are in general most clearly displayed
in embryonic cells, and especially in the egg-cell ^ (Fig. 24). In
the adult tissue-cells the asters are relatively small, the spindle
relatively large and conspicuous. The same is true of plant-cells
in general where the very existence of the asters was at first
overlooked. Plant-mitoses are characterized by the prominence of
the cell-plate (Fig. 25), 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.

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. 108). In some cases the aster contains at its centre
nothing more than a minute deeply staining granule, which doubtless

1 A very remarkable modification of the achromatic figure occurs in the spiral asters^
discovered by Mark ('81) in the eggs of Umax, the astral rays being curved as if the entire
aster had been rotated about its centre. The meaning of this phenomenon is unknown.

^ See p. 224.

58

CELL-DIVISION

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

Fig. 24. — The middle phases of mitosis in the first cleavage of the Ascaris-ftg^. [BOVERI.]
A. Closing prophase, the equatorial plate forming. B. Metaphase; equatorial plate estab-
lished and the chromosomes split ; b, the equatorial plate, viewed en face, showing the four chro-
mosomes. C. Early anaphase; divergence of the daughter-chromosomes (polar body at one
side). D. Later anaphase ; ;). 3., second polar body.

(For preceding stages see Fig. 65 ; for later stages. Fig. 104.)

spindle-fibres are not focussed at a single point, and the spindle
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.^

1 Hacker asserts in a recent paper ('94) that the truncated polar spindles are normal,
and that a centrosome lies at each of the four angles; i.e. two at either pole.

MODIFICATIONS OF MITOSIS

59

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 doubt that a single continuous spireme-thread may be formed
(cf. p. 184), 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. The chromosomes, when fully formed, vary greatly in
appearance. In many of the tissues of adult plants and animals

Fig. 25. — Division of pollen-mother-cells in the lily. [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 certtrosomes still undivided ; lower cell in the late anaphase, cell-plate not yet
formed.

they are rod-shaped and are often bent in the middle like a V (Figs.
21, 33). They often have this form, too, in embryonic cells, as in
the segmentation-stages of the &^^ in Ascai'is (Fig. 24) and other
forms. The rods may, however, be short and straight (segmenting
eggs of echinoderms, etc.), and may be reduced to spheres, as in
the maturation stages of the germ-cells.

CELL-DIVISION

2. Heterotypical Mitosis

Under this name Flemming {[^'J^ first described a peculiar modi-
fication of the division of the chromosomes that has since been shown
to be of very great importance in the early history of the germ-cells,

Fig. 26. — 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).

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. 26), which by its mode of origin is shown to represent two
daughter-chromosomes, each forming half of the ring, united by

MODIFICATIONS OF MITOSIS 6 1

their ends. The ring finally breaks in two to form two U-shaped
chromosomes which diverge to opposite poles 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 represent not two but four chromosomes,
into which they afterwards break up.

3. Bivalent and Plnrivalent Chromosomes

The last paragraph leads to the consideration of certain varia-
tions in the number of the chromosomes. Boveri discovered that the
species Ascans 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 chromo-
somes, while in the other form ("variety univalens") there is but one.
Brauer discovered a similar fact in the phyllopod Artemia, the
number of somatic chromosomes being 168 in some individuals, in
others only 84 (p. 205).

It will appear hereafter 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 Ruckert) 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 p. 203, and
by many facts in the maturation of the germ-cells hereafter con-
sidered. In Ascaris 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
p. 1 1 1, 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 hlmln'icoides, where the normal number is 24.

^ 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 their etymology, is too valuable for general descriptive purposes
to be rejected.

62

CELL-DIVISION

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.

4. Mitosis in the Unicelbdar 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

B

D

Fig. 27. — Mitotic division in Infusoria. [R. Hertwig.]
A-C. Macronucleus of Spirochona, showing pole-plates. D-H. Successive stages in the
division of the micronucleus of /'izra/;2a;i:z«OT. D. The earliest stage, showing reticulum. G. Fol-
lowing stage (" sickle-form ") with nucleolus. E. Chromosomes and pole-plates. F. Late ana-
phase. H. Final phase.

its historical origin. But although traces of mitotic division were
seen in the Infusoria by Balbiani ('58-61), Stein ('59), and others
long before it was known in the higher forms, it is still imperfectly
understood on account of the practical difficulties of observation.
Within a few years, however, our knowledge in this field has rapidly
advanced, and we have already good ground for some important
conclusions.

Mitotic division has now been observed in many of the main divi-
sions of Protozoa and unicellular plants ; but in the present state of
the subject it must be left an open question whether it occurs in all.

MODIFICATIONS OF MITOSIS

63

The essential features of the process appear to be here of the same
nature as in the higher types, but show a series of minor modifications
that indicate the origin of mitotic division from a simpler type.
Four of these modifications are of especial importance, viz. : —

(i) The centrosome or its equivalent lies as a rule inside the
nucleus, thus reversing the rule in higher forms.

(2) The nuclear membrane as a rule remains intact and does not
disappear at any stage.

A ' B C

Fig. 28. — 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 ,^) ; the nucleus meanwhile divides, and one of the daughter-nuclei
afterwards 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.

(3) The asters attain but a slight development, and in some cases
appear to be entirely absent (Infusoria).

(4) The arrangement of the chromatin-granules to form chromo-
somes appears to be of secondary importance as compared with
higher forms, and the essential feature in nuclear division appears to
be the fission of the individual granules.

The basis of our knowledge in this field was laid by Richard
Hertwig through his studies on an infusorian, Spirochona {'77)^ ^^^ ^
rhizopod, Actinosph(Erium ('84). In both these forms a typical spin-

64

CELL-DIVISION

die and equatorial plate are formed inside the luiclear vieuibrane by
a transformation of the nuclear substance. In SpirocJioiia (Fig. 27,
A-C) a 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." Pole-plates of a somewhat different form were
also described in ActinosphcBriiim, and somewhat later by Schewiakoff
('88) in EnglypJia (Fig. 28). Their origin through division of the
"nucleolus" has since been demonstrated by Keuten in Eiiglena

Fig. 29. — Mitosis in the Flagellate Euglena. [KEUTEN.]
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 intra-nuclear
spindle. C. Later stage. D. The nuclear division completed.

(Fig. 29) and Schaudinn in Ainceba. There can therefore be little
doubt that the "nucleolus" in these forms represents an intra-
nuclear centrosome, and that the pole-plates are the daughter-centro-
somes or attraction-spheres. Richard Hertwig's latest work ('95)
indicates that a similar process occurs in the micronuclei of Para-
moeciiim, which at first contain a large " nucleolus " and afterwards
a conspicuous pole-plate at either end of the spindle (Fig. 27, D-H).
The origin of the pole-plates was not, however, positively determined.
These facts indicate, as Richard and Oscar Hertwig have con-
cluded, that the centrosome, in its most primitive form, is an intra-

MODIFICATIONS OF MITOSIS

65

sp

nuclear structure, which may have arisen through a condensation
or differentiation of the "achromatic" constituents. Noctilnca, the
diatoms, and ActinospJicgrijin seem to represent transitions to the
higher types. In the latter form Brauer discovered a distinct cen-
trosome lying in the late anaphase outside the nuclear membrane at
the centre of a small but distinct aster and soon dividing into two,
precisely as in higher forms (Fig. 31,/, J). This centrosome, how-
ever, as Brauer infers, lies within
the nucleus during the resting state
and the earlier stages of division,

and only migrates out into the s

cytoplasm during the late ana- //•

phase, afterward returning to the
nucleus and lying in the " pole-
plate." In the diatoms Biitschli
discovered an extra-nuclear centro-
some and attraction-sphere, and
Lauterborn has traced the forma-
tion of a central spindle from it. A
This spindle, at first extra-nuclear,
is asserted to pass subsequently J^i^^^^^
into the interior of the nucleus.

Ahctiluca, finally, appears to
have attained the condition char-
acteristic of the higher forms.
Here, as Ishikawa has shown, the
cell contains a typical extra-nuclear
centrosome and attraction-sphere
lying in the cytoplasm, precisely
as in Ascaris (Fig. 30). By divi-
sion of centrosome and sphere a
typical central spindle is formed,
about which the nucleus wraps it-
self, and mitosis proceeds much as
in the higher types, except that
the nuclear membrane does not disappear. ^

Regarding the history of the chromatin the most thorough obser-
vations have been made by Schewiakoff in Euglyplia and Brauer in
Actinosphcerimn. In the former case a segmented spireme arises from
the resting reticulum, and long, rod-shaped chromosomes are formed,
which are stated to split lengthwise as in the usual forms of mitosis.
The nuclear membrane persists throughout, and the entire mitotic

^ All of the essential features in this process, as described by Ishikawa, have been con-
firmed by Calkins in the Columbia laboratory.
F

Fig. 30. — Mitosis in the Y\z.geS\.3X& Noctl-
luca.

A. Nucleus (11) in the early prophase;
outside it the attraction-sphere {s), containing
two centrosomes (Ishikawa). B. The mitotic
figure; 7i. the nucleus, containing rod-shaped
chromosomes; s. attraction-sphere; s.p. ex-
tra-nuclear central spindle. (Drawn by G. N.
Calkins from one of his own preparations.)

66

CELL-DIVISION

figure, except the minute asters, is formed inside it (Fig. 28). In
ActinospJiceidimi, on the other hand, there is no true spireme stage, and
no rod-shaped chromosomes are at first formed. The reticulum breaks
up into a large number of granules which give rise to an equatorial
plate, divide by fission, and are distributed to the daughter-nuclei.

o".."""

Fig. 31. — Mhosis \n the rhizopod y4ciiKospk(ermm. [Brauer.]

A. Nucleus and surrounding structures in the early prophase ; above and beiow 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 en face, 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 cenlrosome, 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.

Only in the late anaphase {telophase) do these grannies arrange them-
selves in tlireads (Fig. 31, /), and this process is apparently no more than
a forernnner of the reticular stage. This case is a very convincing
argument in favour of the view that the formation and splitting of chro-
mosomes is secondary to the division of the ultimate chromatin-granules.

MODIFICA riONS OF MITOSIS Qy

(Cf. pp. 7^ and 221.) Richard Hertwig's studies on Infusoria and
those of Lauterborn on Flagellates indicate that here also no longitu-
dinal splitting of the chromatin-threads occurs and that the division
must be referred to the individual chromatin-granules. Ishikawa de-
scribes a peculiar longitudinal splitting of chromosomes in Noctihica,
but Calkins' studies indicate that the latter observer has probably mis-
interpreted certain stages and that the division probably takes place in
a somewhat different manner. A typical spireme and chromosome-
formation has also been described by Lauterborn in the Diatoms ('93).

In none of the foregoing cases does the nuclear membrane dis-
appear. In the gregarines, however, the observations of Wolters
('91) and Clarke ('95) indicate that the membrane does not persist,
and that a perfectly typical mitotic figure is formed.

To sum up : The facts at present known indicate that the unicellu-
lar forms exhibit forms of mitosis that are in some respects transi-
tional from the typical mitosis of higher forms to a simpler type.
The asters may be reduced (Rhizopods) or wanting (Infusoria); the
spindle is typically formed inside the nucleus, either by division of an
intra-nuclear " nucleolo-centrosome " {E?iglejia, Amceba), or possibly
by rearrangement of the chromatic substance without a differentiated
centrosome (Pmicronuclei of Infusoria). In every case the essential
fact in the history of the chromatin is a division of the chromatin-
granules ; but this may be preceded by their arrangement in threads
or chromosomes {EiiglypJia, Diatoms) or may not {Actinosphczrmni).
These facts point tozvards the conclusion that centrosome, spindle, and
chromosomes are all secondary dijferentiations of the primitive nuclear
strncture, and indicate that the asters ajid attraction-spJieres may be
historically a later acquisition developed in the cytoplasm after the dif-
ferentiation of the centrosome.

5. Pathological Mitoses

Under certain circumstances the delicate mechanism of cell-division
may become deranged, and so give rise to various forms of patholog-
ical 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,
occur without discoverable external cause ; and it is a very interest-
ing 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

68

CELL-DIVISION

general groups, as follows: (i) asymuietrical 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
two, and more than one spindle is formed. Under the first group
are included not only the cases of unequal distribution of the daugh-
ter-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-

b. F

Fig. 32. — Pathological mitoses in human cancer-cells. [Galeotti.]
A. Asymmetrical mitosis with unequal centrosomes. B. Later stage, showing unequal dis-
tribution of the chromosomes. C. Quadripolar mitosis. D. Tripolar mitosis. E. Later stage.
F. Tri-nucleate cell resulting.

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. 32). Hansemann, whose conclu-
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 (hyper-chromatic 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

MODIFICATIONS OF MITOSIS

69

that asymmetrical mitoses, exactly like those seen in carcinoma, may
be artificially produced in the epithelial cells of salamanders (Fig.
33) by treatment with dilute solutions of various drugs (antipyrin,
cocaine, quinine).

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-

A

B

Fig. 33. — 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.

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
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 normal
mitosis is certainly the rule in abnormal growths ; and Galeotti's

^ The remarkable polyasters formed in polyspermic fertilization of the egg are de-
scribed at p. 147.

7 O CELL-DIl 'I SI ON

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.

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 Aviphiaster

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 view first clearly stated by Fol,^
that the asters represent in some manner centres of attractive forces
focussed in the centrosome or dynamic centre of the cell. Regarding
the nature of these forces, there is, however, so wide a divergence of
opinion as to compel the admission that we have thus far accom-
plished little more than to clear the ground for a precise investigation
of the subject; and the mechanism of mitosis still lies before us as
one of the most fascinating problems of cytology.

{a) The Theory of Fibrillar Contractility. — The view that has
taken the strongest hold on recent research is the hypothesis of
fibrillar contractility. Blrst suggested by Klein in 1878, this hypoth-
esis was independently put forward by Van Beneden in 1883, and
fully outlined 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 system 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 chromosomes) " in opposite directions.
An important part of the phenomena of (karyo-) kinesis has its efifi-
cient cause, not in the nucleus, but in the protoplasmic body of the
cell."- This beautiful hypothesis was based on very convincing

^ '73. P- 473- '-^ '87, P- 280.

THE MECHANISM OF MITOSIS

71

ac

evidence derived from the study of the Ascaris &g^, and it was
here that Van Beneden first demonstrated the fact, already sus-
pected by Flemming, that the daughter-chromosomes move apart to
the poles of the spindle,
and give rise to the two re-
spective daughter-nuclei.^

Van Beneden describes
the astral rays, both in
Ascaris and in tunicates,
as differentiated into sev-
eral groups (Fig. 34). 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 chro-
mosomes are passively
dragged apart. An oppo-
site group, forming the
"antipodal cone," extend

from the centrosome to K -' . ,'<\f^ "^T^''"'- •. 1 1<^-<S

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 con- ^.^ 34— Slightly schematic figures of dividing eggs

tractions, drag them apart, of Ascaris, illustrating Van Beneden's theory of mitosis.

and thus cause an actual [Van beneden and julin.]

A. Early anaphase; each chromosome has divided

into two. B. Later anaphase during divergence of the
daughter-chromosomes, a.c. Antipodal cone of astral
rays ; c.z. cortical zone of the attraction-sphere ; i. in-
terzonal fibres stretching between the daughter-chromo-
somes ; m.z. medullary zone of the attraction-sphere;
f.c. principal cone, forming one-half of the contractile
spindle (the action of these fibres is reinforced by that of
the antipodal cone) ; s.e.c. sub-equatorial circle, to which
the astral rays are attached.

divergence of the centres.
The remaining astral rays
are attached to the cell-
periphery and d.re limited
by a sub-equatorial circle.
Later observations indi-
cate, 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.

Van Beneden's general hypothesis was accepted in the following
year by Boveri {'88, 2), who contributed many important additional

1 '83, p. 544-

72

CELL-DIVISION

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 irregularly
scattered in the ^gg, are drawn into a position of equilibrium in the
equator of the spindle by the shortening of these rays (Figs. 65, 104);
and that the rays thickeji as they shorten. 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,

B

3

Fig. 35. — 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).

the shortening and thickening of these rays as the daughter-chromo-
somes diverge. In all these operations the behaviour of the rays is
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 fibrillas 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

p. 99.

THE MECHANISM OF MITOSIS

73

carefully by Heidenhain in the salamander. The astral rays here
extend throughout nearly the whole cell (Fig. 35), and are believed
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 Zimmerman ('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

*■" '^ ,^''-^-'^(f.\*^-^.}lf-'^^

A

Fig. 36. — Pigment-cells and asters from the epidermis of fishes. [Zimmerman.]
A. Entire pigment-cell, fi om Blenn'ms. 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, fi om Sargiis.

actively creeping about. Solger and Zimmerman 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. 36).

But although these observations seem to place the theory of fibrillar
contractility upon a firm basis, it has since undergone various modifi-

74 CELL-DIVISION

cations and limitations, which show that the matter is by no means
so simple as it first appeared. The most important of these modifi-
cations are due to Hermann ('91) and Drliner ('95), who have relied
mainly on the study of mitosis in various cells of the salamander, well
known as extremely favourable objects for study. These observers
have demonstrated that in this case the spindle-fibres are of two
kinds which, apparently, differ both in origin and in mode of action.
Hermann showed that the primary amphiaster is formed outside the
nucleus, without connection with the chromosomes, and that the
original spindle persists as a "central spindle" (Figs. 21, 22), which
he regards as composed of non-contractile fibres, and merely forming
a support on which the movements of the chromosomes take place.
The contractile elements are formed by certain of the astral rays
which grow into the nucleus, and become attached to the chromo-
somes, as Boveri described. By the cojitractioji of these latte?" fibres
the chromosomes are now dragged towards the spindle, and around
its equator they are finally grouped to form the equatorial plate. The
fully formed spindle consists, therefore, of two elements ; namely, {a)
the original " central spindle," and (/?) a surrounding mantle of con-
tractile " mantle-fibres " attached to the chromosomes, and originally
derived from astral rays. In the anaphase, as Hermann believes, the
daiighter-chromosoincs are dragged apart solely by the contractile mantle-
fibres, the central spindle-fibi'es being non-contractile and serving as a
support or substratimt along zvhich the chromosomes move. As the
chromosomes diverge, the central spindle comes into view as the in-
terzonal fibres (Fig. 22, G, H). Strasburger ('95) is now inclined to
accept a similar view of mitosis in the cells of plants.

Driiner ('95) in his beautiful studies on the mechanism of mitosis
has advanced a step beyond Hermann, maintaining that the pro-
gressive divergence of the spindle-poles is caused 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
anaphases. This view is supported by the fact that the central
spindle-fibres are always contorted during the metaphases, as if
pushing against a resistance ; and, as Richard Hertwig points out
('95), it harmonizes with the facts observed in the mitoses of in-
fusorian nuclei. The same view is adopted by Braus and by
Reinke. Flemraing ('95) is still inclined, however, to the view that
the divergence of the centres may be in part caused by the trac-
tion of the antipodal fibres, as maintained by Van Beneden and
Boveri.

Heidenhain, finally, while accepting the contractility-hypothesis,
ascribes only a subordinate role to an active physiological contrac-
tility of the fibres. The main factor in mitosis is ascribed to elastic

THE MECHANISM OF MITOSIS 75

tension of the astral rays which are attached at one end to the cen-
trosome, at the other to the cell-periphery. By turgor of the cell
the rays are passively stretched, thus causing divergence of the
spindle-poles and of the daughter-chromosomes to which the spin-
dle-fibres are attached. An active contraction of the fibres is only
invoked to explain the closing phases of mitosis.

{b) Other T/u'07'ies. ~Wsita.se' s ingenious theory of mitosis ('93)
is exactly the opposite of Van Beneden's, assuming that the spindle-
fibres are not pulling but pushing agents, the daughter-chromo-
somes being forced apart by continually lengthening fibres which
grow out from the centres and dovetail in the region of the inter-
zonal fibres. Each daughter-chromosome is therefore connected
with fibres from the aster, not of its own, but of the opposite
side. This view is, I believe, irreconcilable with the movements
of chromosomes observed in multiple asters, and also with those
that occur during the fertilization of the egg, where the chromo-
somes are plainly drawn towards the astral centres and not pushed
away from them.

Biitschli, Carnoy, Platner, and others have sought an explanation
in a totally different direction from any of the foregoing, regarding
the formation of the amphiaster as due essentially to streaming or
osmotic movements of the fluid constituents of the protoplasm, and
the movements of the chromosomes as being in a measure mechan-
ically caused by the same agency. Oscar Hertwig adopts a some-
what vague dynamical view, regarding the formation of the mitotic
figure as due to an interaction between nucleus and cytoplasm, which
he compares to that taking place in a magnetic field between a mag-
net and a mass of iron filings : " The interaction between nucleus and
protoplasm in the cell finds its visible expression in the formation of
the polar centres and astral figures ; the result of the interaction is
that the nucleus always seeks the middle of its sphere of action."^
He gives, however, no hint of his view regarding the nature of the
action or the causes of the chromosomal movements. Ziegler ('95)
accepts a somewhat similar view ; and he has shown that surpris-
ingly close sininlacra of the mitotic figure in many of its different
phases may be produced by placing bent wires (representing the
chromosomes) in the field of a horseshoe magnet strewn with iron
filings.

My own studies on the eggs of echinoderms ('95, 2) and annelids
have convinced me that no adequate hypothesis of the mitotic mech-
anism has yet been advanced. In these, as in many other forms, the
spindle-fibres show no differentiation into central spindle and peri-

-^ Zelle und Geivebe, p. 172.

76

CELL-DIVISION

pheral mable-fibres ; and the chromosomes extend entirely through
the substance of the spindle in its equatorial plane. If there be sup-
porting, as opposed to contractile, fibres, they mu§t be intermingled
with the latter ; and both forms must have the same origin. The

:^U'

-^-^

Ma/.

' 'iH///'/.^^

'!i^\\^-^

' ri

^^spv-^- ;-,.;...;

\-Cv:

kW'y:^

'k

''/ n

/

^^/!t{^

c

\

» Vim I

\

%l/^

\ /

/^i,t!^^^ N

E

D

Fig. 37. — The later stages of mitosis in the egg of the sea-urchin Toxop/ieustes {A-D, X 1000;
E-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 at 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 to D, the centrosphere appears as a large reticulated mass from which the rays pro-
ceed. It is probable that a minute centrosome, or pair of centrosomes, lies near the centre of the
centrosphere, but this is not shown.

daughter-chromosomes appear to move towards the poles through
the substance of the spindle, and do not travel along its periphery as
described by Hermann and Drliner in amphibia and by Strasburger
('93, 2) in the plants. No shortening or thickening of the rays can

THE MECHANISM OF MITOSIS jy

be observed, and the chromosomes proceed to the extreme limit of
the spindle-poles and appear actually to pass into the intei'ior of the
huge reticulated centrosphere. I cannot see how this behaviour of
the chromosomes is to be explained as the result solely of a con-
traction of fibres stretching between them and the centrosphere.
It is certain, moreover, that another factor is at work. Throughout
the anaphases, the centrosphere steadily grows until, at the close,
it attains an enormous size (Fig. 37), and its substance differs chem-
ically from that of the rays, for after double staining with Congo
red (an acid aniline) and hasmatoxylin it becomes bright red while
the rays are blue. It seems probable, therefore, that the movements
of the chromosomes are affected by definite chemical changes occur-
ring in the centrosphere, as Blitschlii and Strasburger^ have main-
tained ; and it is possible that the substance of the spindle-fibres
may be actually taken up into the centrosphere, and the chromo-
somes thus drawn towards it. Strasburger has made the interesting
suggestion, which seems well worthy of consideration, that the move-
ments of the chromosomes may be of a chemotactic character. In
any case, I believe that no satisfactory hypothesis can be framed
that does not reckon with the chemical and physical changes going
on in the centrosphere, and take into account also the probability
of a dynamic action radiating from it into the surrounding struct-
ures. Van Beneden's hypothesis is probably, in principle, correct ;
but, as Boveri himself admits in his latest paper ('95), it seems cer-
tain that other factors are involved besides the contractility of the
achromatic fibres, and the mechanism of mitosis still awaits adequate
physiological analysis.

2. Division of the Chivinosomes

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.

The splitting of the chromosomes is therefore, in Boveri's words,

1 '92, pp. 158, 159. ^ '93, 2. 3 %-], p. 279.

78

CELL-DIVISION

"aji independtnt vital manifestation, an act of reprodiiction on tJie part
of the chromosomes." ^

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
('76) 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 ('72) added the interesting discovery,
that during the mitosis of various tissue-cells of the salamander, the
granules of the spireme-thread divide by fissioji and thus determine the

Fig. 38. — 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.]

longitudinal splitting of the entire chromosome. This discovery was
confirmed by Flemming in the following year ('82, p. 219), and a sim-
ilar result has been reached by many other observers (Fig. 38). The
division of the chromatin-granules may take place at a very early
period. Flemming observed as long ago as 1881 that the chromatin-
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

p. 113.

2 See '81, p. 638.

THE MECHANISM OF MITOSIS

79

reticulum, and long before the division of the centrosome (Fig. 39).
He therefore concludes: "With Boveri I regard the splitting as an
independent reproductive act of the chromatin. The reconstruction
of the nucleus, and in particular the breaking up of the chromosomes
after division into small granules and their uniform distribution
through the nuclear cavity, is, in the first place, for the purpose of

Villi §

F

Fig. 39. — Formation of chromosomes and early splitting of the chromatin-granules in sperma-
togonia oi Ascaris megalocephala, var. bivalens. [Brauer.J

A. Very early prophase ; granules of the nuclear reticulum 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.

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 eqiial 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

80 CELL-DIVISION

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." ^

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
archoplasm, or in the last analysis, the centrosome.

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-
sion of its total viass, but not of its individual elements or chromatin-
granules (Fig. 40).

Before the discovery of mitosis, nuclear division was generally
assumed to take place in accordance with Remak's scheme (p. 45).
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,

1 '93, pp. 203, 204. 2 I.e., p. 205.

DIRECT OR AMITOTIC DIVISION 8 1

many cases of amitotic division were accurately determined, though
very few of them conformed precisely to Remak's scheme. One
such case is that described by Carnoy in the follicle-cells of the
egg in the mole-cricket, where division begins in the fission of the
nucleolus, followed by that of the nucleus. Similar cases have

Fig. 40. — Group of cells with amitotically dividing nuclei ; ovarian follicular epithelium of
the cockroach. [WHEELER.]

been since described, by Hoyer ('90) in the intestinal epithelium of
the nematode RJiabdonenia, by Korscjielt in the intestine of the
annelid OpJiryotrocha, 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. Centrosonie 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 ob-
served ('91) that the nucleus of leucocytes might in some cases divide directly
without the formation of an amphiaster, the attraction-sphere remaining undivided
meanwhile. Heidenhain showed in tlie 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 imcleus is in
this case independent 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 " sperma-

G

82 CELL-DIVISION

togonia" of the salamander. 1 Krause and Flamming 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 obser-
vations 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 granu-
lar 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 (" archoplasm ") 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 "archo-
plasm" of the autumn-stage arises by the breaking down of a definite spherical
attraction-sphere, which is reformed again in the spring in the manner described,
and in this condition the cells may divide either mitotically or amitotically . He
adds the interesting observation, 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 Significance 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
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.), zv/iick are on the ivay
tozvards degeneration. In many cases, moreover, direct nuclear divi-
sion is not followed by fission of the 'cell-body, so that multinuclear

1 '91, p. 628.

^ 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 83

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 {"^"^^ 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 eggs of fishes divide
amitotically, 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 forerunner 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 assimi-
lative activity. Thus, Riige ('90) showed that the absorption of
degenerate eggs in the amphibia is effected by means of leuco-
cytes which creep into the egg-substance. The nuclei of these
cells become enlarged, divide amitotically, 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 Crusta-
cea. 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 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

1 '91, 2, p. 291.

84 CELL-DIVISION

common 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 Aiiilocra ('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 epithelium, new cells are continually
formed in the deeper layers to replace those cast off from the
superficial layers. Dogiel finds in the lining of the bladder of the
mouse that the nuclei of the superficial cells, which secrete the mucus
covering the surface, regularly divide amitotically, giving rise to huge
multinuclear 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
Infusoria, which possess two kinds of nuclei in the same cell, a
macronucleus and a micronucleus. The former is known to be
intimately concerned with the processes of metabolism (cf. p. 165).
During conjugation the macronucleus degenerates and disappears
and a new one is formed from the micronucleus or one of its
descendants. The macronucleus is therefore essentially metabolic,
the micronucleus generative 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 macronucleus than in the micronucleus, and in some
cases the former appears to divide directly while the latter always
goes through a process of mitosis. In view of all these facts and
others of like import Ziegler, like Flemming, concludes that amitosis
is of a secondary character, and that when it occurs the series of
divisions is approaching an end.

This conclusion 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-
producing cells (spermatogonia, etc.), but only in the supporting cells
(Randzellen, StUtzzellen). 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 differentiated, they remain absolutely distinct; amitosis
never takes place in the series which finally results in the formation
of spermatozoa, and the amitotically dividing " sujoporting-cells "
sooner or later perish. Vom Rath thus reached the remarkable con-
clusion 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. 331.)

SUMMARY AND CONCLUSION 8$

Whether this conclusion can be accepted without modification
remains to be seen. Flemming himself regards it as too extreme,
and is inclined to accept Meves' conclusion that amitosis may occur
in the sperm-producing cells of the testis. The same conclusion is
reached by Preusse in the case of insect-ovaries. There can be no
doubt, however, that Flemming's hypothesis in a general way repre-
sents 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

Three distinct elements are involved in the typical mode of cell-
division by mitosis ; namely, the centrosome, the chromosome, and the
cell-body. Of these, the centrosome may be considered the organ
of division par excellence ; for as a rule it leads the way in division,
and under its influence, in some unknown manner, is organized the
astral system which is the immediate instrument of division. This
system appears in the form of two asters, each containing one of the
daughter-centrosomes and connected by a spindle to form an amphi-
aster. It arises as a differentiation or morphological rearrangement
of the general cell-reticulum, the asters being formed from the extra-
nuclear reticulum, the spindle sometimes from the linin-network,
sometimes from the cyto-reticulum, sometimes from both.

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, is formed as a single series of chromatin-granules or
chromomeres which, by their fission, cause the splitting of the thread.
Every individual 'chromatin-granule therefore contributes its quota
to each of the daughter-nuclei.

The mechanism of mitosis is imperfectly understood. There is
good reason to believe that the fission of the chromatin-granules, and
hence the splitting of the thread, is not caused by division of the
centrosome, but only accompanies it as a parallel phenomenon. The
divergence of the daughter-chromosomes, on the other hand, is in
some manner determined by the spindle-fibres developed under the
influence of the centrosomes. There are cogent reasons for the view
that some at least of these fibres are contractile elements which, like

86 CELL-DIVISION

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 contractility
hypothesis is, however, difficult to apply in certain cases, and is prob-
ably an incomplete explanation which awaits further investigation.
The functions of the astral rays are involved in even greater doubt,
being regarded by some investigators as contractile elements like
those of the spindle, by others as rigid supporting fibres like those
of the central spindle. In either case one of their functions is prob-
ably to hold the kinetic centre in a fixed position while the chromo-
somes are pulled apart. Whether they play any part in division of
the cell-body is unknown ; but it must be remembered that the size
of the aster is directly related to that of the resulting cell (p. 51) —
a fact which indicates a very intimate relation between the aster and
the dividing cell-body. On the other hand, in amitosis the cell-body
may divide in the absence of asters.

These facts show that mitosis is due to the co-ordinate play of an
extremely complex system of forces which are as yet scarcely com-
prehended. Its purpose is, however, as obvious as its physiological
explanation is difficult. // is the end of mitosis to divide every part of
the cJiromatiii of the mother-cell equally between the dajighter-nnclei.
All the other operations are tributary to this. We may therefore
regard the mitotic figure as essentially an apparatus for the distri-
bution of the hereditary substance, and in this sense as the especial
instrument of inheritance.

LITERATURE. II

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

Van Beneden, E. — Recherches .sur la maturation de I'oeuf, la fecondation et la

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

mitosique chez I'Ascaride megalocepliale : Bull. Acad. roy. de Belgique, 1887.

III. 14, No. 8.
Boveri, Th. — Zellenstudien: I.Jena. Zeitschr.,XXl. 1887; l\. Ibid. XXII. 1888;

III. Ibid. XXIV. 1890.
Brauer, A. — LJber die Encystirung von Actinosphaerium Eichhorni : Zeitschr.

Wiss. Zool, LVIII. 2. 1894.
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'ol. Centralb., III. 2. 1896.
Flemming, W., '92. — Entwicklung und Stand der Kenntnisseuber Amitose : Merkel

und Bonnet\<; Ergebnisse, II. 1 892 .
Id. — Zelle. (See introductory list. Also general list.)

SUMMARY AND CONCLUSION 8/

Fol, H. — (See List IV.)

Heidenhain, M. — Cytomechanische Studien : Arch. f. Entwickniech., I. 4. 1895.

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

Arch. Mik. Aiiat., XXXVII. 1891 .
Hertwig, R. — Uber Centrosoma und Centralspindel : Sitz.-Ber. Ges. Morph. imd

Phys. MilncJien, 1895, Heft I.
Mark, E. L. — (See List IV.)

Reinke, F. — Zellstiidien : I. Arch. Mik. Anal., XLIII. 1894; II. Ibid. XLIV. 1894.
Strasburger, E. — Karyokinetische Probleme :/(2//r^./. PVtss. Botan. XXVIII. 1895.
Waldeyer, W. — tJber Karyokinese und ilire Beziehungen zu den Befruchtungsvor-

gangen : Arch. Mik. Attat., XXXII. 1888. Q.J.M.S., XXX. 1889-90.

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 which 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 inhibition and of propagation by spon-
taneous Hssion 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.i

" 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 collectively
form the "body," and (2) t\iQ germ-cells, which are of minor signifi-
cance 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 may have the same power of develop-
ment, 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

^ Parthenogenesis, p. 3, 1849.
- Arch. Mik. Anal. XVIIL p. 112, 18S0.
8S

THE GERM-CELLS

89

rise by regeneration to a complete body. The distinction between
somatic and germ-cells is an expression of the physiological division
of labour ; and while it is no doubt the most fundamental and impor-
tant 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. 41), 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-

ft>f

W^^^i^:

f

.Jf?.:^^^

Fig. 41. — Volvox, showing the small ciliated somatic cells and eight large germ-cells (drawn
from hfe by J. H. Emenon).

tion and locomotion, and sooner or later die. Eight or more larger
cells are set asid'e 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
nourished by somatic cells specially set apart for that purpose and
forming distinct sexual organs, the ovaries and spermaries or their
equivalents. Within these organs the germ-cells are carried, pro-
tected, and nourished ; and here they undergo various differentia-
tions to prepare them for their future functions.

90 THE GERM-CELLS

In the earlier stages of embryological development the progeni-
tors of the germ-cells are exactly alike in the two sexes and are in-
distinguishable from the surrounding somatic cells. As development
proceeds, they are first differentiated from the somatic cells and then
diverge very widely in the two sexes, undergoing remarkable transfor-
mations 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 deiitoplasni), 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 centrosome. 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 appara-
tus, 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 destruc-
tive or katabolic processes by which the energy necessary for these
movements is set free.^ When finally matured, therefore, 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 afterwards that the spermatozoon
was proved to be of the same nature.

A. The Ovum

The animal ^gg (Figs. 42, 43 A) 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 viicropyle, through
which the spermatozoon enters (Fig. 45). It contains an enormous
nucleus known as the germinal vesicle, within which is a very conspic-

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

91

uous nucleolus known to the earlier observers as the gejininal spot.
In many eggs the latter is single, but in other forms many nucleoli
are present, and they are sometimes of more than one kind, as in
tissue-cells. 1 In its very early stages the ovum contains a centro-
some, but this afterwards disappears from view, and as a rule cannot
be discovered until the final stages of maturation (at or near the time
of fertilization). It _.-.,rT-TTrr^..,..„

is then found to lie
just outside the ger-
minal vesicle on the
side nearest the egg-
periphery where the
polar bodies are sub-
sequently formed
After extrusion of the
polar bodies (p. 131)
the egg-centrosome
as a rule degenerates
and disappears. The
^gg thus loses the
power of division
which is afterwards
restored during fer-
tilization through the
introduction of a new
centrosome by the
spermatozoon. In
parthenogenetic eggs,
on the other hand,
the egg-centrosome
persists, and the o,^^
accordingly retains the power of division without fertilization. The
disappearance of the egg-centrosome would, therefore, seem to be
in some manner a provision to necessitate fertilization and thus to
guard against parthenogenesis.

The egg-cytoplasm almost always contains a certain amount of
nutritive matter, the yolk or deiUoplasm, in the form of solid spheres
or other bodies suspended in the meshes of the reticulum and vary-
ing greatly in different cases in respect to amount, distribution, form,
and chemical composition.

1 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).

Fig. 42. — Ovarian egg of the sea-urchin Toxopneusies
(X7SO)-

g.v. Nucleus or germinal vesicle, containing an irregular dis-
continuous network of chromatin ; g.s. nucleolus or germinal
spot, intensely stained with hasmatoxylin. The naked cell-body
consists of a very regular network, the threads of which appear
as irregular rows of minute granules or microsomes. Below,
at J, is an entire spermatozoon shown at the same enlargement
(both middle-piece and fiagellum are slightly exaggerated in
size) .

92 THE GERM-CELLS

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. 42); but during the
growth of the Q,gg it may become irregular or even amoeboid (Fig. 58),
and, as Korschelt has shown in the case of insect-eggs, may move
through the cytoplasm towards the source of food. Its structure is
on the whole that of a typical cell-nucleus, but is subject to very great
variation, not only in different animals, but also in different stages of
ovarian growth. Sometimes, as in the echinoderm ovum, the chro-
matin forms a beautiful and regular reticulum consisting of numer-
ous chromatin-granules suspended in a network of linin (Fig. 42).
In other cases, no true reticular stage exists, the nucleus containing
throughout the whole period of its growth the separate daughter-chro-
mosomes of the preceding division (copepods, selachians, amphibia),^
and these chromosomes may undergo the most extraordinary changes
of form, bulk, and staining-reaction during the growth of the Qgg?' It
is a very interesting and important fact that during the growth and
maturation of the ovum a large part of the chromatin of the germinal
vesicle may be lost, either by passing out bodily into the cytoplasm,
by conversion into supernumerary or accessory nucleoli which finally
degenerate, or by being cast out and degenerating at the time the
polar bodies are formed (p. 177).

The nucleolus of the egg-cell is here, as elsewhere, a variable
quantity and is still imperfectly understood. The nucleoli are of
two different kinds, either or both of which may be present. One
of these, the so-called principal nucleolus, is a rounded, usually single
body, staining intensely with the same dyes that colour the chromatin,
and often containing one or more vacuoles. This is typically shown
in the echinoderm ^gg, in the eggs of many annelids, mollusks, and
coelenterates, in some Crustacea, in mammals, and in some other
cases. From its staining-reaction this type of nucleolus appears
to correspond in a chemical sense not with the "true nucleoli" of
tissue-cells, but with the net-knots or karyosomes, such as the nucle-
oli of nerve-cells and of many gland-cells and epithelial cells. The

1 p. 193. ^ P- 245-

THE OVUM 93

second form comprises the so-called "accessory nucleoli," which
stain less intensely, are often numerous, and perhaps correspond
with the true nucleoli of tissue-cells. As growth proceeds, they
usually increase in size and number, and may finally become very
numerous, in which case they often occupy a peripheral position
in the germinal vesicle. This is typically shown in amphibia and
selachians, where there are a large number of nucleoli, which are
at first scattered irregularly through the germinal vesicle but at a
certain period migrate towards the periphery. In some of the
mollusks and Crustacea both forms coexist ; but even closely related
species may differ in this regard. Thus, in Cyclops brevicornis,
according to Hacker, the very young ovum contains a single in-
tensely chromatic nucleolus ; at a later period a number of paler
accessory nucleoli appear; and still later the principal nucleolus
disappears, leaving only the accessory ones. In C. strcnmis, on
the other hand, there is throughout but a single nucleolus. In
some of the mollusks and annelids the " germinal spot " is double,
consisting of a deeply staining principal nucleolus and a paler
accessory nucleolus lying beside it, as in Cyclas and in Nereis

(Fig. 43)-

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 ^Sgiiorea
(Hacker) and in various annelids and echinoderms, afterwards lying
for some time as a " metanucleus " in the egg-cytoplasm before
degenerating. In many cases — for example in amphibia, in sela-
chians, in many Crustacea, annelids, and echinoderms — the chromo-
somes are formed in the germinal vesicle independently of the
nucleoli (Fig. 96), which degenerate in sitn when the membrane of
the germinal vesicle disappears. The evidence is, therefore, very
strong that the nucleoli do not contribute to the formation of the
chromosomes, and that their substance represents passive material
which is of no further direct use. There is, furthermore, strong
evidence that the nucleoli of both kinds are directly or indirectly
derived from the chromatin. Hence we can hardly doubt the
conclusion of Hacker, that the nucleoli of the germ-cells are ac-
cumulations 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

1 Hacker regards the principal nucleolus as a more highly differentiated modification of
the accessory nucleolus, and regards it as a pulsating excretory organ comparable with the
contractile vacuoles of Protozoa.

94 THE GERM-CELLS

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.
It would seem that the nucleoli may likewise represent a portion
of the unused chromatin, more closely aggregated and more or less
modified in a chemical sense.

2. TJie Cytoplasm

The egg-cytoplasm varies greatly in appearance with the varia-
tions of the deutoplasm. In such eggs as those of the echino-
derm (Fig. 42), which have little or no deutoplasm, the cytoplasm
forms a regular reticulum, which is perhaps to be interpreted as
an alveolar structure. Its meshes consist of closely set intensely
staining granules or microsomes embedded in a clearer ground-
substance. The latter, which fills the spaces of the network, is
apparently homogeneous, and contrasts sharply with the micro-
somes in staining capacity. In eggs containing yolk the deutoplasm-
spheres or granules are laid down between the meshes of the net-
work ; and if they are very abundant the latter may be very greatly
reduced, the cytoplasm assuming a pseudo-alveolar structure (Fig. 43),
much as in plant-cells laden with reserve starch. In many cases
a peripheral layer of the ovum, known as the cortical or peri-
vitelline layer, is free from deutoplasm-spheres, though it is continu-
ous with the protoplasmic network in which the latter lie (Fig. 43).
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 towards the
point at which the spermatozoon enters, where a protoplasmic germi-
nal 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 many 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 ^^^ 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 in the annelid

THE OVUM

95

Fig. 43. — Eggs of the annelid Nereis, before and after fertilization, X 400 (for intermediate
stages see Fig. 71).

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 egg is heavily laden with volk, 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 egg 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.l).). Below them lies the mitotic figure, the chromosomes
dividing.

96

THE GERM-CELLS

eii

Nereis (Fig. 43). Often associated with the proper deutoplasm-
spheres are drops of oil, either scattered through the yolk (Fig. 43)
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 egg 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 cases in which
the Q,^^ 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 constitu-
ents they usually cause the Q,^g to lie
with the animal pole turned downwards,
as is the case with some annelids
(^Nereis) and many pelagic fish-eggs.

3. TJie Egg-envelopes

The egg-envelopes fall under three

categories. These are : —
{a) The vitelline membrane, secreted

by the ovum itself.
{U) The chorion, formed outside the
ovum by the activity of the
maternal follicle-cells.
(6-) Accessory envelopes, secreted by
the walls of the oviduct or other
maternal structures after the
ovum has left the ovary.
Only the iirst 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. Renilld), that are
naked throughout their whole develop-
ment. In many others, of which the
sea-urchin is a type, the fresh-laid &%,%
is naked but forms a vitelline mem-
brane almost instantaneously after the

Fig. 44. — Schematic figure of a
median longitudinal section of the
egg of a fiy {Mtisca), showing axes
of the bilateral egg, and the mem-
branes. [From KORSCHELT and
Heider, after Henking and Bloch-

MANN.]

e.7i. The germ-nuclei uniting; ot.,
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 peripheral
or peri-vitelline layer of protoplasm.
The outer heavy line is the chorion,
the inner lighter line the vitelline
membrane, both being perforated by
the micropyle, from which exudes a
mass of jelly-like sutistance.

THE OVUM 97

spermatozoon touches it.^ In other forms (insects, birds) the vitelline
membrane may be present before fertilization, and in such cases the
Qgg is often surrounded by a chorion as well. The latter is usually
very thick and firm and may have a shell-like consistency, its surface
sometimes showing various peculiar markings, prominences, or sculpt-
ured patterns characteristic of the species (insects).^

The accessory envelopes are too varied to be more than touched
upon here. They include 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 Q.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. 44,
45). Where there is but one micro-
pyle, it is usually situated very
near the upper or anterior pole
(fishes, many insects), but it may
be at the opposite pole (some in-
sects and mollusks), or even on Fig. 45. — Upper pole of the egg of r^r^^-

the side (insects). In many insects ^atita. [Ussow.]

.1 • r I. ir J The egg is surrounded by a very thiclc

there is a group of half a dozen or .membrane perforated at m by the 'funnel-

more micropyles near the upper shaped micropyle; below the latter lies the

pole of the ^ZZ^ and perhaps cor- egg-nucleus in the peri-vitelline layer of proto-

. . plasm ; p.b. the polar bodies.

related with this is the fact that

several spermatozoa enter the Q^^, though only one is concerned

with the actual process of fertilization.

The plant ovum, which is usually known as the o'dsphere (Figs.
46, 80), 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 struct-
ure 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

1 That the vitelline membrane does not pre-exist 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.

H

98

THE GERM-CELLS

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 con-
taining pyrenoids, as in Volvox
and many other algae (Fig.
46). In the higher forms
(archegoniate plants), accord-
ing to the researches of
Schmitz and Schimper, the
Qgg contains numerous mi-
nute colourless " leucoplasts,"
which afterwards develop into
green chromatophores or into
the starch-building amylo-
plasts. This is a point of
great theoretical interest; for
the researches of Schmitz,
Schimper, and others have
rendered it highly probable
that these plastids are persistent morphological bodies that arise only
by the division of pre-existing bodies of the same kind, and hence may
be traced continuously from one generation to another through the
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.

Fig. 46. — Germ-cells of Volvox. [OvERTON.]
A. Ovum (oosphere) containing a large central
nucleus and a peripheral layer of chromatophores;
/. pyrenoid. B. Spermatozoid ; cz*. contractile vacu-
oles; e. "eye-spot" (chromoplastid) ; p. pyrenoid.
C. Spermatozoid stained to show the nucleus («).

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 pre-existing in the
parent body. Kolliker, however, did not identify them as cells, but
believed 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

1 Arch. Mik. Afiat., I., '65.

THE SPERMATOZOON

99

morphological nature as the ovum. It is of extraordinary minuteness,
being in many cases less than yfoVo^o" ^^^ bulk of the ovum.^ Its
precise study is therefore difficult, and it is not surprising that our
knowledge of its structure and origin is still far from complete.

Apex or apical body.

Nucleus.

End-knob (?centrosome).
Middle-piece.

. Envelope of the tail.
-Axial filairent

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. 47 : —

1. The micleiis, 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 anilines such as

I methyl-green). It is surrounded by a very

thin cytoplasmic envelope.

2. A minute apex, or apical body, as a
rule of cytoplasmic origin, though appar-
ently derived in some cases from the
nucleus. This lies at the front end of
the head, and in some cases terminates
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.
This body shows the same staining-reaction
as the tip, having an especial afhnity for
"plasma-stains" (acid fuchsin, etc.).

4. The tail, or Jlagell7nn,\r\ part, at least,
a cytoplasmic product develojoed from or
in connection with the " archoplasm " (at-
traction-sphere or "Nebenkern") of the

mother-cell. It consists of a fibrillated axial filament surrounded
by an envelope which sometimes shows a fibrillar structure, sometimes
winds spirally about the axial filament, and is in certain cases differ-

1 In the sea-urchin, Toxopiieustes, I estimate its bulk as being between jq-jooo' ^^^
"JooVo 0" t^^ volume of the ovum. The inequality is in many cases very much greater.

• End-piece.

Fig. 47. — Diagram of the flagel
late spermatozoon.

lOO

THE GERM-CELLS

entiated into a fin-like undulating membrane. The axial filament
may be traced through the middle-piece up to the head, at the base of
which it terminates in a minute body, single or double, known as
the end-hiob, and not improbably representing the centrosome.

There is still some doubt regarding the nature and functions of
these various parts. The nucleus is proved both by its origin and
by its history during fertilization to be exactly equivalent to the
nucleus of the mature ^gg. The middle-piece and the tail represent

Fig. 48. —Spermatozoa of fishes and amphibia. [Ballowitz.]

A. Sturgeon. B. Pike. C. D. Leuciscus. E. Triton (anterior part). F. Triton (posterior
part of flagellum). (7. TPrf/a (anterior part), a. apical body ; (?. end-piece ; y^ fiagellum ; /5. end-
knob (? centrosome) ; m. middle-piece; 71. nucleus; s. apical spur.

the principal mass of the cytoplasm of the sperm-cell, and the mid-
dle-piece is probably to be regarded as merely the thickened basal
portion of the flagellum.

The principal uncertainty relates to the position of the centro-
some. It is certain that in most cases the centrosome or attraction-
sphere lies in the middle-piece ; for from it the centrosome arises
during the fertilization of the &gg, in every accurately known case.
In a few cases, moreover, the middle-piece has been traced back to

THE SPERMATOZOON lOI

the attraction-sphere of the mother-cell, from which the spermato-
zoon is formed in the testis. On the other hand, a few observers
have maintained, apparently on good evidence, that the centrosome
lies, not in the middle-piece, but at the apex (p. 123).

Reviewing these facts from a physiological point of view, we may
arrange the parts of the spermatozoon under two categories as
follows : —

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

These are : —

{a) The nucleus, which contains the chromatin and is to be
regarded as the vehicle of inheritance.

(^) The centrosome, certainly contained in the middle-piece as
a rule, though perhaps lying in the tip in some cases.
This is the fertilizing element par excellence, in Boveri's
sense, since when introduced into the &g^ it causes the
development of the amphiaster by which the ^^% divides.

2. The accessory structures, which play no direct part in fertilization,

viz. : —

(rt) The apex or spur, by which the spermatozoon attaches itself
to the &g% or bores its way into it.

{U) The tail, a locomotor organ which carries the nucleus and
centrosome, and, as it were, deposits them in the q^^ 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. More-
over, as will appear beyond, it is nearly certain that the
contractile fibrillas are derived from the attraction-sphere
of the mother-cell, and therefore arise in the same manner
as the archoplasm-fibres of the mitotic figure — a conclu-
sion of especial interest in its relation to Van Beneden's
theory of mitosis (p. 70).

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 forrns have not yet been fully
determined. The simpler forms, for example those of echinoderms
and some of the fishes (Figs. 48 and 74), conform very nearly to the
foregoing description. Every part of the spermatozoon may, how-

I02

THE GERM-CELLS

H

Fig. 49. — Spermatozoa of various animals. \_A-I, L, from Ballowitz ; J, A", from vON

BRUNN.]

A. (At the left). Beetle (Co/rw), partly macerated to show structure of flagellum ; it con-
sists of a supporting fibre {s.f.) and a fin-like envelope { /.) ; n. nucleus; a. a. apical body divided
into two pans (the posterior of these is perhaps a part of the nucleus). B. Insect {Calalhus) ,
with barbed head and fin-membrane. C. Bird {Phyllopneuste). D. Bii'd {Muscicapa), showing
spiral structure; nucleus divided into two parts (7i^, ?fi) ; no distinct middle-piece. £. Bulfinch ;
spiral membrane of head. F. Gull (La?-us) with spiral middle-piece and apical knob. G. H. Giant
spermatozoon and ordinary form of Tadonia. /. Ordinary form of the same stained, showing
apex, nucleus, middle-piece and flagellum. J-. " Vermiform spermatozoon " and. A', ordinary
spermatozoon of the snail Paliidma. L. Snake {Coluber), showing apical body {a), nucleus,
greatly elongated middle-piece {in), and flagellum (/").

THE SPERMATOZOON IO3

ever, vary more or less widely from it (Figs. 48-50). 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.
The apex sometimes appears to be wanting — e.g. in some fishes
(Fig. 48). 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 spermatozoon appears to penetrate the
ovum {Tritoii). In the mammals it seems to be represented by a
cap-like structure, the so-called "head-cap," which in some forms
covers the anterior end of the nucleus. 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 like diversity
(Figs. 48-50). 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 distin-
guishable 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 limit 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, like
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 minute
bodies lying side by side (Fig. 50, B, D). This body is the only
structure in the middle-piece having the appearance of a centrosome ;
and Hermann conjectures that this is probably its real nature.

The flagellum or tail is merely a locomotor organ which plays
no part in fertilization.' 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 Bal-
lowitz has shown, is composed of a large number of parallel fibrillas,
like a muscle-fibre. This is surrounded by a cytoplasmic envelope,
which sometimes shows a striated or spiral structure, and in which,
or in connection with which, may be developed secondary or acces-

I04

THE GERM-CELLS

sory filaments 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. 48, F^, the
envelope of the axial fila-
ment (" principal filament ")
gives attachment to a re-
markable fin-like membrane,
having a frilled or undulat-
ing free margin along which
is developed a " marginal
filament." Towards the tip
of the tail, the fin, and
finally the entire envelope,
disappears, leaving only the
axial filament to form the
end-piece. After macera-
tion the envelope shows a
conspicuous cross-striation,
which perhaps indicates a
spiral structure such as oc-
curs in the mammals. The
marginal filament, on the
other hand, breaks up into
numerous parallel fibrillae,
while the axial filament re-
mains unaltered (Ballowitz).
A fin-membrane has also
been observed in some in-
sects and fishes, and has
been asserted to occur in
mammals (man included).
Later observers have, how-
ever, failed to find the fin in
mammals, and their obser-
vations indicate that the
axial filament is merely sur-
rounded 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 filaments, both composed of parallel fibrillae, connected by
a membrane and spirally twisted about each other ; a somewhat

Fig. 50. — Spermatozoa of mammals. {A-F from
Ballowitz.)

A. Badger (living). B. The same after staining.
C. Bat ( Vesperugo) . D. The same, fiagelhim and
middle-piece or connecting-piece, showing end-knobs.
E. Head of the spermatozoon of the bat { Rhinolophus)
showing details. F. Head of spermatozoon of tliepig.
G. Opossum (after staining). H. Double spermatozoa
ixova.\!{\ii vas defereiis of the opossum. /. Rat.

h.c. head-cap (apex) ; k. end-knob (? centrosome) ;
m. middle-piece ; n. nucleus (in B, E, F consisting
of two different parts).

THE SPERMATOZOON

105

similar structure occurs in the toad. In some beetles there is a
fin-membrane attached to a stiff axial "supporting fibre" (Fig. 49,
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. 51. — Unusual forms of spermatozoa.
A. B. C. Living amceboid spermatozoa of the crustacean Polyphemus. [Zacharias.]
D. E. Speimatozoa of crab, Dromia. F. Of Ethnsa, G. oi Maja, H. oi Inachus, [Grobben.]
/. Spermatozoon of lobster, //ci;«a;-M. [Herrick.]
y. '&piixm?itozoon of crah, Porcellana. [GROBBEN.]

Many interesting details have necessarily been passed over in the foregoing
account. One of these is the occurrence, in some 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. 49, 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. 49, y. K^ In this case only the smaller spermatozoa are functional
(von Brunn) .

No less remarkable is the conjugation of spermatozoa in pairs (Fig. 50, //), which

io6

THE GERM-CELLS

takes place in the vas deferens in the opossum (Selenka) and in some insects
(Ballowitz, Auerbach). Ballowitz's researches ('95) on the double spermatozoa
of beetles {DyiiscidcB) 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," w^hich 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. 51). In many of
these forms the spermatozoa have no flagellum, and in some cases they
are actively amoeboid ; for example, in the daphnid Polypheimis (Fig.
51, A, B, C) 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-
desmiis), sometimes the form of a hat or helmet having a double brim
{Jiihis). In the latter case the nucleus is a solid disc at the base of
In many decapod Crustacea the spermatozoon consists

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

the hat.

A

B

Fig. 52. — Spermatozoids of C/zaira. [Belajeff.]
A. Mother-cells with reticular nuclei. B. Later

stage, with spermatozoids forming. C. Mature sper-

matozoid (the elongate nucleus black).

3. Paternal Germ-cells of
Plants

In the flowering plants the
male germ-cell is represented
by a "generative nucleus," to-
gether with two centrosomes
and a small amount of cyto-
plasm, lying at the tip of the
pollen-tube (Fig. 80, A). On

the other hand, in a large number of the lower plants (Pteridophytes,
Muscinese, and many others), the male germ-cell is a minute actively
swimming cell, known as the spermatosoid, which is closely analogous

THE SPERMATOZOON

107

to the spermatozoon. The spermatozoids are in general less highly-
differentiated than spermatozoa, and often show a distinct resemblance
to the asexual swarmers or zoospores so common in the lower plants
(Figs. 52, 53). 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 plas-
tids are present in
the form of chromato-
phores, one of which
may be differentiated
into a red " eye-spot,'
as in Volvox and
Fucics (Figs. 41, 53,
A), and they may
even contain contrac-
tile vacuoles (Vo/vox) ;
but both these struct-
ures are wanting in
the higher forms.
These consist only of
a nucleus with a very
small amount of cyto-
plasm, and have typi-
cally a spiral form.
In Chara, where their
structure and devel-
opment have recently
been carefully studied
by Belajeff, the sper-
matozoids have an
elongated spiral form
with two long flagella
attached near the
pointed end which is
directed forwards in swimming (Fig. 52). The main body of the
spermatozoid is occupied by a dense, apparently homogeneous nu-
cleus surrounded 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

^iS- 53- — Spermatozoids of plants. [.4, B, C, E, after
Guignard; D, F, after Strasburger.]

A. Of an alga [Ftcciis) ; a red chromatophore at the right
of the nucleus. B. Liverwort {Pellia). C. Moss {Sphagimni).
D. Maisilia. E. Fern {Angiopteris). F. Fern, Phegopteris (the
nucleus dark).

105 THE GERM-CELLS

and mosses. In the ferns and other pteridophytes a somewhat dif-
ferent type occurs (Fig. 53). 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 may,
however, be cast off when the spermatozoid is set free or at the time
it enters the archegonium. This, according to Strasburger, proba-
bly 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 Cha7'a.

The homologies, or rather analogies, between the respective parts
of the spermatozoid and spermatozoon are not yet very definitely
established, since the history of the spermatozoid in fertilization has
not yet been accurately followed. Strasburger ('92) believes that the
anterior cytoplasmic region, to which the cilia are attached, consists
of "kinoplasm" (archoplasm), and hence corresponds with the mid-
dle-piece of the spermatozoon. If this view be correct, there is, on
the whole, a rather close correspondence between spermatozoid and
spermatozoon, the flagella being attached in both cases to that end of
the cell which contains the centrosome or kinetic centre, the nucleus
lying in the middle, while the opposite end consists of cytoplasm {i.e.
the apex of the spermatozoon, the cytoplasmic vesicle of pterido-
phytes, the basal cytoplasm of Cham, etc.). The attachment of the
flagella in both cases to the archoplasmic region is a significant fact,
for Strasburger believes that they arise from the " kinoplasm " (archo-
plasm), and it is probable that the spermatozoon tail has a similar
origin (p. 126).

C. Origin and Growth 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 believe 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 extern'al stimulus. The nature of the stimulus appears to
vary in different cases. Thus Maupas's experiments seem to show
conclusively that, in rotifers, the differentiation may depend on
temperature, a high temperature tending to produce males, a low

ORIGIN AND GROWTH OF THE GERM-CELLS

109

temperature, females ; while those of Mrs. Treat on lepidoptera and
of Yung on amphibia seem to leave no doubt that the differentiation
here depends on the character of the nutrition, highly-fed individuals
producing a great preponderance of females, while those that are
underfed give rise to a preponderance of males. These and a multi-
tude of related observations by many botanists and zoologists render
it certain that sex as such is not inherited. What is inherited is, in
Busing's words, only the particular manner in which one or the other
sex comes to development. The detcnninatioii of sex is not by in-
heritance, but by the combined effect of external conditions.-^ In
some of the rotifers, however, sex is predetermined from the begin-

Fig. 54. — Germ-cells in the hydro-medusa, Hydractbiia. [BUNTING.]
A. Section through young medusa-bud, with very young ova {ov.) lying in the entoderm;
B. Mature gonophore, showing two ova lying between ectoderm and entoderm.

ning, the eggs being of two sizes, of which the larger produce females;
the smaller, males.

In the greater number of cases, the primordial germ-cells arise in
a germinal epithelium which, in the coelenterates (Fig. 54), 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

1 See DUsing, '84; Geddes, Sex, in Encyclopedia Britannica ; Geddes and Thompson,
The Evolution of Sex; Wa'ase, On the Phenomena of Sex-diffe7'entiation, '92.

I lO

THE GERM-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

^ig- 55- — Origin of the primordial germ-cells and casting out of chromatin in the somatic
c&W^oi Ascaris. [BOVERI.J

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.

into two and finally gives rise to two symmetrical groups of germ-
cells. Haecker has recently traced very carefully the origin of the
primordial germ-cells in Cyclops from a "stem-cell" (Fig. 56) clearly
distinguishable from surrounding cells in the early blastula stage, not

ORIGIN AND GROWTH OF THE GERM-CELLS III

only by its size, but also by its large nuclei rich in chromatin, and by
its peculiar 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
two-cell stage! Moreover, from the outset the progenitor of the germ-
cells dijfers from the somatic cells not only in the greater size and richness
of chromatin of its nuclei, bnt also in its mode of mitosis ; for in all
those blastomeres destined to produce somatic cells a portion of the
chromatin is cast out into the cytoplasm, where it degenerates, and
only in the germ-cells is the sum total of the chromatin retained. In
Ascaris niegalocephala nnivalens the process is as follows (Fig. 55):
Each of the first two cells receives two elongated chromosomes. As
the ovum prepares for the second cleavage, the two chromosomes
reappear in each, but differ in their behaviour (Fig. 55, 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. 55, .s-), all the
chromatin is preserved and the chromosomes do not segment into
smaller pieces. The results are plainly apparent in the 4-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. 55, C). At the ensuing division
(Fig. 55, 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 tlie egg-chromatin handed doiun from the parent. All of the
somatic cells contain ojily a portion of the original germ-substance.
"The original nuclear constitution of the fertilized egg 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

112

THE GERM-CELLS

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.

Fig. 56. — Primordial germ-cells in Cyclops. [HaCKER.]
A. Young embryo, showing stem-cell {st). B. Tlie 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.

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,
is less than in the somatic cells {Fig. 56). Vom Rath, working
in the same direction, has found that in the salamander also the
number of chromosomes in the early progenitors of the germ-cells

' Boveri, '91, p. 437.

2 Cf. p. 321.

GROIVTH AND DIFFERENTIATION OF THE GERM-CELLS II3

is one-half that characteristic of the somatic cells. ^ In both these
cases, the chromosomes are doubtless bivalent, representing two
chromosomes joined together. In Ascaris, in like manner, each of
the two chromosomes of the stem-cell or primordial germ-cells is
probably plurivalent, and represents a combination of several units
of a lower order which separate during the segmentation of the
thread when the somatic mitosis occurs.

D. Growth and Differentiation of the Germ-cells

I. The Ovinn

{a) Grozvth and Nutrition. — Aside from the transformations of the
nucleus, which are considered elsewhere, the story of the ovarian
history of the ^^^ 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 egg 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
Ainceba? More commonly, a definite association is established be-
tween the &g^ and the surrounding cells. In one of the most fre-
quent arrangements the ovarian cells form a regular layer or follicle
about the ovum (Figs. 58, 60), and there is very strong reason to
believe that the follicle-cells are immediately concerned with the con-
veyance of nutriment to the ovum. A number of observers have
maintained that the follicle-cells may actually migrate into the interior
of the &gg, and this seems to be definitely established in the case of
the tunicates.^ Such cases are, in any case, extremely rare ; and,
as a rule, the material elaborated by the nutritive cells is passed
into the (tgg in solution. Very curious and suggestive conditions
occur among the annelids and insects. In the annelids, the nutri-
tive cells often do not form a follicle, but in some forms each Q,g^ is
accompanied by a single nurse-cell, attached to its side, with which
it floats free in the body-cavity. In OpJiryotrocJia, where it has been
carefully described by Korschelt, the nurse-cell is at first much larger

1 Cf. p. 194, Chapter V.

2 It has been asserted that the eggs in such cases feed on the other cells by ingulfing
them bodily, Amoeba-fashion. This is probably an error.

^ See Floderus, '95.
I

114

THE GERM-CELLS

than the egg itself, and contains a large, irregular nucleus, rich in
chromatin (Fig. 57). The egg-cell rapidly grows, apparently at the
expense of the nurse-cell, which becomes reduced to a mere rudi-
ment attached to one side of the Q.^g and finally disappears. There
can hardly be a doubt, as Korschelt maintains, that the nurse-cell is
in some manner connected with the elaboration of food for the grow-
ing egg-cell ; and the intensely chromatic character of the nucleus
is well worthy of note in this connection.

Somewhat similar nurse-cells occur in the insects, where they have
been carefully described by Korschelt. The eggs here lie in a series
in the ovarian " egg-tubes " alternating with nutritive cells vari-

Fig- 57- — Egg and nurse-cell in the annelid, Ophryotrocha. [KORSCHELT.]
A. Young stage, the nurse-cell («), larger than the egg {o), B. Growth of the ovum. C. Late
stage, the nurse-cell degenerating.

ously arranged in different cases. In the butterfly Vanessa, each
Q.g% 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. 58, A). In the ear-wig, Forfic2Lla, the arrangement is still more
remarkable, and recalls that occurring in OphryotrocJia. Here each
Q.g^ 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. 115. In these two cases, again, the nurse-cell is characterized
by the extraordinary development of its nucleus — a fact which
points to an intimate relation between the nucleus and the metabolic
activity of the cell.^

In all these cases it is doubtful whether the nurse-cells are sister-

1 See p. 254.

• GROWTH AND DIFFERENTIATION OF THE GERM-CELLS II5

cells of the Q.g^ 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, LiinibriciLS terres-
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.^

Fig. 58. — Ovarian eggs of insects. [Korschelt.]
A. Egg of the butterfly, Vanessa, surrounded by its follicle ; above, three nurse-cells {n.c^ with
branching nuclei ; g.v. germinal vesicle. B. Egg of water-beetle, Dytlscus, 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.

{b) Differentiation of the Cytoplasm and Deposit of DetUoplasni. —
In the very young ovum the cytoplasm is small in amount and free
from deutoplasm. As the Q,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-mtcleiis appears in the cytoplasm of many
ova, and this is probably concerned in some manner with the growth

1 See McMurrich? '86.

ii6

THE GERM-CELLS

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.

Fig. 59. • — Young ovarian eggs, showing yolk-nuclei and deposit of deutoplasm.

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

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

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

D. Ovarian egg of young duck (3 months) surrounded by a follicle, and containing a " yolk-
nucieus," >'.«. [Mertens.]

Thus Jordan ('93) states that in the newt {Dieinyctyhis) the yolk may
be first formed at one side of the Qgg and afterwards 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
])eriod has an approximately central position. In some amphibia
the deutoplasm appears near the periphery and advances inwards

GROWTH AND DIFFERENTIATION' OF THE GERM-CELLS 11/

towards the nucleus. More commonly it first appears in a zone
surrounding the nucleus (Fig. 59, C, D) and advances thence towards
the periphery (trout, Henneguy ; cephalopods, Ussow). In still others
{e.g. in myriapods, Balbiani) it appears in irregular patches scat-
tered quite irregularly through the ovum (Fig. 59, A). In Bi'ancJii-
P?is the yolk is laid down at the centre of the egg, 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 e.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. Neither do they support the view that the actual polar-
ity of the Qgg exists from the beginning. They probably arise rather
through the varying physiological conditions under which the egg-
formation takes place ; but these have not yet been sufficiently
analyzed.^

The primary origin of the deutoplasm-grains is a question that
really involves the whole theory of cell-action and the relation of
nucleus and cytoplasm in metabolism. The evidence seems per-
fectly clear that in many cases the deutoplasm arises in situ in the
cytoplasm like the zymogen-granules in gland-cells. But there is
now a great body of evidence that seems to show with equal clear-
ness that a part of the egg-cytoplasm is directly or indirectly derived
from the nucleus. There is no question that a large part of the sub-
stance of the germinal vesicle is thrown out into the cytoplasm at the
time of maturation, as shown with especial clearness in the eggs of
amphibia, echinoderms, and some worms {e.g. in Nereis, Fig. 71).
A large number of observers have maintained that a similar giv-
ing off of solid nuclear substance occurs during the earlier stages of
growth ; and these observations are so numerous and some of them
are so careful, that it is impossible to doubt that this process really
takes place. The portions thus cast out of the nucleus have been
described by some authors as actual buds from the nucleus (Bloch-
mann, Scharff, Balbiani, etc.), as separate chromatin-rods (Van Bam-
beke, Erlangef), as portions of the chromatic network (Calkins), or as
nucleoli (Balbiani, Will, Leydig). There is no evidence that such
eliminated nuclear materials directly give rise to deutoplasm-granules.
They would seem, rather, to have the value of food-matters or forma-
tive substances which are afterwards absorbed and elaborated by the
cytoplasm, the deutoplasm being a new deposit in the cytoplasmic
substance. It is, however, a matter of great interest that formed
nuclear elements should be given off into the cytoplasm, in view of
the general role of the nucleus as discussed in Chapter VII.

1 Cf. p. 288.

ii8

THE GERM-CELLS

(c) Yolk-niLcleus.- — The term "yolk-nucleus" has been applied to
various bodies or masses that appear in the cytoplasm of the growing
ovarian egg ; and it must be said that the word has at present no
well-defined meaning. We may distinguish two' extreme types of
"yolk-nuclei" which are connected by various transitional forms.
At one extreme is the yolk-nucleus proper, as originally described by
von Wittich ('45) in the eggs of spiders and later by Balbiani ('93) in

B

Fig. 60.- — Young ovarian eggs of birds and mammals. [Mertens.]
A. Egg of young magpie (8 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 (j), and centrosome. D. Of young thrush surrounded
by follicle and containing besides the nucleus an attraction-sphere and centrosome {s), and a
yolk-nucleus (/. m.). E. Of young chick containing nucleus, attraction-sphere and fatty deuto-
plasm-spheres (black). F. Egg of new-born child, surrounded by follicle and containing nucleus
and attraction-sphere.

those of myriapods, having the form of a single well-defined sphe-
roidal mass which appears at a very early period and persists through-
out the later ovarian history. At the other extreme are " diffused
yolk-nuclei" having the form of numerous irregular and ill-defined
masses scattered through the cytoplasm, as described by Stuhlmann
('86) in the eggs of insects and more recently by Calkins and Foot in
earthworms. An intermediate form is represented in the amphibia

GROWTH AND DIFFERENTIATION OF THE GERM-CELLS I 19

(Jordan, '93) and myriapods (Balbiani, '93), where the ^gg contains a
number of fairly well defined yolk-nuclei. In Lnmbricus the "yolk-
nucleus " first appears as a single irregular deeply staining body
closely applied to the nucleus and afterwards breaks up into numer-
ous smaller bodies (Calkins, '95).

The most diverse accounts have been given of the structure and
origin of these problematical bodies. This is in part owing to the
fact, recently pointed out by Mertens, that two entirely different
structures have been confounded under the one term. One of these
is the attraction-sphere of the young &gg with its centrosome. Such
a "yolk-nucleus" has been described by Balbiani in the eggs of the
myriapod Geophilus (Fig. 59, B). The other is a body, variously
described as arising from the nucleus or in the cytoplasm, which is
not improbably concerned in some manner with the constructive
metabolism involved in the growth of the egg-cytoplasm and perhaps
indirectly concerned with the formation of deutoplasm. It seems
clear that the latter form alone should receive the name of yolk-
nucleus, if indeed the term is worth retaining.

Mertens ('93) has recently described the ova of a number of birds
and mammals (including man) as containing a very distinct attrac-
tion-sphere containing one or more intensely staining centrosomes
(Fig. 60). This has, however, nothing to do with the true yolk-
nucleus which may sometimes be seen in the same Q.gg, lying beside
the attraction-sphere (Fig. 60, D). The latter sooner or later fades
away and disappears. The yolk-nucleus, on the other hand, may long
persist. This observation probably explains the strange result reached
by Balbiani in the case of myriapods {GeopJiihis), where the "yolk-
nucleus" is described as arising by a budding of the nucleus, yet is
identified with an attraction-sphere ! The "yolk-nucleus " of Balbiani
has here the typical appearance of an attraction-sphere, surrounded
by rays and containing two or several centrosomes or centrioles.
Besides this, however, the &gg contains several other bodies which
are described as arising by budding off from the nucleus and per-
haps represent the true yolk-nuclei (Fig. 59, B).

The origin of the yolk-nucleus proper appears to differ in different
cases. Jordan's observations on the newt seem to leave no doubt
that the bodies described as yolk-nuclei in this animal arise in sitit in
the cytoplasm ; and a similar origin of the yolk-nucleus has been
described by a number of earlier observers. On the other hand, a
number of observers have asserted its origin from the nucleus, either
by a process of nuclear budding, by a casting out of the nucleolus of
separate chromatin-rods, or of portions of the chromatic reticulum.
That such a casting-out of nuclear substance occurs during the ova-
rian history of some eggs appears to be well established ; but it is

I20

THE GERM-CELLS

uncertain whether the bodies thus arising have the same physiologi-
cal significance as the "yolk-nuclei" of cytoplasmic origin. Calkins
('95 J i)' working in my laboratory, has brought forward strong evi-
dence that the "yolk-nucleus" of Litnibriciis is derived from a sub-
stance nearly related with chromatin (Fig. 61). The yolk-nucleus

Fig. 61. — Young ovarian eggs of thie earthworm {Lumbricus), showing yolk-nucleus.
{Calkins.]

A. Very early stage ; the irregular yolk-nucleus {y.n.) closely applied to the germinal vesicle
and staining like chromatin. B. Later stage; the yolk-nucleus separating from the germinal
vesicle and changing its staining-power. C. Still later stage; the yolk-nucleus broken up into
rounded bodies staining like the cytoplasm.

here first appears as an irregular granular body lying directly on the
nuclear wall, which in some cases appears to be interrupted, as if yolk-
nucleus and chromatin were directly in continuity. Later the yolk-
nucleus separates from the germinal vesicle and lies beside it in the
cytoplasm. It finally breaks up into a considerable number of sec-
ondary yolk-nuclei scattered through the egg. The action of differ-
ential stains at different periods indicates that the sub.stance of the

GROWTH AND DIFFERENTIATION OF THE GERM- CELTS 121

yolk-nucleus is nearly related with chromatin, if not directly derived
from it. 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 even
after the yolk-nucleus has quite separated from the nuclear mem-
brane. Later, however, as the yolk-nucleus breaks up, it loses its
nuclear staining power, and stains red like the cytoplasm.

This conclusion is, however, disputed in a later work by Foot ('96),
who maintains that the yolk-nucleus in Allolobophora is not of nuclear
but of " archoplasmic " origin, though no relation between it and an
attraction-sphere is established.^ She adds the very interesting dis-
covery that the "polar rings" (cf. p. 150) are probably to be identi-
fied with the yolk-nucleus, or are at least derived from a similar
substance.

Calkins's observations taken in connection with those of Balbiani,
Van Bambeke, and other earlier workers give, however, strong evi-
dence, as I believe, that the "yolk-nucleus" of Liinibriciis is de-
rived, if not from the nucleus, at any rate from a substance nearly
related with chromatin, which is afterwards converted into cyto-
plasmic substance. It is certain, in this case, that the appearance
of the yolk-nucleus is coincident with a rapid growth of cytoplasm ;
but we cannot suppose that the latter grows entirely at the expense
of the yolk-nucleus. More probably the yolk-nucleus supplies certain
materials necessary to constructive metabolism, and it is not impos-
sible that these may be ferments. We may perhaps interpret in
the same manner the elimination of separate nuclear elements {i.e.
not forming a definite yolk-nucleus) as described by Van Bambeke,
Mertens, v. Erlanger, and many earlier writers.

The meaning of the yolk-nuclei of purely cytoplasmic origin is
very obscure, and we have at present really no ground for assigning
to them any particular function. It can only be said that their
appearance coincides in time approximately with the period of great-
est constructive activity in the cytoplasm, but there is no evidence of
their direct participation in the yolk-formation, and we do not know
whether they are active constructive physiological centres, or merely
stores of reserve substances or degeneration-products.

1 Miss Foot's use of the term " archoplasm " largely deprives the word of the definite
meaning attached to it by Boveri. To identify as " archoplasm " everything stained by
Lyons blue is indeed a broad use of the term.

122 THE GERM-CELLS

2. Formation of the Spermatozoon

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 many cases effected during the
two divisions of the primary spermatocyte. In some cases, however
{e.g. in the salamander), the reduced number appears during the divi-
sion of the spermatogonia and may even appear in the very early
germ-cells (cf. p. 194). The reduction of the chromosomes, which is the
most interesting and significant feature of the process, will be con-
sidered in the following chapter, and we are here only concerned with
the transformation of the spermatid 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 generally agreed, further, that the envelope of the
tail-substance is derived from the cytoplasm of the spermatid.
Beyond this point opinion is still far from unanimous, though it is
probable that the other structures — viz. the axial filament, the

1 The terminology, now almost universally adopted, is due to La Valette St. George. Cf.
Fig. 90.

- See Fig. 91.

GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 1 23

middle-piece, and the point — are likewise of cytoplasmic origin; and
it is certain that the middle-piece is in some cases derived from the
attraction-sphere of the spermatid, and contains the centrosome.

As the spermatid develops into the spermatozoon it assumes an
elongate form, the nucleus lying at one end while the cytoplasm is
drawn out to form the flagellum at the opposite end. The origin of
the axial filament is still in doubt. Many authors (for example,
Flemming and Niessing) have described it as growing out from the
micleus ; but more recent work by Hermann, Moore, and others,
shows that this is probably an error and that the axial filament is
derived from the substance of the attraction-sphere.

The greatest uncertainty relates to the origin of the middle-piece
and the apex. By one set of authors the centrosome is believed to
pass into the point of the spermatozoon (Platner, Field, Benda, Pre-
nant) ; by another set, into the middle-piece (Hermann, Wilcox, Cal-
kins). That the latter is a correct view is absolutely demonstrated
by the fact that during fertilization the centrosome in every accu-
rately known case is derived from the middle-piece (amphibia, echino-
derms, tunicates, earthworm, insects, mollusks, etc.). The observations
of Platner and others in support of the other view are, however, too
detailed to be rejected on this ground alone, and it is not impossible
that the position of the centrosome may vary in different forms. The
uncertainty is due to the difficulty of tracing out the fate of the cen-
trosome and archoplasmic structures of the spermatid. It is certain
that each spermatid receives a centrosome or attraction-sphere from
the preceding amphiaster. But besides the centrosome (attraction-
sphere) the spermatid may also contain a second "achromatic" body
known as the paramicleiis (Nebenkern) or mitosome, which has un-
doubtedly been mistaken for the attraction-sphere in some cases ^ and
to this circumstance the existing confusion may be in part due. The
concurrent results of La Valette St. George, Platner, and several
others have shown that the " Nebenkern " is derived from the re-
mains of the spindle-fibres ; but the most divergent accounts of its
later history have been given by different investigators. According
to Platner's studies on the butterfly Pyg(zra ('89), it consists of a larger
posterior and a smaller anterior body, which he calls respectively the
large and small mitosoma (Fig. 62, C). The former gives rise to the
investment of the axial filament of the tail, the latter to the middle-
piece, while the " centrosome " lies at the anterior end of the nucleus
at the " apex" (Fig. 62, D). Field ('95) reaches an essentially similar
result in the echinoderm spermatozoon, the single " Nebenkern "
forming the middle-piece, while the "centrosome" lies at the tip

1 Compare the confusion between yolk-nucleus and attraction-sphere in the ovum, p. 1 19.

124

THE GERM-CELLS

(Fig. 62, B^. Benda describes the " Nebenkern " in the mammals as
consisting of two parts, one of which passes backward and takes
part in the formation of the tail-envelope, while the other passes
forward to form the apex (head-cap or apical knob) and represents
the attraction-sphere (archoplasm). A somewhat similar account
was given by Platner of the " Nebenkern " of pulmonates. Accord-
ing to the more recent work of Moore on elasmobranchs, both
middle-piece and apex are derived from the attraction-sphere, the
centrosome passing into the former (Fig. 62, A).

The work of Platner and Field appears to have been carefully

C

^ I

Fig. 62. — Formation of the spermatozoon from the spermatid.

A. Late stage of spermatid of the shark Scyllium. [MoORE.]

B. Spermatid of starfish Chcetaster. [FIELD.]

C. Spermatid of butterfly Pygcsra. D. Young spermatozoon of the same. [PLATNER.]

a. apical body; a.f. axial filament; c. "centrosome;" e. envelope of tail ; m. middle-piece
(" small mitosoma" of Platner) ; n. nucleus ; p. paranucleus (" Nebenkern," or " large mitosoma"
of Platner).

done, yet there is good reason to believe that both these observers
are in error, since their results are contradicted by the history of the
spermatozoon in fertilization. As regards the insects, Henking's
observations on the fertilization of the butterfly Pieris leave little
doubt that the sperm-centrosome is here derived from the middle-
piece; and, moreover, in the grasshopper Calopteniis, Wilcox ('95) has
traced the centrosome of the spermatid into the middle-piece. In
the case of echinoderms, Boveri, Mathews, and myself, confirmed by
several later observers, have independently traced the sperm-centro-
some to the middle-piece during fertilization, and have shown that

GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 1 25

Fol was in error in referring it to the tip. Field's conclusion is there-
fore almost certainly erroneous, and he has probably confounded the
centrosome with the " Nebenkern " or paranucleus.

Diametrically opposed, moreover, to the results of Platner and
Field are those of Hermann ('89) and Calkins ('95, 2) on amphibia
and earthworms, and
both these observers
have devoted especial
attention to the origin
of the middle-piece.
The evidence brought
forward by the last-
named author, whose
preparations I have
critically examined,
seems perfectly con-
clusive that the at-
traction-sphere or
centrosome passes
into the middle-piece.
The "Nebenkern,"
which is rarely pres-
ent, appears in this
case to take no part
in the formation of
the flagellum, but
degenerates without
further change. In
the salamander the
origin of the middle-
piece has been care-
fully studied by
Flemming and Her-
mann. The latter
('89) has traced- the
middle-piece back to
an "accessory body"
(Nebenkorper), which he believes to be not a " Nebenkern "
(derived from the spindle-fibres), but an attraction-sphere derived
from the aster of the preceding division, as in Lumbriciis. This
body differs greatly from an ordinary attraction-sphere, consisting
of the following three parts lying side by side in the cytoplasm
(Fig. 63). These are : {a) a colourless sphere, {b') a minute rounded
body which stains red with saffranin like the nucleoli or plasmo-

Fig. 63. — Formation of the spermatozoon from the sper-
matid in the salamander. [HERMANN.]

A. Young spermatid showing the nucleus above, and below
the colorless 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.

126 THE GERM-CELLS

somes of the spermatid-nucleus, {c) a ring-shaped structure staining
purple with gentian violet, like the chromatin. The colourless sphere
viltimately vanishes, the red rounded body gives rise to the middle-
piece, while the ring gives rise to the envelope (fin) of the flagellum.
The apex or spur is developed from the nuclear membrane.^ Her-
mann's results on the mouse agree in a general way with those
on the salamander ; but the apex (head-cap) is here derived from
the cytoplasm. A " Nebenkorper" lies in the cytoplasm, consisting
of a pale sphere and a smaller deeply staining body. From the
latter arises the " end-knob," which Hermann accordingly homolo-
gizes with the middle-piece of the salamander spermatozoon, and
from it the axial filament appears to grow out into the flagellum.
The colourless sphere disappears as in the salamander, and the
envelope of the axial filament is derived from the cytoplasm. Moore
('95) describes the flagellum of elasmobranchs as growing out from
the attraction-sphere (archoplasm) of the spermatid (Fig. 62, A).

Summary. — ^The foregoing account shows that our positive know-
ledge of the formation of the spermatozoon still rests upon a some-
what slender basis. But despite the discrepancies in existing
accounts, all agree that the spermatozoon arises by a direct meta-
morphosis of the spermatid, receiving from it a nucleus and a
small amount of cytoplasm containing a centrosome or attraction-
sphere. All agree, further, that the middle-piece is of archoplasmic
origin, being derived, according to some authors, from a true attrac-
tion-sphere (or centrosome); according to others, from a ** Neben-
kern" formed from the spindle-fibres. The former account of its
origin is certainly true in some cases. The latter cannot be accepted
without reinvestigation, since it stands in contradiction to what is
known of the middle-piece in fertilization, and is possibly due to
a confusion between attraction-sphere and " Nebenkern." Similar
doubts exist in regard to the origin of the apex, which is variously
described as arising from the nuclear membrane, from the general
cytoplasm, from the " Nebenkern," and from the centrosome.

Most late observers agree, further, that the flagellum is developed
in intimate relation with the archoplasmic material (attraction-sphere
or " Nebenkern "). This conclusion tallies with that of Strasburger,
who regards the flagella of plant-spermatozoids as derived from the
" kinoplasm " (archoplasm), and it is of especial interest in view of
Van Beneden's hypothesis of the contractility of the archoplasm-
fibrillae. It is, however, possible that the axial filament may be
derived from the nucleus, in which case it would have an origin
comparable with that of the spindle-fibres in many forms of mitosis.

1 Flemming described the middle-piece as arising inside the nucleus ; but Hermann's
observations leave no doubt that this was an error.

STAINING-REACTIONS OF THE GERM-NUCLEI 12/

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
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 afterwards 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, carmin), 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 ^^g
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
cannot 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 (haematoxylin, gentian
violet), red (saffranin), or yellow (iodine), and it is here a manifest
absurdity to speak of " cyanophilous " chromatin (cf. p. 243). 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 alike 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),i 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
described at pp. 245, 246. 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

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

128 THE GERM-CELLS

similar series of facts has been observed in the germ-nuclei of plants
by Strasburger (p. 163). 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
the difference of the staining-reaction is probably due to the fact
that the sperm-chromatin consists of pure or nearly pure nucleic acid,
while the egg-chromatin is a nuclein containing a much higher per-
centage of albumin.

LITERATURE. Ill

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

Arch. Mik. Atiat. XXXII. , 1888; 2. {insects) Zeitschr. Wiss. Zool., L., 1890;

3. {fishes, amphibia, reptiles) Arch. Mik. Anat., XXXVI., 1890; 4. {mafn-

mals) Zeit. IViss. Zool., LI I., 1891.
Van Beneden, E. — Recherches sur la composition et la signification de I'oeuf : Metn.

cour. de VAcad. roy. de s. de Belgique, 1870.
Boveri, Th. — Uber Differenzierung der Zellkerne wahrend der Furcliung des Eies

von Ascaris meg. : Ana/. Anz., 1887.
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 iind Bonnefs Ergebnisse, II., 1892.
Kolliker, A. — Beitrage zur Kenntniss der Geschlechtsverhaltnisse und der Samen-

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

stande ; Zool. Jahrb., III. 1889.
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.
Waldeyer, W. — Eierstock und Ei. Leipzig, 1870.
Id. — Bau und Entwickelung der Samenfaden: Verh. d. Anat. Ges. Leipzig, 1887.

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 v^'eb 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 duplicate 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 probable, 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 gradually comes to an end and is only restored by an
admixtitre of living matter derived from another cell. This operation,
known z.'S, fertilization ox fecimdation, is the essence of sexual repro-
duction ; 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. According to the older and more familiar
"dynamic " hypothesis, brought forward by Blitschli i^T^) and Minot
^TT': '79) '^'^^ afterwards supported by such investigators as Engel-
mann, Hensen, Hertwig, and Maupas, the essential end of sexuality
is rejuvenescence, i.e. the restoration of the growth-energy and the
inauguration of a new cycle of cell-division. Maupas's celebrated
experiments on the conjugation of Infusoria, although not yet ade-

1 Evolution, in Science and Culture, p. 296, from Enc. Brit., 1878.
K 129

I30 FERTILIZATION OF THE OVUM

quately confirmed, 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 conjugation or fertilization actually has such a
dynamic effect is disputed by no one. What is not determined is
whether this is the primary motive for the process- — -i.e. whether
the need of fertilization is a primary attribute of living matter — or
whether it has been secondarily acquired in order to ensure a mixture
of germ-plasms derived from different sources. The latter view has
been urged with great force by Weismann, who rejects the rejuve-
nescence theory in toto and considers the essential end of fertilization
to be a mixture of germ-plasms ("Amphimixis") as a means for the
production, or rather multiplication, of variations which form the
material on which selection operates. On the other hand, Hatschek
('87, i) sees in fertilization exactly the converse function of checking
variations and holding the species true to the specific type. The
present state of knowledge does not, I believe, allow of a decision
between these diverse views, and the admission must be made that
the essential nature of sexual reproduction must remain undetermined
until the subject shall have been far more thoroughly investigated,
especially in the unicellular forms, where the key to the ultimate
problem is undoubtedly to be sought.

A. General Sketch

Among the unicellular plants and animals, fertilization is effected
by means of conjugation, a process in which two or more individuals
permanently fuse together, or in which two unite temporarily and
effect an exchange of nuclear matter, after which they separate, hi
all the higher forms fertilization consists in the permanent fusion of
two germ-cells, one of paternal and one of maternal origi^i. We may
first consider the fertilization 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.

Leeuwenhoek, whose pupil Hamm discovered the spermatozoa
(1677), put forth the conjecture that the spermatozoon must pene-
trate into the o.gg ; but the process was not actually seen until nearly
two centuries later (1854), when Newport observed it in the case of
the frog's (tgg ; and it was described by Pringsheim a year later in one
of the lower plants, CEdigonijim. The first adequate description of
the process was given by Hermann Fol, in 1879,^ though many

^ See P Henogenie, pp. 124 ff., for a full historical account.

GENERAL SKETCH

I.^I

earlier observers, from the time of Martin Barry ('43) onwards, had
seen the spermatozoa inside the egg-envelopes, or asserted its entrance
into the Q.g^.

In many cases the entire spermatozoon enters the o^g^ (mollusks,
insects, nematodes, some annelids, Petroniyzon, axolotl, etc.), and in
such cases the long flagellum may sometimes be seen coiled within
the Q.gg (Fig. 64). Only the nucleus and middle-piece, however, are
concerned in the actual fertilization ; and there are some cases
(echinoderms) in which the tail is left outside the ^gg. At or near

Fig. 64. — Fertilization of the egg of the snail Physa. [Kostanecki and WiERZEjSKi.]
A. The entire spermatozoon lies in the &gg, 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 afterwards disappear, their place being taken by those of the spermatozoon.

the time of fertilization, the Q.gg successively segments off at the upper
pole two minute cells, known as the polar bodies (Figs. 64, 65, 89) 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
matjiration, by which the Q.gg is prepared for fertilization, and we
may defer its consideration to the following chapter.

132 FERTILIZATION OF THE OVUAI

I. The Genn-nuclei in Fei'tilization

The modern era in the study of fertiUzation may be said to begin
with Oscar Hertwig's discovery, in 1875, of the fate of the sperma-
tozoon within the Q^^. Earlier observers had, it is true, paved the
way by showing that, at the time of fertilization, the Q.gg contains
two nuclei that fuse together or become closely associated before
development begins. (Warneck, Biitschli, Auerbach, Van Beneden,
Strasburger.) Hertwig discovered, in the &gg of the sea-urchin
{Toxopiieustes lividiis), that one of these nuclei belongs to the egg,
zvJiile the other is derived from the spennatosoon. 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 kjiown case an
essential phenomenon of fertilization is the imion of a sperm-nucleus,
of pater)uil origin, zvith an egg-mtclens, of matei^nal origin, to form the
piamary nucleus of the embryo. This nucleus, known as the cleavage-
or segmentation-nucleits, gives rise by division to all tJie nuclei of the
body, and hence every nucleus of the child may contain mtclear substance
derived from botJi parents. And thus Hertwig was led to the conclu-
sion ('84), independently reached at the same time by Strasburger,
Kolliker, 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, Ascai'is mcgalocephala, the &^g 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 struct-
ure 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 jDarallel 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.
65) : After the entrance of the spermatozoon, and during the for-
mation of the polar bodies, the sperm-nucleus rapidly enlarges and

GENERAL SKETCH

133

E F

Fig- 65. — Fertilization of the <tgg of Ascaris megalocephala, var. bivalens. [BOVERI.] (For
later stages see Fig. 104.)

A. The spermatozoon has entered the egg, its nucleus is shown at cT; 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 (?, J) in the reticu-
lar stage ; the attraction-sphere {a) contains the dividing centrosome. C. Chromosomes forming
m the germ-nuclei ; the centrosome divided. D. Each germ-nucleus resolved into two chromo-
somes ; attraction-sphere {a) double. E. Mitodc figure forming for the first cleavage ; the chro-
mosomes {c) already split. F. First cleavage in progress, showing divergence of the daughter-
chromosomes towards the spindle-poles (only three chromosomes shown).

134

FERTILIZATION OF THE OVUM

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 (Figs. 65, 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, tJie chromatic portion of which has
been synthetically formed by tJie imion of tivo equal germ-Jiuclei. The

A

B

Fig. 66. — Germ-nuclei and chromosomes in the eggs of nematodes. [Carnoy.]
A. Egg of nematode parasitic in Scylliuni ; the two germ-nuclei in apposition, each containing
four chromosomes ; the two polar bodies above. B. 'Egg oi Fi/aroides ; each germ-nucleus with
eight chromosomes; polar bodies above, deutoplasm-spheres below.

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.

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 Coronilla
4, in Ophiostomnm 6, and in Filaroides 8. A little later Boveri

GENERAL SKETCH 1 35

('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 the medusa
Tiara 14, and in the vi\oVvVi^\L Pterotrachea 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.
In the onion the number is 8 (Strasburger) ; in the annelid OpJiryo-
trocha it is only 2 from each sex (Korschelt). In all these cases the
number contributed by each is one-half the number cJiaracteristic of the
body-cells. The union of two germ-cells thus restores the normal
number, and thus we find the explanation of the remarkable fact
commented on at p. 48 that tJie number of chromosomes iji sexually
produced organisms is alzuays 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
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. And thus we are finally led to the view that chromatin is the
physical basis of inheritance, and that the smallest visible units
of structure by which inheritance is effected are to be sought in the
chromatin-granules or chromomeres.

2. The Centrosome iii Fertilization

The origin of the centrosomes and of the amphiaster, by means of
which the paternal and maternal chromosomes are distributed and
the Q.^'g divides, is still in some measure a matter of dispute. In a
large number of cases, however, it is certainly known that the egg-cen-
trosome disappears before or during fertilization and its place is taken
by a new centrosome zvhich is introduced by the spermatozoon and
divides into two to form the cleavage-anipJiiaster. This has been
conclusively demonstrated in several forms (various echinoderms,
annelids, nematodes, tunicates, mollusks, and vertebrates) and estab-
lished with a high degree of probability in many others (insects, Crus-
tacea). In every accurately known case, moreover, the centrosome
has been traced to the middle-piece of the spermatozoon ; e.g. in
sea-urchins (Hertwig, Boveri, Wilson, Mathews, Hill), in the axolotl
(Fick), in the tunicate Phallusia (Hill), probably in the earthworm,

1 Cf. p. 154.

136

FERTILIZATION OF THE OVUM

AllolobopJiora (Foot), in the butterfly Pieris (Henking), and in the
gasteropod Physa (Kostanecki and Wierzejski). The agreement
between forms so diverse is very strong evidence that this must be
regarded as the typical derivation of the centrosome.^

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

Fig. 67. — 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 d"). C. The two germ-nuclei (cf, ?) 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.

nomena in the sea-urchin Toxopneitstes (Fig. 69). As described at
p. 146, the tail is in this case left outside, and only the head and
middle-piece enter the Q.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

1 Cf. p. 156.

GENERAL SKETCH

137

1 80°, SO that the pointed end is directed outward and the middle-
piece is turned inward (Fig. 69 A-F)} During the rotation a minute
aster is developed about the middle-piece as a centre, and at the

s&S

B

Fig. 68. — Fertilization of the &gg of the gasteropod Pterotrachea. [BOVERI.]
A. The egg-nucleus {E) and sperm-nucleus (^) approaching after formation of the polar
bodies ; the latter shown above {P.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.

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.

138

FERTILIZATION OF THE OVUM

central point a minute intensely staining centrosome may be seen.^
As the sperm-nucleus advances, the aster leads the way, and at the
same time rapidly grows, its rays extending far out into the cytoplasm
and finally traversing nearly an entire hemisphere of the ^gg. The
central mass of the aster comes in contact with the egg-nucleus, di-
vides 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. 70). The nuclei now fuse
to form the cleavage-nucleus. Shortly afterwards the nuclear mem-
brane fades away, a spindle is developed between the asters, and

B

C

D yf.

fff

Fig. 69. — Entrance and rotation of the sperm-head and formation of the sperm-aster in the
sea-urchin Toxopneustes (A.-F., X i6qo; G. //., X 800).

A. Sperm-head before entrance; n, nucleus; m, middle-piece and part of the flagellum.
B. C. Immediately after entrance, showing entrance-cone. D.-F. Rotation of the spei'm-head,
formation of the sperm-aster about the middle-piece (the minute centrosome not shown).
G.H. Approach of the germ-nuclei; growth of the aster.

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. Throughout these changes no trace of an egg-centrosome
is to be discovered. This centrosome, though present in" earlier stages,
has been lost after the polar bodies were formed by the ovarian Qgg.

^ I was unable to find such a centrosome in Toxopneustes, but the observations of Boveri
and Hill prove that it is certainly present in other sea-urchins, and I now believe my own
account to have been at fault in this respect.

GENERAL SKETCH

139

The facts just described are now known to be typical of a large
number of cases. We may, however, distinguish two types of ferti-

Fig. 70. — Conjugation of the germ-nuclei and division of tlie sperm-aster in the sea-urchin
Toxopneustes, X looo. (For later stages see Fig. 37.)

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

lization according as the polar-bodies are formed before or after the
entrance of the spermatozoon. In the first case, well illustrated by
the sea-urchin (Fig. 69), the germ-nuclei conjugate immediately after

140 FERTILIZATIOX OF THE OVUM

entrance of the spermatozoon. In the second and more frequent case
{Ascaris, Fig. 65 ; Physa, Fig. 64; Nereis, Fig. 71 ; Cyclops, Fig. 72),
the sperm-nucleus penetrates for a certain distance, often to the cen-
tre of the Qgg, and then pauses while the polar bodies are formed.
It then conjugates with the reformed egg-nucleus. In this case,
the sperm-aster always divides to form an amphiaster before conju-
gation of the nuclei, while in the first case the aster may be still
undi^iided 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 haxdng,
in the second case, time to form during extrusion of the polar bodies.

It is an interesting and significant fact that the aster or amphiaster
always leads the way in the march towards the egg-nucleus ; and in
many cases it may be far in advance of the sperm-nucleus.^ Boveri
("88, I ) has obser\"ed in sea-urchins that the sperm-nucleus may indeed
be left entirely behind, the aster alone conjugating with the egg-
nucleus and causing division of the Qgg zvithoiit 7inion of the germ-
nuclei, though the sperm-nucleus afterwards 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 demonstration of the fact that it is tJie centro-
some alone tJiat causes division of the egg, and it is therefore tJie fer-
tilising eletnent proper (Boveri, '87, 2). We may therefore conclude
that the end of fertilization is the union of the germ-nuclei and the
equal distribution of their substance, while the active agent in this
process is the centrosome.

The earliest investigators of fertilization, such as Biitschli and Fol,
had no knowledge of the centrosome, and hence no clear idea as to
the origin of the asters, but Fol stated in 1873 that the asters repre-
sented "centres of attraction" lying outside and independent of the
nucleus. Oscar Hertwig showed, in 1875, that in the sea-urchin &gg
the amphiaster arises by the division of a single aster that first
appears near the sperm-nucleus and accompanies it in its progress
toward the egg-nucleus. A similar obser\^ation was soon afterwards
made by Fol ('79) in the eggs of Asterias and Sagitta, and in the
latter case he determined the fact that the astral rays do not centre
in the nucleus, as Hertwig described, bnt at a point in advance of it,
— a fact aftersvards confirmed by Hertwig himself and by Boveri
f'88, I). Hertwig and Fol afterwards found that in cases of poly-
spermy, when several spermatozoa enter the o-gg, each sperm-nucleus
is accompanied by an aster, and Hertwig proved that each of these
might give rise to an amphiaster (Fig. 75).

1 Cf. Kostanecki and Wierzejski, '96.

GEXERAL SKETCH

141

It was Boveri {^^J) who first accurately traced the complete history
of the centrosome and clearly formulated the facts, proving that in
Ascaris a single centrosome is brought in by the spermatozoon and
that it divides to form two centres about which are developed the two

Fig. 71. — Fertilization of the egg oi Nereis, from sections. (X 400.)
A. Soon after the entrance of the spermatozoon, showing tlie minute sperm-nucleus at o^, the
germinal vesicle disappearing, and the first polar mitotic figure forming. The empt}' spaces repre-
sent deutoplasm-spheres (slightly swollen by the reagents), the firm circles oil-drops. B. Sperm-
nucleus {S) 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. 64).

asters of the cleavage-figure. He was thus led to the following con-
clusion, which I believe still accurately expresses the truth : ''The
ripe egg possesses all of the organs and qualities neeessary for division
excepting the centrosome, by zuhich division is initiated. The sPernia-

142

FERTILIZATION OF THE OVUM

tozodu, on the other Jiand, is provided zvith a centrosome, but lacks tJie
substance in which this organ of division may exert its activity.
Through tJie tmion of the two cells in fertilization all of the essential
organs necessaiy for division are broiLght together ; the egg now contains
a centrosome which by its ozvn division leads the zvay in the embryonic
developnicjit} Boveri did not actually follow the disappearance of
the egg-centrosome, but nearly at the same time this process was
carefully described by Vejdovsky in the case of a fresh-water annelid
Rhynclielmis. Here, again, very strong evidence was brought for-

Fig. 72. — Fertilization of the egg in the copepod Cyclops stretiuus. [RUCKERT.]
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
reformed 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 afterwards developed between the two
enoimous sperm-asters ; polar body at the left.

ward to show that the cleavage-amphiaster arises by the division of
a single sperm-aster. Very numerous observations to the same effect
have been made by later observers. Bohm could find in Petromyson
('88) 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 i^%G) had already made similar observations in the snail
Ariojt, and the same result was soon afterwards reached by Brauer
('92) in the case of Branchipus, and by Julin ('93) in Styleopsis.
Pick's careful study of the fertilization of the axolotl ('93) proved in

1 '87, 2, p. 155.

GENERAL SKETCH 1 43

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
7niddle-piece as a centre. The same result was indicated by Foot's
observations on the earthworm ('94), and it was soon afterwards
conclusively demonstrated in echinoderms through the independent
and nearly simultaneous researches of myself on the Q.g^ of Toxo-
pneustes, of Mathews on ATbacia, and of Boveri on Echinus. Nearly
at the same time a careful study was made by Mead ('95) of the
annelid Chceoptems, and of the starfish Asterias by Mathews, both
observers independently showing that the polar spindle contains dis-
tinct 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 Rhynchelmis. Exactly the same result has since been reached by
Hill ('95) in Sph(Ei'echimis and the tunicate Plialliisia, and by Kos-
tanecki and Wierzejski ('96) in Pliysa (Fig. 64) ; and in all of these
the centrosome is likewise shown to arise from the middle-piece.
The origin of the centrosome from the spermatozoon alone has also
been shown by Riickert ('95, 2) in Cyclops (Fig. 72), and is indicated
by Sobotta's work ('95) on the fertilization of the mouse (Fig. ^j).

Such an array of evidence, derived from the study of so many
diverse groups, places Boveri's conception of fertilization (p. 141) on
a very strong foundation, and justifies the conclusion that the origin
of the first cleavage-centrosomes from the spermatozoon alone is a
phenomenon of very wide, if not of universal, occurrence. The
descendants of these centrosomes may be traced continuously into
later cleavage-stages, and there can be little doubt that they are the
progenitors of all the centrosomes of the adult body. Boveri and
Van Beneden, followed by a number of later observers,^ have followed
the daughter-centrosomes through every stage of the first cleavage
into the blastomeres of the two-cell stage, where they persist and give
rise to the centrosomes of the four-cell stage, and so on in later stages.
This is beautifully shown in the Q.g^ of Thalassema (Fig. 73), which
has been carefully followed out in my laboratory by Mr. B. B. Griffin.
The centrosome is here 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
cleavage before the first cleavage takes place (Fig. 73) ! The minute
centrosomes of the second cleavage are therefore the direct descendants
of the sperm-centrosome ; and there is good reason to believe that the

1 See Mead on Chaoptei'us^ '95, and Kostanecki and Wierzejski on Physa, '96.

144

FERTILIZATION OF THE OVUM

continuity is not broken in later stages. An exactly similar process
is described by Kostanecki and Wierzejski in the egg of Pliysa. We
thus reach the following remarkable conclusion : During cleavage
the cytoplasm of the blastomeres is derived from that of the egg, the
centrosonies from the spermatozoon, while the nuclei {chi'omatiii) are

Fig. 172. — Persistence of the centrosomes from cell to cell, in the cleavage of the egg of the
gephyrean Thalassema. [Gkiffin.]

A. Mitotic figure for the first cleavage ; the centrosome already double in each ceritrosphere
(the small black bodies are deutoplasni-spheres). B. Early anaphase ; migration of the centro-
somes to the periphery of the centrosphere. C. Middle anaphase (only one-half of the mitotic
figure shown) ; daughter-amphiaster already formed. D. Telophase ; the egg dividing and nuclei
reforming; the old amphiaster has disappeared, leaving only the daughter-amphiaster in each cell.

UNION OF THE GERM-CELLS 1 45

equally derived frovi both germ-cells ; and certainly it would be hard
to find more convincing evidence that the chromatin is the controlling
factor in the cell by which its specific character is determined.

We now proceed to a more detailed and critical examination of
fertilization.

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 mxany 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-
dence of a definite attraction between the germ-cells, which is
in some cases so marked (for example in the polyp Renilld) 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
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 any point; but there are
some cases in which the point of entrance is predetermined by the
presence of special structures through which the spermatozoon

146

FERTILIZATION OF THE OVUM

enters (Fig. 74). Thus, the starfish &^g, according to Fol, pos-
sesses before fertiHzation a peculiar protoplasmic " attraction-cone "
to which the head of the spermatozoon becomes attached, and through
which it enters the Q.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 towards the point at which the spermatozoon strikes
the &^g and there forming a conical elevation into which the sperm-
head passes. In the sea-urchin (Fig. 74) this structure persists
only a short time after the spermatozoon enters, soon assuming a

/ —

m

n-
a

Fig. 74. — Entrance of the spermatozoon into the egg. A.-G. In the sea-urchin ToA-f^j^wf^/jfef.
H. In the medusa Mitrocoma. [METSCHNIKOFF.] /. In the star-fish Asterias. [FOL.]

A. Spermatozoon of Toxop7ieustes, X 2000; a, the apical body, n, nucleus, m, middle-piece,
f, flagellum. B. Contact with the egg-periphery. C. D. Entrance of the head, formation of the
entfrince-cone and of the vitelline membrane (ti), 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.

ragged flame-shape and breaking up into slender rays. In some
cases the ^'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 AmpJiioxits, 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.

UNION OF THE GERM-CELLS

147

as in echinoderms, mammals, many annelids, etc., while in others
several may enter (insects, elasmobranchs, reptiles, the earthworm,
Petromyzon, 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

Fig. 75. — Pathological polyspermy.

A. Polyspermy in the egg oi Ascaris ; below, the egg-nucleus ; above, three entire spermatozoa
within 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.]

centrosomes (Fig. 75). 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 &g'g 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

148 FERTILIZATION OF THE OVUM

fourth cleavage. Such embryos develop as far as the blastula stage,
but never form a gastrula.^ In cases where several spermatozoa
normally enter the ^gg (physiological polyspermy), only one of the
sperm-nuclei normally unites with the egg-nucleus, the supernumerary
sperm-nuclei either degenerating, or in rare cases — e.g. in elasmo-
branchs and reptiles — living for a time and even dividing to form
"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 O-gg. This is indi-
cated by the following facts. Immature eggs, before the formation
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
(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 by
the polar-bodies after the entrance of the first spermatozoon ; and
•Calberla suggested {Petromyzoii) 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 egg
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.

1 For an account of the internal changes, see p. 261.

2 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 Ilertwig on the ground stated at p. 97.

'•"• 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 1 49

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 egg (echinoderms, Fig. 74).
In other cases the entire spermatozoon enters (amphibia, earthworm,
insects, etc.. Fig. 64), 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. 127). The most important and signifi-
cant result, however, is an immediate resumption by the sperm-nucleus
and sperm-centrosome of the pozver 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
only after formation of the polar bodies ; for when in sea-urchins the
spermatozoa enter immature eggs, as they freely do, they penetrate
but a short distance, and no further change occurs.

ib) The Ointm. — 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 vitelline 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 immediately
after fertilization a peculiar viscid character which it afterwards
loses. In many cases the egg contracts, performs amoeboid move-
ments, or shows wave-like changes of form. Again, the egg-cyto-
plasm may show active streaming movements, as in the formation of
the entrance-cone in echinoderms, or in the flow of peripheral proto-
plasm towards 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-shaped mass
of deeply staining material extending outwards to the periphery.
This has been carefully described by Foot ('94) in the earthworm,

1 I have often observed that the formation of the membrane, in Toxopneustes, proceeds
like a wave from the entrance-point around the periphery, but this is often irregular.

150

FERTTUZATION OF THE OVUM

where it is very large and conspicuous,, and I have since observed it
also in the sea-urchin (Fig. 69).

The most profound change in the ovurai 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
ibef ore 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 €^g^ the germinal vesicle occupies a central position, the

yolk, consisting of deutoplasm-
spheres and oil-globules, is uni-
formly distributed, and at the
periphery of the egg is a zone of
clear perivitelline protoplasm (Fig.
43). Soon after entrance of the
spermatozoon the germinal vesicle
moves towards the periphery, its
membrane fades away, and a radi-
ally directed mitotic figure appears,,
by means of which the first polar
body is formed (Fig. 71). Mean-
while the protoplasm flows towards
the upper pole, the perivitelline
zone disappears, and the Q.^g now
shows a sharply marked polar
differentiation. A remarkable phe-
nomenon, described by Whitman
in the leech ('78), 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
towards the poles, the upper one surrounding the point at which the
polar bodies are formed (Fig. y6). 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. 121).

Fig. 76. — Egg of the leech Clepsine, dur-
ing fertihzation. [Whitman.]

p.b., polar bodies \p.r., polar rings ; cleav-
age-nucleus near the centre.

UNION OF THE GERM-CELLS 151

2. Paths of the Gerni-miclei {Pjv-niiclei) ^

After the entrance of the spermatozoon both germ-nuclei move
through the egg-cytoplasm and finally meet one another. The paths
traversed by each 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 Q.gg and then
pauses while the germinal vesicle moves towards the periphery, and
gives rise to the polar-bodies [Ascaris, annelids, etc.). This significant
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 at-
traction. A second important point, first pointed out by Roux, is
that the path of the sperm-nucleus is curved, its " entrance-path "
into the &g^ forming a considerable angle with its " copulation-path "
towards the egg-nucleus.

These facts are well illustrated in the sea-urchin Q.g^ (Fig- 77),
where the egg-nucleus occupies an eccentric position near the point
at which the polar bodies are formed (before fertilization). Entering
the &g^ 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.
towards a point near the centre of the ^^^. After penetrating a
certain distance its direction changes slightly to that of the copulation-
path, which, again, is directed not precisely towards the egg-nucleus,
but towards a meeting-point where it comes in contact with the
egg-nucleus. The latter does not begin to move until the entrance-
path of the sperm-nucleus changes to the copulation-path. It then
begins to move slowly in a somewhat curved path towards 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 Q.g^.

These facts indicate that the paths of the germ-nuclei are deter-

1 The terms "female pro-nucleus," "male pro-nucleus" (Van Beneden), are often ap-
plied to the germ-nuclei before their union. These should, I think, be rejected in favour of
Hertvvig's terms egg-nucleus ■scn.A. sperm-nudetis, on two grounds: (l) 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. 196), 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. 183).

152

FERTILIZATION OF THE OVUM

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

Fig. 77. — 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 ? ; 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.

the entrance-path of the sperm-nucleus, while both factors probably
operate in the determination of the copulation-path along which it

The real nature of neither factor

travels to meet the egg-nucleus,
is known.

UNION OF THE GERM-CELLS I 53

Hertwig first called attention to the fact — which is easy to observe in the living
sea-urchin egg — 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 are insignificant as compared with the nuclei (as in plants). I have
often observed in cases of dispermy in the sea-urchin, that both sperm-nuclei move
at an equal pace towards 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 character.

3. Union of the Germ-miclei. 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 segmentation-micleiis. As early as 1881, however,
Mark clearly showed that in the slug Lhnax 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 Ascai^is,
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-
cerned, a true fusion of the nuclei never takes place during fertiliza-
tion, and that the paternal and maternal chromatin may remain
separate and distinct in the later stages of development — possibly
throughout life (p. 219). In this regard two general classes may be
distinguished. Tn one, exemplified by some echinoderms, by Amphi-
0X71S, Phalhisia, 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. 65) and other nematodes, by Cyclops {Y'lg. 72),
and by Pterotrachea (Fig. 68), 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-

154

FERTILIZATION OF THE OVUM

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.i 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, the nttmbe^' of cJiromosonies arising from the
germ-miclei is always the same in both, and is one-half the number
characteristic of the tissue-cells of the species. By tJieir union, there-
fore, the germ-nuclei give rise to an equatoi^ial 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.

A Partial List showing the Number of Chromosomes Char-
acteristic OF THE Germ-Nuclei and Somatic Nuclei in
Various Plants and Animals.^

Germ-

Somatic

Name.

Group.

Authority.

Nuclei.

Nuclei.

I

2

Ascaris megalocephala,
var. univalens.

Nematodes.

Van Beneden,
Boveri.

2

4

Id., var. bivalens.

71

»

It

Ophryotrocha.

Annelids.

Korschelt.

[,,]

Styleopsis.

Tunicates.

Julin.

8

Coronilla.

Nematodes.

Carnoy.

)?

Pallavicinia.

HepaticcB.

Farmer.

12

Spiroptera.

Nematodes.

Carnoy.

[»]

Gryllotalpa.

Insects.

vom Rath.

»

Caloptenus.

jy

Wilcox.

['']

■>■>

ALquorea..

Hydromedusae.

Hacker.

i6

Filaroides.

Nematodes.

Carnoy.

5J

Hydrophilus.

Insects.

vom Rath.

It

Phallusia.

Tunicates.

Hill.

J7

Li max.

Gasteropods.

vom Rath.

U

Rat.

Mammals.

Moore.

[.]

Ox, guinea-pig, man.

»j

Bardeleben.

J7

Ceratozamia.

Cycads.

Overton.

:7

Pinus.

Coniferas.

Dixon.

1 Indeed, Boveri has found that in Ascaris both modes occur, though the fusion of the
germ-nuclei is exceptionah (Cf. p. 2i6.)

^ The above table is compiled from papers both on fertilization and maturation. Num-
bers in brackets are inferred.

UNION OF THE GERM-CELLS

155

Germ-

Somatic

Name.

Group.

Authority.

Nuclei.

Nuclei.

8

16

Scilla, Triticum.

Angiosperms.

Overton.

11

11

Allium.

It

Strasburger,
Guignard.

9

18

Echinus.

Echinoderms.

Boveri.

7>

??

Sagitta.

Chaetognaths.

■It

jj

?J

Ascidia.

Tunicates.

tt

11

[22]

Allolobophora.

Annelids.

Foot.

II (12)

22 (24)

Cyclops strenuus. -

Copepods.

Ruckert.

12

24

brevicornis.

II

Hacker.

7?

■)•>

Helix.

Gasteropods.

Platner, vom Rath.

?)

V

Branchipus.

Crustacea.

Brauer.

7?

[»]

Pyrrhocoris.

Insects.

Henking.

5j

jj

Salmo.

Teleosts.

Bohm.

J?

J"

Salamandra.

Amphibia.

Flemming.

5j

??

Rana.

■>■!

vom Rath.

jy

;»

Mouse.

Mammals.

Sobotta.

yj

75

Osmunda.

Ferns.

Strasburger.

??

?7

Lilium.

Angiosperms.

Strasburger,
Guignard.

?j

It

Helleborus.

It

Strasburger.

??

V

Leucojum, Pseonia,
Aconitum.

f>

Overton.

14

28

Tiara.

Hydromedusas.

Boveri.

16

32

Pterotrachea, Carinaria,

Phyllirhoe.

Gastropods.

?5

?>

DO

Diaptomus, Heterocope.

Copepods.

Riickert.

)7

D,]

Anomalocera, Euchasta.

It

vom Rath.

??

DO

Lumbricus.

Annelids.

Calkins.

18

36

Torpedo, Pristiurus.

Elasmobranchs.

Ruckert.

[18(19)]

36(38)

Toxopneustes.

Echinoderms.

Wilson.

84

168

Artemia.

Crustacea.

Brauer.

The above data are drawn from sources so diverse and show so
remarkable a uniformity as to estabhsh 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. 61).
It is probably the case with the gasteropod Arion, where, as described
by Plainer, 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 alike 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-

1 '89, pp. 10, 33.

156 FERTILIZATION OF THE OVUM

nal chromatin, but also the chromosomes, may retain their individu-
ahty throughout development.^ Van Beneden, the pioneer observer in
this direction, was unable to follow the paternal and maternal chro-
matin beyond the first cleavage-nucleus, though he surmised that they
remained distinct in later stages as well ; and 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. 105).
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 egg of variety bivaleus, having two chromosomes, be
fertilized with the spermatozoon of variety imivalens. having one
chromosome, the three chromosomes reappear at each cleavage, at
least as far as the twelve-cell stage (Fig. 106); 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. Centrosome and Archoplasm in Fertilization

We have now finally to consider more critically the history of the
centrosomes in fertilization, already briefly reviewed at p. 135. The
account there given considers only the more usual and typical history
of the centrosome, viz. the degeneration of the egg-centrosome and
the introduction of a new centrosome by the spermatozoon. There is,
however, one phenomenon which indicates a priori the possibility that
other modes of fertilization may occur, namely, parthenogenesis, in
which the &^g develops without fertilization. In this case, as Brauer
('93) has clearly shown in Artemia, the egg-centrosome remaining
after the formation of the polar bodies does not degenerate, but divides
into two to form the cleavage-amphiaster. The degeneration of the
egg-centrosome is therefore not a necessary or invariable phenome-
non, and as a matter of fact several accounts have been given of its
persistence and active participation in the process of fertilization.
These accounts fall under three categories, as follows : —

I. Each germ-cell contributes a single centrosome, one of which

^ Cf. p. 219.

CENTROSOME AND ARCHOPLASM IN FERTILIZATION I 57

forms the centre of each aster of the first mitotic figure (Van Beneden,
in Ascaris, 'Zi, '87, p. 270).

2. Each germ-cell contributes two centrosomes (or one which im-
mediately divides into two), which conjugate, paternal with maternal,
to form those of the cleavage-amphiaster (Fol, in sea-urchins, '91 ;
Guignard, in flowering plants, '91 ; Conklin, in gasteropods, '93).

3. The centrosome is derived not from the spermatozoon, but
from the ^^^ (Wheeler, in the case of Myzostoma, '95).

The first of these accounts, which rested rather on surmise than
on adequate observation, may probably be safely rejected, for it con-
tradicts the universal law that the centrosome divides into two before
cell-division, and is unsupported by later observers (Meyer, Erlanger,
etc.). The second view, as embodied in the statements of Fol, Gui-
gnard, and Conklin, demands fuller consideration. All these authors
agree that each germ-cell contributes two centrosomes, or one which
divides into two during fertilization. The daughter-centrosomes thus
formed conjugate two and two in such a manner that each of the
centrosomes of the cleavage-spindle is formed by the union of a cen-
trosome derived from each germ-cell. It is an interesting and sig-
nificant fact that a conjugation of centrosomes was predicted by
Rabl ('89) on the a priori ground that if the centrosome is a perma-
nent cell-organ, as Boveri and Van Beneden maintain, then a union
of germ-cells must involve a union not only of nuclei, but also of
centrosomes. Unusual interest was therefore aroused when Fol, in
1891, under the somewhat dramatic title of the "Quadrille of Cen-
tres," described precisely such a conjugation of centrosomes as Rabl
had predicted. The results of this veteran observer were very posi-
tively and specifically set forth, and were of so logical and con-
sistent a character as to command instant acceptance on the part of
many authorities. Moreover, a precisely similar result was reached
through the careful studies, in the same year, of Guignard, on the
lily, and of Conklin ('93), on the marine gasteropod Crepidula, a
confirmation which seemed to place the quadrille on a firm basis.
Fol's result was, however, opposed to the earlier conclusions of Boveri
and Hertwig, and. a careful re-examination of the fertilization of the
echinoderm Qgg, independently made in 1894-5 by Boveri {EcJiinus),
by myself {Toxopneiistes), and Mathews {Ai'bacia, Asterias), demon-
strated its erroneous character. In the echinoderm, as in so many
other cases, the egg-centrosome disappears. The cleavage-amphi-
aster arises solely by division of the sperm-aster, and the centrosome
of the latter is derived not from the tip of the spermatozoon, as
asserted by Fol, but from the middle-piece, as already described.
The same result has been since reached by Hill and Erlanger.
Various attempts have been made to explain Fol's results as based

158

FERTILIZATION OF THE OVUM

on double-fertilized eggs, on imperfect method, on a misinterpreta-
tion of the double centrosomes of the cleavage-spindle, yet they still
remain an inexplicable anomaly of scientific literature.

Fig. 78. — Fertilization of the egg of the parasitic annelid Myzostoma. [Wheeler.]
A. Soon after entrance of tlie spermatozoon; the sperm-nucleus at cf ; at ? the germinal
vesicle ; at c the double egg-centrosome. B. First polar body forming at 9 ; «, the cast-out nucle-
olus or germinal spot. C. The polar bodies formed {p.b^ ; germ-nuclei of equal size ; at c the
persistent egg-centrosomes. D. Approach of the germ-nuclei; the egg-amphiaster formed. In
all other known cases this amphiaster is derived from the j/^r/;z-amphiaster.

Serious doubt has also been thrown on Conklin's conclusions by
subsequent research. Kostanecki and Wierzejski ('96) have recently
made a very thorough study, by means of serial sections, of the fertil-

CENTROSOME AND ARCHOPLASM IN FERTILIZATION I 59

ization of the gasteropod Physa, and have reached exactly the same
result as that obtained in the echinoderms. Plere also the egg-centre
degenerates, and its place is taken by a centrosome brought in by
the spermatozoon and giving rise to a sperm-amphiaster, which per-
sists as the cleavage-amphiaster (Fig. 64). A strong presumption is
thus created that Conklin was in error; and if this be the case, the
last positive evidence of a conjugation of centrosomes in the animal
Q.gg disappears.^

In view of this result we may well hesitate to accept Guignard's
conclusions in the case of flowering plants. The figures of this
author show in the clearest manner four centrosomes lying in the
neighbourhood of the apposed germ-nuclei (Fig. 80) ; but the conju-
gation of these centrosomes was an infei^ence, not an observed fact,
and has not been confirmed by any subsequent observer. Until such
confirmation is forthcoming we must receive Guignard's result's with
scepticism. 2

The third view, based upon the single case of Myzostoma as
described by Wheeler ('95), apparently rests on strong evidence,
though its force cannot be exactly estimated until a more detailed
account has been published. In this case no sperm-aster can be
seen at any period, with which is correlated the fact that no middle-
piece can be made out in the spermatozoon. The egg-centrosome,
on the other hand, is stated to persist after the formation of the
second polar body, to become double at a very early period, and
to give rise directly to the cleavage-amphiaster (Fig. 78). I can find
no ground in Professor Wheeler's paper to doubt the accuracy of
his conclusions. Nevertheless, an isolated case, which stands in
contradiction to all that is known of other forms, must rest on irre-
fragable evidence in order to command acceptance. Since, more-
over, the case involves the whole theory of fertilization based on
other animals (cf. p. 141), it must, I think, await further investiga-
tion.

1 Richard Hertvvig has, however, recently published a very interesting observation which
indicates that we may not yet have fully fathomed the facts in the case of echinoderms. If
unfertilized echinoderm-eggs, after formation of the polar-bodies, lie for many hours in
water or be treated with dilute poisons (strychnine), they may form a more or less perfectly
developed amphiaster, and the nucleus may even make an abortive attempt at division. No
centrosomes, however, could be discovered, even by the most approved methods. This
remarkable phenomenon is probably of the same nature as the formation of artificial asters
observed by Morgan (p. 226), but its meaning is not clear.

2 Van der Stricht, in a recent paper on Amphioxus ('95), is inclined to believe that a
fusion between the egg-centre and the sperm-centre occurs; but the evidence is very incom-
plete, and a comparison with the case of Physa indicates that his conclusion cannot be
sustained. The same criticism applies to the earlier work of Blanc ('91, '93) on the trout's
egg-

i6o

FERTILIZATION OF THE OVUM

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
our knowledge of it is less complete, especially in regard to the
history of the centrosome and the archoplasmic structures. It is,
however, sufficient to show that the process is here essentially of
the same nature as in animals in so far as it involves a union of

two germ-nuclei de-
rived from the two
respective sexes.
Many early observers
from the time of
Pringsheim ('55) on-
ward described a con-
jugation of cells in
the lower plants, but
the union of germ-
miclei, as far as I can
find, was first clearly
made out in the flow-
ering plants by Stras-
burger in 1877-8, and
carefully described
by him in 1884.
Schmitz observed a
union of the nuclei

Fig. 179. — Fertilization in Fibular ia. [Cambell.]
A. B. Early stages in the formation of the spermatozoid.
B. The mature spermatozoid; the nucleus lies above in the
spiral turns; below is a cytoplasmic mass containing starch-
grains (cf. the spermatozoids of ferns and oi Marsilia, Fig. 53).
D. Archegonium during fertilization. In the centre the ovum
containing the apposed germ-nuclei (j', ?).

of the conjugating cells of Spirogyra in 1879, and made similar obser-
vations on other algae in 1884. The same has been shown to be true
in Mjtscincce and Ptcridophytes by Strasburger, Cambell, and others

(Fig. 79)-

Up to the present time, however, the only thorough investigation
of fertilization has been made in the case of the flowering plants,
and our knowledge of the process here is due in the first instance to
Strasburger ('84, '88) and Guignard ('91), supplemented by the
work of Belajeff and Overton. The ovum or odsphere of the flower-
ing 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). The ovum
lies in the " embryo-sac," which represents morphologically the female
prothallium or sexual generation of the Pteridophyte, and is itself
embedded in the ovule within the ovary. The male germ-cell is here
non-motile, and is represented by a "generative nucleus," with a

FERTILIZATION IN PLANTS

i6i

small quantity of cytoplasm and two centrosomes (Guignard), lying
near the tip of the pollen-tube (Fig. 80, A), which is developed as an
outgrowth from the pollen-grain and represents, with the latter, a
rudimentary male prothallium or sexual generation. The formation

Fig. 80. — Fertilization of the lily. [GuiGNARn.]
A. The tip of the pollen-tube entering the embryo-sac ; below, the ovum (oosphere) with its
nucleus at $ and two centrosomes; at the tip of the pollen-tube the sperm-nucleus (d") witli two
centrosomes near it. B. Union of the germ-nuclei. C. Later stage of the same, showing the
asserted fusion of the centrosomes. E. The first cleavage-figure in the metaphase. D. Early
anaphase of the same; precocious division of the centrosomes.
M

1 62 FERTILIZATION OF THE OVUM

of the pollen-tube, and its growth down through the tissue of the
pistil to the ovule, was observed by Amici ('23), Brogniard ('26),
and Robert Brown ('31); and in 1833-34 Corda was able to follow
its tip through the micropyle into the ovule. ^

Strasburger ('77-88) 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. On the basis of these observations he
reached, in 1884, the same conclusion as Hertwig, that the essential
phenomena of fertilization is a union of two germ-nuclei, and that
the nucleus is the vehicle of hereditary transmission. Strasburger
did not, however, observe the centrosome in fertilization. This was
accomplished in 1891 by Guignard, who demonstrated in the case of
the lily {Liliiim Mavtagoii) that the generative nucleus as it enters
the egg is accompanied by a small quantity of cytoplasm and by two
centrosomes (Fig. 80). He showed further that the ^^^ also con-
tains two centrosomes; and according to his account the conjugation
of the nuclei is accompanied by a conjugation of the centrosomes, as
already described.

Guignard also first cleared up the history of the chromosomes,
reaching results closely in accord with those of Van Beneden in the
case of Ascaris. The two germ-nuclei do not actually fuse, but
remain in contact, side by side, and give rise each to one-half the
chromosomes of the equatorial plate, precisely as in animals (Fig. 80).
The number of chromosomes from each germ-nucleus is, in the lily,
twelve. The later history is identical with that of the animal Q^g,
each chromosome splitting lengthwise, and the halves passing to
opposite poles of the spindle. Each daughter-nucleus therefore
receives an equal number of chromosomes from the maternal and
paternal germ-nuclei.^

As in the case of animals (p. 127), the germ-nuclei of plants show
marked differences in structure and staining-reaction before their
union, though they ultimately become exactly equivalent. Thus,
according to Rosen ('92, p. 443), on treatment by fuchsin-methyl-blue

1 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 Geoffroy, Needham, and others,
placed himself on the side of Leeuvvenhoek 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.) It is an interesting fact that 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 trachese of the pistil.

2 Guignard's observations on the conjugation of the centrosomes have already been con-
sidered at p. 159. They stand at present isolated as the only precise account of the history
of the centrosomes in plant-fertilization, and no general conclusions on this subject can
therefore at present be drawn.

CONJUGATION IN UNICELLULAR FORMS 1 63

the male germ-nucleus of phanerogams is " cyanophilous," the female
" erythrophilous," as described by Auerbach in animals. Stras-
burger, while confirming this observation in some cases, finds the
reaction to be inconstant, though the germ-nuclei usually show
marked differences in their staining-capacity. These are ascribed by
Strasburger ('92, '94) to differences in the conditions of nutrition ; by
Zacharias and Schwarz to corresponding differences in chemical
composition, the male nucleus being in general richer in nuclein, and
the female nucleus poorer. This distinction disappears during ferti-
lization, 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 "cyanophilous," becomes "erythro-
philous," like the egg-nucleus before the pollen-tube has reached the
^Z^. Within the ^gg 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.

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 judice?'
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, Maupas, and Richard Hertwig, though many valuable ob-
servations 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 ^gg ; namely, that an essential phenomenon

^ Cf. p. 41. 2 Q_ p_ i2g.

1 64

FERTILIZATION OF THE OVUM

of conjugation is a imion of the nuclei of the conjugating cells.
Among the unicellular plants both the cell-bodies and the nuclei
completely fuse. Among animals this may occur ; but in many of
the Infusoria union of the cell-bodies is only temporary, and the con-
jugation consists of a mutual exchange and fusion of nuclei. It is

Second fission.

First fission, after separation.

Differentiation 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. 8r. Diagram showing: the history of the micronuclei during the conjugation of Para-

-inmcium. [Modified from Maupas.]

^Y and Frepresent the opposed macro- and micronuclei in the two respective gametes; circles
represent degenerating nuclei ; black dots, persisting nuclei.

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
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-

CONJUGATION IN UNICELIULAR FORMS 165

logical aspect with the union of germ-cells in the Metazoa. If conju-
gation do not occur, the race rapidly degenerates and dies out ; and
Maupas believes himself justified in the conclusion that conjugation
counteracts the tendency to senile degeneration and causes rejuve-
nescence, as maintained by Biitschli and Minot.^

In Stylonychia pustiilata, 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 tlie cilia, and especially by a more or less complete degradation of the
nuclear apparatus. In Stylonychia pustulata and Onychodroinus gratidis 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
niytilus), after which the micronucleus soon degenerates more or less completely, and
the race dies. It is a very significant fact that towards 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
niacromicleiis and one or more small uiicroiuiclei. 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. Zi).

These facts may be illustrated by the conjugation of Paramcecium
caudatuni, whiqh 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. 82, 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

iCf. p. 129.

Fig. 82. — Conjugation oi Paramcecmm caudatum. \_A-C, after R. HERTWIG; D-K, after
MaupaS.] (The macronuclei dotted in all the figures.)

A. Micronuclei preparing 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. H. Cleavage-nucleus (c), preparing for the first division. /. The cleavage-
nucleus has divided twice. J. 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.

1 66

CONJUGATION IN UNICELLUIAR FORMS

167

B

pronucleus (Fig. 82, 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
times successively, and of the eight resulting bodies four become
macronuclei and four micronuclei (Fig. 82, 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 dis-
tributed in like man-
ner, but in P. caitda-
tiim 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 conju-
gating individuals has
given rise to four
descendants, each
containing a macro-
nucleus and micro-
nucleus derived from
the cleavage-nucleus.
From this time for-
ward fission follows
fission in the usual
manner, both nuclei
dividing at each fis-
sion, until, after many
generations, conjuga-
tion 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 microgameie unites with
a larger niacrogaviete, 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 microgam-
ete one-eighth of the original micronucleus (Fig. 83). 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.

Fig. 83. — Conjugation of Vorticellids. [Maupas.]
A. Attachment of the small free-swimming microgamete to
the large fixed macrogamete; micronucleus dividing in each
{Carckesnim). B. Microgamete containing eight micronuclei;
macrogamete four ( Vorticelld) . C. All but one of the micro-
nuclei 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.

1 68 FERTILIZATION OF THE OVUM

The facts just described show a very close parallel to those observed
in the maturation and fertilization of the Q.gg. 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 egg) 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
these degenerating nuclei or " corpuscules de rebut " with the polar
bodies (p. 175), 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

A

C

Fig. 84. — Conjugation of 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.

is the fact that tJie germ-nuclei tinite wJicn in the form of spindles
or mitotic figiu^es. 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. 82, G\ and this gives good reason
to believe that the chromatin of the two gametes is equally dis-
tributed 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
Noctiluca, which has been studied with some care by Cienkowsky
and Ishikawa (Fig. 84). Here the conjugating animals completely
fuse, but the nuclei are merely apposed and give rise each to one-
half of the mitotic figure. At either pole of the spindle is a cen-
trosome, the origin of which remains undetermined.

It is an interesting fact that in Noctiluca, in the Gregarines, and
probably in some other Protozoa, conjugation is followed by a very

CONJUGATION IN UNICELLULAR FORMS

169

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
process to the fertilization and subsequent cleavage of the ovum is
particularly striking.

The conjugation of unicellular plants shows some interesting

Fig. 85. — Conjugation of Spirogyra. [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
pryrenoids, before union of the nuclei. D. Zygospore after fusion of the nuclei and formation
of the membrane.

features. Here the conjugating cells completely fuse to form a
"zygospore" (Figs. 85, 99), 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 unite,
but in many cases the plastids also (chromatophores). In Spirogyra
some interesting variations in this regard have been observed. In
some species De Bary has observed that the long band-shaped chro-
matophores unite end to end so that in the zygote the paternal and

I/O FERTILIZATION OF THE OVUM

maternal chromatophores lie at opposite ends. In 5. Weberi, on
the other hand, Overton has found that the single maternal chromato-
phore 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. 85). 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 maternal, chromatophores.
In the case of a Spirogyj-a filament having a single chromatophore
it is therefore " wholly immaterial whether the individual cells re-
ceive the chlorophyll-band from the father or the mother " (De Vries),
— a result which, as Wheeler has pointed out, is in a measure analo-
gous to that reached in the case of the centrosome of the animal ^^g?-

F. Summary and Conclusion

All forms of fertilization 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-
cell loses most of its cytoplasm, the main bulk of which, and hence
the main body of the embryo, is now supplied by the &ZZ- ^^^j

1 De Vries's conclusion is, liovvever, not entirely certain; for it is impossible to deter-
mine, save by analogy, whether the chromatophores maintain their individuality in the
zygote.

SUMMARY AND CONCLUSION I /I

more than this, the germ-cells come to differ in their morphological
composition ; for in plants the male germ-cell loses its plastids,
which are supplied by the mother alone, while in most if not all
animals the Q,g^ loses its centrosome, which is then supplied by the
father. The loss of the centrosome by the ^gg is, I believe, to be
regarded as a provision to guard against parthenogenesis and to
ensure amphimixis.

The equivalence of the germ-cells is thus finally lost. Only the
genn-niLclei i^etain their primitive morphological equivalence. Hence
we find the essential fact of fertilization and sexual reproduction to
be a rmion of equivalent nuclei; and to this all otJier processes are
tributary. The substance of the germ-nuclei, giving rise to the
same number of chromosomes in each, is equally distributed to the
daughter-cells and probably to all the cells of the body.

As regards the most highly differentiated type of fertilization and
development we thus reach 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 processes are controlled and from which they receive the spe-
cific stamp of the race. From the father comes the centrosome to
organize the machinery of mitotic division by which the Q.gg 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 woof from the male. What has since been
gained is the knowledge that this web is to be sought in the chro-
matic substance of the nuclei, and that the centrosome is the weaver
at the loom.

LITERATURE IV

Van Beneden, E. — Recherches sur la maturation de I'oeuf, la fecondation et la division

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

mitosique chez PAscaride megalocephale : Bull. Acad. roy. de Belgique, III. 14,

No. 8, 1887.
Boveri, Th. — tjber 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 imd Bonnefs Ergebnisse, I. 1891.
Id. — ijber das Verhalten der Centrosomen bei der Befruchtung des Seeigeleies,

etc.: Verhandl. Phys. Med. Ges. J4^t(rzdiirg, XXIX. 1895.
Fick, R. — iJber die Reifung und Befruchtung des Axolotleies : Zeitschr. IViss.

Zo'ol., LVI. 4. 1893.

172 FERTILIZATION OF THE OVUM

Guignard, L. — Nouvelles etudes sur la fecondation : Ann. d. Sciences nat. Bot.y

XIV. 1891.
Hartog, M. M. — Some Problems of Reproduction, etc.: Qitart. Joitm. Mic. Sci.y

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. MYss.,

II. CI. XVII. 1888-89.
Id. — ijber Befruchtung und Konjugation : yer/i. deutsch. Zo'dl. Ges. Berlin, \2>()2.
Kostanecki, K. v., and Wierzejski, A. — Uber das Verhalten der sogen. achro-

matischen Substanzen im befruchteten Ei (^oi Pkysa) : Arch. mik. Anat., XLVII.

2. 1896.
Mark, E. L. — Maturation, Fecundation, and Segmentation of Liinax campestris :

Bull. Mus. Comp. Zo'dl. Harvard College, Cambridge, Mass., VI. 1881.
Maupas. — Le rejeunissement karyogamique chez les Cilies : Arch. d. Zool., 2"'^

serie, VII. 1889.
Riickert, J. — Uber das Selbstandigbleiben der vaterlichen und mlitterlichen 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 fiir eine Theorie der Zeugung. Jena, 1884.
Id. — iJber Kern- und Zellteilung im Pflanzenreich, nebst einem Anhang liber

Befruchtung. Jena, 1888.
Vejdovsky, F. — Entwickelungsgeschichtliche Untersuchungen, Heft i, Reifung,

Befruchtung und Furchung des Rhynchelmis-Eies. Prag, 1888.
Wilson, Edm. B. — Atlas of Fertilization and Karyokinesis. New York, 1895.

CHAPTER V

OOGENESIS AND SPERMATOGENESIS. REDUCTION OF THE

CHROMOSOMES

" Es kommt also in der Generationenreihe der Keimzelle irgendwo zu einer Reduktion
der urspriinglich vorhandenen Chromosomenzahl auf die Halfte, und diese Za/z/^w-reduk-
tion ist demnach nicht etvva 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 inatiii-a-
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 problems relating to the morphological
constitution of nucleus and chromatin which have an important
bearing on all theories of development, and which now stand in
the foreground of scientific discussion among the most debatable
and interesting of biological problems.

It must be said at the outset that the phenomena of maturation
belong to one of the most difficult fields of cytological research, and
one in which we are confronted not only by diametrically opposing
theoretical views, but also by apparently contradictory results of
observation.

Two fundamentally different views have been held of the manner
in which the reduction is effected. The earlier and simpler view,
which was somewhat doubtfully suggested by Boveri {^"^7, i ), and has
been more recently supported by Van Bambeke ('94) and some others,

1 Zellenstudien, III. p. 62.
173

174

REDUCTION OF THE CHROMOSOMES

assumed an actual degeneration or casting out of half the chromo-
somes during the growth of the germ-cells — a simple and easily
intelligible process. The whole weight of the evidence now goes to
show, however, that this view cannot be sustained, and that rediLctioti
is effected by a rearrangement and redistribution of the nuclear sub-
stance without loss of any of its essential constituents. It is true
that a large amount of chromatin is lost during the growth of the
^SS-"^ It is nevertheless certain that this loss is not directly con-
nected with the process of reduction ; for, as Hertwig 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. We have advanced a certain distance
towards such an explanation and, indeed, apparently have found it

Fig. 86. — Formation of the polar bodies before entrance of the spermatozoon, as seen in the
living ovarian egg of the sea-urchin Toxcpneustes (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
(p. b., I, 2) ; e, the egg-nucleus. In this animal the second polar body fails to divide. For
division of the second polar body see Fig. 64.

in a few specific cases. Yet when the subject is regarded as a
whole, the admission must be made that the time has not yet come
for an understanding of the phenomena, and the subject must there-
fore be treated in the main from an historical point of view.

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 matura-
tion conforms to the same type in both sexes, which show as close a
parallel in this regard as in the later history of the germ-nuclei. Stated

1 Cf. Figs. 71, 88.

GENERAL OUTLINE

175

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 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 of the four cells
forms the "ovum" proper, while the other three, known as t\\& polar
bodies, are minute, rudimentary, and incapable of development (Figs.
64, 71, 86). 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 egg
contains a large amount of protoplasm and yolk, out of which the

Primordial germ-cell.

. Division-peiiod (the number of divi-
sions is much greater).

. Growth-period.

Maturation-period.

Primary oocyte or ovarian egg

Secondary oocytes (egg and

first polar body).

Mature egg and three polar bodies.

Fig. 87. — Diagram showing the genesis of the egg. [After BOVERI.]

main mass of the embryonic body is formed. In the male, therefore,
all of the four cells may become functional ; in the female the func-
tions of development have become restricted to but one of the four,
while the others have become rudimentary (cf. p. 182). The polar
bodies are therefore to be regarded as abortive eggs — a view first put
forward by Mark in 1881, and ultimately adopted by nearly all inves-
tigators.

I. Rediiction in the Female. Formation of the Polar Bodies

As described in Chapter III., the egg 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

1 The parallel was first clearly pointed out by Platner in li
strated by Oscar Hertwig in the following year.

J, and was brilliantly demon-

iy6 REDUCTION OF THE CHROMOSOMES

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 odgonia (Fig. 87), which
are the immediate predecessors of the ovarian Q,g^. At a certain
period these cease to divide. Each of them then grows to form an
ovarian egg, 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 Q-^^.

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 ^^^ 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 afterwards, as in the lamprey-eel, the frog, and
AinpJiioxHS. 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. 71, '^6)\ and in many cases
the first of these divides into two as the second is formed (Fig. 64).

A group of four cells thus arises, namely, the mature ^^g, 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
without further change. The egg-nucleus is now ready for union
with the sperm-nucleus.

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, in some cases at any rate, 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 effected. 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 Purkinje,
von Baer, Bischoff, had no real understanding of the process and
believed the germinal vesicle to disappear at the time of fertilization.

^ See, however, p. 194.

GENERAL OUTLINE

177

To Biitschli (^J^,) Hertwig, and Giard {^"jj^ 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-

D

^ H

Fig. 88. — 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 &gg. 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 ( 5), each con-
taining two single chromosomes (half the somatic number) ; c, the egg-centrosome which now
degenerates and is lost.
N

178

DEDUCTION OF THE CHROMOSOMES

A

C

D

E

F

I

7

Fig. 89. — Formation of the nniar k a-
, ^. The egg wi,t „„ sp ™r,„ „;' ? '" ■*""" "'""""P"'": van .,W„„. [Bovek, 1

-«-..™^o»rerr;,:-"./;ei-s-<s;3£Ef^'^'^^^^^^

GENERAL OUTLINE 1 79

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
zvitlioiit an intervening 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 accurately deter-
mined by Boveri in Ascaris ('87, i), are in a typical case as follows
(Figs. 88, 89) : As the egg prepares for the formation of the first polar
body, the chromatin of the germinal vesicle groups itself in a num-
ber 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 nnniber of tetrads is always one-half the zisnal
number of chromosoines. Thus in Ascaris {fnegalocephala, bivalens)
the germinal vesicle gives rise to two tetrads, the normal number of
chromosomes in the earlier divisions being four ; in the salamander
and the frog there are twelve tetrads, the somatic number of chro-
mosomes being twenty-four (Fleming, vom Rath), etc. 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 Q.gg while the other
passes into the polar body. Both the Q,gg and the first polar body
therefore receive each a number of dyads equal to one-half the usual
number of chromosomes. The Q.gg 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 chromo-
somes, one of which, again, remains in the Q.gg while its sister passes
into the polar body. Both the Q.gg and the second polar body accord-
ingly 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 Q.gg a reticular nucleus, much smaller than the
original germinal vesicle, is now formed.^

Essentially similar facts have now been determined in a consider-
able number of animals, though the form of the tetrads varies greatly,
and there are some cases in which no actual tetrad-formation has been
observed (apparently in the flowering plants). It is clear from the

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.

2 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
similarly halved.

i8o

REDUCTION OF THE CHROMOSOMES

foregoing account that the numerical reduction of chromatin-;//«5^^i-
takes place before the polar bodies are actually formed, through the
operation of forces which determine the number of tetrads within
the germinal vesicle. The numerical reduction is therefore deter-
mined in the grandmother-cell of the ^^^. The actual divisions by
which the polar bodies are formed merely distribute the elements of
the tetrads.

2. Reduction hi the Male. Spermatogenesis

The researches of Platner ('89), Boveri, and especially of Oscar
Hertwig ('90, i) have demonstrated that reduction takes place in the

Primordial germ-cell.

Spermatogonia.

-Division-period (the number of divi-
sions is much greater).

-Growth-period.

Primary spermatocyte.

Secondary spermatocytes,

Spermatids.

Spermatozoa.

- Maturation-period.

Fig. 90. — Diagram showing the genesis of the spermatozoon. [After BOVERI.]

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

GENERAL OUTLINE

i8r

spermatogonia from which the spermatozoa are ultimately formed
(Fig. 90). Like the oogonia, the spermatogonia continue for a time
to divide with the usual (somatic) number of chromosomes ; i.e. four
in Ascaris inegalocepJiala bivalens. Ceasing for a time to divide, they

Fig. 91. — Diagrams showing the essential facts of reduction in the male. The somatic num-
ber of chromosomes is supposed to be four.

A. B. Division of one of the spermatogonia, sliowing 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 persists in the middle-piece of the spermatozoon.

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

1 82 REDUCTION OF THE CHROMOSOMES

history of the chromatin in these tzvo divisions is exactly parallel to
that in the formation of tJie polar bodies (Figs. 91, 92). 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 respec-
tive daughter-spermatocytes. At the ensuing division, which occurs
without the previous formation of a resting reticular nucleus, each
dyad is halved to form two single chromosomes which enter the
respective spermatids (ultimately spermatozoa). From each sperma-
tocyte, therefore, arise four spermatozoa, and each sperm-nucleus
receives half the usual number of single chromosomes. The par-
allel 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 egg, and that
the group of four spermatozoa to which it gives rise is equivalent
to the ripe o.^^ 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
division from the egg-mother-cell (oocyte) in the same manner as
the spermatozoa are formed from the sperm-mother-cell (sperma-
tocyte). 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 of the egg-mother-cell becomes the Q.gg, appropriating to
itself the entire mass of the yolk at the cost of the others which
persist in rudimentary form as the polar bodies." ^

3. Theoretical Significance of Maturation

Up to this point the facts are clear and intelligible. When, how-
ever, we attempt a more searching analysis by considering the origin
of the tetrads and the ultimate meaning of reduction, we find our-
selves in a labyrinth of conflicting observations and hypotheses from
which no exit has as yet been discovered. And we may in this case
most readily approach the subject by considering its theoretical
aspect at the outset.

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 num.ber would be doubled in each succeeding
generation through union of the germ-cells. But why should the
number be constant }

In its modern form this problem was first attacked by Weismann
in 1885, and again in 1887, though many earlier hypotheses regard-

1 '90, I, p. 126.

GENERAL OUTLINE 1 83

ing the meaning of the polar bodies had been put forward. ^ His
interpretation was based on a remarkable paper published by Wil-
helm Roux in 1883,^ in which are developed certain ideas which
afterwards formed the foundation of Weismann's whole theory of in-
heritance and development. Roux argued that the facts of mitosis
are only explicable under the assumption that chromatin is not a
uniform and homogeneous substance, but differs qualitatively in differ-
ent regions of the nucleus ; that the collection 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 conception 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 biophorcs somewhat
like Darwin's " gemmules " (p. 303), each of which has the power of
determining the development of a particular quality. Weismann
conceives these units as aggregated to form units of a higher
order known as "determinants," which in turn are grouped to form

^ Of these we need only consider at this point the very interesting suggestion of Minot
('77), afterwards 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 constituents, 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 fertiliza-
tion 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 Q%t^^. A fatal objection to it, on which both
Strasburger and Weismann have insisted, lies in the fact that male as well as female quali-
ties are transmitted by the egg-cell, while the sperm-cell also transmits female qualities.
The germ-cells are therefore non-sexual; they are physiologically as well as morphologi-
cally equivalent. The researches of Hertwig, Brauer, and Boveri show, moreover, that in
Ascaris, at any rate, all of the four spermatids derived from a spermatocyte become func-
tional spermatozoa, and the beautiful parallel between spermatogenesis and oogenesis thus
established becomes meaningless under Minot's view. This hypothesis must, therefore, in
my opinion, be abandoned.

Balfour probably stated the exact 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."

^ Vber die Bedeutung der Kerntheilungsjiguren.

1 84

REDUCTION OF THE CHROMOSOMES

"ids," the latter being identified with the visible chromomeres or
chromatin-granules. The ids finally are associated in linear groups
to form the " idants " or chromosomes. Since the biophores differ
qualitatively, it follows that the same must be true of the higher units

,i<sr

C

E

I

J

M

/Ao

D

H

7

f

K

90

L

Fig. 92. — Reduction in the spermatogenesis of Ascaris niegalocephala, var. bivalens. [Brauer.] 1
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, B. This shortens (C), and breaks in two to form the two tetrads {D in profile, E viewed
endwise). F. G. H. First division to form two secondary spermatocytes, each receiving two dyads.
/. Secondary spermatocyte. J. K. The same dividing. L. Two resulting spermatids, each with
two single chromosomes and a centrosome.

formed by their aggregation. Hence each chromosome has a dis-
tinct and definite character of its own, representing a particular group
of hereditary qualities. From this it follows that the number of

^ For division of the spermatogonia see Fig. 39 ; for the corresponding phenomena in
var. univalens see Fig. 107.

GENERAL OUTLINE 1 85

specifically distinct chromosomes is doubled by the union of two
germ-cells, a process which if unchecked would quickly lead to an
infinite complexity of the chromatin or germ-plasm. The end of
maturation, or reduction, is therefore to prevent " the excessive
accumulation of different kinds of hereditary tendencies or germ-
plasms " ^ through the progressive summation of ancestral chromatins.

We now come to the vital point of Weismann's hypothesis of
reduction, about which all later researches have revolved. Assuming
with Roux that the different qualities or " ancestral germ-plasms "
are arranged in a linear manner in the spireme-thread and in the
chromosomes derived from it, he ventured the prediction ('87) 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 ordi-
nary cell-division, " by means of which all the ancestral germ-plasms
are equally distributed in each of the daughter-nuclei after having
been divided into halves." This form of division, which he called
"equal division " (Aequationstheilung), was then a known fact. The
second form, at that time a purely theoretical postulate, he assumed
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" (Reduk-
tionstheilung), and suggested ^ that this might be effected either by a
transverse division of the chromosomes, or by the divergence and
separation of entire chromosomes without division. By either method
the number of " ids " would be reduced ; and Weismann argued
that such reducing 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 I believe is incontrovertible, that the
reducing divisions postulated by Weismann actually occur, though
not precisely in the manner conceived by him. Unfortunately, how-
ever, this demonstration has been made in only a few specific cases,
— the complete demonstration, indeed, in but a single group, namely,
the copepod Crustacea, — ^ while careful studies by the most accom-
plished observers have led to an entirely different result in other
cases; namely, in Ascaris and the flowering plants. We are in fact
confronted by an apparent contradiction of so absolute a character
that no middle ground between the conflicting results can at present
be discovered. We may best appreciate the nature of this contra-
diction by a preliminary consideration of the tetrad groups ; for it
is plain that the nature of the maturation-divisions can only be
approached through a study of the origin of the tetrads.
1 Essay VI., p. 366. ^ I.e., p. 375.

1 86 REDUCTION OF THE CHROMOSOMES

B. Origin of the Tetrads
I. General Sketch

It is generally agreed that each tetrad arises by a double division of
a single primary chromatin-rod. 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-Jialf the usual
number of cJirovwsomes, and that this numerical reduction is due to
the fact that the spireme-thread segments into one-half the usual num-
ber of pieces. The contradiction relates to the manner in which the
primary rod divides to form the tetrad. According to one account,
mainly based on the study of Ascaris by Boveri, Hertwig, and Brauer,
and supported in principle by the observations of Guignard and
Strasburger on the flowering plants, each tetrad arises by a double
longitudinal splitting of the primary chromatin-rod caused by the
division of each chromatin-granule into four parts. In this case the
four resulting bodies — i.e. the four chromosomes of the tetrad —
must be exactly equivalent, since all are derived from the same
region of the spireme-thread and consist of equivalent groups of
ids or chromatin-granules (Fig. 102, A). No reducing division can
therefore occvir in Weismann's sense. There is only a reduction in
the number of chromosomes, not a reduction in the number of qualities
represented by the chromatin-granules. This may be graphically
expressed as follows: —

If the original spireme-thread be represented by abed, normal
mitosis consists in its segmentation into the four chromosomes

a — b — c — d, which split lengthwise to form -> t' -' ^,- In matu-

a b c d

ration the thread segments into tzvo portions, ab — cd, each of which

then split into four equivalent portions, giving the equivalent tetrads.

ab

thus, — r

ab

ab , cd

— y and — -.
ab cd

cd XX v V

or . — — , since it is not known

X

L

X

y

cd X X y y

whether ab really is equal to a + b.

The second account, which finds its strongest support in the
observations of Ruckert, Hacker, and vom Rath on the maturation of
arthropods, asserts that each tetrad arises by one longitudinal and one
transverse division of each primary chromatin-rod {Y\^. 1 02, B\ Thus
the spireme abed segments as before into two segments ab and cd.

These first divide longitudinally to form — and ~ and then trans-

ab cd

versely to form — — and -

a\b c

— . Each tetrad therefore consists, not of
d

ORIGIN OF THE TETRADS 1 8/

four equivalent chromosomes, but of two different pairs ; and the
second or transverse division by which a is separated from b, or c
from d, is the reducing division demanded by Weismann's hypoth-
esis. The observations of Riickert and Hacker prove that the
transverse division is accomplished during the formation of the
second polar body.

2. Detailed Evidence

We may now consider some of the evidence in detail, though
the limits of this work will only allow the consideration of some
of the best known cases. We may first examine the case of Ascaris,
on which the first account is based. In the first of his classical
cell-studies Boveri showed that each tetrad appears in the ger-
minal vesicle in the form of four parallel rods, each consisting of
a row of chromatin-granules (Fig. 89, A-C). He believed these rods
to arise by the double longitudinal splitting of a single primary chro-
matin-rod, each cleavage being a 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 sup-
ported this view by further observations in 1890 on the polar bodies
of Sagitta and several gasteropods, in which he again determined, as
he believed, that the tetrads 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 convincing evidence. In 1893, finally, Brauer
made a most thorough and apparently exhaustive study of their origin
in the spermatogenesis 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 reticulum of the resting spermatocytes was traced
with the most painstaking care. The first step observed was a double
splitting of the chromatin-threads in the reticulum, caused by a divi-
sion of the chr()matin-granules into four parts (Fig. 92, A^ From
the reticulum arises a continuous spireme-thread, which from its first
appearance is split into four longitudinal parts, and ultimately breaks
in two to form the two tetrads characteristic of the species. These
have at first the same rod-like form as those of the germinal vesicle.
Later they shorten to form compact groups, each consisting of four
spherical chromosomes. Brauer's figures are very convincing, and,
if correct, seem to leave no doubt that the tetrads here arise by a
double longitudinal splitting of the spireme-thread, initiated even in
the reticular stage before a connected thread has been formed. If

REDUCTION OF THE CHROMOSOMES

this really be so, there can be here no reducing division in Weis-
mann's sense. The reduction of chromatin, caused by the ensuing
cell-division, is therefore only a quantitative mass-reduction, as Hert-
wig and Brauer insist, not a qualitative sundering of different ele-
ments, as Weismann's postulate demands. ^ The work of Strasburger
and Guignard, considered at p. 195, has given in principle the same
general result in the flowering plants, though the details of the pro-
cess are here considerably modified, and apparently no tetrads are
formed.

A

E F

Fig. 93. — 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.

We now return to the second view, referred to at p. 186, which
accords with Weismann's hypothesis, and flatly contradicts the con-
clusions drawn from the study of Ascai^is. This view is based mainly
on the study of arthropods, especially the Crustacea and insects, but
has been confirmed by the facts observed in some of the lower verte-
brata. In many of these forms the tetrads first appear in the form
of closed rings, each of which finally breaks into four parts. First
observed by Henking ('91) in the insect Pyrroc/ioris, they have since
been found in other insects by vom Rath and Wilcox, in various cope-

1 In an earlier paper on Branchipus ('92) Brauer reached an essentially similar result,
which was, however, based on far less convincing evidence.

ORIGIN OF THE TETRADS

189

pods by Riickert, Hacker, and vom Rath, in the frog by vom Rath,
and in elasmobranchs by Moore. 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)
and Hacker ('95, i). All these observers, excepting Wilcox and

c

I I . ^000

• oOO

i#r

i:»

(The

Fig. 94. — Formation of the tetrads and polar bodies in Cyclops, slightly schematic,
full number of tetrads is not shown.) [RiJCKERT.]

A. Germinal vesicle containing eight longitudinally split chromatin-rods (half the somatic
number). B. Shorteiiing 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 division 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 t7-ansverse or
reducing division.

Moore (see p. 201), 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 heterotypi-
cal mitosis (p. 60). The breaking of the ring into four parts involves

190 REDUCTION OF THE CHROMOSOMES

first the separation of these two halves (corresponding with the origi-
nal 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 chro-
mosomes united end to end. This appears with the greatest clear-
ness in the spermatogenesis of Gryllotalpa (Fig. 93). Here 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
give rise to six typical tetrads. An essentially similar account of the
ring-formation is given by vom Rath in Euchczta and Calamis, and
by Riickert in Heterocope and Diaptoinus.

That the foregoing interpretation of the rings is correct, is beauti-
fully demonstrated by the observations of Hacker, and especially of
Riickert, on a number of other copepods {Cyclops, Canthocamptns),
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 strenims, by Riickert ('94), whose
observations, confirmed by Hacker, are quite as convincing as those
of Brauer on Ascaris, though they lead 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. 94),
and finally shorten to form double rods, manifestly equivalent to the
closed rings of DiaptoniiLS. Each of these now segments transversely
to form a tetrad group, and the eleven tetrads then place themselves
in the equator of the spindle for the first polar body (Fig. 94, C\ in such
a manner that the longitudinal 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), 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 chromo-
somes in the egg, and it is a curious fact that this conclusion, which
apparently rests on irrefragable evidence, completely confirms Weis-
mann's earlier views, published in 1887,^ and contradicts the later
interpretation upheld in his book on the germ-plasm.

1 Essay VI.

ORIGIN OF THE TETRADS

191

Hacker ('92) has reached exactly similar results in the case of
Canthocamptus and draws the same conclusion. In Cyclops strenitns
he finds in the case of first-laid eggs a variation of the process which
seems to approach the mode of tetrad formation in some of the lower
vertebrates. In such eggs the primary double rods become sharply

a

Fig. 95. — Diagrams of various modes of tetrad-formation. [Hacker.]
a. Common starting-point, a double spireme-thread in tlie germinal vesicle ; d. common re-
sult, the typical tetrads ; b. c. intermediate stages : at tlie left the ring-formation (as in Diapto??ms,
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
stienuus, according to Hacker, and in the salamander, according to vom Rath).

bent near the middle to form V-shaped loops (Fig. 96, C), which finally
break transversely near the apex to form the tetrad ^ — a process which
clearly gives the same result as before. An exactly similar process
seems to occur in the salamander as described by Flemming and

1 Hacker upholds this account ('95, i) in spite of the criticisms of Riickert and vom
Rath.

192

REDUCTION OF THE CHROMOSOMES

vom Rath. Flemming observed the double V-shaped loops in 1887,
and also the tetrads derived from them, but regarded the latter as
"anomalies." Vom Rath ('93) subsequently found that the double
V's break at the apex, and that the four rods thus formed then draw
together to form four spheres grouped in a tetrad precisely like
those of the arthropods. Still later ('95, i) the same observer traced
a nearly similar process in the frog ; but in this case the four ele-

Fig. 96. — 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. [RUCKERT.]

B. Later stape of the same, condensation and segmentation of the rings. [ROCKERT.]

C. "Cyclops sfrenuus" illustrating Hacker's account of the tetrad-formation from elongate
double rods ; a group of " accessory nucleoli." [Hackf.r]

D. Germinal vesicle of an annelid {Oph>yot!-ocha) showing nucleolus and four chromosomes.
[KORSCHELT.]

ments appear to remain for a short time united to form a ring before
breaking up into separate spheres.

To sum up : The researches of Riickert, Hacker, and vom Rath,
on insects, Crustacea, and amphibia have all led to the same result.
However the tetrad-formation may differ in matter of detail, it is in
all these forms the same in principle. Each primary chromatin-rod
has the value of a bivalent chromosome ; i.e. two chromosomes
joined end to end, ab. By a longitudinal division a ring or double

THE EARLY HISTORY OF THE GERM-NUCLEI 1 93

rod is formed, which represents two equivalent pairs of chromosomes

— r- Durinc^ the two maturation-divisions the four chromosomes

\h
are split apart, — r-:^ and Riickert's observations demonstrate that
a\o

the first division separates the two equivalent dyads, ab and ab, which
by the second division are split apart into the two separate chromo-
somes, a and b. Weismann's postulate is accordingly realized in the
second division. It is clear from this account that the primary
halving of the number of chromatin-rods is not an actual reduction,
since each rod represents two chromosomes. Riickert therefore
proposes the convenient term " pseudo-reduction " for this pre-
liminary halving.! T\\q. actual reduction is not effected until the
dyads are split apart during the second maturation-division.

C. The Early History of the Germ-Nuclei

We may for the present defer a consideration of accounts of reduc-
tion differing from the two already described and pass on to a
consideration of the earlier history of the germ-nuclei. A consider-
able number of observers are now agreed that the primary chromatin-
rods appear at a very early period in the germinal vesicle and are
longitudinally split from the first. (Hacker, vom Rath, Riickert, in
copepods ; Riickert in selachians ; Born and Fick in amphibia ;
Holl in the chick ; Riickert in the rabbit.) Hacker ('92, 2) made the
interesting discovery that in some of the copepods {CantJiocamptus,
Cyclops) these double rods could be traced 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 tj'iie retiaihim
fo7'med in the germinal vesicle (Fig. 97). In the following year Riick-
ert ('93, 2) made a precisely similar discovery in the case of selachians.
After division of the last generation of oogonia the daughter-chro-
mosomes do not give rise to a reticulum, but split lengthwise, and
persist in this condition throughout the entire growth-period of the
Q.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 v/hich
are doubled and arranged in pairs." ^ In the following year ('93)
vom Rath, following out the earlier work of Flemming, discovered
an exactly analogous fact in the spermatogenesis of the salamander.
The tetrads were here traced back to double chromatin-rods, indi-
vidually identical with the daughter-chromosomes of the preceding

1 '93. 2, p. 541. - '92, 2, p. 51.

194

REDUCTION OF THE CHROMOSOMES

spermatogonium-division, which spht lengthwise during the anaphase
and pass into the spermatocyte-nucleus without forming a reticulum.
Flemming had observed in 1887 that these daughter-chromosomes
split in the anaphase, but could not determine their further history.
Vom Rath found that each double daughter-chromosome breaks in
two at the apex to form a tetrad, which passes into the ensuing
spermatocyte without the intervention of a resting stage. ^

It is clear that in such cases the " pseudo-reduction " must take
place at an earlier period than the penultimate generation of cells.
In the salamander Flemming i^'^J^ found that the " chromosomes " of
the spermatogonia appeared in the reduced number (twelve) in at least
three cell-generations preceding the penultimate. Vom Rath ('93)

Fig. 97. — Longitudinal section through the ovary of the copepod Canthoca^nptus. [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.

traced the pseudo-reduction in both sexes back to much earlier stages,
not only in the larvae, but even in the embryo (!). This very remark-
able discovery showed that the pseiido-rediiction might appear in the
early progenitors of the germ-cells during embryonic life — perhaps eveji
during the cleavage. This conjecture has apparently been substan-
tiated by Hacker ('95, 3), who finds that in Cyclops brevicornis the

1 It is certain that these facts do not represent a universal type of maturation, for in
Ascaris there is no doubt that a true reticular resting stage occurs in the primary spermato-
cytes, and probably also in the germinal vesicle. Hacker found, moreover, that the same
species might show differences in this regard; for in Cyclops strenmis the first-laid eggs have
no resting stage, the double daughter-chromosome passing directly into the tetrads, while
in later broods of eggs a daughter-spireme, composed of long double threads, is formed. The
difference is believed by Hacker to be due to the fact that the earlier eggs are quickly laid,
while the later broods arc Ion" retained in the oviduct.

REDUCTION IN THE PI A NTS 195

reduced number of chromosomes (twelve) appears in the primordial
germ-cells which are differentiated in the blastula-stage (Fig. 56).
He adds the interesting 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 develop-
ment 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 penultimate cell-generation, as is certainly the fact
in Ascaris} The foregoing facts pave the way to an examination of
reduction in the plants, to which we now proceed.

D. Reduction in the Plants

Guignard's and Strasburger's observations on reduction in the
flowering plants gave a result which in substance agrees with that
obtained by Boveri and Brauer in the case of Ascaris. These
observers could find absolutely no evidence of a transverse or reduc-
ing division, and asserted that the reduction in number is directly
effected by a segmentation of the spireme-thread into half the usual
number of chromosomes ; i.e. by a process exactly corresponding
with the "pseudo-reduction" of Riickert (see Fig. 25). These
observers find that in the male the chromosomes suddenly appear
in the reduced number (twelve in the lily, eight in the onion) at
the first division of the pollen-mother-cell, from which arise four
pollen-grains. In the female the same process takes place at the
first division of the mother-cell of the embryo-sac. Strasburger
and Guignard agree that in tJie subseg?ient divisions these chromo-
somes do not form tetrads, bitt undergo simple longitudinal split-
ting at each successive division. In case of the male there are
at least four of these divisions ; viz. two divisions to form the
four pollen-grains, a third division to form the vegetative and
generative cell of the pollen-grain, and finally a fourth division
of the generative nucleus in the pollen-tube. In all these mitoses
the reduced number of chromosomes appears, and each division is
followed by a return of the nucleus to the resting state. In the

1 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.

196 REDUCTION OF THE CHROMOSOMES

mother-cell of the embryo-sac the number of divisions before fertiliza-
tion is three, four, five, or sometimes evefi more, the reduced number
persisting throughout. These facts led to the suspicion, first expressed
by Overton in 1892, that the reduced number of chromosomes might be
found in the sexual generation of higher cryptogams (which corresponds
with the cells derived from the pollen-grain, or from the mother-cell of
the embryo-sac). This surmise quickly became a certainty. Overton
himself discovered ('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 Pirms at the same time.
In the following year Strasburger brought the matter to a definite
conclusion in the case of a fern {Osiminda), showing that all the cells
of the prothallimn, from the oi'iginal spore-niotJier-cell onwards to the
formation of the germ-cells, have one-half the number of chromosomes
found in tJie 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, scores of cell-generations
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 four chromosomes dur-
ing division, those of the sexual generation (thallus) eight. It now
seems highly probable that this will be found a general rule.

The striking point in these, as in vom Rath's and Hacker's obser-
vations, 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 animals, and suggests that the four cells arising by
the division of the oogonium {o.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 pro-
thallium of angiosperms 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 relic of the primitive mode of asexual reproduction. ^'^
Could Strasburger's hypothesis be substantiated, it would place the
entire problem, not merely of maturation, but of sexuality itself, in
a new light.

^ '78, p. 262.

REDUCTION IN THE PLANTS

197

Strasburger's hypothesis is, however, open to a very serious a
pi'iori objection, as Hacker has pointed out; for if the account of
"reduction" in the plants given by Guignard and Strasburger be
correct, it corresponds exactly to the " pseudo-reduction " in animals,
and the " chromosomes " of the sexual generation must be bivalent
' like those of the early germ-cells in animals. The recent observa-
tions of Belajeff, Farmer, and especially those of Sargant, give, how-
ever, good reason to believe that both Guignard and Strasburger have
overlooked some of the most essential phenomena of reduction.
These observations have not yet revealed the exact nature of the
process, yet they show that
the first division of the pollen-
mother-cells (in the lily) is of
the hctcrotypical form ; i.e.
that the cJiromosomcs have the
form of rings. It is impos-
sible to avoid the suspicion
that these rings may be of
the same nature as the ring-
shaped tetrads in animals,
though apparently they do
not actually break up into
a tetrad. Until this point
has been cleared up by fur-
ther investigation the nature
of reduction in the plants
remains an open question.

Belajeff and Farmer showed '^e'l o^ ^^e liiy. («
., . .11 1, 1 <:-^£-. after Sargant.)

that as the daughter-chromo

Fig. 98. — Division of the chromosomes (? tetrad-
formaLion) in the first division of the pollen-mother-
after Farmer and Moore;

somes diverge after the first
division they assume a V-
shape, and Miss Sargant's

a. b. Two stages in the ring-formation (hetero-
typical mitosis). c~f. Successive stages, in profile
view, of the separation of the daughter-chromosomes.
g. The daughter-chromosomes, seen en face, at the
moment of separation ; this stage is perhaps to be
very interesting observations interpreted as a tetrad like those occurring in the

give some reason to believe

that the V breaks at the apex precisely as described by Hacker in
Cyclops and vom Rath in the salamander (Fig. 98, g). Should this
prove to be the case the way would be opened for an interpretation
of reduction in the plants agreeing in principle with that of Riick-
ert, Hacker, and vom Rath ; and as far as the plants are concerned,
the a priori objection to Strasburger's interesting hypothesis might
be removed.

198

REDUCTION OF THE CHROMOSOMES

E. Reduction in Unicellular Forms

A reduction of the number of chromosomes as a preparation for
conjugation in the one-celled forms has not yet been certainly deter-
mined, but there are many facts that render it highly probable. In

B

C

D

Fig- 99- — Conjugation of Closterhim. [Klebahn.]
A. Soon after union, four chromatophores. B. Cliromatophores 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.

DIVERGENT ACCOUNTS OF REDUCTION 1 99

the conjugation of infusoria, as already described (p. 165), the original
nucleus divides several times before union, and only one of the result-
ing nuclei becomes the conjugating germ-nucleus, while the others
perish, like the polar bodies. The numerical correspondence be-
tween the rejected nuclei or "corpuscles de rebut" has already been
pointed out (p. 168). Hertwig could not count the chromosomes
with absolute certainty, yet he states ('89) that in Paraviozciuvi
caudaturn, during the final division, the number of spindle-fibres
and of the corresponding chromatic elements is but 4-6, while in
the 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 the final division.^ In the gregarines Wolters('9i)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 chromosomes was not determined.

In the unicellular plants there are indications of a similar process,
but the few facts at our command indicate that the reduction may
here take place not before, but aftei; conjugation of the nuclei. Thus
in the dermids Closterimn and Cosmarimn, according to Klebahn
(Fig. 99), 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 permanent nucleus, while the
other degenerates and disappears. Chmielewski 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 occurrins: after fertilization.

F. Divergent Accounts of Reduction

We can only touch on a few of the accounts of reduction which
differ from both the modes already considered. Of these the most
interesting are observations which indicate the possibility of,

I. The Formation of Tetrads by Conjugation

A considerable number of observers have maintained that reduc-
tion 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 chromo-

1 Cf. Moore on the spermatogenesis of mammals, p. 201,

200 REDUCTION OF THE CHROMOSOMES

somes 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 {Pristi-
iLrus) ; 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 Pristiiiriis 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 segments ; i.e. the reduced number. In this case, therefore,
the preliminary 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 Caloptenus the spireme of the first
spermatocyte first segments into the normal number (twelve) of dumb-
bell-shaped segments, which then become associated in pairs to form
six tetrads. Each of these dumb-bell-shaped bodies is assumed to be
a bivalent chromosome, and the tetrad-formation is therefore inter-
preted as follows : —

abed- I ab-ed-kl a\b e\f /.p.^„^„x

. ■ s » , , ■ %' — \ — ~T — TT' Q^c (lecraas).

(spireme) (segmented spireme) e la S^\ '^

There is, therefore, no longitudinal splitting of the chromosomes.
A careful examination of the figures does not convince me of the
correctness of this conclusion, which is, moreover, inconsistent with
itself on Wilcox's own interpretation. Since each germ-nucleus
receives six chromosomes, the somatic number must be 12, and
Wilcox has observed this number in the divisions of the sperma-
togonia. The 12 dumb-bell-shaped primary segments must there-
fore represent single chromosomes, not bivalent ones, as Wilcox

assumes, and his primary tetrad must therefore be not — p-r? as he

assumes, but either ~ or (if we assume that the normal number of

I
chromosomes undergoes a preliminary doubling) • Until this

contradiction is cleared up Wilcox's results must be received with
considerable scepticism.

The second case, which is perhaps better founded, is that of the
earthworm {LmnbriciLS terrestris), as described by Calkins ('95, 2),
whose work was done under my own direction. Calkins finds, in
accordance with all other spermatologists save Wilcox, that the
spireme-thread splits longitudinally and then divides transversely
into 32 double segments. These then unite, two by two, to form
t6 tetrads. The 32 primary double segments therefore represent

DIVERGENT ACCOUNTS OF REDUCTION 20I

chromosomes of the normal number that have spht longitudinally,

a b ,,^ ,. ^ . a b a x ^ ^

I.e. -> etc., and the formula tor a tetrad is j- or Such

a b a b a X

a tetrad, therefore, agrees as to its composition with the formulas of

Hacker, vom Rath, and Rlickert, and agrees in mode of origin with

the process described by Rlickert in the eggs of Pristiru'its. While

these observations are not absolutely conclusive, they nevertheless

rest on strong evidence, and they do not stand in actual contradiction

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
munber of chromosomes appear in the germinal vesicle (Fig. 96, D).
They are at first single, then double by a longitudinal split, but after-
wards 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 &gg, but meanwhile each of them
again splits into two. Of the four chromosomes thus left in the Qgg
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 fifst polar body is a reducing division (!)
— a result which agrees with the earlier conclusions of Henking on
Pyrrochoris, but differs entirely from those of Rlickert, Hacker, and
vom Rath. The meaning of this remarkable result cannot here be
discussed. A clue to its interpretation is perhaps given by Hacker's
interesting observations on the two modes of maturation in Cantho-
camptiLS, for which the reader is referred to Hacker's paper ('95, i).

Moore ('95) has given an account of reduction in the spermatogen-
esis of mammals and elasmobranchs which differs widely in many
respects from those of all other observers. In both cases there is
said to be a resting stage between the two spermatocyte-divisions,
and in mammals (rat) the reduced number of chromosomes first
appears in the prophase of the last division. In elasmobranchs both
spermatocyte-divisions are of the heterotypical form, with ring-
shaped chromosomes. On all these points Moore's account contra-
dicts those of all other investigators of reduction in the animals,
and he is further in contradiction with Rlickert on the number
of chromosomes. His general interpretation accords with that of
Brauer and Strasburger, reducing divisions being totally denied.
The evidence on which this interpretation rests will be found in his
original papers.

202 REDUCTION OF THE CHROMOSOMES

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 ^Ty) on the
theoretical meaning of maturation the suggestion is made that
parthenogenesis may be due to failure on the part of the Q.gg 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 afterwards maintained by
Van Beneden.^ These authors assumed accordingly that in par-
thenogenetic eggs no polar bodies are formed. Weismann ('86)
soon discovered, however, that the parthenogenetic eggs of Poly-
phevtus (one of the Daphnidae) produce a single polar-body. This
observation was quickly followed by the still more significant dis-
covery by. Blochmann ('88) that in Aphis the parthenogenetic eggs
produce a single polar body zvhile the fertilized eggs produce tzuo.
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 parthenogenesis. Blochmann observed that in insects
the polar bodies were not actually thrown out of the O-gg, but
remained embedded in its substance near the periphery. At the
same time Boveri ('87, i) discovered that in Ascaris the second polar
body might in exceptional cases remain in the &gg and there give
rise to a resting-nucleus indistinguishable 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 : Partheno-
genesis 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 egg of Artemia,
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, tzvo polar-bodies are formed, and Platner discovered the

1 '83, p. 622. 2 Essay VI., p. 359. ^ I.e., p. 73.

MATURATION OF PARTHENOGENETIC EGGS

203

same fact in the butterfly Liparis ('89) — a fact which seemed to con-
tradict Boveri's hypothesis. Brauer's beautiful researches resolved
the contradiction by showing that there are two types of partJienogcne-
sis which may occur in the same animal. In the one case Boveri's

O-oyoboU

c

M^i%

B

Fig. 100. — First type of maturation in the parthenogenetic egg 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 tjie 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.

conception is exactly realized, while the other is easily brought into
relation with it.

{a) In both modes typical tetrads are formed in the germ-nucleus
to the number of eighty-four. In the first and more frequent case
(Fig. 100) but one polar body is formed, which removes eighty-four
dyads, leaving eighty-four in the Qgg. There may be an abortive
attempt to form a second polar spindle, but no division results, and

204

REDUCTION OF THE CHROMOSOMES

the eighty-four dyads give rise to a reticular cleavage-nucleus. From
this arise eighty-four thread-like chromosomes, and the same mtmber
appears in later cleavage-stages.

iU) It is the second and rarer mode that realizes Boveri's concep-
tion (Fig. loi). Both polar bodies are formed, the first removing
eighty-four dyads and leaving the same number in the o-gg. In the

D

E

Fig. lOi. — Second type of maturation in the parthenogenetic egg of Artemia. [Brauer.]
A. Formation of second polar body. B. Return of the second polar nucleus {p.bP) into the
egg; devf-lopment of the egg-amphiaster. C. Union of the egg-nucleus (?) with the second
polar nucleus {p.b.~). D. Cleavage-nucleus and amphiaster. E. First cleavage-figure with
equatorial plate containing i68 chromosomes in two groups of 84 each.

formation of the second, the eighty-four dyads are halved to form
two daughter-groups, each containing eighty-four single chromosomes.
Both these gronps remain in the egg, and each gives rise to a single
reticular nuclcns, as described by Boveri in Ascaris. These two nuclei
place themselves side by side in the cleavage figure, and give rise each
to eigJity-fonr cJiroviosomes, pircisely like tzvo germ-nuclei in ordinary
fcrtilirjation. The one hundred and sixty-eight chromosomes split

SUMMARY AND CONCLUSION 205

lengthwise, and are distributed in the usual manner, and reappear
in the same number hi all later stages. In other words, the second
polar body here plays the part of a sperm-nucleus, precisely as main-
tained 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. It
is impossible to doubt that the chromosomes of the first class are
bivalent; i.e. represent two chromosomes joined together — for that
the dyads have this value is not a theory, but a known fact. It
remains to be seen whether these facts apply to other parthenogenetic
eggs ; but the single case of Arteniia is little short of a demonstration
not only of Hacker's and vom Rath's conception of bivalent chromo-
somes, but also of the more general hypothesis of the individuality
of chromosomes (Chapter VI.). Only on this hypothesis can we
explain the persistence of the original number of chromosomes,
whether eighty-four or one hundred and sixty-eight, in the later stages.
How important a bearing this case has on Strasburger's theory of
reduction (p. 196) is obvious.

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
in the number of chromosomes in the lUtimate germ-cells to one-half the
number chaj^acteristic 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. As soon, however, as we attempt to advance beyond
this point we enter upon doubtful ground, which becomes more and
more uncertain as we proceed. With a few exceptions the reduction
in number first appears in the direct progenitors of the germ-cells by
a segmentation of the spireme-tJiread into one-half the usual number of
rods. This process is, however, not an actual reduction in the num-
ber of chromosomes, but only a preliminary "pseudo-reduction" in
the number of chromatin-;;/«i'i-<?i-. In what we may regard as the
typical case {e.g. Ascaris) the pseudo-reduction first appears at the
penultimate division ; i.e. in the grandmother-cell of the germ-cell
(primary oocyte or spermatocyte). It may, however, appear at a very
much earlier period, even in the embryonic germ-cells, the reduced
number appearing in every succeeding division until the germ-cells
are formed. This is the case in the salamander and in Cyclops. It
appears in its most striking form in the higher plants, where the re-

jo6

REDUCTION OF THE CHROMOSOMES

duced number appears in all the cells of the sexual generation (pro-
thallium, pollen-tube, embryo-sac), beginning with the mother-cell of
the asexual spores from which this generation arises.

In every case we must distinguish carefully between the primary
pseudo-reduction in the number of chromatin-masses, and the actual
reduction in the number of chromosomes ; for the former is in some
cases certainly not an actual halving of the number of chromosomes,
since each of the primary chromatin-rods is proved by its later history
to be bivalent, representing two chromosomes united end to end (sal-
amander, copepods). In these cases the actual reduction takes place

in the course of the last two

A

*t>

A.
V

B

8

the two

divisions (formation of the polar
bodies and of the spermatids),
each bivalent chromatin-rod di-
viding transversely into the two
chromosomes which it repre-
sents, and at the same time
(or earlier) splitting lengthwise.
Each primary rod thus gives
rise to a tetrad consisting of
two pairs of chromosomes which,
by the two final divisions, are
distributed one to each of the
four resulting cells. In the
copepods the first division sepa-
rates the longitudinal halves of
the chromosomes and is there-
fore an " equal division " (Weis-
mann). The second division
corresponds with the transverse
division of the primary rod, and therefore is the "reducing division"
postulated by Weismann.

This result gives a perfectly clear conception of the process of
actual reduction and its relation to the preparatory pseudo-reduction
that precedes it. It has, however, been absolutely demonstrated in
only two groups of animals, viz. the copepods and the vertebrates
(amphibia), and a diametrically opposite result has been reached in
the case of Ascaris (Boveri, Hertwig, Brauer) and in the plants (Gui-
gnard, Strasburger). In Ascaris typical tetrads are formed, but all
observers agree that they arise by a double longitudinal splitting of
the original chromatin-rod. In the plants no tetrads have been ob-
served, but the precise nature of the maturation-divisions is still in
doubt.

We have thus two diametrically opposing results. In the one

Fig. 102. — Diagram contrastin
modes of tetrad-formation.

A. Ascaris-type. Double longitudinal split-
ting of the primary rod ; no reduction in the
number of granules (" ids "). B. Copepod-type.
A longitudinal followed by a transverse division
of the primary rod; the number of granules
halved by the second division.

SUMMARY AND CONCLUSION 20/

case the primary halving in number is a pseudo-reduction, and each
tetrad arises by one longitudinal and one transverse division of
a bivalent chromosome, representing two different regions of the
spireme-thread (Hacker, vom Rath, Riickert, Weismann). In the
other case the primary halving appears to be an actual reduction,
and if tetrads are formed, they arise {Ascaris) by a double longitudi-
nal splitting of the primary rod, and all of its four derivatives repre-
sent the same region of the spireme-thread. Since the latter consists
primarily of a single series of granules ("ids" of Weismann, or
chromomeres), by the fission of which the splitting takes place, the
difference between the two views comes to this : that in the second
case the four chromosomes of each tetrad must represent identical
groups of granules, while in the first case they represent two differ-
ent groups (Fig. 102). In the second case the maturation-divisions
cannot cause a reduction in the number of different kinds of ids.
In the first case the number of ids is reduced to one-half by the
second division by which the second polar body is formed, or
by which two spermatids arise from the daughter-spermatocyte
(Riickert, Hacker, vom Rath).

The first view must obviously stand or fall with the conception of
the primary chromatin-rods as bivalent chromosomes. That this is
a valid conception is in my judgment demonstrated by Brauer's
remarkable observations on Artetnia ; for in this case it is impossi-
ble to escape the conclusion that the "chromosomes" of those
parthenogenetic embryos in which the number is halved are bivalent,
— i.e. have the value of two chromosomes united by their ends, —
and they lend the strongest support to vom Rath's and Hacker's
hypothesis. For if the number of chromosomes be merely the
expression of a formative tendency, like the power of crystalliza-
tion, inherent in each specific kind of chromatin, why should the
chromatin of the same animal differ in the two cases though derived
from the same source in both } Yet if the cleavage-nucleus arises
from eighty-four dyads the same number of chromatin-rods appears
in all later stages ; whereas if the dyads break each into two separate
chromosomes before their union, the number is thenceforward one
hundred and sixty-eight. So great is the force of this evidence that
I think we must still hesitate to accept the results thus far attained
in Ascaris and the plants, and must await further research in this
direction. Until the contradiction is cleared up the problem of
reduction remains unsolved.

208 REDUCriOX OF THE CHROMOSOMES

APPENDIX
I . Accessory Cells of the Testis

It is necessary" to touch here on the nature of the so-caUed •• Sertoli-cells." or
supporting ceUs of the testis in mammals, partly because of the theoretical signifi-
cance attached to them by Minot. partly because of their relations to the question
of amitosis in the testis. In the seminiferous tubules of the mammalian testis, the
parent-cells of the spermatozoa develop from the periphery inwards towards the
lumen, where the spermatozoa are finally formed and set free. At the periphen,- is
a laver of cells next the basement-membrane, having flat, oval nuclei. Within
this, the ceils are arranged in cokmins alternating more or less regularly
with long, clear cells, containing large nuclei. The latter are the Sertoli-cells,
or supporting ceUs ; they extend nearlv 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 ceUs
which lie in columns between the Sertoli-cells. and which undoubtedly represent
spermatogonia, spermatocrtes, and spermatids, though their precise relationship is,
to some extent, in doubt. The innermost of these cells, next the lumen, are sperma-
tids, which, after their formation, are found attached to the Sertoli-ceUs, and are
there converted into spermatozoa without further division. The deeper cells from
which they arise are spermatoc}-tes, and the spermatogonia lie deeper stiU, 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 ("71) at first
regarded the SertoU-ceU as the parent-cell of the group of spermatozoa attached to it,
and the same -view" was afterwards especially advocated by Biondi ("85), and is stUl
maintained by ^linot ("92), who regards the nucleus of the Sertoli-cell as the physio-
logical analogue of the polar bodies, i.e. as containing the female nuclear substance
(■92. p. 77). According to the opposing %dew. first suggested by iMerkel ("71). the
Sertoli-cell is not the parent-cell, but a nurse-cell, the spermatozoa developing trom
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 \-iews, adding the very significant result
that four spermatids arise from each spertnatocyte, precisely as was afterwards
shown to be the case in Ascaris. etc. The ver\" carefiil and thorough work of
Benda and von Ebner leaves no doubt, in my opinion, that mammalian spermato-
genesis conforms, in its main outlines, with that of Ascaris, the salamander, and
other forms, and that Biondi's views, which Minot unfortunately adopts, are without
foundation. If this be the case. Minofs theoretical interpretation of the Sertoli-cell
as the physiological equivalent of the polar bodies, of course collapses.

Various other attempts have been m.ade to discover in the spermatogenesis a
casting out of material which might be compared with the polar bodies, but these
attempts have now* only an historical interest. \'an Beneden and Julin sought such
material in the " residual corpuscles " left behind in the division of the sperm-forming
cells of Ascaris. Other authors have regarded in the same light the - Nebenkern ''
(Waldever) and the "residual globules" (Lankester, Brown) thrown off by the
developing spermatozoa of mammals. All of these ^dews are. like Minot's, wide
of the mark, and they were advanced before the real parallel between spermato-
genesis and ovosrenesis had been made known bv Platner and Hertwig.

APPENDIX 209

2. Ajnitosis in the Early Sex-Cells

Whether the progenitors of the germ-cells ever divide amitoticaUy 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
Aleves ('91), vom Rath C93), and Preusse ("95), leave no doubt whatever that
such divisions occur in the testis of many animals. Vom Rath, however, maintains,
after an extended investigation, that all cells so dividing do not belong in the cvcle
of development of the germ-cells ("93, p. 164) ; that amitosis occurs only in the sup-
porting or nutritive cells (Sertoli-cells, etc.), or in such as are destined to degenerate,
like the •• residual bodies "" of Van Beneden. Aleves 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 these observa-
tions, Flemming ("93) himself now admits the possibility that amitosis may form
part of a normal cycle of development, and Preusse has recently shown that amitosis
may continue through several generations in the early ovarian cells of Hemiptera
without a sign of degeneration.

LITERATURE. V

Van Beneden, E. — Recherches sur la maturation de Poeuf, la fecondation et la

division cellulaire : Arch. Biol., W. 1883.
Boveri, Th. — Zellenstudien, I., III. yena. 1887-90. See also " Befruchtung "

(List IV.).
Brauer, A. — Zur Kenntniss der Spemiatogenese von Ascaris 7)iegalocephala : Arch.

niik. Aiiat., XLII. 1893.
Id. — Zur Kenntniss der Reifung der parthenogenetisch sich entwickelnden Eies

von Artei/iia Saliiia: Arch. niik. Anat., XLIII. 1894.
Hacker, V. — Die Vorstadien der Eireifung (General Review) : Arch. mik. Anat.,

XLV. 2. 1895.
Hertwig, 0. — Vergleich der Ei- und Samenbildung bei Nematoden. Eine Grund-

lage fur cellulare Streitfragen : Arch.ntik. Anat., XXXVI. 1890.
Mark, E. L.— (See List IV.)
Plainer, G. — tJber die Bedeutung der Richtungskorperchen : Biol. Centralb., VIII.

1889.
vom Rath, 0. — Zur Kenntniss der Spermatogenese von Gryllotalpa viilgaris :

Arch mik. Anat., XL. 1892.
Id. — Neue Beitrage zur Frage der Chromatinreduction in der Samen- und Eireife :

Arch. mik. Anat.. XLVI. 1S95.
Ruckert, J. — Die Chromatinreduktion der Chromosomenzahl im Entwicklungsgang

der Organismei;! : Ergebn. d. Anat. u. E7itivick., III. 1893 (1894).
Strasburger, E. — Uber periodische Reduktion der Chromosomenzahl im Entwick-

1894.

CHAPTER VI

SOME PROBLEMS OF CELL-ORGANIZATION

" Wir mussen 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
centrosome. 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. Brticke 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, and suggested the possible existence of
vital units ranking between the molecule and the cell. Many biologi-
cal thinkers since Brlicke'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. I shall
make no attempt to review the a priori arguments that have been
urged in favour of this conception,^ but will rather inquire what
are the extreme conclusions justified by the known facts of cell-
structure.

^ Eletnentarorganismen, 1861, p. 386.
'-^ Anlhropogenie, 1891, p. 104.

3 For an exhaustive review of the subject see Yves Delage, La Sti'nctiire dn protoplasma,
ct les theories sur I'keredite. Paris, 1895.

210

THE NATURE OE CELL-ORGANS 211

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 organ-
ism, 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.^
Nevertheless, 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.

The visible organs of the cell fall under two categories according
as they are merely temporary structures, formed anew in each suc-
cessive cell-generation out of the common structural basis, or per-
manent structures whose identity is never lost since they are directly
handed on by division from cell to cell. To the former category
belong, in general, such structures as cilia, pseudopodia, and the
like ; to the latter, the nucleus, probably also the centrosome, and
the plastids of plant-cells. A peculiar interest attaches to the per-
manent cell-organs. Closely inter-related as these organs are, they
nevertheless have a remarkable degree of morphological indepen-
dence. 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 chromatophores that
neither botanists nor zoologists are as yet able to distinguish with
absolute certainty between those that form an integral part of the
cell, as in the higher green plants, and those that are actually inde-
pendent organisms living symbiotically within it, as is probably the
case with the yellow cells of Radiolaria. Even so acute an investi-
gator as Watase ('93, i) has not hesitated to regard the nucleus
itself — or rather the chromosome — as a distinct organism living in
symbiotic association with the cytoplasm, but having had, in an his-
torical sense, a different origin. It is but a short step from this con-

1 Cf. p. 41.

212 SOME PROBLEMS OF CELL-ORGANIZATION

elusion to the view that the centrosome, too, is such an independent
organism and that the cell is a symbiotic association of at least three
dissimilar living beings ! Such a conception would, however, as I
believe, be in the highest degree misleading, even if with Watase we
limit it to the nucleus and the cytoplasm. The facts point rather to
the conclusion that all cell-organs arise as differentiated areas in the
common structural basis of the cell, and that their morphological
character is the outward expression of localized and specific forms of
metabolic activity.

It is certain that some of the cell-organs are the seat of specific
chemical changes. Chromatin (nuclein) is formed only in the nucleus.
The various forms of plastids have specific metabolic powers, giving
rise to chlorophyll, to pigment, or to starch, according to their nature.
The centrosome, as Biitschli, Strasburger, and Heidenhain have in-
sisted, possesses a specific chemical character to which its remarkable
effect on the cytoplasm must be due.^ Even in regions of the cyto-
plasm not differentiated into distinct cell-organs the metabolic activities
may show specific and constant localization, as shown by the deposit
of zymogen-granules, the secretion of membranes, the formation of
muscle-fibres, and a multitude of related facts. Physiologically,
therefore, no line of demarcation can be drawn between permanent
cell-organs, transient cell-organs, and areas of the cell-substance that
are physiologically specialized but not yet morphologically differen-
tiated into organs. When we turn to the structural relations of cell-
organs, we find, I think, reason to accept the same conclusion in a
morphological sense. The subject may best be approached by a
consideration of the structural basis of the cell and the morphologi-
cal relations between nucleus and cytoplasm.

B. Structural Basis of the Cell

It has been pointed out in Chapter I. that the ultimate structural
basis of the cell is still an open question ; for there is no general
agreement as to the configuration of the protoplasmic network, and
we do not yet know whether the fibrillar or the alveolar structure is
the more fundamental. This question is, however, of minor impor-
tance as compared with the microsome-problem, which is, I think, the
most fundamental question of cell-morphology, and which is equally
pressing whatever view we may hold regarding the configuration of
the network.

Are the granules described as " microsomes " accidental and non-
essential bodies, produced, it may be, by the coagulating effects of

1 Cf. p. 77.

STRUCTURAL BASIS OF THE CELL 213

the reagents, as Fischer's experiments suggest ? Or are they normal
and constant morphological elements that have a definite significance
in the life of the cell ? It is certain that the microsomes are not
merely nodes of the network, or optical sections of the threads, as
the earlier authors maintained ; for the fibrillae may often be seen to
consist of regular rows of granules. Van Beneden gave the first
clear description of the microsomes in this regard in the following
words : " I have often had occasion to note facts that establish the
essential identity of the moniliform fibrillae and the homogeneous
fibrillas of the protoplasm. In my opinion every fibrilla, though it
appear under the microscope as a simple line devoid of varicosities,
is formed at the expense of a moniliform fibril composed of micro-
somes connected with one another by segments of uniting fibrils." ^
Again, in a later work he says of the fibrils of the astral system in
Ascaris : " It is easy to see that the achromatic fibrils are monili-
form, that they are formed of microsomes united by inter-fibrils." ^
Similar observations have been made by many later writers. In the
eggs of sea-urchins and annelids, which I have carefully studied, there
is no doubt that after some reagents, e.^: sublimate-acetic, picro-
acetic, chromo-formic, the entire astral system has exactly the struct-
ure described by Van Beneden in Ascaris. Although the basal
part of the astral ray appears like a continuous fibre, its distal part
may be resolved into a single series of microsomes, like a string of
beads, which passes insensibly into the cytoreticulum. The latter is
composed of irregular rows of distinct granules which stain intensely
blue with haematoxylin, while the substance in which they are em-
bedded, left unstained by haematoxylin, is colored by red acid aniline
dyes, such as Congo red or acid fuchsin.

The difficulty is to determine whether this appearance represents
the normal structure or is produced by a coagulation and partial dis-
organization of the threads through the action of the reagents. A
justifiable scepticism exists in regard to this point; for it is perfectly
certain that such coagulation-effects actually occur in the proteids of
the cell-substance, and that some of the granules there observed have
such an origin." It is very difficult to determine this point in the case
of the cyto-microsomes, owing to their extreme minuteness. The
question must, therefore, be approached indirectly by way of an
examination of the nucleus and its relation to the cytoplasm. Here
we find ourselves on more certain ground and are able to make an
analysis that in a certain measure justifies the hypothesis that the cyto-
microsomes may be true morphological elements having the power of
growth and division like the cell-organs formed by their aggregation.

1 'S3, p. 576, 577. 2 'S7, p. 266.

214 SOME PROBLEMS OF CELL-ORGANIZATION

I. Nucleus and Cytoplasm

From the time of the earlier writings of Frommann ('65, '^f),
Arnold ('67), Heitzmann ('73), and Klein ('78), 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, both chemically and physically,
as the cyto-reticulum, and that the achromatic nuclear membrane is
formed as a condensation of the same substance. Many investi-
gators, among whom may be named Frommann, Leydig, Klein, Van
Beneden, and Reinke, have described the threads 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 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 outside the nucleus, and
there is a pretty general agreement among cytologists, with the
important exception of Boveri, that both spindle-fibres and astral
rays arise by a direct rearrangement of the pre-existing structures.^
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 ; and in the division of the ^^g in the sea-urchin
I have obtained strong evidence that the spindle-fibres are directly
resolved into a portion of the general reticulum. These fibres are
in this case formed inside the nucleus from the linin-network ; and
we have therefore proof positive of a direct genetic continuity be-
tween the latter and the cytoplasmic structures. But more than this,
I have found reason to conclude that in this case a considerable
part of the linin-network is derived from the chromatin, that the
entire nuclear reticulum is a continuous structure, and that it is no
more than a specially differentiated area of the general cell-network
('95, 2). This conclusion finds, I believe, a very strong support in
the studies of Van Beneden, Heidenhain, and Reinke reviewed
beyond (p. 223) ; but the bearing of these only becomes plain after
considering the morphological differentiations of the nuclear net-
work and its transformations during mitosis.

1 Cf. Hammarsten (''95)-

2 The long-standing dispute as to the origin of the nuclear memlirane (whether nuclear
or cytoplasmic) is therefore of little moment.

MORPHOLOGICAL COMPOSITION OF THE NUCLEUS 21 5

C. Morphological Composition of the Nucleus

I. The Chroviatin

(a) Hypothesis of the Individjiality 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 pre-
existing 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, especially by Rabl and Boveri. As a result of a 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 tJie chromatic retictilnm of the resting
micleiis. The reticulum arises through a transformation of the
chromosomes, which give off anastomizing 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 reappear in the ensuing spireme-stage in
nearly or quite the same position 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 de-
scribed the nucleus as showing a distinct polarity having a " pole "
corresponding with the point towards which the apices of the chro-
mosomes converge (i.e. towards the centrosome), and an " anti-
pole " (Gegenpol) at the opposite point {i.e. towards the equator
of the spindle) (Fig. 17). Rabl's hypothesis was precisely
formulated and ardently advocated by Boveri in 1887 and 1888,
and again in i'89i, on the ground of his own studies and those
of Van Beneden on the early stages of Ascaris. The hypothesis
was supported by extremely strong evidence, derived especially from
a study of abnormal variations in the early development of Ascains,
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 in the o.^^- These chromosomes
give rise in the &^^ to a reticular nucleus, indistinguishable from

2l6

SOME PROBLEMS OF CELL-ORGANIZATION

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, how-
ever, abnormally increased from four to five or six (Fig. 103 C, D).
Again, the two chromosomes left in the egg after removal of the

Fig. 103. — Evidence of the individuality of the chromosomes. Abnormalities in the fertiliza-
tion of Ascaris. [BOVERI.]

A. The two chromosomes of the egg-nucleus, accidentally separated, have given rise each to a
reticular nucleus (9, ?) ; the sperm-nucleus below (c?)- 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 ; /. b?' the two chromosomes arising from it, $ the egg-chromo-
somes, (J the sperm-chromosomes. D. Resulting equatorial plate with six chromosomes.

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 afterwards
formed (Fig. 103, A,B). Finally, it sometimes happens that the two
germ-nuclei completely fuse while in the reticular state, as is nor-
mally the case in sea-urchins and some other animals (p. 153). From
the cleavage-nucleus thus formed arise four chromosomes.

MORPHOLOGICAL COMPOSITION OF THE NUCLEUS

21

These remarkable observations show that whatever be the mimber
of chromosomes entering into the formation of a reticular nucleus, the
same number a fterzvards issne from it — a result which demonstrates
that the number of chromosomes is not due merely to the chemical
composition of the chromatin-substance, but to a morphological organ-
ization of the nucleus. A beautiful confirmation of this conclusion
was afterwards made by Boveri ('93, '95, i) and Morgan ('95, 4)
in the case of echinoderms, by rearing larvae from enucleated egg-

Fig. 104. — Evidence of the individuality of the chromosomes in the ^g'g 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 befoire ; centrosomes divided.

fragments, fertilized by a single spermatozoon (p. 258). All the
nuclei of such larvae contain but half the typical number of chromo-
somes,— i.e. nine instead of eighteen, — since all are descended
from one germ-nucleus instead of two !

Van Beneden and Boveri were able, furthermore, to demonstrate
in Ascaris that in the formation of the spireme the chromosomes
reappear in the same position as those which entered into the forma-
tion of the reticulum, precisely as Rabl maintained. As the long

2l8

SOME PROBLEMS OF CELL-ORGANIZATION

chromosomes diverge, their free ends are always turned towards the
middle plane (Fig. 69), and upon the reconstruction of the daughter-
nuclei these ends give rise to corresponding lobes of the nucleus, as
in Fig. 104, which persist throughout the resting state. At the suc-
ceeding division the chromosomes reappear exactly in the same posi-

Fig. 105. — Independence of paternal and maternal chromatin in the segmenting eggs of
Cyclops. [A-C. from RtJCKERT; D. from HACKER.]

A. First cleavage-figure in C. stremms ; complete independence of paternal and maternal
chromosomes. B. Resulting 2-cell stage with double nuclei. C. Second cleavage ; chromosomes
still in double groups. D. Blastomeres with double nuclei from the 8-cell stage of C. brevlcornis.

tion, their ends lying in the nnclcar lobes as before {¥\g. 104, G, H). On
the strength of these facts Boveri concluded that the chromosomes
must be regarded as "individuals" or "elementary organisms," that
have an independent existence in the cell. During the reconstruc-
tion of the nucleus they send forth pseudopodia which anastomose to
form a network in which their identity is lost to view. As the cell

MORPHOLOGICAL COMPOSITION OF THE NUCLEUS 219

prepares for division, however, the chromosomes contract, withdraw
their processes, and return to their " resting state," in which fission
takes place. Applying this conclusion to the fertilization of the o-g^,
Boveri expressed his belief that " 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 remark-
able 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 J lalf of maternal ^ ^

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 re-
markable observations of Riickert, Hacker, Herla, and Zoja on the
independence of the paternal and maternal chromosomes. These
observations, already referred to at p. 1 56, may be more fully reviewed
at this point. Hacker ('92, 2) first showed that in Cyclops strenuus, 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
tennicornis^ each nucleus consists of two distinct though closely united
halves, which Hacker believed to be the derivatives of the two respec-
tive germ-nuclei. The truth of this surmise was demonstrated three
years later by Riickert ('95, 3) in a species of Cyclops, likewise identi-
fied as C. stremms (Fig. 105). The number of chromosomes in each
germ-nucleus is here twelve. Riickert was able to trace the pater-
nal 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 arises, 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 recog-
nizable at a late stage when the germ-layers were being formed.

Finally Victor Herla's remarkable observations on Ascaris ('93)
showed that in Ascai'is not only the chromatin of the germ-nuclei,
but also the paternal and maternal chromosomes, remain perfectly
distinct as far as the twelve-cell stage — certainly a brilliant confirma-
tion of Boveri's conclusion. Just how far the distinction is main-
tained is still uncertain, but Hacker's and Riickert's observations
give some ground to believe that it may persist throughout the
entire life of the embryo. Both these observers have shown that

1 '91, p. 410.

220

SOME PROBLEMS OF CELL-ORGANIZATION

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 egg !
When to these facts is added the evidence afforded by Brauer's
beautiful observations on Arteniia, no escape is left from the
hypothesis of the individuality of the chromosomes in one form or

Fig. io6. — Hybrid fertilization of the egg of Ascaris megalocephala, var. bivalens, by the sper-
matozoon of var. univaleiis. [Herla.]

A. The germ-nuclei shortly before union. B. The cleavage-figure forming; the sperm-nucleus
has given rise to one chromosome (cf ), 'he 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.

another, even though we admit that Boveri's statement may have
gone somewhat too far. The only question is how to state the facts
without introducing obscure conceptions as to what constitutes an
" individual." It is almost certain, as pointed out beyond (p. 221), that
the chromosomes are not the ultimate units of nuclear structure, for
they arise as aggregations of chromatin-grains that have likewise the
power of growth and division. The fact remains — and it is one of

MORPHOLOGICAL COMPOSITION OF THE NUCLEUS 221

the highest significance^ that these more elementary units group
themselves into definite aggregates of a higher order that show a
certain degree of persistent individual existence. It may be said
that the tendency to assume such a grouping is merely a question
of nuclear dynamics, and is due to a "formative force" innate in
the chromatin-substance. This is undoubtedly true ; but it is only
another form of expression for the facts, though one that avoids the
use of the quasi-metaphysical term "individual." Whether a chro-
mosome that emerges from the resting nucleus is individually the
same as one that entered into it can only be determined when we
know whether it consists of the same group of chromatin-granules
or other elementary bodies. It must not be forgotten, however, that
in the case of the &g^ the chromosomes may persist without loss of
their boundaries from one division to another, since no reticulum is
formed (cf. p. 193).

{b) Composition of the Chromosomes. — We owe to Roux^ the first
clear formulation of the view that the chromosomes, or the chro-
matin-thread, consist of successive regions or elements that are
qualitatively different (p. 183). 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 unmis-
takably to the conclusion that these granules are perhaps to be re-
garded as independent morphological elements of a lower grade than
the chromosomes. That they are not artefacts 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 morpholog-
ical nature is, however, even more difficult than in the case of the chro-
mosomes ; 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 unde-
termined. 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 chromatin-disks arranged in a linear series (Van Beneden's
figures of the same object show at most five) which finally fuse to

1 Bedeutung det- Kerntheilungsfiguren, 1883, p. 15.

222 SOME PROBLEMS OF CELL-ORGANIZATION

form an apparently 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 sperma-
togenesis of the same animal (variety bivalens) by Brauer. At the
time the chromatin-grains divide, in the reticulum of the spermato-
cyte-nucleus, they are very numerous. His figures of the spireme-
thread show at first nearly forty granules in linear series (Fig. 92, A).
Just before the breaking of the thread into two the number is
reduced to ten or twelve (Fig. 92, C). Just after the division to form
the two tetrads the number is four or five (Fig. 92, 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. 61) 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 visi-
ble 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- 79) •'' 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. These
units must be capable of assimilation, growth, and division without
loss of their specific character. This I believe is an absolute logical
necessity. 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 assump-
tion that in no manner prejudices our conclusion regarding the
ultimate morphological composition of the chromatin.

CHROMATIN, LININ, AND THE CYTORETICULUM

D. Chromatin, Linin, and the Cytoreticulum

What, now, is the relation of the smallest visible chromatin-grains
to the linin-network and the cytoreticulum ? Van Beneden long
ago maintained ^ that the achromatic network, the nuclear mem-
brane, and the cytoreticulum 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 migJit
be transformed into one another, and zvere 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 Schlo-
ter, finds that the nuclear network contains granules of two
kinds differing in their staining-capacity. The first are the basi-
chromatin granules, which stain with the true nuclear dyes (basic
anilines, etc.), and are identical with the " chromatin-granules " of
other authors. The second are the oxychromatin-granules of the
linin-network, which stain with the plasma-stains (acid anilines, etc.),
and are closely similar to those of the cytoreticulum. These tzvo
forms graduate into one another, and are cojijectnred to be different
pJiases of the same elements. This conception is furthermore sup-
ported by many observations on the behaviour of the nuclear net-
work 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. 244), and is known to vary greatly in bulk.
In certain cases a very large amount of the original chromatic net-
work 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 was forced to the conclusion ('95, 2) that a consid-
erable part of the linin-network, from which the spindle-fibres are
formed, is actually derived from the chromatin.

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 microsomes at one extreme and the chromatin-gran-
ules at the other. And inasmuch as the latter are certainly capable
of growth and division, we cannot deny the possibility that the former
may have like powers. It may well be that our present reagents do
not give us a true picture of these elementary units — that "micro-
somes " are but a rude semblance of reality. That they are never-

1 '83, p, 580, 583. 2 Lc, p. 583.

224 SOME PROBLEMS OF CELL-ORGANIZATION

theless an expression of the morphological aggregation of the proto-
plasmic network out of more elementary units, must, I think, be
accepted as a working hypothesis. Whether they are elementary
organisms in Altmann's sense, whether they have a persistent mor-
phological identity, whether they arise solely by the division of pre-
existing microsomes, or may undergo dissolution and reformation,
whether, in short, they are the self-propagating elementary bodies
postulated by so many eminent naturalists as the essential basis of
the cell, — all these are entirely open questions which the cytology
of the future has to solve.

E. The Centrosome

When we turn to the centrosome, we find clear evidence of the
existence of a cell-organ which, though scarcely larger than a cyto-
microsome, possesses specific physiological powers, assimilates, grows,
divides, and may persist from cell to cell, without loss of identity.
It is far easier to define the centrosome in physiological than in mor-
phological terms. In the former sense Boveri ('95, 2) defines it as a
single permanent cell-organ which forms the dynamic centre of the cell
and multiplies by division to form the centres of the daughter-cells}
A centrosome is necessarily present in all cells at the time of mitosis.
Whether, however, it persists in the resting state of all cells is un-
known. The most careful search has thus far failed to reveal its
presence in many tissue-cells, e.g. in muscle-cells and many gland-
cells ; but these same cells may, under certain conditions, divide by
mitosis, as in regeneration or tumour-formation, and the centrosome
may be hidden in the nucleus, or so minute as to escape observation.
We must, however, remember that the centrosome often disappears
in the mature Q-g^, and the same may be true of some tissue-cells.

Van Beneden's and Boveri's independent identification of centrosome in Asca?-2S
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 pigment-cells of fishes (Solger, '89, '90), in the
spermatocytes of Amphibia (Hermann, '90), in the leucocytes, endothelial cells, con-
nective tissue-cells and lung-epithelium of salamanders (Flemming, '91), in various
plant-cells (Guignard, '91), in the one-celled diatoms (Biitschli, '91), in the giant-
cells and other cells of bone-marrow (Heidenhain, Van Bambeke, Van der Stricht,
'91), in the flagellate Noctiluca (Ishikawa, '91), in the cells of marine algae (Stras-
burger, '92), in cartilage-cells (Van der Stricht, '92), in the cells of cancerous growths
(epithelioma, Lustig and Galeotti, '92), in the young germ-cells as already described,
and finally, in gland-cells (vom Rath, '95), and in nerve-cells (Lenhossek, '95).
They have not yet been found in resting muscle-cells.

1 The fact that the centrosome is double in many cells does not conflict with this defini-
tion, for the douliling is obviously a precocious preparation for the ensuing division.

THE CENTROSOME 225

The earlier observers of the centrosome always found it lying in the cytoplasm,
outside the nucleus. Almost simultaneously, in 1893, three investigators indepen-
dentlv discovered it inside the nucleus of the resting cell, — Wasielewsky, in the
young ovarian eggs (oogonia) of Ascaris ; Brauer, in the spermatocytes of the same
animal ; and Karsten, in the cells of a plant, Psiloticiii (Humphrey states, however,
that Karsten's observations were erroneous). Several later observers have described
a similar intra-nuclear origin of the centrosome, and several of these (Zimmermann,
Lavdovsky, Ki^uten) have followed Wasielewsky in locating it in the nucleolus.
Evidence against this latter view has been brought forward, especially by Humphrey
and Brauer. The latter observer found both nucleoli and centrosome as separate
bodies within the nucleus. He made further the interesting discovery that in

t c 'I

C*

ABC D

>>.,

/

Fig. 107. — Mitosis with intra-nuclear centrosome, in the spermatocytes of Ascaris niegalo-
cephala, var. univalens. [BraUER.]

A. Nucleus containing a quadruple group or tetrad of chromosomes {t) , nucleolus («), and
centrosome {c) . B. C. Division of the centrosome. D. E. F. G. Formation of the mitotic figure,
centrosomes escaping from the nucleus in G.

Ascaris the centrosome lies, in one variety {imivalens^ inside the micleus, in the other
variety {bivalens') 'Outside — a fact which proves that its position is non-essential
(cf.-Figs. 92 and 107). Oscar and Richard Hertwig maintain that the intra-nuclear
position of the centrosome is the more primitive, the centrosome having been
originally differentiated from a part of the nuclear substance. This view is based
in the main on the facts of mitosis in the Infusoria, where the whole mitotic figure
appears to arise within the nuclear membrane (cf. p. 62).

Whether a true centrosome may ever arise de novo is likewise
undetermined. The possibility of such an origin has been conceded
by a number of recent writers, among them Bitrger, Watase, Richard
Hertwig, Heidenhain, and Reinke. The latter author ('94) would

Q

226 SOME PROBLEMS OF CELL-ORGANIZATION

distinguish in the cell, besides the " primary centres " or centrosomes,
secondary and tertiary centres, the latter being single microsomes
formed at the nodes of the network. By the successive aggrega-
tions of the latter may arise the secondary and primary centres as
new formations. Watase ('94) advocates a somewhat similar view,
and states that he has observed numerous gradations between a true
aster and such "tertiary asters" as Reinke describes. Further evi-
dence in the same direction is afforded by Morgan's remarkable,
observations on the formation of "artificial asters" in unfertilized
sea-urchin eggs which have lain for some time in sea-water ('96).
Such eggs often contain numerous asters, each of which contains a
body resembling a centrosome.^ Beside these observations must be
placed those of Richard Hertwig, on the formation of an amphiaster
in ripe unfertilized sea-urchin eggs (p. 159). All these observations
are of high interest in their bearing on the historical origin of the
centrosome ; but they do not prove that the centrosome of the nor-
mal aster ever arises by free formation. On the whole, the evidence
has steadily increased that the centrosome is to be classed among the
permanent cell-organs ; but whether it ranks with the nucleus in this
regard must be left an open question.

The known facts are still too scanty to enable us to state precisely
what a centrosome is in a morphological sense, either as regards its
actual structure or its relation to other parts of the cell. In its sim-
plest form (Fig. 108, ^4) the centrosome appears under the highest
powers as nothing more than a single granule of extraordinary
minuteness which stains intensely with iron-hasmatoxylin, and can
scarcely be distinguished from the cyto-microsomes except for the
fact that it lies at the focus of the astral rays. In this form it
appears at the centre of the young sperm-aster in various animals
— for example in the sea-urchin (Boveri), in CJiCBtopterns (Mead),
and in Nereis? In almost all cases, however, the centrosome after-
wards assumes a more complex structure and becomes surrounded
by certain envelopes, the relation of which, on the one hand, to the
centrosome and, on the other hand, to the astral rays have not yet
been fully cleared up.

Boveri, whose observations have been confirmed by Brauer, Hacker,
and others, described the centrosome in the cleavage-asters of Ascaris
as a small sphere containing a minute central granule ; and Brauer's
careful studies on the spermatogenesis of the same animal showed

1 I have had the privilege of examining Professor Morgan's preparations, and can confirm
his statement that these eggs contain but a single nucleus and hence are not polyspermic.

2 This appearance is not due to the shrinkage of a larger and more complex structure, as
some authors have suggested; for in Nereis such a structure — i.e. the centrosphere — is
afterwards developed around the centrosome.

THE CENTROSOME 22/

that both these structures are persistent and that division of the sphere
is preceded by division of the granule (Fig. 107). The central granule
is exactly like the simple centrosome of the sperm-aster as described
above, but we do not yet know with certainty the genesis of the
sphere surrounding it, and hence cannot state whether this is part
of the centrosome proper or a part of the centrosphere surrounding
it. The former view is adopted by Boveri, who suggests the word
" centriole " for the central granule ; and, according to his observa-
tions on, Ascaris and on sea-urchins, the simple centrosome of the
original sperm-aster enlarges to form the sphere, while the centriole
afterwards appears within it. In the case of Thalasseina, however,
Griffin's observations leave no doubt that the central granule per-
sists in its original form from its first appearance in the sperm-aster
through every stage of the cleavage-amphiaster, dividing during the
early anaphase in each aster and giving rise to the centrosomes of
the daughter-asters in which it again appears as a simple granule at
the focus of the rays without a trace of surrounding envelopes
(Fig. 73). In the cleavage-amphiaster it is surrounded by a some-
what vague, rounded mass (apparently representing the entire " cen-
trosome " of Boveri and Brauer), which in turn lies in a reticulated
centrosphere, from which the rays radiate. Both these structures
disappear during the late anaphase, leaving only the central granule.
Here, therefore, the true centrosome certainly corresponds to the
central granule or centriole ; and all the surrounding structures be-
long to the centrosphere.

As soon as we look further we find apparent departures from this
simple type of centrosome. In leucocytes Heidenhain finds at the
centre of the centrosphere not one or two, but always three, and some-
times four, granules, which he conceives as centrosomes forming a
central group or microcentrum. In the giant-cells of bone-marrow
the central group consists of a very large number (a hundred or
more) of such granules, each of which is again conceived as a
"centrosome" (Fig. 11, D). In the sea-urchin [Ec/mins) Boveri states
that the original simple centrosome of the sperm-aster enlarges
greatly to form a relatively large, well-defined sphere in which
appear numerous granules (centrioles), which he would compare
individually with the elements of Heidenhain's "central group."
I have given a somewhat similar account of the facts in Toxopnett-
stes, describing the centrosphere as a reticulated mass derived from
an original granule or centrosome at the focus of the rays,^ and many

1 Professor Boveri informs me that I was in error in attributing to him the view that the
entire central mass of the aster — i.e. the centrosphere — here represents the centrosome.
The large spherical centrosome of Echinus is surrounded by a clear area which he regards
as the centrosphere.

228 SOME PROBLEMS OF CELL-ORGANIZATION

Other investigators have been unable to find a distinct body to be
identified as a centrosome within the pentrosphere. As far as the
sea-urchins are concerned, there is, I think, good reason to doubt not
only my own former conclusions, but also those of Boveri. Both vom
Rath ('95, 2) and Hill ('95) find at the centre of the centrosphere in
sea-urchins a distinct black granule (" centrosome "), which becomes
double in the early anaphase precisely as in Thalassenia. More-
over, Griffin's studies under my direction show that the minute single
centrosome of Thalassenia entirely loses its staining-power after cer-
tain reagents and only comes into view after other treatment.^ I am
now, therefore, inclined to believe that many if not all of the accounts
asserting the absence of a minute central centrosome in the centro-
sphere are based on unsuitable methods, and that in most of such
cases, if not in all, it is really present.

However this may be, it is now certainly known that the centro-
some is in some cases a granule so small as to be almost indistin-
guishable from the microsomes ; that in this form it is able to organize
the surrounding cytoplasm into the astral system ; and that in this
form it may be handed on by division from cell to cell. It may well
be that in some cases such a centrosome may multiply to form a cen-
tral group, as in leucocytes and giant-cells'; that it may enlarge to
form a granular or reticular sphere, as Boveri describes ; and that
the individual granules within such a sphere do not have the value
of centrosomes. Such secondary morphological modifications do not
affect the physiological significance of the centrosome as a perma-
nent cell-organ, but they have an important bearing on the question
of its relation to the other constituents of the cell.

The latter question has not been definitely answered. Butschli,
who has been followed by Erlanger, regards the centrosome as a
small differentiated area in the general alveolar structure ; and he
describes it in the sea-urchin as actually made up of a number of
minute vesicles (Fig. 8, B). Burger ('92) suggested that the entire
attraction-sphere and aster arise by a centripetal movement of micro-
somes to form a radiating group the centre of which (centrosome) is
represented by a condensed mass of the ground-substance. Watase
('93, '94) added the very interesting suggestion that the centrosome is
itself nothing other than a microsome of the same morphological
nature as those of the astral rays and the general thread-work, differ-
ing from them only in size and in its peculiar powers.^ Despite the

1 The centrosome disappears after fixation with sulilimate-acetic, but is perfectly shown
after pure sublimate or picro-acetic. See Science, Jan. lo, i8g6.

■'' 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 lilce the nucleus, but a temporary body formed at the focus of the astral

THE ARCHOPLASMIC STRUCTURES 229

ambiguity of the word "microsome" Watase's suggestion is full of
interest, indicating as it does that the centrosome is morphologically
comparable to other elementary bodies existing in the cytoplasmic
structure, and which, minute though they are, may have specific
chemical and physiological properties.

F. The Archoplasmic Structures
I. Asters and Spindle

The asters and attraction-spheres have a special interest for the
study of cell-organs ; for these are structures that may divide and
persist from cell to cell or may lose their identity and reform 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 struct-
ural basis. Two sharply opposing views of these structures are now
held. Boveri i^'^'^, 2), who has been followed in a measure by Stras-
burger, maintains that the attraction-sphere of the resting cell is com-
posed of a distinct substance, '^ arcJioplasni,'' consisting of granules
or microsomes aggregated 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 reticulum as the roots of a plant grow into
the soil, and at the close of mitosis are again withdrawn into the cen-
tral mass, breaking up into granules meanwhile, so that each daugh-
ter-cell receives one-half of the entire archoplasmic material of the
parent-cell. This material is, however, wholly distinct from that of
the general reticulum, not, as many earlier observers have maintained,
identical with it. Boveri was further inclined to believe that the
individual granules or archoplasmic microsomes were " independent
structures, not the nodal points of a general network," and that the
archoplasmic rays arose by the arrangement of these granules in

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. In the latter case the astral rays are con-
ceived as actual derivatives of the centrosome which, as it were, spins them out in the cyto-
plasm. " The aster, from this point of view, may be considered as a physiological device
for concentrating the cytoplasmic substance in a form which can be spun out again into
filaments in the direction which will produce a definite physiological effect" ('94, p. 284).
This part of Watase's conception is, on the whole, I think, opposed to the facts, though it
certainly explains the inpushing of the nuclear membrane during the prophases of mitosis.
It is impossible to believe that the rays of the enormous sperm-aster are developed out of
the minute granule at their centre or that they flow back into it at the close of division.
The centrosome increases in size during the formation of the aster, decreases during its
disappearance, which is the reverse of what the hypothesis demands. Many other argu-
ments in the same direction might be urged.

230 SOME PROBLEMS OF CELL-ORGANIZATION

rows without loss of their individuaUty.^ In a later paper on the
sea-urchin ('95) this view is somewhat modified by the admission that
in this case the archoplasm may not pre-exist as formed material, but
that the rays and fibres may be a new formation, crystallizing, as it
were, out of the protoplasm about the centrosome as a centre,^ but
having no organic relation with the general reticulum.

Strong evidence against the archoplasm-theory has been brought
forward by many investigators, and I believe it to be in principle
untenable. Nearly all recent workers have accepted in one form 'or
another the early view of Biitschli, Klein, and Van Beneden that the
astral rays and spindle-fibres, and hence the attraction-sphere, arise
through a morphological rearrangement of the pre-existing protoplas-
mic network, under the influence of the centrosome. Although this
view may be traced back to the early work of Fol ('73) and Auerbach
('74), it was first clearly formulated by Biitschli ^^G), 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 centrosome (Fig.
8, B). The fibrous appearance of the astral rays is an optical delu-
sion, for they are not fibres, but flat lamellae forming the walls of
elongated closed chambers. This view has more recently been urged
by Reinke and Eismond.

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 ('78), who regarded the aster as due to "a radiar
arrangement of what corresponds to the cell-substance," the latter
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

1 '88, 2, p. 80. ^ I.e., p. 40.

^ For a very careful review of the early views on this subject, see Mark, Lhnax., 1881.

* '92, 2, pp. 158-169.
^ It is interesting to note that in the same place Klein anticipated the theory of fibrillar
contractiHty, both the nuclear and the cytoplasmic reticulum being regarded as contractile
{I.e., p. 417)-

'■' 'S3, P- 592. ' '83, p. 576.

THE ARCHOPLASMIC STRUCTURES 23 I

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 ('87). "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, Eismond, Reinke, etc., have ob-
served that the astral fibres branch out peripherally into the general
reticulum and become perfectly continuous with its meshes. This is
very clearly shown in the formation of the sperm-aster about the
middle-piece of the spermatozoon. In the sea-urchin {Toxopneiistes)
the formation of the rays from the cytoplasmic reticulum can be fol-
lowed step by step, and there can, I think, be no doubt that the astral
rays arise by a direct transformation or morphological rearrangement
of the pre-existing structure, and that they extend themselves at their
outer ends, as the sperm-aster moves through the egg-substance, by
progressive differentiation out of this reticulum.^ Once formed, how-
ever, the rays may possess a considerable degree of persistence and
may actively elongate by growth. Only thus can we explain the
pushing in of the nuclear membrane by the ingrowing spindle-fibres
during the prophases of mitosis in certain forms (p. 50) and the
bending of the rays when two asters collide, as recently described by
Kostanecki and Wierzejski ('96). It seems certain, furthermore, that
during the rotation of the amphiaster in the formation of the polar
bodies (Fig. 71) and in similar cases, the spindle, at least, moves bodily.
The substance of the spindle or of the asters may, moreover, persist
in the resting cell, after the close of mitosis, as the attraction-
sphere or paranucleus (Nebenkern), and in such cases the term
" archoplasm " may conveniently be retained for descriptive purposes.
To regard the archoplasm as a primary and independent constituent
of the cell would, however, as I believe, be an error.

■^ '^3. P- 550. " I.e., p. 263. 3 /_^_^ p_ 275. 4 I.e., p. 280. ° '95, 2, p. 446.

2 32 SOME PROBLEMS OF CELL-ORGANIZATION

2. The Attraction-spJiere

The foregoing conception of the asters receives a strong support
from the study of the attraction-sphere in resting cells. It is agreed
by all observers that this structure is derived from the aster of the
dividing cell ; but there is still no general agreement regarding its
precise mode of origin from the aster, and the subject is confused by
differences in the terminology of different authors. There are some
cases in which the entire aster persists throughout the resting cell
(leucocytes, connective tissue-cells) and the term " attraction-sphere "
has by some authors been applied to the whole structure. As origi-
nally used by Van Beneden, however,^ the word was applied (in
Ascaris) not to the entire aster but only to its central portion — a
spherical mass bounded by a circle of microsomes from which the
astral rays proceed. At the close of division the rays fade away in
the general network, leaving only the central sphere containing the
ccntrosome. Boveri's account of the same object was entirely differ-
ent; for he conceived the attraction-sphere (" archoplasm-sphere ")
of the resting cell as representing the entire aster, the rays being
withdrawn towards the centrosome and breaking up into a mass of
granules. Later workers have proposed different terminologies, which
are at present in a state of complete confusion. Fol ('91) proposed
to call the centrosome the astroceiitre, and the spherical mass sur-
rounding it (attraction-sphere of Van Beneden) the astrospJiere.
Strasburger accepted the latter term and proposed the new woid
" centrosphere " for the astrosphere and the centrosome taken to-
gether.^ This terminology has been accepted by most botanists and by
some zoologists. A new complication was introduced by Boveri ('95),
who applied the word "astrosphere" to the entire aster o.y.zXM'&wQ.
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 " centrosphere," which may be
understood as equivalent to the " astrosphere " of Fol.

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, the centrosphere has a radiate structure, being
traversed by rays which stretch between the centrosome and the
peripheral microsome-circle (Figs. 34, 108, G). In the other form,
described by Vejdovsky in the eggs of Rliynchelniis, by Solger and
Zimmermann in pigment-cells, by myself in sea-urchin eggs and in

i'83, p. 548. 2.52, p. 51.

THE ARCHOPLASMIC STRUCTURES

^IZ

Nereis, by Riickert in Cyclops, and in a number of other cases, the
centrosphere has a non-radiate reticular structure (Figs. 71, 108, E).
In some cases no centrosome has been found in this sphere ; but for
reasons ah'eady stated (p. 228) I incHne to believe that a centrosome
is really present.

In many, if not in all cases of both types, the sphere consists of an
outer and an inner zone, the latter enclosing the centrosome ; but the
relation of the inner zone to the centrosome still remains, in a meas-

//\

\\\l|//,

H

Fig. 108. — Diagrams illustrating various descriptions of centrosome and centrosphere.
A. Simplest type; only a minute centrosome at the focus of the ravs (sperm-aster in many
forms). B. Rays proceeding directly from a centrosome of considerable size within which is a
central granule. Example, Brauer's description of the spermatocytes oi Ascaris. C. Rays pro-
ceeding from a clear centrosphere (astrosphere of Strasburger) , enclosing a centrosome like
the last but with no central granule (in flowering plants according to Guignard, Strasburger,
and others). D. An extremely minute centrosome lying in the middle of a large reticulated cen-
trosphere (eg. Hill's description of the sperm-aster in sea-urchins and tunicates). E. Like the
last, but with a small spherical body surrounding the centrosome (examples, the eggs of Ihalas-
sema And Ne?-eis). F. No centrosome as distinguished from tlie reticulated centrosphere. E-x-
amples in the pigment-cells of fishes according to Zimmerman, in the eggs of echinoderms ac-
cording to Wilson ; many similar accounts have been given, but all are open to question. G. In
Ascaris, according to Van Beneden, outside the centrosome lie the cortical and medullary zones
of the attraction-sphere. H. The same according to Boveri. The centrosome contains a cen-
tral granule or centriole (cf. 5.) ; outside this is a clear zone (medullary zone of Van Beneden),
and outside this a vaguely defined granular zone, probably corresponding to Van Beneden's
cortical zone.

ure, in doubt. Van Beneden described the centrosphere in Ascaris
as consisting of an outer cortical and an inner medullary zone, both
of which were conceived as only a modification of the inner region of
the aster. Boveri's account is somewhat different. The centrosome
is described as surrounded by a clear zone (" heller Hof "), — probably
corresponding with Van Beneden's "medullary zone," — -while the
" cortical zone " of the latter author is not recognized as distinct from
the aster (or archoplasm-sphere). The centrosome itself contains a

234

so ATE PROBLEMS OF CELL-ORGANIZATION

minute central granule or centriole. This discrepancy between Boveri
and Van Beneden was cleared up in a measure by Heidenhain's
beautiful studies on the asters in leucocytes, and the still more
thorough later work of Driiner on the spermatocyte-divisions of the
salamander. In leucocytes (Fig. 35) 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 out-
side the centrosphere. Driiner
found that a whole series of such
concentric circles might exist (in
the cell shown in Fig. 109 no
less than nine), but that the inner-
most two are often especially
distinct, so as to mark off a cen-
trosphere composed of a medul-
lary and a cortical zone precisely
as described by Van Beneden.
These observations show conclu-
sively that the centrosphere of
the radial type is merely the inner-
most portion of the aster, which
acquires an apparent boundary
through the especial development
of a ring of microsomes. And

Fig. loy. — spciiiKitogonium of salaman-
der. [Druner.]

The nucleus lies below. Above is the
enormous 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.

thus Van Beneden's original view
is confirmed, that not only the

aster as a whole, but also the centro-
sphere, is but a modified area of the
general cytoplasmic thread-work.
Heidenhain points out that there are many cases — for instance,
the young sperm-aster — in which there is at first no clearly marked
central sphere, and the rays proceed outward directly from the centro-
some. The sphere, in such cases, seems to arise secondarily through
a modification of the inner ends of the astral rays. Heidenhain there-
fore concludes that the centrosome is the only constant element in the
sphere, the latter being a secondary formation and not entitled to rank
as a persistent cell-organ, though it may in certain cases persist and
divide like the centrosome. Vom Rath, who has made a very careful
study of the attraction-spheres in a large number of cells among both
vcrtebrata and invertebrata, arrives at a nearly similar view, though
he lays greater stress on the differentiation and independence of the
sphere. In asters of dividing cells he could find in many cases no

THE ARCHOPLASMIC STRUCTURES 235

limit between sphere and aster, though in other cases it is distinctly
present. In the resting cell, on the other hand, the boundary of the
sphere is often very sharply marked, so that the sphere appears as a
well-defined spherical body. The origin of such a definite sphere from
the aster has not been very definitely determined, but Driiner's obser-
vations indicate that it arises in the manner described by Van Bene-
den, through the disappearance of the more peripheral portions of the
astral rays. It is, in other words, the persistent centrosphere.^

The genesis of the reticular type of centrosphere is not so well
determined. In Nereis the aster (maturation-asters, sperm-aster)
has at first nothing more than a minute centrosome at its centre.
This becomes surrounded at a later period by a large reticulated
centrosphere, showing no sign of radial arrangement, that appears
to arise by a transformation of the inner ends of the astral rays.
A nearly similar account is given by Hill in the case of the sperm-
aster in Strongylocentrotus and Phallitsia. In these latter cases the
centrosphere shows no differentiation into cortical and medullary
zones. In Thalassema and Nereis, on the other hand, the minute cen-
trosome becomes surrounded by a somewhat vague body distinctly
different from the reticulum of the outer centrosphere, and this
body perhaps represents a "medullary zone." This body, with the
centrosome, corresponds very nearly to the "centrosome" of Ascaris
with its " centriole " or central granule as described by Boveri and
Brauer; but in TJialassema Griffin's observations show conclusively
that the minute central granule alone is the centrosome, and that
the surrounding body does not persist after division. I cannot
avoid the suspicion that the body described by Boveri as the
" centrosome " in Echinus may represent this medullary region of
the centrosphere, and that he, like myself, may have overlooked the
centrosome. Nor does it seem impossible that the " centriole " or
central granule of Ascaris (Boveri, Brauer) may likewise represent
the true centrosome. These questions can only be cleared up by
further investigation.

To sum up: The history of the " archoplasmic " structures gives
strong ground for the conclusion that attraction-spheres, asters, and
spindle are, like the nucleus, differentiations of the general cell-netivork,
zvhich is, as it zvere, mo2ilded by the centrosome into a specific form.
If this be well founded, the word "archoplasm" has no significance
save in a topographical or descriptive sense. In this light it is an
interesting fact that the aster or attraction-sphere may either persist
and divide, like a permanent cell-organ, or may disappear and re-form
in successive cell-generations.

^ The same general result is indicated in the case of plants, though the phenomena have
here been less carefully examined.

<.l6 SOME PROBLEMS OF CELL-ORGANIZATION

G. Summary and Conclusion

A minute analysis of the various parts of the cell leads to the
conclusion 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. Permanent organs, such as the nucleus or centrosome,
are constant areas in the same basis, which never are formed de novo^
but arise by the division of pre-existing areas of the same kind.
These two extremes are, however, connected by various interme-
diate 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.

The facts point strongly to the conclusion, which has been espe-
cially urged by De Vries and Wiesner, that in many if not in all
cases the division of cell-organs is in the last analysis brought about
by the division of more elementary masses of which they are made
up; and furthermore that the degree of permanence depends on the
degree of cohesion inamfested by these masses. The clearest evi-
dence in this direction is afforded by the chromatic substance of the
nucleus, the division of which does not take place as a single mass-
division, but through the fission of more elementary discrete bodies
of which it consists or into w^hich it is resolved before division.
Several orders of such bodies are visible in the dividing nucleus,
forming a series of which the highest term is the plurivalent chro-
mosome, the lowest the smallest visible dividing basichromatin-grains,
while the intermediate terms are formed by the successive aggrega-
tion of these to form the chromomeres of which the dividing chromo-
somes consist. Whether any or all of these bodies are "individuals "
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 properties, and
which form the elementary morphological units of the nucleus.

There is reason to believe that the linin-network is likewise com-
posed of minute bodies, the oxychromatin-granules, which are closely
similar in appearance to the smallest chromatin-grains, and differ
from them only in chemical nature as shown by the difference of
staining-power. Whether the oxychromatin-granules have also the
power of growth and division is unknown ; but if, as Van Beneden
and Heidenhain maintain, the basichromatin- and oxychromatin-gran-

SUMMARY AND CONCLUSION 237

viles be only different modifications of the same element, a presump-
tion certainly exists that they have such powers. When we extend
this comparison to the cytoplasm, the ground becomes more uncer-
tain. It seems well established that the cytoreticulum is of the same
nature as the linin-network. If this be admitted, we are led to accept
on a priori grounds that some at least of the cytomicrosomes are not
artefacts, but morphological bodies comparable with those of the linin
and chromatin networks, and like them capable of growth and division.
This conclusion is, as yet, no more than a somewhat doubtful inference.
In the centrosome, however, we have a body, no larger in many cases
than a "microsome," which is positively known to be a persistent
morphological element, having the power of growth, division, and
persistence in the daughter-cells. Probably these powers of the cen-
trosome would never have been discovered were it not that its stain-
ing-capacity renders 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 oxy-
chromatin-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
to-day not entirely without the support of facts.

LITERATURE. VI

Van Beneden, E. — (See List IV.)

Van Beneden and Julin. — La segmentation cliez les Ascidiens et ses rapports avec

Torganisation de la larve : Arch. Biol., Y- 1884.
Boveri, Th. — Zellenstudien. (See List IV.)

Briicke, C. — Die Elementarorganismen : Wiener Sits.-Ber., XLIV. 1861 .
Biitschli, 0. — Protoplasma. (See List I.)
Hacker, V. — ^Uber den heutigen Stand der Centrosomenfrage : Ver/i. d. deiitscJi.

Zoijl. Ges. 1894.
Heidenhain, M. — (See List I.)
Herla, V. — Etude des variations de la mitose cliez Tascaride megalocephale : ArcJi.

Biol., XIII. 1893.
Nussbaum, M. — Uber die Teilbarkeit der lebendigen Materia: Arch. inik. Anat.,

XXVI. 1886.'
Rabl, C. — Uber Zellteilung : Morph. Jahrb., X. 1885.
Riickert, J. — (See List IV.)

De Vries, H. — I ntracellulare Pangenesis : Jena, \?>?,c).

Watase, 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. — ArchoiDlasm, Centrosome, and Chromatin in the Sea-urchin

Egg: Jour7i. Morph. ,Yo\. XI. 1895.

CHAPTER VII

SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY

" Les phenomenes fonctionnels ou de depense vitale aiiraient done leur siege dans le
protoplasme celhdaire.

" Le noyau est un appareil de syntJiese organique, r instrument de la production, le gernie
de la cellule." Claude Bernard.^

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 chemi-
cal 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 investiga-
tion is beset 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 complicated character which differ very widely in dif-
ferent 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 an all-pervading substratum
which in the greater number of cases, perhaps in all, has the form of

1 Le(ons sur les phenomenes de la vie, I., 1878, p. 198.
238

CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM 239

a sponge-like network. Cell-organs, such as the nucleus, the spindle
and asters, the centrosome, are to be regarded as specialized areas
in this network, just as the visible organs of the multicellular body
are specialized regions in the all-pervading cellular tissue. And pre-
cisely as the various organs and tissues are the seat of special chemi-
cal activities leading to the formation and characteristic transformation
of specific substances, — as for instance haemoglobin is characteristic
of the red blood-corpuscles, or chlorophyll of the assimilating tissues of
plants, — so in the cell the various morphological regions are areas
of specific chemical activities and are characterized by the presence
of corresponding substances. The morphological differentiation of
cell-organs is therefore in a way the visible expression of underlying
chemical specializations ; and these are in the last analysis reducible
to differences of metabolic action.

I. The Proteids and their Allies

The most important chemical compounds found in the cell are the
group of 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 exten-
sive 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. Halliburton ('93) employs the word " proteids "
as synonymous with albnminoiis substances, including under them the
various forms of albnniin (egg-albumin, cell-albumin, muscle-albumin,
vegetable-albumins), glolndin (fibrinogen, vitellin, etc.), and the pep-
tones (diffusible hydrated proteids). This author places in a sepa-
rate class of albuminoids another series of nearly related substances
(reckoned by some chemists among the "proteids"), examples of
which are gelatine, mucin, and especially nnclein, and the nncleo-
albiimins. The three last-named bodies are characterized by the
presence of phosphorus, in which respect they show a very definite
contrast to the "proteids," many of which, such as egg-albumin, con-
tain no phosphorus, and others only a trace. By Hammarsten and
some others the word "proteid" is, however, employed in a more
restricted sense, being applied to substances such as the nucleins
and nucleo-proteids, of greater complexity than the albumins and
globulins. The latter, together with the nucleo-albumins, are classed
as albuminous bodies (Eiweisskorper).^

1 See Hammersten, '95, p. 16.

240 SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PLIYSLOLOGY

The distribution of these substances throughout 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 betzveen niicleiLS 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, nnclein and mtcleo-proteids,
which as the names indicate, forms its main bulk and its most con-
stant 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. TJie Nnclein Seines

Nuclein was first isolated and named by Miescher in 1871, by
subjecting cells to artificial gastric digestion. The cytoplasm is
thus digested, leaving only the nuclei ; and in some cases, for in-
stance pus-cells and spermatozoa, it is possible by this method to
procure large quantities of nuclear substance for accurate quanti-
tative analysis. The results of analysis show it to be a complex
albuminoid substance, rich in phosphorus, for which Miescher gave
the chemical formula C29H49NgP3022- Later analyses gave some-
what discordant results, as appears in the following table of per-
centage-compositions : ^ —

Pus-cells.

Spermatozoa of Salmon.

Human Brain.

(Hoi'pe-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
substances; namely, (i) an organic acid rich in phosphorus, to which
he gave the name nucleic acid, and (2) a form of albumin. Moreover,

1 From Halliburton, '91, p. 203. [The oxygen-percentage is omitted in this table.]

CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM 24 1

the nuclein may be synthetically formed by the re-combination of
these two substances. Pm'e nucleic acid 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 con-
tains no sulphur and a maximum of phosphorus (over 9.5 %), might
be uncombined nucleic acid itself. Kossel accordingly drew the
conclusion, based on his own work as well as that of Liebermann,
Altmann, Malf atti, and others, that " what the histologists designate
as cJiromatiii consists essentially of combinations of nucleic acid with
more or less albumin, and in some cases may even be free nucleic
acid. The less the percentage of albumin in these compounds, the
nearer do their properties approach those of pure nucleic acid, and
we may assume that the percentage of albumin in the chromatin
of the same nucleus may vary according to physiological condi-
tions." ^ In the same year Halliburton, following in part Hoppe-
Seyler, stated the same view as follows. The so-called " nucleins "
form a series leading downward from nucleic acid thus : —

(i) Those containing no albumin and a maximum (9-10 %) of phos-
phorus (pure nucleic 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 included pyrenin (nucleoli)
2CCi^ plastin (linin). These graduate into

(4) Those containing a minimum (0.5 to i %) of phosphorus —

the nucleo-albumins, which occur both in the nucleus and in
the cytoplasm (vitellin, caseinogin, etc.).

Finally, we reach the globulins and albumins, especially character-
istic of the cell-substance, and containing no nucleic 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
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-
i'93, p. 158. 2 .93^ p. 574.

242 SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY

change, some constituents being elaborated, others breaking down
into simpler products." ^ These conclusions established a probability
that the chemical differences between chromatin and cytoplasm,
striking and constant as they are, are differences of degree only ;
and they opened the way to a more precise investigation of the
physiological 7'dle of nucleus and cytoplasm in metabolism.

3. Stainhig-rcactions of the Nuclein-series

We may now bring these facts into relation with the staining-
reactions of chromatin and cytoplasm when treated with the aniline
dyes. These dyes are divided into two main classes,^ viz. the
" basic " anilines and the " acid " anilines, the colouring-matter playing
the part of a base in the former and of an acid in the latter. The
basic anilines {e.g. methyl-green, Bismarck brown, saffranin) are in
general "nuclear stains," having a strong affinity for chromatin,
while the acid anilines (acid fuchsin, Congo red, eosin, etc.) are
"plasma-stains," colouring more especially the cytoplasmic elements.
We owe to Malfatti and Lilienfeld the very interesting discovery
that the various members of the miclein series shozv an affinity for
the basic dyes in direct proportion to the amount of nucleic acid
{as '}neasured by the amoitnt of phosphorus) they contain. Thus the
nuclei of spermatozoa, known to consist of nearly pure nucleic acid,
stain most intensely with basic dyes, those of ordinary tissue-cells,
which contain less phosphorus, less intensely. Malfatti ('91) tested
various members of the nuclein-series, synthetically produced as
combinations of egg-albumin and nucleic acid from yeast, with a
mixture of red acid fuchsin and basic methyl-green. With this
combination free nucleic acid was coloured pure green, nucleins
containing less phosphorus became bluish-violet, those with little
or no phosphorus pure red. Lilienfeld' s more precise experiments
in this direction ('92, '93) led to similar results. His starting-point
was given by the results of Kossel's researches on the relations of
the nuclein group, which are expressed as follows:^ —

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 nu-
cleins 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 "
(Hammarsten) has therefore been suggested for this substance. True nucleins containing
a large percentage of albumin are distinguished as micleo-proteids. They may be split into
albumin and nucleic acid, the latter yielding as cleavage-products the nuclein bases. Pseudo-
nucleins containing a large percentage of albumin are designated as niicleo-albtimins, which
in like manner split into albumin and paranucleic or pseudo-nucleic acid, which yields no
nuclein bases. (See Hammarsten, '94.)

2 See Ehrlich, '79.

2 From Lilienfeld, after Kossel, '92, p. 129.

CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM 243

Nucleo-albumin (i % of P or less),
by peptic digestion splits into

Peptone

Nuclein (3-4 % P),
by treatment with acids splits into

Albumin

Nucleic acid (9-10 % P),
heated with mineral acids splits into

Phosphoric acid

Nuclein bases
(adenin, guanin, etc.).

(^ carboJiydrate.')

Now, according to Kossel and Lilienfeld, the principal nucleo-
albumin (nucleo-proteid) in the nucleus of leucocytes is nucleo-Jiiston^
containing about 3 % of- phosphorus, which may be split into a form
of 7mclein playing the part of an acid, and an albuminoid base, the
histon of Kossel ; the nuclein may in turn be split into albumin and
nucleic acid. These four substances — albumin, nucleo-histon, nu-
clein, nucleic acid — thus form a series in which the proportion of
phosphorus, i.e. of nucleic 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, nucleic acid intense green. " We
see, therefore, that the principle that determines the staining of the
nuclear substances is always the nucleic 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 towards blue." ^
Lilienfeld explains the fact that chromatin in the cell-nucleus seldom
appears pure green on the assumption, supported by many facts,
that the proportion of nucleic 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 consist, like the chromatin of spermatozoa,
of pure or nearly pure nucleic acid. Very interesting and con-
vincing 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 is coloured red, the albumin green. This is a beautiful
demonstration of the fact that staining-reagents cannot be logically
classified according to colour, but only according to their chemical

^ I.e., p. 394.

244 SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY

nature. Such terms as "erythrophilous," " cyanophilous," and the
Hke have therefore no meaning apart from the chemical compo-
sition both of the dye and of the substance stained.^

The constancy and accuracy of these reactions await further test,
and until this has been carried out we should be careful not to place
too implicit a trust in the staining-reactions as an indication of chemi-
cal nature, especially as they are known to be affected by the pre-
ceding mode of fixation. They afford, nevertheless, a rough method
for the micro-chemical test of the proportion of nucleic acid present
in the nuclear structvu'es, and this in the hands of Heidenhain has
led to some suggestive results. 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 substance shows a
very sharp differentiation. The chromatic network and the chromo-
somes 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 sus-
pended 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,
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
basichromatin-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. nucleic
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 are regulated by cer-
tain physiological 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-

1 Cf. p. 127. 2.9^^ p, ^43. 3/.f.,p. 547. */.^.,p. 54S.

CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM

245

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 ^2,^, and several observers
have maintained its entire disappearance at one period. Rlickert
first carefully traced out the history of the chromatin in detail in the

>-^^^"^

-#i^^

Fig. no. — Chromosomes of the germinal vesicle in the shark Pj-istiurus, at different periods,
drawn to the same scale. [RiJCKERT.]

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.

eggs of sharks, and his general results have since been confirmed by
Born in the eggs of Triton. In the shark Pristiurus Riickert ('92, i)
finds that the chromosomes, which persist throughout the entire
growth-period of the ^gg, undergo the following changes (Fig. 1 10) :
At a very early stage they are small, and stain intensely with nuclear
dyes. During the growth of the Q.gg they undergo a great increase
in size, and progressively lose tJieir 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

246 SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY

(Fig. 1 10, A). As the &gg approaches its full size, the chromosomes
rapidly diminish in size, the radiating threads disappear, and the stain-
ing-capacity increases (Fig. 1 10, ^5). They are finally again reduced to
minute intensely staining bodies which enter into the equatorial plate
of the first polar mitotic figure (Fig. no, C). How great the change
of volume is may be seen from the following figures. At the beginning
the chromosomes measure, at most, 12 jjl (about goVo'^'^-) ^^ length and
|-/x 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 //- in length and i yu, 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
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 Qgg, points to an intimate connection between
the nuclear activity and the formative energy of the cytoplasm.

If these facts are considered in the light of the known stain-
ing-reaction of the nuclein series, we must admit that the follow-
ing conclusions are something more than mere possibilities. We
may infer that the original chromosomes contain a high percent-
age of nucleic acid ; that their growth and loss of staining-power is
due to a combination with a large amount of albuminous substance
to form a lower member of the nuclein series, perhaps even a nucleo-
albumin; 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
diminution in bulk and resumption of staining-capacity coincides with
the cessation of this activity. This result is in harmony with the
observations of Schwarz and Zacharias on growing plant-cells, the
percentage 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
nucleic 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

CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM 247

admit of a safe generalization. They are, however, enough to indi-
cate the probabihty that chromatin may pass through a certain cycle
in the life of the cell, the percentage of albumin increasing during
the vegetative activity of the nucleus, decreasing in its reproductive
phase. In other words, a combination of albumin with nuclein or
■ nucleic acid is an accompaniment of constructive metabolism. As
the cell prepares for division, the combination is dissolved and the
nuclein-radicle or nucleic acid is handed on by division to the daugh-
ter-cells. It is a tempting hypothesis, suggested to me by Mr. A. P.
Mathews on the basis of Kossel's work, that the nuclein is in a chem-
ical sense the formative centre of the cell, attracting to it the food-
matters, entering into loose combination with them, and giving them
off to the cytoplasm in an elaborated form. Could this be estab-
lished, we should have a clue to the nuclear control of the cell
through the process of synthetic metabolism. Claude Bernard
advanced a nearly similar hypothesis two score years ago ^j"^), main-
taining that the cytoplasm is the seat of destructive metabolism, the
nucleus the organ of constructive metabolism and organic synthesis,
and insisting that the role of the nucleus in nutrition gives the key
to its significance as the organ of development, regeneration, and
inheritance.^

That the nucleus is especially concerned in synthetic metabolism
is now becoming more and more clearly recognized by physiological
chemists. Kossel concludes that the formation of new organic matter
is dependent on the nucleus,^ and that nuclein in some manner plays
a leading role in this process ; and he makes some interesting sugges-
tions regarding the synthesis of complex organic matters in the living
cell with nuclein as a starting-point. Chittenden, too, in a review of
recent chemico-physiological discoveries 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 qualities 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).

1 " II semble done que la cellule qui a perdu son noyau soit sterilisee au point de vue de
la generation, c'est a 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 \q germe de nutrition de la cellule; il attire autour
de lui et elabore les materiaux nutritifs " ('78, p. 523).

- Schiefferdecker und Kossel, GewebeleJwe^ p. 57.

248 SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY

B. Physiological Relations of Nucleus and Cytoplasm

How nearly the foregoing facts bear on the problem of the form-
ative power of the cell in a morphological sense 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
different point of view.

Briicke 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 phenome-
non, and that both are expressions of the nuclear activity. We have
now to consider some of the evidence that the formative power of the
cell, in a morphological sense, 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, position and
movements of the nucleus in actively growing or metabolizing cells ;
and from the history of the nucleus in mitotic cell-division, in fer-
tilization, and in maturation.

I. Experiments on Unicellular Organisms

Brandt ^Jj) long since observed that enucleated fragments of
ActinospJicerinm soon die, while nucleated fragments heal their wounds
and continue to live. The first decisive comparison between nucle-
ated and non-nucleated masses of protoplasm was, however, made by
Moritz Nussbaum in 1884 in the case of an infusorian, Oxytjdcha.
If one of these animals be cut into two pieces, the subsequent
behaviour of the two fragments depends on the presence or absence
of the nucleus or a nuclear fragment. The nucleated fragments
quickly heal the wound, regenerate the missing portions, 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 indispensable for the forma-
tive energy of the cell. The experiment was soon after repeated by
Gruber ('85) in the case of St en tor, another infusorian, and with the
same result (Fig. 1 12). Fragments possessing a large fragment of the
nucleus completely regenerated within twenty-four hours. If the nu-

PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM 249

clear fragment were smaller, the regeneration 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 com-
pleted the formation of the peristome. Lillie ('96) has recently
found that Stcntor may by
shaking be broken into frag-
ments of all sizes, and that
nucleated fragments as small
as 2V the volume of the entire
animal are still capable of
complete regeneration. All
non-nucleated fragments per-
ish.

These studies of Nussbaum
and Gruber formed a prelude
to more extended investiga-
tions in the same direction
by Gruber, Balbiani, Hofer,
and especially Verworn.
Verworn ('88) proved that
in Polystomella, one of the
Foraminifera, nucleated frag-
ments are able to repair the
shell, while non-nucleated
fragments lack this power.
Balbiani ('89) showed that
although non-nucleated frag-
ments of infusoria had no

Fi^. III. — Stylonychla, 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.

power of regeneration, they might nevertheless continue to live and
swim actively about for many days after the operation, the con-
tractile vacuole, pulsating as usual. Hofer ('89), experimenting on
Amceba, found that non-nucleated fragments might live as long
as fourteen days after the operation (Fig. 113). 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 digest food, and
the power of adhering to the substratum. Nearly at the same time
Verworn ('89) published 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

250 SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY

accounts of his predecessors in regard to regeneration, added many
extremely important and significant results. Non-nucleated frag-
ments both of infusoria {e.g., LacJirymaria) and rhizopods {Poly-
stoniella, 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, however, the poiver
of digestion and secretion. Ingested food-matters may be slightly

B

C

Fig. 112. — Regeneration in the unicellular animal Stentor. [Gruiier.]
A. Animal divided into three pieces, each containing a fragment of the nucleus. B. The
three fragments shortly afterwards. C. The three fragments after twenty-four hours, each regen-
erated to a perfect animal.

altered, but are never completely digested. The non-nucleated frag-
ments are unable to secrete the material for a new shell {Polysto-
inella) or the slime by which the animals adhere to the substratum
{Ainceha, 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 connection with the nerve-cell, while the remaining portion inva-
riably degenerates.

These beautiful observations prove that destructive metabolism, as

PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM 25 1

manifested by co-ordinated forms of protoplasmic contractility, may
go on for some time undisturbed in a mass of cytoplasm deprived of
a nucleus. On the other hand, the formation of new chemical or
morphological products by the cytoplasm only takes place in the pres-
ence of a nucleus. These facts form a complete demonstration that
the nucleus plays an essential part not only in the operations of syn-
thetic metabolism or chemical synthesis, but also in the inorpJiological

1/1 ' C '^:{

c-S

%, %,

^ m ...^

Fig. 113. — Nucleated and non-nucleated fragments of Amceba. [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.

determination of these operations, 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 unicellular plants. Klebs
observed as. long ago as 1879 that naked protoplasmic fragments of
Vaiccheria and other algae were incapable of forming a new cellulose
membrane if devoid of a nucleus ; and he afterwards showed ('87)

2 52 SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY

that the same is true of Zygnenia and CEdigoninni. By plasmolysis
the cells of these forms may be broken up into fragments, both
nucleated and non-nucleated. The former surround themselves with
a new wall, grow, and develop into complete plants ; the latter, while
able to form starch by means of the chloroph3dl they contain, are
incapable of utilizing it, and are devoid of the power of forming a
new membrane, and of growth and regeneration. ^

Although Verworn's results confirm and extend the earlier work of
Nussbaum and Gruber, he has drawn from them a somewhat different
conclusion, based mainly on the fact, determined by him, that a
nucleus deprived of cytoplasm is as devoid of the power to regenerate
the whole as an enucleated mass of cytoplasm. From this he argues,
with perfect justice, that the formative energy cannot properly be
ascribed to the nucleus alone, but is rather a co-ordinate activity of
both nucleus and cytoplasm. No one will dispute this conclusion;
yet in the light of other evidence it is, I think, stated in somewhat
misleading terms which obscure the significance of Verworn's own
beautiful experiments. It is undoubtedly true that the cell, like any
other living organism, acts as a whole, and that the integrity of all of
its parts is necessary to its continued existence ; but this no more pre-
cludes a specialization and localization of function in the cell than in
the higher organism. The experiments certainly do not prove that
the nucleus is the sole instrument of organic synthesis, but they no
less certainly indicate its especial importance in this process. The
sperm-nucleus is unable to develop its latent capacities without be-
coming associated with the cytoplasm of an ovum, but its significance
as the bearer of the paternal heritage is not thereby lessened one iota.

2. Position and Movements of the Xnclens

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 impos-
sible, and we must be content to consider only the well-known
researches of Haberlandt i^yj) 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 determined the very significant fact that local growth of the
cell-wall is always preceded by a movement of the nucleus to the

1 Palla ('90) has disputed this result, maintaining that enucleated masses of protoplasm
pressed out from pollen-tubes might surround themselves with membranes and grow out
into long tubes. Later observations, however, by Acqua ('91), throw doubt on Palla's
conclusion.

PHYSIOLOGICAL RELATIOXS OF NUCLEUS AND CYTOPLASM 253

point of growth. Thus, in the formation of epidermal cells the
nucleus lies at first near the centre, but as the outer wall thickens,
the nucleus moves towards it, and remains closely applied to it
throughout its growth, after which the nucleus often moves into
another part of the cell (Fig. 114, A, B). That this is not due
simply to a movement of the nucleus towards the air and light is
beautifully shown in the coats of certain seeds, where the nucleus

Fig. 114. — Position of the nuclei in growing plant-cells. [Haberlaxdt.]
A. Young epidermal cell of Luziila with central nucleus, before thickening of the membrane.
B. Three epidermal cells of Momtera, during the thickening of the outer wall. C. Cell from ihe
seed-coat of Scopuliua 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.

moves not to the outer, but to the inner wall of the cell, and here
the thickening takes place (Fig. 114, C). The same position 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
primarv outgrowth alwavs takes place from the immediate neighbour-
hood of the nucleus, which is carried outward and remains near the
tip of the growing hair (Fig. 1 14, D, E). The same is true of the

2 54 SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY

rhizoids of fern-prothallia and liverworts. In the hairs of aerial
plants this 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 VaucJieria 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 morphologi-
cal 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 elaboration 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 con-
centrated into a very short period. Here the nucleus forms a
labyrinthine network (Fig. ii, 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 p. 114. Here the develop-
ing 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 Forficnla each o.^^ is accompanied
by a single large nutritive cell (Fig. 115), which has a very large
nucleus rich in chromatin (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

1 Lc, p. 99.

PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM 255

,f^i.

of such cells at one pole of the egg from which the latter is believed
to draw its nutriment (Fig. 58). A very interesting case is that of
the annelid OphryotrocJia, referred to at p. 114. Here, as described
by Korschelt, the ^gg floats in the perivisceral fluid, accompanied
by a nurse-cell having a very large chromatic nucleus, while that of
the &gg is smaller and
poorer in chromatin.
As the Q.gg completes
its growth, the nurse-
cell dwindles away and
finally perishes (Fig. 57).
In all these cases it
is scarcely possible to
doubt that the Q.gg is

in a measure relieved /^V-' . i<:.. n

of the task of elaborat-
ing cytoplasmic products
by the nurse-cell, and
that the great develop-
ment 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 towards the
same conclusion. Per- g 1'
haps the most sugges-
tive of these relate to
the nucleus of the o^gg

during its ovarian his-
tory. In many of the
insects, as in both the

Fig. 115. — Upper portion oi the ovary in the earwig
Fofjiciila, showing eggs and nurse-cells. [KORSCHELT.]

Below, a portion of the nearly ripe egg {e), showing deuto-
plasm-spheres and germinal vesicle {gv). Above it lies the
nurse-L-ell {ti) with its enormous bi'anching nucleus. Two
cases referred to above si^ii^cessively younger stages of egg and nurse are shown above.

the egg-nucleus at first occupies a central position, but as the
egg bsgins to grow, it moves to the periphery on the side turned
towards 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 tgg. Most suggestive of all is the case
of the water-beetle Dytisais, in which Korschelt was able to observe
the movements and changes of form in the living object. The eggs

256 SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY

here lie in a single series alternating 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, the egg contains accumulations of similar
granules, which extend inwards in dense masses from the nutritive
cells to the germinal vesicle, which they may more or less completely
surround. The latter meanwhile becomes amoeboid, sending out long
pseudopodia, which are always directed towards the principal mass of
granules (Fig. 58). The granules could not be traced into the nucleus,
but the latter grows rapidly during these changes, proving that mat-
ter must be absorbed by it, probably in a liquid form.^

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 its constructive
phase. On the whole, therefore, the behaviour of the nucleus in this
regard is in harmony with the result reached by experiment on the
one-celled forms, though it gives in itself a far less certain and con-
vincing result.

We now turn to evidence which, though less direct than the experi-
mental proof, is scarcely less convincing. This evidence, which has
been exhaustively discussed by Hertwig, Weismann, and Strasburger,
is drawn from the history of the nucleus in mitosis, fertilization, and
maturation. It calls for only a brief review here, since the facts have
been fully described in earlier chapters.

3. Tlie Nuclei ts in Mitosis

To Wilhelm Roux ('83) we owe the first clear recognition of
the fact that the transformation of the chromatic substance dur-
ing mitotic division is manifestly designed to effect a precise di-
vision 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 divi-
sion 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 undergoes 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

1 Some observers have maintained that the nucleus may take in as well as give off solid
matters. This statement rests, however, on a very insecure foundation.

PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM 257

must represent different qualities, and second, that the apparatus of
mitosis is designed to distribute these quahties, according to a
definite law, to the daughter-cells. The particular form in which
Roux and Weismann developed this conception has now been gener-
ally rejected, and in any form it has some serious difficulties in its
way. We cannot assume a precise localization of chromatin-ele-
ments 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 matu-
ration of the ^g^, and on the other hand in the Protozoa a small
fragment of the nucleus is able to regenerate the whole. Neverthe-
less, 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 scrupu-
lous 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 through-
out 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 posi-
tion 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 Fertilisation

The foregoing argument receives an overwhelming reinforce-
ment from the facts of fertilization. Although the ovum supplies
nearly all the cytoplasm for the embryonic body, and the sper-
matozoon 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 contributed by the two germ-cells, though apparently
different, become in the end exactly equivalent in every visible
respect — in structure, in staining-reactions, and in the number and
form of the chromosomes to which each gives rise. But further-
more the substance of the two germ-nuclei is distributed with abso-
lute 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 cer-
tainty by the remarkable observations of Riickert, Zoja, and Hacker,
described in Chapters IV. and VI. The conclusion is irresistible
that the specific character of the cell is in the last analysis deter-
mined by that of the nucleus, that is by the chromatin, and that in
the equal distribution of paternal and maternal chromatin to all the
cells of the offspring we find the physiological explanation of the

258 SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY

fact that 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 enucleated by shaking, and
that spermatozoa would enter
the enucleated fragments and
cause them to segment. Boveri
proved that such fragments
would even give rise to dwarf
larvae, indistinguishable from
the normal in general appear-
ance and differing from the
latter only in size and in the
very significant fact that their
nuclei contain only half the nor-
mal number of chromosomes.
Now, by fertilizing enucleated
egg-fragments of one species
{SphcerccJiimis graiuilaris) Avith
the spermatozoa of another
{Echijiiis viicrotiibercidat7is), Bo-
veri obtained in a few instances
dwarf Plutei shozving purely
paternal characteristics (Fig.
116). 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, effected by the
nucleus alone.^ Boveri's result
is unfortunately not quite conclusive, as has been pointed or.t
by Seeliger and Morgan, yet his extensive experiments establish, I
think, a strong presumption in its favour. Should they be positively
confirmed, they would furnish a practical demonstration of inheritance
through the nucleus.

^ The ccntrosome is left out of account, since it is frequently derived from one sex only.

Pig. 116. — Normal and dwarf larvas of the
sea-urchin. [BOVERI.]

A. Dwarf Pluteus arising from an enucleated
egg-fragment ot Splicer echhius gratmlaris , fertilized
with spermatozoon of Echinus microtube7-culatus ,
&nd s\\o\^vag purely pate?-?2al characters. B. Nor-
mal Pluteus of Echinus microtuberculatus.

THE CENTKOSOME 259

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 are expressly designed to establish a perfect
equivalence between them at the time of their union, and, more re-
motely, a perfect equality of distribution to the embryonic cells.
The cytoplasm, on the other hand, undergoes a special and per-
sistent differentiation in each to effect a secondary division of labour
between the germ-cells. When this is correlated 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

Nearly all investigators have now accepted Van Beneden's and
Boveri's conclusion that tJie cent7'osome is an organ for cell-division,
and that in this sense it represents the dynamic centre of the cell (cf.
p. 56). This is most clearly shown in the ordinary fertilization of the
ovum, in which process, as Boveri has insisted, it is the centrosome
that is the fertilizing element par excellence, since its introduction
into the egg confers upon the latter the power of division, and hence
of development. Boveri's interesting observations on " partial fertil-
ization" in the sea-urchin referred to at p. 140 afford a beautiful illus-
tration of this point. In certain exceptional cases the (fg^ may divide
before conjugation of the germ-nuclei has occurred, the sperm-nucleus
lying passive in the cytoplasm until after the first cleavage and then
conjugating with, one of the nuclei of the two-celled stage. The Q,gg
is \\QXQ. fertilized — i.e. rendered capable of division — by the centro-
some, which separates from the sperm-nucleus, approaches the egg-
nucleus, and gives rise to the cleavage-amphiaster as usual.

Again, Boveri has observed that the segmenting ovum of Ascaris
sometimes contains a supernumerary centrosome that does not enter
into connection with the chromosomes, but lies alone in the cytoplasm
(Fig. 117). 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-

26o SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY

tion-sphere. The fate of such eggs was not determined, but they
form a complete demonstration that it is the centrosome and not the
nucleus that is the active centre of cell-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. ii8). The cleavage-nucleus,
therefore, arises by the union of three nuclei and four centrosomes.
Such eggs invariably divide at the first cleavage into four equal blas-
tomeres, each of which receives one of the centrosomes. The latter
must, therefore, be the centres of division.^

The statement that the centrosome is an organ for cell-division
does not, however, express the whole truth ; for 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

Fig. 117. — Eggs of Ascarls with supernumerary centrosome. [BOVERI.]
A. First cleavage-spindle above, isolated centrosome below. B. Result of the ensuing division.

cell-body as a whole ; and, moreover, amitotic division may appar-
ently take place independently of the centrosome. The role of the
centrosome and attraction-sphere in gland-cells (where they are some-
times very large) and in the nerve-cells is still wholly problematical.
It would seem, therefore, that the primary function of the centrosome
is to organize an astral system, of which it forms the focus, that is
primarily an apparatus for mitotic division, but may secondarily
become devoted to other functions. The nature of the energy by
which this organization takes place is almost wholly in the dark.
The extraordinary resemblance of the amphiaster to the lines of
force in a magnetic field has impressed many observers, but Roux
has proved that the axis of the mitotic figure is not affected, during
its formation, by a powerful electro-magnet. The molecules or micro-

1 This phenomenon was first observed by Hertwig, and afterwards by Driesch.
repeatedly observed the internal changes in the living eggs of Toxopneustes.

I have

SUMMARY AND CONCLUSION

261

somes of the fibres must be in some manner polarized by an influence
emanating from the centrosome, but in the present state of know-
ledge it would be useless to speculate on the nature of this influence.
One fact, however, should be borne in mind, namely, that the centro-
some differs chemically from the substance of the fibres as shown by
its staining-reactions ; and this may form a clue to the further inves-
tigation of this most interesting problem.

The principal point in connection with our present theme is that
the centrosome cannot be regarded as taking any important part in

Fig. 118. — 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.

the general metabolism of the cell, nor can it be an organ of inheri-
tance ; for on the one hand it is absent or so small as to be indistin-
guishable in many actively metabolizing cells, such as those of the
pancreas or kidney, or the older ovarian eggs, and, on the other hand,
in fertilization it may be derived from one sex only. The conclusion
regarding inheritance would not be invalidated, even if it could be
positively shown that in some cases both germ-cells might contribute
a centrosome ; for a single case of its one-sided origin would be con-
clusive, and many such are actually known.

D. Summary and Conclusion

All of the facts reviewed in the foregoing pages converge, I think,
to the conclusion drawn by Claude Bernard, that the nucleus is the
formative centre of the cell in a chemical sense, and through this is
the especial seat of the formative energy in a morphological sense.
That the nucleus has such a significance in synthetic metabolism is
proved by the fact that digestion and absorption of food, growth, and

262 SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY

secretion cease with its removal from the cytoplasm, while destructive
metabolism may long continue as manifested by the phenomena of
irritability and contractility. It is indicated by the position and move-
ments of the nucleus in relation to the food-supply and to the forma-
tion 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 substance known as nucleic acid plays a leading part
in the constructive process. During the vegetative phase of the cell
this substance appears to enter into combination with proteid or
albuminous substance to form a nuclein. During its mitotic or repro-
ductive phase the albumin is split off, leaving the substance of the
chromosomes as nearly pure nucleic acid. When this is correlated
with the fact that the sperm-nucleus, which brings with it the pater-
nal heritage, likewise consists of nearly pure nucleic acid, the pos-
sibility is opened that this substance may be in a chemical sense not
only the formative centre of the nucleus but also a primary factor in
the constructive processes of the cytoplasm.

The, role of the nucleus in constructive metabolism is intimately
related with its Tole 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
metabolic action of which they are the outward expression. That
the nucleus is in fact a primary factor in morphological as well as
chemical synthesis is demonstrated by experiments on unicellular
plants and animals, which prove that the power of regenerating lost
parts disappears with its removal, though the enucleated fragment
may continue to live and move for a considerable period.

This fact establishes the presumption that the nucleus is, if not the
actual seat of the formative energy, at least the controlling factor in
that energy, and hence the controlling factor in inheritance. This
presumption becomes a practical certainty when we turn to the
facts of maturation, fertilization, and cell-division. All of these con-
verge to the conclusion that the chromatin is the most essential ele-
ment in development. 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 ele-
ments are brought together and by mitotic division distributed with
exact equality to the embryonic cells. The result proves that the
spermatozoon is as potent in inheritance as the ovum, though the
latter contributes an amount of cytoplasm which is but an infini-

SUMMARY AND CONCLUSION 263

tesimal fraction of that supplied by the ovum. The centrosome,
finally, is excluded from the process of inheritance, since it may be
derived from one sex only.

LITERATURE. VII

Bernard, Claude. — Le9ons sur les Phenomenes de la Vie: ist ed. 1878; 2d ed.

1885. Paris.
Chittenden, R. H. — Some Recent Chemico-physiological Discoveries regarding the

Cell: Am. Nat., XXVIII., Feb., 1894.
Haberlandt, G. — tjber die Beziehungen zwischen Funktion und Lage des Zellkerns.

Fischer, 1887.
Halliburton, W. D. — A Text-book of Chemical Physiology and Pathology. London,

1891.
Id. — The Chemical Physiology of the Cell {Goiddstonian Lectures): Brit. Med.

Joitrn . 1 893 .
Hammarsten, 0. — Lehrbuch der physiologische Chemie. 3d ed. Wiesbaden, 1895.
Hertwig, 0. & R. — Uber den Befruchtungs- und Teilungsvorgang des tierischen

Eies unter dem Einfluss ausserer Agentien. Jena, 1887.
Kolliker, A. — Das Karyoplasma und die Vererbung, eine Kritik der Weismann'schen

Theorie von der Kontinuitat des Keimplasmas : Zeitschr. iviss. Zo'ol., XLIV.

1886.
Korschelt, E. — Beitrage sur Morphologic und Physiologic des Zell-kernes : Zo'dl.

Jahrb. Anat. u. Ontog., IV. 1889.
Kossel, A. — IJber die chemische Zusammensetzung der Zelle : Arch. Anat. n. Phys.

1891.
Lilienfeld, L. — Uber die Wahlverwandtschaft der Zellelemente zu FarbstofFen :

Arch. Anat. 11. Phys. 1893.
Malfatti, H. — Beitrage zur Kenntniss der Nucleine : Zeitschr. Phys. Chem., XVI.

1891.
Riickert, J. — Zur Entwicklungsgeschichte des Ovarialeies bei Selachiern : Aii. Anz.^

VII. 1892.
•Sachs, J. — Vorlesungen iiber Pflanzen-physiologie. Leipzig, 1882.
Id. — Stofif und Form der Pflanzen-organe : Gesaninielte AbJiandliingen, II. 1893.
Verworn, M. — Die Physiologische Bedeutung des Zellkerns : Arch, fiir die Ges.

Phys., XLI. 1892.
Id. — Allgemeine Physiologic. Jena, 1895.

Zacharias, E. — Uber Chromatophilie : Ber. d. deiitsch. Bot. Ges. 1893.
Id. — tJber des Verhalten des Zellkerns in wachsenden Zellen : Flora, 81. 1895.
Whitman, C. 0. — The Seat of Formative and Regenerative Energy : Jottrn. Morph.,

II. 1888.

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 niorphologisch ahnliche Elemente entstehen, und dass die in einer
embryonischen Organ-Anlage enthaltenden Zellen, so gering auch ihre Zahl sein mag,
dennoch die ausschliessliche ungegliederte Anlage fiir sammtliche Formbestandtheile der
spateren Organe enthalten." Remak.^

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 &gg 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, bringing with it a new centrosome,
arouses in the Qgg 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 descendants 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,

1 Unlersuchu7igen, 1855, p. 140.
264

GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 265

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
sometimes so clearly marked and appear so early that with the very
fiijst cleavage the position in which the embryo will finally appear in
the egg maybe exactly predicted. Such " promorphological " rela-
tions of the segmenting Q,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 laws 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, with reference
to the general laws of cell-division.

2. The Promorphological 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 are
variants of a single type which has been moulded this way or that
by special conditions, and which is itself an expression of two general
laws of cell-division, first formulated by Sachs in the case of plant-
cells. These are :

1. The cell typically tends to divide into eqital parts.

2. Each nezv plane of division tends to intersect the preceding plane
at a rischt ans'le.

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 {periclijtes). 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

266

CELL-DIVISION AND DEVELOPMENT

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

Fig. 119. — Geometrical relations of cleavage-planes in growing plant-tissues. [From Sachs,
after various authors.]

A. Flat ellipsoidal germ-disc of Melobesia (Rosanoff) ; nearly typical relation of elliptic
periclines and hyperbolic anticlines. B. C. Apical view of terminal knob on epidermal hair of
Phiguicola. 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 (Strasburger) ; circular
or spheroidal type transitional to ellipsoidal. F. Root-cap of Egiiisetum (Nageli and Leitgeb) ;
modified circular type. G. Cross-section of leaf-vein, Trichomanes (Prantl) ; ellipsoidal type with
incomplete periclines. H. Embryo of Alisma ; typical ellipsoid type, pericline incomplete only
at lower side. /. Growing point of bud of the pine {Abies) ; typical paraboloid type, both anti-
clines and periclines having the form of parabolas (Sachs).

right angles. All these schemes are, mutatis mutandis, directly con-
vertible into the corresponding solid forms in three dimensions.

Sachs has shown in the most beautiful manner that all the above
ideal types are closely approximated in nature, and Rauber has applied

GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 26/

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 various lower plants, in the embryos of flowering plants, and
elsewhere (Fig. 119). The paraboloid form is according to Sachs
characteristic of the growing points of many higher plants ; and
here too the actual form is remarkably similar to the ideal scheme
(Fig. 119, /).

For our purpose the most important form is the sphere, which is
the typical shape of the egg-cell; and all forms of cleavage are deriv-
atives of the typical division of a sphere in accordance with Sachs's
laws. 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 &g^ 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 most typical 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. 120, constructed from Selenka's drawings.^ The
first cleavage is vertical, or meridional, passing through the egg-axis
and dividing the Q.g^ 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
Qgg into equal octants. The order of division is thus far exactly
that demanded by Sachs's law 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 wall consisting of a single layer of cells in
which anticlinal cleavages occur in regular rectangular succession.
The fourth cleavage is again meridional, giving two tiers 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.

This simple and regular mode of division forms a type to which
nearly all forms of cleavage may be referred ; but the order and form

1 Cf. also Fig. 3.

26^

CELL-DIVISION AND DEVELOPMENT

of the divisions is endlessly varied by special conditions. These
modifications are all referable to the three following causes : —

1. Disturbances in the rhythm of division.

2. Displacement of the cells.

3. Unequal division of the cells.

The first of these requires little comment. Nothing is more com-
mon than a departure from the mathematical regularity of division.
The variations are sometimes quite irregular, sometimes follow a
definite law, as, for instance, in the annelid Nereis (Fig. 122), where
the typical succession in the number of cells is with great constancy

Fig. 120. — Cleavage of the ovum in the holothurian Synafta (slightly schematized). [After
Selenka.]

A-E. Successive cleavages to the 32-cell stage. F. Blastula of 128 cells.

2, 4, 8, 16, 20, 23, 29, 32, 37, 38, 41, 42, after which the order is more
or less variable. The meaning of such variations in particular cases
is not very clear. They are certainly due in part to variations in the
amount of deutoplasm ; for, as Balfour long since pointed out ('75),
the rapidity of division in any part of the ovum is in general inversely
proportional to the amount of deutoplasm it contains. Exceptions
to this law are, however, known.

The second series of modifications, due to displacements of the
cells, are probably due to mutual pressure, however caused,^ which

1 The pressure is probably due primarily to an attraction between the cells {cytotropisin
of Roux), but may be increased by the presence of membranes, by turgor, or by special
processes of growth.

GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS

269

leads them to take up the position of least resistance or greatest
economy of space. In this regard the behaviour of tissue-cells in
general has been shown to conform on the whole to that of elastic
spheres, such as soap-bubbles when massed together and free to
move. Such bodies, as Plateau and Lamarle have shown, assume a
polyhedral form and tend towards such an arrangement that the aira

, Fig. 121. — Cleavage o{ Polygordhts, from life.

A. Four-cell stage, from above. B. Corresponding view of 8-cell stage,
same (contrast Fig. 120, C). D. Sixteen-cell stage from the side.

C. Side view of the

of S7i7'face-contact between them is a ininiimnn. 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 sur-
face they assume the form of 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

270 CELL-DIVISION AND DEVELOPMENT

('86) and Errara ('86, '87) have pointed out that in almost all cases
the cells tend to alternate or interlock so as to reduce the contact-area
to a minimum. Thus arise many of the most frequent modifications
of cleavage. Sometimes, as in Synapta, the alternation of the cells is
effected through displacement of the blastomeres after their forma-
tion. 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 posi-
tion of least resistance from the beginning. This form of cleavage,
especially characteristic of many worms and mollusks, is typically
shown by the o^^^ of Polygordms (Fig. 121). The four-celled stage is
nearly like that of Synapta, though even here the cells slightly inter-
lock. 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 like-
wise oblique, but at right angles to the third, so that all of the cells
interlock as shown in Fig. 121, D. This alternation regularly recurs
in the later cleavages.

This form of cleavage beautifully illustrates Sachs's second law
operating under modified conditions, and the conclusion is irresistible
that the modification is at bottom a result of the same forces as those
operating in the case of soap-bubbles. 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 ^-g^, 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. 122). It is a curious fact
that the direction of the displacement is extremely constant, the
upper quartet in the eight-cell stage being rotated in all but a few
cases to the right, or with the hands of a watch. 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.

The third class of modifications, due to unequal division of the cells,
leads to the most extreme types of cleavage. Such 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 inequality first appears varies greatly in different forms.
In Polygordius (Fig. 121) the first marked inequality appears at the
fifth cleavage; in sea-urchins it appears at the fourth (Fig. 3); in
AmpJiioxus at the third (Fig. 123); in the tunicate Clavclina at the
second (Fig. 126); in Nereis at the first division (Figs. 43, 122). The

GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS

271

extent of the inequality varies in like manner. Taking the third
cleavage as a type, we may trace every transition from an equal divi-
sion (echinoderms, Polygordius), through forms in which it is but
slightly marked {Ampkioxtcs, frog), those in which it is conspicuous
{Nereis, Lymn(2a, Polyclades, Petroniyson, etc.), to forms such as Clep-
sine, where the cells of the upper quartet are so minute as to appear
like mere buds from the four large lower cells (Fig. 123). At the

Fig. 122. — ■ Cleavage of Nereis. An example of a spiral cleavage, unequal from the beginning
and of a marked mosaic-like 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 ceils 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; p. girdle of prototrochal
cells which give rise to the ciliated belt.

extreme of the series we reach the partial or meroblastic cleavage,
such as occurs in the cephalopods, in many fishes, and in birds and
reptiles. Here the lower hemisphere of the ^g'g 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 o.^^ (Fig. 124).

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-

2/2

CELL-DIVISION AND DEVELOPMENT

ular succession at a fixed point, thus giving rise to long cords of cells
(Fig. 125). 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.

Fig. 123. — The 8-cell stage of four different animals showing gradations in the inequality of
the third cleavage.

A. The leech Clepsine (Whitman). B. The chsetopod Rhymchelmis (Vejdovsky). C. The
lamellibranch Unio (Lillie). D. Aviphioxus.

Unequal division still awaits an explanation. The fact has already
been pointed out (p. 51) that the inequality of the daughter-cells is
preceded, if not caused, by an inequality of the asters ; but we are
still almost entirely ignorant of the ultimate cause of this inequality.
In the cleavage of the animal egg unequal division is closely con-
nected with the distribution of yolk — a fact generalized by Balfour

GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 2/3

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 Q.^^ from which they arise. Thus, in all telolecithal ova, where
the deutoplasm is mainly stored in the lower or vegetative hemi-
sphere, 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 macromcres
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 cleav-
age in the lower hemisphere.

.><rV^^

Fig. 124. — Partial or meroblastic cleavage in the squid Loligo. [Watase.]

B

Balfour's law undoubtedly explains a large number of cases, but
by no means all ; for innumerable cases are known in which no cor-
relation can be made out between the distribution 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 inapplicable to the in-
equalities of the first two divisions in annelids and gasteropods. It
is conspicuously inadequate in the history of individual 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. 122, E), undergoes an inva-
riable order of division, three unequal divisions being followed by
an equal one, then by three other unequal divisions, and again by an
equal. This cell contains no deutoplasm and undergoes no percepti-

2-7 A

CELL-DIVISION AND DEVELOPMENT

ble changes of substance. The cause of the definite succession of
equal and unequal divisions is here wholly unexplained.

Such cases prove that Balfour's law is only a partial explanation,
and is 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

Fig. 125. — Embryos of the earthworm Allolobophora faitida, showing teloblasts or apical cells.
A. Gastrula from the ventral side. B. The same from the riglit side; w/, the terminal telo-
blasts or primary ??tesoblasts, which bud forth the mesoblast-bands, cell by cell; t, lateral teloblasts,
comprising a neuroblast, tib, from which the ventral nerve-cord arises, and two nepkroblasts, 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 rnesoblasts enlarged; one in division.

GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 2/5

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, but 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.
I believe there is reason to accept the conclusion that this relation
is one of position, however caused. A central position of the mitotic
figure results in an equal division ; an eccentric position caused by a
radial movement of the mitotic figure, in the direction of its axis
towards the periphery, leads to unequal division, and the greater the
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. 71, 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 ai^ea that falls ivitJiin the spJiere of influence
of the centrosonie ; 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 Q.gg would
probably divide equally.

The causes that determine the position of the amphiaster are
scarcely known. It has been proved by experiment that in some
cases this position may be determined by mechanical causes. Thus,
Driesch has shown that when the eggs of sea-urchins are flattened
by pressure, the amphiasters all assume the position of least resist-
ance, i.e. parallel to the flattened sides, so that the cleavages are all
vertical, and the o.g'g segments as a fiat plate of eight, sixteen, or
thirty-two cells (Fig. 135). This is totally different from the normal
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 mqroblastic eggs, like that of the squid or bird, is a
mechanical result of the accumulation of yolk by which the forma-
tive 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 towards
the upper pole. Even here, however, the hypothesis of a merely
mechanical displacement probably does not touch the root of the

^ In the cleavage of gasteropod eggs.

2/6 CELL-DIVISION AND DEVELOPMENT

matter ; for it will not account for the eccentric position of the
spindle in the formation of the polar bodies or in teloblasts. Neither
will it explain the eccentric position of the horizontal spindle in such
cases as the first cleavage of the annelid Q,gg. In Nereis, for exam-
ple (Figs. 43, 122), the inequality of the first cleavage is predeter-
mined long before actual division both by an eccentric position of
the spindle and an inequality in the asters, neither of which can be
referred to an unequal horizontal distribution of the yolk.^ In this
and many similar cases we must assume more subtle causes lying
in the organization of the cytoplasmic mass, or rather of the egg
as a whole ; but these deeper causes still lie beyond our grasp.
Unequal division, which plays so important a part in development,
still therefore awaits a final explanation, and until this is forthcom-
ing we have but a vague comprehension of the primary factors of
growth.

Hertivigs Development of SacJis' s Lazv. — We have now to consider
two additional laws of cell-division 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 . TJie nucleus tends to take tip a position at the centre of its sphere
of injlnence, 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 law explains not only the mode of division in fl.attened
eggs, but also the normal succession of the division-planes according
to Sachs's second law. The first division of a homogeneous 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
lies 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 &gg while at most only half its diameter. Division is there-
fore transverse to the long axis and vertical to the first two planes.

Hertwig's second law has caused much discussion and has been
shown to have many exceptions, as for instance in the cambium-cells
of plants and in columnar epithelium. While undoubtedly one of
the most important laws of cell-division thus far determined, it only
pushes the analysis a stage further back, and leaves unexplained

1 In an earlier paper I made the erroneous statement that the first cleavage-spindle of
N'ereis lies centrally in the egg. Later and more careful studies by means of sections prove
that this was incorrect.

GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 2/7

the nature of the forces that determine the position of the spindle-
axis. Pfliiger^ assumed that this position must be that of least
resistance to the elongation of the spindle, which is obviously in the
long axis of the protoplasmic mass ; and the same view has been
advocated by Braem and Driesch. Now, there can of course be no
doubt that the final direction of the spindle, like that of any body, is
the position of least resistance, i.e. the position of equilibrium de-
termined by the resultant of all the forces operating upon it. The
undetermined point is whether these forces are of a simple mechani-
cal nature, such as pressure and the like, or of a more subtle physio-
logical character. Roux seeks them in the "tractive forces" of the
protoplasmic mass modified by an innate predisposition to a partic-
ular form and succession of divisions that has its seat in the nucleus.
Heidenhain identifies them with conditions of intra-cellular tension
determined by the astral rays.

It cannot be doubted that all these forces may play a part in de-
termining the position of the spindle, but it must be confessed that
the problem is still very far from a solution. In some cases Hert-
wig's law is directly opposed to the facts, the spindle lying trans-
versely to the axis of the protoplasmic mass. In other cases, as for
instance in the division of some Protozoa {Eiiglypha, t. Schewiakoff)
and in segmenting ova {Crepidida, t. Conklin), the protoplasmic
elongation leads the way, and may be fully determined before the
spindle is formed. In still other cases the reverse is true, as in the
formation of the polar bodies, where the spindle forms and rotates
into position before the ^gg shows any corresponding change of
form. In many ova we can assign no mechanical cause for the rota-
tion, such as the pressure of deutoplasm and the like ; and even
when deutoplasm is present, its position is such that we should
expect a horizontal rather than a vertical position of the polar
spindles were it a mechanical result of the presence of deutoplasm.

The ultimate determination of the planes of division is probably to
be sought in those influences that determine the movements of the
centrosomes. Sachs's law of rectangular succession is primarily a
result of the fact that the daughter-centrosomes typically diverge,
and so determine the spindle-axis, in a line which is at right angles
to the axis of the mother-spindle ; hence the ensuing cleavage is
vertical to the last.^ What we do not really understand is the prin-
ciple by which this typical succession is modified. The pressure-
experiments prove that the modifications may be produced by simple

1 '84, p. 613.

2 In this we find also an explanation of the fact first observed by Roux in the frog's egg,
and confirmed by me in the sea-urchin egg, that the first plane of cleavage passes through
the sperm-track and hence approximately through the entrance-point of the spermatozoon.

2/8 celC-division and development

*

mechanical means. The history of division in the cambium-cells
and columnar epithelium seems to show that neither direct pressure
nor the shape of the cells caused by it can be the ultimate cause.
The succession of divisions, always in the same plane, in apical cells
and in teloblasts, is directly related with a deeply lying law of growth
that affects the whole developing organism, and we cannot at pres-
ent distinguish in such cases between cause and effect ; for whether
the apical growth of the body as a whole is caused by local condi-
tions within the apical cells, or the reverse, is undetermined. This
unsatisfactory result shows how far we still are from an understand-
ing of the fundamental laws of growth and their relation to cell-
division, and how vast a field for experimental research lies open in
this direction.

B. Promorphological Relations of Cleavage

The cleavage of the ovum has thus far been considered merely as
a problem of cell-division. We have now to regard it in a far more
interesting and suggestive aspect ; namely, in its morphological rela-
tions to the body to which it gives rise. From what has been said
thus far it might be supposed that the Q.^g simply splits up into indif-
ferent cells which, to use the phrase of Pfiuger, have no more definite
relation to the structure of the adult body than have snow-flakes to the
avalanche to which they contribute. Such a conclusion would be
totally erroneous. It is a remarkable fact that in a very large num-
ber 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 onwards we are able to predict the exact future of
every individual cell. In this regard the cleavage of the ovum often
goes forward with a wonderful clock-like precision, giving the impres-
sion of a strictly ordered series in which every division plays a defi-
nite 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 further back to the regions of the undivided
and even unfertilized ovum. The egg, therefore, may exhibit a dis-
tinct 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. Proniorpliology of the Ovmn

(a) Polarity and the Egg-axis. — It was long ago recognized by
von Baer ('34) that the unsegmented egg of the frog has a definite
egg-axis connecting two differentiated poles, and that the position of

PROMO RfiHOLOGICAL RELATIONS OF CLEAVAGE 279

the embryo is definitely related to it. The great embryologist
pointed out, further, that the early cleavage-planes also are defi-
nitely related to it, the first two passing through it in two meridians
intersecting each other at a right angle, while the third is transverse
to it, and is hence equatorial. ^ Remak afterwards recognized the
fact '-^ that the larger cells of the lower hemisphere represent, broadly
speaking, the " vegetative layer " of von Baer, i.e. the inner germ-
layer or entoblast, from which the alimentary organs arise ; while
the smaller cells 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, afterwards
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 deiitoplasmic, the latter terms referring to the fact
that the nutritive deutoplasm is mainly stored in the lower hemi-
sphere, and that development is therefore more active in the upper.
The polarity of the ovum is accentuated by other correlated phe-
nomena. 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 eccentrically in the upper hemisphere, and the
polar bodies are formed at the upper pole. Even in cases where
the deutoplasm is equally distributed or is wanting — if there really
be such cases — an egg-axis is still determined by the eccentricity
of the nucleus and the corresponding 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. This
relation is, it is true, somewhat variable in different animals, yet the
evidence indicates that as a rule it is constant in a given species. It
is a very general rule that the upper pole, as marked by the posi-
tion of the polar bodies, lies in the median plane at a point which is
afterwards found to lie at or near the anterior end. Throughout
the annelids and mollusks, for example, the upper pole is the point
at which the cerebral ganglia are afterwards 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. It is not, however, a
universal rule. The recent observations of Castle ('96), which are
in accordance with the earlier work of Seeliger, show that in the

1 The third plane is in this case not precisely at the equator, but considerably above it,
forming a " parallel " cleavage.
"'55-P- 130-

28o CELL-DIVISION AND DEVELOPMENT

tunicate Ciona the usual relation is reversed, the polar bodies being
formed at the vegetative {i.e. deutoplasmic) pole, which afterwards
becomes the ventral side of the larva. In Ascaris Boveri's observa-
tions seem to show that the position of the polar bodies has no con-
stant relation to the adult axes, and Hacker describes a similar vari-
ability in the copepods (Fig. 130). My own observations on the
echinoderm-egg indicate that here also the primitive egg-axis has
an entirely inconstant and casual relation to the gastrula-axis. It
may perhaps still be possible to show that these exceptions are only
apparent, and the principle involved is too important to be accepted
without further proof. As the facts now stand, however, they seem
to admit of no other conclusion than that the relation of the primitive
egg-axis to the adult axes is not absolutely constant, and may in par-
ticular cases be variable. To admit this is to grant that this relation
is not of a fundamental character, and that the axes of the adult are
not predetermined from the beginning, but are established in the o.g'g
in the course of development.

{b) Axial Relations of the Primaiy 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
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 frogs egg coincides with the median plane of the
adnlt 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 Pfliiger and Roux i^^j). It
was placed beyond all question by a remarkable experiment by Roux
('88), 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. 131)-

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)
in the Q,g^ of the tunicate Clavelina (Fig. 126), and by Watase ('91)
in the eggs of cephalopods (Fig. 127). 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 Q.g% be distin-
guished with the greatest clearness, but also the anterior and poste-
rior regions, and the dorsal and ventral aspects. These discoveries

PROAIORPHOLOGICAL RELATIONS OF CLEAVAGE

2«I

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 conclusion was soon proved to have been premature. In one
series of forms, not the first but the second cleavage-plane was found

Fig. 126. — Bilateral cleavage of the tunicate egg.
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;
/, posterior end ; /, left; r, right side. [Orientation according to Castle.]

to coincide with the future long axis {Nereis, and some other annelids ;
Cj'epidiila, 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 {Clepsijie and other leeches ;
Rhynchelmis 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

282

CELL-DIVISION AND DEVELOPMENT

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
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. 309).

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-

A

P

Fig. 127. — Bilateral cleavage of the squid's egg. [WaTASE.]
A. Eight-cell stage. B. The fifth cleavage in progress. The first cleavage (a-/) coincides
with the future median plane ; the second {l-r') is transverse.

tive forces by which development is determined may or may not
coincide with those controlling the cleavage, gives us some hope of
disentangling the complicated factors of development through a com-
parative study of the different forms.

{c) Other P romorphological 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 o^^^, sometimes even before it is
laid. This is the case, for example, in the cephalopod Qgg, as shown

PROMORPIIOLOGICAL RELATIONS OF CLEAVAGE

by Watase (Fig. 128). Here the form of the new-laid egg, 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 o.'g^ slightly flattened on one side,
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-fiirrozvs are bilaterally arranged zvith respect to the plane of
symmetry in the undivided egg ; and the same is true of the later
development of all the bilateral parts.

/

Fig. 128. — Outline of unsegmented squid-s egg, to show bilaterality. [Watase.]
A. From right side. B. From the posterior aspect.
a-p, antero-posterior axis ; d-v, dorso-ventral axis ; /, left side ; r, right side.

Scarcely less striking is the case of the insect o-gg, as has been
pointed out especially by Hallez, Blochmann, and Wheeler (Figs.
44, 129). In a large number of cases the Q.gg is elongated and
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 result of a
study of the cockroach {Periplaneta), the water-beetle {Hydivphilus),
and the locust {Lociista) that " the egg-cell possesses the same orienta-
tion 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

^ See Wheeler, '93, p. 67.

284

CELL-DIVISION AND DEVELOPMENT

result, and concluded that even when the Q,g'g 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

a

m

a

\

V:

A

^

\ d

-n

\i

\

B

P

\

Fig. 129. — 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; v, ventral, d, dorsal aspect. (These letters
refer to the fatal position of the embryo, which is nearly diametrically opposite to that in which it
first develops) ; in, micropyle ; near/ is the pedicle by which the egg is attached.

forwards towards the upper end of the oviduct, and hence towards
the head-end of the mother.^

1 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).

PROMORPHOLOGICAL RELATIONS OF CLEAVAGE 285

2. Meaning of the PromorpJiology of the 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 conveniently be
considered at this point. Two fundamentally different interpreta-
tions of the facts have been given. On the one hand, it has been
suggested 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 exis-
tence even though invisible, and is revealed to observation through
polar differentiation, bilateral symmetry, and other obvious characters
in the unsegmented &gg. On the other hand, it has been maintained
by Pfiiiger, 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 pre-embryonic history. Thus the Qgg of a bilateral
animal is not at the beginning 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
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-
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 pre-existing cause. That the &^% possesses no fixed
and predetermined cytoplasmic localization with reference to the
adult parts, has, I think, been demonstrated through the remarkable
experiments of Driesch, Roux, and Boveri, which show that a frag-
ment of the Q.gg may give rise to a complete larva (p. 308). 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 Pfluger ('84),
Born ('85), and Schultze ('94) have shown, the cytoplasmic materials

1 See p. 298. 2 Cf. pp. 299, 300.

'.60

CELL-DIVISION AND DEVELOPMENT

may be entirely re-arranged 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. Finally, it is becoming more and more doubt-
ful whether the relation of the egg-axis to the adult axis has so deep
a significance as was at first assumed. This relation has been found

Fig. 130. — Variations in the axial relations of the eggs of Cyclops. From sections of the eggs
as tliey 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) ; «, b, and c, show three different positions.

to vary not only in nearly related forms (insects), but even in the
same species {Ascaris, according to Boveri and others ; Toxopnciistes,
according to my own observations; copepods, according to Hacker).

All these and many other similar facts force us, I think, to the
conclusion that the promorphological features of the Q.gg are as truly
a result of development as the characters coming into view at later

PROMORPHOLOGICAL RELATIONS OF CLEAVAGE iS/

Stages. They are gradually established during the pre-embryonic
stages, and the o-gg, when ready for fertilization, has already accom-
plished 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 afterwards 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 pre-existing organization of the o.^^ 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 pre-organized 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-
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 pinori 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
shozvs a definite relation to the .ovarian, tissues, the germinal vesicle
lying near the point of attachment or of food-supply. Mark made

1 '81, p. 512. 2 'gi^ p_ 280.

288 CELL-DIVISION AND DEVELOPMENT

-the pregnant suggestion, in iSSi, that the primary polarity of the &g^
might be determined by " the topographical relation of the egg (when
still in an indifferent state) to the remaining cells of the tnaternal 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 Q,g^ takes its origin. If, in cases where the Q.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 p. 40, the nucleus usually lies towards the base of the
cell, near the source of food, while the characteristic cytoplasmic
products, such as zymogen granules and other secretions, appear in
the outer portion.^ The exact conditions under which the ovarian
&gg develops are still too little known to allow of a positive conclu-
sion regarding Mark's suggestion. Moreover, 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 towards 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
to the periphery, the deutoplasm become massed at one pole, and
the polarity of the egg come into view {Nereis, Figs. 43 and 71).^ In
such cases the axis of the Q,gg is not improbably 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 Q,gg.
These considerations show that the 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.

1 '81, p. 515. 2 Hatschek has suggested the same comparison (^Zoologie, p. 112).

■' The immature egg of Nereis shows, however, a distinct polarity in the arrangement of
the fat-drops, which form a ring in the equatorial region.

THE ENERGY OF DIVISION 289

C. The Energy of Division

The causes by which cell-division is incited and by which its cessa-
tion is determined are as yet scarcely comprehended, and the ques-
tions that they suggest merge into the larger problem of the general
control of growth. All animals and plants have a limit of growth,
which is, however, much more definite in some forms than in others,
and which differs in different tissues. During the individual devel-
opment 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
equilibrium is established, some of the cells ceasing to divide and
perhaps losing this power altogether (nerve-cells), others dividing
only under special conditions (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 equi-
librium is attained is an hereditary character, which in many cases
shows an obvious relation to the environment, and has therefore prob-
ably been determined and is maintained by natural selection. From
the cytological point of view the limit of body-size appears to be cor-
related with the total nmnber of cells formed rather than with their
individual size. This relation has been carefully studied by Conklin
('96) in the case of the gasteropod Crepidula, an animal which varies
greatly in size in the mature condition, the dwarfs having in some
cases not more than ^V ^^^ volume of the giants. The eggs are,
however, of the same size in all, and their nnmbe7'- is proportional to
the size of the adult. The same is true of the tissue-cells. Measure-
ments of cells from the epidermis, the kidney, the liver, the alimen-
tary 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.^

Morgan has examined the same question experimentally through
a comparison of normal larvae of echinoderms and Aviphioxus with
dwarf larvae of the same species developed from egg-fragments ("95, i ;
'96). Broadly speaking, his results agree with Conklin's, though they
show that the relation is by no means simple or constant. If unseg-
mented eggs of sea-urchins {SpJicBvechinns) be shaken to pieces,
fragments of all sizes are obtained which may segment and pro-
duce blastulas and gastrulas ranging down to ^-^ the volume of
the normal size. Dwarfs are also obtained from isolated blasto-
meres of two-, four-, or eight-cell stages. In both cases the num-

^ See Amelung ('93) and Strasburger ('93).

290 CELL-DIVISION AND DEVELOPMENT

ber of cells in the blastula just before invagination is approximately
proportional to the size of the blastula, though the smaller frag-
ments show a tendency to produce a somewhat larger number, and
their cells are, therefore, somewhat smaller than in the larger forms.
The same is true in Amphioxus. Morgan, therefore, draws the very
interesting conclusion that "the ultimate size of the cells produced
by repeated division determines when the division shall come to
an end for a certain stage of the ontogeny." ^ This conclusion
is, however, subject to exception; for Morgan finds that the dwarf
larvae show a tendency to use the same number of cells in the
formation of certain organs as the full-sized individuals. Thus the
dwarf blastulas tend to invaginate the same number of cells to
form the archenteron as in the normal forms ; and in Amphioxus
the notochord of the dwarfs consists in cross-section of three cells,
as in normal individuals, irrespective of the total number of cells. It
is clear, therefore, that there is another factor, besides the size of the
cells, to be taken into account, and the whole subject awaits further
investigation.

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-
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 Ainphioxus (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 succession up to the thirty-eight-cell stage,
when a long pause takes place, and when the divisions are re-
sumed 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 num-
ber remaining fixed thereafter.^ Again, the number of cells produced
for the foundation of particular structures is often definitely fixed,
even when their number is afterwards increased by division. In

^ '95> P- "9-

2 This organ, doubtfully identified by me as the head-kidney, is probably a mucus-gland

(Mead).

3 Cf. Fi". 122.

THE ENERGY OF DIVISION 29 1

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. In
Echinus, according to Morgan, the number of cells used in the forma-
tion of the archenteron is approximately one hundred ; in SpJicercchinus
the number is approximately fifty.

Perhaps the most interesting numerical relation of this kind are
those recently discovered in the division of teloblasts, where the num-
ber of divisions is directly correlated with the number of segments or
somites. It is well known that this is the case in certain plants
{CJiaraccce), where the alternating nodes and internodes of the stem
are derived from corresponding single cells successively segmented
off from the apical cell. Vejdovsky's observations on the annelid
DcndrobcEua 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
accurately 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. McMurrich
('95) has recently demonstrated that the mesoblasts always divide
exactly sixteen times, the ectoblasts thirty-two (or thirty-three) times,
before relinquishing their teloblastic 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-naupliar region
of the embryo {i.e. from the second maxilla backward). In other
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 further
into the nature of development. It must be confessed that the
specific causes that incite or inhibit cell-division are scarcely known.
The egg-cell is in most cases stimulated to divide by the entrance of
the spermatozoon, but in parthenogenesis exactly the same result is

292 CELL-DIVISION AND DEVELOPMENT

produced by a different cause. 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 in-
jection of foreign substances into the blood; by mechanical press-
ure, 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 betv/een 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 gaining
ground that the immediate causes of division, whatever their ante-
cedents, are to be sought in this direction. The most promising field
for their investigation seems to lie in the direction of cellular pathology
through the study of tumours and other abnormal growths. The work
of Ziegler and Obolonsky indicates that the cells of the liver and
kidney may be directly incited to divide through the action of arsenic
and phosphorus; and several others have reached analogous results
in the case of other tissues and other poisons. The formation of
galls seems to leave no doubt that extremely complex and charac-
teristic abnormal growths may result from specific chemical stimuli,
and some pathologists have held a similar view in regard to the origin
of abnormal growths in the animal body.

Suggestive as these results are, they scarcely touch the ultimate
problem. The unknown factor is that which determines and main-
tains the normal equilibrium. 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 neigh-
bouring tissues or organs. The removal or lessening of this resistance
through injury or disease causes a resumption of growth and division,
leading either to the regeneration of the lost parts or to the forma-
tion 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 re-
generation. To speak of such a "barrier" or "resistance" is, how-
ever, 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 com-

CELL-DIVISION AND GROWTH 293

pensatory 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 regeneration.^ Again, in
the case of coelenterates, Loeb and Bickford have shown that the
tubularian hydroids are able to regenerate the tentacles at both ends
of a segment of the stem, while the polyp CeriantJms can regenerate
them only at the distal end of a section (Fig. 142). In the latter case,
therefore, the body possesses an inherent polarity which cannot be
overturned by external conditions.

D. Cell-Division and Growth

The relation between cell-division and growth has already been
touched upon at pp. 41 and 265. The direction of the division-
planes in the individual cells evidently stands in some causal rela-
tion with the axes of growth in the body, as is especially 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 phenomena
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 groivth 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).

The opinion has of late rapidly gained ground that the same is
true in principle of animal growth, and this view has been urged
by many writers, among whom may be mentioned Rauber, Hertwig,
and especially Whitman, whose line essay on the Inadequacy of the
Cell-theory of Development ('93) marks a distinct advance in our
point of view. It is certain that in the earlier stages of develop-
ment, and in a less degree in later stages as well, the character of
growth and division in the individual cell is but a local manifesta-
tion of a formative power pervading the organism as a whole ; and

1 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.

294 CELL-DIVISION AND DEVELOPMENT

the general truth of this view has been in certain cases conclusively
demonstrated by experiment.^ It has, however, become clear that
this conclusion can be accepted only with certain reservations ; for
as development proceeds, the cells may-acquire so high a degree
of independence that profound modifications may occur in special
regions through injury or disease, without affecting the general equi-
librium 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 organ-
ization of the entire mass ; and in the following chapter we shall see
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.

LITERATURE. VIII

Berthold, G. — Studieii 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.

Joiirn. M. S., XXXVIII. i, 1895.
Errara. — Zellformen und Seifenblasm : Tagebl. der 60 Versamvihiiig deiUscher

Natiirforscher 7md Aerzte zn Wiesbaden. 1887.
Hertwig, 0. — Das Problem der Befruchtung und der Isotropic des Eies, eine

Theorie der Vererbung. Jena, 1884.
Hofmeister. — Die Lehre von der Pflanzenzelle. Leipzig, 1867.
McMurrich, J. P. — Embryology of the Isopod Crustacea: Journ. Morph.,X.l. i.

1895.
Mark, E. L. — Limax. (See List IV.)
Rauber, A. — Neue Grundlegungen zur Kenntniss der Zelle : Morph. JaJi7-b.,V\\\.

1883.
Sachs, J. — Pflanzenphysiologie. (See List VII.)
Sedgwick, H. — On the Inadequacy of the Cellular Theory of Development, etc.:

(Quart. Joiirn. Mic. Sci., XXXVII. i. 1894.
Strasburger, E. — Ueber die Wirkungssphare der Kerne und die Zellgrbsse : Histo-

logische Beitrdge, V . 1 893 .
Watase, S. — Studies on Cephalopods ; I., Cleavage of the Ovum : Jorirn. 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 : Jorirn. Morph., VI. 3. 1892.
Id. — Amphioxus and the Mosaic Theory of Development: Journ. Morph., VIII.

3- 1893.

1 Cf. p. 312.

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 pre-existing 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 tie fern Einsicht in die Zeugungs- und Entwickelungsgeschichte der organischen
Korper zu fiihren und zu zeigen, wie dieselben weder vorgebildet sind, noch auch, wie man
sich gewohnlich denkt, aus ungeformter Masse in einem bestimmten Momente pldtzlich
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 parental 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 pro-
cesses going on in the parental body. What gives development its
marvellous character is the rapidity 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. Hozv 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

^ Evolution, Science and Culture, p. 291.

2 Variation of Animals and Plants, II. p. 398.

3 Enttvick. der Thiere, II., 1837, P- ^•

295

296 THEORIES OF INHERITANCE AND DEVELOPMENT

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 fundamental aspects as great a riddle as they were
to the Greeks. What we have gained is a tolerably precise acquaint-
ance with the external aspects of development. The gross errors of
the early preformationists have been dispelled.^ We know that the
germ-cell contains no predelineated embryo ; that development is
manifested, on the one hand, by a continued process of cell-division,
on the other hand, by a process of differentiation, through which
the cells gradually assume diverse forms and functions, and so
accomplish a physiological division of labour. But we have not yet
fathomed the inmost structure of the germ-cell, and the means by
which the latent adult characters that it involves are made actual
as development proceeds. And it should be clearly understood that
when we attempt to approach these deeper problems we are com-
pelled to advance beyond the solid ground of fact into a region of
more or less doubtful and shifting hypothesis, where the point of
view continually changes as we proceed. It would, however, be an
error to conclude that modern hypotheses of inheritance and develop-
ment are baseless speculations that attempt a merely formal solution
of the problem, like those of the seventeenth and eighteenth cen-
turies. They are a product of the inductive method, a direct out-
come of accurately determined fact, and they lend to the study of
embryology a point and precision that it would largely lack if limited
to a strictly objective description of phenomena.

All discussions of development are now revolving about two cen-
tral hypotheses, a preliminary examination of which will serve as
an introduction to the general subject. 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 considered
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

^ Cf. Introduction, p. 6.

2 I venture to suggest this term as an English equivalent for the awkward expression
" Organbildende Keimbezirke " of His.

THE THEORY OF GERMINAL LOCALIZATION 297

been advocated by some of the foremost students of development.
It is maintained that, although the embryo is not ^rQ,-foruied in
the germ, it must nevertheless be ^XQ.-determined m. the sense that the
^gg contains definite areas or definite substances predestined for the
formation of corresponding parts of the embryonic body. The first
definite statement of this conception is found in the interesting and
suggestive work of Wilhelm His ('74) entitled Unsere Kdrpevfoin}^..
Considering the development of the chick, he says : " It is clear, on
the one hand, that every point in the embryonic region of the blasto-
derm must represent 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 backwards 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 Q.gg.
According to this principle, the germ-disc contains the organ-germs
spread out in a flat plate, and, conversely, every point of the germ-
disc reappears in a later organ ; I call this the principle of organ-
forming germ-regions r ^ His thus conceived the embryo, not as
Y)rt-formed, but as having all of its parts ^XQ.-localized in the egg-
protoplasm (cytoplasm).

A great impulse to this conception was given during the following
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
predelineated, we can say that it is predetermined. The ' Histo-
genetic sundering ' of embryonic elements begins with the cleavage,

1 I.e., p. 19.

2 It is clear from the context that by " substance" Lankester had in mind the cytoplasm,
though this is not specifically stated.

3 '77, p. 14.

298 THEORIES OF INHERITANCE AND DEVELOPMENT

and every step in the process bears a definite and invariable relation
to antecedent and subsequent steps. ... It is, therefore, not sur-
prising 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, Flemming who gave the first specific statement of the
matter from the cytological 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 develop-
ment wherein one Q.gg differs from another, and it will be possible to
look for it witJi 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? In the following year Van Beneden
pointed out how nearly this conception 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 animal 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 ('88) on the frog's egg. On killing one of the
blastomeres of the two-cell stage by means of a heated needle the
uninjured half developed in some cases into a perfectly formed half-
larva (Fig. 131), accurately representing 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

1 '78, p. 49.

^ Zellsubstanz, '82, p. 70 ; the italics are in the original.

^ '83. P- 571-

* The accuracy of this result was disputed by Oscar Hertvvig ('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 perfect half-larva, to a larva with irregular defects, or to a whole
larva of half-size, according to circumstances (p. 319).

THE THEORY OF GERMINAL LOCALIZATION

299

independently 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.

Fig. 131. — Half-embryos of the frog (in transverse section) arising from a blastomere of the
2-ceIl 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; ;«./], medullary fold ; ;«j., meso-
blast-plate.

In an able series of later works Whitman has followed out the sug-
gestion made in his paper of 1878, already cited, pointing out how
essential a part is played in development by the cytoplasm and insist-
ing that cytoplasmic pre-organization must be regarded as a leading
factor in the ontogeny. Whitman's interesting and suggestive views

1 I.e., p. TO.

300 THEORIES OF INHERITANCE AND DEVELOPMENT

are expressed with great caution and with a full recognition of the
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 organ-
ization 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 organ-
ism exists before cleavage sets in, and persists throughout every
stage of cell-multiplication."^ In so far as this view involves the
assumption that the organization of the egg-cytoplasm at the be-
ginning of cleavage is a primordial character of the Q-g^, Whitman's
conception must, I think, be placed on the side of the localization
theory ; but his point of view can only be appreciated through a
study of his own writings.

All of these views, excepting those of Roux, lean more or less
distinctly towards 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 o.g'g 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 Q.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 unsegmented &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 best be 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. It was the

^ '93, p. 115. - /.(■., p. 112. 3 Theorie der Abstajninungslelwe, 1884.

THE IDIOPLASM THEORY 3OI

great merit of Nageli's hypothesis to consider inheritance as 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
tfopJioplasvi. Hereditary traits are the outcome of a definite molec-
ular organization of the idioplasm. The hen's ^^g differs from the
frog's because it contains a different idioplasm. The species is as
completely contained in the one as in the other, and the hen's Q.%g
differs from a frog's 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 dynamic properties of the micellae themselves. Dur-
ing development the idioplasm undergoes a progressive transforma-
tion of its substance, not through any material change, but through
dynamic alterations of the conditions of tension and movement 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 biologi-
cal investigation were led to locate the idioplasm in the nucleus,
and succeeding researches have rendered it more and more highly
probable that it is to be identified with clironiatin. The grounds
for this conclusion, which have already been stated in Chapter
VII., may be. here again briefly reviewed. The beautiful experi-
ments of Nussbaum, Gruber, and Verworn proved that the regenera-
tion of differentiated cytoplasmic structures in the Protozoa can only
take place when nuclear matter is present (cf. p. 248). 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

302 THEORIES OF INHERITANCE AND DEVELOPMENT

gives rise to chromosomes of the same number, form, and size. Van
Beneden and Boveri proved (p. 134) 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 effect
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 niiclejis contains the physical basis of
inheritance, and that chr^ontatin, its essential constituent, ts the idio-
plasm postulated in Ndgeli s theory. This conclusion is now widely
accepted ; and notwithstanding certain facts which at first sight may
seem opposed to it, I believe it rests upon a basis so firm that it may
be taken as one of the elementary data of heredity. To accept it is,
however, to reject the theory of germinal localization in so far as it
assumes a pre -localization 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 fact 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 therefore be determined by the paternal chromatin in
the germ-nucleus, and not by a pre-determination of the egg-cyto-
plasm.

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

THE ROUX-WEISMANN THEORY OF DEVELOPMENT 303

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 gennnules,
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 mi-
nute self-propagating organisms, every one of which predetermines
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 z^S\&& pang ens, are
contained in the nucleus, migrating thence into the cytoplasm step
by step during ontogeny, and thus determining the successive stages
of development. The same view was afterwards accepted by Hert-
wig 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 ^%^, 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,

1 The neo-pangenesis of De Vries differs 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 pre-exist 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 specific substances from the nucleus, vi'e need not, however, necessarily adopt the
pangen-liypothesis.

304 THEORIES OF INHERITANCE AND DEVELOPMENT

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
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-VVeismann hypothesis has already been touched on at
p. 183. 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 daughter-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-nucleus, tJierefore, receives a specific form of cJironiatin 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 fur-
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,

THE ROUX-WEISMANN THEORY OE DEVELOPMENT 305

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
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, (i) 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 case {^' honiceokinesis,'' integral
or quantitative division), the resulting nuclei remain precisely equiva-
lent. In the second case {" ketei^okinesis,'' 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
ox germ-plasm (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 bi-
ophores, 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 wh'ich 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 en 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 slightly
modified germ-plasm to those parts capable of regeneration.

1 Essay IV., p. 193, 1885. 2 Germ-plasm, pp. 76, 77. ^ I.e., p. 15.

X

3o6

THEORIES OF INHERITANCE AND DEVELOPMENT

E. Critique of the Roux-Weismann Theory

From a logical point of view the Roux-Weismann theory is unas-
sailable. Its fundamental weakness is its ^?/ir?i-z'-metaphysical char-
acter, which indeed almost places it outside the sphere of legitimate
scientific hypothesis. Not a single visible phenomenon of cell-divi-

Fig. 132. — Half and whole cleavage in the eggs of sea-urchins.
A. Normal 16-cell stage, showing the four micromeres above (from Driesch, after Selenka),
B. Half 16-cell stage developed from one blastomere of the 2-cell stage after killing the other by
shaking (Driesch). C. Half blastula resulting, the dead blastomere at the right (Driesch).
D. Half-sized 16-cell stage of Toxopueustes, viewed from the micromere-pole (the eight lower cells
not shown). This embryo, developed from an isolated blastomere of the 2-cell stage, segmented
like an entire normal ovum.

sion gives even a remote suggestion of qualitative division. All the
facts, on the contrary, indicate that the division of the chromatin is
carried out with the most exact equality. The theory of qualita-
tive division was suggested by a totally different order of phenom-
ena, and is an explanation constructed ad hoc. Roux, it is true, was
led to the hypothesis through an examination of mitosis ; but it is

CRITIQUE OF THE ROUX-WEISMANN THEORY

307

safe to say that he would never have maintained in the same breath
that mitosis is expressly designed for quantitative and also for qual-
itative division, had he fixed his attention on the actual phenomena
of mitosis alone. The hypothesis is in fact as complete an a priori
assumption as any that the history of scholasticism can show, and
every fact opposed to it has been met by equally baseless subsidiary
hypotheses, which, like their principal, relate to matters beyond the
reach of observation.

Such an hypothesis cannot be actually overturned by an appeal to
fact. When, however, we make such an appeal, the improbability of

Fig. 133. — Normal and dwarf gastrulas of Amphioxus.
A. Normal gastrula. B. Half-sized dwarf, from an isolated blastomere of the 2-cell stage.
C. Quarter-sized dwarf, from an isolated blastomere of the 4-cell stage.

the hypothesis becomes so great that it loses all semblance of reality.
It is rather remarkable that Roux himself led the way in this direc-
tion. In the course of his observations on the development of a half-
embryo from one of the blastomeres of the two-cell stage he determined
the significant fact that the half-embryo afterzvards regenerated the
missing half, and gave rise to a complete embryo. Essentially the
same result was reached by later observers, both in the frog (Endres,
Walter, Morgan) and in a number of other animals, with the impor-
tant addition that the half-formation is sometimes characteristic of
only the earliest stages and may be entirely suppressed. In 1891
Driesch was able to follow out the development of isolated blasto-

3o8

THEORIES OF INHERITANCE AND DEVELOPMENT

meres of sea-urchin eggs separated by shaking to pieces the two-
cell and four-cell stages. Blastoraeres thus isolated segment as if
still forming part of an entire larva, and give rise to a half- (or quar-
ter-) blastula (Fig. 132). The opening soon closes, however, to form a

Fig. 134. — Dwarf and double embryos of Amphioxus.
A. Isolated blastomere of the 2-celI stage segmenting like an entire egg (cf. Fig, 123,/)).
B. Twin gastrulas from a single egg. C. Double cleavage resulting from the partial separation,
by shaking, of the blastomeres of the 2-cell stage. D. E. F. Double gastrulas arising from such
forms as the last.

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 gave rise to double embryos like
the Siamese twins. Shortly afterwards the writer obtained similar
result in the case of Amphioxus, but here tJie isolated blastomere scg-

CRITIQUE OF THE ROUX-WEISMANN THEORY 309

nients from the beginning like an entire ovjivi of diminished size (Figs.
133, 124). The same result has since been reached by Morgan in the
teleost fishes, and by Zoja in the medusae. The last-named experi-
menter was able to obtain perfect embryos not only from blasto-
meres 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
y^g- the normal size !

These experiments gave a fatal blow to the entire Roux-Weismann
theory ; for the results showed that the cleavage of the ovum does not
in these cases sunder different materials, either nuclear or cytoplasmic,
but only splits it up into a number of similar parts, each of which
may give rise to an entire body of diminished size.

The theory of qualitative nuclear division has been practically

Fig. 135. — Modification of cleavage in sea-urchin eggs by pressure.
A. Normal 8-cell stage of Toxop7ieustes. B. Eight-cell stage of Echinus segmenting under
pressure. Both forms produce normal Plutei.

disproved in another way by Driesch, through the pressure-experi-
ments already mentioned at p. 275. Following the earlier experiments
of Pflliger and Roux on the frog's ^'g'g, 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 from the
normal (Fig. 134) ; yet such eggs when released from pressure continue
to segment, zvithont rearrangement of the nuclei, and give rise to per-
fectly normal larvae. I have repeated these experiments 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 differentia-
tion of the cells appears very early. In the normal development of
this animal the archenteron arises from four large cells or macro-
meres (entomeres), which remain after the successive formation of
three quartets of micromeres (ectomeres) and the parent-cell of the

3IO

THEORIES OF INHERITANCE AND DEVEIOPMENT

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 charac-
teristic appearance, differing from that of the other blastomeres in
its pale non-granular character and in the presence of large oil-drops.

Fig. 136. — Modification of cleavage by pressure in Nereis.
A. B. Normal 4- and 8-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 to trocho-
phores with eight instead of four entoderra-cells. Numerals designate the successive cleavages.

If unsegmented eggs be subjected to pressure, as in Driesch's echino-
derm 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. 136, 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

ON THE NA TURE AND CA USES OF DIFFERENTIA TION 3 1 1

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 trochophores contain-
ing eight instead of four macromeres, which have the typical clear
protoplasm containing oil-drops. In this case there can be no doubt
whatever that four of the entoblastic nuclei were normally destined
for the first quartet of micromeres (Fig. 136, B), from which arise the
apical ganglia and the prototroch. Under the conditions of the
experiment, however, they have given rise to the nuclei of cells
which differ in no wise from the other entoderm-cells. Even in a
highly differentiated type of cleavage, therefore, the nuclei of the
segmenting ^gg are not specifically different, as the Roux-Weismann
hypothesis demands, but contain the same materials even in cells that
undergo the most diverse subsequent fate. But there is, furthermore,
very strong reason for believing that this may be true in later stages
as well, as Kolliker insisted in opposition to Weismann as early as
1886, and as has been urged by many subsequent writers. The strong-
est 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 small fragment of the body is able to repro-
duce 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 limitation arises through specifica-
tion 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 prob-
lem 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. dijferentiationf 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 Q.gg.
These changes are sooner or later accompanied by the cleavage
of the ^gg into cells whose boundaries may sharply mark the

312 THEORIES OF INHERITANCE AND DEVELOPMENT

areas of differentiation. What gives these cells their specific char-
acter ? Why, in the four-cell stage of an annelid egg, 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. 122,
137, B). There cannot be a fixed and necessary relation of cause
and effect between the various regions of the Q.gg which these blas-
tomeres represent and the adult parts arising from them ; for, as we
have seen, these relations may be artificially altered. A portion of
the egg which under normal conditions 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
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
migration of the pangens, and so correlates the operations of devel-
opment t Both Driesch and Oscar Hertwig have attempted to
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 Q.gg 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-
ment. Both at first assumed not only that the nuclei are equivalent,
but also that the cytoplasmic regions of the Q,gg are isotropic, i.e.
primarily composed of the same materials and equivalent in struct-
ure. Hertwig insisted that the organism develops as a whole as the
result of a formative power pervading the entire mass ; that differen-
tiation is but an expression of this power acting at particular points ;
and that the development of each part is, therefore, dependent on
that of the whole. ^ "According to my conception," said Hertwig,
" each of the first two blastomeres contains the formative and differ-
entiating 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 blasto-
mere." ^ Again, in a later paper: — "The egg is a specifically

1 Whitman had strongly urged this view several years before, and a nearly similar concep-
tion lay at the bottom of Herbert Spencer's theory of development. Cf. pp. 41, 293.

2 '92, I, p. 481.

ON THE NATURE AND CAUSES OF DIFFERENTIATION 313

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.^^,, 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 prod-
ucts according to its position with reference to the entire organism
(blastula, gastrula, etc)."^

Driesch expressed the same view with great clearness and pre-
cision shortly after Hertwig : — " The fragments {i.e. cells) produced
by cleavage are completely equivalent or indifferent." "The blasto-

A

B

Fig. 137. — Diagrams contrasting the value of the blastomeres in polyclades and annehds.

A. Plan of cleavage in the polyclade egg (constructed from the figures of Lang). B. Corre-
sponding plan of the annelid egg. In both cases the ectoblast is unshaded, with the exception of
X ; the mesoblast is ruled in vertical lines and the entoblast in horizontal. In both, three succes-
sive quartets of micromeres are budded forth from the four primary cells A. B. C. D. In the
polyclade the first quartet is ectoblastic, the second and third mesoblastic. In the annelid all three
quartets are ectoblastic, while the mesoblast (M) arises from the posterior cell of a fourth quartet
of which the remaining three are entoblastic.

meres 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 develop-
ment." "^ ''The relative position of a blastomeve in the whole de-
termines in general zvhat develops from it ; if its position be changed,
it gives rise to something dijferent ; in other words, its prospective
value is a function of its position.'" ^

This conclusion undoubtedly expresses a part of the truth, though,
as will presently appear, it is too extreme. The relation of the part

^ That is, in the specifically organized chromatin within the nucleus.
^ '93, P- 793- ^ Studien IV. p. 25. •* Studien TV. p. 39.

314

THEORIES OF INHERITANCE AND DEVELOPMENT

to the whole must not, however, be conceived as a merely geometri-
cal or mechanical one ; for, in different species of eggs, blastomeres
may exactly correspond in origin and relative position, yet have
entirely different morphological value. This is strikingly shown by

Fig. 138. — Partial larvae of the ctenophore Beroe. [Driesch and Morgan.]
A. Half i6-cell stage, from an isolated blastomere. B. Resulting larva, with four rows of swim-
ming plates and three gastric pouches. C. One-fourth i6-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 unsegmented egg.
F. Similar larva with five rows of plates, from above.

a comparison of the polyclade egg with that of the annelid or
gasteropod (Fig. 137). In both cases three quartets of micromeres
are successively budded off from the four cells of the four-cell
stage in exactly the same manner. The iirst quartet in both gives
rise to ectoderm. Beyond this point, however, the agreement ceases ;

ON THE NATURE AND CAUSES OF DIFFERENTIATION 315

for the second and third quartets form mesoblast in the polyclade,
but ectoblast in the anneUd and gasteropod ! In the latter forms
the mesoblast lies in a single cell belonging to a fourth quartet of
which the other three cells form entoblast. This shows conclusively
that the relation of the part to the whole is of an exceedingly subtle
character, and that the nature of the individual blastomere depends,
not merely upon its geometrical position, but upon its physiological
relation to tJie inJierited organization of which it forms a part.

Meanwhile, and subsequently, however, facts were determined
that threw doubts on the hypothesis of cytoplasmic isotropy and
led Driesch to a profound modification of his views, and in a
measure rehabilitated the theory of cytoplasmic localization. Whit-
man, Morgan, and Driesch himself showed that the cytoplasm of
the echinoderm o,^^ is not strictly isotropic, as Hertwig assumed ;
for the ovum possesses a polarity predetermined before cleavage
begins, as proved by the fact that a group of small cells or micro-
meres always arises at a certain point which may be precisely located
before cleavage by reference to the eccentricity of the first cleavage-
nucleus. ^ Experiments on the eggs of other animals proved that the
predetermination of the cytoplasmic regions may be more extensive.
In the Q,gg of the ctenophore, for example, Driesch and Morgan
('95), confirming the earlier observations of Chun, proved that an
isolated blastomere of the two- or four-cell stage gives rise not to a
whole dwarf body, but to a half- or quarter-body, as Roux had
observed in the frog^ (Fig. 135, y^-Z*). But, more than this, these
experimenters made the interesting discovery that if a part of the
cytoplasm of an imsegmented ctenophore-egg were removed, the
remainder gave rise to an incomplete larva, showing certain defects
zvhich represent the portions removed (Fig. 138, E, F). Again,
Crampton 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. 139).

These results demonstrate that the ovum may show a high degree
of cytoplasmic- localization and that in such cases cleavage may be in
fact a mosaic-work, as Roux maintained in case of the frog. But
they also show that the localization, and the resulting mosaic-like
cleavage, is not determined by specific differences in the nuclei ; for
in the ctenophore the fragment of an nnseginentcd &gg, though con-
taining an entire nucleus, gives rise to a defective larva, and in Nereis
the nuclei may be shifted about at will without altering the develop-

1 Cf. Fig. 77.

2 The larva is, however, not a sU-ict partial one, since it makes an abortive attempt to
form the normal number of gastric pouches.

3i6

THEORIES OE INHERITANCE AND DEVELOPMENT

ment. And if the germinal localization is not directly determined by
the nuclei it must here be determined by a pre-organization of the
cytoplasmic substance. How is this result to be reconciled with the
experiments on AnipJiioxiis and the echinoderms, and with the more
general conclusion that the ultimate determining causes of differentia-

Fig. 139. — Partial development of isolated blastomeres of the gasteropod egg, Ilyanassa.
[Crampton.]

A. Normal 8-cell stage. B. Normal 16-cell stage. C. Half 8-ceU stage, from isolated blasto-
mere of the 2-cell stage. D. Half 12-cell stage succeeding. E. Two stages in the cleavage of an
isolated blastomere of the 4-cell stage ; above a one-fourth 8-cell stage, below a one-fourth 16-cell
stage.

tion are to be sought in the nucleus ? The difficulty at once disap-
pears when we recall 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

ON THE NATURE AND CAUSES OF DIFFERENTIATION 31/

between Amphioxus on the one hand, and the 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.

We thus arrive at the central point of my own conception of devel-
opment, and of Driesch's later views, which were developed in a most
able and suggestive though somewhat abstruse manner in his Analy-
tische Theorie der organisclicn Entuncklnng i^c^d^), and slightly modified
in a later paper published jointly with Morgan ('95, 2). The gist of
Driesch's theory is as follows. All the nuclei are equivalent, and all
contain the same idioplasm equally distributed to them by mitotic
division. Through the influence of this idioplasm the cytoplasm of
the ^gg, or of the blastomeres derived from it, undergoes specific and
progressive changes, each change reacting upon the nucleus and thus
inciting a new change. These changes differ in different regions of
the egg because of pre-existing differences, chemical and physical,
in the cytoplasmic structure ; and these form the conditions ("Form-
bildungsfaktoren ") under which the idioplasm operates. Some of
these conditions are purely mechanical, such as the shape of the
ovum, the distribution of deutoplasm, and the like. Others, and
probably the more important, are far more subtle, such as the distri-
bution of different chemical substances in the cytoplasm, and the
unknown polarities of the cytoplasmic molecules.

A nearly related conception was developed with admirable clear-
ness by Oscar Hertwig ('94) nearly at the same time. Both
Driesch and Hertwig thus retreated in a measure towards the
theory of germinal localization in the cytoplasm, which both had at
first rejected; but only to a middle ground which lies between the
two extremes of the strict predestination theory and the theory of
cytoplasmic isotropy. For these writers now maintain that the initial
cytoplasmic localization of the formative conditions is of hmited extent
and determines only the earlier steps of development. With each
forward step new conditions (chemical differentiations and the like)
are established which form the basis for the ensuing change, and so
on in ever-increasing complexity. This view is substantially the same
as that which I have myself urged in several earlier works, and I have
pointed out how it enables us to reconcile the apparent contradiction
between the partial development of isolated blastomeres of such
forms as the ctenophore, on the one hand, with the total development
of such forms as Amphioxus or the echinoderm, on the other. In the
latter case we may suppose the cytoplasmic differentiation to be but
feebly established at the beginning, and the blastomeres remain for a
time in a plastic state, which enables them on isolation to revert to
the condition of the original entire ovum. In the former case the
initial differentiation is more extensive or more rigidly fixed, so that

3ii

THEORIES OF INHERITANCE AND DEVELOPMENT

the development of the blastomere is from the begmning hemmed
in by the cytoplasmic conditions, and its powers are correspondingly
limited. In such cases the cleavage may exhibit more or less of
a mosaic-like character, and the theory of cytoplasmic localization
acquires a real meaning and value.

That we are here approaching the true explanation is indicated by

'siit''^^'/- ■-

Fig. 140. — Double embryos of frog developed from eggs inverted when in the 2-ceIl 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.

certain very remarkable and interesting experiments on the frog's
Q.gg 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 circumstances, and which indicate, furthermore, that these circum-
stances lie in a measure in the arrangement of the cytoplasmic
materials. This most important result, which we owe especially to

ON THE NATURE AND CAUSES OF DIFFERENTIATION 319

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 towards the lower side,
while the nucleus and protoplasm rise. A new axis is thus established
in the egg, which has the same relation to the body-axes as in the
ordinary development (though the pigment retains its original arrange-
ment). This proves that in eggs of this character (telolecithal) the
distribution of deutoplasm, or conversely of protoplasm, is one of the
primary formative conditions of the cytoplasm ; and the significant
fact is that by artificially ehanging this distribution the axis of the
embryo is shifted. Oscar Schultze ('94) discovered that if the Q.g^ be
turned upside down when in the two-cell stage, a whole embryo (or
half of a double embryo) might 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. 140).
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 blastomere. If, after destruction of one blastomere, the
other be allowed to remain in its normal position, a half-embryo always
results,^ precisely 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
equilibrium like that of an entire ovum.

This beautiful experiment gives most conclusive evidence that each
of the 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
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 both of Amphi-
0X71S and the "echinoderms. In Amphioxns the isolated blastomere
usually segments like an entire ovum of diminished size. This is,
however, not invariable, for a certain proportion of the blastomeres
show a more or less marked tendency to divide as if still forming part
of an entire embryo. The sea-urchin Toxopneustes reverses this
rule, for the isolated blastomere of the two-cell stage usually shows a
perfectly typical half-cleavage, as described by Driesch, but in rare

1 Anat. Anz., X. 19, 1895. ^ Three cases.

^ Eleven cases observed. * Nine cases observed.

320 THEORIES OF INHERITANCE AND DEVEIOPMENT

cases it may segment like an entire ovum of half -size (Fig. 132, D) and
give rise to an entire blastula.^ We may interpret this to mean that
in Aniphioxns the differentiation of the cytoplasmic substance is at
first very slight, 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 we have the opposite extreme to Aniphioxns, the cytoplasmic
conditions having been so firmly established that they cannot be
altered, 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 isotropy. Primarily the egg-cytoplasm is isotropic in
the sense that its various regions stand in no fixed and necessary rela-
tion with the parts to which they respectively give rise. Secondarily,
however, it may undergo differentiations through which it acquires a
definite regional predetermination which becomes ever more firmly
established as development advances. This process does not, how-
ever, begin at the same time, or proceed at the same rate in all eggs.
Hence the eggs of different animals may vary widely in this regard
at the time cleavage begins, and hence may differ as widely in their
power of response to changed conditions.

The origin of the cytoplasmic differentiations existing at the be-
ginning of cleavage has already been considered (p. 285). 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-
work within which the subsequent operations take place, in a more
or less fixed course, and which itself becomes ever more complex as
development goes forward. If the cytoplasmic conditions be artifi-
cially altered by isolation or other disturbance of the blastomeres, a
readjustment may take place and development may be correspond-
ingly altered. Whether such a readjustment 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.

1 I have observed this only twice. In both cases the cleavage up to the sixteen-cell stage
was exactly like that of the entire egg except that the micromeres were relatively larger, as
shown in the figure.

THE NUCLEUS IN LATER DEVELOPMENT 321

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
leaves wholly 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. There are, however, a multitude of well-known facts which
cannot be explained, even approximately, under this assumption.
The power of a single cell to produce the entire body is in general
limited 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 regenerate
one another save in a few exceptional cases. In asexual reproduction,
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.
How can we explain this progressive specification of the tissue-cells
and how interpret the differences in this regard between related
species } To these questions the Roux-Weismann theory gives a
definite and intelligible answer; namely, that differentiation sooner or
later iniwlves a, specification of the mtclear substance which differs in
degree in different cases. When we reflect on the general role of the
nucleus in metabolism and its significance as the especial seat of the
formative power, we may well hesitate 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 trans-
formation 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 re-
markable observations on the nuclei of the primordial germ-cells in

322 THEORIES OF INHERITANCE AND DEVELOPMENT

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 tJie genn-miclei retain the entire ancestral Jieritage. Boveri
himself has in a measure pointed out the significance of his discovery,
insisting that the specific development of the tissue-cells is condi-
tioned by specific changes in the chromatin that they receive,^ though
he is careful not to commit himself 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 zvithoat recourse
to tlie theory of qualitative division. We do not need to assume that
the unused chromatin is cast out bodily ; for it may 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 application 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 doAvn 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 the morphological formative power,
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,
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.

1 '91, p. 433.

THE EXTERNAL CONDITIONS OF DEVELOPMENT 323

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
established 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 cr3^stallization, begins with the youngest ovarian Q.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.

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 relation is ; and it has been espe-

324

THEORIES OF INHERITANCE AND DEVELOPMENT

cially 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 referred to the works especially of Herbst. I shall only
consider one or two cases which may serve to bring out the g"eneral
principle that they involve. Every living organism at every stage

of its existence reacts to its environ-
ment by physiological and morpho-
logical 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 of these animals
be made to develop in sea-water con-
taining no lime-salts, the larva fails to
develop not only its calcareous skele-
ton, but also its ciliated arms, and a
larva thus results that resembles in
some particulars an entirely different
specific form ; namely, the Tornaria
larva of Balanoglossns. This result
is not due simply to the lack of neces-
sary material ; for Herbst showed
that the same result is attained if a
slight excess of potassium chloride be
added to sea-water containing the nor-
mal amount of lime (Fig. 141). In
the latter case the specific metabolism
of the protoplasm is altered by a particular chemical stimulus, and a
new form results.

The changes thus caused by slight chemical alterations in the
water may be still more profound. Herbst ('92) observed, for ex-
ample, 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
larva, one half of which represents the archenteron, the other half
the ectoblast. Moreover, a much larger number of the blastula-cells

Fig. 141. — Normal and modified
larvae of sea-urchins. [HERBST.]

A. Normal Pluteus {Stro?7gylocentro-
tus). B. Larva {^Sphcer echinus) at the
same stage as the foregoing, developed
in sea-water containing a slight excess
of potassium chloride.

THE EXTERNAL CONDITIONS OF DEVELOPMENT 325

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
differentiated into cells having the histological character of the normal
entoblast ! One of the most fundamental of embryonic differentia-

Fig. 142. — Regeperafion in coelenterates {A. B. from LOEB ; C. D. from BiCKFORD).
A. Polyp {Ceriantlms) producing new tentacles from the aboral side of a lateral wound.
B. Hydroid ( Tubularia) 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.

tions 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 gave a similar result ('91). It
has long been known that Tiibula?'ia, like many other hydroids, has

326 THEORIES OF INHERITANCE AND DEVELOPMENT

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
Titbulm-ia 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 eacJi efid (Fig. 142) ; 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 complicated structure is
called forth by the stimulus of the external environment.

These cases must suffice for our purpose. They prove incontesta-
bly that normal develop'ment is in a greater or less deg7'ee 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
Qg^ 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 idioplasm 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
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 ;

PREFORMATION AND EPIGENESIS 327

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. This remains true however we
may conceive the morphological nature of the idioplasm — whether as
a microcosm of invisible germs or pangens, as conceived by De Vries,
Weismann, and Hertwig, as a storehouse of specific ferments as
Driesch suggests, or as 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, both by 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 Q.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. Both are necessary to development ; the nucleus alone
suffices for the inheritance of specific possibilities of development.

J. Preformation and Epigenesis. The Unknown Factor in

Development

We have now arrived at the furthest 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
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.

328 THEORIES OF INHERITANCE AND DEVELOPMENT

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 this 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 difihculty 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,
we but throw the problem one stage further back, and, as Weismann
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

1 Germ-plasm, p. 14.

2 Evolution, Science and Culture, p. 296.

■^ Germinal Selection, January, 1896, p. 284.

PREFORMATION AND E PI GENESIS 329

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 properties of the idioplasm involve 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 causality as this,
the embryologist may well be content to reserve his judgment on a
problem a hundredfold 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 pre-existing 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 on which, as far as I can see, the study of the cell has
not thus far thrown a ray of light. Whatever position we take on
this question, the same difficulty is encountered; namely, the origin
of that co-ordinated fitness, 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 ma)-^
be true, as Schwann himself urged, that the adaptive power of
living beings differs in degree only, not in kind, from that of unor-
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 ojoeration of natural causes. But

1 See Wolfi", '95, and Miiller, '96.

2 See Wolff, '94, for an admirably clear and forcible discussion of this case.

330 THEORIES OE INHERITANCE AND DEVEIOPMENT

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 play of physical forces upon it 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 limit 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 repress the hope
that step by step the way may yet be opened to an understanding of
inheritance and development.

LITERATURE. IX

Boveri, Th. — Ein geschlechtlich erzeugter Organismus ohne miitterliche Eigen-

schaften : Sitz.-Ber. d. Ges. f. Morph. unci Phys. in Milncheji, V. 1889. See

also Arch. f. Entw/n. 1895.
Brooks, W. K. — The Law of Heredity. Baltimore, 1883.

Driesch, H. — Analytische Theorie der organischen Entwicklung. Leipzig, 1894.
Herbst, C. — Uber die Bedeutung der Reizphysiologie flir die kausale Auffassung

von Vorgangen in der tierischen Ontogenese : Biol. Centralb., XIV., XV.

1894-95.
Hertwig, 0. — Altere und neuere Entwicklungs-tljeorieen. Berlin, 1892.
Id. — Urmund und Spina Bifida: Arch. Jtiik. Anat., XXXIX. 1892.
Id. — Uber den Werth der ersten Furchungszellen fUr die Organbildung des

Embryo : Arch. niik. Anat., XLII. 1893.
Id. — Zeit und Streitfragen der Biologie. Berlin, 1894.
His, W. — Unsere Korperform und das physiologische Problem ihrer Entstehung.

Leipzig, 1874.
Loeb, J. — Untersuchungen zur physiologischen Morphologie : I. Heteromorphosis.

Wilrzbtirg, 1891. II. Organbildung und Wachsthum. IVilrzbnrg, 1892.
Id. — Some Facts and Principles of Physiological Morphology: Wood^s Holl Biol.

Lectures. 1 893 .
Nageli, C. — Mechanisch-physiologische Theorie der Abstammungslehre. M'nn-

chen 71. Leipzig, 1884.
Roux, W. — Uber die Bedeutung der Kernteilungsfiguren. . Leipzig, 1883.

PREFORMATION AND EPIGENESIS 33 I

Roux, W. — iJber das kiinstliche Hervorbringen halber Embryonen durch Zerstorung
eiiier der beiden ersten Furchungskugeln, etc. : Virchow's Archiv, 114. 1888.

Sachs, J. — Stoffund Form der Fflanzenorgane : Ges. Abhandlungen, II. 1893.

Weismann, A. — Essays upon Heredity, First Series. Oxford, 1891.

Id. — Essays upon Heredity, Second Series. Oxford, 1892.

Sd. — Aussere Einfliisse als Entwicklungsreize. Jena, 1894.

Whitman, C. 0. — Evolution and Epigenesis : lVood''s H oil Biol. Lectures. 1894.

Wilson, Edm. B. — On Cleavage and Mosaic-work: Arch, fur Entwicklungsm.,
III. I. i8g6.

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'matiii (see Chromatin), the non-staining substance of the nucleus, as

opposed to chromatin ; comprising the ground-substance and the Hnin-network.

(Flemming, 1880.)
[Akaryo'ta] (see Karyota), non-nucleated cells. (Flemming, 1882.)
Ale'cithal (d-priv. ; Ae'/ct^os, the yolk of an egg), having little or no yolk (applied

to eggs). (Balfour, 1880.)
Aniito'sis (see Mitosis), direct or amitotic nuclear division; mass-division of

the nuclear substance without the formation of chromosomes and amphiaster.

(Flemming, 1882.)
Ani'phiaster (dfJi4>h on both sides; da-rrjp, a star), the achromatic figure formed

in mitotic cell-division, consisting of two asters connected bv a spindle. (Fol,

1877.)

Aniphipy 'renin (see Pyrenin), the substance of the nuclear membrane.
(SCHWARZ, 1887.)

Aniy'loplasts (a/xuAov, starch; TrAao-ro?, irXdaaeti', form), the colourless starch-
forming plastids of plant-cells. (Errara, 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). (PflIjger, 1883.)

Antherozo'id, the same as Sperniatozoid.

Anti'podal cone, the cone of astral rays opposite to the spindle-fibres. (Van
Beneden, 1883.)

Archiam'phiaster (dpxi- = first, -|- amphiaster), the amphiaster by which the first
or second polar body is formed. (Whitman, 1878.)

Ar'clioplasnia or Archoplasm. (ap^wv, a ruler), the substance from which the
attraction-sphere, the astral rays and the spindle-fibres are developed, and of
which they consist. (Boveri, 1888.)

As'ter (dcTTT/p, a star), i. The star-shaped structure surrounding the centrosome.
(Fol, 1877.) [2. The star-shaped group of chromosomes during mitosis (see
Kary aster). (Flemming, 1892.)]

[As'troccEle] (do-xT/p, a star ; koIAos, hollow), a term somewhat vaguely applied to
the space in which the centrosome lies. (Fol, 1891.)

As'trosphere (see Centrospliere) . 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.)

333

334 GLOSSARY

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] (aiuTos, self), applied by Altmann to bacteria and other minute organ-
isms, conceived as independent solitary " bioblasts." (i8go.)

Axial filament, the central filament, probably contractile, of the spermatozoon-
fiagellum. (Eimer, 1874.)

Basichro 'matin (see Chromatin), the same as chromatin in the usual sense.
That portion of the nuclear network stained by basic aniline dyes. (Heidenhain,

1894-)

Bi'oblast (jSto^, life ; ^SAao-ros, a germ), the hypothetical ultimate supra-molecular
vital unit. Equivalent to //«i'(?///i?, etc. First used by Beale. Afterwards identi-
fied by Altmann as the "granulum."

Bi'ogen (^tos, life: -yev^s, producing), equivalent to p/asoz/w, etc. (Verwokn,
1895.)

Bi'ophores (/?tos, life; -<^opos, bearing), the ultimate supra-molecular vital units.
Equivalent to the pangens of De Vries, the plasomes of Wiesner, etc. (Weismann,

1893-)
Bivalent, applied to chromatin-rods representing two chromosomes joined end to

end. (Hacker, 1892.)
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.)
Centriole, a term applied by Boveri to a minute body or boches (" Central-korn ")

within the centrosome. In some cases not to be distinguished from the centro-
some. (Boveri, 1895.)
Centrodes'mus (K€VTf)ov, centre ; Secr|aos, a band), the primary connection between

the centrosomes, forming the beginning of the central spindle. (Heidenhain,

1894.)
Centrole'cithal (KeVrpov, centre ; AeKt^os, yolk), that type of ovum in which the

deutoplasm is mainly accumulated in the centre. (Balfour, 1880.)
Cen'trosome {Kevrpov, centre ; au)[jLa, body), a cell-organ generally regarded as the

active centi-e of cell-division and in this sense as the dynamic centre of the cell.

Ulider 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 (xXmpo?, green ; TrAacrros, form), the green plastids or chlorophyll-
bodies of plant ancl animal cells. (Schimper, 1883.)
Chro'matin (xpSy/xa, colour), the deeply staining substance of the nuclear network

and of the chromosomes, consisting of nuclein or nucleic acid. (Flemming,

1880.)
Chro'matophore (xp^^p-a. colour; -<^opos, bearing), a general term applied to the

colored plastids of plant and animal cells, including chloroplastids and chromo-

plastids. (Schaarschmidt, 1880; Schmitz, 1882.)
Chro'matoplasm (';)(pw/via, colour; TrXdapa, anything formed or moulded), the sub-
stance of the chromatoplasts and other plastids. (Stkasi5URGER, 1882.)

GLOSSARY 335

Chro'moniere (;)(pw/xa, colour; fxepo?, a part), the individual chromatin-granules of
which the chromosomes are made up. Identified by VVeismann as the " id."'
(FoL, 1 891.)

Chronioplas'tids {^^poj/xa, colour ; TrAacrTO?, form), the coloured plastids or pigment-
bodies other than the chloroplasts, in plant-cells. (Schimper, 1883.)

Chro'nio.sonies {-^puifxa, colour ; crtu/xa, 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'philons (kwos, blue; <^iXetv, to love), having an especial affinitv for blue
or green dyes. (Auerbach.)

Cy'taster (kvto<;, hollow (a cell); a<TTi]p, star), the same as Aster, i. See Kary-
aster. (Flemming, 1882.)

[Cy'toblast] (kvtos, hollow (a cell) ; ySAacrros, 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 (kuVos, hollow (a cell) ; x^^Ads, juice), the ground-substance of the
cytoplasm as opposed to that of the nucleus. (Strasburger, 1882.)

Cy'tode (kwto?, hollow (a cell) ; etSo?, form), a non-nucleated cell. (Hackel, 1866.)

Cytodie'resis (kuVos, hollow (a cell) ; Soatpecri?, division), the same as Mitosis.
(Carnoy, 1885.)

Cytohy'aloplasma (kvto<;, hollow (a cell) ; {I'aXos, glass ; TrXaa-jxa, anything formed),
the substance of the cytoreticulum in which are embedded the microsomes ;
opposed to nucleohyaloplasma. (Strasburger, 1882.)

Cy'tolymph (kwtos, hollow (a cell) ; lyvipha, clear water), the cytoplasmic ground-
substance. (Hackel, 1891.)

Cytomi'crosomes (see Microsome), microsomes of the cytoplasm; opposed
to nucleomicrosomes. (Strasburger, 1882.)

Cytomi'tome (kwVos, hollow (a cell) ; lUiTW/ua, from /xtVo?, thread), the cytoplasmic
as opposed to the nuclear thread-work. (Flemming, 1882.)

Cytoretic'ulum, the same as Cytomitome. (Strasburger, 1882.)

Cy'tosome (kwtos, hollow (a cell) ; crai/x,a, bod}',) the cell-body or cytoplasmic
mass as opposed to the nucleus. (Hackel, 1891.)

Der'matoplasm (8ep/xa, skin), the living protoplasm asserted to form a part of the
cell-membrane in plants. (Wiesner, 1886.)

Der'matosomes (Se'p/xa, skin ; crCypLa, 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] (S£VT(epo<i), second ; see Hyalosome), the nucleus remaining
in the egg after formation of the first polar body. (Van Beneden, 1883.)

Deu'toplasm (SevT{epo?), second ; TrAacr/xa, anything formed), yolk, lifeless food-
matters deposited in the cytoplasm of the egg; opposed to "protoplasm." (Van
Beneden, 1870.)

Directive bodies, the polar bodies. (Fr. Muller, 1848.)

Directive sphere, the attraction-sphere. (Guignard, 1891.)

Dispermy, the entrance of two spermatozoa into the egg.

336 GLOSSARY

Dispi'l-eme (see Spireme), that stage of mitosis in which each daughter-nucleus

has given rise to a spireme. (Flemming, 1882.)
Dy 'aster (Sua?, two; see Aster, 2), the double group of chromosomes during the

anaphases of cell-division. (Flemming, 1882.)
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

Bexeden. (O. Hertwig, 1875.)
Enchyle'ma (eV, in; x^^^o?- juice), i- The more fluid portion of protoplasm,

consisting of "hyaloplasma." (Hanstein, 1882.) 2. The ground-substance

(cytolymph) of cytoplasm as opposed to the reticulum. (Carnoy, 1883.)
Bner'gid, the cell-nucleus together with the cytoplasm lying within its sphere of

influence. (Sachs, 1892.)
Equatorial plate, the group of chromosomes lying at the equator of the spmdle

during mitosis. (Van Beneden, 1875.)
Erythro'philous (ipv0p6<;, red; c^tAeTv, to love), having an especial affinity for

red dyes. (Auerbach.)
Ga'mete (yafxiry, wife ; ya/xerTjs, 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 ; (SXaaros, germ), a term applied by Minot to the mature

o-erm-cells. The female genoblast (egg, or " thelyblast ") unites with the male

(spermatozoon or " arsenoblast ") to form an hermaphrodite or indifferent cell.

(Minot, 1877.)
G-erminal 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.)
Heterole'cithal (erepos, different; Ae/ct^os, yolk), having unequally distributed

deutoplasm (includes telolecithal and centrolecithal). (Mark, 1892.)
Heterotyp'ical mitosis (erejoos, 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] (0A.0;, whole; crxt'^etr, to split), direct nuclear division. Amitosis.

(Flemming, 1882.)
Homole'cithal (6/xos, the same, uniform; \€klOo<;, yolk), equivalent to alecithal.

Having little deutoplasm, equally distributed, or none. (Mark, 1892.)
Homoeotyp'ical mitosis (o/jlolo?, like; see Mitosis), a form of mitosis occurring

in the spermatocytes of the salamandei^, differing from the usual type only in the

shortness of the chromosomes and the irregular arrangement of the daughter-
chromosomes. (Flemming, 1887.)
Hy'aloplasma (ilaA-os, glass; TrAacr/xa, anything formed), i. The ground-sub-
stance of the cell as distinguished from the granules or microsomes. [Hanstein,

1880.] 2. The ground-substance as distinguished from the reticulum or "spon-

o-ioplasm." (Leydig, 1885.) 3. The exoplasm or peripheral protoplasmic zone

in plant-cells. (Pfeffer.)
Hy'alosomes (uaXos, glass ; o-w/xa, body), nucleolar-like bodies but slightly stained

by either nuclear or plasma stains. (Lukjanow, 1888.)
[Hy'groplasma] (iiypds, wet ; TrXdcrfia, something formed), the more liquid part

of protoplasm as opposed to the firmer stereoplasm. (Nageli, 1884.)
Id, the hypothetical structural unit resulting from the successive aggregation of

biophores and determinants. Identified by Weismann as the chromomere, or

chromatin-granule. (Weismann, 1891.)

GLOSSARY 337

Idant, the hypothetical unit resulting from the successive aggregation of biophores,

determinants, and ids. Identified by Weismann as the chromosome. (Weis-

MANN, 1 891.)
Id'ioblasts (I'Stos, one's own, ySAao-ro's, germ), the liypothetical uhimate units of the

cell; the same as biophores. (O. Hertwig, 1893.)
Id'ioplasm (tStos, one's own; 7rAao-//,a, a thing formed), equivalent to the germ-
plasm of Weismann. The substance, iiow generally identified with chromatin,

which by its inherent organization involves the characteristics of the species.

The physical basis of inheritance. (Nageli, 1884.)
Id'iosome ((,810;, one's own; crw^ua, body), the same as idioblast or plasome.

(Whitman, 1893.)
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 (to-os, equal; Tpoirrj, a turning), the absence of predetermined axes (as

applied to the egg). (Pflijger, 1883.)
[Ka'ryaster] {Kapvov, nut, nucleus ; see Aster, 2), the star-shaped group of chromo-
somes in mitosis. Opposed to cytaster. (Flemming, 1882.)
Karyenchy'nia (Kapvov, nut, nucleus; ev, in; x^M^s* juice), the ^'nuclear sap."

(Flemming, 1882.)
Karyokine'sis (Kapvov, nut, nucleus ; KLvrjcra, 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; Atjctis, 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'crosome (see Microsome), the same as nucleo-microsome.
Ka'ryomite, the same as chromosome [? Schiefferdecker].
Karyomi'tome (Kapvov, nut, nucleus; /iiVw/xa, from p.iro'i, 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 (Ka,ovoy, nut, nucleus ; TrAao-jaa, a thing formed) , nucleoplasm. The

nuclear as opposed to the cytoplasmic substance. (Flemming. 1882.)
Ka'ryosome (Kapvov, nut, nucleus; croj/xa, 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; OtJkt), case, box), the nuclear membrane.

(Hackel, 1891.)
Ki'noplasm (kiveiv, to move; ■rrXda-p.a, a thing formed), equivalent to archoplasm ;

opposed by Strasburger to the " trophoplasm " or nutritive plasm. (Stras-

BURGER, 1892.)
[Lanthanin] (XavOavetv, to conceal), equivalent to oxychromatin. (Heiden-

hain, 1892.)

338

GLOSSARY

Leucoplas'tids (XevKo's, white ; TrXaaros, 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
reticulum. (ScHWARZ, 1887.)

Maturation, the final stages in the development of the germ-cells. More spe-
cifically, the processes by which the reduction of the number of chromosomes
is effected.

Metakine'sis (see Metaphase) {fxtra, beyond {i.e. further) ; KLVvrjais, movement),
the middle stage of mitosis, when the chromosomes are grouped in the equa-
torial plate. (Flemming, 1882.)

Metanu'cleus, a term applied to the egg-nucleus after its extrusion from the
germinal 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 daera, after, beyond; 7rAao-/xa, a thing formed), a term collectively
applied to the lifeless inclusions (deutoplasm, starch, etc.) in protoplasm as op-
posed to the living substance. (Hanstein, 1880.)

Micel'la, one of the ultimate supra-molecular units of the cell. (Nageli, 1884.)

Microcen'trum, the dynamic centre of the cell, consisting of one or more centro-
somes. (Heidenhain, 1894.)

Mi'cropyle (^atKpos, small; irvXrj, 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, t. Robert
Brown.]

Mi'crosome (/xtKpo's, small ; o-w/xa, body), the granules as opposed to the ground-
substance of protoplasm. (Hanstein, 1880.)

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 (/xtrw/xa, from /xtVos, a thread), the reticulum or thread-work as opposed
to the ground-substance of protoplasm. (Flemming, 1882.)

[Mitoschi'sis] (|U.tros, thread ; axt^^-i-v, to split), indirect nuclear division ; mito-
sis. (Flemming, 1882.)

Mito'sis (/Atros, 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 {jXiTO';, a thread ; o-w/xa, body), a body derived from the spindle-fibres
of the secondary spermatocytes, giving rise, according to Platner, to the mid-
dle-piece and the tail-envelope of the spermatozoon. Equivalent to the Neben-
kern of La Valette St. George. (Platner, 1889.)

Nebenkern (Paranucleus), a name originally applied by Blitschli (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.

Nucleic acid, a complex organic acid, rich in phosphorus, and an essential
constituent of chromatin.

GLOSSARY

339

Nuclein, the chemical basis of chromatin ; a compound of nucleic acid and
albumin. (Miescher, 1874.)

Nucleo-albumin, a nuclein having a relatively high percentage of albumin.
Distinguished from nucleo-proteids by containing paranucleic acid which yields
no xanthin-bodies.

Nucleochyl'ema (;!^i;Ads, juice), the ground-substance of the nucleus as opposed
to that of the cytoplasm. (Strasburger, 1882.)

Nucleohy'aloplasma (see Hyaloplasm), the achromatic substance (linin) in
which the chro matin-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 {&gg-) nucleus. (Van
Beneden, 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 nucleic acid, the latter yielding xanthin-bodies.

CEdematin (^olSrjixa, a swelling), the granules or microsomes of the nuclear ground-
substance. (Reinke, 1893.)

O'ocyte (Ovocyte), (wov, egg; kvto?, 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 egg. (Boveri, 1891.)

Oogen'esis, Ovogenesis (ww, egg? yeVecrts, 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.

Oogo'nium, Ovogonium (cJoj/, egg ; yovy, generation), i. The primordial mother-
cell from which arises the egg and its follicle. (Pfluger.) 2. The descendants
of the primordial germ-cell which ultimately give rise to the oocytes or ovarian
eggs. (Boveri, 1891.)

Ookine'sis (wov, egg; Kivrjcn?, 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^v?, acid ; see Chromatin), that portion of the nuclear substance
stained by acid aniline dyes. Equivalent to "linin" in the usual sense.
(Heidenhain, 1894.)

Pangeu'esis (ttSs (Trai/-), all; yeVecrts, production), the theory of gemmules, ac-
cording to which hereditary traits are carried by invisible germs thrown off by
the individual cells of the body. (Darwin, 1868.)

Pangens (ttSs (ttuv-), all; -yevrj?, producing), the hypothetical ultimate supra-
molecular units of the idioplasm, and of the cell generally. Equivalent to
gemmules, micellae, idioblasts, biophores, etc. (De Vries, 1889.)

Panmeri'stic (irav. all; /Aepos, part), relating to an ultimate protoplasmic structure
consisting of independent units. See Pangen.

Parachro'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 pro-
toplasm, 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.

340 GLOSSARY ■

Par'aplasm (irapa, beside; -rrXaa-fxa, something formed), tlie less active portion of
the cell-sulDstance. Originally applied by Kupfter to the cortical region of the
cell (exoplasm), but now often applied to the ground-substance. (Kupffer,

1875.)

Periplast (jrtpi, around; TrAao-ros, form), a term somewhat vaguely applied to the
attraction-sphere. The term daughter-periplast is applied to the centrosome.
(Vejdovsky, 1888.)

Plas'mosome {irXda-jxa, something formed (i.e. protoplasmic) ; aw/Aa, body), the
true nucleolus, distinguished by its affinity for acid anilines and other " plasma-
stains." (Ogata, 1883.)

Pla'some (TrAao-jna, a thing formed; crw/xa, body), the ultimate supra-molecular
vital unit. See Biophore, Pangen. (Wiesner, 1890.)

Plas'tid (TrAacrrds, form). I. A cell, whether nucleated or non-nucleated. (Hackel,
1866.) 2. A general term applied to permanent cell-organs (chioroplasts, etc.)
other than the nucleus and centrosome. (Schiimper, 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-netwoik (Zacharias) and the
cytoreticulum (Carnoy). (Reinke and Rodewald, 1881.)

Pluriva'lent (phes, more ; valere, to be worth), applied to chromatin-rods that
have the value of more than one chromosome sejisn 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 ravs 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'niatin (see Chromatin), the substance of true nucleoli, or plasmosomes.
Equivalent to paranuclein of O. Hertwig. (Pfitzner, 1883.)

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 egg. (Van Beneden,

1875.)
[Prothy'alosome] (see Hyalosome). an area in the germinal vesicle {oi A scans)

by which the germinal spot is surrounded, and which is concerned in formation

of the first polar body. (Van Beneden, 1883.)
Pro'toblast (7rpa)ro9, first ; ^Aaards, a germ), a naked cell, devoid of a membrane.

(KOLLIKER.)

Pro'toplasm (Trpwros, first; TrXdcrfxa, a thing formed or moulded), i. The living
substance of the cell, comprising cytoplasm and karyoplasm. (Purkyne, 1840;
H. VON MOHL, 1846.) 2. The cytoplasm as opposed to the karyoplasm.

Pro'toplast (TTpwTos, first; TrAao-rds, formed), i. The protoplasmic body of the
cell, including nucleus and cytoplasm, regarded as a unit. Nearly equivalent to
the energid of Sachs. (Hanstein, 1880.) 2. Used by some authors synony-
mously with plastid.

[Pseudochro'matin] (see Chromatin), the same as prochromatin. (Pfitzner,
1886.)

GLOSSARY 341

Pseiidonu'clein (see Nuclein), the same as the paranuclein of Kossel. (Ham-

MARSTEN, 1894.)

Psetido-reduction, the prehminary 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 ; i.e. relating to the nucleus), the substance
of true nucleoli. Equivalent to the paranuclein of Hertwig. (Schwarz, 1887.)

Pyre'noid (irvprjv, 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.

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'niatid (cnrepixa, seed), the final cells which are converted without further
division into spermatozoa ; they arise by division of the secondary spermatocytes
or " Samenmiitterzellen." (La Valette St. George, 1886.)

Sper'matoblasts (crTrep/xa, seed; ySAacrTos, 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-7rep|ua, seed ; kwtis, 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 {cnrepfxa, seed; kijtos, 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 seconda?y sper^~
f/iatocytes, each of which gives rise to two spermatids (ultimately spermatozoa).
(La Valette St. George, 1876.)

[Sperniatogeni'ma] (o-Trepfia, seed; gemma, bud), nearly equivalent to spermato-
cyst. DiiTers in the- absence of a surrounding membrane. [In mammals,
La Valette St. George, 1878.]

Spermatogen'esis (awepfjia, seed ; yevecrts, origin), the phenomena involved in
the formation of the spermatozoon. Often used more specifically to denote the
process of reduction in the male.

Sperniatogo'niuni ("• Ursamenzelle " ) (airepfxa, seed ; yovr'j, 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 ((nripfjia, seed; ixepo?, a part), the chromatin-granules into
which the sperm-nucleus resolves itself after entrance of the spermatozoon. (In
Petromyzon, Bohm, 1887.)

Sper'niatosome {(jTrippa., seed ; awjxa, body), the same as spermatozoon. (La
Valette St. George, 1878.)

Spermatozo'id (see Spermatozoon), the ciliated paternal germ-cell in plants.
The word was first used by von Siebold as synonymous with spermatozoon.

Spermatozo'on (cnripfxa, seed; ^(J^ov, 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, i8gi.)

342 GLOSSARY

Spi'reme ((nreiprjixa, 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 {awoyyLOv, a sponge ; TrXdafxa, a thing formed), the cytoreticulum.

(Leydig, 1885.)
Ste'reoplasm (arepeo?, 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.)
Te'loblast (reAos, end; /^Aao-rd?, a germ), large cells situated at the growing end

of the embryo (in annelids, etc.), which bud forth rows of smaller cells.

(Whitman, Wilson, 1887.)
Telole'cithal (re'Ao?, end ; AeKi^os, yolk), that type of ovum in which the yolk is

mainly accumulated in one hemisphere. (Balfour, 1880.)
Telophases, Telokine'sis (reXos, end), the closing phases of mitosis, during

which the daughter-nuclei are re-formed. (Heidenhain, 1894.)
To'noplasts (two?, tension ; TrAao-rds, form), plastids from which arise the vacuoles

in plant-cells. (De Vries, 1885.)
Trophoplasm (jpo^ir), nourishment; 7rAacr/xa). 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

archoplasm. (Strasburger, 1892.)
Tro'phoplasts (rpocfn]. nourishment; TrAao-rds, form), a general term, nearly equiv-
alent to the "plastids" of Schimper, including " anaplasts " (amyloplasts),

"autoplasts" (chloroplasts), and chromoplasts. (A. Meyer, 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 (Cvyov, a yoke), the cell produced by the fusion of two

conjugciting 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 Untersuchungeit
('5o-'55), Huxley on the Cell-theory ('53), and Tyson's Cell-doctrine ('78). An
exhaustive review of the earlier literature on protoplasm, nucleus, and cell-
division will be found in Flemming's Zellsiibstanz ('82), and a later review of
theories of protoplasmic structure in Butschli's Protoplasuia ('92). The earlier
work on mitosis and fertilization is very thoroughly reviewed in Whitman's Clep-
sine ('78), FoPs Henogenie ('79), and Mark's Liinax ('81). For more recent
general literature-lists see especially Hertwig's Zelle 2iiid Gewebe ('93), Yves
Delage ('95), Henneguy's Cellule ('96), and the admirable reviews by Flemming,
Boveri, Ruckert, Roux, and others in iMerkel and Bonnet's Ergebnisse ('91 -'94).

The titles are arranged in alphabetical order, according to the system adopted in
Minot's Hujitan 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 Physiologic.

A. m. A. Archiv fiir mikroscopische Anatomic.

A. Entm. Archiv fiir Entvvicklungsmechanik.

B. C. Biologisches Centralblatt.

C. R. Comptes Rendus.

y. M. Journal of Morphology.

J. Z. Jenaische Zeitschrift.

M. A. Miiller's Archiv.

M.J. Morphologisches Jahrbuch.

Q. J. Quarterly Journal of Microscopical Science.

Z. A. Zoologischer Anzeiger.

Z. w. Z. Zeitschrift fiir vvissenschaftliche Zoologie.

ACQUA, '91. Contribuzione alia conoscenza della cellula vegetale : Afalplghia,
V. — Altman, R., '86. Studien liber 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 den Zellen : Leipzig. —
Amelung, E., '93. tJber mittlere Zellgrosse : Flora, p. 176. — Arnold, J., '79.

343

344 GENERAL LITERATURE-LIST

tiber feinere Struktur der Zellen, etc. : Vircho'w''s Arch., 1879. (See earlier papers.)

— Auerbach, L., '74. Organologische Studien : Breslau. — 16.., '91. (Jber einen
sexuellen Gegensatz in der Chromatophilie der Keimsubstanzen : Sitzimgsber . der
KojiigL preuss. Akad. d. IViss. Berlin, XXXV.

VON BAER, C. E., '28, '37. tjber Entwickelungsgeschichte der Thlere. Beo-
bachtung und Reflexion: I. Konigsberg, 1828; II. 1837. — Id., '34. Die Metamor-
phose des Eies der Batrachier: Milller's Arch. — Balbiani, E. G., '64. Sur la
constitution du germe dans I'oeuf animal avant la fecondation : C. R., LVIII. —
Id., '76. Sur les phenomenes de la division du noyau cellulaire : C. R , XXX., Octo-
ber, 1876. — Id., '81. Sur la structvu'e du noyau des cellules salivares chez les larves
de Chironomus : Z. A., 1881, Nos. 99, 100. — Id., '89. Recherches experimen-
tales sur la merotomie des Infusoires cilies : Reciieil Zool. Suisse, January, 1889. —
Id., '91, 1. Sur les regenerations successives du peristome chez les Stentors et sur
le role du noyau dans ce phenomene : Z. A., 372, 373. — Id., '91, 2. Sur la struc-
ture et division du noyau chez les Spirochona gemmipara : Ann. d. Micrographie.
Id., '93. Centrosome et Dotterkern : Joitrn. de Panat. et de la physioL, XXIX. —

— Balfour, F. M., '80. Comparative Embryology : I. 1880. — Ballowitz, '88-'91.
Untersuchungen liber die Struktur der Spermatozoen : i. (birds) A. m. A., XXXII.,
1888; 2. (insects) Z. iv. Z., LX., 1890; 3. (fishes, amphibia, reptiles) A. ni. A.
XXXVI., 1890; 4. (mammals) Z. w. Z., 1891. — Id., '89. Fibrillare Struktur und
Contractilitat : Arch. ges. Phys., XLVI. — Id., '91, 2. Die innere Zusammensetz-
ung des Spermatozoenkopfes der Saugetiere : Centralb. f. Phys., V. — Id., '95.
Die Doppelspermatozoa der Dytisciden : Z. w. Z., XLV., 3. — Van Bambeke, C,
'93. Elimination d'elements nucleaires dans I'oeuf ovarien de Scorpasna scrofa :
A. B., XIII. I. — De Bary, '58. Die Conjugaten. — Id., '62. tJber den Bau
und das Wesen der Zelle : Flora, 1862. — Id., '64. Die Mycetozoa : 2d Ed., Leip-
zig.— 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. — Bechanip and Ester, '82. De la constitution elemen-
taire des tissues : Mo7itpellier. — Belajeff, '94, 1. Zur Kenntniss der Karyokinese
bei den Pflanzen : Flora, 1894, Erganzungsheft. — Id., '94, 2. tJber Bau und
Entwicklung der Spermatozoiden der Pflanzen: Flora, LIV. — Benda, C, '87.
Untersuchungen iiber den Bau des funktionirenden Samenkenkalchens einiger Sau-
gethiere : A. in. A. — Id., '93. Zellstrukturen und Zelltheilungen des Salaman-
derhodens : Verh. d. Anat. Ges., 1893. — Van Beneden, E., '70. Recherches
sur la composition et la signification de I'ceuf : Mem. cour. de VAc. roy. d. S. de
Belgique, 1870. — Id., '75. La maturation de I'oeuf, la fecondation et les premieres
phases du developjDement embryonnaire des mammiferes d'apres des recherches
faites chez le lapin : Bull. Ac. roy. de Belgique, XI. — Id., '76, 1. Recherches
sur les Dicyemides : Bull. Acad. Roy. Belgique, XLI., XLII. — Id., '76, 2.
Contribution a I'histoire de la vesicule germinative et du premier noyau embryon-
naire: Ibid., XLI.; also Q.J-, XVI. — Id., '83. Recherches sur la maturation 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 : Bull. Acad. Roy. Belgique, 3me ser., VII. — Van Beneden, E.,
et Neyt, A., '87. Nouvelles recherches sur la fecondation et la division mitosique
chez I'Ascaride me'galoce'phale : Ibid., 1887. — Bergh, R. S., '94. Vorlesungen liber
die Zelle und die einfachen Gewebe : Wiesbaden. — Id., '95. Uber die relativen
Theilungspotenzen einiger Embryonalzellen : //. Entni., II., 2. — Bernard, Claude.
Lemons sur les Phenomenes de la Vie: ist Ed. 1878, 2d Ed. 1885, Paris. — Ber-
thold, G., '86. Studien liber Protoplasma-mechanik : Leipzig. — Bickford, E. E.,

GENERAL LITERATURE-LIST 345

'94. Notes on Regeneration and Heteromorphosis of Tubularian Hydroids : J. M.,
IX., 3. — Biondi, D., '85. Die Entwicklnng der Spermatozoiden : A. m. A., XXV.

— Blanc, H., '93. Etude sur la fecondation de Poeuf de la truite : Ber. Natur-
forsch. Ges. su Freiburg, VIII. — Blochmann, F., '87, 2. Uber die Richtungs-
korper bei Insekteneiern : Af. J., XII. — Id., '88. tJber die Richtungskorper bei
unbefruclitet sich entwickelnden Insekteneiern: Veili. natiirh. ined. Ver. Heidel-
berg, N. F., IV., 2. — Id., "89. iJber die Zalil der Riclitungskorper bei befruchteten
und unbefruchteten Bieneneiern : M. J. — Id., '94. (Jber die Kerntheiking bei
Euglena: B. C, XIV. — Bolim, A., '88. tJber Reifung und Befruclitung des Eies
von Petromyzon Planeri : A. ni. A., XXXII. — Id., '91. Die Befruclitung des
Forelleneies : Sitz.-Ber. d. Ges.f. Morph. ?/. Phys. Munchen, VII. — Boll, Fr., '76.
Das Princip des Waclistliums : Berlin. — Bonnet, C, 1762. Considerations sur
les Corps organises: Ainsterdani. — Born, G-., '85. Uber den Einfluss der
Scliwere auf das Froscliei : A. in. A., XXIV. — Id.. '94. Die Structur des Keim-
blaschens im Ovarialei von Triton tasniatus : A. ni. 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. Uber die Bedeutung der Richtungskorper: Sitz.-Ber. Ges.
Morph. u. Phys. Miinchen, II. — Id., '87, 1. Zellenstudien, Heft I. : J. Z., XXI. —
Id., '87, 2. Uber die Befruchtung der Eier von Ascaris jnegalocephala : Sitz.-Ber.
Ges. Morph. Phys. Munchen, III. — Id., '87, 2. Uber den Anteil des Spermatozoon
an der Teilung des Eies: Sitz.-Ber. Ges. Morph. Phys. M'unchen, III., 3. — Id.,
'87, 3. tJber Differenzierung der Zellkerne wahrend der Furchung des Eies von,
Ascaris meg.: A. A., 1887. — Id., '88, 1. Uber partielle Befruchtung: Sitz.-Ber.
Ges. Morph. Phys. Munchen, IV., 2.— Id., '88, 2. Zellenstudien, II.: J. Z.,
XXII. — Id., '89. Ein geschleehtlich erzeugter Organismus ohne miitterliche
Eigenschaften : Sitz.-Ber. Ges. Morph. Phys. Munchen, V. Trans, in Am. Nat.,
March, '93.— Id., '90. Zellenstudien, Heft III.: J. Z., XXIV. —Id., '91. Be-
fruchtung: Merkel und Bonnet\^ Ergebnisse, I. — Id.. '95, 1. Uber die Befruch-
tungs-und Entwickelungsfaihigkeit kernloser Seeigel-Eier, etc.: A. Entni., II., 3. —
Id., '95, 2. Uber das Verhalten der Centrosomen bei der Befruchtung des Seeige-
leies, nebst allgemeinen Bemerkungen iiber Centrosomen und Verwandtes : Verh. d.
Physikal.-nied. Gesellschaft zu Wiirzburg, N. F., XXIX., i.— Braem, F., '93.
Des Prinzip der organbildenden Keimbezirke und die entwicklungsmechanischen
Studien von H. Driesch : B. €., XIII., 4. 5. — Brandt, H., '77. Uber Actino-
sphaerium Eichhornii : Dissertation, Halle, \Zii . — Brass, A., '83-4. Die Organi-
sation der thierischen Zelle : Halle. — Brauer, A., '92. Das Ei von Branchipus
Grubii von der Bildung bis zur Ablage : Abh.preuss. Akad. Wi.ss., '92. — Id., '93,1.
Zur Kenntniss der Reifung des parthenogenetisch sich entwickelnden Eies von
Artemia Salina : A. ni. A., XLIII. — Id., '93, 2. Zur Kenntniss der Spermato-
genese von Ascaris megalocephala : A. jn. A., XLII. — Id., '94. Uber die En-
cystierung von Actinosphaerium Eichhornii: Z. w. Z., LVIIL, 2. — Braus, '95.
iiber Zellteilung und Wachstum des Tritoneies: J. Z., XXIX. — Brooks, W. K.,
'83. The Law of Heredity: Baltimore. — Brown, H. H., '85. On Spermato-
genesis in the Rat: Q. J., XXV. — Brown, Robert, '33. Observations on the
Organs and Mode of Fecundation in Orchideae and Asclepiadese : Trans. Linn.
Soc, 1833.— Brunn, M- "^o"? '89. Beitrage zur Kenntniss der Saraenkorper
und ihrer Entwickelung bei Vogeln und Saugethieren : A. m. A., XXXIII. —
Brucke, C '61. Die Elementarorganismen : Wiener Sitzber., XLIV., 1861.

— Burger, O., '91.- Uber Attractionsspharen in den Zellkorpern einer Leibes-
fliissigkeit: A. A., VI. — Id., 92. Was sind die Attractionsspharen und ihre
Centralkorper ? A. A.. 1892. — De Bruyne, C, '95. La sphere attractive dans
les cellules fixes du tissu conjonctif: Bull. Acad. Sc. de Belgique, XXX. — -

346 GENERAL LITERATURE-LIST

Butschli, O., '73. Beitrage zur Kenntniss der freilebenden Nematoden : Nova acta
acad. Car. Leopold, XXXVI. — Id., '75. Vorlaufige Mitteilungen iiber Unter-
suchungen betrefFend die ersten Entwickelungsvorgange im befruchteten Ei von
Nematoden und Schnecken : Z. iu. Z., XXV. — Id., '76. Studien iiber die ersten
Entwickelungsvorgange der Eizelle, die Zellteilung und die Konjugation der Infu-
sorien : Ad/i. des Seiickenb. Natin-forsclier-Ges., X. — Id., '91. tJber die soge-
nannten Centralkorper der Zellen und ilire Bedeutung : Verh. Naturhist. Med. Ver.
Heidelberg, 1891. — Id., '92, 1. tJber die kiinstliclie Naclialimung der Karyoki-
netisclien Figuren : Ibid., N. F., V. — Id., '92, 2. Untersuchungen liber mikro-
skopisclie Schaume und das Protoplasma (full review of literature on protoplasmic
structure): Leipzig {Engelmami). — Id., '94. Vorlaufige Bericht iiber fortgesetzte
Untersuchungen an Gerinnungsschaumen, etc. : Verh. Naturhist. Ver. Heidelberg, V.

CALKINS, G. N., '95, 1. Observations on the Yolk-nucleus in the Eggs of
Lumbricus : Travis. N.Y. Acad. Sci., June, 1895. — Id., '95, 2. The Spermato-
genesis of Lumbricus : J. M., XL, 2. — Carnoy, J. B., '94. La biologic cellulaire :
Liege. — Id., '85. La cytodierese des Arthropodes : La Celhde,!. — Id., '86. La
cytodierese de I'oeuf: La Celhde, III. — Id., '86. La vesicule germinative et les
globules polaires chez quelques Nematodes: La Cellule, III. — Id., '86. La seg-
mentation de I'oeuf chez les Nematodes : La Cellule, III., i. — Calberla, E., '78. Der
Befruchtungsvorgang beim Ei von Petromyzon Planeri : Z.w.Z., XXX. — Campbell,

D. H., '88-9. On the Development of Pilularia globuhfera : Ami. Bot., II. —
Castle, "W. E., '96. The Early Embryology of Ciona intestinalis : Bidl. Mies. Coinp.
Zool., XXYll., 7. — Cliabry, L., '87. Contributions a I'embryologie normale et
pathologique des ascidies simples: Paris, 1887. — Chittenden, R. H., '94. Some
Recent Chemico-physiological Discussions regarding the Cell : Aufi. Nat., XXVIII. ,
Feb., 1894. — Chun, C, '90. tJber die Bedeutung der direkten Zelltheilung : Sitzb.
Schr. Physik.-Okon. Ges. K'onigsberg, 1890. — Id., '92, 1. Die Dissogonie der
Rippenquallen : Festschr. f. Leuckart, Leipzig, 1892. — Id., '92, 2. (In Roux, '92,
p. 55): Verh. d. Auat. Ges., VI., 1892. — Clapp, C. M., '91. Some Points in
the Development of the Toad-Fish: J. M., V. — Clarke, J. Jackson, '95. Ob-
servations on various Sporozoa : Q. J., XXXVII., 3. — Cohn, Ferd., '51. Nach-
trage zur Naturgeschichte des Protococcus pluvialis : Nova Acta, XXII. — Conklin,

E. G-., '94. The Fertilization of the Ovum: Biol. Led., Marine Biol. Lab., JVood's
Hod, Boston, 1894. — Id., '96. Cell-size and Body-size: Pept. of Am. Morph.
Soc, Science, III., ]'2iTv. 10, 1896. — Crampton, H. E., '94. Reversal of Cleavage
in a Sinistral Gasteropod : Ann. N. V. Acad. Sci., March, 1894. — Crampton and
Wilson, '96. Experimental Studies on Gasteropod Development (H. E. Cramp-
ton). Appendix on Cleavage and Mosaic-Work (E. B. Wilson) : A. Enttn., III., i-

DEL AGE, YVES, '95. La Structure du Protoplasma et les Theories sur I'here-
dite et les grands Problemes de la Biologic Generale : Paris, 1895. — Denioor, J.,
'95. Contribution a I'etude de la physiologic de la cellule (independance fonction-
elle du protoplasme et du noyau) : A. B., XIII. — Dogiel, A. S., '90. Zur Frage
iiber das Epithel der Harnblase : A. m. A., XXXV. — Driesch, H. Entwicklungs-
mechanische Studien ; I., II., 1892, Z. w. Z., LIII. ; III.-VL, 1893, /^/^., LV.; VII.-
X., 1893: Mdt. Zool. St. Neapel, XL, 2.— Id., '94. Analytische Theorie der
organischen Entwicklung: Leipzig. — Id., '95, 1. Von der Entwickelung einzelner
Ascidienblastomeren : A. Entm., I., 3. — Driesch and Morgan, '95, 2. Zur Analysis
der ersten Entwickelungs stadien des Ctenophoreneies : Ibid., II., 2. — Driinei-,
L., '94. Zur Morphologie der Centralspindel : J. Z., XXVIII. (XXI.). — Id., '95.
Studien iiber den Mechanismus der Zelltheilung: Ibid., XXIX.. 2. — Diising, C,
'84. Die Regulierung des Geschlechtsverhaltnisses : Jena, 1884.

GENERAL LITERATURE-LIST 34/

VON EBNER, V., '71. Untersuchungen liber den Bau der Samencansilchen und
die Entwicklung der Spermatozoiden bei den Saugethieren und beim Menschen :
Inst. PJiys. 71. Hist. G?-az., 1871 {Leipzig). — Id., 88. Zur Spermatogenese bei
den Saugethieren: A. m. A., XXXI. — Ehrlich, P., '79. tJber die specifisclien
Graiiulationen des Blutes : A. A. P. {P/iys.), i^jc), p. 573. — Eismond, J., '95.
Einige Beitrage zur Kenntniss der Attraktionsspliaren und der Centrosomen : A. A.,
X. — Endres and "Walter, '95. Anstichversuche an Eiern von Rana fusca : A.
Eiitm., II., I. — Engelniann, T. "W., '80. Zur Anatomie und Physiologie der
Flimmerzellen : Arch. ges. Phys., XXIII. —r von Erlanger, R., '96, 1. Die neuesten
Ansicliten liber die Zelltheilung und ilire Mechanik : ZooL Centralb., III., 2. — Id.,
'96, 2. Zur Befruchtung des Ascariseies nebst Bemerkungen liber die Struktur des
Protoplasmas und des Centrosomes : Z. A., XIX. — Id., '96, 3. Neuere Ansicliten
iiber die Struktur des Protoplasmas : Zo'dl. Centralb., III., 8, 9. — Errara, '86. Eine
fundamentale Gleichgevvichtsbedingung organischen Zellen : Ber. Deiitsch. Bot.
Ges. J 1886. — Id., '87. Zellformen und Seifenblasm : TagebL der 60 Versanimlimg
deidscJier Natiirforscher imd Aerzte zii Wiesbaden., 1887.

FARMER, J. B., '93. On nuclear division of the pollen-mother-cell of Lilium
Martagon: Ann. Bot.,Yl\., 27. — Id., '94. Studies in Hepaticae : Ibid.,Ylll., 2().

— Id., '95, 1. tJber Kernteilung in Lilium-Antheren, besonders in Bezug auf die
Centrosomenfrage : Flora, 1895, p. 57. — Farmer and Moore, '95. On the essential
similarities existing between the heterotype nuclear divisions in animals and plants :
A. A., XI., 3. — Fick, R., '93. tJber die Reifung und Befruchtung des Axolotleies :
Z. w. Z., LVI., 4. — Fiedler, C, '91. Entwickelungsmechanische Studien an
Echinodermeneier : Festschr. Ndgeli n. Kolliker, Zurich, 1891. — Field, Gr. W., '95.
On the Morphology and Physiology of the Echinoderm Spermatozoon : J. M., XI. — ■
Fischer, A., '94. Zur Kritik der Fixierungsmethoden der Granula : A. A., IX., 22.

— Id. '95. Neue Beitrage zur Kritik der Fixierungsmethoden: Ibid.,X. — Flem^
ming, "W., '79. Beitrage zur Kenntniss der Zelle und ihre Lebenserscheinungen,
I. : A. m. A., XVI. — Id., '79. tJber das Verhalten des Kerns bei der Zelltheilung,
etc.: Virchow's Arch., LXXVII. — Id., '80. Beitrage zur Kenntniss der Zelle und
ihrer Lebenserscheinungen, II. : A. ni. A., XIX. — Id., '81. Beitrage zur Kenntnis
der Zelle und ihrer Lebenserscheinungen, III. : Ibid., XX. — Id. ,'82. Zellsubstanz,
Kern und Zellteilung: Leipzig, 1882. — Id., '87. Neue Beitrage zur Kenntnis der
Zelle : A. m. A., XXIX. — Id., '88. Weitere Beobachtungen liber die Entwickelung
der Spermatosomen bei Salamandra maculosa: Ibid., XXXI. — Id., '91-4. Zelle,
I.-IV. : Ergebn. Anat. n. Entwicklungsgesch. {Merkel and Bonnet), 1891-94. — Id.,
'91, 1. Attraktionsspliaren u. Centralkorper in Gewebs- u. Wanderzellen : A. A.

— Id., '91, 2. Neue Beitrage zur Kenntnis der Zelle, II. Teil : A. m. A., XXXVII.

— Id., '95, 1. tJber die Struktur der Spinalganglienzellen : Verhandl. der anat.
Gesellschaft in Basel, 17 April, 1895, p. 19. — Id., '95, 2. Zur Mechanik der Zell-
theilung: A. m. A., XLVI. — Floderus, M., '96. Uber die Bildung der Follikel-
hullen bei den Asci'dien : Z. w. Z., LXL, 2. — Fol, H., '73. Die erste Entwickelung
des Geryonideies : J. Z., VII. — Id., '75. Etudes sur le developpement des Mol-
lusques. — 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 I'henogenie; Mem. de
la Soc. de physiqice et d'hist. nat., Geneve, XXVI. — Id., '91. Le Quadrille des
Centres. Un episode nouveau dans I'histoire de la fecondation : Arch, des sci. phys.
et nat., 15 Avril, 1891 ; also, ^. A., 9-10, 1891. — Foot, K., '94. Preliminary
Note on the Maturation and Fertilization of Allolobophora : /. M., IX., 3, '94. —
Id., '96. Yolk-nucleus and Polar Rings: Ibid., XII., i.— Frenzel, J., '93. Die
Mitteldarmdrlise des Flusskrebses und die amitotische Zelltheilung: A. m. A.,

348 GENERAL LITERATURE-LIST

XLI. — Fromman, C, '65. tJber die Stmktur der Bindesubstanzzellen des
Ruckenmarks: Centrl. f. med. IViss., III., 6. — Id., '75. Zur Lehre von der
Structur der Zellen : y. Z., IX. (earlier papers cited). — Id., '84. Untersuchungen
iiber Struktur, Lebenserscheinungen und Reactionen thierischer und pflanzlicher
Zellen: y.Z., XVII.

GALEOTTI, GrINO, '93. tJber experimentelle Erzeugung von Unregelmassig-
keiten des karyokinetischen Processes: Bei. ziir patholog. Anat. u. z. Allg. Pathol.,
XIV., 2, Jena, Fischer, 1893. — Gardiner, "W., '83. Continuity of Protoplasm in
Vegetable Cells: Phil. Trans., CLXXIV. — Garnault, '88, '89. Sur les pheno-
menes de la fe'condation chez Helix aspera et Arion empiricorum : Zool. Anz., XL,

XII. Geddes and Thompson. The Evolution of Sex. — Gegenbaur, C, '54.

Beitrage zur naheren Kenntniss der Schwimrapolypen : Z. w. Z., V. — Van
Gehuchten, A.. '90. Recherches histologiques sur I'appareil digestif de la larve de
la Ptychoptera contaminata: La C^?/////^, VI. — Giard, A., '77. Sur la significa-
tion morphologique des globules polaires : Revue scientifique, XX. — Id., '90. Sur
les o-lobules polaires et les homologues de ces ele'ments chez les infusoires cilies :
Bulletin scientifique de la France et de la Belgique, XXII. — Grobben, C, '78.
Beitrage zur Kenntniss der mannlichen Geschlechtsorgane der Dekapoden : Arb.
Zool. Inst. lVien,l. — Gruber, A., '84. tJber Kern und Kerntheilung bei den
Protozoen : Z. iv. Z., XL. — Id., 85. tJber klinstliche Teilung bei Infusorien:
B. C, IV., 23; v., 5. — Id., '86. Beitrage zur Kenntniss der Physiologie und
Biologic der Protozoen : Ber. JVaturf. Ges. Freiburg, I. — Id., '93. Mikroscopische
Vivisektion: Ber. d. Naturf. Ges. su Freiburg, VII., i — Guignard, L., '89.
Developpement et constitution des Antherozoides : Rev. gen. Bot., I. — Id., '91, 1.
Nouvelles etudes sur la fecondation : Ann. d. Sciences Nat. Bot., XIV. — Id., '91, 2.
Sur I'existence des " spheres attractives " dans les cellules vegetales : C. R., 9 Mars.

HABERLANDT, G., '87. tJber die Beziehungen zwischen Funktion und Lage
des Zellkerns: Fischer, 1887. — Hackel, E., '66. Generelle Morphologic. —
Id., '91. Anthropogenic, 4th ed., Leipzig, 1891. — Hacker, V., '92, 1. Die
Furchung des Eies von ^quorea Forskalea : A. vi. A., XL. — Id., '92, 2. Die
Eibildung bei Cyclops und Camptocanthus : Zool. Jahrb., V. — Id., '92,3. Die
hetcrotypische Kerntheilung im Cyclus der generativen Zellen : Ber. naturf. Ges.
Freiburg, VI. — Id., '93. Das Keimblaschen, seine Elemente und Lagcverander-
ungen : A. m. A., XLI. — Id., '94. Uber den heutigen Stand der Centrosomen-
frage : Verhandl. d. deutschen Zool. Ges., 1894, p. 11. — Id., '95, 1. Die Vorstadicn
der Eireifung : A. m. A., XLV., 2. — Id., '95, 2. Zur Fragc nach dem Vorkommen
der Schein-Reduktion bei den Pflanzen : Lbid., XLVI. Also Ann. Bot., IX. —
Id., '95, 3. tJber die Selbstandigkeit der vaterlichcn und miittcrlichen Kernsbe-
standtheile wahrend der Embryonalentwicklung von Cyclops : A. m. A., XLVI., 4. —
Hallez, P., '86. Sur la loi de I'orientation de I'embryon chez les insectes : C. R.,
103, 1886. — Halliburton, W. D., '91. A Text-book of Chemical Physiology and
Pathology : London. — Id., '93. The Chemical Physiology of the Cell : {Gould-
stonian Lectures') Brit. Med. 7^;/;-;^. — Hammar, J. A., '96. Ubcr einen
primarcn Zusammenhang zwischen den Furchungszellen des Seeigeleics : A. m. A.,
XLVIL, I. — Hammar.sten, O., '94. Zur Kenntniss der Nuclco-proteids : Zeit.
Phys. Chem., XIX. — Id., '95. Lehrbuch der physiologischcn Chemie, 3e Ausgabe :
Wiesbaden, 1895. — Hansemann, D., '91. Karyokinese und Cellularpathologie :
Berl. Klin. Wochenschrift, No. 42. — Id.. '93. Spezificitat, Altruismus und die
Anaplasie der Zellen: Berlin, 1893. — Hanstein, J., '80. Das Protoplasma als
Trager der pflanzlichen und thierischen Lebensverrichtungen . Heidelberg. —
Harvey, Wm., 1651. Exercitationes de Generationc Animalium : London. Trans.

GENERAL LITERATURE-LIST 349

in Sydenham Soc, X., 1847. — Hartog, M. M., '91. Some Problems of Reproduc-
tion, etc: Q. 7., XXXIII.— Hatschek, B., '87. IJber die Bedeutung der
geschlechtlichen Fortpflanzung : Prager Med. Wochenschrift, XLVI. — Id., '88.
Lehrbuch der Zoologie. — Heidenhain, M., '93. Uber Kern und Protoplasma:
Festchr. z. t,Q-Jahr. Doct07-jub. von v. K'dlliker : Leipzig. — Id., '94. Neue Unter-
suchungen liber die Centi'alkorper und ihre Beziehungen zum Kern und Zellen-
protoplasma: A. 7n. A., XLIII. — Id., '95. Cytomechanische Studien : A. Entm..,
I., 4. — Heitzmann, J., '73. Untersuchungen liber das Protoplasma: Sitzb. d. k.
Acad. IJ'iss. ll'ien, LXVII. — Id., '83. Mikroscopische Morphologic des Thier-
korpers im gesunden und kranken Zustande: IVien, 1883. — Henking, H. Unter-
suchungen liber die ersten Entvvicklungsvorgange in den Eiern der Insekten, I.,
II., III.: Z: TV. Z., XLIX., LI., LIV., 1 890-1 892. — Henle, J., '41. Allgemeine
Anatomic: Leipzig. — Henneguy, L. F., '91. Nouvelles recherches sur la divi-
sion cellulaire indirecte : Joiirn. Anat. et Physiol, XXVII. — Id., '93. Le Corps
vitellin de Balbiani dans I'oeuf des Vertebres : Ibid.. XXIX. — Id., '96. Lemons
sur la cellule : Paris. — Hensen, V., '81. Physiologic der Zeugung : Hermann'' s
Physiologie, VI. — Herbst, C. Experimentelle Untersuchungen liber den Ein-
fluss der veranderten chemischen Zusammensetzung des umgebenden Mediums
auf die Entwicklung der Thiere, I.: Z. w. Z., LV.. 1892; II., Mitt. Zool. St.
JVeapel, XL, 1893; III.-VL, Arch. Entm., IL, 4, 1896. — Id., '94, '95. tJber
die Bedeutung der Reizphysiologie flir die Kausale Auffassung von Vorgangen
in der tierischen Ontogenese : Biol. Centralb., XIV., XV., 1894, 1895. — Herla,
v., '93. Etude des variations de la mitose chez I'ascaride megalocephale : A. B.,
XIII. — Herlitzka, A., '95. Contributo alio studio della capacita evolutiva dei
due primi blastomeri nell' uove di Tritone : A. Entm., IL, 3. — Hermann, F., '89.
Beitrage zur Histologic des Hodens : A. m. A., XXXIV. — Id., '91. Beitrag zur
Lehre von der Entstehung der karyokinetischen Spindel : Ibid.., XXXVII. —
Id., '92. Urogenitalsystem, Struktur und Histiogenese der Spermatozoen : Merkel
nnd Bonnet^s Ergebnisse, II. ^ Hertwig, O., '75. Beitrage zur Kenntnis der
Bildung, Befruchtung und Teilung des tierischen Eics, I.: M. J., I. — Id., '77.
Beitrage, etc, II. : Ibid., III. — Id., '78. Beitrage, etc, III. : Ibid., IV. —Id., '84.
Das Problem der Befruchtung und der Isotropic des Eies, eine Theorie der Vcrer-
bung: J. Z., XVIII. — Id., '90, 1. Vergleich der Ei- und Samenbildung bei
Nematoden. Eine Grundlage flir cellulare Streitfragen : A. in. A., XXXVI. —
Id , '90, 2. Experimentelle Studien am tierischen Ei vor, wahrend und nach der
Befruchtung: J. Z., 1890. — Id., '92, 1. Urmund und Spina Bifida: A. m. A.,
XXXIX. — Id., '92, 2. Aeltere und neuere Entwicklungs-theorieen : Berlin. —
Id., "93, 1. Uber den Werth der ersten Furchungszellen flir die Organbildung des
Embr3'o : A. m. A., XLII. — Id., 93, 2. Die Zelle und die Gewebe : Fischer, Jena,
1893. — Id., '94. Zeit und Streitfragen der Biologic : Berlin. — Hertwig, O. and
R., '86. Experimentelle Untersucliungen liber die Bedingungen der Bastardbe-
fruchtung: y. Z, XIX. — Id., "87. tJber den Befruchtungs- und Teilungsvorgang
des tierischen Eies itnter dem Einfluss ausserer Agentien : Ibid., XX. — • Hertwig,
R., '77. liber den Bau und die Entwicklung der Spirochona gemmipara : Ibid.,
XI.^ — Id., '84. Die Kerntheilung bei Actinosphsrium Eichhorni : /(J/V/., XVII. —
Id., '88. Uber Kernstruktur und ihre Bedeutung flir Zellteilung und Befruclitung :
Ibid., IV., 1888. — Id., '89. Uber die Konjugation der Infusorien: AbJi. der bayr.
Akad. d. IFiss., IL, CI., XVIL— Id., '92. Uber Befruchtung und Conjugation:
Ferh. deutsch. Zool. Ges., Berlin. — Id., '95. Uber Centrosoma und Centralspindel :
Sitz.-Ber. Ges. Morph. jtnd Phys., Munchen, 1895, Heft I. — Heuser, E., '84.
Beobachtung liber Zelltheilung : Bot. Cent. — Hill, M. D., '95. Notes on the
Fecundation of the Egg of Sphcerechinns grannlaris and on the Maturation and
Fertilization of the Egg of Phallusia inammillata : Q. J., XXXVIII. — His, "W.,

350 GENERAL LITERATURE-LIST

'74. — Unsere Korperform und das physiologische Problem ihrer Entstehung:
Leipzig. — Hofer, B., '89. Experimentelle Untersuchungen liber den Einfluss des
Kerns auf das Protoplasma : J. Z., XXIV. — Hofmeister, '67. Die Lehre von der
Pflanzenzelle : Leipzig, 1867. — Holl, M., '90. Uber die Reifung der Eizelle des
Y\.vX\xi%: Sit zb. Acad. Wiss. Wien., XCIX., 3. — Hooke, Robt, 1665. Mikro-
graphia, or some physiological Descriptions of minute Bodies by magnifying Glasses :
London. — Hoyer, H., '90. tJber ein flir das Studium der "direkten" Zelltheilung
vorzuglich geeignetes Objekt : A. A.,V . — 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. Keview, XII. — Id., '78. Evolution in Biology,
Enc. Brit., 9th ed., 1878 ; Science and Ctdture, N. Y., 1882.

ISHIKAWA, M., '91. Vorlaufige Mitteilungen liber die Konjugations-
erscheinungen bei den Noctiluceen : Z. A., No. 353, 1891. — Id., '94. Studies on
Reproductive Elements : II., Noctilnca miliaris, Sur., its Division and Spore-forma-
tion ; Journ. College of Sc. Imp. Univ. Japan, VI.

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. Camp. Zool., XXII., 3. — Jordan, E. O., '93. The Habits and Development
of the Nevirt: J. M., VIII., 2. — Jordan and Eycleshymer, '94. On the Cleav-
age of Amphibian Ova: J. M., IX., 3, 1894. — Julin, J., '93, 1. Structure et
de'veloppement des glandes sexuelles, ovogenese, spermatogenese et fecondation
chez Styleopsis grossularia : Bull. Sc. de France et de Belgique., XXIV. — Id.,
'93, 2. Le corps vitellin de Balbiani et les elements des Metazoaires qui corre-
spondent au Macronucleus des Infusoires cilies : Bull. Sc. de France et de
Belgique, XXIV.

KEUTEN, J., '95. Die Kerntheilung von Euglena viridis Ehr: Z. w. Z., LX.
Kienitz-Gerloff, F., '91. Review and Bibliography of Researches on Proto-
plasmic Connection between adjacent Cells: in Bot. Zeitting,XlAX. — Klebahn,
'92. Die Befruchtung von QEdigonium : Zeit. wiss. Bot., XXIV. — Id. Die
Keimungvon Closterium und Cosmarium : Jahrb.f. zviss. Bot.,XXll. — Klebs, G.,
'84. iJber die neueren Forschungen betreffs der Protoplasmaverbindungen benach-
barter Zellen : Bot. Zeit., 1884. — Id., '87. Uber den Einfluss des Kerns in der
Zelle: B. C, VII. — Klein, E., '78-9. Observations on the Structure of Cells
and Nuclei : Q. 7., XVIII., XIX. — von Kolliker, A., '41. Beitrage zur Kenntnis
der Geschlechtsverhaltnisse und der Samenfllissigkeit wirbelloser Tiere : Berlin. —
Id., '44. Entwicklungsgeschichte der Cephalopoden : Zuric/t. —Id., '85. Die
Bedeutung der Zellkerne fiir 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 : Z. iv. Z., XLIII. —Id., '89. Hand-
buch der Gewebelehre, 6th ed. : Leipzig. — Korschelt, E., '89. Beitrage zur Mor-
phologie und Physiologie des Zell-Kernes : Zool. Jahrb. Anat. n. Ontog., IV. —
Id.. '93. tJber Ophryotrocha puerilis : Z. w. Z., LIV. — Id., '95. Uber Kern-
theilung, Eireifung und Befruchtung bei Ophryotrocha picerilis : Z. w. Z.,!^^. —
Kossel, A., '91. tJber die chemische Zusammensetzung der Zelle : Arch. Anat.
u. Fhys. — Id., '93. Uber die Nucleinsaure : Ibid., 1893. — von Kostanecki, '91.
liber Centralspindelkorperchen bei karyokinetischer Zellteilung : Anat. Hefte, 1892,
dat. 91. — Kostanecki and Wierzejski, '96. tJber das Verhalten der sogenann-
ten achromatischen Substanzen im befruchteten Ei : A. m. A., XLII., 2. — Kiihne,
W., '64. Untersuchungen liber das Protoplasma und die Contractilitat. — Kupffer,

GENERAL LITERATURE-LIST 35 I

C, '75. tJber DifFerenzierung des Protoplasma an den Zellen thierischer Gewebe :
Schr. nahir. Ver. Schles.-Holst., I., 3. — Id., '90. Die Entwicklung von Petromy-
zon Planeri: A. m. A., XXXV.

LAMEERE, A., '90. Recherches sur la reduction karyogamique : Briixelles. —
Lauterborn, R., '93. tJber Bau und Kerntheilung der Uiatomeen : Verh. d.
Naturh. Med. Ver. in Heidelberg, 1893. — Id., '95. Protozoenstudien. Kern- und
Zellteilung von Ceratium hirundinella O. F. M. : Z. w. Z., XLIX. — La Valette St.
George, '65. Ueber die Genese der Samenkorper : A. m. A., I. — Id., '67. Uber
die Genese der Samenkorper, II., (Terminology) : A. m. A., III. — Id., '76. — Die
Spermatogenese bei den Ampliibien : Ibid., XII. — Id., '78. Die Spermatogenese bei
den Saugethieren und dem Menschen : Ibid., XV. — Id. Spermatologische Beitrage,
I.-V. : A. m. A., XXV., XXVII., XXVIII., and XXX., 1885-87. - Lankester, E.
Ray, '77. Notes on Embryology and Classification : London. — Lavdovsky, M., '94.
Von der Entstehung der chromatischen und achromatischen Substanzen in den
tierischen und pflanzlichen Zellen: Merkel tind Bonnefs Anat. Hefte, IV., 13. —
von Lenhossek, M., '95. Centrosom und Sphare in den Spinalganglien des
Frosches; A. m. A., XLVI. — Leydig, Fr. '54. Lehrbuch der Histologie des
Menschen und der Thiere : Frankfurt. — Id., '85. — Zelle und Gewebe, Bonn. —
Id., '89. Beitrage zur Kenntniss des thierischen Eies im unbefruchteten Zustande :
SpengeVs Jahrb. Anat. Ont., III. — Lilienfeld, L., '92, '93. tJber die Verwand-
schaft der Zellelemente zu gewissen Farbstoffen : Verh. Phys. Ges., Berlin, 1892-3.
— Id., '93. tJber die Wahlverwandtschaft der Zellelemente zu Farbstoffen:
A. A. P., 1893. — Lillie, F. R., '95. The Embryology of the Unionids: J. M.,
X. — Id., '96. On the Limit of Size in the Regeneration of Stentor : Rept. Am.
Morph. Soc. Science., III., Jan. 10, 1896. — Loeb, J., '91-2. Untersuchungen zur
physiologischen Morphologie. I. Heteromorphosis : IViirzbnrg, i8gi. II. Organ-
bildung und Wachsthum : Wiirzbnrg, 1892. — Id., '93. Some Facts and Principles
of ph3'siological Morphology: PVood's Holl Biol. Lectures, i?)()2,. — Id., '94. tJber
die Grenzen der Theilbarkeit der Eisubstanz : A. ges. P., LIX., 6, 7. — Lowit, M.,
'91. tJber amitotische Kerntheilung: B. C, XI. — Lukjanow, '91. Grundzuge
einer allgemeinen Pathologie der Zelle: Leipzig. — Lustig and Galeotti, '93.
Cytologische Studien liber 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. — McMurrich, J. P., '86. A Contribution to
the Embryology of the Prosobranch Gasteropods : Studies Biol. Lab. Johns Hopkins
Univ., III. — Id., '95. Embryology of the Isopod Crustacea: /. M., XL, i. —
Maggi, L., '78. I plastiduH nei cihati ed i plastiduli liberamente viventi : Atti. Soc.
Ital. Sc. Nat. Milano, XXI. (also later papers). — Malf atti, H., '91. — Beitrage zur
Kenntniss der Nucleine : Zeit. Phys. Chem., XVI. — Mark, E. L., '81. Maturation,
Fecundation and Segmentation of Limax campestris : Bidl. Mus. Comp. Zo'ol. Har-
vard College, VI. — Maupas, M., '88. Recherches experimentales sur la multipli-
cation des Infusoires cilies : Arch. Zool. Exp., 2me serie, VI. — Id., '89. Le
rejeunissement karyogamique chez les Cilies: Ibid., 2me serie, VII. — Id., '91.
Sur le determinisme de la sexualite chez THydatina senta : C. R-, Paris. — "M-ea-A,
A. D., '95. Some observations on maturation and fecundation in Cha^topterus
pergamentaceus Cuv. : y. M., X., i . — Merkel, F., '71. Die Stlitzzellen des mensch-
lichen Hodens : Mutter's Arch. — Mertens, H., '93. Recherches sur la signifi-
cation du corps vitelHn de Balbiani dans I'ovule des Mammiferes et des Oiseaux :
A. B., XIII. — Metschnikoff, E., '66. Embryologische Studien an Insecten:
Z. iv. Z., XVI. — Meves, F., '91. (Jber amitotische Kernteilung in den Sperma-

352 GENERAL LirERATURE-LISr

too-onien des Salamanders, und das Verhalten der Attraktionsspharen bei derselben :
A. A., 1891, No. 22. — Id., '94. iJber eine Metamorphose der Attraktionssphare
in den Spermatogonien von Salamandra maculosa: A. m. A., XLIV. — Id., '95.
iJber die Zellen des Sesambeines der Achillessehne des Frosches {Raiia tein-
poraria) und liber ihre Centralkorper : Ibid.^ XLV. — Msyer, O., '95. Cellulare
Untersuchungen an Nematodeneiern : J. Z., XXIX. (XXIL). — Mikosch, '94.
liber Struktur im pflanzlichen Protoplasma : Verhandl. d. Ges. deiitscher Natii?-/.
und Arzte, 1894; Abteil f. Pflanzeiiphysiologie u. Pflanzenanatomie. — Minot,
C. S., '77. Recent Investigations of Embryologists : Proc. Host. 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 A7/i. Nat., February, 1880. — Id., '87.
Theorie der Genoblasten: B. C, II., 12, 1887; also, Proc. Bost. Soc. Nat. Hist.,
XIX., 1877. — Id., '91. Senescence and Rejuvenation : /i?//;-;?. Phys.,X\\., 2.. —
Id., '92. Human Embryology: New York. — von Mohl, Hugo, '46. (Jber die
Saftbewegung im Innern der Zellen : Bot. Zeittnig. — Moore, J. E. S., '93. Mam-
malian Spermatogenesis: A.A.,YUl. — 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. Entiii., 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 Fragments of Echinoderm-eggs : Arch. Etitin., II., 2. — Id., '95, 5.
The Formation of the Fish-embryo : J. 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. Entm., III., 2. — Muller, E., '96. Uber die
Regeneration der Augenhnse nach Exstirpation derselben bei Triton: A. in. A.,
XLVIL, I.

NAGELI, C, '84. Mechanisch-physiologische Theorie der Abstammungslehre :
Mimchen 71. Leipzig, 1884. — Nageli und Schwendener, '67. Das Mikroskop.
(See later editions.) Leipzig. — Newport, G. On the Impregnation of the
Ovum in the Amphibia: PhiL Tratis., 185 1, 1853, 1854. — Nussbaum, M., '80.
Zur Differenzierung des Geschlechts im Tierreich : A. m. A., XVIII. — Id., '84, 1.
Tiber Spontane und Kunstliche Theilung von Infusorien: Verh. d. naturh. Ver.
preuss. Rhiiieland, 1884. • — Id., '84, 2. tJber die Veranderungen der Geschlechts-
producte bis zur Eifurchung : A. m. A^,XX\\\. — Id., '85. — LJber die Teilbarkeit
der lebendigen Materie, I. : A. in. A., XXVI. — Id., '94. Die mit der Entwickelung
fortschreitende Differenzierung der Zellen : Sitz.-Ber. d. niederrhein. Gesellschaft f.
Natur- n. Heilkunde, Bonn, 5 Nov., 1894; also B. C, XVI., 2, 1896.

OGATA, '83. Die Veranderungen der Pancreaszellen bei der Secretion :
A. A. P. — Oppel, A., '92. Die Befruchtung des Reptilieneies : A. ni. A , XXXIX.
— Overton, C. E., '88. (Jber den Conjugationsvorgang bei Spirogyra : Ber.
deutsch. Bot. Ges., VI. — Id., '89. Beitrag zur Kentniss der Gattung Volvox : Bot.
Cejitrb., XXXIX. — Id., '93. Uber die Reduktion der Chromosomen in den Kernen
der Pflanzen : Vierteljahrschr. naturf. Ges. Zihich, XXXVIII. Also Anii. Bot.,
VII., 25.

PALADINO, G., '90. I ponti intercellulari tra 1' novo ovarico e le cellule folli-
colari, etc.: A. A., V. — Palla, '90. Beobachtungen uber Zellhautbildung an
des Zellkerns beraubten Protoplasten. Flora, 1S90. — Pfitzner, W., '82. Uber

GENERAL LITERATURE-LIST 353

den feineren Bau der bei der Zelltheilung auffretenden fadenformigen Differenzier-
ungen des Zellkerns : M. y.. VII. — Id., '83. Beitrage zur Lehi'e vom Baue des
Zellkerns und seinen Theilungserscheinungen : A, m. A., XXII. — Pfluger, E., '83.
ttber den Einfluss der Schwerkraft auf die Theilung der Zellen : I., Arch. ges.
Phys.,XXXl.; II., Ibid., XXXII.; abstract in Biol. Centb., III., 1884. — Id., '84.
iJber die Einwirkung der Schwerkraft und anderer Bedingungen auf die Richtung
der Zelltlieilung : Arch. ges. Phys., XXXIV. — Id., '89. Die allgemeinen Lebenser-
scheinungen : Bonn. — Platner, G., '86. tJber die Befruchtung von Arion enipiri-
coruni: A. vi. A., XXXVII. — Id., '89,1. (Jber die Bedeutung der Richtungs-
korperchen : B. C, VIII. — Id., '89, 2. Beitrage zur Kenntniss der Zeile und
ihrer Teilungserscheinungen, I.-VI. : A. vi. y^., XXXIII. — Poirault and Raci-
borski, '96. tJber konjugate Kerne und die konjugate Kerntheilung : B. C, XVI.,
I. — Prenant, '94. Sur le corpuscule central: Bull. Soc. Sci., Nancy, 1894. —
Preusse, F., '95. tJber die amitotische Kerntheilung in den Ovarien der Hemi-
pteren : Z. iv. Z., LIX., 2. — Prevost and Dumas, '24. Nouvelle theorie de la
generation: Ann. Sci. Nat., \., II. — Pringslieim, N., '55. tJber die Befruchtung
der Algen : Monatsb. Berl. Akad., 1855-6. — Purkyne : Jahrb. f. iviss. Kritik,
1840.

RABL, C, '85. tJber Zellteilung: M. J., X. — Id., '89, 1. tJber Zelltheil-
ung: A. A., IV. — Id., '89, 2. tJber die Prinzipien der Histologie: Verh. Anat.
Ges., III. — vom Rath, O., '91. tJber die Bedeutung der amitotischen Kernteilung
im Hoden : Zo'ol. Anz., XIV. — Id., '92. Zur Kenntniss der Spermatogenese von
Gryllotalpa znclgaris : A. m. A., XL. — Id., '93. Beitrage zur Spermatogenese
von Salamandra : Z. w. Z., LVII. — Id., '94. tJber 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., XLVL- — 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 Kentniss der Zelle : M. J.,
VIII. — Rawitz, B., '95. Centrosoma und Attraktionsphare in der ruhenden Zelle
des Salamanderhodens : A. m. A., XLIV., 4. — Reinke, Fr. '94. Zellstudien.
I., A. in. A., XLIII.; II., Ibid., XLIV., 1894. — Id., '95. Untersuchungen liber
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 : UntersucJi. aus d. bot. Inst. G'dttingen, II. — Remak, R., '41. Uber
Theilung rother Blutzellen beim Embryo: Med. Ver. Zeit., 1841. — Id., '50-5.,
Untersuchungen liber die Entwicklung der Wirbelthiere : Berlin, 1850-55. — Id.,
'58. Uber die Theilung der Blutzellen beim Embryo: Milllers Arch., 1858. — ■
Retzius, G., '89. Die Intercellularbrlicken des Eierstockeies und der F'ollikelzellen :
Verh. Anat. Ges., i88g. — 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. — Rompel, '94. Kentrochona
Nebaliae n. g. n. sp., gin neues Infusor aus der Familie der Spirochoninen. Zugleich
ein Beitrag zur Lehre von der Kernt.eilung und dem Centrosoma: Z. w. Z., LVIII.,
4. — Rosen, 92. Uber tinctionelle Unterscheidung verschiedener Kernbestand-
theile und der Sexual-kerne : Cohn^s Beitr. z. Biol. d. Pflaiizen, V. — Id., '94. Neueres
liber die Chromatophilie der Zellkerne : Schles. Ges. vdterl. Knit, 1894. — Roux,
W., '83, 1. tJber die Bedeutung der Kernteilungsfiguren : Leipzig. — Id., '83, 2.
tiber die Zeit der Bestimmung der Hauptrichtungen des Froschembryo : Leipzig. —
Id., '85. Uber die Bestimmung der Hauptrichtungen des Froschembryos im Ei,
und liber die erste Theilung des Froscheies : Breslauer artzl. Zeitg., 1885. — Id.,
'87. Bestimmung der medianebene des Froschembryo durch die Kopulationsricht-
ung des Eikernes und des Spermakernes : ^. //;. ^., XXIX. — Id., '88. tJber das
2 A

354 GENERAL LITERATURE-LIST

klinstliche Hervorbringen halber Embryonen durch Zerstorung einer der beiden
ersten Furchungskugeln, etc.: Virckow''s Arc/uv, 114. — Id., '90. Die Entwickel-
ungsmechanik der Organismen. Wien, 1 890. — Id., '92, 1. Entwickelungsmechanik :
Me?'kel 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 Entwickelungshypothesen : An.
Hefte, Feb., 1893. — Id., '93, 2. Uber die Spezifikation der Furchungzellen, etc.:
B. C, XIII., 19-22. — Id., '94, 1. tJber den " Cytotropismus" der Furchungszellen
des Grasfrosches : Arch. Entm., L, i, 2. — Id., '94, 2. Aufgabe der Entwickel-
uncrsmechanik, etc.: Arch. Entni., I., i. Trans, in Biol. Lectures, Wood''s Holl,
1894. — Riickert, J., '91. Zur Befruchtung des Selachiereies : A. A., VI. — Id.,
'92, 1. Zur Entwicklungsgeschichte des Ovarialeies bei Selachiern : A. A., VII.—
Id., '92, 2. liber die Verdoppelung der Chromosomen im Keimblaschen des Se-
lachiereies: Ibid., VIII. — Id., '93, 2. Die Chromatinreduktion der Chromosomen-
zahl im Entwicklungsgang der Organismen: Merkel and Bonnet, Erg., III. — Id.,
'94. Zur Eireifung bei Copepoden : An. Hefte. — Id., '95,1. Zur Kenntniss des
Befruchtungsvorganges : Sitsb. Bayer. Akad. Wiss., XXVI., i. — Id., '95, 2. Zur
Befruchtung von Cyclops streniius : A. A., X., 22. — Id., '95, 3. Uber das Selb-
standigbleiben der vaterlichen und mliterliclien Kernsubstanz wahrend der ersten
Entwicklung des befruchteten Cyclops-Eies : A. m. A., XLV., 3. — Hiige, G., '89.
Vorgange am Eifollikel der Wirbelthiere : M. J., XV. — Ryder, J. A., '83. — The
microscopic Sexual Characteristics of the Oyster, etc. : Bull. U. S. Fish. Cotnm.,
March 14, 1883. Also, Ann. Mag. Nat. Hist., XII., 1883.

SABATIER, A., '90. De la Spermatogenese chez les Locustides; Comptes
Rend., CXI., '90. — Sachs., J., '82. Vorlesungen iiber Pflanzen-physiologie ; Leip-
zig.— Id. iiber die Anordnung der Zellen in jiingsten Pflanzentheile : Arb. Bat.
Inst. Wtlrzburg, II. — Id., '92. Physiologische Notizen, II., Beitrage zur Zel-
lentheorie: Flora, 1892, Heft I. — Id., '93. StofF und .Form der Pflanzen-organe ;
Gesammelte Abhandbing, II ., 1 893. — Id., '95. Physiologische Notizen, IX., weitere
Betrachtungen liber Energiden und Zellen: Flora, LXXXI., 2. — Sala, L., '95.
Experimentelle Untersuchungen iiber die Reifung und Befruchtung der Eier bei
Ascaris megalocephala ; A. m. A., XL. — Sargant. Ethel, '95. Some details of
the first nuclear Division in the Pollen-mother-cells of Liliimi juartagon ; Journ.
Roy. Mic. Sac, 1895, part 3. — Schafer, E. A., '91. General Anatomy or Histol-
ogy: in Quaiii's Anatomy, I., 2, loth ed., London. — Schaudinn, F., '95. tJber
die Theilung von A?nosba binucleata Gruber : Sitz.-Ber. Ges. N'at?irforsch., Frennde,
Berlin, Jahrg. 1895, No. 6. — Id., '96. Uber den Zeugungskreis von Par amoeba
Eilhardi: Sitz.-Ber. Ges. Naturforsch., Frennde, Berlin, 1896, Jan. 13. — Sche-wi-
akoff, "W., '88. tJber die karyokinetische Kerntheilung der Euglypha alveolata:
M. J., XIII. — Schiefferdecker and Kossel, '91. Die Gewebe des Menschlichen
Kbrpers : Brataischweig. — Schiniper, '85. Untersuchungen iiber die Chlorophyll-
korper, etc. : Zeitsch. wiss. Bot., XVI. — Schleicher, W., '78. Die Knorpelzell-
theilung. Ein Beitrag zur Lehre der Theilung von Gewebezellen : Centr. med. Wiss.
Berlin, 1878. Also A. m. A., XVI, 1879. — Schleiden, ^- J-» '38. Beitrage zur
Phytogenesis : Micller''s Archiv, i%2>^. [Trans, in Sydenham Soc, XII.: London,
1847.] — Schloter, G-., '94. Zur Morphologic der Zelle : A. 7n. A., XLIV., 2. —
Schniitz, '84. Die Chromatophoren der Algen. — Schneider, A., '73. Unter-
suchungen iiber Plathelminthen : Jahrb. d. oberhess. Ges. f. Natur- Heilkunde,
XIV., Giessen. — -Schneider, C, '91. Untersuchungen iiber die Zelle: Arb. Zool.
Inst. VVien, IX., 2. — Schottlander, J., '88. Uber Kern und Zelltheilungsoor-
gange in dem Endothel der entziindeten Hornhaut: A. m. A., XXXI. — Schultze,
Max, '61. Uber Muskelkorperchen und das was man eine Zelle zu nennen hat :

GENERAL LITERATURE-LIST 355

Arch. Anat. Phys., 1861. — Scliultze, O., '87. Untersuchungen iiber die Reifung
und Befruchtung des Amphibien-eies : Z. w. Z.,XLV. — Id., '94. Die kunstliche
Erzeugung von Doppelbildungen bei Froschlarven, etc. : Arch. Eiitni., I., 2. —
Schwann, Th., '39. Mikroscopisclie Untersuchungen liber die Uebereinstimmung
in der Structur und dem Wachsthum der Thiere und Pflanzen : Berlin. [Trans, in
Sydenham Soc.,X\\.: London, 1847.] — Schwarz, Fr., '87. Die Morphologische
und chemische Zusammensetzung des Protoplasmas : Breslan. — Schweigger-
Seidel, O., '65. tlber die Samenkorperclien und ilire Entwickelung : A. m. A., I.

— Sedgwick, A., '85-8. Tlie Development of the Cape Species of Peripatus,
I.-VI.: Q. 7., XXV.-XXVIII. — Id., '94. On the Inadequacy of the Cellular
Theory of Development, etc.: Q. J., XXXVII., i. — Seeliger, O., '94. Giebt es
geschlechtlicherzeugte Organismen ohne miitterliche Eigenschaften ? : A. Ent., I., 2.

— Selenka, E., '83. Die Keimblatter der Echinodermen : Stiidien iiber Entwick.,
II, Wiesbaden, 1883. — Sertoli, E., '65. Dell' esistenza di particolori cellule
ramificate dei canaliculi seminiferi del testicolo umano : // Morgagni. — Sied-
lecki, M., '95. tJber die Struktur und Kerntheilungsvorgange bei den Leucocy-
ten der Urodelen: A)iz. Akad. Wiss., Krakau, 1895. — Sobotta, J., '95. Die
Befruchtung und Furchung des Eies der Maus : A. ni. A., XL. — Solger, B.,
'91. Die radiaren Strukturen der Zellkorper im Zustand der Ruhe und bei der
Kerntheilung : Berl. Klin. Wochenschr.,.XX., 1891. — Spallanzani, 1786. Ex-
periences pour servir a I'histoire de la generation des animaux et des plantes :
Geneva. — Strasburger, E., '75. Zellbildung und Zelltheilung : ist ed., Jena,
1875.— Id., '77. tJber Befruchtung und Zelltheilung : J. Z., XI. — Id., '80. Zell-
bildung und Zellteilung: 3d ed. — Id., '82. IJber den Theilungsvorgang der Zell-
kerne 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 Untersuchungen iiber den Befruchtungsvorgang bei den Phaneroga-
men, als Grundlage fiir eine Theorie der Zeugung: Jena, 1884. — Id., '88. Uber
Kern- und Zellteilung im Pflanzenreich, nebst einem Anhang Uber Befruchtung :
Jena. — Id., '89. Uber das Wachsthum vegetabilischer Zellhaute: Hist. Bei.,
II., Jena. — Id., '91. Das Protoplasma und die Reizbarkeit : Rektoratsrede, Bonn,
Oct. 18, 1891. Jena, Fischer . — !&., '92. Histologische Beitrage, Heft IV. : Das
Verhalten des Pollens und die Befruchtungsvorgange bei den Gymnospermen,
Schwarmsporen, pflanzliche Spermatozoiden 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
Zelltheilungsfragan : A. A^NWl.,^. 177. — Id., '94. Uber periodische Reduktion
der Chromosomenzahl im Entwicklungsgang der Organismen: B. C, XIV. — Id.,
'95. Karyokinetische Probleme : Jahrb. f. wiss. Botanik,XXS[\\\.,\.^'^^vi A&x
Stricht, O., '92. Contribution a I'etude de la sphere attractive: A. B., XII., 4. —
Id., '95, 1. La maturation et la fecondation de I'oeuf d'Amphioxus lanceolatus :
Bull. Acad. Roy. Belgiqice, 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.
Versaniml. in Strassburg, 1895, p. 223. — Id., '95, 3. Contributions a I'etude de
la forme, de la structure et de la division du noyau : Bidl. Acad. Roy. Sc. Belgique,
XXIX. — Strieker, S., '71. Handbuch der Lehre von den Geweben : Leipzig. —
Stulilmann, Fr., '86. Die Reifung des Arthropodeneies nach Beobachtungen an
Insekten, Spinnen, Myriopoden und Peripatus : Ber. Natnrf. Ges. Freibnrg, I. —
Swaen and Masquelin, '83. Etude sur la Spermatogenese : A. B., IV.

THOMA, R., '96. Text-book of General Pathology and Pathological Anatomy :
Trans, by A. Bruce, London. — Thomson, Allen. Article " Generation " in Todd's
Cyclopedia. — Id. Article '-Ovum" in Todd's Cyclopedia. — Tyson, Jame.'s, "78.
The Cell-doctrine: 2d ed., Philadelphia.

356 GENERAL LITERATURE-LIST

USSOW, M., '81. Untersuchungen iiber die Entwickelung der Cephalopoden :
A7-ch. Biol., II.

VEJDOVSKY, P., '88. Entwickelungsgeschichtliche Untersuchungen, Heft I. :
Reifung, Befruchtungund Furchungdes Rhynchelmis-Eies : Prag, i888. — Verworn,
M. '88. Biologische Protisten-.studien : Z. w. Z., XLVL — Id., '89. — Psycho-
physiologische Protisten-studien : Jena. — Id., '91. Die physiologisclie Bedeutung
des Zellkerns : Ffliige7'''s Arch. f. d. ges. Physiol.., LI. — Id., '95. Allgemeine
Physiologie: Jena. — Virchow, R., '55. Cellular-Path ologie : Arch. Path. Anat.
Phys., VIII., I. — Id., '58. Die Cellularpathologie in ihrer Begriindung auf physio-
logische und pathologische Gewebelehre : Berlin, 1858. — De Vries, H., '89.
Intracellulare Pangenesis : Jena.

WALDEYER, W., '70. Eierstock und Ei : Leipzig. — Id., '87. Bau und
Entwickelung der Samenfaden : Verh. d. Anat. Leipzig, 1887. — Id., '88. tJber
Karyokinese und ihre Beziehungen zu den Befruchtungsvorgangen : A. m. A.,
XXXII. [Trans, in Q- J.~\ — Id., '95. Die neueren Ansichten liber den Bau und
das Wesen der Zelle : Deiitsch. Med. Wochenschr., No. 43, flf., Oct. ff., 1895. —
Warneck, N. A., '50. Ueber die Bildung und Entwickelung des Embryos bei
Gasteropoden : Bull. Soc. Imp. Nat. Moscou, XXIII., i. — Watase, S., '91.
Studies on Cephalopods ; 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 : J. M., VIII., 2. — Id., '94. Origin of the centro-
some : Biological Lectures, Wood''s Holl, 1894. — Weismann, A., '83. tJber
Vererbung: Jena. — Id., '85. Die Kontinuitat des Keimplasmas als Grundlage
einer Theorie der Vererbung: _/i?;^<2. — Id., '86, 1. Richtungskorper bei partheno-
genetischen Eiern: Zool. Anz., No. 233. — Id., '86, 2. Die Bedeutung der sexuel-
len Fortpflanzung flir die Selektionstheorie : Jena. — Id., '87. tJber die Zahl der
Richtungskorper und liber ihre Bedeutung flir die Vererbung: Jena. — 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, i8g2. — Id. ,'93. The Germ-plasm : New Vork. — Id., '94.
Aeussere Einflusse als Entwicklungsreize : Jena. — Wheeler, W. M., '89. The
Embryology of Blatta Germanica and Doryphora deceinlineata : J. M., III. —
Id., '93. A Contribution to Insect-embryology: Ibid., VIII. i. — Id., '95. The
Behavior of the Centrosomes in the Fertilized Egg oi Myzostonia glabnini : Ibid.,X^.

— Whitman, C. O., '78. The Embryology of Clepsine : Q. J., XVIII. — Id., '87.
The Kinetic Phenomena of the Egg during Maturation and Fecundation : J. 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 Holl Biol. Lectures,
1893. — Id., 94. Evolution and Epigenesis : Ibid., i8g4. — Wiesner, J., '92. Die
Elementarstruktur und das Wachstum der lebenden Substanz : Wien. — 'WilcGX,
E. v., '95. Spermatogenesis of Caloptenus and Cicada: Bull, of the Museum
of Comp. Zool., Harvard College, Vol. XXVII., Nr. i. — Will, L., '86. Die
Entstehung des Eies von Colymbetes : Z. w. Z., XLIII. — Wilson, Edm. B.,
'92. The Cell-lineage of Nereis: J. M., VI., 3. — Id.. '93. Amphioxus and the
Mosaic Theory of Development : Ibid., VIII . 3. — Id.. '94. The Mosaic Theory of
Development: M^'ood^s Holl Biol. Led., 1894. — Id.. '95, 1. Atlas of Fertilization
and Karyokinesis : New York, Macmillan. — Id., '95, 2. Archoplasm, Centro-
some, and Chromatin in the Sea-urchin Egg: /. M., XI. — Id., '96. On Cleavage
and Mosaic-work: A. Entm.. III., i. — Wilson and Mathews, '95. Maturation,
Fertilization, and Polarity in the Echinoderm Egg: /. M., X., I. — Wolff, Caspar

GENERAL LITERATURE-LIST 357

Priedrich, 1759. Theoria Generationis. — "Wolff, Gustav, '94. Bemerkungen
zum Darvvinismus mit einem experimeiitellen Beitrag zur Physiologie der Entwick-
lung: B. C, XIV., 17.- — Id., '95. Die Regeneration der Urodelenlin.se: Arch.
Enti/2., I., 3. — Woltei's, M., '91. Die Conjugation und Sporenbildung bei
Gregarinen: A. in. A., XXXVII.

ZACH ARIAS, O., '85. tJber die amoboiden Bewegungen der Spermatozoen
von Polyphemus pediculus : Z. w. Z., XLI. — Zacharias, E., '93, 1. (Jber die
chemische Beschaffenheit von Cytoplasma und Zellkern : Ber. deutsch. bot. Ges.,
II., S- — Id., '93, 2. Liber Chromatophilie : Ibid., 1893. — Id., '95. tJber das
Verhalten des Zellkerns in wachsenden Zellen : flora, 81, 1895. — Id., '94. tJber
Beziehungen des Zellenwachstums zur Beschaffenheit des Zellkerns : Berichte der
deiitschen botan. Gesellschaft, XII., 5. — Ziegler, B., '88. Die neuesten Arbeiten
liber Vererbung und Abstammungslehre und ihre Bedeutung fiir die Pathologic :
Beitr. zur path. Anat., IV. — Id., '92. Lehrbuch der allgemeinen pathologischen
Anatomie und Pathogenese, 7th ed. : Jena.- — Ziegler, H. E., '87. Die Entsteh-
ung des Blutes bei Knochenfischenembryonen : A. in. A. — Id., '91. Die biolo-
gische Bedeutung der amitotischen Kerntheilung im Tierreich : B. C, XI. — Id., '94.
tJber das Verhalten der Kerne im Dotter der meroblastischen Wirbelthiere : Ber.
Natiirf. Ges. Freiburg, 1894. — Id., '95. Untersuchungen Uber die Zelltheilung :
Verhaiidl. d. deutsch. Zool. Ges., 1895. — Ziegler and vom Rath. Die amitotische
Kerntheilung bei den Arthropoden : B. C, XI. — • Zimmermann, A., '93,1. Bei-
trage zur Morphologic und Physiologie der Pflanzenzelle : Tubingen. — Zimmer-
mann, K. W., '93. 2. Studien liber Pigmentzellcn, etc: A. m. A., XLI. —
Zoja, R., '95, 1. Sullo sviluppo dei blastomeri isolati dalle uova di alcunc meduse :
A. Eiitni., I., 4; II., I ; II., IV. — Id., '95, 2. Sulla independenza della cromatina
paterna e materna nel nucleo dellc cellule embrionali : A. A., XL, 10.

INDEX OF AUTHORS

Altmann, granule-theory, 21, 22, 28, 31, 224;

nuclein, 240.
Amici, pollen-tube, 162.
Aristotle, epigenesis, 6.
Arnold, fibrillar theory of protoplasm, 19;

leucocytes, 83; nucleus and cytoplasm,

214.
Auerbach, 5; double spermatozoa, 106;

staining-reactions, 127; fertilization, 132.

von Baer, cleavage, 9; cell-division, 46;

egg-axis, 278; development, 295.
Balbiani, spireme-nuclei, 25, 26; mitosis in

Infusoria, 62; chromatin-granules, 78;

yolk-nucleus, 116-121; regeneration in

Infusoria, 249.
Balfour, polar bodies, 183; Unequal division,

273-

Ballowitz, structure of spermatozoa, 34, 100-
104; double spermatozoa, 106.

Van Bambeke, deutoplasm and yolk-nucleus,
116, 117, 121; elimination of chromatin,
116; reduction, 173.

Barry, fertilization, 131.

De Bary, conjugation, 163, 169; cell-division
and growth, 293.

Beale, cell-organization, 21, 22.

Bechamp and Estor, microsome-theory, 21;
microzymas, 22.

Belajeff, spermatozoids, 106, 107; reduction
in plants, 197.

Bellonci, polymorphic nuclei, 82.

Benda, spermatogenesis, 123, 124; Sertoli-
cells, 208.

Van Beneden, cell-theory, i, 4; protoplasm,
19; nuclear membrane, 28; centrosome
and attraction-sphere, 36, 53, 70, 224,230,
232, 233; cell-polarity, 39, 40; cell-divi-
sion, 46, 51, 52; origin of mitotic figure,
53,54; theory of mitosis, 70-75 ; division
of chromosomes, 77 ; fertilization of As-
caris, 134; continuity of centrosomes, 143;

germ-nuclei, 153, 154; centrosome in fer-
tilization, 157; theory of sex, 183; par-
thenogenesis, 202; microsomes, 213;
nucleus and cytoplasm, 214; individuality
of chromosomes, 217; nuclear microsomes,
223; promorphology of cleavage, 281;
germinal localization, 298.

Van Beneden and Julin, first cleavage-plane,
280.

Bergmann, cleavage, 9; cell, 13.

Bernard, Claude, nucleus and cytoplasm,
238, 247; organic synthesis, 248, 261,
326.

Berthold, protoplasm, 19.

Bickford, regeneration in coelenterates, 293,

325-

Biondi, Sertoli-cells, 208.

Biondi-Ehrlich, staining-fluid, 121.

Bischoff, cell, 13.

Bizzozero, cell-bridges, 42.

Blanc, fertilization of trout, 159.

Blochmann, insect-egg, 96; budding of
nucleus, 117; polar bodies, 202; bilater-
ality of ovum, 283.

Bohm, fertilization in fishes, 142.

Bolsius, nephridial cells, 32.

Bonnet, theory of development, 6, 328.

Born, chromosomes in Triton-agg, 245;
gravitation-experiments, 285.

Boveri, centrosome, named, 36, 49 ; a per-
manent organ, 56; in fertilization, 124,
135, 140, 141; continuity of, 143; defini-
tion of, 224; structure, 226, 227; func-
tions, 259; archoplasm, 51, 121, 229;
origin of mitotic figure, 53, 55; mitosis in
Ascaris, z^S; \zx\e\.\es oi Ascaris,6i; the-
ory of mitosis, 71, 72; division of chromo-
somes, 77; origin of germ-cells, no, in,
322; fertilization of Ascaris, 133, 134; of
Pterotrachea, 137; of Echinus, 143, 157;
theory of fertilization, 140, 141 ; of par-
thenogenesis, 202; partial fertilization.

3S9

36o

INDEX OF AUTHORS

140, 259; reduction, 173; maturation in
Ascaris, i'](j, lij ; tetrads, 221 ; centriole,
227, 235; attraction-sphere, 233; egg-
fragments, 258; position of polar bodies,
280.

Brandt, symbiosis, 37; regeneration in Pro-
tozoa, 248.

Brauer, bivalent chromosomes, 61 ; mitosis
in rhizopod, 65; fission of chromatin-
granules, 78; deutoplasm, 117; fertiliza-
tion in Branchipus, 142; parthenogenesis
in Artemia, 156, 202-205; spermatogene-
sis in Ascaris, 184, 187; tetrads, 222;
intra-nuclear centrosome, 225.

Braus, mitosis, 74.

Brogniard, pollen-tube, 162.

Brooks, heredity, 10.

Brown, Robert, cell-nucleus, 13; pollen-
tube, 162.

Briicke, cell-organization, 21, 210, 237, 249.

von Brunn, spermatozoon, 102, 105.

Buffon, organization, 21.

Bunting, germ-cells, 109.

Burger, centrosome, 228.

Biitschli, 5; protoplasm, 17-19; diffused
nuclei, 23; artefacts, 31 ; asters, 34, 230;
cell-membrane, 38; mitosis, 46, 53, 75;
centrosome in diatoms, 65, 224; rejuve-
nescence, 129; cyclical division, 163;
polar bodies, 175; nature of centrosome,
228.

Calberla, micropyle, 148.

Calkins, mitosis in Noctiluca, 65, 67; yolk-
nucleus, 117-121; origin of middle-piece,
123, 125; reduction, 200.

Campbell, fertilization in plants, 160.

Carnoy, muscle-fibre, 34; mitosis, 75; ami-
tosis, 81-83; germ-nuclei, 134.

Castle, egg-axis, 279 ; bilateral cleavage, 281.

Chittenden, organic synthesis, 247.

Chmielevvski, reduction in Spirogyra^ 199.

Chun, amitosis, 83; partial development of
ctenophores, 315.

Clapp, first cleavage-plane, 282.

Clarke, mitosis in gregarines, 67.

Cohn, cell, 13.

Conklin, size of nuclei, 52; union of germ-
nuclei, 153; centrosome in fertilization,
157, 158; unequal division, 275; cell-size
and body-size, 289.

Corda, pollen-tube, 162.

Crampton, reversal of cleavage, 270 ; experi-
ments on snail, 315.

Darwin, evolution, 2, 4; inheritance, 7, 295 ;

variation, 9; pangenesis, 10, 303; gem-
mules, 21, 22.

Dogiel, amitosis, 84.

Driesch, dispermy, 147; fertilization of egg-
fragments, 148; pressure-experiments, 275,
282,309; isolated blastomeres, 308; theory
of development, 312, 317, 328; experi-
ments on ctenophores, 315; ferment-
theory, 327.

Driiner, spmdle-fibres, 35; central spindle,
74, 76; aster, 234.

Diising, sex, 109.

von Ebner, Sertoli-cells, 208.

Eismond, structure of aster, 34.

Elssberg, plastidules, 22.

Endres, experiments on frog's egg, 307.

Engelmann, inotagmata, 22; ciliated cells,

30, 31, 34; rejuvenescence, 129.
von Erlanger, asters, 34; elimination of

chromatin, 117, 121 ; fertilization, 157;

centrosome, 228.
Eycleshymer, first cleavage-plane, 282.

Farmer, reduction in plants, 196, 197.

Fick, fertilization of axolotl, 135, 142.

Field, formation of spermatozoon, 1 23-125:
staining-reactions, 127.

Fischer, artefacts, 31, 213.

Flemming, protoplasm, 19, 31 ; chromatin,
24; cell-bridges, 42; cell-division, 46;
splitting of chromosomes, 51; mitotic fig-
ure, 53; heterotypical mitosis, 60; leuco-
cytes, 72; theory of mitosis, 74; division
of chromatin, 78; amitosis, 80-84, 209;
axial filament, 123; middle-piece, 125, 126;
rotation of sperm-head, 137; spermato-
genesis, 193, 194; astral rays, 231; ger-
minal localization, 298.

Floderus, follicle-cells, 113.

Fol, I, 5, 46; amphiaster, 49, 53; theory of
mitosis, 70; sperm-centrosome, 125; fer-
tilization in echinodermS, 130, 157; poly-
spermy, 140; attraction-cone, 146; vitel-
line membrane, 148; asters, 230.

Foot, yolk-nucleus and polar rings, 119, 121,
150; archoplasm, 121 ; fertilization in
earthworm, 136, 143; entrance-cone, 149.

Foster, cell-organization, somacules, 22.

Frommann, protoplasm, 19; nucleus and
cytoplasm, 214.

Galeotti, pathological mitoses, 67-69.
Gallon, inheritance, 7.
Gardiner, cell-bridges, 42.
Garnault, fertilization in Arion. 155.

INDEX OF AUTHORS

361

Geddes and Thompson, theory of sex, 90.
Van Gehuchten, spireme-nuclei, 25; nuclear

polarity, 26; muscle -fibre, 34.
Giard, polar bodies, 177.
Gilson, spireme-nuclei, 26.
Graf, nephridial cells, 32.
Griffin, fertilization, centrosomes in Thalas-

sema^ 143, 144; structure of centrosome,

235-
Grobben, spermatozoa, 105.
Gruber, diffused nuclei, 23, 26; regeneration

in Sientoi\ 248.
Guignard, mitosis in plants, 59, 78; sperma-

tozoids, 107; fertilization in plants, 157,

159, 161; reduction, 195; centrosome,

224.

Haacke, gemmse, 22.

Haberlandt, position of nuclei, 252.

Hackel, inheritance, 5; cell-organization,
21,22,210; epithelium, 40; cell-state, 41.

Hacker, polar spindles of Ascaris, 58; bi-
valent chromosomes, 61, 62; nucleolus,
91,93; primordial germ-cells, no, II2;
germ-nuclei, 156, 193, 194, 219; reduc-
tion in copepods, 189, 191; polar bodies,
280.

Hallez, promorphology of ovum, 283.

Halliburton, proteids, 239; nuclein, 240, 241.

Hamm, discovery of spermatozoon, 7, 130.

Hammar, cell-bridges, 43.

Hammarsten, proteids, 239.

Hansemann, pathological mitoses, 67, 68.

Hanstein, metaplasm, 15; microsomes, 21.

Hartsoeker, spermatozoon, 7.

Harvey, inheritance, 5; epigenesis, 6.

Hatschek, cell-polarity, 39,40; fertilization,
130.

Heidenhain, nucleus, 24, 25; basichromatin
and oxychromatin, 27, 244; cell-polarity,
39; position of centrosome, 40; leuco-
cytes, 72, 73; theory of mitosis, 74; ami-
tosis, 81 ; staining-reactions, 127, 144;
nuclear microsomes, 223; microcentrum,
227; asters, 234; position of spindle, 277.

Heider, insect-egg, 96.

Heitzmann, theory of organization, 42; nu-
cleus and cytoplasm, 214.

Henking, fertilization, 124, 136; insect-egg,
96; tetrads, 188; reduction, 201.

Henle, granules, 21.

Henneguy, deutoplasm, 117.

Hansen, rejuvenescence, 129.

Herbst, development and environment, 324.

Herla, independence of chromosomes, 156,
219.

Hermann, spermatogonia, 16; central spin-
dle, 52, 74, 76; division of chromatin,
78; spermatozoon, 123-126; staining-
reactions, 127; centrosome, 224.

Herrick, spermatozoon, 105.

Hertvvig, O., i, 5, 7, 15, 21; idioblasts, 22;
cell-division, 46; bivalent chromosomes,
61; pathological mitoses, 67; theory of
mitosis, 75; rejuvenescence, 129; fertiliza-
tion, 132; middle-piece, 135; polyspermy,
140; paths of germ-nuclei, 153; matura-
tion, 175, 180-182; polar bodies, 177;
inheritance, 257,302; laws of cell-division,
276; cleavage-planes, 282; theory of de-
velopment, 312, 317, 322, 328.

Hertwig, O. and R., origin of centrosome,
64; egg-fragments, 145; polyspermy, 148.

Hertwig, R., mitosis in Protozoa, 63, 64, 67;
central spindle, 74; amphiasters in un-
fertilized eggs, 159, 226; conjugation, 167;
reduction in Infusoria, 199.

Hill, fertilization, 135, 143, 157; centro-
sphere, 235.

His, germinal localization, 297.

Hofer, regeneration in Amceba, 249.

Hoffman, micropyle, 148.

Hofmeister, cell-division and growth, 293.

Hooke, R., cell, 13.

Hoyer, amitosis, 81.

Humphrey, centrosome, 225.

Huxley, protoplasm, 3; germ, 5, 295; fer-
tilization, 129, 171 ; evolution and epi-
genesis, 328.

Ishikawa, Nodihica^ mitosis, 65, 67; conju-
gation, 168.

Jordan, deutoplasm and yolk-nucleus, 116,

119; first cleavage-plane, 282.
Julin, fertilization in Siyleopsis, 142.

Karsten, centrosome, 225.

Keuten, mitosis in Eiiglena^ 64.

Klebahn, conjugation and reduction in des-
mids, 199.

Klebs, pathological mitosis, 67, 68; cell-
membrane, 251.

Klein, nuclear membrane, 28; theory of
mitosis, 70, 230; amitosis, 84; nucleus
and cytoplasm, 214; asters, 230.

von Kolliker, i, 5, 7, 9, 13; epithelium, 40;
cell-division, 45; spermatozoon, 98, 122;
inheritance, 257, 302; development, 31 1.

Korschelt, nucleus, 25; amitosis, 81, 83;
movements and position of nuclei, 92,
254-256; insect-egg, 96; nurse-cells, 113,

362

INDEX OF AUTHORS

114; ovarian ova, 115; fertilization, 135;
tetrads in Ophryotrocha, 201 ; physiology
of nucleus, 252, 254-256; polarity of
egg, 287.

Kossel, chromatin, 241; nuclein, 243; or-
ganic synthesis, 247.

Kostanecki, position of centrosome, 40.

Kostanecki and Wierzejski, fertilization of
P/iysa, 131, 136, 143, 159; continuity of
centrosonies, 144; collision of asters, 231.

Krause, polymorphic nuclei, 82.

Kupffer, cytoplasm, 29.

Lamarck, inheritance, 10.

Lamarle, minimal contact-areas, 269.

Lankester, germinal localization, 297.

Lauterborn, mitosis in diatoms, 65, 67.

Lebrun, position of centrosome, 40.

Leeuvvenhoek, spermatozoon, 7 ; fertiliza-
tion, 130.

von Lenhossek, nerve-cell, 16, 33; centro-
some, 224.

Leydig, cell, 14; protoplasm, 17; cell-mem-
brane, 38; spermatozoa, 106; elimination
of chromatin, 117.

Lilienfeld, staining-reactions of nucleins,
242, 243.

Lillie, regeneration in Siejttor, 249.

Loeb, regeneration in coelenterates, 293,
325;* theory of development, 322; envi-
ronment and development, 324.

Lustig and Galeotti, pathological mitoses,
68; centrosome, 224.

Maggi, granules, 21.

Malfatti, staining-reactions of nucleins, 242.

Mark, spiral asters, 57; germ-nuclei, 153;

polar bodies, 175; promorphology of

ovum, 287.
Mathews, pancreas-cell, 31 ; fertilization of

echinoderms, 124, 135, 143, 157; nucleic

acid, 247.
Maupas, sex in Rotifers, 108; rejuvenes-
cence, 129; conjugation of Infusoria, 165,

168.
McMurrich, gasteropod development, 115;

metamerism in isopods, 291.
Mead, fertilization of Chcetopterus, 143;

sperm-centrosome, 226.
Merkel, Sertoli-cells, 208.
Mertens, yolk-nucleus and attraction-sphere,

116-121.
Metschnikoff, insect-egg, 284.
Meves, amitosis, 81-85, 209.
Miescher, nuclein, 240.
Mikosch, protoplasm, 31.

Minot, rejuvenescence, 129; cyclical divi-
sion, 163; theory of sex, 183; Sertoli-cells,
208; parthenogenesis, 202.

von Mohl, protoplasm, 13.

Moore, spermatozoon, 123-126; reduction,
189, 201.

Morgan, fertilization of egg-fragments, 148;
effect of fertilization, 149; numerical rela-
tions of cells, 288; isolated blastomeres,
309; experiments on ctenophores, 315;
on frog's egg, 319.

Nageli, cell-organization, 21 ; micellae, 22,
301; polioplasm, 29; idioplasm-theory,
300.

Newport, fertilization, 130; first cleavage-
plane, 280.

Niessing, axial filament, 123.

Nissl, chromophilic granules, ^ilii 34-

Nussbaum, germ-cells, 88; regeneration in
Infusoria, 248; nucleus, 321.

Overton, germ-cells of Volvox, 98; conjuga-
tion of Spii'ogyra, 169, 170; reduction,
196.

Owen, germ-cells, 88.

Paladino, cell-bridges, 42.

Peremeschko, leucocytes, 83.

Pfeffer, hyaloplasm, 29; chemotaxis of germ-
cells, 145.

Pfitzner, cell-bridges, 42; chromatin-gran-
ules, 78.

Pfliiger, position of spindle, 277; first
cleavage-plane, 280 ; gravitation-experi-
ments, 285; isotropy, 278.

Plateau, minimal contact-areas, 269.

Platner, mitosis, 75 ; formation of spermato-
zoon, 123-125; fertilization of Arion;
maturation, 175, 180.

Pouchet and Chabry, development and en-
vironment, 324.

Prenant, spermatozoon, 123.

Preusse, amitosis, 85, 209.

Prevost and Dumas, cleavage, 9.

Pringsheim, Hautschicht, 29; fertilization,
130. ^

Purkyne, protoplasm, 13.

Rabl, nuclear polarity, 26; cell-polarity, 39,
40, 52; centrosome in fertilization, 157;
individuality of chromosomes, 215.

Ranvier, blood-corpuscles, 38.

vom Rath, nucleus, 26; bivalent chromo-
somes, 61 ; amitosis, 82-84; early germ-
cells, 112; reduction, 189, 192; tetrads.

INDEX OF AUl'HOIiS

363

193; centrosome, 224; attraction-sphere,

234-

Rauber, cell-division and growth, 293.

Ravvitz, spermatogonium, 15; amitosis, 82.

Redi, genetic continuity, 21.

Reichert, cleavage, 9, 46.

Reinke, pseudo-alveolar structure, 19; nu-
cleus, 26, 27, 223; oedematin, 28; cyto-
plasm, 29; asters, 34, 226, 231 ; central
spindle, 74; nucleus and cytoplasm, 214.

Remak, cleavage, i, 9, 264; cell-division,
4Si 46 ; egg-axis, 279.

Retzius, muscle-tibre, 34 ; cell-bridges, 42 ;
end-piece, 104.

Robin, germinal vesicle, 46.

Rosen, staining-reactions, 162.

Roux, cell-organization, 2i; meaning of mi-
tosis, 51, 183, 221, 256; position of spindle,
277; first cleavage-plane, 277, 280; frog-
experiments, mosaic theory, 298; theory
of development, 303; post-generation,

307-

Riickert, pseudo-reduction, 61, 193; fertili-
zation of Cyclops^ 142; independence of
germ-nuclei, 156, 219; reduction in cope-
pods, 189; early history of germ-nuclei,
193, 245; reduction in selachians, 200;
history of germinal vesicle, 245.

Riige, amitosis, 83.

Ryder, staining-reactions, 127.

Sabatier, amitosis, 82.

Sachs, energid, 14 ; laws of cell-division,
265; cell-division and growth, 293; de-
velopment, 322.

St. George, La Valette, spermatozoon, 7, 98 ;
spermatogenesis (terminology), 122.

Sala, polyspermy, 147.

Sargant, reduction in plants, 197.

Schafer, protoplasm, 17.

Scharff, budding of nucleus, 117.

Schaudinn, mitosis in Amceba, 64.

Schevviakoff, mitosis in Euglypha, 63-65.

Schimper, plastids, 98.

Schleicher, karyokinesis, 46.

Schleiden, cell-theory, i ; cell-division, 7 ;
nature of cells, 13 ; fertilization, 162.

Schloter, granules, 28, 223.

Schmitz, plastids, 98 ; conjugation, 160.

Schneider, discovery of mitosis, 46.

Schottlander, multipolar mitosis, 69.

Schultze, M., cells, i, 13, 14; protoplasm,
19.

Schultze, O., gravitation-experiments, 285 ;
double embryos, 318.

Schwann, cell-theory, i ; the egg a cell, 6 ;

origin of cells, 7; nature of cells, 13 ; or-
ganization, 41 ; adaptation, 329.

Schwarz, protoplasm, 19; linin, 24; chem-
istry of nucleus, 28 ; nuclei of growing
cells, 246.

Schweigger-Seidel, spermatozoon, 7,98, 122.

Sedgwick, cell-bridges, 43.

Seeliger, egg-fragments, 258 ; egg-axis, 279.

Selenka, double spermatozoa, 106.

Sobotta, fertilization of mouse, 136, 143.

Solger, pigment-cells, 73 ; attraction-sphere,
224.

Spallanzani, spermatozoa, 7.

Spencer, physiological units, 21, 22 ; devel-
opment, 328.

Strobe, multipolar mitoses, 69.

Strasburger, i, 5 ; cytoplasm, 15 ; proto-
plasm, 19 ; Kornerplasma, 29; centro-
sphere, 49, 232 ; origin of amphiaster,
53 ; multipolar mitoses, 69 ; theory of
mitosis, 74, 76, 77 ; amitosis, 83 ; sper-
matozoids, 107, 108, 126; kinoplasm, 108,
126; staining-reactions of germ-nuclei,
128; fertilization in plants, 135, 160,
162 ; reduction, 188, 195 ; theory of
maturation, 196; organization, 210; in-
heritance, 257, 302 ; action of nucleus,
322.

zur Strassen, primordial germ-cells in Asca-
ris, III.

Van der Stricht, amitosis, 82 ; attraction-
sphere, 224 ; fertilization in Amphioxus,
159.

Stuhlmann, yolk-nucleus, 119.

Tangl, cell-bridges, 42.

Thiersch and Boll, theory of growth, 292.

Treat, sex, 109.

Ussow, micropyle, 97; deutoplasm, 1 17.

Vejdovsky, centrosome, 55 ; fertilization in
Rhynchehnis^ 142 ; metamerism in an-
nelids, 291.

Verworn, cell-physiology, 4 ; cell-organiza-
tion, 21 ; biogens, 22 ; regeneration in
Protozoa^ 249 ; nucleus and cytoplasm,
252 ; inheritance, 327.

Virchow, i ; cell-division, 8, 9, 21, 45-47 ;
protoplasm, 19 ; cell-state, 41.

De Vries, organization, 21 ; pangens, 22 ;
tonoplasts, 37 ; plastids, 170 ; chromatin,
183 ; panmeristic division, 236 ; pangene-
sis, 303; development, 312.

Waldeyer, nucleus, 27 ; cytoplasm, 29 ; cell-

364

INDEX OF AUTHORS

membrane, 38 ; chromosomes, 47 ; amito-
sis, 83.

Walter, frog-experiments, 307.

Wasielevvsky, centrosome, 225.

Watase, theory of mitosis, 75 ; staining-
reactions of germ-nuclei, 127; nucleus and
cytoplasm, 211 ; asters, 226; theory of
centrosome, 228 ; astral rays, 231 ; cleav-
age of squid, 273, 283; promorphology
of ovum, 283, 287.

Weismann, inheritance, 10, 11, 302; cell-
organization, 21; biophores, 22; ids, 27;
somatic and germ cells, 88 ; amphimixis,
130; maturation, 183-185; constitution
of the germ-plasm, 183, 305; partheno-
genesis, 202 ; theory of development, 303-
305. 328.

Went, vacuoles, 37.

Wheeler, amitosis, 8i ; insect-egg, 97;
fertilization in Rlyzostorna^ 157— 159;
plastids, 170 ; bilaterality of ovum,
283.

Whitman, on Harvey, 6 ; cell-organization,
21 ; idiosome, 22 ; polar rings, 150 ; cell-
division and growth, 293 ; theory of de-
velopment, 297, 299.

Wiesner, cell-organization, 21, 137 ; pla-
some, 22 ; panmeristic division, 236.

Wilcox, sperm-centrosome, 123, 124; re-
duction, 189, 200.

Will, chromatin-elimination, 117.

Wilson, fertilization in sea-urchin, 135, 136,
143; paths of germ-nuclei, 152; origin
of linin, 223 ; astral rays, 231 ; centro-
sphere and centrosome, 232-235 ; di-
spermy, 260 ; pressure-experiments, 309 ;
first cleavage-plane, 277 ; experiments on
Amphioxtcs, 308, 319 ; theory of develop-
ment, 317.

von Wittich, yolk-nucleus, 1 18.

Wolff, C. F., epigenesis, 6.

Wolff, G., regeneration of lens, 329.

Wolters, mitosis in gregarines, 67 ; polar
body in gregarines, 199.

Yung, sex, 109.

Zacharias, E., nucleoli, 24, 25 ; of meristem,
27 ; staining-reactions, 127 ; nuclein in
growing cells, 246.

Zacharias, O., amoeboid spermatozoa, 105.

Ziegler, artificial mitotic figure, 75 ; amito-
sis, 83, 84.

Zimmerman, pigment-cells, 73.

Zoja, independence of chromosomes, 156,
219 ; isolated blastomeres, 309.

INDEX OF SUBJECTS

Achromatic figure (see Amphiaster), 50;
varieties of, 57; nature, 229.

Actinosph(Erium, mitosis, 63, 66; regenera-
tion, 248.

Adaptation, 329.

yEqtcorea, metanucleus, 93.

Albumin, 239, 241.

Allolobophora, fertilization, 136; teloblasts,
274.

Amphiaster, 49; asymmetry of, 51, 275;
origin, 49, 75, 231; in amitosis, 81 ; in
fertilization, 134, 140, 142, 156; nature,
260; position, 275-277.

Amitosis, 46, 80; biological significance, 82;
in sex-cells, 209.

Amceba^ 4; mitosis, 64; experiments on,
249.

Amphibia, spermatozoa, 100.

Amphimixis, 130, 171.

Amphioxzis^ fertilization, 153, 159; polar
body, 176; cleavage, 270, 271; dwarf
larvse, 289, 307; double embryos, 308.

Amphipyrenin, 29.

Amyloplasts, 37; in plant-ovum, 98, 160.

Anaphases, 47, 51 ; in sea-urchin egg, 77.

A7iiiocra, gland-cells, nuclei, 26; amitosis.
84.

Anodonta, ciliated cells, 30.

Antipodal cone, 71.

Archoplasm, 51; in developing sperma-
tozoa, loi, 123, 126, in spermatozoids,
108, 126; and yolk-nucleus, 121; nature
of, 229-231.

Argonauta, micropyle, 97.

Arion^ germ-nuclei, 155.

Artefacts, in protoplasm, 31, 213.

Arteinia, chromosomes, 49, 61, 205; par-
thenogenetic maturation, 202-205.

Ascaris, chromosomes, 49; mitosis, 52, 58,
71, 78; primordial germ-cells, no, 332;
fertihzation, 132-134, 141 ; polyspermy,
147; polar bodies, 179; spermatogenesis.

180-182, 184; individuality of chromo-
somes, 215-218; intranuclear centrosome,
225; attraction-sphere, 233; supernum-
erary centrosome, 259.

Aster, 34; asymmetry, 51 ; spiral, 57;
structure and functions, 71 ; in amitosis,
81 ; in fertilization, 138, 157; nature of,
229-231 ; finer structure, 231, 233, 244;
relative size, 275.

Asterias, spermatozoa, 127; sperm-aster,
140 ; fertilization, 143, 146.

Astrocentre, 232.

Astrosphere, 232.

Attraction-cone, 146.

Attraction-sphere, 36, 53, 54; in amitosis,
81; of the ovum, 119; of the spermatid,
125; in resting cells, 224; nature of,
232-235.

Axial filament, 99; origin of, 123.

Axis, of the cell, 38; of the nucleus, 26,
215; of the ovum, 40, 278-280, 298, 319.

Axolotl, fertilization, 131.

Bacteria, nuclei, 23.

Basichromatin, 27, 223; staining-reactions,
223, 245.

Bioblast, 22.

Biogen, 22.

Biophore, 22, 183, 305.

Birds, blood-cells, 46; spermatozoa, 102;
young ova, 119.

Blastomeres, displacement of, 270; in-
dividual history, 273; prospective value,
280, 313; rhythm of division, 290; de-
velopment of single, 298, 307-309, 315,
319; in normal development, 312.

Blennitis, pigment-cells, 73.

Braiichipus, yolk, 117; sperm-aster, 142;
reduction, 188.

Calaiius, tetrads, 190.
Cancer-cells, mitosis, 6

365

366

INDEX OF SUBJECTS

Canthocamptus^ reduction, 190 ; ovarian
eggs, I93r 194-

Cell, in general, 3; origin, 7, 8; name, 13;
general sketch, 14; polarity of, 38; as a
structural unit, 41; structural basis, 16-22,
212; physiology and chemistry, 238; size
and numerical relations, 289-292; in in-
heritance, 7, 295, 328; differentiation of,
311-315; independence of, 323.

Cell-bridges, 42.

Cell-division (see Mitosis, Amitosis), general
significance, 9, 45; general account, 45;
types, 46; Remak's scheme, 46; indirect,
47; direct, 80; cyclical character, 129,
164; equal and reducing or qualitative,
185, 304, 305; relation to development,
264; Sachs's laws, 265; rhythm, 268, 289;
unequal, 270-276; of teloblasts, 271;
energy of, 289; relation to metamerism,
291 ; causes, 292; relation to growth,
293; and differentiation, 312, 323.

Cell-membrane, 38.

Cell-organs, 37; nature of, 21 1; temporary
and permanent, 211, 236.

Cell-organization, 21 ; general discussion,
210-237.

Cell- plate, 52.

Cell-state, 41.

Cell-theory, general sketch, i-io.

Central spindle, 49, 52; origin and function,

. 74-

Centrosome, 17; general sketch, 36; posi-
tion, 39; in mitosis, 49; a permanent
organ, 54, 224; dynamic centre, 56;
historical origin, 67; functions, 70, 259;
in amitosis, 81 ; of the ovum, 91 ; of the
spermatozoon, 99, loi, 123; in fertiliza-
tion, 135, 141, 144, 156-159, 171 ; degen-
eration of, 135, 142, 171, 224; continuity,
143, 227; in parthenogenesis, 156, 203;
nature, 224-229; intra-nuclear, 64, 225;
effect on cytoplasm, 36, 212; supernum-
erary, 260.

Centrosphere, 36, 49, 77; nature of, 232.

Ceratozainia, reduction, 196.

Ceriantfuis^ regeneration in, 293.

ChcEtopterus^ fertilization, 143; sperm-centro-
some, 226.

Chara, spermatozoids, 106.

C/iironomus, spireme-nuclei, 26.

Chorion, 96.

Chromatic figure, 50; origin, 53; varieties,
59; in fertilization, 134.

Chromatin, 24; in meristem, 27; in mitosis,
47; in cancer-cells, 68; of the egg-nucleus,
92; elimination of, in cleavage, iii, in

oogenesis, 117, 121; staining-reactions,
127, 243, 244; morphological organization,
78, 80, 183-185, 215-222, 304, 305;
chemical nature, 28, 241-244; relations
to linin, 223; physiological changes, 244-
247; as the idioplasm, 257, 301, 302; in
development. 321, 322, 326.

Chromatin-granules, 27; in mitosis, 78; in
reduction, 206; general significance, 221,
222, 305 ; relations to linin, 223.

Chromoplast, 37.

Chromatophore, 37, 211 ; in the ovum, 98;
in fertilization, 169.

Chromomere (see Chromatin-granule), 27,
221.

Chromosomes, 27; in mitosis, 47-52; num-
ber of, 48, 49, 154, 219; variation of, 59;
bivalent and plurivalent, 61, 190, 205—
207; division, 77; of the primordial germ-
cell, III; in fertilization, 134, 135, 154;
independence in fertilization, 156, 219;
reduction, 173; in early germ-nuclei, 193;
conjugation of, 199; in parthenogenesis,
203,204; individuality of, 215-221 ; com-
position of, 221, 304, 305; chemistry,
243; history in germinal vesicle, 245; in
dwarf larvae, 258.

Ciliated cells, 30, 34.

Ciona, egg-axis, 280.

Claveliua^ cleavage, 270, 281.

Cleavage, in general, 9; geometrical rela-
tions, 265-278; Sachs's laws, 265; modi-
fications of, 268; spiral, 270; reversal of,
270; meroblastic, 271; under pressure,
275? 309; Hertwig's laws, 276; promor-
phology of, 278; bilateral, 280; rhythm,
290; mosaic theory, 299; half cleavage,
308; and development, 309-320, 323;
partial, 315.

Cleavage-nucleus, 153.

Cleavage-planes, 267; determination of, 277;
axial relations, 280-285, 287.

Clepsine, nephridial cell, 32; polar rings,
150; cleavage, 270.

Closieritim^ conjugation and reduction, 198.

Cockroach, amitosis, 81; orientation of egg,
283.

Coelenterates, germ-cells, 109; regeneration,

325-

Conjugation, in unicellular animals, 163-168;
unicellular plants, 169; physiological mean-
ing, 129, 165.

Contractility, theory of mitosis, 70-74; in-
adequacy, 77.

Copepods, reduction, 190.

Coi'ixa, ovum, 284.

INDEX OF SUBJECTS

367

Crepidula, fertilization, 157; position of
spindles, 277; dwarfs and giants, 2S9.

Cross- furrow, 270.

Crustacea, spermatozoa, 105, 106. '

Ctenophores, experiments on eggs, 314.

Cyclas, ciliated cells, 30.

Cyclops^ ova, 93 ; primordial germ-cells, 112;
fertilization, 142, 156, 218; reduction, 189-
191 ; attraction-sphere, 233; axial rela-
tions, 286.

Cytolymph, 17.

Cytoplasm, 15, 29, 213, 236; of the ovum,
97, 115, 170; of the spermatozoon, 107;
morphological relations to nucleus, 214;
to archoplasm, 230-235; chemical rela-
tions to nucleus, 238, 240, 241; physio-
logical relations to nucleus, 248; in
inheritance, 297, 298, 327; in develop-
ment, 315-320; origin, 327.

Dendrobcena, metamerism, 291.

Determinants, 183, 305.

Deutoplasm, 90, 91, 94; deposit, 115; ar-
rangement, 117, 279; effect on cleavage,
273; re-arrangement by gravity, 285, 319.

Development, i, 6; and cell-division, 264;
mosaic theory, 298; theory of Nageli, 301 ;
Roux-Weismann theory, 303; of single
blastomeres, 298, 307, 315; of egg-frag-
.ments, 217, 258, 285, 315; Hertwig's
theory, 312, 317; Driesch's theory, 313,
317; partial, 315; half and whole, 319;
nature of, 320; external conditions, 323;
and metabolism, 326; unknown factor,
327; rhythm, 328; adaptive character,

329-
Diatoms, mitosis, 67; centrosome, 224.
Diemyctyhis^ yolk, 116; yolk-nuclei, 119.
Differentiation, 264, 296; theory of De Vries,

303; of Weismann, 305; nature and

causes, 311-320; of the nuclear substance,

321 ; and cell-division, 323.
Dispermy, 147, 260.
Double embryos, 308, 319.
Dwarfs, formation of, 258, 289, 307-309,

315; size of cells, 589.
Dyads (Zweiergrappen), 179, 181, 184, 189;

in parthenogenesis, 203-205.
Dyaster, 51.

Dycyemids, centrosome, 36.
Dytiscus, ovarian eggs, 115, 256.

Earthworm, ova, 115; spermatozoon, 125;
yolk-nucleus, 121 ; fertilization, 13c;; polar
rings, 150; spermatogenesis, 200; telo-
blasts, 274.

Echinoderms, spermatozoa, 123; fertiliza-
tion, 143, 157; polyspermy, 147; dwarf
larvse, 258, 289; half cleavage, 306; eggs
under pressure, 309; modified larvae, 324.

EcJiinus, fertilization, 143, 157; centrosome,
235; dwarf larvEe, 258; number of cells,
291.

Egg-axis, 278; promorphological signifi-
cance, 279, 298; determination, 285, 287,
322; alteration of, 319.

Egg-centrosome, 91, 119; degeneration of,
91, 138, 141, 142, 171; asserted persist-
ence, 157-159-

Egg-fragments, fertilization, 97, 145, 148;
development, 217, 258, 285, 289, 315.

Elasmobranchs, spermatozoon, 100, 124;
germinal vesicle, 193, 245; reduction, 200.

Embryo-sac, 160.

Enchylema, 17.

Endoplasm, 29.

End-piece, 100, 104.

End-plate, 64.

Envelopes, of the egg, 96.

Epigenesis, 6, 305, 327, 328.

Equatorial plate, 49, 58, 66; formation, 74.

EuchcBta, tetrads, 190.

Etiglena, mitosis, 64.

Euglypha^ mitosis, 63, 277.

Evolution (preformation), 6, 298, 305, 327,
328.

Evolution, theory of, 3,

Exoplasm, 29.

Fertilization, general aspect, 7; physiologi-
cal meaning, 129; general sketch, 1 30;
^As-caris, 132; mouse, 136; sea-urchin,
138; Ne7-eis, 141; Cyclops, 142; Thalas-
sema, Chatopterus, 143; pathological, 148;
partial, 140, 259; of Myzosioma, 159; in
plants, 160; egg-fragments, 97, 145, 258;
Boveri's theory, 141; general aspect, 171;
Minot's theory, 183; Maupas on, 165.

Fishes, pigment-cells, 73; periblast-nuclei,
83; spermatozoa, 100; young ova, 1 16;
dwarfs, 309.

Flagellates, diffused nuclei, 23.

Follicle, of the egg, 113.

Forjicula, nurse-cells, 115, 255.

Fragmentation, 46.

Fritillaria, spireme, 78.

Frog, ganghon-cell, 16, 33; tetrads, 192;
egg-axis, 278; first cleavage-plane, 280,
282, 298; Roux's puncture experiment,
299; post-generation, 307; pressure-ex-
periments, 309; effect of gravity on the
egg, 285, 319; development of single

368

INDEX OF SUBJECTS

blastomeres, 299, 319; double embryos,
319-

Ganglion-cell, 33; centrosome in, 16, 224.

Gemmfe, 22.

Gemmules, 22, 303.

Genoblasts, 183.

Geophilns, deutoplasm, 1 16; yolk-nucleus,

119.
Germ, 5, 295.
Germ-cells, gen-eral, 7-1 1 ; detailed account,

88; of plants, 97, 106; origin and growth,

108; growth and differentiation, 1 13;

union, 130, 145; results of union, 149;

maturation, 173; early history of nuclei,

193; in inheritance, 295.
Germinal localization, theory of, 296-300,

317-

Germinal spot, 91.

Germinal vesicle, 90; structure, 92 ; in ma-
turation, 179; early history, 193, 245;
movements, 150, 255; position, 287.

Germ-nuclei, of the ovum, 92; of the sper-
matozoon, 99, 103; of plants, ic6, 162;
staining-reactions, 127, 162; in fertiliza-
tion, 132, 153, 160, 257; equivalence, 132,
155, 171; paths, 151; movements, 153;
union, 53; independence, 156, 219; in
Infusoria, 165; early history, 165.

Giant-cells, 25; microcentrum, 227, 228.

Globulin, 239, 241.

Granules (see Microsomes), of Altmann, 21 ;
nuclear, 28; chromophilic, 34; in general,
223.

Gravity, effect on the egg, 96, 286, 319.

Gregarines, mitosis, 67; polar body, 199.

Ground-substance, of protoplasm, 19; of
nucleus, 25.

Growth, and cell-division, 41, 278,293, 312;
laws of, 292.

Gryllotalpa, reduction, 188.

Heterocope, tetrads, 190.

Heterokinesis, 305.

Histon, 243.

Homrfiokinesis, 305.

Ilydrophilits, orientation of egg, 283.

Id, 27; in reduction, 185; in inheritance,

305-
Id ant, 305.
Idioblast, 22, 328.
Idioplasm, theory of, 300; as chromatin,

302; action of, 301, 327, 329.
Idiosome, 22.
Ilyanassa, partial development, 317.

Infusoria, nuclei, 23, 165; mitosis, 62; con-
jugation, 164; reduction, 199.

Inheritance, of acquired characters, 10, 329;
Weismann's theory, 1 1 ; through the nu-
cleus, 5, 135, 248, 257, 262, 302, 327; and
metabolism, 326.

Inotagmata, 22.

Insect-eggs, 96, 283, 284.

Interzonal fibres, 51, 74.

Isopods, metamerism, 291.

Isotropy, of the egg, 285, 287, 312, 315.

Karyokinesis (see Mitosis), 46, 47.
Karyokinetic figure (see Mitotic figure), 50.
Karyoplasm, 15.
Karyosome, 24.

Kinoplasm (archoplasm), in spermatozoids,
108, 126.

Lanthanin, 27.

Leucocytes, structure, 72; division, 83; cen-
trosome, 224, 227; attraction-sphere, 234.

Leucoplasts, of plant-ovum, 98*

Lilium, mitosis, 59; spireme, 78; fertiliza-
tion, 160; reduction, 195-197.

Liinax, spiral asters, 57; germ-nuclei, 153.

Linin, 24, 28; relations to cytoreticulum and
chromatin, 214, 223.

Lociista, orientation of egg, 283.

Loligo, cleavage, 273, 282, 283.

Macrogamete, 167.

Macromeres, 273, 311, 313.

Mammals, spermatozoa, 104; young ova,
119.

Mantle-fibres, 52, 74.

Maturation (see Reduction), 131 ; theoreti-
cal significance, 182; of parthenogenetic
eggs, 202 ; nucleus in, 259.

MedusEe, dwarf embryos, 309.

Meristem, nuclei of, 246.

Metamerism, 291.

Metaphase, 47.

Metaplasm, 15.

Micellae, 22, 301, 327.

Microcentrum, 227.

Microgamete, 167.

Micromeres, 273, 311, 313.

Micropyle, 90, 97.

Microsomes, 21; of the egg-cytoplasm, 94;
nature of, 212, 228, 237; of the astral
systems, 213, 214; of the nucleus, 214,
223; relation to centrosome, 229; stain-
ing-reactions, 244.

Microzyma, 22.

Mid-body, 52, 56.

INDEX OF SUBJECTS

369

Middle-piece, 99, 100, 103; origin, 123,
125; ill fertilization, 135, 137, 143, 157.

Mitosis, 46; general outline, 47; modifica-
tions of, 57; heterotypical, 60; in uni-
cellular forms, 62; pathological, 67;
multipolar, 69; mechanism of, 70-75;
physiological significance, 86, 171, 183,
256; in fertilization, 134; Roux-Weismann
conception of, 104, 305.

Mitosome, 123.

Mitotic figure (see Mitosis, Spindle), 50;
origin, 53; varieties, 57.

Mouse, formation of spermatozoon, 126;
fertilization, 136.

Mtisca, ovum, 96.

Myriapods, spermatozoa, 106; yolk-nucleus,
117.

Jl/yzostoiua, fertilization, 159.

Nebenkern, pancreas-cells, 31 ; archoplas-
mic, 53; of spermatid, 123, 125.

AWtui'tis, pancreas-cells, 31.

Nematodes, germ-nuclei, 134.

Nereis^ asters, 34; perivitelline layer, 94;
ovum, 95; deutoplasm, 96; fertilization,
141 ; sperm-centrosome, 226; attraction-
sphere and centrosome, 233; cleavage,
271, 276; pressure-experiments on, 309,
310.

Nerve-cell, 33.

Net-knot, 24.

Noctihica^ mitosis, 65 ; conjugation, 168.

Nuclear stains, 28, 242.

Nucleic acid, 29, 240; staining-reactions,
243; physiological significance, 247.

Nuclein, 17, 29, 239, 240; staining-reac- .
tions, 242-246; physiological significance,
247.

Nucleo-albumin, 239 ; relations to iiuclein,
241, 243.

Nucleo-proteid, 242, 243.

Nucleolus, 14, 24; in mitosis, 49; of the
ovum, 91-93; physiological meaning, 93;
relation to centrosome, 64, 225.

Nucleoplasm, 15.

Nucleus, general stuucture and functions,
22; finer structure, 27; polarity, 26, 215;
chemistry, 28; in mitosis, 47, 256; of the
ovum, 92; of the spermatozoon, 98, 103;
relation to cytoplasm, 214, 238; morpho-
logical composition, 215; in organic syn-
thesis, 247, 262; physiology, 248; position
and movements, 252-256; in fertilization,
257; in maturation, 259; in later devel-
opment, 321, 327; in metabolism and
inheritance, 326; in inheritance and de-

velopment, 302-311, 317; control of the
cell, 322.
Nurse-cells, 113, 114, 255.

(Edigoniiim, fertilization, 1 30; membrane,

252.
Oocyte, 175, 176.
Oogenesis, 173, 175.
Oogonium, 175, 176.
Oosphere, 97.
Ophiyotrocha, amitosis, 81 ; nurse-cells, 113;

tetrads, 192, 201.
Organism, nature of, 41.
Organization, 41, 210; of the nucleus, 222;

of the cytoplasm, 223; of the egg, 299,

327-

Origin of species, 4.

Osiimnda^ reduction, 196.

Ovary, 89; of Canthocamptus, 194.

Ovum, in general, 5, 6; detailed account,
90; nucleus, 92; cytoplasm, 94; envel-
opes, 96; of plants, 97; origin and growth,
113; fertilization, 129; effects of spermato-
zoon upon, 149; maturation, 175; parthe-
nogenetic, 202; promorphology, 278, 285;
bilaterality, 282; in development, 311,

323-
Oxychromatin, 27, 223; staining-reactions,

244.
Oyster, germ-nuclei, staining-reactions, 127.

Pallavicinia, chromosomes, 49 ; reduction,
196.

Pahidina, dimorphic spermatozoa, 104, 106.

Pangens, 22, 303, 305, 312, 322, 328.

Pangenesis, 10, 303, 305, 312.

Parachromatin, 29.

Paralinin, 29.

Paramcecium^ mitosis, 62; conjugation, 165;
reduction, 199.

Paranucleus, 53; of the spermatid, 123.

Parthenogenesis, centrosome in, 156; preven-
tion of, 91, 176; theories of, 202; polar
bodies in, 202-205.

Petromyzon, fertilization, 142, 147.

Phallusia, fertilization, 143, 153; centro-
sphere, 235.

Physa, fertilization, 131, 144; reversed cleav-
age, 270.

Physiological units, 22.

Pieris, spinning-gland, 25, 254.

Pigment-cells, 72.

Pilularia, fertilization, 160.

Pintis, reduction, 196.

Plant-cells, plastids, 37; membranes, 38;
mitosis, 57, 59; cleavage-planes, 266.

370

INDEX OF SUBJECTS

Plasma-stains, 28, 242.

Plasmosome, 24.

Plasome, 22.

Plastids, 37; of the ovum, 98, 160; of the
spermatozoid, 107; conjugation of, 169;
in fertilization, 170, 171; independence,
211.

Plastidule, 22.

Polar bodies, 131 ; nature and mode of
formation, 175-180 ; division, 179; in
parthenogenesis, 202-205.

Polarity' of the nucleus, 26, 215 ; of the cell,
38; of the ovum, 278, 279, 298; determi-
nation of, 285, 287, 322.

Polar rings, 121, 150.

Pole-plates, 64.

Pollen-grains, formation, 59; division, 195.

Pollen-tube, 106, 161, 162.

Polyclades, cleavage, 313.

Polygordius, cleavage, 269.

Polyspermy, 140, 147; prevention of, 148.

Polystoniella, regeneration, 249, 250.

Porcellio, amitosis, 82.

Predelineation, 297.

Preformation (see Evolution).

Principal cone, 71.

Promorphology (see Cleavage, Ovum).

Pronuclei, 151.

Prophase, 47.

Proteids, 239.

Prothallium, 160; chromosomes in, 196.

Protoplasm, 3, 15; structure, 17, 212; chem-
istry, 238.

Protoplast (see Plastid).

Pseudo-alveolar structure, 19, 94.

Pseudo-reduction, 61, 193, 194, 197, 205.

PterotJ'achea, germ-nuclei, 135, 137, 153.

Ptychoptera, spireme-nuclei, 25.

Pygczra, formation of spermatozoon, 123.

Pyrenin, 25, 29.

Pyrenoid, 98.

Pyrrochoris, tetrads, 188.

Quadrille of centres, 157.

Reduction, general, 173; general outline,
174; parallel between the two sexes, 180,
182; theoretical significance, 182-185;
detailed account, 1 86-1 93; in plants,
195-197; Strasburger's theory of, 196;
in unicellular forms, 198; by conjugation,
199; modes contrasted, 206.

Regeneration, 293, 294; Weismann's theory,
305; in frog-embryo, 307; nature of, 323;
in coelentgrates, 225; of lens, 329.

!\ejuvenescence, 129, 165.

Renilla, ovum, 96, 145.
Rhabdoneina, amitosis, 81.
Rhynchelmis, fertilization, 142; cleavage,
272.

Sagitta, number of chromosomes, 49; pri-
mordial germ-cells, no; germ-nuclei, 135;
sperm-aster, 140.

Salamander, epidermis, 2, 16, 20; sperma-
togonia, 15, 16, 234; nuclei, 24; mitosis
in, 54-56, 60; pathological mitosis, 69;
leucocytes, 72; spermatocyte, 78; amito-
sis, 82; spermatozoa, 125; tetrads, 191,
192.

Sargiis, pigment-cells, 73.

Segmentation (see Cleavage).

Selaginella, spermatozoids, 145.

Senescence, 130, 165.

Sertoli-cells, 183, 208.

Sex, 7; determination of, 109; nature of,
130; Minot's theory of, 183, 208.

Siphonophores, amitosis, 83.

Soma, II.

Somacule, 22.

Somatic cells, 88; number of chromosomes,
176.

Spermary, 89.

Spermatid, 122, 180; development into
spermatozoon, 122-126.

Spermatocyte, 122, 180; of Ascaris, 180,
225.

Spermatogenesis (see Reduction), 173; gen-
eral outline, parallel with oogenesis, 180-
182.

Spermatogonium, 122, 180; early history,
194; of salamander, 234.

Spermatozeugma, 106.

Spermatozoid, structure and origin, 106,
126; in fertilization, 145, 160.

Spermatozoon, discovery, 7; structure, 98;
essential parts, loi ; giant, 105 ; double,
106 ; unusual forms, 106; of plants, 107 ;
formation, 123 ; in fertilization, 131, 136;
entrance into ovum, 145; physiological
significance, 90, 141, 171.

Sperm-centrosome, 99, lOl ; position, 123;
in fertilization, 135, 138, 141, 143, 144,

156-159, 171-
Sperm-nucleus, 99, loi, 103; origin, 122;
in fertilization, 132, 153 ; rotation, 137 ;
path in the egg, 151 ; in inheritance, 252,

257-
Splicer echinus, fertilization, 143 ; number of

cells, 291 ; hybrids, 258.
Spindle (see Amphiaster, Central spindle),

49= 57; origin, 48, 53, 74, 214; in Pro-

INDEX OF SUBJECTS

Z7^

tozoa, 64-67 ; conjugation of, i68 ; nature
of, 230, 231 ; position, 276-278.

Spireme, 27, 47, 77, 193.

Spirochona, mitosis, 62, 63.

Spirogyra, conjugation, 169; reduction,
199.

Spongioplasm, 17.

Spontaneous generation, 5.

Stem-cells, iii, 112.

Steiitor, regeneration, 249, 250.

Stylonychia, senescence, 165.

Symbiosis, 211.

Synapta, cleavage, 267, 268.

Syncytium, 42.

Telophase, 47, 52.

Teloblasts, 271, 291.

Tetrads (Vierergruppen) , 179; origin, 186
in Ascaris, 187 ; in arthropods, 188-193
ring-shaped, 188; in amphibia, 191, 192
origin by conjugation, 199 ; formulas for
186, 193, 200, 201.

Thalassema, fertilization, 143 ; centrosome,
228; attraction-sphere, 235.

ThalassicoUa, experiments on, 250.

Tonoplast, 37.

Toxopiiensies, cleavage, 8 ; mitosis, 76 ;
ovum, 91 ; spermatozoon, 99; fertili-
zation, 146 ; paths of germ-nuclei,

152; polar bodies, 174; double cleavage,

260.
Trachelocerca, diffused nuclei, 26.
Trophoplasm, 301.
Tubidaria, regeneration, 293, 325.
Tunicates, egg-axis, 280; cleavage, 281.

Unicellular organisms, 3 ; mitosis, 62 ; con-
jugation, 163; reduction, 198; experi-
ments on, 248-252.

Unio, cleavage, 272.

Vacuole, 37.

Vanessa, ovarian egg, 115.
Variations, 9 ; origin of, 329, 330.

Vauchei'ia, membrane, 251, 254.
Vitelline membrane, 96; of egg- fragments,
97; formation of, 146; function, 148.

Volvox, germ-cells, 89, 98.

Vorticella, conjugation, 167.

Yellow cells (of Radiolaria), 37, 211.
Yolk (see Deutoplasm), 90, 94.
Yolk-nucleus, 115, 118.
Yolk-plates, 94.

Zwischenkorper (mid-body), 52,
Zygne?na, membrane, 252.
Zygospore, 169.

Columbia University Biological Series.

EDITED BY

HENRY FAIRFIELD OSBORN,

Da, Costa Professor of Zoology in Columbia Cnivernity .

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 Heney Fairfield

OSBORN.

II. Amphioxus and the Ancestry of the TertehrateSc

By Arthur Willey.
III. Fishes^ Living- and Fossil. By Bashford Dean".
IT. The Cell in Development and Inheritance. By

Edmujstd B, Wilson.

Two other vohimes are in preparation.

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 ProfenHor of Zoology in Columbia Unit'ersity .
8vo. Cloth. $2.00, net.

THs 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 vipon critical studies of the original authorities, partly
u23on 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 attemj)ting to establish a comjolete conti-
nuity of thought in the growth of the various elements in the
Evolution idea, and especially in the more critical and exsict
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 ANTiciPATioisr and Interpret atiois^ of IsTature.
II. Among 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 Daravin.
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 WSLLEY5 B.Sc. Lond.,

Tutor in Biology, Columbia Uiviversity ; Balfour Student of the
University of Cainbridge.

8vo. Cloth. $2.50, net.

The purpose of this vohime is to consider the problem of the
ancestry of the Vertebrates from the standpoint of tlie anat-
omy and development of Amphioxns 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
Geoflt'roy St. Hilaire down to our own daj^, 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 Amphioxns
as compared with that of higher Vertebrates. The third chapter
deals with the embryonic and larval develojDment of Amphioxns,
while the fourth deals more briefly with the anatomy, embryology,
and relationships of the Ascidians; then the other allied forms,
Balanogiossus, Cejihalodiscus, 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 plaee^-

The work contains an extensive bibliography, full notes, and
135 illustrations.

TABLE OF CONTENTS.
IXTRODUCTIOK.

Chapter I, Aisr atomy of Amphioxus.
11.. Ditto.

III. Development of Amphioxus.

IV. The Ascidians.

V. The Protochordata ix their Relation to
THE Problem of Vertebrate Descent.

III. FISHES, LIVING AND FOSSIL.

AJV INTBODUCTOBY STUDY.

BY

BASHFORD DEAN, Ph.D., Columbia,

Instructor in Biology, Columbia University.
8vo. Cloth. I2.50, 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 Dipnoans, 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 Diuichthys,
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 G-iinther. 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-lishes. Their Appearance in Time and their
Di.stributiou.
II. The Lampreys. Their Position -with Reference to Fishes. Bdel-
lostoma, Myxiue, Petromyzon, Palaeospondylus.

III. The Shark Group. Anatomical Characters. Its Extinct Members,

Acauthodiau, Cladoselachid, Xeuacanthid, Cestracionts.

IV. Chimaeroids. Structures of Callorhyuchus and Chimaera. Squalo-

raja aud Myriacanthus. Life-habits aud Probable Relationships.
V. Teleostomes, The Forms of Recent " Ganoids." Habits aud Dis-
tribution. The Relations of Prominent Extinct Forms. Crosso-
pterygiaus. Typical " Bony Fishes. "

VI. The Evolution of the Groups of Fishes. Aquatic Metamerism.
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. Summaries.