KiHiffiS
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Marine Biological Laboratory
WOODS HOLE. MASSACHUSETTS
IN MEMORY OF
Edward Gardiner Gardiner
1854-1907
I
THE CELL
IN DEVELOPMENT AND INHERITANCE
Columbia SEnibrrsi'tu Biological Series.
EDITED BY
HENRY FAIRFIELD OSBORN.
I
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. LV.
THE CELL
IN
Development and Inheritance
BY
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
Copyright, 1896,
By the MACMILLAN COMPANY.
3 3//
ICorliioolj ^rcss
J. S. Ciisliing & Co. - Berwick & Smith
Norwood Mass. I'.S.A.
2E0 mg Jricuti
THEODOR BOVERI
V
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 Zcllc iind Gctvcbe
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 egg, — a subject which had
been thrown into such confusion by Fol's extraordinary account of
the " Quadrille of Centres " in echinoderms that it seemed for a time
impossible to form any definite conception of the cell in its relation
to inheritance. By a fortunate coincidence the same task was inde-
pendently undertaken, nearly at the same time, by several other
investigators. The concordant results of these researches led to a
decisive overthrow of Fol's conclusions, and the way was thus cleared
for a return to the earlier and juster views founded by Hertwig,
Strasburger, and Van Beneden, and so lucidly and forcibly developed
by Boveri.
The rapid advance of discovery in the mean time has made it
seem desirable to amplify the original plan of the work, in order to
render it useful to students as well as to more general readers ; and
to this end it has been found necessary to go over a considerable
vii
Vlll PREFACE
part of the ground already so well covered by Hertvvig.^ This book
does not, however, in any manner aim to be a treatise on general
histology, or to give an exhaustive account of the cell. It has rather
been my endeavour to consider, within moderate limits, those features
of the cell that seem more important and suggestive to the student
of development, and in some measure to trace the steps by which our
present knowledge has been acquired. A work thus limited neces-
sarily shows many gaps ; and some of these, especially on the botani-
cal side, are, I fear, but too obvious. On its historical side, too, the
subject could be traced only in its main outlines, and to many
investigators of whose results I have made use it has been impossible
to do full justice.
To the purely speculative side of the subject I do not desire to
add more than is necessary to define some of the problems still to be
solved ; for I am mindful of Blumenbach's remark that while Drelin-
court rejected two hundred and sixty-two "groundless hypotheses"
of development, " nothing is more certain than that Drelincourt's
own theory formed the two hundred and sixty-third." ^ I have no
wish to add another to this list. And yet, even in a field where
standpoints are so rapidly shifting and existing views are still so
widely opposed, the conclusions of the individual observer may have
a certain value if they point the way to further investigation of the
facts. In this spirit I have endeavoured to examine some of the more
important existing views, to trace them to their sources, and in some
measure to give a critical estimate of their present standing, in the
hope of finding suggestion for further research.
Every writer on the cell must find himself under a heavy obliga-
tion to the works of Van Beneden, Oscar Hertwig, Flemming, Stras-
burger, and Boveri ; and to the last-named author I have a special
sense of gratitude. I am much indebted to my former student,
Mr. A. P. Mathews, for calling my attention to the importance of
the recent work of physiological chemists in its bearing on the
problems of synthetic metabolism. The views developed in Chap-
ter VII. have been considerably influenced by his suggestions, and
this subject will be more fully treated by him in a forthcoming work ;
but I have endeavoured as far as possible to avoid anticipating his own
special conclusions. Among many others to whom I am indebted
for kindly suggestion and advice, I must particularly mention my
ever helpful friend. Professor Henry F. Osborn, and Professors
J. E. Humphrey, T. H. Morgan, and F. S. Lee.
In copying so great a number of figures from the papers of other
1 Henneguy's Le^otis sur la cellule is received, too late for further notice, as this volume
is going through the press.
- Allen Thomson.
> PREFACE ix
inv^estigators, 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. VV.
Columbia Univeksitv, Xew York,
July, 1S96.
TABLE OF CONTENTS
INTRODUCTION
List of Figures
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 ...........
Literature ..........•••••
PAGE
XV
I
12
CHAPTER I
General Sketch of the Cell
A. General Morphology of the Cell .
B. Structural Basis of Protoplasm
C. The Nucleus ....
1. General Structure
2. Finer Structure of the Nucleus
3. Chemistry of the Nucleus .
D. The Cytoplasm
E. The Centrosome
F. Other Cell-organs .
G. The Cell-membrane
H. Polarity of the Cell
I. The Cell in Relation to the Multicellular Body
Literature, I. ...... .
14
17
27
28
29
30
37
38
38
41
43
CHAPTER II
Cell-Division
A. Outline of Indirect Division or Mitosis .
B. Origin of the Mitotic Figure ....
C. Modifications of Mitosis .....
1. \^arieties of the Mitotic Figure .
2. Heterotypical Mitosis ....
Bivalent and Plurivalent Chromosomes
Mitosis in the Unicellular Plants and Animals
Patliological Mitoses .....
I). The Mechanism of Mitosis .....
1. Function of the Amphiaster
()/) Theory of I'ihrillar Cijutractility .
{l>) Other Theories ....
2. Division of the Chromosomes
xi
J-
4-
5-
47
53
57
57
00
61
62
f'7
70
70
70
75
77
Xll
TABLE OF CONTENTS
E.' Direct or Amitotic Division
1. General Sketch ......
2. Centrosome and Attraction-sphere in Amitosis
3. Biological Significance of Amitosis
F. Summary and Conclusion .....
Literature, II. . . •
PAGE
80
80
81
82
85
86
CHAPTER III
The Gekm-Cells
A. The. Ovum
1. The Nucleus
2. The Cytoplasm .
3. The Egg-envelopes
B. The Spermatozoon
1. The Flagellate Spermatozoon
2. Other Forms of Spermatozoa
3. Paternal Germ-cells of Plants
C. Origin and Growth of the Germ-cells .
D. Growth and Differentiation of the Germ-cells
1. The Ovum .....
(«) Growth and Nutrition .
(^) Differentiation of the Cytoplasm.
() Composition of the Chromosomes
Chromatin, Linin, and the Cytoret
The Centrosome .
The Archoplasmic Structures
1. Asters and Spindle
2. The Attraction-sphere
Summary and Conclusion
culum
the Chromosomes
A.
B.
C.
D.
E.
F.
Literature, VI. ....
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
Experiments on Unicellular Organisms
Position and Movements of the Nucleus
The Nucleus in Mitosis
The Nucleus in Fertilization
The Nucleus in Maturation
C. The Centrosome . . . ■
D. Summary and Conclusion
I.
2.
3-
4-
5-
Literature, VII.
PAGE
198
199
199
202
205
208
208
209
209
211
212
214
215
221
223
224
229
229
232
236
237
238
239
240
242
248
248
252
256
257
259
259
261
26'?
CHAPTER VIII
Cell-Division and Development
A. Geometrical Relations of Cleavage-forms ....
B. Promorphological Relations of Cleavage ....
1. Promorphology of the Ovum . . . • •
(a) 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 .
265
278
278
278
280
282
285
XIV
TABLE OF CONTENTS
C. The Energy of Division
D. Cell-division and Growth
Literature, VIII. .
289
293
294
CHAPTER IX
Theories of Inhekhance and Development
The Theory of Germinal Localization .
The Idioplasm Theory ....
Union of the Two Theories .
The Roux-Weismann Theory of Development
Critique of the Roux-Weismann Theory
On the Nature and Causes of Differentiation
The Nucleus in Later Development
The External Conditions of Development
Development, Inheritance, and Metabolism
Preformation and Epigenesis. The Unknown Factor in Development
A.
B.
C.
D.
E.
F.
G.
H.
I.
J-
Literature, IX.
Glossary . . . .
General Literature-List
Index ok Authors .
Index of Subjects
296
;oo
;o2
306
3"
321
526
327
33
o
343
359
365
LIST OF FIGURES
lo.
II.
12.
13-
14.
15-
16.
17-
18.
19-
20.
21.
22.
23-
24.
26.
27.
28.
29.
30.
31-
32.
33-
34-
35-
36.
37-
38.
39-
40.
41-
42-
PAGE
Epidermis of larval salamander 2
Amceba Proteus 4
Cleavage of the ovum in Toxopneustes 8
Diagram of inheritance 11
Diagram of a cell 14
Spermatogonium of salamander 15
Group of cells, showing cytoplasm, nu-
cleus, and centrosome 16
Alveolar or foam-structure of proto-
plasm, according to Biitschli 18
Living cells of salamander, showing
fibrillar structure 20
Nuclei from the crypts of Lieberkiihn. 24
Special forms of nuclei 25
Diffused nucleus in Trackelocerca. ... 26
Ciliated cells 30
Nephridial cell of Clepsine 32
Nerve-cell of frog 33
Diagram of dividing cell 35
Diagrams of cell-polarity 39
Remak's scheme of cell-division 46
Diagram of the prophases of mitosis. . 48
Diagram of later phases of mitosis. ... 50
Prophases in salamander cells 54
Metaphase and anaphases in salaman-
der cells 55
Telophases in salamander cells 56
Middle phases of mitosis in Ascaris-
eggs 58
Mitosis in pollen-mother-cells of lily. . 59
Heterotypical mitosis 60
Mitosis in Infusoria 62
Mitosis in Euglypha 63
Mitosis in Euglena 64
Mitosis in Noctiluca 65
Mitosis in Actiiiosphcsrium 66
Pathological mitoses in cancer-cells.. . 68
Pathological mitosis caused by poisons 69
Mechanism of mitosis in Ascaris 71
Leucocytes 72
Pigment-cells 73
Mitosis in the egg of Toxopneustes. ... 76
Nuclei in the spireme-stage 78
Early division of chromatin in Ascaris 79
Amitotic division 81
Volvox 89
Ovum of Toxopneustes gi
43-
44-
45-
46.
47-
48.
49-
50-
51-
52-
53-
54-
55-
56.
57.
58.
59-
60.
61.
62.
63-
64.
65-
66.
67.
68.
69.
70.
71-
72.
73-
74-
75-
76.
77-
79-
80.
81.
82.
PAGE
Ovum of Nereis 95
Insect-egg 96
Micropyle in Argonauta 97
Germ-cells of / 'olvox 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 Ckara 106
Spermatozoids of various plants 107
Germ-cells of Hydractinia 109
Primordial germ-cells of Ascaris, no
Primordial germ-cells of Cyclops 112
Egg and nurse-cell in Opkryotrocha . . . 114
Ovarian eggs of insects 115
Young ovarian eggs of various animals 116
Young ovarian eggs of birds and mam-
mals iiS
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 'Ihalas-
seina 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 Infusori.i. . 164
Conjugation of Paramaccium 166
XVI
LIST OF FIGURES
83-
84.
85-
86.
87.
88.
8g.
90.
91-
92.
93-
94-
95-
96.
97-
98.
99.
102.
103.
104.
106.
107.
108.
109.
no.
PAGE
Conjugation of I 'orticella 167 112.
Conjugation of .\octiliica 168 j 113.
Conjugation of Spi7-ogyra 169 !
Polar bodies in Toxopneustes 174 j 114.
Genesis of the egg 175 ' 1 15.
Diagram of formation of polar bodies 177 ' 116.
Polar bodies in .-{scans 178 i
Genesis of the spermatozoon 180 j 117.
Diagram of reduction in the male. .. . 181 ! 118.
Spermatogenesis of Asca) is 184 '
Tetrads of Gryllotalpa 188 I 119.
Tetrads and polar bodies in Cyclops.. 189
Diagrams of tetrad-formation in ar- ! 120.
thropods 191 121.
Germinal vesicles and tetrads 192 122.
Ovary of Caiitkocamptiis 194 123.
Possible tetrad-formation in the lily. . 197 124.
Conjugation and reduction in Closte- 125.
rnim 198 126.
First type of parthenogenetic matura- 127.
tion in Ariemia 203 128.
Second type of parthenogenetic mat- 129.
uration in .Irtemia 204 130.
Modes of tetrad-formation contrasted 206 131.
.Abnormalities in the fertilization of 132.
.hcaris 216 133.
Individuality of chromosomes in .As-
caris 217 134.
Independence of chromosomes in fer-
tilization of Cyclops 218 135.
Hybrid fertilization of .4scaris 220
Mitosis with intra-nuclear centrosome' 136.
in .Ascar/s 225
Diagram of different types of centro- , 137.
some and centrosphere 233
Structure of the aster in spermatogo- 138.
nium of salamander 234 139.
History of chromosomes in the germi- 140.
nal vesicle of sharks 245 141.
Nucleated and enucleated fragments
of Stylonychia 249 142.
P.AGE
Regeneration in Stentor 250
Nucleated and enucleated fragments
of Amoeba 25 1
Position of nuclei in plant-cells 253
Ovary of Forficula 255
Normal and dwarf larvae of sea-
urchins 258
Supernumerary centrosome in .Ascaris 260
Cleavage of dispermic egg of Toxo-
pneustes 261
Geometrical relations of cleavage-
planes in plants 266
Cleavage of Synapta 268
Cleavage of Polygordius 269
Cleavage of Nereis 271
Variations in the third cleavage 272
Meroblastic cleavage in the squid. . . . 273
Teloblasts of the earthworm 274
Bilateral cleavage in tunicates 281
Bilateral cleavage in Loligo 282
Eggs of Loligo 283
Eggs and embryos of Corixa 284
Variations in axial relations of Cyclops 286
Half-embryos of the frog 299
Half and whole cleavage in sea-urchins 306
Normal and dwarf gastrulas of Amptii-
oxus 307
Dwarf and double embryos of .Amphi-
o.vus 308
Cleavage of sea-urchin eggs under
pressure 309
Cleavage of yV^rifw-eggs under press-
ure 310
Diagrams of cleavage in annelids and
polyclades 313
Partial larvae of ctenophores 314
Partial cleavage in Ilyaiiassa 316
Double embryos of frog 318
Normal and modified larvse of sea-
urchins 324
Regeneration in coelenterates 325
INTRODUCTION
-ooJ^^Oo-
"Jedes Thier erscheint ah eine Sunune vitaUr Einheiten, von denen jede den vollen
Charakter des Lebens an sich tragt." ViRCHOW.'
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 Qgg, 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
^ Cellnlarpathologie , p. 12, 1S5S.
B I
^ 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 witli 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-
a
F IWt'-'^^-'^^
\
I "^l^^^^r
X
b
did triumphs. The study of micro.scopical 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 oHgin of living
forms; and ev;en at the present day the fundamental problems k
organization with which the cell-theory deals, are far less accessible
o2Trt "T""^\ '^'" '^"■'^^ suggested by the more obvious,
cxteinal characters of plants and animals. Only within a few years
INTR OD UCTION 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
ox protoplasm, 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 he taken with 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
■;-.v-..N
• -■.;-■:■-■*«- ■• '-.{1 ;-i f ■• c (""n ^i^'To '.V ~
C V
Fig. 2. — Amxba 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 ccll-
pJiysiologyy ^
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, AUgeiiteine Physiologic, p. 53, 1895.
INTRODUCTION
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 ccll-nncleus as the vehicle of inheritance, made independently and
almost simultaneously in 1884-85 by Oscar Hertwig, Strasburger,
Kblliker, and Weismann, must be recognized as the first definite
advance '1 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 ^zg^ 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 egg is the terminus from which all fowls, male
and female, have sprung, and to which all their lives tend — it is the
result which nature has proposed to herself in their being. And
thus it comes that individuals in procreating their like for the sake
of their species, endure forever. The egg, I say, is a period or por-
tion of this eternity."^
This passage appears at first sight to be a close approximationto
the modern doctrine of germinal continuity about which all theories
of heredity are revolving. To the modern student the germ is, m
Huxley's words, simplv 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
superficiallv 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 vieu- in 1866— only 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 heredUary
characters, the external plasma on the other hand for accommodation or adaptation to the
external world" {Gen. Morph., p. 287-9).
-De Genera Hone, 1651; Trans., p. 271.
3 Evolution in Biology, 1878; Science and Culture, p. 291.
6 INTRODUCTION
the influence of a mysterious " calidniii iuuatiiiu,'' out of not-living
matter. Whitman, too, in a recent brilHant essay,i has shown how-
far Harvey was from any real grasp of the law of genetic continuity,
which is well characterized as the central fact of modern biology.
Neither could the great physiologist of the seventeenth century have
had the remotest conception of the actual structure of the egg. The
cellular structure of living things was not comprehended until nearly
two centuries later. The spermatozoon was still undiscovered, and the
nature of fertilization was a subject of fantastic and baseless specu-
lation. For a hundred years after Harvey's time embryologists
sought in vain to penetrate the mysteries enveloping the beginning
of the individual life, and despite their failure the controversial writ-
ings of this period form one of the most interesting chapters in the
history of biology. By the extreme "evolutionists" or " praeforma-
tionists" the &gg was believed to contain an embryo fully formed in
miniature, as the bud contains the flower or the chrysalis the butter-
fly. Development was to them merely the unfolding of that which
already existed ; inheritance, the handing down from parent to child
of an infinitesimal reproduction of its own body. It was the service
of Bonnet to push this conception to its logical consequence, the
theory of cviboitcmcnt or encasement, and thus to demonstrate the
absurdity of its grosser forms ; for if the egg contains a complete
embryo, this must itself contain eggs for the next generation, these
other eggs in their turn, and so ad infinitiiui, 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 observation that the egg
does not at first contain any formed embryo 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, previously non-
existent as such. This is somewhat as Harvey, himself following
Aristotle, had conceived it — a process of epigoicsis as opposed to
evolution. Later researches established this conclusion as the very
foundation of embryological science.
But although the external nature of development was thus deter-
mined, the actual structure of the egg and the mechanism of inheri-
tance remained for nearly a century in the dark. It was reserved
for Schwann (1839) ^I'ld his immediate followers to recognize the
fact, conclusively demonstrated by all later researches, that the ej^x
is a eel! having the same essential structure as other cells of the
'^Evolution and Epigenesis, Wood's HoU Biological Lectures, 1894.
INTRODUCTION 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 observers as parasitic
animalcules or infusoria, a view which gave rise to the name spcruia-
to.zoa (sperm-animals) by which they are still generally known. ^ As
long ago as 1786, however, it was shown by Spallanzani that the
fertilizing power must lie in the spermatozoa, not in the liquid in
which they swim, because the spermatic fluid loses its power when
filtered. Two years after the appearance of Schwann's epoch-mak-
ing work Kolliker demonstrated (1841) that the spermatozoa arise
directly from cells in the testis, and hence cannot be regarded as
parasites, but are, like the ovum, derived from the parent-body. Not
until 1865, however, was the final proof attained by Schweigger-
Seidel and La Valette St. George that the spermatozoon contains
not only a nucleus, as Kolliker believed, but also cytoplasm. It
was thus shown to be, like the Qgg, a single cell, peculiarly modified
in structure, it is true, and of extraordinary minuteness, yet on the
whole morphologically equivalent to other cells. A final step was
taken ten years later (1875), when Oscar Hertwig established the
all-important fact that fertilization of the egg is accomplished by
its union with one spermatozoon, and one only. In sexual repro-
duction, therefore, each sex contributes a single cell of its own body
to the formation of the offspring, a fact which beautifully tallies
with the conclusion of Darwin and Galton that the sexes play, on
the whole, equal, though not identical parts in hereditary trans-
mission. The ultimate problems of sex, fertilization, inheritance,
and development were thus shown to be cell-problems.
Meanwhile, during the years immediately following the announce-
ment of the cell-theory the attention of investigators was especially
focussed upon the question : How do the cells of the body arise .-^
Schwann and Schleiden held that cells might arise in two different
ways ; viz. either by the division or fission of a pre-exi.sting mother-
cell, or by " free cell-formation," new cells arising in the latter case
not from pre-existing cells, but by crystallizing, as it were, out of
a formative or nutritive substance, termed the " cytoblastema." It
was only after many years of painstaking research that " free ccll-
1 The discovery of the spermatozoa is generally accredited to Ludwig Ilamm, 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
A
B
H
C
1
Fig. 3. — Cleavage of the ovum of the sea-urchin Toxopneusles, X 330, from hfe. The suc-
cessive divisions up t'o the i6-cell stage {H) occupy about two hours. / is a section of the embryo
(blastula) of three hours, consisting of approximately 128 cells surrounding a central cavity or
blastocoel.
celbila 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, filr Path. Ana/., VIII., p. 23, 1855.
INTR ODUC TION 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 ^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 ^fgg has spHt 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 egg,
was observed long before its meaning was understood. It seems to
have been first definitely described in the case of the frog's egg, by
Prevost and Dumas (1824), though earlier observers had seen it; but
at this time neither the &gg nor its descendants were known to be
cells, and its true meaning was first clearly perceived by Bergmann,
Kolliker, Reichert, von Baer, and Remak, some twenty years later.
The interpretation of cleavage as a process of cell-division was fol-
lowed by the demonstration that cell-division does not begin with
cleavage, but can be traced back into the foregoing generation ; for the
egg-cell, as well as the sperm-cell, arises by the division of a cell pre-
existing in the parent-body. It is therefore deidved by direct descent
from an egg-cell of the foregoing generation, and so on ad infimtnm.
Embryologists thus arrived at the conception so vividly set forth by
Virchow in 1858 ^ of an uninterrupted series of cell-divisions extend-
ing backw^ard 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 acquired char-
1 See tlie quotation from the original edition of the Celliilarpatliologie at the head of
Chapter II., p. 45.
lO INTRODUCTION
acters ; i.e. changes that arise in the course of the individual Ufe as
the effect of use and disuse, or of food, cHmate, and the Hke. The
inheritance of congenital characters is now universally admitted, but
it is otherwise v^'ith 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 XXVIl.
- The Im7u of Heredity, Baltimore, 18S3.
3 Ueber Vcrerhiin:^, 1S83. See Essays jipon Heredity, I., by A. Weismann, C'laremloii
Press, Oxford, 1S89.
iNrRODUcriox
1 1
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, w^e 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 germ-cell, not from the parent-body, and the
germ-cell owes its characteristics not to the body which bears it, but
to its descent from a pre-existing germ-cell of the same kind. Thus
the body is, as it were, an offshoot from the germ-cell (Fig. 4). As
" .S Line of succession.
(T) Line of inheritance.
G
Fig. 4. — Diagram illustrating Weismann's theory of inheritance.
G. The germ-cell, which by division gives rise to the body or soma (5) and to new germ-cells
(f?) 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-
1 2 INTR OD UCTION
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.
SOxME GENERAL WORKS ON THE CELL-THEORY
Bergh, R. S. — Vorlesungen liber die Zelle unci die einfachen Gewebe : Wiesbaden,
1S94.
Delage, Yves. — La Structure du Protoplasma et les Theories sur Tlieredite et les
grands Problemes de la Biologie Generale : Paris, 1895.
Geddes & Thompson. — The Evolution of Sex : Ne-d) York, 1890.
Henneguy, L. F. — Lecons sur la cellule : Paris. 1896.
Hertwig, 0. — Die Zelle und die Gewebe: Fischer, Jena, 1892. Translation, pub-
lished by Macinillaii, Londoti and N'ew York, 1895.
Huxley, T. H. — Review of the Cell-theory : British and Foreign Medico-Chirurgical
Review, XH. 1853.
Minot, C. S. — Human Embryology: New York, 1892.
Remak, R. — Laitersuchungen liber die Entwicklung der Wirbelthiere : BerliUy
1850-55.
Schleideri, M. J. — Beitrage zur Phytogenesis : Miiller's Archiv, 1838. Translation
in Sydenham Soc, XH. 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. Philadelpliia. 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. IVew 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 n3.Vi\Q 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 bv
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 nucleus,'^ which in turn may contain a still
^ UnteisHchungen, p. 227, 1839.
- 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 (1840) to designate the
formative material of young animal embryos.
■* First described by Robert Brown in 1833.
13
14
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.
I Plasmosnme or
true nucleolus.
Chromatin-
network.
Nucleus -i
Linin-network.
Karyosome or
net-knot.
Plastids lying in the
cytoplasm.
Vacuole.
Lifeless bodies (meta-
plasm) suspended in
the cytoplasmic reticu-
lum.
Fig. 5. — Diagram of a cell. Its basis consists of a thread-work (viitome, or teticuluiii) com-
posed of minute granules (//uc/oso/?ies) 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 (^Elora, '92, p. 57) to designate the
essential living i)art 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-
jiriale term has not come into general use. (See also Elora, '95, p. 405.)
GENERAL MORPHOLOGY OE THE CELL
15
protoplasm which forms its living basis is a viscid, translucent,
granular substance, often forming a network or sponge-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 nictaplasiu (Hanstein) in contradis-
tinction to the Xw-xw-^ protoplasm ; but
this convenient term is not in general
use. Among the lifeless products of
the protoplasm must be reckoned
also the cell-wall or vicDibranc 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 iincleiis, which lies in its
interior (Fig. 5). Both structurally Fig. 6. — A resting cell {ipermatogo-
oiArl /^Ko,-.-,;,^oU,r frU^r.^ 4- ^ 4- 1 riiiim) from the testis of the salamander,
and cnemicaliy these two parts show , • .u . ■ 1 . m, .11
J 1 li o oi.wvv showing the typical parts. Above, the large
differences of so marked and constant nucleus, with scattered masses of chro-
a character that they must be re- "i^^''". .''"i™work and membrane.
■' Around it, the cyloplasuiic tlircad-work.
garded as the most important of all Below, the attract'ion-sphere (a) and cen-
protoplasmic differentiations. The ^'■«^o'»*= W- [After Rawitz.]
nuclear substance is therefore often designated as nucleoplasm or
karyoplasm ; 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.
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 com.mon structural basis, morphologically continuous
A
B
C
D
P'g- 7- — Various cells showing the typical parts.
A. From peritoneal epithelium of the salamander-larva. Two centrosomes at the right.
Nucleus showing net-knots. [P'LEMMING.]
B. Spermatogonium of frog. Attraction-sphere (aster) containing a single centrosome.
Nucleus with a smgle |ilasniosome. [Hermann.]
C. Spinal ganglion-cell of frog. Attraction-sphere near the centre, containing a single centro-
some with several centrioles. [Lenhossek.]
D. Spermatocyte of Proleus. 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 tliat the nucleus may move actively throui^h the cvtoplasm, as occurs iluring
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 1/
"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 niicli'iH, 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 centrosovie (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 ground-substance, cyto-
lyiupJi, or cncJiylcma, is more liquid, while the other, the spongio-
plasni or reticulum, 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
i8
GENERAL SKETCH OE THE CELL
zoologists and botanists regard protoplasm as essentially a liquid, or
I'ather 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
r
A
c
Fig. 8. — Alveolar or foam-structure of protoplasm, according to Biitschli. [BuTSCHLI.]
A. Epidermal cell of the earthworm. B. Aster, attract ion-sphere, and centrosome from sea-
urchin egg. C. Intra-capsular protoplasm of a radiolarian {Thalasucolla) 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-
1 " Wabenslriik/ur. "
STRUCTURAL BASIS OF PRO'J'OPI.ASM IQ
blance to actual protoplasm, and that drops of oil-emulsions suspended
in water may even exhibit amoeboid changes of form.
Opposed to Butschli's conception is the view, first clearly set forth
by Frommann and Arnold ('65-'67), and now maintained by such
authorities as Flemming, 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
tlircad-ivork or rcticiduvi ("Geriistwerk," "Fadenwerk" of German
writers) in contradistinction to the more liquid gnv -d-substance. 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 t)ne of very wide occurrence.
20
GENERAL SKETCH OF THE CELL
\
-^i:f-. S^"
*^.
I /
r^(
/_
:y7
A
. ' ,, ^ ^ i._"^
\
^.,
H^lfff^-^-
VI
1
C
D
^ ;•
-\
iogens (Ver-
worn) ; inicrozy»ins (Bechamp and Estor) ; getnmct (Haacke).
THE XL- CLE us 23
eel], and through this is especially concenicd 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-activit)^ 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 St rue tare
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
cdiate Infusoria the body contains nuclei of two kinds, viz. a large
macronnclcus and one or more smaller inicronuclei. 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 Ti^acJielocerca 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
cell-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, 1 1): —
a. The nuclear membrane, a well-defined delicate wall which gives
the nucleus a sharp contour and differentiates it clearly from the
surrounding cytoplasm.
/;. The nuclear reticulum. This, the most essential part of the
nucleus, forms an irregular branching network or reticulum 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 (Flemmino)
on account of its very marked staining capacity wlien treated witli
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 linin (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 plasmosovics (Figs. 5, 7, B,
10), are of spherical form, and
by treatment with differential
stains such as ha;matoxylin and
eosin are found to consist typi-
T- k'^,'-m°'~T"° ""'''" ^'■°'" ^'^'^ ">'P'^ °^ cally of a central mass stainino-
Lieberkuhn in the salamander. [Heidenhain.I in,/ <-! >. i ^
The character of the chromatin-net.ork , ^^'^ Cytoplasm, SUrrOUuded
(^a«c/zw;«a//«) is accurately shown. The upper '^Y ^ shell which Staius like
nucleus contains three plasmosomes or true chromatin. Those of the Other
nucleoli ; the lower, one. A few fine linin-threads r "J. c ui uic OLUer
(oxychromathi) are seen in the upper nucleus lOrm, the " Uet-kuotS " (NetZ-
running off from the chromatin-masses. The \x\Ol<
r^y^yi
.v>'
.'-v
yy-.'V'
5
^^^--iaa"y..>S'?:'i;fe*vr; '""'"''•*"'''
C E
Fig. II. — Special forms of nuclei.
A. Permanent spireme-nucleus, salivary gland of Chironotn us larva. Chromatin in a single
thread, composed of chromatin-discs (chromomeres), terminating at each end in a true nucleolus
or plasmosome. [BalbiaN'I.]
B. Permanent spireme-nuclei, intestinal epithelium of dipterous larva Ptychoptera. [VAN
Gehuchten.] C. The same, side view.
D. Polymorphic ring-nucleus, giant-cell of bone-marrow of the rabbit ; <■, a group of centro-
somes or centrioles. [Heidenhain.]
E. Branching nucleus, spinning-gland of butterfly larva {Pier is). [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
r.ubstance (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 epithelia.1 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 {CJiiroiiomus), 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. w, 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 Aui-
locra 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. Fmbryonic cells are in general
Fig. 12. — An infusorian, Trachelo-
cerca, with diffused nucleus consi=ting of
scattered chromatin-granules. [Gruber.]
^ ( 'f. the polarity of tlie cell,
THE NUCLEUS 27
characterized by the large size of tlie 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. Fi)ur St nut lire 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 chrojuosovies (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 {cJu'oniomeres, 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
cJirouiosoines (Figs. 7, D, 38, 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 oxycJironiatin-^x-^xwXo^'s, from the basic/i7'0]natiu-gr:x\M\\ts
of the chromatic network. Like the latter, the oxychromatin-granules
are suspended in a non-staining clear substance, for whicli 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. Chemistry of the Nucleus
The chemical nature of the various nuclear elements will be considered in
Chapter VII., and a brief statement will here suffice. The following classification
of the nuclear substances, proposed by Schwarz in 1887. has been widely accepted,
though open to criticism on various grounds.
1. CJiroDiatin. The chromatic substance (basichromatin) of the network and of
tliose nucleoli known as net-knots or karyosomes.
2. Li)ii)i. The achromatic network and the spindle-fibres arising from it.
THE CYTOPLASM 29
3. Paralinin. The ground-substance.
4. Pyrenin or Parachromatin. The inner mass of trae nucleoH.
5. Amphipyrenin. The substance of the nuclear membrane.
CJiroiiiatin is probably identical with nitclein (p. 240). which is a compound of
«7^c/^/t rt67V/ (a com ple.x 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 Unin 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. Amphipyrenin 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 endoplasm in which the
nucleus lies, and an outer cortical substance or exoplasm (ectoplasm)
from which the more differentiated products of the cytoplasm, such
as cilia, trichocysts, and membrane, take their origin. Indications of
a similar differentiation are often shown in the tissue-cells of higher
plants and animals,^ though it may take the form of a polar 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 reason 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 ils
importance has since been urged by Waldeyer, Reinke, and others. The cortical layer is
by Kupffer termed paraplasm, by Pfeffer hyaloplasm, by Pringsheini the Haiiischiclit. The
medullary zone is termed by Kupffer, protoplasm, sensu strictti ; by Strasburger Korner-
plastna. bv Nageli polioplasrn.
2 Cf. p'. 38. '
3 See Kupffer ('90), pp. 473-476-
^o
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
\ \ ■■.
:as^-;
(OinBiitiiMiinnitiaiunuiiiif
i;;;;f.'!!;;:;;;.'i,'::i!ti!i:;|i;;i;.'i,'
!j;;|ii;;j;;;;;;;;;|;i!i;fi;||;;;i!!f
B
uw*«i»"»*"'!"*,"'";"""i:
C
D
Fig. 13. — Ciliated cells, showing cytoplasmic fibrill^ terminating in a zone of peripheral
microsomes to which the cilia are attached. [Engelmann.]
A. From intestinal ii\)\\.\-\fi\\\xm oi A>iodo)ita. B. Yxom. g\\\ o{ Aiwdonta. 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 protcids, 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-
serv^cd in microscopical sections. Bijtschli 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 sinudacruui 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 j^reserved 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 fibrillse 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 Nccturus, 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 Ncbenkcni. 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.
?^^^iiu
•"■•■■■'■.•'• • "••'i
ijpfr--;-
%ry ■-:-■■:-.
*• * :• <-
• *z*\
Fig. 14. — Section through a nephridial cell of the leech, Clepsine (drawn bv 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 inem-
brane, is traversed by a very distinct linin-network, contains nuinerous scattered chromatin-
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 Bofeius and Graf in the ne-
THE CYTOPLASM
II
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,
';S?»«»."tv
Fig. 15. — Spinal ganglion-cell of the frog. [VON Lenhossek.]
The nucleus contains a single intensely chromatic nucleolus, and a paler linin-network with
rounded chromatin-granules. The cytoplasmic fibrillae are faintly shown passing out into the
nerve-process below. (They are figured as far more distinct by Plemming.) The dark cyto-
plasmic masses are the deeply staining " chromophilic granules" (.\issl) of unknown function.
(The centrosome, which lies near the centre of the cell, is shown in Fig. 7, 6".) 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-
D
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
fibrillse corresponding in number with the cilia as if continuous with
their bases (Fig. 13).^ In nerve-fibres the threads form closely set
parallel fibrillae which may be traced into the body of the nerve-cell ;
here, according to most authors, they break up into a network in
which are suspended numerous deeply staining masses, the "chromo-
philic granules" of Nissl (Fig. 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 fibrillae group themselves
in two radiating systems or asters, which are in some manner the
immediate agents of cell-division. Similar radiating systems of fibres
occur in amoeboid cells, such as leucocytes (Fig. 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 fibrillae
being regarded as the optical sections of thin plates or lamellae
which form the walls of closed chambers filled by a more liquid
substance. Biitschli, followed by Reinke, Eismond, Erlanger, and
others, 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, B). 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
^ 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.
Aster.
Spindle.
Chromosomes forming the equatorial plate.
Fig. 16. — Diagram of the dividing cell, showing the mitotic" figure and its relation to the cyto-
reticuluni.
different cells and even in different physiological phases of the
same cell ; and that it is imj^ossible 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 \'I.
36 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 dynaviic 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 Qgg. 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 centrosphere (Figs. 5, 6, 7).- 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 P.overi ('88, 2, p. 68). '
2 Cf. p. 229.
^ 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 3.s p last ids or proto-
plasts {¥\g. 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 cJiromatopJiorcs or clironioplasts,
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 OF 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 Aviceba 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
" pelhcle " of Blitschli — 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 Q^Yi^^Or^ ; 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 orga/ac
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 {^"^l) 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 egg, which is so important
a factor in development, with that of the tissue-cells; for the egg-
r-A t
tt
A
Van Beneden.
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, B in a gland-cell, 6" 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 jn 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 Q,g^ is one of the primary con-
ditions of development, and we have here, as I believe, a clue to its
origin.^
' Cf. pp. 288, 320.
THE CELL LN RELATLON TO THE MULTLCELLULAR 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 /// 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
^ Unkrsuckinigen, p. igi.
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
judicc. 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
1 Cf. Chapters VIII., IX.
2 For a fuller discussion see pp. 293 and 311.
THE CELL IX RELATION TO THE MULTICELLULAR BODY 43
bridges not only with one another, but also xcit/i the ovnvi, a conclu-
sion which, if estabhshed 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 Fcripatits 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 Ainphioxiis 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 Elementarorganismen 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. — Untersuchungen iiber mikroskopische Schaume und das Protoplasma.
Leipzig (Engelmann), 1892.
Engelmann, T. W. — Zur Anatomie und Physiologic der Flimmerzellen : Arch. ges.
Pliys..XyA\\. 1880.
von Erlanger, R. — Neuere Ansichten liber die Struktur des Protoplasmas : Zool.
Cetitralbl., III. 8, 9. 1896.
1 See also Introductory list, p. 12.
44 GENERAL SKETCH OF THE CELL
Flemming, W. — Zellsubstanz, Kern unci Zellteilung. Leipzig, 1882.
Id. — Zelle: Merkel unci Bo7i)iefs Ergeb)iisse,\.-\N . 1891-94. (Admirable reviews
and literature-lists.)
Heidenhain, M. — Uber Kern und Protoplasma : FestscJir. z. ^o-jahr. Doctorjiib.
71011 V. KolUker. Leipzig, 1893.
Klein, E. — Observations on the Structure of Cells and Nuclei: (2uart. Joiirn. Mic.
Sci.,XN\\\. 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 (2!iain's Anatomy, I. 2, loth
ed. London, 1891.
Schiefferdecker & Kossel. — Die Gewebe des Alenschlichen Kbrpers. Braiinschweig,
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:
DcjiiscJi. j]Led. JJ'oc/iensc/ir., Oct., Nov., 1895.
Wiesner, J. — Die Elementarstruktur u. das Wachstum der lebenden Substanz :
Wien, Lioldcr. 1892.
Zimmerman, A. — Beitrage zur Morphologic und Physiologie der Pflanzenzelle.
Tilbitigen, 1893.
CHAPTER II
CELL-DIVISION
" Wo eine Zelle entsteht, da muss eine Zelle vorausgegangen sein, ebenso wie das Thier
nur aus dein Thieve, die Pflanze nur aus der Ptianze entstehen kann. Auf diese Wt-ise 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 nio"en
nun ganze Ptlanzen oder thierische Organismen oder integrirende Theile derselben sein, ein
ewiges Gesetz der coniininrlichen Enhvicklung besteht." Virchow.^
The law of genetic cellular continuity, first clearly stated by Vir-
chow in the above words, has now become one of the primary data
of biology. 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 cc/l-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 (41), Kolliker ('44),
and others showed that an essential part of the process is a division
of both the nucleus and the cell-body. In 1855 (/.r., 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 Celhdarpathologie, p. 25, 1858.
2 Cf. Introtluction, p. 9.
•5 For a full historical account of this period, see Remak, Untersuchun^en ilber die Ent-
wicklung der Wirhdthiere, 1855, pp. 164-180.
■* Unlersuchiingeti, p. 175.
45
46
CELL-DIVISION
with the division of the nucleolus, 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 Karyokiiiesis. It soon ap-
peared, however, that this mode
d ' €
Fig. 18. — Direct division of blood-cells in
the embryo cluck, illustrating Remak's scheme.
[Remak.]
a-e. Successive stages of division; / Cell of division waS not of UniVCr-
dividing by mitosis. g^l occurrcncc ; and that cell-
division is of two widely different types, which Van Beneden ('76)
distinguished as fragvientatwn, 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 aviitosis (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-
OUTLINE OF IXDIRECr DIVISIOX OR MITOSIS 47
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 JMctapJiasc, which involves the most essential step in the
division of the nucleus; (3) the Anaphases, in which the nuclear
material is distributed; (4) the Telophases, in which the entire cell
divides and the daughter-cells are formed.
I. Prophases. — {a) The Nucleus. As the cell prepares for division
the most conspicuous fact is a transformation of the nuclear 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-DIVISION
Every species of plant or animal has a fixed and characteristic num-
ber of chromosomes, zvJiich regularly recurs in the division of all of its
cells; and in all forms arising by sexual reproduction the number is
D
Fig. 19. —Diagrams showing the prophases of mitosis.
A Resting-cell with reticular nucleus and true nucleolus ; at c the attraction-sphere contain-
in'^ two centrosomes. B. Earlv prophase ; the chromatin forming a continuous spireme, nucleolus
sti'll present- above, the amphh.ster {a). C. D. 'IVo different types of later prophases; C. Dis-
appearance'of the primary spindle, divergence of the centrosomes to opposite poles of the nucleus
(examples manv plant-cells, cleavage-stages of many eggs). /.). Persistence of the' primary
spindle (to form' in some cases the "cent.al spindle"), fading of the nuclear membrane, ingrowth
of the astral ravs segmentation of the spireree-thread to form the chromosomes (examples, epi-
d-rmal cells of salamander, formation of the polar bodies). E. Later prophase of tvpe C ; fadmg
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). I. The mitotic
figure established ; e.p. The equatorial plate of chromosomes. (Cf. Figs. 16. 21, 24.)
OUTLINE OF INDIRECT DIVISION OR MITOSIS 49
cveji. 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 hepatic Pallavicinia and
some of the nematodes, 8 ; and in Ascaris, another thread-worm, 4 or
2. In the crustacean Artcmia it is 168.1 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 interestins:
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 Aviphiastcr. Meanwhile, more or less nearly parallel with
these changes in the chromatin, a complicated structure known as the
aviphiastcr {Yo\, 'yj) 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 asfer 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 ccntrosome (Boveri,
'88), which may be surrounded by a spherical mass known as the
centrosphcre (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 mure complete list see p. 154.
50
CELL-DIVISION
disappears, and the two asters pass to opposite poles of the nucleus
(most plant 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
may be pushed in by the spindle-fibres for some distance before its
Fig. 20. — Diagrams of the later phases of mitosis.
G. Metaphase ; spHtting of the chromosomes {e. p.) ; «. The cast-off nucleolus. H. Ana-
phase; the daughter-chromosomes diverging, between them the interzonal fibres (/./), or central
spindle; centrosomes already doubled in anticipation of the ensuing division. /. Late anaphase
or telophase, showing division of the cell-body, mid-body at the equator of the spindle and begm-
ning reconstruction of the daughter-nuclei. J. Division completed.
disappearance (Fig. 19, C, E). 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 karyokinctic or mitotic fiirurc. It may be described as
consi-sting of two distinct parts; namely, i, the chromatic figure,
formed by the deeply staining chromosomes ; and, 2, the achromatic
OUTUNE OF INDIRECT DTVrSION OR MITOSIS 5 1
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 arcJioplasni (Boveri, "i^), but this term is not applied
to the centrosome within the aster.
2. Metaphasc. — The prophases of mitosis are, on the w^hole, pre-
paratory in character. The metapJiasc, 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 network is converted into a tJiread^ ivhich, zvhether
continuous or discontinuous, splits throughout its entire length into
two exactly equivalent halves. The splitting of the chromosomes,
discovered by Flemming in 1880, is the most significant and funda-
mental operation of cell-division ; for by it, as Roux first pointed out
('83), the entire substance of the chromatic network is precisely halved,
and the daughter-nuclei receive precisely equivalent portions 0/ chro-
matin from the 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 Qgg, though the latter is of gigantic size as compared with the
former. On the other hand, the size of the asters varies with that
of the daughter-cells (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 grou^is of
daughter-chromosomes are connected by a bundle of achromatic
fibres, stretching across the interval between them, and known as the
interzonal fibres QX cotinecting fibres? In some cases, these differ \\\ a
1 It was this fact that led Flemming to employ the woxA "mitosis" (/iiros, a thread).
- This stage is termed by Flemming the dyaster, a term which should, however, be aban-
doned in order to avoid confusion with the earlier word amphiaster. The latter convenient
and appropriate term clearly has priority.
^ Verbinduugsfasern of German authors; filaments reiinissanls of Van Beneden.
,2 CELL-DIVISION
marked degree from the other spindle-fibres ; and they are believed
by many observers to have an entirely different origm and function.
A view now widely held is that of Hermann, who regards these fibres
as belono-ing to a central spindle, surrounded by a peripheral layer
of manUe-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. — \x^ the final phases of mitosis, the entire cell
divides in two in a plane passing through the equator of the spmdle,
each of the daughter-cells receiving a group of chromosomes, half
of the spindle, and one of the asters with its centrosome. Meanwhile,
a dauo-hter-nucleus is reconstructed in each cell from the group of
chrom^osomes 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, contort.ed,
and closely crowded to form a danghter-spireuie, 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 m the seg-
mentino- eggs of Ascaris (Van Beneden, Boveri). In other cases, as
in man^'y segmenting ova, each chromosome gives rise to a hollow
vesicle after which the vesicles fuse together to produce a smgle
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 m which
it lies. , .J ui 11
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
daughtertcells 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 \.\iQ. paranucleus or Nebenkeru (Fig. 62).
The aster may in some cases entirely disappear, together with the
centrosome (as occurs in the mature <^g,^). In a large number of
cases, however, the centrosome persists, lying either outside or 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 attraction-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 docs 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,
•* Zellsiibstanz, p. 226.
54
CELL-DIVISION
which leads the Avay 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
•7^^
■^
Sf-*:
\
B
A
' 'l \
Z>
Fig. 21. — The prophases in cells (spermatogonia and spermatocytes) of the salamander.
[DrUner.]
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 spmdle 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-sphere from the pre-existing
OA'/CLY 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 (^?>7, 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.
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 Btsneden) 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 ofifice
1 '87, p. 279.
2 '88, pp. 151, etc.
56
CELL-DIVISION
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-rr/A-. The centrosome rep-
resents the dynamic 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; gianules of the "mid-body" or 7.ivischenkdiper v.i 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- 77 ■)
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 Figiwe
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-celP (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
^ A very remarkable modification of the achromatic figure occurs in the spiral asters,
ch.o
mosomes. C. Early anaphase; divergence of the daughter-chromosomes (polar body at one
side) . D. Later anaphase ; p.b., second polar body.
(For preceding stages see Fig. 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 m a transverse
row of granules (maturation-spindles of Ascans, and some plant-cells).
It is not entirely certain, however, that such spindles observed m
preparations represent the normal structure during life.^
1 Hacker asserts in a recent paper ('94) that the truncate>-''--»'?5>.
Pig- 30- — Mitosis in the Flagellate .V(7t//-
A. Nucleus (fi) in the early prophase;
outside it the attraction-sphere [s), containing
two centrosomes (Ishikawa). B. The mitotic
figure; n. the nucleus, containing rod-shaped
chromosomes; s. attraction-sphere; s./>. ex-
tra-nuclear central spindle. (Drawn by G. N.
Calkins from one of his own preparations.)
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 EiiglypJia and Brauer in
Actuwsphcerinin. 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
.AH of the essential features in this process, as described by Ishikawa, have been con-
lirmed by Calkins in the Columbia laboratory.
F
66
CELL-DIVISION
figure, except the minute asters, is formed inside it (Fig. 28). In
ActinospJicerimn, 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.
sO,"."
.oo'">\
^^-in^t.
'0<'
00
■ o
O O0O6OO
Fig. 31. — Mitosis in the rhizopod ^t/ww/^«r2w;«. [BRAUER.]
A. Nucleus and surrounding structures in the early prophase ; above and below the reticular
nucleus lie the semilunar " pole-plates," and outside these the cytoplasmic masses in which the
asters afterward develop. B. Later stage of the nucleus. D. Mitotic figure in the metaphase,
showing equatorial plate, intra-nuclear spindle, and pole-plates {p.p.). C. Equatorial plate,
viewed e/t face, consisting of double chromatin-granules. E. Early anaphase. F. G. Later ana-
phases. H. Final anaphase. /. Telophase; daughter-nucleus forming, chromatin in loop-shaped
threads ; outside the nuclear membrane the centrosome, already divided, and the aster. J. 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 granules arrange them-
selves in threads (Fig. 31,/), and this process is apparently no more than
a forerunner 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.
MODIFICATIONS OF MITOSIS
67
(Cf. pp. y?, 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 divi^sion
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 " {Eicglena, Amceba), or possibly
by rearrangement of the chromatic substance without a differentiated
centrosome (.?micronuclei 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 {Euglypha, Diatoms) or may not {Actinosphcenimi).
These facts point toivards the conclusion Jhat centrosome, spindle, and
chromosomes are all secondary differentiations of the primitive nuclear
staictiire, and indicate that the asters and attraction-spheres 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
o-eneral groups, as follows: (i) asyinvictrical 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-
Pig, 22. — 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-
^ B
Fig. 33. — Pathological mitoses in epidermal cells of salamander caused by poisons.
[Galeotti.]
.-/. Asymmetrical mitosis after treatment with 0.05% antipyrin solution. D. 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
1 The remarkable polyasters formed in polyspermia fertilization of the egg are de-
scribed at p. 147.
/
-O CELL-DIVISION
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. Fi/nctioji of the AmpJiiastcr
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-
phshed 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 fasciniiting problems of cytology.
{a) The Theory of Fibrillar Coiitraetility. — The view that has
taken the strongest hold on recent research is the hypothesis of
fibrillar contractility. First 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" (/.r. 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 effi-
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
K-ac
cz-
a.c
evidence derived from the study of the Ascaris Qgg, 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
Asca7-is 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
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-
tractions, drag them apart,
and thus cause an actual
divergence of the centres.
The remaining astral rays
are attached to the cell-
periphery and are limited
by a 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-
m.z.
Fig. 34. — Slightly schematic figures of dividing eggs
of Ascaris, illustrating Van Beneden's theory of mitosis.
[Van Beneden and Julin.]
A. Early anaphase; each chcomosome 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;
p.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 ravs are attached.
72
CELL-DI VISION
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 Q,g^, are drawn into a position of equilibrium in the
equator of the spindle by the shortening of these rays (Figs. 65, 104);
and that tJie rays thicken as tJicy sJiortcn. 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
/
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 fibrillae there can be no doubt." ^
Very convincing evidence in the same direction is afforded by
pigment-cells and leucocytes or wandering-cells, in both of which
there is a very large permanent aster (attraction-sphere) even in the
resting-cell. The structure of the aster in the leucocyte, where it
was first discovered by Flemming in 1891, has been studied very
1 '88, 2, p. 99.
THE MECHANISM OF MITOSIS
71
carefully by Heidenhaiii 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
Fig. 36. — Pigment-cells and asters from the epidermis of fishes. [ZIMMERMAN.]
.-/. Entire pigment-cell, fi cm Blennius. The central clear space is the central mass of the aster
Irom 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, fiom 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 Driiner ('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 i;i 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 noii-contmctilc 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 contraction of these latter 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, («)
the original " central spindle," and {b) 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
daughter-chromosomes are dragged apart solely by the contractile mantle-
fibres, the central spindle fibres being non-contractile and serving as a
support or substratum along zvhich the chromosomes viove. As the
chromosomes diverge, the central spindle comes into view as the m-
terzonal fibres (Fig. 22, G, H). Strasburger ('95) is now inclined to
accept a similar view of mitosis in the cells of plants.
Druner ('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. Flemming ('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.
{!)) Other Theories. — Watase'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 occin- during the fertilization of the Qgg, 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 sininhxcra 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 spnidle in its equatorial plane. If there be sup-
porting, as opposed to contractile, fibres, they must be intermingled
with the latter; and both forms must have the same origin. The
\^^v^^M■/ '■/
-^*-
^§^;^^i^
M
"M.
#ifi^-^^^^^
lil'
^o ^-
0^
^gm^n'm^
c
■"■>•'■
D
E
pig_ 27. — The later stages of mitosis in the egg of the sea-urchin Toxopiienstes {A-D, X 1000;
L-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. £. Immediately
after division, the asters undivided; the spindle has disappeared. K 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 ot the
centrosphere, but this is not shown.
daughter-chromosomes appear to move towards the poles tJinnigh
Ihe substance of the spindle, and do not travel along its periphery as
described by Hermann and Drtiner 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 interior 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 haematoxylin it becomes bright red while
the ra^'s are blue. It seems probable, therefore, that the movements
of the chromosomes are affected by definite chemical changes occur-
ring in the centrosphere, as Butschli^ 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 CJiromosomes
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 'S;, p. 279.
7S
CELL-DIVISION
''ail independent vital manifestatio)i, an act of reproduction on the part
of the cJiroviosoines.'" ^
All of the recent researches m 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
{^'j6) 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 fission and thus determine the
Fig. 38. — Nuclei in the spireme-stage.
A. From the endosperm of the Hly, 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 si.\ nucleoli. Endosperm of Fritlllaria. [Flem-
ming.]
lon^i^itudinal 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 i88r 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 Asearis
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
i'88, 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
B
Fig. 39. — Formation of chromosomes and early splitting of the chromatin-granules in sperma-
togonia of Ascaris megalocephala, var. bivalens. [Bkauer.J
A. Very early prophase; granules of the nuclear reticulum already divided. D. Spireme;
the continuous chromatin-thread split throughout. C. Later spireme. D. Sliortenmg of tlie
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 opmion
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 cono-eries 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 o-ive 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 mass, 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. " ''■'^•' P- -°5-
DIRECT OR AMITOTIC DIVISION
8i
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 Rhabdoncma, by Korschelt in the intestine of the
annelid Ophryotrocha, and in a few other cases. In many cases, how-
ever, no preliminary fission of the nucleolus occurs ; and Remak's
scheme must, therefore, be regarded as one of the rarest forms of
cell-division ( ! ).
2. Centrosome and Attraction-Sphere in Amitosis
The behaviour of the centrosome in amitosis forms an interesting question
on account of its bearing on the mechanics of cell-division. Flemming 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 the following year, however, that in some
cases leucocytes containing two nuclei (doubtless formed by amitotic division)
might also contain two asters connected by a spindle. Both Heidenhain and
Flemming drew from this the conclusion that direct division of the nucleus is in
this case 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-
82 CELL-DIVISION
too-onia" of the salamander. ^ Krause and Flemming observed that in the autumn
many of these cells show peculiarly-lobed and irregular nuclei (the " polymorphic
nuclei " of Bellonci) . These were, and still are by some writers, regarded as
deo-enerating 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,
encirclino' 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 initotieally or aniitotically. 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 tlie 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.), w/iic/i arc on tlic zvay
toivards 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 lirst 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 ('88) 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 nrtritive-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 Anilocm ('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 R^th 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 " supporting-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. 33 1-)
SUMMARY AND CONCLUSION 85
Whether this conckision can be accepted without modification
remains to be seen. Flemming himself regards it as too extreme,
and is incHned 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 aviphi-
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-granulc 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 centrosomcs. There arc 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 tlic cud of mitosis to divide every part of
the e/iroiuatin of the inother-eell equallv betiveen the dajighter-uiielei.
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. Breslaii, 1874.
Van Beneden, E. — Recherches .sur la maturation de Poeuf, la fecondation et la
division cellulaire : Arcti. de Biol., IV. 1883.
Van Beneden & Neyt. — Nouvelles recherches sur la fecondation et la division
mitosique chez I'Ascaride megalocephale : Bull. Acad. roy. de Belgique, 1887.
III. 14, No. 8.
Boveri, Th. — Zellenstudien: \. Jena. Zedschr., XXI. 1887; U. Il^id. XXII. 1888;
III. Ibid. XXIV. 1890.
Brauer, A. — Qber die EncN'stinmg von Actinosphaerium Eichhorni : Zcitschr.
IViss. Zool., LVIII. 2. 1894.
Driiner, L. — Studien Uber den Mechanismus der Zelltheilung. Jena. Zedsckr.,
XXIX., II. 1894.
Erlanger, R. von. — Die neuesten Ansichten iil)er die Zellthcilung und ihre Mechanik :
Zool. Centralb., III. 2. 1896.
Flemming, W.,'92. — Entwicklung und Stand der Kenntnisse liber Amitose : Merkel
und Boruief-i Ergebnisse. II. 1 892 .
Id. — Zellc. (See introdnctdiv list. Also general list.)
SUMMARY AND CONCLUSION 8/
Fol, H. — (See List IV.)
Heidenhain, M. — Cytomechanische Studien : Arch.f. Entwickmech., 1. 4. 1895.
Hermann, F. — Beitrag zur Lehre von der Entstehung der karyokinetischen Spindel :
Arch. Mik. Aiiai., XXXVII. 1891 .
Hertwig, R- — Ober Centrosoma und Centralspindel : Sitz.-Ber. Ges. Morph. nnd
Pliys. Miinc/ien, 1895, Heft I.
Mark, E. L. — (See List IV.)
Reinke, F. — Zellstudien : I. Arch. Mik. Anat., XLIII. 1894; II. Ibid. XLIV. 1894.
Strasburger, E. — Karyokinetische Vrohltmc : Jahrb. f. Wiss.Botan. XXVIII. 1895.
Waldeyer, W. — IJber Karyokinese und ihre Beziehungen zu den Befruchtungsvor-
^ransen : Arch. Mik. Anat., XXXII. 1888. Q.J.M.S., XXX. 1889-90.
CHAPTER III
THE GERAI-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 precedmg 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) \X-\..
is then found to lie .^^':'B0m:fMmi^
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
;7::;^^v^;•■.v■•i>•'v^'•.^^:; e> "
t •". jT-J; '■'■J ■^'^'V■•!^' ''»*V *^\>i.'
M
egg
thus loses the
power of division
Avhich 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 egg
"•-J&.Vf.C\i^''-'""'"'
Fig. 42. — Ovarian egg of the sea-urchin Toxopneustes
(X750)-
g.v. Nucleus or germinal vesicle, containing an irregular dis-
continuous network of chromatin ; g-.s. nucleolus or germinal
spot, intensely stained with haeniatoxylin. The naked ceii-body
consists of a very regular network, the threads of which appear
as irregular rows of minute granules or microsomes. Below,
at s, is an entire spermatozoon shown at the same enlargement
(both middle-piece and flagellum are slightly exaggerated in
size).
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 denioplasjii, 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 litUe deutoplasm (ccelenterates, echinoderms,
many annelids, and some copepods), while it is multiple in large eggs heavily laden with
'.«.). 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
I
GROWTH AND D/FFEEENTIATION OF THE GERM-CELLS I IQ
(Jordan, '93) and niyriapods (Balbiani, '93), where the &gg contains a
number of fairly well defined yolk-nuclei. In Lumbricus 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 o-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 {GeopJiilns), 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 Q.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 situ 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, i), working in my laboratory, has brought forward strong evi-
dence that the "yolk-nucleus" of Liivibricns is derived from a sub-
stance nearly related with chromatin (Fig. 61). The yolk-nucleus
Fig. 61. — Young ovarian eggs of the earthworm (Lumbricus) , showing yolk-nucleus.
[Calkins.]
A. Very early stage ; the irregular yolk-nucleus (>'.«.) 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 Q^,g. The action of differ-
ential stains at different periods indicates that the substance of the
GROWTH AND DIFFERENTIATION OF THE GERM-CELIS 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. i 50) 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 Lmnbriciis 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 impo.s-
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
Owino- 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 speinnatogonia}
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 I.a Valette St. George. Cf.
Fig. 90.
- See Fig. 91.
GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 123
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
nucleus; 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 paranucleus (Nebenkern) or initosome, 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 Pygcera ('89), it consists of a larger
posterior and a smaller anterior body, which he calls respectively the
large and small niitosonia (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
^ Compare the confusion between yolk-nucleus and attraction-sphere in the ovum, p. 119.
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
Jl
C
Fig. 62. — Formation of the spermatozoon from the spermatid.
A. Late stage of spermatid of the shark Scyllium. [Moore.]
B. Spermatid of starfish ChcBtaster. [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 ; /. 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 Picris leave little
doubt that the sperm-centrosome is here derived from the middle-
piece; and, moreover, in the grasshopper Caloptenits, 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
■ ' ' f '% i^' »iJ
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 bv 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 fiagellum, 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
Fig. 63. — Formation of the spermatozoon from the sper-
matid in the salam.mder. [HERMANN.]
A. Young spermatid showing the nucleus above, and below
the colorless sphere, the ring, and tlie 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,
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 Liunbricus. 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-
126 THE GERM-CELLS
somes of the spermatid-nucleus, ic) a ring-shaped structure staining
purple with gentian violet, like the chromatin. The colourless sphere
ultimately 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. M.oore
('95) describes the flagellum of elasmobranchs as growing out from
the attraction-sphere (archoplasm) of the spermatid (Fig. 62, A).
Sin/nnary. — 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-
fibrilte. 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.
' Flemminp; described the middle-piece as arising inside the nucleus ; liut 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,
hsematoxylin). He was thus led to regard the chromatin of the egg
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 (hsematoxylin, 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 tha.t 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
1 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 Liber die Strulvtur der Spermatozoen : I. {birds)
Arch. Mik. Anat. XXXII., 1888; 2. {insects) Zeitschr. Wiss. ZooL, L., 1890;
3. {fishes, ainpJiibia, reptiles) Arch. Mik. Anat., XXXVI., 1890; 4. {niain-
inals) Zeit. M'iss. Zoo/., LI I., 1891.
Van Beneden, E. — Recherches sur la composition et la signification de Poeuf : Mem.
coi/r. de I Acad. ray. de s. de Bclgiqne, 1870.
Boveri, Th. — tJber Differenzierung der Zellkerne wahrend der Furchung des Eies
von Ascaris meg. : Anat. Ans., 1887.
Brunn, M. von. — Beitrage zur Kenntniss der Samenkorper und ihrer Entwickelung
bei Vogeln und Saugetliieren : Arch. Mik. Anat., XXXIII., 1889.
Hacker, V. — Die Eibiidung bei Cyclops und Camptocanthus : Zool. Jahrb., V.,
1892. (See also List V.)
Hermann, F. — Urogenitalsystem ; Struktur und Histiogenese der Spermatozoen :
Mer/cel und Bonnet^' Ergebnisse, II., 1892.
Kblliker, A. — Beitrage zur Kenntniss der Geschlecbtsverhaltnisse 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, ptlanz-
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. Aiiat. 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 web of which the warp is derived
from the female and the woof from the male." HuXLEY.i
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
admixture of living- matter derived from anotJier cell. This operation,
known :is fertilisation ox fecundation, 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 Biitschli ('76) and Minot
i'n^ '79) and 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. Bril., 1878.
K 129
130 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. In
all the higher forms fertilisation consists in the pennancnt fusion of
tzvo genn-cells, one of paternal and one of maternal origin. We may
first consider the fertilization of the animal &^g, 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 egg ; but the process was not actually seen until nearly
two centuries later (1854), when Newport observed it in the case of
the frog's ^^^\ and it was described by Pringsheim a year later in one
of the lower plants, GLdigoniiDn. The first adequate description of
the process was given by Hermann Fol, in 1879,^ though many
^ See I Ilhtogenie, pp. 124 ff., for a full liistorical 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.gg.
In many cases the entire spermatozoon enters the Q,gg (mollusks,
insects, nematodes, some annelids, Petromyzon, axolotl, etc.), and in
such cases the long fiagellum may sometimes be seen coiled within
the Qg^ (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 egg. At or near
.0 •^, 00 o.r,; ^ d»
Fig. 64. — Fertilization of the egg of the snail P/iysa. [Kostanfxki and Wierzejski.]
A. The entire spermatozoon lies in the egg, its nucleus at the right, fl.igellum 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 egg successively segments off at the upper
pole two minute cells, known as the po/ar 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
maturation, by which the egg is prepared for fertilization, and we
may defer its consideration to the following chapter.
132 FERTILIZATIOX OF THE OVUM
I. The Gcnu-iiiiclci in Frrti/i. 'nation
The modern era in the study of fertilization may be said to begin
with Oscar Hertwig's discovery, in 1875, of the fate of the sperma-
tozoon within the egg. Earlier observers had, it is true, paved the
way by showing that, at the time of fertilization, the Qgg contains
Hvo nuclei that fuse together or become closely associated before
development begins. (Warneck, Biitschli, Auerbach, Van Beneden,
Strasburger.) Hertwig discovered, in the O-gg of the sea-urchin
( Toxopnciistcs lividiis), that one of these nuclei belongs to the egg,
ivhile the other is derived from the sperniatozodn. 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 hnoivn ease an
essential phenomenon of fertilirjation is the union of a sperm-nucleus,
of paternal origin, ivitJi an egg-nucleus, of maternal origin, to form the
primary nnclens of the embryo. This nucleus, kjiozvn as the cleavage-
or segmentation-7iucleus, gives rise by division to all the nuclei of the
body, and hence every nnclens of tJie child may contain nuclear substance
derived from both parents. And thus Hertwig was led to the conclu-
sion ('84), independently reached at the same time by Strasburger,
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, Ascaris mcgalocepJiala, the egg 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
sjze totally disappears during fertilization, but that the two nuclei
undergo a parallel series of structural changes which demonstrate
their precise morphological equivalence down to the minutest detail ;
and here, again, later researches, foremost among them those of
Boveri, Strasburger, and Guignard, have shown that, essentially, the
same is true of the germ-cells of other animals and of plants. The
facts in Ascaris (variety bivalens) are essentially as follows (Fig.
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 ^
Fig. 65. - Fertilization of the egg of Ascaris megalocepkala, var. bivaleus. [BOVERI.] (tor
later stages see Fig. 104.)
A. The spermatozoon has entered the egg, its nucleus is shown a. c?; ^''^^f^ '"^^I'^^J-^^Z^
lar mass of '■ archoplasm" (attraction-sphere) ; above are the clos.ng phase n the ^ -» "J °
the second polar body (two chromosomes in each nucleus). B. ^^''^-''f'^'i^'J^^^:^^^^^
lar stage ; the attraction-sphere (a) contains the dividmg centrosome. C. '-^'^'^^;'''^'; '^^^^^^l
in the ^e m-nuclei; the centrosome divided. D. Each germ-nucleus resolved mto t^vo chromo
so nesT attraction-;phere (.) double. E. Mitot.c figure forming for the ^rst cleavage ^he chro-
mosomes (.) alreadv split. /••. First cleavage in progress, showmg 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, the chromatic portion of zuhich has
been syjithctically formed by the union of two equal germ-nuclei. The
A
B
Fig. 66. — Germ-nuclei and chromosomes in tlie eggs of nematodes. [Carnoy.]
A. Egg of nematode parasitic in Scyllium ; the two germ-nuclei in apposition, each containing
four chromosomes; the two polar bodies above. B. Y^gg oi Filar aides ; 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 mater)ial
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 Ophiostomum 6, and in Filaroides 8. A little later Boveri
GENERAL SKETCH 135
('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\oWvi'i\i PterotracJica 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 Ophryo-
troclia it is only 2 from each sex (Korschelt). In all these cases the
iiuuibcr contributed by each is onc-Jialf the number characteristic 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 ninnber of chromosomes in sexually
produced organisms is ahvays evcji}
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 in 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 tljat the egg-cen-
trosome disappears before or during fertilization and its place is taken
by a nczv ccjitrosonte which is introduced by the spcj'matododn and
divides into tivo to form the cleavage-amphiaster. 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
Allolobophoj-a (Foot), in the butterfly Picris (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 tlie egg of the mouse. [SOKOTTA.]
A. Tlie ovarian egg still surrounded by the follicle-cells and the membrane {:./., zona pel-
lucida) ; the polar spindle formed. B. Egg immediately after entrance of the spermatozoon
(sperm-nucleus at ,'^). C. The two germ-nuclei (j', ? ) 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 Toxopnc/istcs (Fig. 69). As described at
p. 146, the tail is in this case left outside, and only the head and
middle-piece enter the Qgg. Within a few minutes after its entrance,
and while still very near the periphery, the lance-shaped sperm-head,
carrying the middlcrpiece at its base, rotates through nearly or quite
1 Cf. p. 156.
GENERAL SKETCH
m
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
B
Fig. 68. — Fertilization of the egg of the gasteropod Pterotracbea. [BOVERI.]
A. Tlie egCT-nucleus {E) and sperm-nucleus (5) 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.
' 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.
1^.8
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 egg. 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
i
m X
B
C
D '-^'f^
Fig. 69. — Entrance and rotation of tlie sperm-head and formation of tlie sperm-aster in the
sea-urchin Toxopiiensfes (A.-F., X 1600; G. //., X 800).
A. Sperm-head before entrance; «, nucleus; m, middle-piece and part of the flagellum.
B. C. Immediately after entrance, showing entrance-cone. D.-F. Rotation of the sperm-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 ^g^.
1 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 lielieve 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-niiclei and division of the sperm-aster in the sea-urchin
Toxopneustes, X 1000. (For later stages see Fig. 37.)
. /. Union of the nuclei, extension of the aster. D. 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
I40 FERTILIZATION 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 Q.gg, 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
undivided at the time of union. This difference is doubtless due
merely to a difference in the time elapsing between entrance of the
spermatozoon and conjugation of the nuclei, the amphiaster having,
in the second case, time to 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.^ Bovori
('88, i) has observed 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 egg without wiion of the gcrni-
nuclci, 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 // is the centro-
soDie a/one that ca?ises division of the egg, and it is therefore the fer-
tilising element proper (Boveri, '8"/, 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 egg
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 observation 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, but at a point in advance of it,
— a fact afterwards confirmed by Hertwig himself and by Boveri
('88, I ). Hertwig and Fol afterwards found that in cases of poly-
spermy, when several spermatozoa enter the 0.^2,, each sperm-nucleus
is accompanied by an aster, and Hertwig proved that each of these
might give rise to an amphiaster (Fig. 75).
^ Cf. Kostanecki and Wierzejski, '96.
GENERAL SKETCH
141
It was Boveri ("87) who first accurately traced the complete history
of the centrosome and clearly formulated the facts, proving that in
Ascans 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 of A'^^ifM, from sections. ( X 400.)
. /. Soon after the entrance of the spermatozoon, showing the minute sperm-nucleus at Q., the
germinal vesicle disappearing, and the first polar mitotic figure forming. The empty spaces repre-
sent deutoplasm-spheres (slightly swollen by the reagents), the firm circles oil-chops. B. Sperm-
nucleus (d") 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 eger possesses all of the organs and qualities necessary for division
excepting the centrosome, by zvhich division is initiated. The sperma-
142
FERTILIZATION OF THE OVUM
to::odn, on the other hand, is provieled zvitJi a centrosome, bnt lacks the
substance in ivJiich this organ of division may exert its activity.
Through the union of the tzvo cells in fertilization all of the essential
organs necessary for division are brought togetJier ; the egg notv contains
a centrosome which by its own division leads the ivay in the embryonic
development} 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
Rhynchelmis. Here, again, very strong evidence was brought for-
Fig. 72. — Fertilization of the egg in tlie copepod Cyclops stienuus. [RuCKERT.]
A. Sperm-nucleus soon after entrance, the sperm-aster dividing. B. The germ-nuclei ap-
proaching; c?,the enlarged sperm-nucleus with a large aster at each pole; $,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
eiioimous 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 Petromyzon
('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 ('86) had already made similar observations in the snail
Arion, 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
middle-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 ^g^ of Toxo-
pncustes, of Mathews on Aj-bacia, and of Boveri on EcJiimis. Nearly
at the same time a careful study was made by Mead ('95) of the
annelid Chceoptents, 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 Rhyjichelmis. Exactly the same result has since been reached by
Hill ('95) in SpJi(zrcchiniis and the tunicate Phallusia, and by Kos-
tanecki and Wierzejski ('96) in PJiysa (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. 67).
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.gg of Thalassemia (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 Chisopierus, '95, and Kostanecki and Wierzejski on Physa, '96.
144
FERTFLTZATIOX OF THE OVUM
continuity is not broken ni later stages. An exactly similar process
is described by Kostanecki and Wierzejski in the &^^ of PJiysa. We
thus reach the following remarkable conclusion : During cleavage
the cytoplasm of tlic blastoinercs is derived from that of the egg, the
centrosomcs from the spermatozoon, ivhile the nuclei {cJiromatin) are
Fig. .73. — Persistence of the centrosomes from cell to cell, in ihc cleavage of the egg of the
gephyiean Thalasscina. [GRIFFIN.]
A. Mitotic figure for the first cleavage ; the centrosome already double in each centrosphere
(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 from both gcnn-cclls ; 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 docs not lie within the scope of this work to consider the
innumerable modes by which the germ-cells are brought together,
further than to recall the fact that their union may take place inside
the body of the mother or outside, and that in the latter case, both
eggs and spermatozoa are as a rule discharged into the water, where
fertilization and development take place. The spermatozoa may
live for a long period, either before or after their discharge, without
losing their fertilizing power, and their movements may continue
throughout this period. In many cases they are motionless when
first discharged, and only begin their characteristic swimming move-
ments after coming in contact with the water. There is clear evi-
dence of a definite attraction between the germ-cells, which is
in some cases so marked (for example in the polyp Rcnilld) 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 Pfeffcr'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 Selaginclla
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 Q^g, according to Fol, pos-
sesses before fertilization a pecuhar protoplasmic " attraction-cone "
to which the head of the spermatozoon becomes attached, and through
which it enters the Qgg. In some of the hydromedusse, 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 ^gg 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
lilO . ■
Fig. 74. — Entrance of the spermatozoon into the egg. A.-G. In the sea-urchin Toxppneustes.
//. In the medusa il/z/wcowi?. [Metschnmkoff.] /. In the star-fish .-:/.f/tV7aj. [FoL.]
A. Spermatozoon of Toxopneustes, X 2000; a, the apical body, ;/, nucleus, m, middle-piece,
f, fiagellum. D. Contact with the egg-periphery. C. D. Entrance of the head, formation of the
entrance-cone and of the vitelline membrane (y), leaving ihe 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 Q.gg remains naked, even after fertilization, as appears to
be the case in many ccelenterates. More commonly a vitelline mem-
brane is quickly formed after contact of the spermatozoon, — e.g.
in Amphioxiis, 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
H7
as in cchinoderms, mammals, many annelids, etc., while in others
several may enter (insects, elasmobranchs, reptiles, the earthworm,
Petromyzoii, 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 .{scans ; below, the egg-nucleus ; above, three entire spermatozoa
within the egg. [Sala.]
D. 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. HERlWic;.]
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 egg may dcv^elop 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 FERTILIZATIOX 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 *^'g^^ (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 Q^^. 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 {Pctnv/iyrjon) 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.
^ For an account of the internal changes, see p. 261.
- The Hertwigs attribute this to a diminished irritability on the part of the egg-substance.
Normally requiring the stimulus of only a single spermatozoon f(jr the formation of the vitel-
line meml:>rane, it here demands the more intense stimulus of two, three, or more before
the membrane is formed. Tliat the membrane is not present before fertilization is admitted
by Hertwig 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. Innncdiatc 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 Q.g^ (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 spcjin-nucleus
and sperm-eentrosoj/ie of the poiver 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 formatioji 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.
(/;) The Ovum. — The entrance of the spermatozoon produces an
extraordinary effect on the Qg%, 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 Q.g^ 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 7'oxopitftts/es^ proceeds
like a wave from the entrance-point around the periphery, but this is often irregular-
I50
FERTILIZATION 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 ovum is, however, the migration
of the germinal vesicle to the periphery, and the formation of the
polar bodies. In many cases either or both these processes may occur
before contact with the spermatozoon (echinoderms, some vertebrates).
In others, however, the egg awaits the entrance of the spermatozoon
(annelids, gasteropods, etc.), which gives it the necessary stimulus.
This is well illustrated by the Q^g of Nereis. In the newly-dis-
charged egg the germinal vesicle occupies a central position, the
yolk, consisting of deutoplasm-
spheres and oil-globules, is 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 egg near either pole and advance thence
towards the poles, the upper one surrounding the point at which the
polar bodies are formed (Fig. 76). 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 Clepsiiic, dur-
ing fertilization. [Whitman.]
I>.b., polar bodies ; p.r., polar rings ; cleav-
age-nucleus near the centre.
UNION OF THE GERM-CELLS 151
2. Paths of the Gcrui-nnclei {Pro-nuclei) ^
After the entrance of the spermatozoon both germ-nuclei move
through the egg-cytoplasm and finally meet one another. The paths
traversed by 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 tgg 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 Q.gg forming a considerable angle with its "copulation-path "
towards the egg-nucleus.
These facts are well illustrated in the sea-urchin ^gg (Fig- 77),
where the egg-nucleus occupies an eccentric position near the point
at which the polar bodies are formed (before fertilization). Entering
the Qgg 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 ^g%. 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 ^gg.
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
Hertwig's terms egg-niuleiis -icnA sperin-mideus, on two grounds: (i) The germ-nuclei are
true nuclei in every sense, differing from the somatic-nuclei only in the reduced number of
chromosomes. As the latter character has recently been shown to be true also of the
somatic nuclei in the sexual generation of plants (p. 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 Toxopiu'iistes. From camera drawings of the transparent living eggs.
In all the figures the original position of the egg-nucleus (reticulated) is shown at 9 ; the point
at which the spermatozoon enters at E (entrance-cone). Arrows indicate the paths traversed by
the nuclei. At the meeting-point {M) the egg-nucleus is dotted. The cleavage-nucleus in its
final position is ruled in parallel lines, and through it is drawn the axis of the resulting cleavage-
figure. The axis of the egg is indicated by an arrow, the point of which is turned away from the
micromere-pole. Plane of first cleavage, passing near the entrance-point, shown by the curved
dotted line.
the entrance-path of the sperm-nucleus, while both factors probably
operate in the determination of the copulation-path along which it
travels to meet the egg-nucleus. The real nature of neither factor
is known.
UX/OX OF THE GKKM-CELLS 153
Hertwisi 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 toi^ether 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 mav l^e 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 Gcnn-nnclci. The Chromosomes
t
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 clca-eage- or segmentation-niteleus. As early as 1881, however,
Mark clearly showed that in the slug Limax this is not the case, the
two nuclei merely becoming apposed without actual fusion. Two
years later appeared Van Beneden's epoch-making work on Ascaris,
in which it was shown not only that the nuclei do not fuse, but that
they give rise to two independent groups of chromosomes which
separately enter the equatorial plate and whose descendants pass
separately into the daughter-nuclei. Later observations have given
the strongest reason to believe that, as far as the chromatin is con-
cerned, a true fusion of the nuclei never takes place during fertiliza-
tion, and that the paternal and maternal chromatin jnay remain
separate and distinct in the later stages of development — possibly
throughout life (p. 219). In this regard two general classes may be
distinguished. In one, exemplified by some echinoderms, by Amphi-
oxns, P/iail/isia, 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 rej^re-
sented by Ascaris (Fig. 65) and other nematodes, by Cje/ops {¥\g. 72),
and by Pterotrachea (Fig. 6?>), 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. ^ 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 tunnber of chromosomes arising frovi the
gcrui-mtclei is alivays the same in both, and is one-half the number
characteristic of the tissne-cells of the species. By their nnion, tJiere-
fore, the germ-iuiclci give rise to an equatorial plate co7itaining the
typical nnmbcr 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-
Nuclei.
Somatic
Nuclei.
Name.
Group.
Authority.
I
2
Ascaris megalocephala,
var. univalens.
Nematodes.
Van Beneden,
Boveri.
2
4
Id., var. bivalens.
77
7?
If
??
Ophryotrocha.
Annelids.
Korschelt.
T>
["]
Styleopsis.
Tunicates.
Julin.
4
8
Coronilla.
Nematodes.
Carnoy.
?9
1?
Pallavicinia.
HepaticEe.
Farmer.
6
12
Spiroptera.
Nematodes.
Carnoy.
v>
U
Gryllotalpa.
Insects.
vom Rath.
If
??
Caloptenus.
77
Wilcox.
GO
*?
/Equorea.
Hydromedusse.
Hacker.
8
i6
Filaroides.
Nematodes.
Carnoy.
))
77
Hydrophilus.
Insects.
vom Rath.
J)
j;
Phallusia.
Tunicates.
Hill.
■)■>
?;
Li max.
Gasteropods.
vom Rath.
77
u
Rat.
Mammals.
Moore.
f>
[.]
Ox, guinea-pig, man.
77
Bardeleben.
■>■>
77
Ceratozamia.
Cycads.
Overton.
}}
77
Pinus.
Conifera?.
Dixon.
1 Indeed, Boveri has found that in Ascaris both modes occur, though the fusion of the
germ-nuclei is exceptional. (Cf. p. 216.)
2 The above table is compiled from papers both on fertilization and maturation. Num-
bers in brackets are inferred.
IWION OF THE GERM-CELLS
155
Germ-
Somatic
Name.
Group.
AUTHOKITV.
Nl'CLEI.
Nuclei.
8
16
Scilla, Triticum.
Angiosperms.
Overton.
?5
■>■>
Allium.
>>
Strasburger,
Guignard.
9
18
Echinus.
Echinoderms.
Boveri.
55
??
Sagitta.
Chaitognaths.
j>
*•!
^9
Ascidia.
Tunicates.
?7
II
[22]
Allolobophora.
Annelids.
Foot.
II (12)
22 (24)
Cyclops strenuus.
Copepods.
Rlickert.
12
24
brevicornis.
■>i
Hacker.
■>•!
Helix.
Gasteropods.
Platner, vom Rath.
j»
If
Branchipus.
Crustacea.
Brauer.
j;
D,]
Pynhocoris.
Insects.
Henking.
)j
>>
Salmo.
Teleosts.
Bohm.
jj
5«
Salamandra.
Amphibia.
Flemming.
^j
?7
Rana.
>7
vom Rath.
J7
Mouse.
Mammals.
Sobotta.
4^
V
Osmunda.
Ferns.
Strasburger.
??
»
Lilium.
Angiosperms.
Strasburger,
Guignard.
jy
J>
Helleborus.
?>
Strasburger.
??
7?
Leucojum, Paeonia,
Aconitum.
1^
Overton.
14
28
Tiara.
Hydromedusae.
Boveri.
16
32
Pterotrachea, Carinaria,
•
Phyllirhoe.
Gastropods.
•\1
„
[..]
Diaptomus, Heterocope.
Copepods.
Rlickert.
^y
[•,]
Anomalocera, Euchaeta.
*>%
vom Rath.
J?
D,]
Lumbricus.
Annelids.
Calkins.
18
36
Torpedo, Pristiurus.
Ellasmobranchs.
Rlickert.
[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 Avion, 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-bodics,
and that the germ-nuclei are exactly alike at the time of union. ^ We
may here briefly refer to remarkable recent observations by Ruckert
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-
ality 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 I
remained distinct in later stages as well ; and Rabl and Boveri {j
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
A scans the egg of variety diva/ens, having two chromosomes, be
fertilized with the spermatozoon of variety nnivalcns 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 « /r/t?;-/ the possibility that
other modes of fertilization may occur, namely, partJicnogcncsis, in
which the Q.gg develops without fertilization. In this case, as Brauer
('93) has clearly shown in Arteviia, 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
1 Cf. p. 219.
CENTROSOME AXD ARCHOPLASM LV FERTILIZATION I 5/
forms the centre of each aster of the first mitotic figure (Van Beneden,
in Ascaris, '83, "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 egg (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 Crcpidnla, 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 egg, independently made in 1894-5 by Boveri {Echinus),
by myself {Toxopneiistes), and Mathews {Arbacia, Astcrias), 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 OVO'M
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. 1^8. — Fertilization of the egg of the parasitic annelid Myzostoma. [WHEELER.]
A. Soon after entrance of tlie spermatozoon ; the sperm-nucleus at J ; at ? the germinal
vesicle ; at c the double egg-centrosome. B. First polar body forming at ? ; n, 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 jr/t'r;«-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 P/iysa, and have reached exactly the same
result as that obtained in the echinoderms. Here 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
&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 inference, not an observed fact,
and has not been confirmed by any subsequent observer. Until such
confirmation is forthcoming we must receive Guignard's results with
scepticism.^
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 Ilertwig 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.
- Van der Stricht, in a recent paper on Amphioxtts ('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
i6o
FERriLIZAriOiX 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 gcnii-
nnclci, as far as I can
lind, 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
of the conjugating cells of Spirogyra in 1879, and made similar obser-
vations on other algce in 1884. The same has been shown to be true
in MuscinccE. and Ptcridophytcs 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 oosphere 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 (chromatophorcs, 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
Fig. 79. — Fertilization in Pibularia. [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 tlie centre the ovum
containing the apposed germ-nuclei (d", ?).
FERTILIZATION IX 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
D
Fig. 80. —Fertilization of the lily. [Guionaru.]
A. The tip of the pollen-tube entering the embryo-sac ; below, the ovum (ocisphere) with its
nucleus at ? and two centrosomes; at the tip of the pollen-tube the sperm-nucleus (cf) with two
centrosomes near it. B. Union of the germ-nuclei. C. Later stage of the same, showing the
asserted fusion of the centrosomes. R. 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 pollcn-tube, and its growth clown 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 Martagon) 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 o.^^ 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 egg,
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 eighteentli 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 Francjois Geoffroy, Needham, and others,
placed himself on the side of Leeuwenhoek and the spermatists, maintaining that the pollen
supplied the embryo which entered the ovule through the micropyle. (The latter had been
described by Grew in 1672.) 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 \vas excited to development l)y an aura or vapour emanating from the pollen
and entering through the tracheae of the pistil.
'^ 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
&g^^. Within the egg both stain exactly alike. These facts indicate,
as Strasburger insists, that the differences between the germ-nuclei
of plants are as in animals of a temporary and non-essential character.
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 jiidicc?
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 diflficulties 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
Butschli, 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 ii-g^ ; namely, that an essential phenomenon
' Cf. p. 41. 2Cf. p. 129.
164
FERTILIZATION OF THE OVUM
of conjugation is a union 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, alter separation.
Difierentiation 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. 81. — Dingrnm showin? the history of the mictonuclei during the conjugation of Para-
vimciuin. [Modified from MauPAS.]
A' and 1" represent 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
inauo-urates a new cycle, and is obviously comparable in its physio-
CONJUGATIOX IX UNICELLULAR FORMS 1 65
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 Stvloiivchia pustulata, whicli Maupas followed continuously from the end of
February until July, the tirst conjugation occurred on April 29th. after 128 bi-parti-
tions ; and tlie epidemic reached its height three weeks later, after 175 bi-partitions.
The descendants of individuals prevented from conjugation died out through -senile
degeneracv," after 316 bi-partitions. Similar facts were observed in many other
forms. The degeneracy is manifested by a very marked reduction in size, a partial
atrophy of the cilia, and especially by a more or less complete degradation of the
nuclear apparatus. In Stylonyclna pttstulata and 0)iychodroniiis grandis this process
especially affects the micronucleus, which atrophies, and finally disappears, though
the animals still actively swim, and for a time divide. Later, the macronucleus
becomes irregular, and sometimes breaks up into smaller bodies. In other cases,
the degeneration first afiects the macronucleus, which may lose its chromadn,
undergo fatty degeneration, and may finally disappear altogether {Stylonychia
mytilus), 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 ]\Iaupas points out, that the degeneration of the cytoplasmic organs
is clue to disturbances in nutrition caused bv the degeneration of the nucleus.
-&^
The more essential phenomena occurring during conjugation are
as follows. The Infusoria possess two kinds of nuclei, a large
viacnmuclcns and one or more small micromtclci. 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 gcrvi-uuclc7is,
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 bv division to both macronuclei
and micronuclei of the offspring of the conjugating animals (Fig. 81 ).
These facts may be illustrated by the conjugation of Paj-ania'ciiiin
caiidatiini, which possesses a single macronucleus and micronucleus,
and in which conjugation is temporary and fertilization mutual. The
two animals become united by their ventral sides and the macronu-
cleus of each begins to degenerate, while the micronucleus divides
twice to form four spindle-shaped bodies (Fig. 82, A, B). Three of
these degenerate, forming the " corpusculcs 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
1 Cf. p. 129.
Fig. 82. — Conjugation oi Paramcsairm caudatum. \A-C, after R. Hertwu;; D-K, after
Maupas.] (Tlie macronuclei dotted in all the figures.)
4 Micronuclei preparing for their first division. i?. Second division. C. Third division:
three polar bodies or " corpuscules de rebut," and one dividing germ-nucleus m each animal. D.
Exchange of the germ-nuclei. E. The same, enlarged. F. Fusion of the germ-nuc ei. G. The
same enlarged. H. Cleavage-nucleus (t). preparing for the first division /. The cleavage-
nucleus has divided twice. J. After three divisions of the cleavage-nuc eus ; macronucleus
breaking up. K. Four of the nuclei enlarging to form new macronuclei. The first fission soon
takes place.
CONJUGATION IN UNICELLULAR FORMS
167
5
pronucleus (Fig. ^2, 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 /'. cauda-
tuui 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-
Fig. 83. — Conjugation of Vorticellids. [Maupas.]
A. Attachment of the small free-swimming microgamete to
the large fixed macrogamete ; micronucleus dividing in each
{Carchesiuni). B. Microgamete containing eight micronuclei;
macrogamete four {Voiiice/hi) . 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.
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 microgamete unites with
a larger macroi^aj/iftr, 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.
i68
FERTILIZATIOX OF THE OVUM
The facts just described show a very close parallel to those observed
in the maturation and fertilization of the ^^g. In both cases there
is a union of two similar nuclei to form a cleavage-nucleus or its
equivalent, equally derived from both gametes, and this is the pro-
genitor of all the nuclei of the daughter-cells arising by subsequent
divisions. In both cases, moreover (if we confine the comparison
to the 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
B
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 the gcnn-niiclci unite zchen in the form of spindles
or mitotic figures. These spindles consist of achromatic fibres, or
"archoplasm," and chromosomes, but no asters or undoubted cen-
trosomes have been thus far seen in them. During union the
spindles join side by side (Fig. 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
D
Fig. 85. — Conjugation of Spirogyra. [OVERTON.]
A. Union of the conjugating cells {S. communis). D. The typical, though not invariable,
mode of fusion in 5. 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
lyO FERTILIZATION OF THE OVUM
maternal chromatophores lie at opposite ends. In 5. Wcbcri, 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 Spirogyra 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 ^gg?-
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 Qgg. But,
1 De Vries's conclusion is, however, not entirely certain; for it is impossible to deter-
mine, save by analogy, whether the chromatophores maintain their individuality in the
zygote.
SUMMARY AXD 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 egg loses its centrosome, which is then supplied by the
father. The loss of the centrosome by the egg is, I believe, to be
regarded as a provision to guard against parthenogenesis and to
ensure amphimixis.
The equivalence of the germ-cells is tints finally lost. Only the
germ-nuclei retain their primitive morphological equivalence. Hence
zve find the essential fact of fertilization and sexual reproduction to
be a union of equivalent nuclei; and to this all other 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 egg 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 Toeuf, la fe'condation et la division
ccllulaire: Arc/i. Biol.,lY. 1883.
Van Beneden and Neyt. — Nouvelles recherches sur la fecondation et la division
mitosique chez TAscaride megalocephale : Bull. Acad. roy. de Belgique, III. 14,
No. 8, 1887.
Boveri, Th. — tjber den Anteil des Spermatozoon an der Teilung des Eies : Sit2.-
Bcr. d. Ges.f. Morph. 11. P/iys. in Munchen. B. III.. Heft 3. 1887.
Id. — Zellenstudien, II. 1888.
Id. — Befmchtung : Merkel wui Bonnefs Ergebnissc. I. 1891 .
Id. — Uber das Verhalten der Centrosomen bei der Befruclitung des Seeigeleies,
etc.: Ver/miidl. P/iys. Med. Ges. IVnrsbitrg, XXIX. 1895.
Fick, R. — ijber die Reifung und Befmchtung des Axolotleies : Zeitschr. U'/ss.
Zo'dl., LVI. 4. 1893.
1/2 FERTILIZATION OF THE OVUM
Guignard, L. — Nouvelles etudes sur la fecondation : Aim. d. Sciences nat. Bot.y
XIV. 1891.
Hartog, M. M. — Some Problems of Reproduction, etc.: (luart. Jouni. Mic. Sci.,
XXXIII. 1 89 1.
Hertwig, 0. — Beitrage zur Kenntniss der Bildung, Befruchtung und Teilung des
tierischen Eies, I.: Morpli. Jalirb.. I. 1875.
Hertwig, R. — Uber die Konjugation der Infusorien: Abh. d. bayr. Akad. d. ll'iss.,
II. CI. XVII. 1888-89.
Id. — Uber Befruchtung und Konjugation : ]'crli. deittsch. Zool. Ges. Berlin, i%c)2.
Kostanecki, K. v., and Wierzejski, A. — Uber das Verhalten der sogen. achro-
matischen Substanzen im befruchteten Ei {oi P/iysa) : Arch. mik. Anal, XLVII.
2. 1896.
Mark, E. L. — Maturation, Fecundation, and Segmentation of Liviax campestris:
Bull. Miis. Co/up. Zool. Harvard Collei^e, Cambridge. Mass., VI. 1881.
Maupas. — Le rejeunissement karyogamique chez les Cilie's : Arch. d. Zool., 2"'"
se'rie. VII. 1889.
Riickert, J. — Uber das Selbstandigbleiben der vaterlichen und mlitterlichen Kern-
substanz walirend der ersten Entwicklung des befruchteten Cyclops-Eies : Arch.
jnik. Anal, XLV. 3. 1895.
Strasburger, E. — Neue Untersuchungen liber den Befruchtungsvorgang bei den
Phanerogamen. als Grundlage fiir eine Theorie der Zeugung. JeJia, 1884.
Id. — Uber Kern- und Zellteilung im Pflanzenreich, nebst einem Anhang iiber
Befruchtung. Jena. 1888.
Vejdovsky, F. — Entvvickelungsgeschichtliche Untersuchungen. Heft i, Reifung,
Befruchtung und Furchung des Rhynchelmis-Eies. Brag. 1888.
Wilson, Edm. B. — Atlas of Fertilization and Karyokinesis. New York, 1895.
CHAPTER V
OOGENESIS AND SPERMATOGENESIS. REDUCTION OF THE
CHROMOSOMES
" Es komnit also in der Generationenreihe der Keimzelle irgendwo zu eiiier Reduktion
der urspriinglich vorhandenen Chromosomenzahl auf die Hiilfte, und diese Za/iUn-reduk-
tion ist demnach nicht etwa nur ein theoretisches Postulat, sondern eine Thatsache."
BovERl.i
Van Beneden's epoch-making discovery that the nuclei of the con-
iuo-atinsf srerm-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 uiatui-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 i^'&'J, i ), and has
been more recently supported by Van Bambeke ('94) and some others,
1 Zelleiistudien, 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 reduction
is effected by a rearrangement and redistribution of tlie nnclear 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
It is nevertheless certain that this loss is not directly con-
po'ir
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 Toxopneusfes (X 365).
A. Preliminary change of form in the germinal vesicle. D. The first polar body formed, the
second forming. C. The ripe egg, ready for fertilization, after formation of the two polar bodies
(p.b.,1, 2); ^, 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 OUTLIAE
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\\Q. 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 q^^
contains a large amount of protoplasm and yolk, out of which the
Primordial germ-cell.
Oogonia.
Primary oocyte or ovarian egg.
Secondary oocytes (egg and
first polar body).
^Mature egg and three polar bodies.
Division-period (the number of divi-
sions is much greater).
Growth-period.
" Maturation-period.
Fig. 87. — Diagram showing the genesis of the egg. [After Boaeri.]
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. Reduction in tJie Female. Formation of the Polar Bodies
As described in Chapter III., the Qgg 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
^ The parallel was first clearly pointed out by Plainer in 1889, ^f^' was brilliantly demon-
strated by Oscar Hcrtwig in the following year.
1/6 KEDUCriOX 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 oogonia (Fig. 87), which
are the immediate predecessors of the ovarian egg. 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 egg.
In this condition the egg-cell remains until near the time of fertili-
zation, when the process of maturation proper — i.e. the formation of
the polar bodies — takes place. In some cases, e.g. in the sea-urchin,
the polar bodies are formed before fertilization while the ^gg is still
in the ovary. More commonly, as in annelids, gasteropods, nema-
todes, they are not formed until after the spermatozoon has made its
entrance; while in a few cases one polar body may be formed before
fertilization and one afterwards, as in the lamprey-eel, the frog, and
Ampliioxus. 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, 86); 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 egg, 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
\77
To Butschli ('76,) Hertwig, and Giard {'yj) 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
G
Fig. 88. -Diagrams showing the essential facts in the maturation of the egg. The somatic
number of chromosomes is supposed to be four.
.^.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
ngure. c. 1 he mUotic figure has rotated into position, leaving the remains of the germinal
vesicle at gv D. Formation of the first polar body: each tetrad divides into two dvads.
Hi;,!/. polar body formed; two dyads in it and in the egg. F. Preparation for the second
division G. Second polar body forming and the first dividing; each dyad divides into two
single chromosomes H. Final result; three pol.ar bodies and the egg-nucleus ( 9 ). each con-
taining two single chromosomes (half the somatic number); ^. the egg-cemrosome which now
degenerates and is lost.
178
REDUCTION OF THE CHROMOSOMES
.4:s?>-^ .:.
w^^^m^^cm^^-^'
F
I
^^
K
Fig. 89. — Formation of the polar bodies in Ascaris megalocephala, var. bivaleiis. [BOVERI.]
A. The egg with the spermatozoon just entering at J" ; the germinal vesicle contains two rod-
shaped tetrads (only one clearly shown), the number of chromosomes in earlier divisions having
been four. B. The tetrads seen in profile. C. The same in end view. D. First spindle forming
(in this case inside the germinal vesicle). E. First polar spindle. F. The tetrads dividing.
G. First polar body formed, containing, like the egg, two dyads. H. I. The dyads rotating into
position for the second division. J. The dvads dividing. K. Each dyad has divided into two
single chromosomes, completing the reduction. (For later stages see Fig. 65.)
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
wit/ioNt ail 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 Qgg 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). T/ie number of tetrads is ahvays one-half the usual
number of chromosomes. Thus in Ascaris {megalocephala, bivalens)
the germinal vesicle gives rise to two tetrads, the normal number of
chromosomes in the earlier divisions being four ; in the 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 egg while the other
passes into the polar body. Both the &gg and the first polar body
therefore receive each a number of dyads equal to one-half the usual
number of chromosomes. The Qgg 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 egg while its sister passes
into the polar body. Both the Qgg 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 egg 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.
^ 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 c\\xovci?L\Jvi\-})iasscs
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 Q.gg. The actual divisions by
which the polar bodies are formed merely distribute the elements of
the tetrads.
2. Reduction ill 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-ce
Spermatogonia.
Division-period (the number of divi
sions is much greater).
Maturation-period.
Growth -period.
Primarj- spermatocyte.
Secondary spermatocytes.
Spermatids.
Spermatozoa.
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 Q.^^,'' 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
l8l
spcruiatogotiia 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 vicgaloccpliala bivalcns. Ceasing for a time to divide, they
Fig. 91. — Diagrams showing the essential facts of reduction in the male. The somatic num-
ber ot chromosomes is suppos(;d to be four.
A. B. Division of one of the spermatogonia, sliowing the fuH number (four) of chromosomes.
C. Primary spermatocyte prt- p.iiing for division ; tlie 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 s/^n-inaton'fc\s\ 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 KEDUCTIOX OF THE CHROMOSOMES
history of the chromatin in these tzvo divisions is exactly parallel to
that in tJic formation of the 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 ^gg 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 ^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 Significa7ice 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 number 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.
GEXERAL 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 biopJiorcs 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
A'an Beneden in his great work on Ascaris ('83). A fatal objection to it, on which both
Strasburger and Weismann have insisted, lies in the fact that male as well as female 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."
^ tJber die Bedeutung der Kerntheilungsfiguren.
1 84
REDUCTION OF THE CHROMOSOMES
" ids," the latter being idcntiiied 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
Fig. 92. — Reduction in the sperm.ifogenesis of Ascarh megalocephala, var. bivalens. [Brauer.] ^
A-G. Successive stages in the division of the primary spermatocyte. The original reticuhmi
undergoes a very early division of the chromatin-granules which then form a doubly split spireme-
tliread, D. This shortens (C), and breaks in two to form the two tetrads {D in profile, E viewed
endw ise) . 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 centrosonie.
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
1 For division of the spermatogonia see Pig. 39 ; for the corresponding phenomena in
var. iiiiii'alens 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 " 1 through the progressive summation of ancestral chromatins.
We now come to the vital point of Weismann's hypothesis of
reduction, about w^hich 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, w^hich 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 ]:)resent
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. ^ l-c, p. 375-
1 86 REDUCTION OF THE CHROMOSOMES
B. Origin of the Tetrads
I. General SketcJi
It is generally agreed that each tetrad arises by a double division of
a single primary chromatin-rod. Nearly all obser\'ers agree further
that the number of primary rods at their first appearance in the
o-erminal vesicle or in the spermatocyte-nucleus is one-Jialf the usual
nuviber of chromosomes, 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
lojigitudinal 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. \02, A). No reducing division can
therefore occur 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 -. -r* -> -,- In matu-
^ ^ abed
ration the thread segments into tn'o portions, ab — cd, each of which
then split into four equivalent portions, giving the equivalent tetrads,
ab Mb , cd i cd . x x y y ■ v • ^ \
thus, — r\—r and — 7 — -, . or . =^ — , smce it is not known
ab\ab cd 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 eacJi primaiy chroinatiji-ivd {¥'\g. \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 -j-. Each tetrad therefore consists, not of
a b c \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 accompHshed 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 Asearis,
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 Asearis, 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 Asearis, 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 chromatin-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
i88
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 Ascaris. 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 PyrrocJwris, they have since
been found in other insects by vom Rath and Wilcox, in various cope-
1 In an earlier paper on Braiichipiis ('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 {Gryllotalpa, '92), and
has been thoroughly elucidated by the later work of Riickert ('94)
and Hacker ('95, i). All these observers, excepting Wilcox and
°00§ 0\\ • •; ... , . Cl_
OOOO
0 0 1)0
^O 0 ° °
o
^J^^
D
Fig. 94. — Formation of the tetrads and polar bodies in Cyclops, slightly schematic. (The
full number of tetrads is not shown.) [RiicKERT.]
A. Germinal vesicle containing eight longitudinally split chroniatin-rods (half the somatic
number). D. Shortening of the rods; transverse division (to form the tetrads) in progress.
C. Position of the tetr.ids in the first polar spindle, tlie 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 transvetse 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
IQO 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 Euclueta and Calaiiiis, and
by Riickert in Hcterocope and Diaptouius.
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, Canthocamptus),
in which rings are not formed, since the splitting of the primary
chromatin-rods is complete. The origin of the tetrads has here been
traced with especial care in Cyclops strciiiiiis, 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 Diaptouius. 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
[91
Hacker ('92) has reached exactly similar results in the case of
CantJiocamptus and draws the same conclusion. In Cyclops stt'enuiis
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 the germinal vesicle ; d. common re-
sult, the typical tetrads; b. c. intermediate stages: at the left the ring-formation (as in Diaptomus,
Gryllotalpa, Heterocope) ; middle series, complete splitting of the rods (as in Cyclops according to
Ruckert, and in Cauthocamptns) ; at tlie right by breaking of the V-shaped rods (as in Cyclops
sti cnuHs, 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
^ Hacker upholds this account ('95, i) in spite of the criticisms of RUckert 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 eg^s, showing chromosomes, tetrads, and nucleoli.
A. A copepod (Hcterocope) showing eight of the sixteen ring-shaped tetrads and the nucleo-
lus. [RUCKERT.]
B. Later stape of tlie same, condensation and segmentation of the rings. [ROCKERT.]
C. "Cyclops sdeniiif'," illustrating Hacker's account of the tetrad-formation from elongate
double rods; a group of " accessory nucleoli." [Hackf.r]
D. (jerminal vesicle of an annelid {Ophrvofi-ocha) showing nucleolus and four chromosomes.
[K(_)RSCHF,l/r.]
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 193
rod is formed, which represents two equivalent pairs of chromosomes
ab
ab
During the two maturation-divisions the four chromosomes
are spHt apart, — nr» and Riickert's observations demonstrate that
a\b
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. Ruckert therefore
proposes the convenient term " pseudo-reduction " for this pre-
liminary halving.^ The 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, Ruckert, in
copepods ; Ruckert in selachians ; Born and Pick in amphibia ;
Holl in the chick; Ruckert in the rabbit.) Hacker ('92, 2) made the
interesting discovery that in some of the copepods {Cajithocaviptus,
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 true retictibnn
formed in tJie germinal vesicle (Fig. 97). In the following year Rlick-
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
Qg^^. Ruckert 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-clivision, 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 ('87) 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 Canthocamptus. [HACKER.]
og. The youngest germ-cells or oogonia (dividing at ^^. -) ; 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 larva;, but even in the embryo (!). This very remark-
able discovery showed that the pseudo-reduction miglit appear in the
early progenitors of the germ-cells during embryonic life — perhaps even
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 strenuus 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 are long 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
m 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
m the reduced number (twelve in the lily, eight in the onion) at
the finst division of the pollen-mother-cell, from which arise four
pollen-grains. In the female the same process takes place at the
f^rst division of the mother-cell of the embryo-sac. Strasburger
and Guignard agree that in the subsequent divisions these chromo-
somes do not form tetrads, but 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
'It may be recallc-,1 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 even 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 Ccmtozaniia divide with the reduced number, namely
eight ; and Dixon observed the same fact in Pinits at the same time.
In the following year Strasburger brought the matter to a definite
conclusion in the case of a fern {Osmnnda), showing that all the crlls
of the protJiallimn, from the original spore-motJier-cell omvarcis to the
fonnation of the germ-cells, have ouc-half the nmnher of cJiroviosomcs
found in the asexual generation, namely twelve instead of twenty-four;
in other words, the reduction takes place in the formation of the spore
from which the sexual generation arises, 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 tJie 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 (egg 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.
1 '78, p. 262.
REDUCTIOX IX THE PLANTS
197
Strasburger's hypothesis is, however, open to a very serious a
priori 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 lictcro typical form ; i.e.
that the cJiromosonies 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 _.._-... ,., , ,-,, > a
'■ . Fig. 98. — Division of the chromosomes (? tetrad-
remains an open question. . formanon) in the first division of the pollen-mother-
cell of the lily. (a. b. after FARMER and MoORE;
c-g. after Sargant.)
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.
i^. The daughter-chromosomes, seen en face, at the
shape, and Miss Sargant's moment of separation -, this stage is perhaps 10 be
very interesting observations interpreted as a tetrad like those occurring in the
-' '^ . salamander.
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.
Belajeff and Farmer showed
that as the daughter-chromo-
somes diverge after the first
division they assume a V-
198
liEDUCTION 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
E
H
Fig. 99. — Conjugation of Closteriitm. [Klebahn.]
A. Soon after union, four chromatophores. B. Chromatophores leduced to two, nuclei
distinct. C. Fusion of the nuclei. D. First cleavage of the zygote. E. Rrsulting 2-cell stage.
/•: Second cleavage. G. Resulting stage, each cell bi-nuoleate. H. Separation of the cells;
one of the nuclei in each enlarging to form the permanent nucleus, the other (proliably 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 Pammocciuui
ca7idatinn, 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 gregarinesWolters('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 after, conjugation of the nuclei. Thus
in the dermids Clostcrinm and Costnarii/ni, 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 occurring 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 Format ioii 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 Ruckert, 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-
iirus) ; 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 Pristinnis 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 Caloptcnus 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 : —
^bcd-l ab-cd-kl a^ 4(, etc. (tetrads).
(spireme) (segmented spireme) c\a K^'^
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 — -^> as he
^ ^ c\d
assumes, but either - or (if we assume that the normal number of
chromosomes undergoes a preliminary doubling) . • Until this
o r ^ ^ b\ 0
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 {Lwnbriciis tejTestris), 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
16 tetrads. The 32 primary double segments therefore represent
DIVERGENT ACCOUXTS OF REDUCTION 20I
chromosomes of the normal number that have spHt longitudinally,
a b , , r 1 r , . a b a x „ ,
I.e. T» etc., and the lormula 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 Ruckert, and agrees in mode of origin with
the process described by Ruckert in the eggs of Pristiiirus. 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 OpJiryotrocJia ('95), which
are very difficult to reconcile with anything known in other forms.
The typical somatic number of chromosomes is here four. The same
minibcr 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 O-^Z, but meanwhile each of them
again splits into two. Of the four chromosomes thus left in the (t^^,
two are passed out into the second polar body, while the two remain-
ing in the Q'gg give rise to the germ-nucleus. From this it follows
that the formation of the first polar body is a reducing division (!)
— a result which agrees with the earlier conclusions of Henklng on
PyiTochoris, but differs entirely from those of Ruckert, 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 Cautho-
caniptns, 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 Ruckert 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 i^Jj) on the
theoretical meaning of maturation the suggestion is made that
parthenogenesis may be due to failure on the part of the &g^ 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 ^'^^)
soon discovered, however, that the parthenogenetic eggs of Poly-
pJicuins (one of the Daphnidse) produce a single polar-body. This
observation was quickly followed by the still more significant dis-
covery by Blochmann ('88) that /// ApJiis the partJicnogcjietic eggs
produce a single polar body lohile the fertilized eggs produce txvo.
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 egg, but
remained embedded in its substance near the periphery. At the
same time Boveri (^7, i) discovered that in Ascaris the second polar
body might in exceptional cases remain in the Q.g^ 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 ^^^j:^ 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 ^
€r
(yoo^om
B
:■■'■■ '^'^.
E
X' -'-'<^2-
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 the egg-centrosome and aster. E. G. Division of the aster and formation
of the cleavage-figure; the equatorial plate consists of 84 apparently single but in reality bivalent
chromosomes.
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. too) but one polar body is formed, which removes eighty-four
dyads, leaving eighty-four in the o.^^. 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 munber
appears in later cleavage-stages.
{b) 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 egg. In the
~^"m
D
/•:
Fig. loi. — Second type of maturation in the parthenogenetic egg of Artetnia. [Brauer.]
A. Formation of second polar body. B. Return of the second polar nucleus {p.b:-) into the
egg ; devvlopment of the egg-amphiaster. C. Union of the egg-nucleus ( 9 ) 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.
BotJi these groups remain in the egg, and each gives rise to a sttigle
reticular nucleus, as described by Boveri in Ascaris. These tiva nuclei
place the VIS elves side by side in the cleavage figure, and give rise each
to eighty-four chromosomes, precisely like tivo germ-nuclei in ordinary
fertilization. The one hundred and sixty-eight chromosomes split
SUMMARY AA'D CONCLUSION 205
lengthwise, and are distributed in the usual manner, and reappear
ill the same unuiber in 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 Artemia 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 reductioji
in the number of chromosoines in the ttltimate genn-cells to one-half the
number eJiaracteristic 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-thread into one-ha^f the ns?tal number of
rods. This process is, however, not an actual reduction in the num-
ber of cJiromosomcs, but only a preliminary " pseudo-reduction " in
the number of chromatin-///c?'i'.s-rjr. 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, how^ever, 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
A
.t-
(
r-H
■■■■1 1'
>
h-1
^' ■ <
h-H
■'1 r
t -<
'■■1 ■'
tH
'"• r
1— 1
"i r
h-H
"n
,-\
'< r
^
( 1-1 \
(I— (>
(HI
cluced 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 cJiromosonics,
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
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 VVeismann.
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 contrasting the two
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 AXD 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 A^'temia ; 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 REDUCTION OF THE CHROMOSOMES
APPENDIX
I . Accessory Cells of the Testis
It is necessary to touch here on the nature of the so-called " SertoH-cells," or
supporting cells of the testis in mammals, partly because of the theoretical signifi-
cance attached to them by iMinot, 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
lum.en, where the spermatozoa are finally formed and set free. At the periphery is
a layer of cells next the basement-membrane, having flat, oval nuclei. Within
this.' the cells are arranged in columns alternating more or less regularly
with long, clear cells, containing large nuclei. The latter are the Sertoli-cells,
or supporting cells ; they extend nearly through from the basement-membrane to
the lumen, and to their inner ends the young spermatozoa are attached by their
heads, and there complete their growth. Tlie spermatozoa are developed from cells
which lie in columns between the Sertoli-cells, and which undoubtedly represent
spermatogonia, spermatocytes, 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-cells, and are
there converted into spermatozoa without further division. The deeper cells from
which they arise are spermatocytes, and the spermatogonia lie deeper still, being
probably represented by the large, rounded cells.
Two entirely different interpretations of the Sertoli-cells were advanced as long
ago as 1871, and both views still have their adherents. Von Ebner ('71) at first
regarded the Sertoli-cell 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 .still
maintained by Minot ('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 view, first suggested by Merkel (71), the
Sertoli-cell is not the parent-cell, but a nurse-cell, the spermatozoa developing from
the columns of rounded cells, and becoming secondarily attached to the Sertoli-cell,
which serves merely as a support and a means of conveying nourishment to the
growing spermatozoa. This view was advocated by Brown ('85), and especially by
Benda ("87). In the following year ('88), von Ebner himself abandoned his early
hypothesis and strongly advocated Benda\s views, adding the very significant result
that four spermatids arise from each spermatocyte, precisely as was afterwards
shown to be the case in Ascaris, etc. The very careful 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, Minot's theoretical interpretation of the Sertoli-cell
as the physiological equivalent of the polar bodies, of course collapses.
Various other attempts have been made 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. Van 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 tlie same light the '■ Nebenkern "
(Waldeyer) and the "residual globules" (Lankester, Brown) thrown off by the
developing spermatozoa of mammals. All of these views are, like iMinofs, wide
of the mark, and they were advanced before the real parallel between spermato-
genesis and ovogenesis had been made known by Platner and Hertwig.
APPENDIX 209
2. Atrn'tosis in the Early Sex-Cells
Whether the progenitors of the germ-cells ever divide amitotically is a question
of high theoretical interest. Numerous observers have described amitotic division
in testis-cells, and a few also in those of the ovary. The recent observations of
Meves (91), vom Rath ('93), and Preusse C95), 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 cycle
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 ot 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 Toeuf, la fecondation et la
division cellulaire : Arch. Biol., \W . 1883.
Boveri, Th. — Zellenstudien. I., III. Jena, 1887-90. See also " Befruchtung "
(List IV.).
Brauer, A. — Zur Kenntniss der Spermatogenese von Ascaris megalocepJiala : Arch.
jiiik. Aiiat., XLII. 1893.
Id. — Zur Kenntniss der Reifung der parthenogenetisch sich entwickelnden Eies
von Artcmia Saliiia : Arch. mile. Aitaf., XLIII. 1894.
Hacker, V. — Die \'orstadien der Eireifung (General Review) : Arch. inik. Atiat.,
XLV. 2. 1895.
Hertwig, 0. — Vergleich der Ei- und Samenbildung bei Nematoden. Eine Grund-
lage fiir cellulaire Streitfragen : Arch. mile. Anal., 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 vulgaris :
Arch mik. Anal.. XL. 1892.
Id. — Neue Beitrage zur Frage der Chromatinreduction in der Samen- und Eireife :
Arch. mik. Aiiat., XLVI. 1895.
Ruckert, J. — Die Chromatinreduktion der Chromosomenzahl im Entwicklungsgang
der Organismen: Ergebn. d. Anat. u. Ejitwick., III. 1893 (1894).
Strasburger, E. — Uber periodische Reduktion der Chromosomenzahl im Entwick-
lungsgang der Organismen : Biol. Centralb.,XlW 1894.
CHAPTER VI
SOME PROBLEMS OF CELL-ORGANIZATION
" Wir miissen deshalb den lebenden Zellen, abgesehen von der Alolecularstructur 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 Oi-ganization
bezeichnen." Brucke.i
"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. Brlicke 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 Briicke's time have in one form or other accepted
this conception, which indeed lies at the root of nearly all recent
attempts to analyze exhaustively the phenomena of cell-life. 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.
^ Ele7ncntaro7-ganismen, 1861, p. 386.
'^ Aiiihropogenie, 1891, p. 104.
3 For an exhaustive review of the subject see Yves Delage, La Slructiirc dit protoplasina,
ct les theories sur Vheredite. Paris, 1895.
210
THE NATURE OE CELL-ORGAXS 211
A. The Nature of Cell-organs
The cell is, in Briicke's words, an cloncntary 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
ca.se 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.
2 12 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 Butschli, 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 fibrillas and the homogeneous
fibrillae 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.g. 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 ^%Z> P- 576, 577- ■' '87- P- 266.
2 14 SOME PROBLEMS OF CELL-ORGANIZATION
I. Xuclcus and Cytoplasm
From the time of the earlier writings of Frommann ('65, '6f),
Arnold ('67), Heitzmann ('73), and Klein i^j?)), down to the present,
an increasing number of observers have held that the nuclear reticu-
lum is to be conceived as a modification of the same structural basis
as that which forms the cytoplasm. The latest researches indicate,
indeed, that true chromatin (nuclein) is confined to the nucleus.^
But the whole weight of the evidence now goes to show that the
linin-network is of the same nature, 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 (tg2, 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.
' Cf. Hammarsteii ("95).
-The long-standing dispute as to tlie origin of the nuclear menihranc (whether nuclear
or cytoplasmic) is therefore of little moment.
JfONF/rOLOG/CAL COMPOSITION OF yilE NUCLEUS 215
C. Morphological Composition of the Nucleus
I. The CJironiatin
(a) Hypothesis of the Individuality of the Chromosomes. — It
may now be taken as a well-established fact that the nucleus is
never formed dc 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 ehromosomes do not lose their individuality at the
close of division, but persist in the chromatic reticulum of the resting
nucleus. 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 1891, 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 Ascaris,
the force of which has, I think, been underestimated by the critics
of the hypothesis. Some of this evidence may here be briefly
reviewed. In some cases, through a miscarriage of the mitotic
mechanism, one or both of the chromosomes destined for the second
polar body are accidentally left in the egg. These chromosomes
give rise in tlie egg 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 Ascaru. [BOVERI.]
A. The two chromosomes of the egg-nucleus, accidentally separated, have given rise each to a
reticular nucleus (?, ?) ; the sperm-nucleus below (cf). B. Later stage of tlie 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, 9 'h^ egg-chromo-
somes, cf 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 sins^le 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. I53)- From
the cleavage-nucleus tlius formed arise four chromosomes.
MORPHOLOGICAL COMPOSITION OF THE NUCLEUS
217
These remarkable observations show that whatever be the number
of cJirovwsonies entering into the formation of a reticular miclens, the
same number afterwards issue from it — -a. result which demonstrates
that the number of chromosomes is not clue 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 egg of Ascaris. [BOVEKI.]
E. Anaphase of the first cleavage. F. Two-cell stage with lobed nuclei, the lobes formed by
the ends of the chromosomes. G. Early prophase of the ensuing division ; chromosomes re-form-
ing, centrosomes dividing. H. Later prophase, the chromosomes lying with their ends in the
same position as before ; centrosomes divided.
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 OE 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 RUCKERT; D. from Hacker.]
A. First cleavage-figure in C.strenuus; complete independence of paternal and maternal
chromosomes. D. 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. breviconiis.
tion, tJicir cuds lying in the nuclear lobes as before (Fig. 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
MORPHOI.OCICAL COMPOSiriON OF THE NUCLEUS 219
prepares for division, however, the chromosomes contract, withdraw
their processes, and return to their '' resting state," in which iission
takes place. Applying this conclusion to the fertilization of the egg,
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 tJiat in all cells derived in the regular course
of division from the fertilized egg, one-half of the chroviosonies are of
strictly paternal origin, the other Jialf 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 Ruckert, 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
tenuicornis)&2iQh 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 Ruckert (95, 3) in a species of Cyclops, likewise identi-
fied as C strenuus {¥\g. 105). The number of chromosomes in each
germ-nucleus is here twelve. Ruckert 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 Ascaris 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 tzoo distinct
groups, and Ruckert 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 Artcmia, no escape is left from the
hypothesis of the individuality of the chromosomes in one form or
B
C
Fig. io6. — Hybrid fertilization of the egg of Ascaris megalocephala, var. bivaUiis, 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), the egg-nucleus to two (9). C. Two-cell stage dividing,
showing the three chromosomes in each cell. D. Twelve-cell stage, with the three distinct chro-
mosomes still shown in the primordial germ-cell or stem-cell.
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 XUCLEUS 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 egg the chromosomes may persist without loss of
their boundaries from one division to another, since no reticulum is
formed (cf. p. 193).
{b) Compositio)i of the Cliromosomes. — 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 Bedeuliing der Kertitheilungsfiguren^ 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 viorpJiological composition of the chromatin.
CHROMATIN, LIN IN, AND THE CYTORETICULUM 223
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 Beneclen 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 tJie chromatic and achromatic microsomes miirht
be transformed into one another, and zuc7'-e 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 arc conjectured to be different
phases 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 arc never-
1 '83, p, 580, 583. ^ 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 zuhich forms the dynamic centre of the cell
and mnltiplics 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 egg, and the same may be true of some tissue-cells.
Van Beneden's and Boveri's independent identification of centrosome in Ascaris
as a permanent cell-organ ('Sy) 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. 'Sg. '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 (Blitschli, '91), in the giant-
cells and other cells of bone-marrow (Heidenhain, Van Bambeke, Van der Stricht,
'91), in the flagellate Nottiliica (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 doubling 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-
dently discovered it inside the nucleus of the resting cell, — Wasielewsky, in the
vouno- ovarian eggs (oogonia) of Ascaris ; Brauer. in the spermatocytes of the same
animal; and Karsten. in the cells of a plant. Psilotitm (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, Knuten) have followed Wasielewsky in locating it in tlie 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
^-^ — //
A
♦
B
C
D
E
^^^'^
a
Fig. 107. — Mitosis with intra-nuclear centrosome, in the spermatocytes of Ascaris megalo-
ccphala, var. univalciis. [BraUKR.]
A. Nucleus containing a quadruple group or tetrad of chromosomes (/), nucleolus (//), and
centrosome {c). B. C. Division of the centrosome. D. E. F. G. Formation of the mitotic figure,
ccntrosomes escaping from the nucleus in G.
Ascaris the centrosome lies, in one variety {mtivalcns) inside the nucleus, in the other
variety {bivalens') outside — z. 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 tlie facts of mitosis in the Infusoria, where the whole mitotic tigure
appears to arise within the nuclear membrane (cf. p. 62).
Whether a true centrosome may ever arise de novo is Hkewise
undetermined. The possibiHty of such an origin has been conceded
by a number of recent writers, among them Biirger, Watase, Richard
Hertwig, Heidenhain, and Reinke. The latter author ('94) would
Q
226 SOME FJWBLEMS OF CELL-ORGANIZAriOM
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 vmfertilized
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, A) the centrosome appears under the highest
powers as nothing more than a single granule of extraordinary
minuteness which stains intensely with iron-haematoxylin, and can
scarcely be distinguished from the cyto-microsomes except for the
fact that it 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), iji Chcetoptcnis (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 Asearis
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 contirm
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 J
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 T/ialassenia, however,
Grififin'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. \\,D). In the sea-urchin {Echinus) 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 Toxopjicn-
stcs, 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 tliat 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 regard?
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 centrosphere. As far as the
sea-urchins are concerned, there is, I think, good reason to doubt not
only my own former concUisions, 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 Thalasscma. More-
over, Grifhn's studies under my direction show that the minute single
centrosome of Thalasscvia 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 cjuestion has not been definitely answered. Biitschli,
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 tlic centrosome is
itself nothing other than a viicrosonic 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 sublimate-acetic, Init is perfectly shown
after pure sublimate or picro-acetic. .See Science, Jan. lo, 1S96.
- 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 like 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 ('88, 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, " arc/ioplasni,'' 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 he 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 conce]5tion is, on the whole, I think, opposed to the facts, though it
certainly explains the injiushing of the nuclear membrane during the prophases of mitosis.
It is im|jossible 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 individuality.^ In a later paper on the
sea-urchin ('95) this view is somewhat modified by the admission that
in this case the archoj^lasm 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 ^"jQ), 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, Umax, 1881.
* '92, 2, pp. 158-169.
^ It is interesting to note that in the same place Klein anticipated the theory of fibrillar
contractility, both the nuclear and the cytoplasmic reticulum being regarded as contractile
{I.e., p. 417).
'■' '83. P- 59-- ' '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 {Toxopncnstcs)
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 lea.st, 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.
1 '5
3, p. 550. - I.e., p. 263. 3 i_c_^ p. 275. * I.e., p. 280. 5 '95, 2, p. 446.
232 SOME PROBLEMS OF CELL-ORGAXJZATION
2. The Attraction-Sphere
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 astroccntre, and the spherical mass sur-
rounding it (attraction-sphere of Van Beneden) the astrosphere.
Strasburger accepted the latter term and proposed the new word
" centrosphere " for the astrosphere and the centrosome taken to-
gether.2 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 r;////v ^^sVr/- exclusive
of the centrosome, in which sense the phrase "astral sphere" had
been employed by Mark in 1881. The word "astrosphere" has
therefore a double meaning and would better be abandoned in favour
of Strasburger's convenient term "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 Drliner 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 RJiyncIichiiis, by Solger and
Zimmermann in pigment-cells, by myself in sea-urchin eggs and in
^'83, p. 548. -'92, p. 5i-
THE ARCHOPLASMIC STRUCTURES
233
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 incline 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-
-:^Wm>
B
-///,
/
/Ijfr;
t F G H
Fig. 108. — Diagrams illustrating various descriptions of centrosome and centrosphere.
A. Simplest type; only a minute centrosome at the focus of the ra\s (sp'rm-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 of Ascaris. C. Ra\s pro-
ceeding from a clear centrosphere (astrosphere of Strasburger) , enclosing a centro>ome like
the last but with no central granule (in flowering plants according to Guignard, Strasburger,
and others). D. An e.xtremely minute centrosome lying in the middle of a large reticulated cen-
trosphere {e£'. Hill's description of the sperm-aster in sea-urchins and tunicates). £. Like the
last, but with a small spherical body surrounding the centrosome (examples, the eggs of Ihalas-
scma 2,x\6. Nereis). F. No centrosome as distinguished from tiie rrticulated centrosphere. Ex-
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 tlie cortical and medullary zones
of the attraction-sphere. H. The same according to Boveri. The centrosome contains a cen-
tral granule or centriole (cf. B.) ; outside this is a clear zone (medullary zone of Van B- neden),
and outside this a vaguely defined granular zone, probably corresponding to \"an Beneden's
cortical zone.
ure, in doubt. Van Beneden described the centrosphere in Ascaris
as consisting of an outer cortical and an inner vicdiillary 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
SOME PROBLEMS OF CELL-ORGAXIZATION
minute central granule or ccntriolc. 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
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
dividt^ like the centrosome. Vom Rath, who has made a very careful
study of the attraction-spheres in a large number of cells among both
vertebrata 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
Fig. 10^. — SnjcniKitogonium of salaman-
der. [DiU'NF.R.]
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.
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 Xcrcis 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 Strongyloccutrotus and Phallnsia. In these latter cases the
centrosphere shows no differentiation into cortical and medullary
zones. In TJialasseina 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 Tlialassema 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 Echiinis 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 attractiou-spheres, asters, and
spindle are, like the nucleus, differcntiatioiis of the general cell-netivork,
which is, as it zvere, moulded 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.
1 The same general result is indicated in the case of plants, though the phenomena have
here been less carefully examined.
236 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 dc 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 tJic degree of permanence depends on the
degree of coJiesion nianfested 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 which 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 beheve that the linin-network is likewise com-
posed of minute bodies, the oxychromatin-granules, which are closely
similar m 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 23/
ules 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 chez les Ascidiens et ses rapports avec
Torganisation de la larve : Arch. Biol.. V. 1884.
Boveri, Th. — Zellenstudien. (See List IV.)
Briicke, C. — Die Elementarorganismen : Wiener Sitz.-Ber., WAN . 1861.
Biitschli, 0. — Protoplasma. (See List I.)
Hacker, V. — Uber den heutigen Stand der Centiosomenfrage : I'erh. d. deiitsch.
Zool. ues. 1894.
Heidenhain, M. — (See List I.)
Heria, V. — Etude des variations de la mitose chez Tascaride megalocephale : /Ire/i.
/>VV7/.. XIII. 1S93.
Nussbaum, M. — Uber die Teilbarkeit der lebendigen Alaterie: Arc/i. mile. Anat.,
XXVL 1886.
Rabl, C. — Uber Zellteilung : Morph. Jahrh.. X. 1885.
Riickert, J. — (See List IV'.)
De Vries, H. — Intracellulaie Pangenesis: yena, 1889.
Watase, S. — Homology of the Centrosome: Joitrn. Morph., VIII. 2. 1893.
Id. — On the Nature of Cell-organization : U'ooi/s Holl Biol. Lectures. 1893.
Wiesner, J. — Die Elementarstruktur und das Wachstum der lebenden Substanz :
Wien, 1892.
Wilson, Edm. B. — .\rchoplasm. Centrosome, and Chromatin in the Sea-urchin
Egg: Joitrii. Morph.., \'ol. XI. 1895.
CHAPTER VII
SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY
" Les phenomenes fonctionnels ou de depense vitale anraieut done leur siege dans le
protoplasme celhilaire.
" Le noyau est un appareil de syntlu'se organiqiu, I' instrument de la production, le gertne
de la celllller 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 wddely 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
^ I.e^ons sur les pkenomefies de la 77>, L, 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 Protcids and tJicir 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 albuminous substajices, including under them the
various forms of albumin (egg-albumin, cell-albumin, muscle-albumin,
vegetable-albumins), globulin (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 nuclein, and the nucleo-
albumiiis. 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, emplovcd 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).i
1 See Hammersten, '95, p. 16.
240
SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY
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 between widens 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, unclein and nncleo-firoteids,
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. The Nnclein Series
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 m-
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 C.gH^gN.PgO.^^. Later analyses gave some-
what discordant results, as appears in the following table of per-
centage-compositions : ^ —
C
H
N
P
PrS-CELLS.
(Hoi>pe-Seyler.)
49.58
7.10
15.02
2.28
Spermatozoa of Salmon.
(Miescher.)
36.11
5-15
13.09
5-59
Human Brain.
(v. Jaksch.)
50.6
7.6
13.18
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 omittea in this table.]
CHEMICAL KELATIOXS OF NUCLEUS AXD CYTOPLASM 241
the nuclein may be synthetically formed by the re-combination of
these two substances. Pure 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 lo\ver 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 o\\\\ work as well as that of Liebermann,
Altmann, Malfatti, and others, that " what the histologists designate
as chromatin 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 mav 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)
2i\\(\ 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-
1 '93, p. 158. 2 '53^ p. 5-4.
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 ivle of nucleus and cytoplasm in metabolism.
3. Staining-rcactioiis of tJic Ahiclein-scries
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 tJic various members of tJie nuclcin series shoiv an affinity for
tJie basic dyes in direct proportion to the amount of niie/eie acid
{as measured by the amount 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 "
(Ilammarsten) 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
ali)umin and nucleic acid, the latter yielding as cleavage-products the nuclein bases. Pseudo-
nucleins containing a large percentage of albumin are designated as nucleo-allniimns, 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.
^ From Lilienfeld, after Kossel, '92, p. 129.
CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM 243
Nucleo-albi(ijti)i (i % of P or less),
by peptic digestion splits into
Peptone
Nudein (3-4 % P),
by treatment with acids splits into
Albumin
Nncleic acid (9-10 % P).
heated with mineral acids splits into
Phosphoric acid
Nnclein bases
(adenin, guanin, etc.).
i^A carbohydrate.^
Now, according to Ko.ssel and Lilienfeld, the principal nucleo-
albumin (nucleo-proteid) in the nucleus of leucocytes is micleo-Jiiston,
containing about 3 % of phosphorus, which may be split into a form
of nnclein 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 coloin-, but onlv according to their chemical
1 I.e., p. 394.
244 ^OME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY
nature. Such terms as " erythrophilous," " cyanophilous," and the
like 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 structures, 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
verv 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-
iCf. p. 127. 2 '94, p. 543. 3/.^., p. 547. s
W^iM^
r^>^
n
Fig. 113. — Nucleated and non-nuoleated fragments of Amxba. [HOP'ER.]
A. D. An Ammha 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.
dctcr»iination 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
Vanchcria and other algae were incapable of forming a new cellulose
membrane if devoid of a nucleus ; and he afterwards showed ('87)
252 SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY
that the same is true of Zygnciiia and CEdigoniitvi. 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 chlorophyll 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 Xiieleus
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 (j"]) 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 CgO, throw doubt on Palla's
conclusion.
PHYSIOLOGICAL RELATIONS 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. [tiAKERLAXnT.]
A. Young epidermal cell of Lmula with central nucleus, before thickening of the membrane.
B. Three epidermal cells of Momtera, during the thickening of the outer wall. C. Cell from the
seed-coat of Scopulina during the thickening of the inner wall. D. E. Position of the nuclei dur-
ing the formation of branches in the root-hairs of the pea.
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
primary 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. 114, D, E). The same is true of the
254 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 Vauchcria the
o-rowincf resrion, 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 larvas (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 Forficiila each ^gg 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 I.e., p. 99.
PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM 255
of such cells at one pole of the o.^^ from which the latter is believed
to draw its nutriment (Fig. 58). A very interesting case is that of
the annelid OpJiryotrocha, referred to at p. 114. Here, as described
by Korschelt, the ^g^ floats in the perivisceral fluid, accompanied
by a nurse-cell having a very large chromatic nucleus, while that of
the egg is smaller and
poorer in chromatin.
As the ^tgg 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 egg is
in a measure relieved
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
haps the most sugges-
tive of these relate to
the nucleus of the Q^'g
during its ovarian his-
tory. In many of the
Fig. 115. — Upper portion of the ovary in the earwig
Forjicula, showing eggs and nurse-cells. [KORSCHELT.]
Below, a portion of the near!)' ripe egg {e), showing deuto-
plasm-spheres and germinal vesicle (£'v). Above it lies the
mseCtS, as in both the mnse-cell («) with its enormous branching nucleus. Two
cases referred to above successivelyyounger stages of egg and nurse are shown above,
the egg-nucleus at first occupies a central jjosition, but as the
egg begins 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 Qgg. Most suggestive of all is the case
of the water-beetle Dytiscns, 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 egg, 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. The Xnclcus in Mitosis
To Wilhelm Roux ('83) w^e 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.
PHYSrOLOGICAL KELATIOXS OF NUCLEUS AND CYTOPLASM 257
must represent different qualities, and second, that the apparatus of
mitosis is designed to distribute these qualities, according to a
definite law, to the daughter-cells. The particular form in which
Roux and VVeismann 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 Qgg), 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-divi?ion
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 Xitclcits ill Fertilization
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
m 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 analvsis 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
s
258 SOME ASPECTS OF CELL-CIIEMISTKY AND CELL-PHYSIOLOGY
fact that every part of the latter may show the characteristics ot
either or both parents. , • i .4.
Boveri ('89. '95, i ) has attempted to test this cone usion by a most
ingenious and beautiful experiment; and although his cone usions do
^ not rest on absolutely certam
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
larva;, 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
{SphcvrccJiinus granulans) with
the spermatozoa of another
( Ecliiniis Diicrotiiberculatns), Bo-
veri obtained in a few instances
dwarf Plutei showing purely
paternal characteristics ( h ig.
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.i Boveri's result
is unfortunately not quite conclusive, _ as has ^een pointed^^^t
bv Seeli-er and Morgan, yet his extensive experiments establish, 1
hink a s? ong presumption in its favour. Should they be positively
fonfii-med, the' would furnish a practical demonstration of mheritance
through the nucleus.
Fig. 116. — Normal and dwarf larvte of the
sea-urchin. [BOVERI.]
A. Dwarf Pluteus arising from an enucleated
eg<^-fragment ot Sp/uerechnnis gra>ii„°^''^'''- ' '""
repeateclh observed U,e Internal changes in .be bv.ng eggs of r«../»e„./«.
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 Toxoptieustes.
A. One sperm-nucleus has united with the egg-nucleus, shown at <^. ^ ; the other lies above
Both sperm-asters have divided to form amphiasters («, b and ., d) . B The cleavage nuc^eu^
formed by unton of the three germ-nuclei, ,s surrounded by the fo^r asters. C. ResJt of the fi.^
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. SUMM.-XRV 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
metabohsm 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 stammg-
capacity undergone by the chromatin during the cycle of cell-life,
taken in connection with the researches of physiological chemistson
the chemical composition and staining-reactions of the nuclem-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 o the
chromosomes as nearly pure nucleic acid. When this is correlated
with the fact that the sperm-nucleus, which brmgs 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 m
the constructive processes of the cytoplasm.
The role of the nucleus in constructive metabolism is intimately
related with its rdle in morphological synthesis and thus m inheri-
tance • for the recurrence of similar morphological characters must in
the last analysis be due to the recurrence of correspondmg fornis o
metabolic action of which they are the outward expression. That
the nucleus is in fact a primary factor in morphological aswe 1 as
chemical synthesis is demonstrated by experiments on unicellular
plants and animals, which prove that the power of regenerating los
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 m
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-
vero-e 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 he
spermatozoon is as potent in inheritance as the ovum though the
latter contributes an amount of cytoplasm which is but an infini-
SUMMARY AXD 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. — Le(;ons sur les Phenomenes de la Vie: ist ed. 1878; 2d ed.
1885. Finis.
Chittenden, R. H. — Some Recent Chemico-physiological Discoveries regardinjj the
Cell: Am. Nat., XXVIII., Feb., 1894.
Haberlandt. G. — Uber die Beziehungen zwischen Funktion und Lage des Zellkerns.
Fisc/wr. 1887.
Halliburton, W. D. — A Text-book of Chemical Physiology and Pathology. London,
1 89 1.
Id. — The Chemical Physiology of the Cell {Gouldstonian Lectures): Brit. Med.
JoilDl. 1893.
Hammarsten. 0. — Lehrbuch der physiologische Chemie. 3d ed. Wiesbaden, 1895.
Hertwig, 0. & R. — Uber den Befruchtungs- und Teilungsvorgang des tierischen
Eies unter dem Eintiuss Jiusserer 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 Morphologie und Physiologic des Zell-kernes : Zo'ol.
Jahrb. Anat. 11. Onto_i^.. 1\'. 1889.
Kossel, A. — Uber die chemische Zusammensetzung der Zelle : Arc/i. Anat. u. Fhys
1S91.
Lilienfeld, L. — Uber die Wahlverwandtschaft der Zellelemente zu Farbstoffen :
.bxh. Anat. 11. Fhys. 1893.
Malfatti, H. — Beitrage zur Kenntniss der Nucleine : Zeitschr. Fhys. Chevi., XVI
1891.
Riickert, J. — Zur Entwicklungsgeschichte des Ovarialeies bei Selachiern : An. Anz.
VII. 1892.
Sachs, J. — Vorlesungen uber Pflanzen-physiologie. Leipzig, 1882.
Id. — Stoff und Form der Pflanzen-organe : Gesammelte Abhandlnngen, II. 1893.
Verworn, M. — Die Phvsiologische Bedeutung des Zellkerns : Arch, fiir die Ge<;.
Fhys.,XL\. 1892.
Id. — Allgemeine Physiologic. Jena, 1895.
Zacharias, E. — Uber Chromatophilie : Ber. d. dentsch. Bot. Ges. 1893.
Id. — Uber des Verhalten des Zellkerns in wachsenden Zellen : Flora. 81. 1895.
Whitman, C. 0. — The Seat of Formative and Regenerative Energv : Journ. Morph..
II. 1888.
CHAPTER VIII
CELL-DIVISION AND DEVELOPMENT
" Wir koiinen demnach endlich den Satz aufstellen, dass siimmtliche im entwickelten
Zustande vorhandenen Zellen oder Aequivalente von Zellen durch eine fortschreitende
Gliederung der Eizelle in morphologisch ahnliche Elemente entstehen, und dass die in einer
embryonischen Organ-Anlage enthaltenden Zellen, so gering auch ihre Zahl sein mag,
dennoch die ausschliessliche ungegliederte Anlage filr sammtliche Formbestandtheile der
spateren Organe enthalten." Remak.i
Since the early work of Kolliker and Remak it has been recog-
nized that the cleavage or segmentation of the ovum, with which
the development of all higher animals begins, is nothing other than
a rapid series of fnitotic cell-divisions by which the egg splits up
into the elements of the tissues. This process is merely a contin-
uation of that by which the germ-cell arose in the parental body.
A long pause, however, intervenes during the latter period of its
ovarian life, during which no divisions take place. Throughout this
period the egg leads, on the whole, a somewhat passive existence,
devoting itself especially to the storage of potential energy to be used
during the intense activity that is to come. Its power of division
remains dormant until the period of full maturity approaches. The
entrance of the spermatozoon, bringing with it a new centrosome,
arouses in the egg a new phase of activity. Its power of division,
which may have lain dormant for months or years, is suddenly raised
to the highest pitch of intensity, and in a very short time it gives
rise by division to a myriad of 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 Untersucliiingen, 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
first cleavage the position in w^hich the embryo will finally appear in
the ^%g may be exactly predicted. Such " promorphological " rela-
tions of the segmenting egg 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. TJic Geometrical Relations of Cleavage-fonns, with reference
to the general laws of cell-division.
2. T/ie PromorpJiological 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. TJie cell typically tends to divide into equal parts.
2. EacJi new plane of division tends to intersect the preceding plane
at a rig J it angle.
In the simplest and least modified forms the direction of the
cleavage-planes, and hence the general configuration of the cell-
system, depends on the general form of the dividing mass ; for, as
Sachs has shown, the cleavage-planes tend to be either vertical to the
surface (anticlines) or parallel to it (periclines). 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 iiyperbolic anticlines. B. C. Apical view of terminal knob on epidermal iiair of
Pinguicola. B. shows the ellipsoid type, C. the circular (spherical type), somewhat modified
(only anticlines present). D. Growing point of Salvinia (Pringsheim) ; typical ellipsoid type,
the single pericline is however incomplete. E. Growing point of Azolla (Strasburger) ; circular
or spheroidal type transitional to ellipsoidal. F. Root-cap of Eqiiisetum (Niigeli and Leitgeb) ;
modified circular type. G. Cross-section of leaf-vein, Triclioinanes (Prantl) ; ellipsoidal type with
incomplete periclines. H. Embryo of Alismci; typical ellipsoid type, pericline incomplete only
at lower side. /. Growing point of bud of the pine {Ad/£s) ; 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 26y
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 i^gg in the initial stages
successively into halves, quadrants, and octants, the periclines being
parallel to the surface so as to separate the inner ends of these cells
from the outer. No case is known in which this order is accurately
followed throughout, and the periclinal cleavages are of compara-
tively rare occurrence, being found as a regular feature of the early
cleavage only in those cases where the primary germ-layers are sepa-
rated by delamination. The simplest and 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 Qgg of the holothurian Synapta, as shown in
the diagrams. Fig. 120, constructed from Selenka's drawings. ^ The
first cleavage is vertical, or Jiuridional, passing through the egg-axis
and dividing the egg into equal halves. The second, which is also
meridional, cuts the first plane at right angles and divides the egg
into quadrants. The third is horizontal, or equatorial, dividing the
(tgg into equal octants. The order of division is thus far exactly
that demanded by Sachs's 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.
268
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 {Y\g. 122), where
the typical succession in the number of cells is with great constancy
D E
Fig. 120. — Cleavage of the ovum in the holothurian Synapta (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, n, 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. E.xceptions
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
^ The pressure is probably due primarily to an attraction between the cells {cyto/ropisin
of Koux), but may be increased by the presence of membranes, by tur_<^(ir, or by sjiccial
processes of gr. \o.
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 egg, 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 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 egg 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 t2,g.
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
1 '93, p. 115. - /.<■., p. 112. 3 j'/igorie der Abstaviinungslehre, 1884.
THE IDIOPLASM THEORY 3OI
oreat merit of Nageli's hypothesis to consider inheritance as effected
iDy 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
trophoplasui. Hereditary traits are the outcome of a definite molec-
ular organization of the idioplasm. The hen's egg 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 niiccUcB.
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 micellse ; 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 chroiiiatiu. 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 INIIERITANCE 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, Ruckert, 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 nucleus contains the physical basis of
inheritance, ajid that chromatin, its essential constituent, is the idio-
plasm postulated in Nageli 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 Qg'g. 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-IVEISMANN THEORY OF DEVELOPMENT 303
RoLix, Dc 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 gcinvuilcs,
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 czWo.^ 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 ^2,2,, 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 tlie germ-cell, and those of the tissue-cells are derived from this
source in' 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, we need not, however, necessarily adopt the
pangen -hypothesis.
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-Weismann 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
dijfcrcnt 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 correspoudiiig
differentiation in the daughter-eells. 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, therefore, receives a specific form of chroniatiti 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 OF 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 biopJiorcs are conceived to be successively ag-
gregated in larger and larger groups ; namely, (i) dcteinniiiants, 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 {'' lioiiiccokinesis,'" integral
or quantitative division), the resulting nuclei remain precisely equiva-
lent. In the second case {'' /wtcrokinesis,'' qualitative or dijfcrential
division), the daughter-cells receive different groups of chromatin-
elements, and hence become differently modified. During ontogeny,
through successive qualitative divisions, the elements of the idioplasm
or genn-plasni (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 07ie 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 which the hereditary substance is progressively disinte-
grated. In order to account for heredity through successive genera-
tions, Weismann is obliged to assume that, by means of quantitative
or integral division, a certain part of the original germ-plasm is car-
ried on unchanged, and is finally delivered, with its original architecture
unaltered, to the germ-nuclei. The power of regeneration is explained,
in like manner, as the result of a transmission of unmodified or slightly
modified germ-plasm to those parts capable of regeneration.
1 Essay IV., p. 193, 1885. - Gerni-plasin, 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 (///cri-Z-metaphysical char-
acter, which indeed almost places it outside the sphere of legitimate
scientific hypothesis. Not a single visible phenomenon of cell-divi-
C
D
Fig. 132. — Half and whole cleavage in the eggs of sea-urchins.
A. Normal i6-cell stage, showing the four micromeres above (from Diiesch, after Selenka).
B. Half i6-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 i6-cell stage of Toxopneiistes, 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
A B
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 afterivards regenerated the
missing Iialf, and gave rise to a complete embryo. Essentiall)' 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 OE INHERITANCE AND DEVEIOPMENT
meres of sea-urchin eggs separated by shaking to pieces the two-
cell and four-cell stages. Blastomeres thus isolated segment as if
still forming part of an entire larva, and give rise to a half- (or 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-cell stage segmenting like an entire egg (cf. Fig, 123, Z)).
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 AnipJiioxus, but here the isolated blastomere seg-
CRITIQUE OF THE ROUX-WEISMANN THEORY
309
ments from the beginning like an entire ovitni of diuiinisJicd size (Figs.
133, 124). The same result has since been reached by Morgan in the
teleost fishes, and by Zoja in the medusa. 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
^^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 Toxopneustes. 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 Pfliiger and Roux on the frog's Q^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, zvitJiont 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
lO
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 entoderm-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 XATURE AND CAUSES OE DIFFERENTIATION 31I
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. dijfcrcntiationf 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 egg.
These changes are sooner or later accompanied by the cleavage
of the egg 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 ^ZZ 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 .^ 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 egg 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 Qgg 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.
ox 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 annelids.
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 (;!/) 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 blastoniere in the zvliole de-
termines in general ivJiat develops from it; if its position be changed,
it gives rise to something different ; in other words, its prospective
value is a function of its position^' **
This conclusion undoubtedly expresses a part of the truth, though,
as will presently appear, it is too extreme. The relation of the part
1 That is, in the specifically organized chromatin within the nucleus.
93. P- 793-
Stuilien IV. p. 25.
^ Studien IV. 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 Beroc. [DRiESCii 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 first quartet in both gives
rise to ectoderm. Beyond this point, however, the agreement ceases :
ox THE NA TURE AND CA USES OF DIFFERENTIA TION 3 I 5
for the second and third quartets form mesoblast in the polyclade,
but ectoblast in the annelid 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 the inJicritcd 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 Qgg 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 egg 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 frog2(Fig. 135, A-D). But, more than this, these
experimenters made the interesting discovery that if a part of the
cytoplasm of an jmscgDicntcd ctenophore-egg were removed, the
remainder gave rise to an incomplete larva, sJioiving certain defects
li'hich 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 X.o 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 nnsegmentcd 'i'^i,^^, 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.
- The larva is, however, not a strict partial one, since it makes an abortive attempt to
form the normal number of gastric pouches.
i6
THEORIES OF 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 Ampliioxjis and the echinoderms, and with the more
o-eneral conclusion that the ultimate determining causes of differentia-
Fig. 139. — Partial development of isolated blastomeres of the gasteropod egg, llyanassa.
[CRAMI'TON.]
A. Normal 8-cell stage. B. Normal i6-cell stage. C. Half 8-cell 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 i6-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 317
between AinpJiioxus on the one hand, and the snail or ctenophorc
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 Atialy-
tiscJtc Thcoric dcr organiscJicn Entivickliing{^()^), 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 egg, 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 Qgg 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 limited 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 ctenophorc, on the one hand, with the total development
of such forms as Aviphioxus 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
3i8
THEORIES OF INHERITANCE AND DEVELOPMENT
the development of the blastomcre is from the beginning 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
Fig. 140. — Double embryos of frog developed from eggs inverted when in the 2-cen stage.
[O. SCHULTZE.]
A. Twins with heads turned in opposite directions. B. Twins united Ijack 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 changing this distribution the axis of the
embryo is shifted. Oscar Schultze ('94) discovered that if the ^^^ 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 w^hole half-sized dwarf might be formed, according to the
position of the blastouiere. 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 Aniphi-
oxHS and the echinoderms. In Aniphioxns 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 Toxopncustes 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. Aiiz., 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 AmphioxiLS 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 Amphioxus, 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 32 1
G. The Nucleus in Later Development
Tlie 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 dijfcrcntiatiou sooner or
later involves a specification of the nuclear substance ivhich differs in
degree in different cases. W'hen 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 i)art 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 Nus.sbaum, 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 the gcrm-nuclci retain the entire ancestral heritage. Boveri
hirnself 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.i though
he is careful not to commit himself to any definite theory. It hardly
seems possible to doubt that in Asca7ds 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 ivitJiout recourse
to the 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 down by Nageli, regards the
action as essentially dynamic, i.e. as a propagation of molecular
movements from nucleus to cytoplasm in a manner which might be
compared to the transmission of a nervous impulse. When, however,
we consider the role of the nucleus in synthetic metabolism, and the
relation between this process and 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-
mg pangens, and these bemg, 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.
^ '91. P- 433-
THE EXTERXAL CONDITIONS ofi DEVELOPMENT 323
It is but a step from this to the very interestini;" 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—/.^, chromatin — in a less
highly modified condition, and the differences between the various
tissues in this regard, or between related organisms, would find a
natural explanation.
Development may thus be conceived as a progressive transforma-
tion of the egg-substance primarily incited by the nucleus, first mani-
festing itself by specific changes in the cytoplasm, but sooner or later
involving in some measure the nuclear substance itself. This process,
which one is tempted to compare to a complicated and progressive
form of crystallization, begins with the youngest ovarian egg 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 Condition.s 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-
]24
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 general
principle that they involve. Every living organism at every stage
of its existence reacts to its envn-on-
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. I40- Ii"^
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 m 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 invagmate to
form a typical gastrula, but evaghiatcs 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 {Strongyloce7itro-
tus). B. Larva {Spkcsreckhiits) 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. — Regeneration in coelenterates (. /. D. from LoEB ; C. D. from BiCKFORD).
./. Polyp {Cerianthus) producing new tentacles from the aboral side of a lateral wound.
B. Hydroid ( Ttibularia) 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 Ttibularia.
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 Tubularia, 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
Tiibularia 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 end (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 nonnnl developineiit is in a greater or less degree tJie response
of tJie developing oiganisni to iiormal 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 direetly eansed by the external conditions ; for the
egg 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-
ino- 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 brinoino- the foreooins; 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 12 J
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 Vervvorn 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 &g^ 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 .' 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 difficulty confronts us under any theory we can frame. If
with Herbert Spencer we assume the germ-plasm to be an aggrega-
tion of like units, molecular or supra-molecular, endowed with prede-
termined polarities which lead to their grouping in specific forms,
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 Gerin-plas//i , p. 14.
2 Evohition, Science and Culture^ p. 296.
^ Gerniinal Selection, January, 1S96, p. 284.
PREFORMATION AND EPIGENESIS 329
epigencsis has now arrived at a stage where it has Httle meaning
apart from the general problem of physical causality. What we
know is that a specific kind of livdng 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 w^e are made to realize, as by a flash of
light, how far we still are from a solution of this problem. ^ It may
be true, as Schwann himself urged, that the adaptive power of
living beings differs in degree only, not in kind, from that of unor-
ganized bodies. It is true that we may trace in organic nature long
and finely graduated series leading upward from the lower to the
higher forms, and we must believe that the wonderful adaptive mani-
festations of the more complex forms have been derived from simpler
conditions through the progressive operation of natural causes. But
1 See Wolff, '95, and MuUer, '96.
2 See Wolff, '94, for an admirably clear and forcible discussion of this case.
330 THEORIES OF INHERITANCE AND DEVELOPMENT
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 Orgaiiismus ohne miitterliche Eigen-
schaften: Sitz.-Ber. d. Ges. f. Morph. unci P/iys. hi Miinchen, V. 1889. See
iX'i.o ArcJi. f. Eiitiviii . 1895.
Brooks, W. K.— The Law of Heredity. Ballimorc, 1883.
Driesch, H. — Analytische Theorie der organischen Entwicklung. Leipzig, 1894.
Herbst, C — Uber die Bedeutung der Reizphysiologie fur die kausale Auffassung
von Vorgangen in der tierischen Ontogenese : BioL Centralb., XIV., XV.
1894-95.
Hertwig, 0. — Altere and neuere Entwicklungs-theorieen. Berlin, 1892.
Id._Urmund und Spina Bifida: Arch. mik. Anat.. XXXIX. 1892.
Id. — iJber den Werth der ersten Fiu-chungszellen fur die Organbildung des
Embryo: Arch. mik. Anat., XIA\. 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 Morphologic: I. Heteromorphosis.
Wiirzburg, \?>()i . II. Organbildung und Wachsthum. IVnrzdurg, iSg2.
Id. — Some Facts and Principles of Physiological Morphology: J food's Moll Biol.
Lectures, i S93 .
Nageli, C. — Mechani.sch-physiologische Theorie der Abstammungslehre. MYin-
chen u. Leipzig, 1884.
Roux, W. — Uber die Bedeutung der Kernteilungsiiguren. Leipzig. 1883.
PREFORMATION AND EPIGENESIS 33!
Roux, W. — iJber das klinstliche Hervorbringen lialber Embryonen durch Zerstorung
einer der beiden ersten Furchungskugeln, etc. : Vircliow's Archiv, 114. 1888.
Sachs, J. — StofFund Form der Pfianzenorgaiie : Ges. Ah/iandlungeji, II. 1893.
Weismann, A. — Essays upon Heredity, First Series. Oxford, 1891.
Id. — Essays upon Heredity, Second Series. Oxford, 1892.
Id. — Aussere Einflusse als Entwicklungsreize. Jena, 1894.
Whitman, C. 0. — Evolution and Epigenesis : IVood^s Holl Biol. Lectures. 1894.
Wilson, Edm. B. — On Cleavage and Mosaic-work: Arch, fiir Entwicklungsm.,
III. I. 1896.
GLOSSARY
rObsolete terms are enclosed in brackets. The name and date refer to the first use of the word ;
LUDsoiete ^^^^^^^^^^t changes of meaning are indicated m the definition.]
Achro'matin (see Chromatin), the non-staining substance of the nucleus, as
opposed to chromatin: comprising the ground-substance and the hnm-netwo,k.
(Flemmixg. 1880.)
TAkaryo'ta] (see Karyota). non-nucleated cells. (Flemmixg. 1882.)
kte cithal (ipriv. : A.'k.^o., the yolk of an egg), having httle or no yolk (applied
to eo-o-sV (Balfour. 1880.) . .
Amito'sls (see Mitosis), direct or amitotic nuclear division; mass-division ot
the nuclear substance without the formation of chromosomes and amphiaster.
AtipMalteM^aJx^?' on both sides; aar^p. a star), the achromatic tigure formed
in mitotic cell-division, consisting of two asters connected by a spindle. (FOL.
Amphipy'renin (see Pyrenin). the substance of the nuclear membrane.
Amyloplasts '[Z^Xov, starch: TrXacrro., 7rU/xa, skin), the living protoplasm asserted to fomr a part of the
cell-membrane in plants. (Wiesner, 18S6.)
Der'matosomes (8e'p/xa, skin ; o-w/xa. body), the plasomes which form the cell-mem-
brane. (WiESXER, 1886.)
Determinant, a hypothetical unit formed as an aggregation of biophores, determin-
ing the development of a single ceil or independently variable group ot cells.
(Wels-maxx, 1 891.)
[Deuthy'alosome] (8euT(epos), second ; see Hyalosome), the nucleus remaining
in the egg after formation of the tirst polar body. (Vax Bexedex. 1883.)
Deu'toplasm (SturCepos), .second ; TrAatr/xu, anything formed), yolk, lifeless food-
matters deposited in the cytoplasm of the &%% ; opposed to -protoplasm."' (Van
Benedex, 1870.)
Directive bodies, the polar bodies. (Fr. Ml'LLER, 1848.)
Directive sphere, the attraction-sphere. (Guigxard. 1891.)
Dispermy, the entrance of two spermatozoa into the egg.
336 GLOSSARY
Dispi'reme (see Spireme), that stage of mitosis in wliich eacli daugliter-nucleus
has given rise to a spireme. (Flemming, 1882.)
Dy 'aster (8m?. two; see Aster. 2). the double group of chromosomes during the
anaphases of cell-division. (FlemiMING, 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
Beneden. (O. Hertwig, 1875.)
Enchyle'ma (eV. in; x"'^os, juice), i. The more fluid portion of protoplasm,
consisting of "hyaloplasma." (Hanstein, 1882.) 2. The ground-substance
(cvtolvmph) of cytoplasm as opposed to the reticulum. (Carnov, 1883.)
Biier'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 spindle
during mitosis. (Van Beneden. 1875.)
Erythro'philous (ipvOpo^, red; c^tAeu'. to love), having an especial affinity for
red dves. (Auerbach.)
Ga'mete (ya/xeVr/, wife: yajxh-q^. husband), one of two conjugating cells. Usually
applied to the unicellular forms.
G-em'mule (see Pangen). one of the ultimate supra-molecular germs of the cell
assumed by Darwin. (Darwin. 1868.)
[Gre'noblasts] (ycVos, se.x ; /^Aacrros. germ), a term applied by Minot to the mature
germ-cells. The female genoblast (egg, or '• thelyblast ") unites with the male
(spermatozoon or •• arsenoblast ") to form an hermaphrodite or indifferent cell.
(ATixoT. 1877.)
Germinal spot, the nucleolus of the germinal vesicle. (Wagner, 1836.)
Germinal vesicle, the nucleus of the egg before formation of the polar bodies.
(PuRKiNjE. 1825.)
Germ-plasm, the same as idioplasm. (Weismann.)
Heterole'cithal (eVepos. different; AeKt^os, yolk), having unequally distributed
deutoplasm (includes telolecithal and centrolecithal). (Mark, 1892.)
Heterotyp'ical mitosis (crepo?, 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] (0A05. whole ; or_;^i'^eti/. to split), direct nuclear division. Amitosis.
(Flemming, 1882.)
Homole'cithal (6/xo?. the same, uniform; AeKt^os, yolk), equivalent to alecithal.
Having little deutoplasm, equally distributed, or none. (Mark. 1892.)
Homceotyp'ical mitosis (o/xoto?. like: see Mitosis), a form of mitosis occurring
in the spermatocytes of the salamander, differing from the usual tvpe only in the
shortness of the chromosomes and the irregular arrangement of the daughter-
chromosomes. (Flemming, 1887.)
Hy'aloplasma (ilaAo?, glass; TrXdafxa, 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-
gioplasm." (Levdig, 1885.) 3. The exoplasm or peripheral protoplasmic zone
in plant-cells. (Pfeffer.)
Hy'alosomes ( JaAo?. glass ; aw/xa, body), nucleolar-like bodies but slightly stained
by eitlier nuclear or plasma stains. (Lukjanow. 1888.)
[Hy'groplasma] (vyp6<;, wet; TrAarr/xa. 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 tlie successive aggregation of biophores,
determinants, and ids. Identified by Weismann as the chromosome. (Weis-
MANX, 1 891.)
Id'ioblasts (tStos, one's own, /^Aucrro?. germ), the hypothetical ultimate units of the
cell; the same as biophores. (O. Hertwig. 1893.)
Id'ioplasm (I'Sios, one's own; 7rAao-/Aa, a thing formed), equivalent to the germ-
plasm of Weismann. The substance, now generally identified with chromatin.
which bv its inherent organization involves the characteristics of the species.
The physical basis of inheritance. (Nageli, 18S4. )
Id'iosoine (18105, one's own; awfxa, body), the same as idioblast or plasome.
(WaiTMAX. 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., iS8r.)
Iso'tropy (i,'cros, equal; rpoTrr;, a turning), the absence of predetermined axes (as
applied to the egg). (Pfluger, 1883.)
[Ka'ryaster] {Kapvov, nut, nucleus ; see Aster. 2). the star-shaped group of chromo-
somes in mitosis. Opposed to cytaster. (Flem.mixg, 1882.)
Karyenchy'ma (Kapvov, nut. nucleus; ev, in; x^^/x-o's, juice), the "nuclear sap."
(Flemmixg, 1882.)
Karyokine'sis (Kapvov, nut, nucleus; KtVr;o-ts, 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; Avert?, 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; p.LTo}p.a, from /AtVos, a thread), the nuclear
as opposed to the cytoplasmic thread-work. (Flemmixg. 1882.)
Karyomito'sis (Kapvov, nut. nucleus; see Mitosis), mitosis. (Flemmixg,
1882.)
Ka'ryon (Kapvov. nut. nucleus), the cell-nucleus. (Hackel. 1891.)
Ka'ryoplasm ( Kaovoi/. nut. nucleus ; TrAao-jua. a thing formed), nucleoplasm. The
nuclear as opposed to the cytoplasmic substance. (FLE^^MIXG. 1882.)
Ka'ryosome {Kapvov, nut. nucleus; o-w/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. (Platxek. 1886.) 3. Caryo-
some. The cell-nucleus. (Wata.se, 1894. )
[Karyo'ta] {Kapvov, nut. nucleus), nucleated cells. (Fle.mmixg. 1882.)
Karyothe'ca (Kapvov, nut. nucleus; 6y]Kr], case. box), the nuclear membrane.
(H.ACKKL. 1S91.)
Ki'noplasm (klvuv, to move; irXaiTp-'i, a thing formed), equivalent to archoplasm ;
opposed ])y Strasburger to the '• trophoplasm " or nutritive plasm. (Stras-
burger, 1892.)
[Lanthanin] {kavOavw, to conceal), equivalent to oxychromatin. (Heidex-
haix, 1892.)
z
338
GLOSSARY
Leucoplas'tids (Xet-Kos, white; TrAaoros, 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. (Schw'ARZ. 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) duera, beyond {i.e. further) ; K^vvrjcn^, 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 extnision 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 (/xera. after, beyond; TrAacr/Lia, 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 (^i/cpds, small; ttvXt], 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?. small ; aw/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
Iwse of the flagellum. (Schweigger-Seidel, 1865.)
Mid-body (•• Zwischenkbrper"). 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 (/Atrcofta, from /aitos, a thread), the reticulum or thread-work as opposed
to the ground-substance of protoplasm. (Flemming, 1882.)
[Mitoschi'sis] du-tro?, thread; cr;>^t'^etv, to split), indirect nuclear division; mito-
sis. (Fle.mming, 1882.)
Mito'sis (|U,tTo?. a thread), indirect nuclear division typically involving: a. the
formation of an am]3hiaster; ^, conversion of the chromatin into a thread
(spireme) ; c, segmentation of the thread into chromosomes ; d, splitting of the
chromosomes. (Flemming, 1882.)
Mi'tosome (ixlto?, a thread ; (rw/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 wliich yields
no xanthin-bodies.
Nucleochylema (x^Aos, 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 chromatin-granules are suspended. (Strasburger, 1882.)
Nucleomi'crosomes (see Microsome), the nuclear (chromatin) granules as
opposed to those of the cytoplasm. (Strasburger, 1882.)
Nu'cleoplasm. i. The reticular substance of the (egg-) nucleus. (Van
Benedex, 1875.) -• "^^^^ 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 {oihr]ixa, a swelling), the granules or microsomes of the nuclear ground-
substance. (Reixke. 1893.)
O'ocyte (Ovocyte), (cJdi/. egg; Kt'ro?, hollow (a cell)), the ultimate ovarian egg
before formation of the polar bodies. The primary oocyte divides to form the
first polar bodv and the secondary oocyte. The latter divides to form the second
polar body and the mature egg. (Boveri. 1891.)
Oogen'esis, Ovogenesis (wdv, egg; yeVeo-is, origin), the genesis of the egg after
its origin by division from the mother-cell. Often used more specifically to
denote tlie process of reduction in the female.
Oogo'nium, Ovogonium (cJdi'. egg ; yovrj. 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 (cJdi/, egg; KLvrjcn?. 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 (ofws, acid ; see Chromatin), that portion of the nuclear substance
stained by acid aniline dyes. Equivalent to -linin" in the usual .sense.
(Heidenhain, 1894.)
Pangenesis (7r5s (Trai/-), all; yeVecrt?, production), the theory of gemmules, ac-
cording to which hereditary traits are carried by invisible germs thrown off by
the individual cells of the Ixidy. (Darwin. 1868.)
Pangeas (ttSs (vrav-), all; -yei'T^?. 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 (-rrav. all; /aepo?. part), relating to an ultimate protoplasmic structure
consisting of independent units. See Pangen.
Parachro matin (see Chromatin), the achromatic nuclear substance (linin of
Scliwarz) 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. (Fle.m.mixc;. 1892.)
Paranu'clein (see Nuclein). the .substance of true nucleoli or plasmosomes.
Pyrenin of Schwarz. (O. Hertwtg, 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'aplasni {Trapd, beside; TrAacr/xa, something formed), the less active portion of
the cell-substance. Originally applied by Kupffer to the cortical region of the
cell (exoplasm), hut now often applied to the ground-substance. (Kupffer,
I875-)
Per'iplast (TrepL around ; TrAacrros, form), a term somewhat vaguely applied to the
attraction-sphere. The term daughter-periplast is applied to the centrosome.
(\'FjnovsKV, 1888.)
Plas'mosome (TrXdcrfxa, something formed (/.c. protoplasmic) ; crw/xa. body), the
true nucleolus, distinguished by its affinity for acid anilines and other •' plasma-
stains."' (Ogata, 1883.)
Pla'sonie (TrAacr/xa, a thing formed; croj/xa. body), the ultimate supra-molecular
vital unit. See Biophore. Pangea. (Wiesner, 1890.)
Plas'tid (TrAacTTos, form), i. A cell, whether nucleated or non-nucleated. (Hackel,
1866.) 2. A general term applied to permanent cell-organs (chloroplasts, etc.)
other than the nucleus and centrosome. (Schimper. 1883.)
Plas'tidule. the ultimate supra-molecular vital unit. (Elssberg, 1874; Hackel,
1876.)
Plas'tin, a term of vague meaning applied to a substance related to the nucleo-
proteids and nucleo-albumins constituting the linin-netwoik (Zacharias) and the
cytoreticulum (Carnoy). (Reinke and Rodewald, 1881.)
Pluriva'lent {plus, more ; 7'alere. to be worth), applied to chromatin-rods that
have the value of more than one chromosome sensn strictii. (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 Roiiix.
1862.)
Polar corpuscle, the centrosome. (Va\ Bexeden. 1876.)
Polar rays (Polradien). a term sometimes applied to all of the astral ravs 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. 1S77.)
Polyspermy, the entrance into the ovum of more than one spermatozoon.
Prochro 'matin (see Chromatin), the substance of true nucleoli, or plasmosomes.
Equivalent to paranuclein of O. Hertwig. (Pfitzner, 1883.)
Pronuclei, the germ-nuclei during fertilization; /.t'.. 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. ( Vax Bexeden,
1875.)
[Prothy'alosome] (see Hyalosome). an area in tlie germinal vesicle {oi Ascan's)
by which the germinal spot is surrounded, and which is concerned in formation
of the first polar body. (Van Bexeden, 1S83.)
Pro'toblast (Trpwro?. first : ^Aaoros, a germ), a naked cell, devoid of a membrane.
(KOLLIKER.)
Pro'toplasm (TrpoiTos. first ; TrAacr/xa. a thing formed or moulded), i. The living
substance of the cell, comprising cytoplasm and karyoplasm. (Purkyxe. 1840;
H. VON MoHL. 1846.) 2. The cytoplasm as opposed to the karyoplasm.
Pro'toplast (TrpwTo?. first; TrAaord?. formed), i. The protoplasmic body of the
cell, including nucleus and cytoplasm, regarded as a unit. Nearly equi\-alent to
the energid of Sachs. (Han.steix. 1880.) 2. Used by some authors synony-
mous! v with plastid
[Pseudochro'matin] (see Chromatin), the same as prochromatin. (Pfitz.ner,
1886.)
GLOSSARY 341
Pseudonu'cleiii (see Nuclein). the same as the paranuclein of Kossel. (Ham-
MARSTKX. 1894.)
Pseudo-reduction, the preliminary halving of the number of chromatin-rods as a
prelude to the formation of the tetrads and to the actual reduction in tlie number
of chromosomes in maturation. (RuCKERT, 1894.)
Pyre'nin {-n-vpi'^v. 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 {Trvprjv, 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. (Sch.mitz, 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 tlie mammalian
testis to which the developing spermatozoa are attached. (Equivalent to - sper-
matoblast"' as originally used by vox Ebxer, 1871.)
Spermatid {cnrepixa, seed), the tinal 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 {aTrep/xa, seed; /iJAaards. 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, (vox Ebxer, 1871.)
Sper'matocyst (o-7re'p/i,a, seed ; Kuort?, 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.)
Spermatocyte ( o-Trepjua, seed ; kv'tos, hollow^ (a cell)), the cells arising from the
spermatogonia. The primary spermatocyte arises by growth of one of the last
generation of spermatogonia. By its division are formed two secondary sper-
matocytes, each of which gives rise to two spermatids (ultimately spermatozoa).
(La Valette St. George. 1876.)
[Spermatogem'ma] {cnr(.pp.a. seed; gemma, bud), nearly equixalent to spermato-
cyst. Differs in the absence of a surrounding membrane. [In mammals.
La Valette St. George, 1878.]
Spermatogen'esis {(nvipp-a, seed; yeVecns. origin), the phenomena involved in
the formation of the spermatozoon. Often used more specifically to denote the
process of reduction in the male.
Spermatogo'nium (" Ursamenzelle " ) {(Jiripjxa, seed; yovr/. 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 (a-n-epixa, seed; /xepo?. a part), the chromatin-granules into
which the sperm-nucleus resolves itself after entrance of the spermatozoiin. (In
Petroiiiyson. Bohm. 1887.)
Sper'matosome ( o-Trep/xu. seed; a^p-a, body), the same as spermatozoon. (La
\\vlette St. George, 1878.)
Spermatozo'id (see Spermatozoon), the ciliated paternal germ-cell in plants.
Tlie word was first used by von Siebold as synonymous with spermatozoon.
Spermatozoon {(j-Kkppxu seed; ^woi/, animal), the paternal germ-cell of animals.
(Lei:l\vk\hoek. 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. Hertwtg. 1875.)
Sper'mocentre. the sperm-centrosome during fertilization. (FoL, 1891.)
342 GLOSSARY
Spi'reme ((T7reip>7jaa. 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 {(nroyyiov, a sponge; 7rAacr/x,a, a thing formed), the cytoreticulum.
(Levdig, 1885.)
Ste'reoplasm {uTepto'i, solid), the more solid part of protoplasm as opposed to the
more fluid " hygroplasm." (Nageli, 1884.)
Substantia hyalina, the protoplasmic ground-substance or "hyaloplasm."
(Levdig. 1885.)
Substantia opaca, the protoplasmic reticulum or " spongioplasm." (Leydig,
1885.)
Te'loblast (re'Aos, end; /^AacrTo?, 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 (reAos. end ; AeKt^os. yolk), that type of ovum in which the yolk is
mainlv accumulated in one hemisphere. (Balfour, 1880.)
Telophases. Telokine'sis (reAos, end), the closing phases of mitosis, during
which the daughter-nuclei are re-formed. (Heidenhain, 1894.)
To'noplasts (two?, tension ; TrAao-ro?, form), plastids from which arise the vacuoles
in plant-cells. (De Vries, 18S5.)
Trophoplasni (rpocfiy, nourishment ; irXdafxa) . 1 . 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 (rpocfyy]. nourishment; TrAao-ro?, 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" bv Carus,
1850.]
Zy'gote or Zy'gospore (^uyoV, a yoke), the cell produced by the fusion of two
conjugating cells or gametes in some of the lower plants.
GENERAL LITERATURE-LIST
The following list includes only the titles of works actually referred to in the text
and those immediately related to them. For more complete bibliography the reader
is referred to the literature-lists in the special works cited, especially the following.
For reviews of the early history of the cell-theory see Remak"s Untersiiduuigen
C50-'55). Huxley on the Cell-theory ('53), and Tyson's Cell-doetrine (^8). An
exhaustive review of the earlier literature on protoplasm, nucleus, and cell-
division will be found in Flemming's Zellsubstanz ('82), and a later review of
theories of protoplasmic structure in Butschli's Protoplasnia ('92). The earlier
work on mitosis and fertilization is very thoroughly reviewed in Whitman's Clep-
sine C78). Fol's Henogenie ('79), and Mark^s Umax ('81). For more recent
general literature-lists see especially Hertwig's Zelle iind Gewebe ('93), Yves
Delage ("95), Henneguy's Cellule ('96), and the admirable reviews by Flemming,
Boveri, Rlickert, Roux, and others in Merkel and Bonnet's Ergebnisse ("91 -'94).
The titles are arranged in alphabetical order, according to the system adopted in
Minot's Human Embryology. Each author's name is followed by the date of publi-
cation (the first two digits being omitted, except in case of works published before
the present century), and this by a single number to designate the paper, in case
two or more works were published in the same year. For example, Boveri, Th.,
'87, 2, denotes the second paper published by Boveri in 1S87.
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. in. .4. Archiv fiir mikroscopische Anatomic.
A. Entm. Archiv fur Entwicklungsmcchanik.
B. C. Biologischcs Centralblatt.
C. R. Comptes Rendus.
/. i\I. journal of Morphology.
J. Z. Jenaischc Zeitschrift.
M. A. Miiller's Archiv.
M.J. Morphologisches Jahrbuch.
Q. J. Quarterly Journal of Microscopical Science.
• Z. A. Zoologischer Anzeiger.
Z. ru. Z. Zeitschrift fiir wissenschaftliche Zoologie.
ACQUA, "91. Contribuzione alia conoscenza della cellula vegetale : Malpighia,
V. — Altmaii. R., "86. Studicn liber die Zelle. 1. : Leipzig. — Id., '87. Die Genese
der Zcllen: Leipzig. — lA., '89. Uber Nucleinsaure : A. A. /'., p. 524. — Id.,
'90. "94. Die Elementarorganismen und ihre Beziehung zu den Zellen : Leipzig. —
Amelung, E.. '93. IMjer mittlere Zellgrcisse : Elora. p. 176. — Arnold, J., '79.
343
344 GENERAL LITERATURE-LIST
Uber feinere Struktur der Zellen, etc. : Virclunv^s Arch., 1879. (See earlier papers.)
— Aiierbach. L.. '74. Organologische Studien : Ii?-eslaii. — Id.. "91. Uber einen
se.xuellen Gegensatz in der Chromatophilie der Keim.substanzen : Sitziingsber. der
KonigL preiiss. Akad. d. W/ss. Berlin, XXXV^
VON BAER. C. E., '28, '37. Uber Entwickelung.sgescliiclite der Thiere. Beo-
bachtung und Relie.xioii : I. Koiiigsberg, 1828; II. 1837. — Id., '34. Die Metamor-
phose des Eies der Batrachier: MYiUer^s Arch. — Balbiani. E. G., "64. Sur la
constitution du germe dan.s I'oeuf animal a\ant la fecondation : C. R., LVIII. —
Id., '76. Sur les phcnomenes de la division du noyau cellulaire : C. R , XXX., Octo-
ber, 1876. — Id., "81. Sur la structure du noyau des cellules salivares chez les larves
de Chironomus : Z. .1.. 1881, Nos. 99, 100. — Id., "89. Recherches experimen-
tales sur la merotomie des Infusoires cilies : Reciieil ZooL Suisse, Januarv, 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 : your)i. de Tauat . et de la phvsiol ., XXIX. —
— Balfour, F. M.. "80. Comparative Embryology: I. 1S80. — Ballowitz, ■88-"91.
Untersuchungen iiber die Struktur der Spermatozoen : i. (birds) A. in. A., XXXII.,
1888; 2. (insects) Z. iv. Z., LX., 1890; 3. (fishes, amphibia, reptiles) .-/. ///. A.
XXXVI., 1890: 4. (mammals) Z. iv. Z., 1891. — Id.. "89. Fibrillare Struktur und
Contractilitat : Arch. ges. Rhys., XLVI. — Id., "91, 2. Die innere Zusammensetz-
ung des Spermatozoenkopfes der Saugetiere : Centralb. f. Rhys., V. — Id., "95.
Die Doppelspermatozoa der Dytisciden : Z. «'. Z.. XLV., 3. — "Van Bambeke. C,
"93. Elimination d'elements nucleaires dans I'oeuf ovarien de Scorp;tna scrofa :
A. B., XIII. I. — De Bary. "58. Die Conjugaten. — Id.. "62. Uber den Bau
und das Wesen der Zelle : RIora, 1862. — Id.. "64. Die Alycetozoa : 2d Ed., Leip-
zig.— Barry, M. Spermatozoa observed within the JMammiferous Ovum: Rhil.
Trans., 1843. — Beale, Lionel S.. "61. On the Structure of Simple Tissues of the
Human Body: London. — Bechamp and Ester, '82. De la constitution elemen-
taire des tissues : Montpellier. — Belajeff, "94, 1. Zur Kenntniss der Karvokinese
bei den Pflanzen : Flora, 1894. Erganzungsheft. — Id., "94, 2. Uber Bau und
Entwicklung der Spermatozoiden der Pflanzen: Flora, LIV. — Benda. C, "87.
Untersuchungen iiber den Bau des funktionirenden Samenkenkalchens einiger Sau-
gethiere : A. ni. A. — Id., '93. Zellstrukturen und Zelltheilungen des Salaman-
derhodens : I'crh. d. Anal. Ges., 1893. — "Van Beneden, E., '70. Recherches
sur la composition et la signification de I'oeuf: Ulem. cour. de TAc. roy. d. S. de
Belgique, 1870. — Id., '75. La maturation de Toeuf, la fecondation et les premieres
phases du developpement 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., \W . — "Van Beneden and
Julin, '84. 1. La segmentation chez les Ascidiens et ses rapports av^c Torgani-
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 TAscaride megaloctfphale : Ibid.. 1887. — Bergh, R. S., "94. Vorlesungen Uber
die Zelle und die einfachen Gewebe : Wiesbaden. — Id., "95. Uber die relativen
Theilungspotenzen einiger Embryonalzellen : . /. Entin., II., 2. — Bernard, Claude.
Legons sur les Phenomenes de la Vie: ist Ed. 1878, 2d Ed. 1885, Raris. — Ber-
tliold, G., "86. Studien iiber Protoplasma-mechanik : Leipzig. — Bickford, E. E.,
GENERAL LITERATURE-LIST 345
'94. Notes on Regeneration and Heteromoqshosis of Tubulaiian Hydroids : /. M.,
IX., 3. — Biondi, D.. '85. Die Entwicklung der Spermatozoiden : A. m. A., XXV.
— Blanc, H.. '93. Etude sur la fccondation de I'neuf de la truite : Ber. Natiir-
forsch. Ut's. zii FrL-il>i/>\^\ VIII. — Blochmann, F., '87.2. Liber die Richtungs-
korper bei Insekteneiern : M. J.. XII. — Id., "88. ('ber die Richtungskorper bei
imbefruclitet .sich entwickelnden Insekteneiern: Verli. natiirh. med. Ver. Heidel-
berg, N. F., IV.. 2. — Id.. '89. L'ber die Zahl der Riclitungskorper bei befruchteten
und unbefruchteten Bieneneiern : J/. J. — Id., '94. Cber die Kerntheilung bei
Euglena : B. C, XIV. — Bohm. A.. '88. Uber Reifung und Befruchtung des Eies
von Petromyzon Planeri : A. iii. A., XXXII. — Id.. '91. Die Befruclitung des
Forelknieies : Sitz.-Be?'. d. Ges.f. Morph. it. Phys. MYiiiclwn. VII. — Boll, Fr., '76.
Das Princip des Wachstliums : Berlin. — Bonnet, C, 1762. Considerations sur
les Corps organises: .linsterdain. — Born. G.. "85. ('ber den Einfluss der
Schwere auf das Froschei : A. in. A.. XXIV'. — Id.. '94. Die Structur des Keim-
blaschens im Ovarialei von Triton taeniatus : A. ;//. A., XLIII. — Bonrne, G. C,
'95. A Criticism of tlie Cell-theory ; being an answer to Mr. Sedgwick's Article on
the Inadequacy of the Cellular Theory of Development: O. y., XXXVIII., i. —
Boveri, Th., "86. Uber die Bedeutung der Richtungskorper: Sitz.-Ber. Ges.
Morph. 11. FJiys. Miincheii. II. — Id.. '87.^. Zellenstudien, Heft I. : J. Z., XXL —
Id., '87, 2. Uber die Befruchtung der Eier von Ascaris niegalocephala : .Sitz.-Ber.
Ges. Morph. Phys. Miuuheii, III. — Id., '87, 2. Uber den Anteil des Spermatozoon
an der Teilung des Eies: Sitz.-Ber. Ges. Morph. Phys. MYinchieiu III., 3- — Id.,
'87. 3. Uber Dififerenzierung der Zellkerne wahrend der Furchung des Eies von
Ascaris meg.: A. A., 1887. — Id., '88, 1. Uber partielle Befruchtung: Sitz.-Ber.
Ges. Morph. Phys. Miincheiu IV.. 2.— Id., '88, 2. Zellenstudien,'' II. : J. Z.,
XXII. — Id., "89. Ein geschlechtlich erzeugter Organismus ohne nflitterliche
Eigenschaften : Sitz.-Ber. Ges. Morph. Phys. M'lhiehen,^ . Trans, in Am. Nat.,
March, '93.— Id., '90. Zellenstudien, Heft III.: J.Z., XXIV. —Id., '91. Be-
fruchtung: Merkel imd BonnePs Ergebiiissc, I. — Id.. '95, 1. Uber die Befruch-
tungs-und Entwickelungsfahigkeit kernloser Seeigel-Eier, etc: A. Entiii., II., 3. —
Id., '95, 2. Uber das Verhalten der Centrosomen bei der Befruchtung des Seeige-
leies, nebst allgemeinen Bemerkungen uber Centrosomen und Verwandtes : Verh. d.
Physikal.-iiied. Gesellschaft zu Wiirzburg, N. F., XXIX., i. — Braem, F., '93.
Des Prinzip der organbildenden Keimbezirke und die entwicklungsmechanischen
Studien von H. Driesch : B. C, Xlll., 4, 5. — Brandt, H., "77. Uber Actino-
sphxrium lOichhornii : Dissertation, Halle, i?,J7 . — 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. preiiss. A/cad. ll'iss., '92. — Id., '93. 1.
Zur Kenntniss der Reifung des parthenogenetisch sich entwickelnden Eies von
Artemia Salina : A. m. A., XLIII. — Id.. '93, 2. Zur Kenntniss der Spermato-
genese von Ascaris megalocephala : ./. ni. A., XLII. — Id.. '94. Uber die En-
cystierung von Actinosphirrium Eichhornii : Z. w. Z., LVllL, 2. — Braus. '95.
Uber Zellteilung und Wachstum des Tritoneies: J. Z., XXIX. — Brooks. "W. K.,
'83. The Law of Heredity: Baltimore. — 'Rxovvw, H. H., "85. On Spermato-
genesis in the Rat: Q. J., XXV. — Brown, Robert. "33. Observations on the
Organs and Mode of Fecundation in Orchides and AsclepiadecX : Trans. Linn.
Sac, 1833. — Brunn. M. von, '89. Beitrjige zur Kenntniss der Samenkiirper
und ilircr Entwickelung bei Vogeln und Saugethieren : A. m. A., XXXIII. —
Briicke, 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
Centralkdrper ? A. .L. 1892. — De Bruyne, C, '95. La sphere attractive dans
les cellules fixes du tissu conjonctif : Bull. Acad. Sc. de Belgique, XXX. —
34b GENERAL LITERATURE-LIST
Biitschli, O.. "73. Beitrage zur Kenntniss der freilebenden Nematoden : Nova acta
acad. Car. Leopold, XXXVI. — Id., '75. Vorlaufige Mitteilungen iiber Unter-
suchuiigen betrefifend die ensten Entwickelungsvorgange im befmchteten Ei von
Nematoden und Schnecken : Z. w. Z.. XXV. — Id., "76. Studien iiber die ersten
Entwickelungsvorgange der Eizelle. die Zellteilung und die Konjugation der Infu-
sorien : AbJi. des Senckeiib. A^atiirforscher-Ges., X. — Id., "91. Uber die soge-
nannten Centralkorper der Zellen und ilire Bedeutung : I'erJi. Au-iturhist. Med. Ver.
Heidelberg, 1891. — Id., '92. 1. Uber die kiinstliche Nachalnnung der Karyoki-
netischen Figuren : Ibid., N. F., V. — Id.. "92. 2. Untersuclumgen iiber mikro-
skopische Schaume und das Protoplasma (full review of literature on protoplasmic
structure): Leipzig {Engeliuanii'). — Id.. "94. Vorlaufige Bericht iiber fortgesetzte
Untersuchungen an Gerinnungsschaumen, etc. : l^erh. Nat iir hist. Ver. Heidelberg, V.
CALKINS. G. N.. "95. 1. Observations on the Yolk-nucleus in the Eggs of
Lumbricus: Trans. A^.V. Acad. Sci.. June. 1895. — Id.. "95. 2. The Spermato-
o-enesis of Lumbricus : '7. M.. XL. 2. — Carnoy, J. B.. "94. La biologic cellulaire :
Liege. — Id., '85. La cytodierese des Arthropodes : La Cellule A. — Id.. '86. La
cytodierese de I'oeuf: La Cellule, III. — Id., '86. La vesicule germinative et les
globules polaires chez quelques Nematodes: La Cellule, III. — Id., "86. La seg-
mentation de Toeuf chez les Nematodes : La Cellule, III., i. — Calberla. E.. "78. Der
BefiT.chtungsvorgang beim Ei von Petromyzon Planeri : Z. %u. Z., XXX . — Campbell,
D. H., '88-9. On the Development of Pilularia globulifera : A/ui. Bot., II. —
Castle. "W. B.. '96. The Early Embryology of Ciona intestinalis : Bull. Miis. Coinp.
Ztfi>7., XXVII. , 7. — Chabry. 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 : Ann. Nat., XXVIII.,
Feb., 1894.— Chun. C, '90. Uber die Bedeutung der direkten Zelltheilung : Sitzb.
Schr. Fhysik.-Okon. Ges. Konigsberg, 1890. — Id., '92, 1. Die Dissogonie der
Rippenquallen : Festschr. f. Lei/ckart, Leipzig, 1892. — Id., '92, 2. (In Roux, "92,
p. 55): J'er/i. d. Anat. Ges., VI.. 1892. — Clapp, C. M.. '91. Some Points in
the Development of the Toad-Fish: J. J/.. 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., IVood's
Holl, Boston, 1894. — Id., '96. Cell-size and Body-size: Rept. of Am. MorpJi.
Sac, Science All.. ]^-^. 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. Entm., III., i.
DELAGE. "Y"VES, "95. La Structure du Protoplasma et les The'ories sur I'here-
dite et les grands Problemes de la Biologie Generale : Paris. 1895. — Demoor, 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.-VI.. 1893, Z*?-/'^/.. LV. ; VII.-
X., 1893: Mitt. Zool. St. Neapel, XL, 2.— Id., '94. Analytische Theorie der
organischen Entwicklung : Leipzig. — Id.. '95. 1. Von der Entwickelung einzelner
Ascidienblastomeren : A. Entni.A-. 3- — Driesch and Morgan. "95. 2. Zur Analysis
der ersten Entwickelungs stadien des Ctenophoreneies : Ibid., II., 2. — Driiner,
L., "94. Zur Morphologic der Centralspindel : J. Z, XXVIII. (XXI.). — Id., '95.
Studien iiber den Mechanismus der Zelltheilung: Ibid., XXIX.. 2. — Diising, C,
'84. Die Regulicrung des Geschlechtsverhailtnisses : Jena, 1884.
GENERAL LITERATURE-LIST 347
VON EBNER, V.. '71. Untersuchungen uber den Bau der Samencanalchen und
die Entwickluag der Spermatozoiden bei den Saiugethieren und beim Menschen :
Inst. Phys. u. Hist. Graz., 1871 {Leipzig). — 1di.,QQ. Zur Spermatogenese bei
den Saiugethieren: A. m. A., XXXI. — Ehrlich, P.. "79. Uber die specifischen
Granulationen des Blutes : A. A. P. {Phys.), 1879, P- 573- — Eismond, J., "95.
Einige Beitrage zur Kenntniss der Attraktionsspharen und der Centrosomen : A. A.,
X.— Endres and Walter, "95. Anstichversuche an Eiern von Rana fusca : A.
Entin., II., I. — Eiigelmann, T. W., "80. Zur Anatomic und Physiologic der
Flimmerzellcn : Arch.ges. Phys., XXIII. — von Erlanger, R.. "96, 1. Die ncuesten
Ansichten liber die Zelltheilung und ihre Mechanik : Zool. Centralb., III., 2. — Id..
'96, 2. Zur Befmchtung des Ascariseies nebst Bemerkungen liber die Struktur des
Protoplasmas und des Centrosomes : Z. A., XIX. — Id.. "96, 3. Neuere Ansichten
liber die Stmktur des Protoplasmas : Zool. Centralb., III.. 8. 9. — Errara, "86. Eine
fundamentale Gleichgewichtsbedingung organischen Zellen : Ber. Deutsch. Bot.
Ges., 1886. — Id., "87. ZcUformcn und Seifenblasm : Tagebl. der 60 Versammlnng
deiitscher Naturforscher und Aerzte zii Wiesbaden, 1887.
FARMER, J. B.. "93. On nuclear division of the pollen-mother-ccll of Lilium
Martagon: Ann. Bot., VII., 27.— Id., "94. Studies in Hcpatica? : Ibid., VIII., 29.
— Id., "95. 1. tJber Kernteilung in Lilium-.Antheren, besonders in Bezug auf die
Centrosomcnfrage : Flora, 1895. p. 57. — Farmer and Moore. "95. On the essential
similarities e.xisting between the heterotype nuclear divisions in animals and plants :
A. A., XL, 3. — Fiok, R., '93. ttber die Reifung und Befruchtung des Axolotleies :
Z. w. Z., LVI., 4. — Fiedler. C, "91. Entwickelungsmechanische Studien an
Echinodermeneier : Festschr. Xijgeli u. Ko Hiker. Zurich. 1891. — Field, G. "W., "95.
On the Morphology and Physiology of the Echinoderm Spermatozoon : /. M.. XI. —
Fischer. A., '94. Zur Kritik der Fixierungsmethoden der Granula : A. A., IX., 22.
— Id. '95. Neue Beitrage zur Kritik der Fixiemngsmethoden : Ibid.,X. — Flem-
ming, W., "79. Beitrage zur Kenntniss der Zelle und ihre Lebenserscheinuno-en.
I. : A. >n. A., XVI. — Id., "79. Tber 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. 11. Enttvickhingsgesch. {Merkel and Bonnet), 1891-94. — Id.
'91, 1. Attraktionsspharen u. Centralkorper in Gewebs- u. Wanderzellen : A. A.
— Id., "91, 2. Neue Beitrage zur Kenntnis der Zelle, II. Teil : A. in. A., XXXVII.
— Id., '95, 1. Uber 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., XLVL — Floderus. M., '96. C'ber die Bildung der Follikel-
hlillen bei den Ascidien : Z. iu. 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. i\at. et Phys. Geneve, L\'I1I. See also Arch. Zool. Exp., VI. — Id.,
'79. Recherches sur la fecondation et la commencement de rhenogenie; Mem. de
la Soc. de physique 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, /i. 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.,X\\.. i. — Frenzel, J.. '93. Die
Mitteldarmdrlise des Flusskrebses und die amitotische Zelltheilung: A. m. A..,
348 GENERAL LITERArURE-LIST
XLI. — Fromman, C, '65. Uber die Struktur der Bindesubstanzzellen des
Rlickenmarks : Centrl. f. iiied. IJ'/ss.. III.. 6. — Id.. "75. Zur Lehre von der
Structur der Zellen : J. Z.. IX. (earlier papers cited). — Id., "84. Untersuchungen
liber Struktur, Lebenserscheinungen und Reactionen thierischer uiid pflanzlicher
Zellen: /. Z., XVII.
GALEOTTI, GINO, '93. Uber experimentelle Erzeugung von Unregelmassig-
keiten des karyokinetischen Processes: Be/, siir patholog. Anat. u. s. 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 fecondation chez Helix aspera et Arion empiricorum : Zool. Anz., XI.,
XII. — Geddes and Thompson. The Evolution of Sex. — Gegenbaiir, C. '54.
Beitrage zur naheren Kenntniss der Schwimmpolypen : Z. w. Z., V. — Van
Gehnchten. A.. "90. Recherches histologiques sur Tappareil digestif de la larve de
la Ptvchoptera contaminata: La Celliile.,V\. — Giard, A., "77. Sur la significa-
tion morphologique des globules polaires : Revue scientifiquc, XX. — Id., '90. Sur
les globules polaires et les homologues de ces elements chez les infusoires cilies :
BuUeti)i scientifique de la France et de la Belgiqiie, XXII. — Grobben, C, '78.
Beitrage zur Kenntniss der mannlichen Geschlechtsorgane der Dekapoden : Arb.
Zool. Lnst. \]ien,\. — Gruber, A.. "84. Uber Kern und Kerntheilung bei den
Protozoen : Z. iu. Z., XL. — Id., 85. Uber klinstliche Teilung bei Infusorien:
B. C, IV., 23; v., 5. — Id., "86. Beitrage zur Kenntniss der Physiologic und
Biologic der Protozoen: Ber. Natnrf- Ges. F'reibnrg, I. — Id., '93. Mikroscopische
Vivisektion : Ber. d. Naturf. Ges. zn Freiburg, VII., i — Guignard, L.. "89.
Developpement et constitution des Antherozoides : Rct. 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. Uber die Beziehun-it. and Foreign Med.-Chir. Beview, XII. — Id., "78. Evolution in Biology,
Enc. Brit., 9th ed., 187S ; Science and Culture, N. Y., 1882.
ISHIKA"WA, M., "91. Vorlautige Alitteilungen liber die Konjugations-
erscheinungen bei den Noctiluceen : Z. A., No. 353, 1891. — Id., "94. Studies on
Reproductive Elements : II.. iVoctiluca 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. Conip. Zo'dl.. XXII., 3. — Jordan. E. O., "93. The Habits and Development
of the Newt: J. J/., VIII., 2. — Jordan and Eycleshymer. "94. On the Cleav-
age of Amphibian Ova: J. J/.. IX.. 3, 1894. — ■ Julin, J., "93. 1. Structure et
developpement des glandes sexuelles, ovogenese, spermatogenese et fecondation
chez Styleopsis grossularia: L)ull. 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. iu. Z., LX.
— Kienitz-Gerloff, F., '91. Review and Bibliography of Researches on Proto-
plasmic Connection between adjacent Cells: in Bot. Zeitung, XIAX. — Klebahn,
'92. Die Befruchtung von Qidigonium : Zeit. iviss. Bot., XXIV. — Id. Die
Keimung von Closterium und Cosmarium : JaJirb. f. wiss. Bot.,XXll. — Klebs. G.,
'84. L'ber die neueren Forschungen betretfs 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. Beitraigezur Kenntnis
der Geschlechtsverhaltnisse und der Samenfllissigkeit wirbelloser Tiere : Berlin. —
Id., '44. Entwicklungsgeschichte der Cephalopoden: Zurich. — Id.. "85. Die
Bedeutung der Zellkerne flir die Vorgange der Vererbung : Z. w. Z., XLII. — Id.,
'86. Das Karyoplasma und die Vererbung. eine Kritik der Weismann'schen
Theorie von der Kontinuitat des Keimplasmas : Z. w. Z., XLIII. —Id., "89. Hand-
buch der Gewebelehre, 6th ed. : Leipzig. — Korsclielt. E.. '89. Beitrage zur Mor-
phologic und Physiologie des Zell-Kernes : Zool. Jahrb. Anat. u. Ontog., IV. —
Id.. '93. L'ber Ophryotrocha puerilis : Z. w. Z.. LIV. — Id.. "95. t^ber Kern-
tlieilung, Eireifung und Befrachtung bei Ophryotrocha puerilis: Z. iv. Z.,'LX. —
Kossel, A.. '91. Uber die chemische Zu.sammensetzung der Zelle : Arch. Anat.
u. Phys. — Id., '93. Uber die Nucleinsaure : Ibid., 1893. — von Kostanecki. '91.
Uber Centralspindelkorperchen bei karyokinetischer Zellteilung : Anat. Hefte. 1892,
dat. gi. — Kostanecki and "Wierzejski. '96. Uber das Verhalten der sogenann-
ten achromatischen Substanzen im befruciiteten Ei : A. m. A., XLII.. 2. — Kiihne.
"W., "64. Untersuchungen liber das Protoplasma und die Contractilitait. — Kupffer,
GENERAL LITERATURE-LIST 351
C, '75. t'ber Differenzierung des Protoplasma an den Zellen thierischer Gewebe :
Schr. natur. V'er. Schles.-Holst., I., 3. — Id.. "90. Die Entwicklung von Petromy-
zon Planeri : A. m. A., XXXV.
LAMEERE. A.. "90. Recherches sur la reduction karyogamique : Bnixelles. —
Lauteiborn, R., "93. Uber Bau und Kerntlieilung der Diatomeen : Verh. d.
Naturh. Med. Ver. in Heidelberg, 1893. — Id.. "95. Protozoenstudien. Kern- und
Zellteilung von Ceratium iiirundinella O. F. M. : Z. iv. 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. 111. A., III. — Id., '76. — Die
Spermatogenese bei den Amphibien : Idid.,Xll. — Id., '78. Die Spermatogenese bei
den Saugethieren und dem Menschen : //>id., XV. — Id. Spermatologi.sche Beitrage,
I.-V. : A. w. 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 prianzliclien Zellen: Mcrkel und Bonnet's Anat. Hefte, IV., 13. —
von Lenhossek, M., "95. Centrosom und Sphare in den Spinalganglien des
Frosches ; A. in. A.. XLVI. — Leydig. Fr. "54. Lehrbuch der Histologic des
Menschen und der Thiere : Frankfurt. ^\di.. "85. — Zelle und Gewebe. Bonn. —
Id., "89. Beitrage zur Kenntniss des thierischen Eies im unbefruchteten Zustande :
Spengel's Jahrb. Anat. Ont., III. — Lilienfeld. L.. '92. "93. tlber die Verwand-
schaft der Zellelemente zu gewissen Farbstotien: I'er/i. Fhys. Ges., Berlin, 1892-3.
— Id., "93. t'ber die Wahlverwandtschaft der Zellelemente zu Farbstoffen :
A. A. /'., 1893. — Lillie. F. R . "95. The Embryology of the Unionid?e: J. Af.,
X. — Id.. "96. On the Limit of Size in the Regeneration of Stentor: Be/>t. Am.
JMorplt. Soc. Science.. III., Jan. 10. 1896. — Loeb. J., "91-2. Untersuchungen zur
phvsiologischen iMorphologie. I. Heteromorphosis : U "/irzbnrg, iSgi. II. Organ-
bildung und Wachsthum : U'iirzburg, 1892. — Id.. '93. Some Facts and Principles
of physiological Morphology : Wood's Holl Biol. Lectures, 1893. — Id.. "94. L'ber
die Grenzen der Theilbarkeit der Eisubstanz : A. ges. P., LIX., 6, 7. — Lowit. M..
■91. rber amitotische Kerntheilung : I). C. XI. — Lukjanow. "91. ("rrundzlige
einer allgemeinen Pathologic der Zelle: Leipzig. — Lustig and Galeotti, "93.
Cytologische Studien uber pathologische menschliche Gewebe: Beitr. Path. Anat..
XIV.
MACALLXJM, 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: /. J/.. XI.. i. —
Maggi, L., '78. I plastiduli nei ciliati ed i plastiduli liberamente viventi : .4tti. Soc.
Ital. .Sc. Nat. Milano, XXI. (also later papers). — Malfatti. H.. "91. — Beitrage zur
Kenntniss der Nucleine : Zeit. Phys. Cheni.. XVI. — Mark. E. L.. '81. Maturation.
Fecundation and Segmentation of Lima.K campestris : Bull. Jfus. Conip. Zo'ol. Har-
vard College, VI. — Maupas. M.. "88. Recherches experimentales sur la multipli-
cation des Infusoires cilies : Arch. Zool. Pxp., 2me .serie. VI. — Id.. '89. Le
rejeunissement karyogamique chez les Cilies: Ibid., 2me .serie, VII. — Id.. "91.
Sur le determinisme de la se.xualite chez FHydatina senta: C. R-, Paris. — Mead,
A. D.. '95. Some observations on maturation and fecundation in Chaetopterus
pergamentaceus Cuv. : J M., X.. i . — Merkel. F.. "71. Die Stlitzzellen des mensch-
lichen Hodens : JIuller's Arch. — Meitens, H., "93. Recherches sur la signifi-
cation du corps vitellin de Balbiani dans I'ovule des Mammiferes et des Oiseaux :
A. B.. XIII. — Met.schnikoff. E.. '66. Embryologische Studien an Insecten:
Z.ti'. Z.. XVI. — Meves, F.. '91. C'ber amitotische Kernteilung in den Sperma-
352
G ENERA L L I TERA T I ^RE-LIS T
togonien des Sahimanders, und das Verhalten der Attraktionsspharen bei derselben :
A. .-/.. 1891. No. 22. — Id., '94. Uber eine Metamorphose der Attraktionssphare
in den Spermatogonien von Salamandra maculosa: A. m. A., XLIV. — Id., '95.
Uber die Zellen des Sesambeines der Achillessehne des Frosches {Kana tein-
poraria) und iiber ihre Centralkorper : Ibid.. XLV. — Msyer. O.. '95. Cellulaire
Untersuchungen an Nematodeneiern : J. Z., XXIX. (XXII.)- — Mikosch. "94.
Uber Struktur im pflanzlichen Protoplasma : I'erhandl. d. Ges. deiitschcr Xaturf.
und Arzte, 1894; Abteil f. Pflansenphysiologie u. Pflaiizenanattviiie. — Minot,
C. S., '77. Recent Investigations of Embryologists : Froc. Bost.Soc. Nat. Hist.,
XIX. — Id.. "79. Growth as a Function of Cells : /Z'/<'/., XX. — Id., '82. Theorie
der Genoblasten: B. C, II., 12. See also A//i. Nat., February, 1880.— Id., "87.
Theorie der Genoblasten: B. C. II., 12, 1887; also, Pfoc. Post. Soc. Nat. Hist.,
XIX., 1877. — Id., '91. Senescence and Rejuvenation:/^';/;-;/. /V/i'.r., XII., 2. —
Id., '92. Human Embryology: New York. — von Mohl, Hugo, "46. Uber die
Saftbewegung im Innern der Zellen : Bot. Zeitung. —lAooxe. J. E. S.. "93. Mam-
malian Spermatogenesis: ^. .-i., VIII. — Id., '95. On the Structural Changes in
the Reproductive Cells during the Spermatogenesis of Elasmobranchs : Q,. J.,
XXX\'III. Morgan, T. H.. '93. E.xperimental Studies on Echinoderm Eggs:
A. .-/., IX., 5, 6. — Id., '95. 1. Studies of the "Partial"" Larvae of Sphaerechinus :
A. Entm.. 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. Iiiiti/i., II., 2. — Id., "95, 5.
The Formation of the Fish-embryo : J. J/.. X.. 2. — Id., "96. 1. On the Production
of artiticial archoplasmic Centres: Rept. of the Am. Morph. Soc., Science. III.,
January 10, 1896. — Id., '96, 2. The Number of Cells in Larvce from Isolated
Blastomeres of Amphioxus : Arch. Eutni.. III., 2. — Muller. E., "96. Uber die
Re"-eneration der Augenlinse nach Exstirpation derselben bei Triton: A. jji. A.,
XLVII., I.
NAGELI. C, "84. Mechanisch-physiologische Theorie der Abstammungslehre :
Miinchen //. Leipzig. 1884. — Nageli und Schwendener, "67. Das Mikroskop.
(See later editions.) Leipzig. — Newport, G. On the Impregnation of the
Ovum in the Amphibia: Phil. Trans.. 185 1. 1853. 1854. — Nussbaum, M., "80.
Zur Dift'erenzierung des Geschlechts im Tierreich : A. in. A.. XVIII. — Id., '8?:, 1.
i'ber Spontane und Kiinstliche Theilung von Infusorien: I'erJi. d. naturh. I'er.
preiiss. Rhineland. 1884. — Id., '84, 2. ("ber die Veranderungen der Geschlechts-
producte bis zur Eifurchung : A. in. A., XXIII. — Id., "85. — Uber die Teilbarkeit
der lebendigen Materie. I. : A. in. A., XXVI. — Id., "94. Die mit der Entwickelung
fortschreitende Differenzierung der Zellen : Sitz.-Ber. d. niederrhein. Gescllschaft f.
Natiir- //. Heiliciinde, 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. in. A , XXXIX.
— Overton, C. E., "88. Uber den Conjugationsvorgang bei Spirogyra : Per.
deutsch. Bot. Ges.. VI. — Id., "89. Beitrag zur Kentniss der Gattung Volvox : Pot.
Ceiitrb: XXXIX. — Id., "93. Uljer die Reduktion der Chromosomen in den Kernen
der PHanzen : Vierteljahrschr. natiirf. Ges. ZYirich, XXXVllI. Also Ann. Pot.,
VII., 25.
PALADINO, G , "90. I ponti intercellulari tra I' novo ovarico e le cellule folli-
colari, etc.: A. A., V. — Palla. "90. Beobachtungen liber Zellhautbildung an
des Zellkerns beraubten Protoplasten. P''/ora, 1890. — Pfitzner, "W., "82. Uber
GENERAL LITERA TURK-LIST 353
den feineren Ban der bei der Zelltheilung auffretenden fadenfdrmigen Differenzier-
ungen des Zellkerns : J/. J.. VII. — Id., "83. Beitrage zur Lehre vom Baue des
Zellkenis und seinen Theilungseischeinungen : A. 111. A.. XXII. — Pfluger, E.. "83.
Uber den Einfluss der Schwerkraft auf die Theilung der Zellen : 1., Arch. i^es.
F/iys.. XXXI.: II.. /diW., XXXII.; abstract in BioL Centb., III., 1884. — Id., "84.
Uber die Einwirkung der Schwerkraft und anderer Bedingiingen auf die Rich tun o-
der Zelltheilung : Arch. o-cs. Fhys.. XXXIW — Id.. '89. Die allgemeinen Lebenser-
scheinungen : Bonn. — Platner. G., "86. Tber die Befruchtung von .Ir/o/i empiri-
coriim: A. m. A., XXXVII. — Id., "89.1. L'ber die Bedeutung der Richtungs-
korperchen : B. C. VIII. — Id.. "89. 2. Beitrage zur Kenntniss der Zelle und
ihrer Teilungserscheinungen, I.-VI. : ./. m. A., XXXIU. — Poirault and Raci-
borski. "96. l'ber konjugate Kerne und die konjugate Kerntheilung : B. C, XVI.,
I . — Prenant. "94. Sur le corpuscule central : Bi/IL Soc. Sci., Nancy, 1 894. —
Preusse. F., "95. Uber die amitotische Kerntheilung in den Ovarien der Hemi-
pteren : Z. w. Z.. LIX.. 2. — Prevost and Dumas. '24. Nouvelle the'orie de la
generation: .liiii. Sci. N'at.. \.. II. — Pringsheim. N.. "55. L'ber die Befruchtuno-
der Algen: Moiiatsb. Berl. Akad., 1855-6. — Purkyne : JaJirb. f. u. Z.,
LX.. I. — Raiiber. A.. '83. Neue Grundlegungen zur Kentniss der Zelle: M. y.,
VIII. — Rawitz. B.. "95. Centrosoma und Attraktionsphiire in der nihenden Zelle
des Salamanderhodens : A. iii. A., XLIV.. 4. — Reinke. Fr. '94. Zellstudien.
\., A. m. A.. XLIIL; II.. /bid.. XLIV.. 1894. — Id.. "95. Untersuchungen iiber
Befruchtung und Furchung des Eies der Echinodermen : Sitz.-Ber. Akad. d. JJ'iss.
Berlin. 1895, June 20. — Reiuke and Rodewald. "81. Studien liber das Proto-
plasma : Untersuch. aiis d. bot. Inst. Gidii/n^eii. II. — Remak. R.. "41. t'ber
Theilung rother Blutzellen beim Embryo: Med. Ver. Zeit., 1841. — Id., "50-5.,
Untersuchungen liber die Entwicklung der Wirbelthiere : Berlin. 1850-55. — Id.,
'58. t'ber die Theilung der Blutzellen beim Embryo: MYillers Arch.. 1858. —
Retzius, G., "89. Die Intercellularbriicken des Eierstockeies und der Follikelzellen :
Verh. Anal. Ges., 1889. — Rhumbler. L.. '93. t'ber Entstehung und Bedeutung
der in den Kernen vieler Protozoen und im Keimblaschen von Metazoen vorkom-
menden Binnenkorper (Nucleolen) : Z. %u. Z.. LVI. — Rompel. '94. Kentrochona
Nebaliae n. g. n. sp.. ein neues Infusor aus der Familie der Spirochoninen. Zugleich
ein Beitrag zur Lehre von dei" Kernteilung und dem Centrosoma: Z. ia. Z.. LVIII.,
4. — Rosen, 92. t'ber tinctionelle L^nterscheidung verschiedener Kernbestand-
theile und der Sexual-kerne : Cohn's Beitr. z. Biol. d. Pflanzen, V. — Id., '94. Neueres
Uber die Chromatophilie der Zellkerne : Schles. Ges. vHIerl. Kiilt. 1894. — Roux.
"W.. '83, 1. t'ber die Bedeutung der Kernteilungsfiguren : Leipzig. — Id., '83, 2.
t^ber die Zeit der Bestimmung der Hauptrichtungen des Froschembryo : Leipzig. —
Id . "85. L'ber die Bestimmung der Hauptrichtungen des Froschembryos im Ei,
und iiber die erste Theilung des Froscheies : Breslaner iirtzl. Zeitg.. 1885. — Id.,
'87. Bestimmung der medianebene des f^roschembryo durch die Kopulationsricht-
ung des Eikernes und des Spermakernes : A. rn. A..XX\X. — Id., "88. Uber das
2 A
354 GENERAL LITERATURE-LIST
klinstliche Hervorbringen halber Embryonen durch Zerstorung einer der beiden
ersten Furchungskugehi. etc.: Virchow's Archiv, 1 14. — Id., '90. Die Entwickel-
ungsmechanik derOrganismen. ^fVev/, 1890. — Id., '92.1. Entwickelungsmechanik :
M'erkel and Bonnet, Erg., II. — Id., "92. 2. Uber das entwickelungsmechanische
Vermogen jeder der beiden ersten Fuicliungszellen des Eies : Verh. Anat. Ges.,
VI. — id., '93, 1. Uber Mosaikarbeit und neuere Entwickelangshypothesen : An.
Hefte, Feb., 1893. — Id., '93, 2. Uber die Spezifikation der Furchungzellen, etc.:
B. c] XIII., 19-22.— Id., '94, 1. Uber den " Cytotropismus" der Furchungszellen
des Gra.sfrosches : Arc/i. Entni., I., i, 2. — Id., '94, 2. Aufgabe der Entvvickel-
unc^smechanik. etc: Arch. Entni., I., i. Trans, in Biol. Lectures, n'ood's Noll,
1894. — Rtickert, J., '91. Zur Befruchtung des Selachiereies : A. A., VI. — Id.,
'92. 1. Zur Entwicklungsgeschichte des Ovarialeies bei Selachiern : A. A., VII.—
Id. '92, 2. Uber die Verdoppelung der Chromosomen ini 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. — ld., '95,1. Zur Kenntniss des
Befruchtungsvorganges : Sitsb. Bayer. Akad. ll'iss., XXVI., i. — Id., '95, 2. Zur
Befruchtung von C}'c/o/>s strenuiis : A. A., X., 22. — Id., '95, 3. Uber das Selb-
standigbleiben der vaterlichen und mliterlichen Kernsubstanz wahrend der ersten
Entwicklung des befruchteten Cyclops-Eies : A. ni. A., XLV^, 3. — Ruge, 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. Comin.,
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 liber Pflanzen-physiologie ; Leip-
zig, Id. Uber die Anordnung der Zellen in jungsten Pflanzentheile : Arb. Bot.
Inst. UTerzbnrg, II. — Id., '92. Physiologische Notizen, II., Beitrage zur Zel-
lentheorie: Flora, 1892, Heft I. — Id., '93. Stoff und Form der Pflanzen-organe ;
Gesammelte Abhandhtng, II., 1893. — Id., '95. Physiologische Notizen, IX., weitere
Betrachtungen liber Energiden und Zellen: Flora, LXXXI., 2. — Sala, L., '95.
E.xperimentelle Untersuchungen liber die Reifung und Befruchtung der Eier bei
Ascaris megalocephala ; A. ni. A., XL. — Sargant. Ethel, '95. Some details of
the first nuclear Division in the Pollen-mother-cells of Lilium martagon ; Joitrn.
Roy. Mic. Soc, 1895, part 3. — Schafer, E. A.. '91. General Anatomy or Histol-
oo-y: in Quain's Anatomy, I., 2, loth ed., London. — Schaudinn. F., '95. Uber
die Theiluno- von Antaba binucleata Gmber: Sitz.-Ber. Ges. Naturforscli., Freunde,
Berlin, Jahrg. 1895, No. 6. — Id., '96. Uber den Zeugungskreis von Paranuvba
Eilliardi: Sitz.-Ber. Ges. Naturforsch., Freunde, Berlin, 1896, Jan. 13. — Schewi-
akoff, "W., '88. tJber die karyokinetische Kerntheilung der Euglypha alveolata:
M. J., XIII. — Schiefferdecker and Kossel, "91. Die Gewebe des Menschlichen
Korpers : Braunsc/nccig. — Schimper, "85. Untersuchungen liber die Chlorophyll-
korper, etc. : Zeitsch. loiss. Bot., XVI. — Schleicher, W., '78. Die Knorpelzell-
theiluno-. Ein Beitrag zur Lehre der Theilung von Gewebezellen : Ccntr. i/ied. Wiss.
Berlin^iSjS. Also A. m. A., XVI, 1879. — Schleiden, M. J., '38. Beitrage zur
Phytogenesis : Aliiller's Arc/iiv, \%^%. [Trans, in Sydenham Soc, XII.: London,
1847.] — Schloter, G., '94. Zur Morphologie der Zelle : A. in. A.. XLIV.. 2. —
Schmitz, '84. Die Chromatophoren der Algen. — Schneider, A., '73. LTnter-
suchungen liber Plathelminthen : Jahrb. d. oberhess. Ges. f. Natur- Heilkunde,
XIV., Giesscn. — Schneider, C, "91. Untersuchungen liber die Zelle: Arb. Zool.
Inst. Jl'ien, IX.. 2. — Schottlander, J., '88. Uber Kern und Zelltheilungsoor-
gilnge in dem Endothel der entziindeten Hornhaut: A. in. A.. XXXI. — Schultze,
Max, '61. liber Muskelkdrperchen und das was man eine Zelle zu nennen hat :
CENEKAL LITERATURE-IJSr 355
Arch. Anat. P/iys., 1861. — Schultze, O., "87. Untersuchungen iiber die Reifung
unci Befmchtung des Amphil)ien-eies : Z. w. Z., XLV. — Id., '94. Die kunstliche
Erzeugung von Doppelhildungen bei Frosclilarven, etc.: Arcli. Entiii.^ I., 2. —
Schwann. Th., '39. Alikroscopisclie Untersuchungen liber die Uebereinstimmung
in der Structur und dem Wachsthum der Tiiiere und Pfianzen : Berlin. [Trans, in
Sydenham Soc.,X\\.: London, 1847.] — Schwarz, Fr., '87. Die iMorphologische
und chemische Zusammensetzung des Protoplasmas : Breslait. — Schweigger-
Seidel. O., "65. Uber die Samenkorperclien und ihre Entwickelung : A. m. A., I.
— Sedgwick. A.. "85-8. The Development of the Cape Species of Peripatus,
I-VI.: (2- 7- XXV.-XXVIII. — Id., "94. On the Inadequacy of the Cellular
Theory of Development, etc.: Q. J., XXXVII., i. — Seeliger. 6., "94. Giebt es
geschlechtlicherzeugte Organismen ohne miitterliche Eigenschaften ?: A. Ent., I.. 2.
— Selenka, E., '83. Die Keimblatter der Echinodermen : Stitdien uber Entwick.,
II, Wiesbaden, 1883. — Sertoli, E., '65. Dell' esistenza di particolori cellule
ramificate dei canaliculi seminiferi del testicolo umano : // Morgagni. — Sied-
lecki, M., '95. Uber die Struktur und Kerntheilungsvorgange bei den Leucocy-
ten der Urodelen : Anz. Akad. It'iss., Krakaii, 1895. — Sobotta, J., '95. Die
Befruchtung und Furchung des Eies der Maus : A. m. A., XL. — Solger. B.,
'91. Die radiaren Strukturen der Zellkorper im Zustand der Ruhe und bei der
Kerntheilung: Berl. Klin. Vl'ochenschr., XX., 1891. — Spallanzani, 1786. Ex-
periences pour servir a I'histoire de la generation des animaux et des plantes :
Geiieva. — Strasbnrger, E., '75. Zellbildung und Zelltheilung : ist ed., Je)ia,
1875. — Id., -77. i'ber Befruchtung und Zelltheilung : J. Z., XI. — Id., '80. Zell-
bildung und Zellteilung : 3d ed. — Id. ,'82. Uber den Theilungsvorgang der Zell-
kerne und das Verhaltniss der Kerntheilung zur Zelltheilung: A. ni. A.. XXI. —
Id.. '84. 1. Die Controversen der indirecten Zelltheilung: Ibid., XXIII. — Id.,
'84. 2. Neue Untersuchungen liber den Befruchtungsvorgang bei den Phaneroga-
men. als Grundlage fur eine Theorie der Zeugung: Jena, 1884. ^ Id., '88. Uber
Kern- und Zellteilung im Pflanzenreich, nebst einem Anhang liber Befruchtung :
Jena. — Id., '89. Vh^x das Wachsthum vegetabilischer Zellhaute : Hist. Bei.,
II., Jena. — Id., "91. Das Protoplasma und die Reizbarkeit : Rektoratsrede. Bonn,
Oct. 18, 1891. Jena, Fisc/ier. — IA.,'92. Histologische Beitrage, Heft IV. : Das
Verhalten des Pollens und die Befruchtungsvorgange bei den Gymnospermen,
Schuarmsporen, pflanzliche Spermatozoiden und das Wesen der Befruchtung:
Fischer, Jena, 1892. — Id., '93. 1. Uber die Wirkungssphare der Kerne und die
Zellengrcisse : Hist. Beitr., V. — Id., '93, 2. Zu dem jetzigen Stande der Kern- und
Zelltheilungsfragan : A. A.,V\\\., \). 177. — Id., '94. Uber periodische Reduktion
der Chromosomenzahl im Entwicklungsgang der Organismen: B. C, XIV. — Id.,
'95. Kuryokinetische Probleme : Jahrb. f. luiss. Botanik, XXVIII., i . — Van der
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 dWmphioxus lanceolatus :
Bnll. Acad. Roy. Bel^iqitc, XXX., 2. — Id., '95, 2. De Torigine de la figure achro-
matique de I'ovule en mitose chez le Thvsanozoon Brocchi : Verhandl. d. anat.
I'ersaninil. in Strassburg 1895, p. 223. — Id., "95, 3. Contributions a I'etude de
la forme, de la structure et de la division du noyau : Bull. Acad. Roy. Sc. Belgiqne,
XXI.X. — Strieker. S., '71. Handbuch der Lehre von den Geweben : Leipzig. —
Stuhlmann, Fr., '86. Die Reifung des Arthropodeneies nach Beobachtungen an
Insekten. Spinnen, Myrio])oden und Peripatus: Ber. Natiirf. Ges. Freiburg, I. —
Swaen and Masquelin. "83. Etude sur la Spermatogenese : A. />., 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, James. "78.
The Cell-doctrine: 2d ed., Philadelphia.
356 GENERAL LITERATURE-LIST
USSOW, M., '81. Untersuchungen liber die Entwickelung der Cephalopoden :
Arch. Biol.. II.
VEJDOVSKY, F., '88. Entwickelungsgeschichtliche Untersuchungen, Heft I. :
Reifung, Befruchtungund Furchungdes Rhynchelmis-Eies : Prag, i888. — Verworn,
M. '88. Biologische Protisten-studien : Z. -w. Z., XLVI. — Id.. "89. — Psyclio-
physiologische Protisten-studien: Jena. — Id., '91. Die physiologisclie Bedeutung
des Zellkerns : I'/iiiger's Arch. f. d. ges. Pliysiol.. LI. — Id.. "95. Allgemeine
Physiologie : Jena. — Virchow, R.. '55. Cellular-Pathologie : Arcli. Patli. Anat.
Pliys.. V'lII., I. — Id., "58. Die Cellularpathologie in ihrer Begrlindung auf phvsio-
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 : I'erh. d. Anal. Leipzig. 1887. — Id.. "88. Uber
Karyokinese und ihre Beziehungen zu den Befruchtungsvorgangen : A. m. A.,
XXXII. [Trans, in Q.J.^ — Id., '95. Die neueren Ansichten iiber den Bau und
das Wesen der Zelle : Deiitscli. Med. Woc/iensc/ir.. No. 43, IT.. Oct. ff., 1895. —
Warneck, N. A., '50. Ueber die Bildung und Entwickelung des Embryos bei
Gasteropoden : Bull. Soc. Imp. Nat. Moscon. XXIII.. i. — Watase, S.. "91.
Studies on Cephalopods ; I.. Cleavage of the Ovum : J. J/.. IV.. 3. — Id., '92. On
the Phenomena of Sex-differentiation: Ibid.. VI., 2, 1892. — Id.. '93. 1. On the
Nature of Cell-organization: IVood^s Hall Biol. Lectures. 1893. — Id., '93, 2.
Homology of the Centrosome: J. M., VIII., 2. — Id.. '94. Origin of the centro-
some : Biological Lectures, IVood^s Holl. 1894. — "Weismann. A.. '83. Uber
Vererbung : Je/ia. — Id.. "85. Die Kontinuitat des Keimplasmas als Grundlage
einer Theorie der Vererbung: Jena. — Id., '86. 1. Richtungskorper bei partheno-
genetischen Eiern: Zool. Ans., No. 233. — Id.. "86. 2. Die Bedeutung der se.xuel-
len Fortpflanzung fiir die Selektionstheorie : Jena. — Id., '87. Uber die Zahl der
Richtungskorper und iiber ihre Bedeutung fiir 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 Hereditv,
Second Series : O.x'ford, \%()2. — Id. .'93. The Germ-plasm : iVew y'ork. — Id., "94.
Aeussere Einflusse als Entwicklungsreize : Jena. — "Wheeler, "W. M., '89. The
Embryology of Blatta Gernianica and Doryphora decenilmeata : J. M.. III. —
Id.. '93. A Contribution to Insect-embryology: //;/>/.. VIII. i. — Id.. '95. The
Behavior of the Centrosomes in the Fertilized Egg oi Myzostonia glabruin : Ibid.. X.
— 'Whitman, C. O.. '78. The Embryology of Clepsine : (J. /., XVIII. — Id.. '87.
The Kinetic Phenomena of the Egg during Maturation and Fecundation : J. AL. I.. 2.
— Id., 88. The Seat of Formative and Regenerative Energy: /bid.. II. — Id. .'93.
The Inadequacv of the Cell-theorv of Development: Wood's Noll Biol. Lectures,
1893. — Id., 94. Evolution and Epigenesis : 7/'/^/., 1894. — Wiesner, J.. '92. Die
Elementarstruktur und das Wachstum der lebenden Substanz : IJ'ie//. — "Wilcox,
E. v.. '95. Spermatogenesis of Caloptenus and Cicada: Bull, of the Museum
of Camp. Zool., Harvard College. Vol. XXVII., Nr. i. — Will, L.. '86. Die
Entstehung des Eies von Colymbetes : Z. w. Z., XLIII. — 'Wilsou, Edni. 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: U'ood^s Holl Biol. Led.. 1894. — Id.. '95. 1. Atlas of Fertilization
and Karyokinesis : New I'ork. Macmillan. — Id.. '95. 2. Archoplasm, Centro-
some, and Chromatin in the Sea-urchin Egg: J. J/.. XI. — Id.. '96. On Cleavage
and Mosaic-work: A. Entm.. III., i. — Wilson and Mathe'ws. "95. Maturation,
Fertilization, and Polarity in the Echinoderm Egg: /. J/., X., i. — Wolff, Caspar
GENERAL LITERATURE-LIST 357
Friedrich, 1759. Theoria Generationis. — Wolff. Gustav. 94. Bemerkungen
zum Darwinismus mit einem experimentellen Beitrag zur Physiologic der Entwick-
lung: B. C. XIV.. 17. — Id.. "95. Die Regeneration der Urodelenlinse : .Irc/i.
Entin., I., 3. — Welters, M.. "91. Die Conjugation und Sporenbildung bei
Gregarinen : ./. ///. ./.. XXXV'II.
ZACH ARIAS. O., '85. Uber die amoboiden Bewegungen der Spermatozoen
von I'()l\ plit-mus pediculus : Z. iv. Z., XLI. — Zacharias, E.. "93. 1. L'ber die
chemische Beschatfenheit von Cytoplasma und Zellkern : Ber. deutsch. bot. Ges.,
II., 5. — Id.. "93, 2. Uber Chroniatophilie : Ibid., 1893. — Id., '95. t'ber da.s
Verhalten des Zellkerns in \vach.senden Zellen : flora, 'i\. 1895. — Id., "94. I'ber
Beziehungen des Zellenwachstums zur Beschaffenheit des Zellkerns : Berichte der
deutschen botan. Gesellschaft, XII., 5. — Ziegler. E.. "88. Die neuesten Arbeiten
liber Vererbung und Abstammungslehre und ihre Bedeutung fur die Pathologie :
Beitr. zur path. Aiiat.. IV. — Id.. "92. Lehrbuch der allgemeinen pathologischen
Anatomie und Pathogenese, 7th ed. : Jena . —Ziegier. H. E., "87. Die Entsteh-
ung des Blutes bei Knochenfischenembryonen : A. m. .4. — Id.. "91. Die biolo-
gische Bedeutung der amitotischen Kernthcilung im Tierreich : B. C. XI. — Id., "94.
Uber das Verhalten der Kerne im Dotter der meroblastischen Wirbelthiere : Ber.
N'ati/rf. Ges. Freiburg, 1894. — Id.. "95. Untersuchungen Uber die Zelltheilung :
Verhandl. d. deutsch. Zool. Ges., 1895. — Ziegler and vom Rath. Die amitotische
Kerntheilung bei den Arthropoden : /S'. C. XI. ^Zimmermann. A., '93,1. Bei-
trage zur Morphologic und Physiologic der Pflanzenzelle : Tubingen. — Zimmer-
mann. K. "W.. '93. 2. Studien iiber Pigmentzellen. etc: A. m. A., XLI. —
Zoja, R., "95, 1. Sullo sviluppo dei blastomeri isolati dalle uova di alcune medusc :
A. Entvi., I., 4: II.. I : II.. I\'. — Id.. '95. 2. .Sulla independenza della cromatina
paterna e materna nel nucleo delle cellule emljrionali: A. A.. XI.. 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; une(iual division,
273-
Ballov\itz, 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 T)-iton-tgg, 245;
gravitation-experiments, 285.
Boveri, centrosome, named, 36,49; a jier-
manent organ, 56; in fertilization, 124,
135, 140, 141 ; continuity of, 143; defini-
tion of, 224; structure, 226, 227; func-
tions, 259; archoplasm, 51, I2i, 229;
origin of mitotic figure, 53, 55 ; mitosis in
Ascaris, >,?>■, varieties of ./jir^? ;-/.?, 61 ; the-
ory of mitosis, 71, 72; division of cliromo-
somes, 77; origin of germ-cells, iio, ill,
322; fertilization of .hearts, 133, 134; of
Pterotrachea, 137; of Echinus, 143, 157;
theory of fertilization, 140, 14I; of par-
thenogenesis, 202; partial fertilization.
559
36o
INDEX OF AUTHORS
140, 259; reduction, 173; maturation in
Asca>-is, i-jg, iSj; 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 i?;-rt«f/«)>;«, 142; parthenogenesis
in Ai-iemia, 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 Noctilnca, 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.
Chmielewski, reduction in Sfirogyra, 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, inutagmata, 22; ciliated cells,
30, 31, 34; rejuvenescence, 129.
von Erlanger, asters, 34; elimination of
chromatin, 117, 121 ; fertilization, 157;
centrosome, 22S.
Eycleshymer, first cleavage-plane, 282.
Farmer, reduction in plants, 196, 197.
Pick, fertilization of axolotl, 135, 142.
Field, formation of spermatozoon, 123-125:
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 memlirane, 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.
GaltDH, inheritance, 7.
Cardiner, cell-bridges, 42.
Garnault, fertilization in A?-ioii. 155.
INDEX OF AUTHORS
361
Geddes and Thompson, theory of sex, 90.
Van Gehuchten, spireme-nuclei, 25; nuclear
polarity, 26; muscle -hbre, 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 Stentor, 248.
Guignard, mitosis in plants, 59, 78; sperma-
tozoiids, 107; fertilization in plants, 157,
159, 161 ; reduction, 195; centrosome,
224.
Haacke, gemmae, 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, I12;
germ-nuclei, 156, 193, 194, 219; reduc-
tion in copepods, 189, 191; polar bodies,
280.
Hallez, promorphology of ovum, 2S3.
Halliburton, proteids, 239; nuclein, 240, 241.
Hamm, discovery of spermatozoon, 7, 130.
Hammar, cell-bridges, 43.
Hamraarsten, 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.
HeiHer, 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.
Hertwig, 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 Amceha, 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-
tihzation, 129, 171 ; evolution and epi-
genesis, 328.
Ishikawa, A'octiluca, mitosis, 65, 67; conju-
gation, 168.
Jordan, deutoplasm and yolk-nucleus, 116,
119; first cleavage-plane, 282.
Julin, fertilization in Styleopsis, 142.
I Karsten, centrosome, 225.
i Keuten, mitosis in Eiigletia^ 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, 311.
Korschelt. nucleus, 25; amitosis, 81, 83;
movements and position of nuclei, 92,
254-256; insect-egg, 96; nurse-cells. 113,
36:
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
Physa, 131, 136, 143, 159; continuity of
centrosomes, 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.
Leeuwenhoek, 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; eUmination
of chromatin, 117.
Lilienfeld, staining-reactions of nucleins,
242, 243.
Lillie, regeneration in Stentor, 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 Chcctoptertis, 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 blaslomeres,
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, ^t„ 34.
Nussbaum, germ-cells, 88; regeneration in
Infusoria, 248; nucleus, 321.
Overton, germ-cells of Vohwx, 98; conjuga-
tion of Spirogyra, 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 .4rion;
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 AUTHORS
363
193; centrosome, 224; attraction-sphere,
234-
Rauber, cell-division and growth, 293.
Rawitz, spermatogonium, 15; amitosis, 82.
Redi, genetic continuity, 21.
Reichert, cleavage, 9, 46.
Reinke, pseudo-alveolar structure, 19; nu-
cleus, 26, 27, 223; fedematin, 28; cyto-
plasm, 29; asters, 34, 226, 231; central
spindle, 74; nucleus anti cytoplasm, 214.
Reniak, cleavage, l, 9, 264; cell-division,
45' 46 ; egg-axis, 279.
Retzius, muscle-tibre, 34 ; cell-bridges, 42 ;
entl-pic'ce, 104.
Robin, germinal vesicle, 46.
Rosen, staining-reactions, 162.
Roux, cell-organization, 21; 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, 6r, 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 Amxha. 64.
Schewiakoff, mitosis in Etiglypha, 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, 32S.
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, 1 19.
Tangl. cell-bridges, 42.
Thiersch and Boll, theory of growth, 292.
Treat, sex, 109.
Ussow, micropyle, 97; deutoplasm. 117.
Vejdovsky, centrosome, 55 ; fertilization in
Rhynchelmis^ 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.
Wasielewsky, 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, 1 83-1 85 ; constitution
of the germ-plasm, 183, 305; partheno-
genesis, 202; theory of development, 303-
305. 328.
Went, vacuoles, 37.
Wheeler, amitosis, 81 ; insect-egg, 97 ;
fertilization in Alyzostonia^ ^57" '59''
plastids, 170; bilateralitv 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.
W'ill, chromatin-ehmination, 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-
sperniy, 260 ; pressure-experiments, 309 ;
first cleavage-plane, 277 ; experiments on
Amphioxus, 308, 319; theory of develop-
ment, 317.
von Wittich, yolk-nucleus, 118.
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.
Actinosphariitm, mitosis, 63, 66; regenera-
tion, 248.
Adaptation, 329.
ALqieort'a, 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.
Aiiitvlm, 4; mitosis, 64; experiments on,
249.
Amphibia, spermatozoa, 100.
Amphimixis. 130, 1 71.
Aniphio.xiis^ fertilization, 153, 159; polar
body, 176; cleavage, 270, 271; dwarf
larvce, 289, 307; double embryos, 308.
Amphipyrenin, 29.
Amyloplasts, 37; in plant-ovum, 98, 160.
Anaphases, 47, 51 ; in sea-urchin egg, 77.
Aniioci-a, gland-cells, nuclei, 26; amitosis.
84.
Auodonta, 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.
Argoiiauta, micropyle, 97.
Arioii, germ-nuclei, 155.
Artefacts, in protoplasm, 31, 213.
Artemia, chromcsomes, 49, 61, 205; par-
thenogenetic maturation, 202-205.
Ascaris, chromosomes, 49; mitosis, 52, 58,
71, 78; primordial germ-cells, no, 332;
fertilization, 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.
As/erias, 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.
Bleuiiius, pigment-cells, 73.
Branchipiis, yolk, 117; sperm-aster, 142;
reduction, 1 88.
Calanus, tetrads, 190.
Cancer-cells, mitosis, 68.
366
INDEX OF SUBJECTS
Canthocainptus^ reduction, 190 ; ovarian
eggs, I93> 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
signilicance, 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.
Ceraiozainia, reduction, 196.
Cerianthns^ regeneration in, 293.
Chcrtopterus, fertilization, 143; sperm-centro-
some, 226.
Chara, spermatozoids, 106.
Chii-onomtis, spireme-nuclei, 26.
Chorion, 96.
Chromatic figure, 50; origin, 53; varieties,
59; in fertilization, 134.
Chromatin, 24; in meristeni, 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 larv£e, 258.
Ciliated cells, 30, 34.
Ciona, egg-axis, 280.
Clavelina, 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.
Closteriuf7i, 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.
Corixa, ovum, 284.
INDEX OF SUBJECTS
367
Crepidula^ fertilization, 157; position of
spindles, 277; dwarfs and giants, 289.
Cross- furrow, 270.
Crustacea, spermatozoa, 105, 106.
Ctenophores, experiments on eggs, 314.
Cyclas, ciliated cells, 30.
Cyclops, ova, 93 ; primordial germ-cells, 1 1 2 ;
fertilization, 142, 156,218; reduction, 189-
191; attraction-sphere, 233; axial rela-
tions, 286.
Cytolyniph, 17.
Cytoplasm, 15, 29, 213, 236; of the ovum,
97, 115, 170; of the spermatozoon, 107;
morphological relations to nucleus, 214;
to archuplasm, 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.
Dendrobcsna, 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.
Dieinyctyliis, yolk, 116; yolk-nuclei, 1 19.
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, 289.
Dyads (Zweiergrappen), 179, 181, 184, 189;
in parthenogenesis, 203-205.
Dyaster, 51.
Dycyemids, centrosome, 36.
Dytisciis, ovarian eggs, 115, 256.
Earthworm, ova, 115; spermatozoon, 125;
yolk-nucleus, I2i; fertilization, 135; polar
rings, 150; spermatogenesis, 200; telo-
blasts, 274.
Echinoderms, spermatozoa, 123; fertiliza-
tion, 143, 157; polyspermy, 147; dwarf
larva:, 258, 289; half cleavage, 306; eggs
underpressure, 309; modified larvse, 324.
Echinus, fertilization, 143,157; centrosome,
235; dwarf larvje, 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.
Endoplasni, 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.
£?/c/i(2'/'y conjugation, 199 ; formulas for,
186, 193, 200, 201.
Tka/asse>?ia, fertilization, 143 ; centrosome,
228 ; attraction-sphere, 235.
Thalassicolla, experiments on. 250.
Tonoplast, 37.
Toxopneustes, 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.
Tiibularia, 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.
Vaticheria, 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.
Zygnema, membrane, 252.
Zygospore, 169.
V I
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