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AGRICULTURAL AND BIOLOGICAL PUBLICATIONS
CHARLES V. PIPER, CONSULTING EDITOR

AN INTRODUCTION TO CYTOLOGY

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AN INTRODUCTION TO

BY

LESTER W. SHARP

CORNELL UNIVERSITY

"The most important discoveries of the laws, methods
and progress of nature have nearly always sprung from
the examination of the smallest objects which she
contains, and from apparently the most insignificant
enquiries." — Lamarck, Philosophic Zuoloyiquc.

FIRST EDITION

McGRAW-HILL BOOK COMPANY, INC.

NEW YORK: 370 SEVENTH AVENUE

LONDON: 6 & 8 BOUVERIE ST., E. C. 4

1921

COPYRIGHT, 1921, BY THE
MGGRAW-HILL BOOK COMPANY, INC.

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CONTENTS

PAGE
PREFACE vii

CHAPTER I

HISTORICAL SKETCH 1

The discovery of the cell — Preformation and epigenesis — Early theories of
cell-formation — Early observations on the cell contents — The foundation
of the cell theory — Elaboration of the cell theory — The protoplasm doctrine
— The new conception of the cell — Fertilization and embryogeny — The be-
ginning of the modern period in cytology — Bibliography 1.

CHAPTER II

PRELIMINARY DESCRIPTION OF THE CELL 23

Description of the cell — The differentiation of cells — Bibliography 2.

CHAPTER III

PROTOPLASM 32

Physical properties — Protoplasm as a colloidal system — Microdissection —
Chemical nature of protoplasm — Varieties of protoplasm — The plasma
membrane — Protoplasmic connections — Vacuoles — Protoplasm as the sub-
stratum of life— Micromeric theories — Chemical theories — Conclusion —
Bibliography 3.

CHAPTER IV

THE NUCLEUS 59

Occurrence — General characters — Nucleoplasmic ratio — Structure of nucleus
— Nuclei of bacteria and other protista — The function of the nucleus —
Bibliography 4.

CHAPTER V

THE CENTROSOME AND THE BLEPHAROPLAST 76

The centrosome — Occurrence and general characters — Individuality —
Centrosomes in Algae — Fungi — Bryophytes — Conclusion — The blepharo-
plast — Occurrence — In flagellates — In thallophytes— In byrophytes — In
pteridophytes — In gymnosperms — In animals — Conclusion — Bibliography 5.

CHAPTER VI

PLASTIDS AND CHONDRIOSOMES 103

Plastids — General nature and occurrence — Leucoplasts — Chromatophores —
Starch — The pyrenoid — Elaioplasts and oil bodies — The eyespot — The
individuality of the plastid — Chondriosomes — General nature and occur-
rence— Physico-chemical nature — Origin and multiplication — Function —
Relation of chondriosomes to plastids — Conclusion — Bibliography 6.

CHAPTER VII

METAPLASM; POLARITY 133

Metaplasm — Extruded chromatin — The senescence of the cell — Polarity —
Metabolic gradient — Bibliography 7.

xi

xii CONTENTS

CHAPTER VIII

PAGE

SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 143

Somatic mitosis — Preliminary sketch of mitosis — Detailed description of
the behavior of the chromosomes in somatic mitosis — Chromomeres —
Summary — The individuality of the chromosome — The frequent persistence
of visible chromosome limits in the resting reticulum — Prochromosomes —
Persistence of parental chromosome groups after fertilization — Size and
shape of chromosomes — Chromosome number — Discussion and Conclusions
— Bibliography 8.

CHAPTER IX

THE ACHROMATIC FIGURE, CYTOKINESIS, AND THE CELL WALL 175

The achromatic figure — In higher plants — In animals — Intranuclear
figures— Origin of the figure — The mechanism of mitosis — Cytokinesis —
In thallophytes — In microsporocytes — In animals — Mechanism of furrowing
—The cell wall — The primary wall layer — Secondary and tertiary wall
layers — The physical nature of the cell wall — The chemical nature of the
cell wall — The walls of spores — Bibliography 9.

CHAPTER X

OTHER MODES OF NUCLEAR DIVISION 202

Cyanophyceae — Protozoa — Other cases in plants — Amitosis — Amitosis and
heredity — Bibliography 10.

CHAPTER XI

THE REDUCTION OF THE CHROMOSOMES 219

Discovery — The stage in the life cycle at which reduction occurs — The
meaning of reduction — Interpretations based on Weismann's theory —
Somatic and heterotypic mitoses compared — Modes of chromosome reduc-
tion— Scheme A — Scheme B — Comparison of Schemes A and B — Reduction
with chromosome tetrads — Numerical reduction without qualitative reduc-
tion— Synapsis, or chromosome conjugation — Relationship of the synaptic
mates — The stage at which conjugation occurs — The nature of the synaptic
union — Chromomeres — Other opinions on the heterotypic prophase —
Bibliography 11.

CHAPTER XII

FERTILIZATION 273

Fertilization in animals — The gametes — The fusion of the gametes —
Fertilization in protozoa — The physiology of fertilization — Fertilization
in plants — Algse — Fungi — Bryophytes and pteridophytes — Gymnosperms —
Angiosperms — Chromosome behavior — Endosperm — Bibliography 12.

CHAPTER XIII

APOGAMY, APOSPORY, AND PARTHENOGENESIS 311

Apogamy — Apospory — Parthenogenesis in animals — Bibliography 13.

CHAPTER XIV

THE ROLE OF THE CELL ORGANS IN HEREDITY 323

The law of genetic continuity — The role of the nucleus — The promorphology
of the ovum — Plastid inheritance — Aleurone inheritance — General con-
clusions.

CONTENTS xiii

CHAPTER XV

PAGE

MENDELISM AND MUTATION 336

Mendelism — A typical case of Mendelian inheritance — The cytological
basis of Mendelism — Mutation — Mutations accompanied by changes
in chromosome number — Bearing on the origin of species and varieties —
Mutations accompanied by no change in chromosome number — Conclusion.

CHAPTER XVI

SEX 354

Experimental evidence for sex-determination — Sex-chromosomes — Sex-
chromosomes and Mendelism — Experimental alteration of the sex ratio —
Metabolic theories of sex — General discussion.

CHAPTER XVII

LINKAGE -. . 378

A typical case of linkage — Sex-linkage — Non-disjunction — Linkage groups —
The chiasmatype theory — Application of the chiasmatype theory to the
problem of linkage — General discussion — Other theories of linkage — Value
of chromosome theory of heredity.

CHAPTER XVIII

WEISMANNISM AND OTHER THEORIES 398

Darwin's hypothesis of pangenesis — DeVries's theory of intracellular
pangenesis — Nageli's idioplasm theory — Weismann's theory — Some modern
aspects of Weismannism — Non-factorial theories — A chemical theory of
heredity — Conclusion — Bibliography 14 (for Chapters XIV-XVIII).

INDEX. . 427

CHAPTER I

HISTORICAL SKETCH

The history of cytology falls naturally into three periods, of which
the first begins with the discovery of the cell by Robert Hooke in 1665,
the second with the foundation of the Cell Theory by Schleiden and
Schwann in 1838-9, and the third with the important researches of
Strasburger, Hertwig, Biitschli, and others between 1870 and 1880. In
the present sketch attention will be confined almost entirely to the first
two periods, the work of the third, or modern, period being dealt with in
the other chapters of the book.

Prior to the seventeenth century attempts to analyse the structure
of organisms were necessarily unsatisfactory. Aristotle (384-322 B.C.)
in his De Partibus Animalium distinguished the "homogeneous parts"
and the "heterogeneous parts," the former corresponding in general to
what we classify as tissues (bone, fat, cartilage, flesh, blood, lymph,
nerve, membrane, nails, hair, skin, vessels, tendon, etc.), and the latter
being the larger members of the body (head, face, hands, feet, trunk,
etc.). Theophrastus, the pupil and successor of Aristotle, taught in his
Historia Plantarum that the plant body is composed of "sap," "veins,"
and "flesh." Aristotle's classification was developed further by Galen
(131-201 A.D.) and by his followers. Although we no longer regard the
above components as elementary parts, but rather as tissues and organs,
the ancients may be pardoned for not carrying the analysis further, for they
did not possess the necessary instruments. Something was then known
about the refraction of light, but it was not until many centuries later
that suitable lenses were available. The first compound microscope was
brought out in 1590 by J. and Z. Janssen, spectacle makers of Middle-
burg, Holland; and during the first part of the seventeenth century
other improved models were designed by other workers. These instru-
ments in the hands of men possessed of scientific curiosity soon led to
many significant discoveries. A new world was opened to the eye of
science, and the compound microscope has since remained an instru-
ment of extraordinary value in biological research.

1

2 INTRODUCTION TO CYTOLOGY

The Discovery of the Cell. — Cytology may be said to have begun with
the discovery of the cell by Robert Hooke (1635-1703) in 1665. Hooke,
who lived in London and has been described as a man of eccentric appear-
ance and habits, showed a remarkably varied scientific activity. For a
time he was a professor of geometry, and later became an architect. He
performed many original experiments in mechanics and for a number of
years was curator of experiments to the Royal Society. His interest in
optics led him to examine all sorts of objects with the compound micro-
scope. In charcoal and later in cork and other plant tissues he found
small honeycomb-like cavities which he called "cells." He had no dis-
tinct notion of the cell contents, but spoke of a "nourishing juice,"
which he inferred must pass through pores from one cell to another.
His many observations were embodied in his Micrographia (1665), a
large work illustrated with 83 plates. The chapter containing his re-
marks on cells is entitled "Of the schematisme or texture of cork and
the cells and pores of some other such frothy bodies." Quaint and crude
as it now appears to us, the Micrographia takes its place as the earliest
cytological classic.

Three other names even more prominent in the early history of micro-
scopy are those of Malpighi, Grew, and Leeuwenhoek. Marcello Mal-
pighi (1628-1694), an Italian physiologist and professor of medicine at
Bologna, Pisa and Messina, is best known for his important pioneer work
in anatomy and embryology. Most of'his observations on plants were
included in his Anatome Plantarum (1675) and had to do largely with the
various kinds of elements making up the body of the vascular plant.
Malpighi made a distinct step in advance in studying tissues with the
cell as a unit; a clear fore-shadowing of the Cell Theory is seen in his
remarks concerning the importance of the "utriculi" in the structure of
the body. At Pisa Malpighi was associated with G. A. Borelli, who was
one of the first to use the microscope on the tissues of higher animals.

Nehemiah Grew (1641-1712) was an English physician and botanist.
He began a careful study of plant structure in 1664, and in 1670 read his
first important paper before the Royal Society. Further contributions
followed at intervals until 1682, when all of them were published under
the title The Anatomy of Plants. Like Malpighi, an abstract of whose
first work on plants was presented to the Royal Society in 1671, Grew was
interested in tissues, and gave particular attention to the combinations
of these tissues in different plant organs. He was strongly impressed
by the manner in which the cells, which he also called "vesicles" and
"bladders," appeared to make up the bulk of certain tissues : " . . . the paren-
chyma of the Barque," he said, "is much the same thing, as to its con-
formation, which the froth of beer or eggs is, as a fluid, or a piece of fine
Manchet, as a fixed body" (p. 64). He further believed the walls of
the cells to be composed of numerous extremely fine fibrils : in the vessels

HISTORICAL SKETCH 3

or longitudinal elements these fibrils were wound in the form of a close
spiral, while the vessels themselves were bound together by a transverse
series of interwoven threads. He accordingly compared the structure
of the plant with that of a basket, and with "fine bone-lace, when the
women are working it upon the cushion" (p. 121).

Antony van Leeuwenhoek (1632-1723) of Delft is remembered for
his pioneer researches in the field of microscopy. He constructed a
number of simple lenses of high power, and with these he was able to see
for the first time certain protozoa, bacteria, and other minute forms of life.
In the course of his investigations he observed the cells ("globules")
in the tissues of higher organisms. His work, in spite of the fact that
it was carried on without any definite plan, brought to light a number
of important facts, but in general his accomplishments do not bear
favorable comparison with those of Grew and Malpighi.

Preformation and Epigenesis. — After the death of Leeuwenhoek
there ensued a period during which the actual investigation of the cell
and the structure of organisms remained practically at a standstill. At
that time, however, certain speculations were indulged in which should
be recorded here, not because they can be regarded as scientific cytology
but because of the influence they exerted upon the formulation of many
cytological problems in later years. These speculations resulted in the
division of the biologists of the day into two schools, the main question
at issue being the manner in which the embryo develops from the egg.
The two theories formulated in answer to this question have been Called
the Preformation Theory and the Theory of Epigenesis.

According to the Preformation Theory, the basis for which was laid
in the seventeenth century works of Swammerdam, Malpighi, and
Leeuwenhoek, the egg contains a fully formed miniature individual,
which simply unfolds and enlarges as development proceeds. Because
of this unfolding the theory was also known as the Theory of Evolu-
tion, a phrase which has a quite different connotation today. In the
eighteenth century the preformation idea was carried to an absurd
extreme by Bonnet (1720-1793) and others, who argued that if the egg
contains the complete new individual, the latter must in turn contain
the eggs and individuals of all future generations successively encased
within it, like an infinite series of boxes one within another. This theory
of encasement (emboitemenf) was a logical deduction from the since
abandoned premise that everything, including organisms for all time,
had been formed by one original creation, and that nothing could there-
fore be formed anew. The preformationists soon became separated into
two groups: the spermists or animalculists, and the ovists. By the
former the new individual was supposed to be encased in the sperma-
tozoon, and figures were actually published showing a small human figure,
or "homunculus," within the sperm head. The ovists, on the contrary,

4 INTRODUCTION TO CYTOLOGY

held that the individual is encased in the egg. A bitter strife was carried
on over this question by the two groups of preformationists, and various
interesting compromises were made. But all extreme forms of preforma-
tionism were to disappear in the light of more critical investigations,
which went far to support the opposing Theory of Epigenesis.

Two of the early champions of the Theory of Epigenesis were William
Harvey (1578-1667; Exercitationes de Generatione Animalium, 1651),
and Caspar Friedrich Wolff (1733-1794; Theoria Generationis, 1759).
As the result of many careful observations on the embryogeny of the
chick Wolff was able to show beyond question that development is
epigenetic: neither egg nor spermatozoon contains a formed embryo;
development consists not in a process of unfolding, but in "the continual
formation of new parts previously non-existent as such" (Wilson).
Here there was room for the principle of true generation, or "the
production of heterogeneity out of homogeneity." The Theoria Genera-
tionis is to be regarded as one of the really great contributions to
biological science, for the Theory of Epigenesis, to which it furnished
substantial support, later became established with modifications as a
fundamental principle of embryology, particularly through the work of
von Baer in the nineteenth century.

In commenting on preformation and epigenesis Whitman (1894)
emphasizes the fact that the tendency of modern biology has not been to
show the entire falsity of either or both of these views, but to seek out the
germs of truth possessed by each, and to relate them to modern biological
conceptions. "The two views missed the mark by over-shots in contrary
directions," says Whitman. The one theory claimed too much preforma-
tion: everything was preformed at the start. The other theory claimed
too much postf ormation : everything was formed anew. Our present
position, although it excludes both views in their crude original form,
involves in a new sense both conceptions. When we say that the egg is
organized, possessing an architecture or mechanism in its cytoplasm or
nucleus which largely predetermines development, we are making a
modernized statement of the preformation idea. When we say that the
parts of the individual are in no way delineated in the egg, but are mainly
determined by external conditions during the course of development,
we are speaking in terms of modern epigenesis. "The question is no
longer whether all is preformation or all postf ormation; it is rather this :
How far is post-formation to be explained as the result of pre-f ormation, and
how far as the result of external influences?^ When it is borne in mind,
therefore, that one of the outstanding problems of modern cytology is
that of identifying the factors involved in the development of an organ-
ized and highly differentiated individual from an organized but relatively
undifferentiated egg cell, it is at once evident that our sketch of cyto-
logical history would be incomplete without the above reference to the
early Theories of Preformation and Epigenesis.

HISTORICAL SKETCH 5

Early Theories of Cell-formation. — The researches of Hooke,
Malpighi, and Grew in the seventeenth century had shown that "cells,"
or "globules," are important structural elements in organisms. When
attention was again directed to such matters in the eighteenth century
there was very soon felt a need for a theory which would account for the
origin of cells. We may briefly review some of the suggestions which were
offered.

One of the earliest theories of cell-formation was that put forward by
Wolff in the Theoria Generationis. According to Wolff, every organ is
at first a clear, viscous fluid with no definite structural organization.
In this fluid cavities (Blaschen; Zellen) arise and become cells, or, by
elongation, vessels. These may later be thickened by deposits from the
" solidescible " nutritive fluid. The cavities, or cells, are not to be
regarded as independent entities; organization is not effected by them,
but they are rather the passive results of an organizing force (vis essen-
tialis) inherent in the living mass. Three important points in Wolff's
theory should be noted because of the relation they bear to subsequent
conceptions of the role of cells : the spontaneous origin of the cell, the
organization of parts by differentiation in a homogeneous living mass,
and the passive role of the cell in this organizing process. This theory
was adopted in 1801 by C. F. Mirbel (1776-1854), who further believed
that the cells communicate through pores in their walls.

K. Sprengel (1766-1833) stated that cells originate in the contents
of other cells as granules or vesicles which absorb water and enlarge.
Sprengel's observations seem to have been very poorly made, for he
evidently mistook starch grains for the "vesicles" which were supposed
to grow into new cells. But Sprengel's theory was upheld by L. C.
Treviranus (1779-1864) in a work appearing in 1806, and both men fought
many years for its support. Kieser (1812) further developed the theory
that granules in the latex are "cell germs" which later hatch in the inter-
cellular spaces to form new cells.

With a much clearer understanding of the nature of the problems
involved a number of excellent observations were made by J. J. Bern-
hardi in 1805, by H. F. Link and K. A. Rudolphi in 1807, and by J. J. P.
Moldenhawer in 1812. It is to be regretted that the deserved attention
was not given to their views, for they promised to lead in the right
direction.

A number of years later Mirbel, in a work on Marc/ianto'a (183 1-1833),
distinguished three modes of cell-formation: (1) the formation of cells on
the surface of other cells, (2) the formation of cells within older cells, and
(3) the formation of cells between older cells. The first mode apparently
represented the budding of the germ tube arising from the spore, while the
second and third modes were formulated as the result of a misinterpreta-
tion of the process of cell-multiplication in growing gemmae.

6 INTRODUCTION TO CYTOLOGY

Hugo von Mohl (1805-1872), in spite of his many valuable observa-
tions on the growth of algae, in 1835 agreed essentially with Mirbel. He
made a step in advance, however, when he described carefully for the
first time the division of a cell. We shall see further on that von Mohl's
later researches contributed largely to the upbuilding of an adequate
theory of the cell.

F. J. F. Meyen (1804-1840) held that there are three fundamental
forms of elementary organs: cells, spiral tubes, and sap vessels. He
noted the wide occurrence of cell-division but did not describe the process
in detail. Meyen apparently made the first attempt to distinguish cell-
division from the free cell-formation described by previous workers. It
has been pointed out by Sachs that if this short step had been clearly
taken earlier the peculiar theory of cell-formation later developed by
Schleiden would have been impossible. Von Mohl also had made obser-
vations ruling out Schleiden's idea, but his excessive caution prevented
him from making a decisive statement on the subject. H. J. Dutrochet
(1776-1847) in 1837 described the body as being composed of solids and
fluids, the former being aggregations of cells of a certain degree of firmness,
and the latter, such as blood, being made up of cells freely floating. He
believed that although the cell contents may be more or less solid, the
highest degree of vitality is compatible only with the liquid condition.
He further recognized muscle fibers as elongated cells.

To all the above workers the important elementary unit was the
"globule." It was customary to refer to this conception as the Globular
Theory, in contradistinction to the curious and fanciful Fiber Theory
put forth by Haller (1708-1777) many years before (1757), according to
which the organism is made up of slender fibers cemented together
by "organized concrete." For some the term "globule" stood for the
granules seen in the cell contents, whereas for others it meant the cell
itself. As observations multiplied and ideas became more definite the
Cell Theory of Schleiden and Schwann was more and more distinctly fore-
shadowed. Before turning to the Cell Theory, however, we must notice
briefly a few observations which had been made on the cell contents.

Early Observations on the Cell Contents. — Although the true nature
and significance of the contents of cells were not recognized until many
years later, a number of early investigators had seen protoplasm and had
been impressed by certain of its activities. As early as 1772 Corti, and a
few years later Fontana (1781) saw the rotation of the "sap" in the
Characeae and other plants. After being long forgotten these facts
were rediscovered by L. C. Treviranus (1811) and G. B. Amici (1819),
whereupon Horkel, an uncle of Schleiden, called attention to the earlier
work of Corti. Protoplasmic circulation of the more complex type was
discovered in the stamen hairs of Tradescantia by Robert Brown in 1831,
and other workers, especially Meyen, soon added other cases.

HISTORICAL SKETCH 7

During the first third of the nineteenth century no name is of greater
interest to cytologists than that of Robert Brown (1773-1858). Al-
though he is famous chiefly for his great taxonomic monographs and his
morphological work, he is known in cytology as the man who is usually
given the credit for the discovery of the nucleus, which he announced
in 1831. Although it was Brown who was impressed by the probable
importance of the nucleus, and who concluded in 1833 that it is a normal
cell element, certain other observers, notably Fontana, who described a
nucleus in 1781, and Meyen, who saw it in Spirogyra in 1826, should
share the honor for its discovery. The phenomenon which has since
been known as "Brownian movement" was seen by Brown in 1827.

The first period in the development of our subject is seen to have
been one in which there was a tendency to indulge in speculation to an
extent quite unwarranted by the facts at hand. As we have already
pointed out, however, this speculation was of considerable importance to
us, in that it had to do with questions which later became central prob-
lems of cytology. Carefully made observations were meanwhile in-
creasing in number and varie(y$, and the time eventually became ripe
for the formulation of a theory which would correlate these data and give
a definite trend to cytological investigations. Such a theory was soon
forthcoming.

The Foundation of the Cell Theory. — The year 1838 marks an epoch
in the history of biology. In this and the following year Schleiden and
Schwann founded the Cell Theory, which, in view of its enormous in-
fluence upon all branches of biological science, may be regarded as second
in importance only to the Theory of Evolution. We have seen that cells
had been observed by various workers during many years, and had been
recognized as being constantly present in the bodies of living organisms,
but it remained for Schleiden and especially Schwann to formulate a
comprehensive theory embracing the known facts and affording a start-
ing point for further researches.

The Cell Theory stated primarily that the body is composed entirely of
cells and their products, the cell being the unit of structure and function
and the primary agent of organization. Subsidiary to this was Schleiden's
theory of cell-formation, which should not be confused with the main
thesis just stated.

Matthias Jakob Schleiden (1804-1881) is one of the most prominent
and interesting characters in botanical history. He studied law at
Heidelberg, medicine at Gottingen, and botany at Berlin, where he met
Schwann and Robert Brown. The association of these men undoubtedly
meant much to the future of botany and zoology. Eventually Schleiden
became Professor of Botany at Jena, where he remained for 23 years.
Schleiden was famous not merely because of his own work, but chiefly as
the result of the tremendous impetus which he gave to investigation.

He sought to place botany on a scientific footing equal to that of physics
and chemistry, and insisted upon accurate observation and developmental
studies as the basis of morphology. Sachs says: " Endowed with some-
what too great love of combat, and armed with a pen regardless of the
wounds it inflicted, ready to strike at any moment, and very prone to
exaggeration, Schleiden was just the man needed in the state in which
botany then was."

Theodor Schwann (1810-1882) was associated as a student with
Johannes M tiller, the great physiologist, first at Wiirzburg and later at
Berlin. It was in the latter place that he put forth his statement of the
Cell Theory. Immediately afterward he went to Louvain, where he was
a professor for nine years, and later transferred to Liege. In disposition
he contrasted strongly with Schleiden, being described as "gentle and
pacific."

It is said that Schleiden, while dining with Schwann, discussed with
him some of his ideas regarding cells in plants, which he had been studying
in his laboratory. Schwann had been making similar observations on
animals, and after the meal the two went to Schwann's laboratory, where
they came to the conclusion that cells are fundamentally alike in both
kingdoms. Schleiden's treatise on the subject, Beitrage zur Phylogenesis,
appeared in 1838 and dealt mainly with the origin of cells. Robert Brown
had recently discovered the nucleus, and about it Schleiden built up his
theory of "free cell-formation," which was essentially as follows: In the
general cell contents or mother liquor (" cytoblastema ") there are formed,
by a process of condensation, certain small granules (later called "nu-
cleoli" by Schwann). Around these many other granules accumulate,
thus forming nuclei ("cytoblasts"). Then, "as soon as the cytoblasts
have attained their full size, a delicate transparent vesicle appears upon
their surface." This vesicle in each case enlarges and forms a new cell,
and, since it arise? upon the surface of the cytoblast (nucleus), "the
cytoblast can never lie free in the interior of the cell, but is always en-
closed [i.e., imbedded] in the cell wall ..." Schleiden thus regarded
new cell-formation as endogenous ("cells within cells") rather than the
result of cell-division. With respect to the main proposition of the Cell
Theory he says in the opening paragraphs: " . . . every plant developed
in any higher degree, is an aggregate of fully individualized, independent,
separate beings, even the cells themselves. Each cell leads a double
life: an independent one, pertaining to its own development alone;
and another incidental, in so far as it has become an integral part of a
plant. It is, however, easy to perceive that the vital process of the in-
dividual cells must form the first, absolutely indispensable fundamental
basis, both as regards vegetable physiology and comparative physiology
in general; . . ."

Schleiden shared the results of his observations, including his errors,

HISTORICAL SKETCH 9

with Schwann, who was the one to formulate the Cell Theory in a com-
prehensive manner. Schwann announced the theory in concise form in
1838, and in 1839 published a very full account under the title "Mikro-
skopische Untersuchungen iiber die Uebereinstimmung in der Struktur und
dem Wachsthum der Thiere und Pflanzen." He says: "The elementary
parts of all tissues are formed of cells in an analogous, though very diver-
sified manner, so that it may be asserted that there is one universal prin-
ciple of development for the elementary parts of organisms, however different,
and that this principle is the formation of cells." And further: "The
development of the proposition that there exists one general principle
for the formation of all organic productions, and that this principle is
the formation of cells, as well as the conclusions which may be drawn from
this proposition, may be comprised under the term Cell Theory . . . "
" . . .all organized bodies are composed of essentially similar parts,
namely, of cells . . . "

Elaboration of the Cell Theory. — The Cell Theory at once became
established as one of the main foundation stones of biological research,
but it underwent considerable modification as investigations proceeded.
The main thesis, that the body is composed of cells and their products,
remained, but other ideas associated with this in the minds of Schleiden
and Schwann, particularly that concerning free cell-formation, were
superseded. Soon after the formulation of the Cell Theory its elabora-
tion was begun by Unger, von Mohl, and Nageli, who based their con-
clusions on observations of a very high order. Franz Unger (1800-1870),
in two works appearing in 1844 on vegetable growing points and the
growth of internodes, argued for the origin of cells by division. Von
Mohl, in two treatises (1835, 1844), maintained that there are two meth-
ods of cell-formation: by division and by the formation of cells within
cells. He thought the "primordial utricle" (protoplast) must be ab-
sorbed to make way for the two new ones, or, less probably, the old one
must divide into two. Like Schleiden, he thought the nucleus must be
incorporated in the cell wall, but later (1846) concluded that it lies in
the primordial utricle. It was in his paper of 1846 that von Mohl in-
troduced the term "protoplasm" in its present sense.

Carl von Naj^li (1807-1891) in 1844 produced an exhaustive treatise
on the nucleus, cell-formation, and growth. In algae and the micro-
sporocytes of angiosperms he clearly showed that cells multiply b}'
division, and Schleiden was forced to admit that this might be "a second
kind of cell-formation." The continuation of Naegli's researches in
1846 completely overthrew Schleiden's conception of free cell-formation,
establishing the significant fact that all vegetative cell-formation is by
cell-division. Many similar observations had been made by Unger and
von Mohl, but Nageli elaborated a broad theory which took into account
all of the data at hand. He distinctly defined cell-division and free

10 INTRODUCTION TO CYTOLOGY

cell-formation, and showed that what had been taken for the latter was
only a special case of the former. Nageli's conclusions were supported
by new evidence furnished by other investigators, who further demon-
strated that not only vegetative cells but also those reproductive cells
(in thallophytes) which Nageli thought in some cases might be formed
freely, originate by a modified process of cell-division. It was now clear
that cells arise only from preexisting cells, a conception which had been
emphasized by Remak (1841) and which Virchow (1855) expressed in the
dictum "omnis cellula e cellula."

Opinions concerning the origin of the nucleus and its role in cell-
division varied greatly among these workers, reliable observations being
as yet insufficient to allow the formulation of any definite conclusion.
In 1841 Henle believed with Schleiden and Schwann that the nucleus was
formed by the aggregation of "elementary granules," and that it was not
constantly present. Goodsir looked upon the nucleus as the reproduc-
tive organ of the cell. Von Kolliker in 1845 asserted that nuclear divi-
sion precedes the division of the cell, and Remak, as a result of his
observations on blood cells in the chick embryo, formulated a definite
theory of cell-division (1841, 1858). He believed cell-division to be a
"centrifugal" process: the nucleolus, nucleus, cytoplasm, and cell mem-
brane were supposed to divide in turn by simple constriction. Just such
a process, though evidently very exceptional, has been observed at a
more recent date by Conklin (1903). In describing a case of nuclear
division Wilhelm Hofmeister (1824-1877) stated that the membrane of
the nucleus dissolved, the nuclear material then separating into two
masses around which new membranes were formed (1848, 1849). It was
generally believed, however, that the origin of nuclei by division was
of rare occurrence, and that ordinarily the nucleus dissolved just before
cell-division, two new ones forming de novo in the daughter cells. Von
Mohl (1851), who in the main agreed with Hofmeister, wrote as follows:
"The second mode of origin of a nucleus, by division of a nucleus already
existing in the parent-cell, seems to be much rarer than the new produc-
tion of them . . . " And again, " . . . it is possible that this process
[nuclear division] prevails very widely, since ... we know very little
yet respecting the origin of nuclei. Naegli thinks that the process is
quite similar to that in cell-division, the membrane of the nucleus form-
ing a partition, and the two portions separating in the form of two dis-
tinct cells."

It was not until many years later, in connection with researches upon
fertilization and embrybgeny, that the behavior of the nucleus in cell-
division became known in detail, and its probable significance pointed
out. In 1879 Eduard Strasburger (1844-1912) announced definitely
that nuclei arise only from preexisting nuclei. W. Flemming was led
to the same conclusion by his studies on animal cells, and expressed
it in the dictum "omnis nucleus e nucleo" (1882). (See footnote, p. 143.)

HISTORICAL SKETCH 11

The Protoplasm Doctrine. — The Cell Theory and all of its corollaries
were placed in a new light with the development of a more adequate
conception of the significance of protoplasm. To its discoverers the
cell meant nothing more than the wall surrounding a cavity; they spoke
only in the vaguest terms of the "juices" present in cellular structures.
The founders of the Cell Theory held a position but little in advance of
this; they observed the cell contents but regarded them as of relatively
slight importance. Even those who had been impressed by the phe-
nomenon of protoplasmic streaming were not aware of the significance of
the substance before their eyes.

Felix Dujardin (1801-1860) in 1835 described the "sarcode" of the
lower animals as a substance having the properties of life. Von Mohl
had seen a similar substance in plant cells, and in 1846, as noted above,
he called it "Schleim," or "Protoplasma," the latter term having been
used shortly before by Purkinje in a somewhat different sense. Nageli
and A. Payen (1795-1871) in 1846 recognized the importance of proto-
plasm as the vehicle of the vital activity of the cell ; and Alexander Braun
(1805-1877) in 1850 pointed out that swarm spores, which are cells, con-
sist of naked protoplasm. An important point was reached when Payen
(1846) and Ferdinand Cohn (1850) concluded that the "sarcode" of
the animal and the "protoplasm" of the plant are essentially similar
substances. In the words of Cohn :

"The protoplasm of the botanist, and the contractile substance and sarcode
of the zoologist, must be, if not identical, yet in a high degree analogous sub-
stances. Hence, from this point of view, the difference between animals and
plants consists in this; that, in the latter, the contractile substance, as a primordial
utricle, is enclosed within an inert cellulose membrane, which permits it only to
exhibit an internal motion, expressed by the phenomena of rotation and circula-
tion, while, in the former, it is not so enclosed. The protoplasm in the form
of the primordial utricle is, as it were, the animal element in the plant, but which
is imprisoned, and only becomes free in the animal; or, to strip off the metaphor
which obscures simple thought, the energy of organic vitality which is manifested
in movement is especially exhibited by a nitrogenous contractile substance, which
in plants is limited and fettered by an inert membrane, in animals not so."

Protoplasm was now studied more intensively than ever. H. A.
de Bary (1831-1888), working on myxomycetes and other plant forms,
and Max Schultze (1825-1874), investigating animal cells, demonstrated
the correctness of Cohn's view. The work of Schultze was especially
important in that it firmly established in 1861 the Protoplasm Doctrine,
namely, that the units of organization are masses of protoplasm, and that
this substance is essentially similar in all living organisms. The cell,
according to Schultze, is "a mass of protoplasm containing a nucleus,,
both nucleus and protoplasm arising through the division of the corres-
ponding elements of a preexisting cell." The cell wall, upon which the

12 INTRODUCTION TO CYTOLOGY

early workers had focussed their attention, turned out to be of secondary
importance. The cell was thus seen to be primarily the organized
protoplasmic mass, to which Hanstein in 1880 applied the convenient
term protoplast.

Extensive studies on the physical nature of protoplasm were soon
undertaken by Kuhne (1864), Cienkowski (1863), and de Bary (1859,
1864); and there later followed the well-known structural theories of
Klein, Flemming, Altman, and Biitschli. (See Chapter III.)

Von Mohl as early as 1837 held that the plastid is a protoplasmic
body. The classic researches of Nageli (1858, 1863) on plastids and
starch grains laid the foundation for our knowledge of these bodies, which
was greatly extended in later years by Meyer (1881, 1883, etc.) and
Schimper (1880, etc.). (See Chapter VI.)

It would be difficult to overestimate the value, both practical and
theoretical, of the Protoplasm Doctrine, for its establishment has not
only led to knowledge by which the conditions of life have been materially
improved, but has also been an important factor in assisting man to a
modern, rational outlook on organic nature, in which he has learned to
include himself. It is not too much to say that the identification of
protoplasm as the material substratum of the life processes was one of
the most significant events of the nineteenth century. The doctrine
was furnished with a popular expression by Huxley in his well-known
essay, The Physical Basis of Life (1868).

The New Conception of the Cell. — The conception of the cell had now
developed into something quite different from what it had been in the
minds of the founders of the Cell Theory. The cell was now recognized
as a protoplasmic unit, and the ideas of these men concerning the origin
and multiplication of cells had been overthrown. Future researches
were to show more clearly the importance of the cell in connection with
development and inheritance, and certain limits were to be set to the
conception of the cell as a unit of function and organization. To Schlei-
den and Schwann the multicellular plant or animal appeared as little
more than a cell aggregate, the cells being the primary individualities;
the organism was looked upon as something completely dependent upon
their varied activities for all its phenomena. "The cause of nutrition
and growth," said Schwann, "resides not in the organism as a whole,
but in the separate elementary parts — the cells." This elementalistic
conception of the organism as an aggregate of independent vital units
governing the activities of the whole dominated biology for many years,
notwithstanding its severe criticism by Sachs, de Bary, and many other
later writers who pointed out that, owing to the high degree of physio-
logical differentiation among the various tissues and organs, the cell
cannot be regarded merely as an independent unit, but as an integral
part of a higher individual organization, and that as such the exercise

HISTORICAL SKETCH 13

of its functions must be governed to a considerable extent by the organ-
ism as a whole (Wager). Such divergence of opinion led to much dis-
cussion over the question of organic individuality, which remains as
one of the important problems of modern biology.

But in spite of all these changes we should not forget the great service
rendered by Schleiden and Schwann in the formulation of the Cell Theory.
Huxley (1853) estimated the value of their contribution in the following
lines :

"Doubtless the truer a theory is — the more appropriate the colligating
conception — the better will it serve its mnemonic purpose, but its absolute
truth is neither necessary to its usefulness, nor indeed in any way cognizable by
the human faculties. Now it appears to us that Schwann and Schleiden have
performed precisely this service to the biological sciences. At a time when the
researches of innumerable guideless investigators, called into existence by the
tempting facilities offered by the improvement of microscopes, threatened to
swamp science in minutiae, and to render the noble calling of the physiologist
identical with that of the 'putter-up' of preparations, they stepped forward with
the cell theory as a colligation of the facts. To the investigator, they afforded
a clear basis and a starting point for his inquiries; for the student, they grouped
immense masses of details in a clear and perspicuous manner. Let us not be
ungrateful for what they brought. If not absolutely true, it was the truest
thing that had been done in biology for half a century."

Fertilization and Embryogeny. — In Plants. — Although it was known
to the ancients that there is in plants something analogous to the sexual
reproduction seen in animals, ideas of the organs and processes involved
were very vague. Like Grew and others in the seventeenth century, the
botanists of antiquity were aware of the fact that the pollen in some way
influences the development of the ovary into a fruit with seeds. Definite
proof that the stamens are (to speak somewhat loosely) the male organs
was furnished in the well-known experiments of R. J. Camerarius (1691).
But in spite of the excellent work of J. G. Koelreuter (1761), C. K.
Sprengel (1793), and K. F. Gaertner (1849), all of whom proved the
correctness of this conclusion, the idea of sexuality in plants was vigor-
ously combatted in certain quarters for many years.

An important step in advance was made when G. B. Amici (1830)
followed the growth of the pollen tube from the pollen grain on the stigma
down to the ovule. Schleiden (1837) and Schacht (1850,1858) took up
the study and made a curious misinterpretation: they regarded the ovule
as merely a place of incubation for the end of the pollen tube, which they
supposed to enter the ovule and enlarge to form the embryo directly.
The work of Amici (1842), Tulasne (1849), and others showed the falsity
of this notion, but an acrimonious discussion raged about the subject
for a number of years, Schleiden (1842, 1844) using the most vigorous
language in support of his position. After Hofmeister (1849) had fol-

14 INTRODUCTION TO CYTOLOGY

lowed the process with his characteristic thoroughness there could remain
no doubt concerning the error of Schleiden and Schacht. Hofmeister
clearly demonstrated that the embryo arises, as Amici contended, not
from the end of the pollen tube, but from an egg contained in the ovule,
the egg being stimulated to development by the pollen tube. He was
wrong, however, in supposing that the tube did not open, but that a
fertilizing substance diffused through its wall.

It was in the algae that the union of the sperm cell with the egg cell
(the act of fertilization) was first seen in the case of plants. In 1853
Thuret saw spermatozoids attach themselves to the egg of Fucus, and in
1854 he showed that they are necessary to its development. The actual
entrance of the spermatozoid into the egg was first observed in 1856 by
Nathanael Pringsheim (1824-1894) in (Edogonium. The fusion of the
parental nuclei was seen by Strasburger (1877) in Spirogyra, but he
thought they thereupon dissolved. This error was corrected shortly
afterward by Schmitz (1879), who was thus the first to show clearly that
the central feature of the sexual process in plants is the union of two
parental nuclei to form the primary nucleus of the new individual.

That the same process occurs in fertilization in the higher plants
was demonstrated by Strasburger, who in 1884 described the union of the
egg nucleus with a nucleus brought in by the pollen tube.. In 1898 and
1899 S. Nawaschin and L. Guignard completed the story by describing
the phenomenon of double fertilization, whereby the second male nucleus
contributed by the pollen tube unites with the two polar nuclei to form
the primary endosperm nucleus. The subsequent work of Strasburger
and others on the gymnosperms and angiosperms greatly cleared up the
whole matter of fertilization and embryogeny in these plants. This
work belongs to the modern period of cytology.

In Animals. — It is probable that the spermatozoon was first seen in
1677 by Ludwig Hamm, a pupil of Leeuwenhoek. The credit for the
discovery, however, is usually given to Leeuwenhoek, since it was he who
brought the matter to the attention of the Royal Society and pursued
such studies further. He asserted that the spermatozoa must penetrate
into the egg, but it was thought at that time and for many years after-
ward that they were parasitic animalcules in the spermatic liquid; hence
the name "spermatozoa."

Although L. Spallanzani (1786) is usually said to have shown by his
filtration experiment that the spermatozoon is the fertilizing element,
it is pointed out by Lillie (1916) that Spallanzani did not draw the correct
conclusion: he even denied that the spermatozoon is the active element,
holding rather that the fertilizing power lies in the spermatic liquid. It
was Prevost and Dumas who corrected this mistake and demonstrated
the true role of the spermatozoon (1824). The spermatozoon was later
shown by Schweigger-Seidel (1865) and La Valette St. George (1865) to

HISTORICAL SKETCH 15

be a complete cell with its nucleus and cytoplasm, as von Kolliker had
maintained. That Schwann (1839) had been right in considering the
egg as a cell was shown by Gegenbaur in 1861. The polar bodies formed
at the time the egg matures are said to have been first seen by Carus
(1824). Biitschli (1875) showed them to be formed as the result of the
division of the egg nucleus, and Giard (1877) and Mark (1881) interpreted
them as abortive eggs.

The penetration of the spermatozoon into the egg was not actually
seen until Newport (1854) observed it in the case of the frog. In 1875
O. Hertwig (b. 1849) announced the important discovery that the two
nuclei seen fusing in the fertilized egg are furnished by the egg and the
spermatozoon — by the two parents. The role of the nucleus in fertiliza-
tion was thus demonstrated in animals only shortly before it was in
plants, and it is interesting to note that the first complete description of
the union of the germ cells in animals was given by H. Fol in the same
year (1879) that Schmitz described clearly the process in plants. It was
now evident that fertilization in both kingdoms consists in the union of
two cells (gametes), one from each parent (in dioscious forms), and that
the central feature of the process is the union of the two gamete nuclei, the
new individual therefore deriving half of its nuclear substance from each
parent.

Although the cleavage of the fertilized animal egg to form the embryo
had been seen many years previously, it was first definitely described by
Prevost and Dumas in 1824 for the frog. At that time neither the egg nor
the products of its division were known to be cells. The true meaning of
cleavage was elucidated by M. Barry, who held that the blastomeres are
cells and that their division is preceded by the division of their nuclei,
and by a number of later writers, including A. von Kolliker, who traced
in detail the long series of changes by which the multiplying embryonic
cells become differentiated into the various tissues and organs. Embry-
ogeny was thus shown to be a process of cell-division and differentiation,
the fertilized egg cell initiating a series of divisions giving rise to all the
cells of the body, and to the germ cells. The life cycle was now recognized
as a cell cycle ; and since the egg is the direct descendant of the egg of the
previous generation it became evident, as Virchow pointed out in 1858,
that there has been an uninterrupted series of cell-divisions from the
beginnings of life on the earth in the remote past down to the organisms
in existence today. The statement of this conception is known as the
Law of Genetic Continuity. In the words of Locy (1915) :

"The conception that there is unbroken continuity of germinal substance
between all living organisms, and that the egg and the sperm are endowed with an
inherited organization of great complexity, has become the basis for all current
theories of heredity and development. So much is involved in this conception
that ... it has been designated (Whitman) 'the central fact of modern

16 INTRODUCTION TO CYTOLOGY

biology.' The first clear expression of it is found in Virchow's Cellular Pathology,
published in 1858. It was not, however, until the period of Balfour, and through
the work of Fol, Van Beneden (chromosomes, 1883) Boveri, Hertwig, and
others, that the great importance of this conception began to be appreciated,
and came to be woven into the fundamental ideas of development.''

The Beginning of the Modern Period in Cytology. — As Wilson (1900,
p. 6) points out, the great significance of the many facts brought to light
in the early days of cytology lies in the relation which they bear to the
Theory of Evolution and to the problems of heredity, though for many
years this was only vaguely realized. Darwin, aside from his Hypothesis
of Pangenesis, scarcely mentioned the theories of the cell; and not until
many years later was the cell investigated with reference to these matters.
Researches on the origin of the germ cells, nuclear division, and fertiliza-
tion, which brought the Cell Theory and the Theory of Evolution into
intimate association, began shortly after 1870 with the works of Schneider
(1873), Auerbach (1874), Fol (1875, etc.), Biitschli (1875, etc. ),O.Hertwig
(1875, etc.), van Beneden (1875, etc.), Strasburger (1875, etc.), Flemming
(1879, etc.), and Boveri (1887, etc.). These men laid the foundations for
the work which has followed; and their researches, greatly aided by the
development of new refinements in microtechnique, ushered in modern
cytology. A powerful stimulus to investigation was given when the
zoologists Hertwig, von Kolliker and Weismann, and the botanist Stras-
burger, concluded independently and almost simultaneously (1884-J885)
that the nucleus is the vehicle of heredity, an idea which Haeckel had put
forward as a speculation in 1866. The announcement of this conception
led to an even more intensive study of the nucleus and of its role in
heredity, a study which is now in progress, and which, more than any
other one thing, can be said to characterize the work of our modern period.

Bibliography 1

A, Works dealing wholly or in part with the history of cytology, and general works on

the cell:
AGAR, W. E. 1920. Cytology, with Special Reference to the Metazoan Nucleus.

London.

BOVERI, TH. 1891. Befruchtung. Ergeb. d. Anat. u. Entw. 1 : 386-485.
BUCHNER, P. 1915. Prakticum der Zellenlehre. 1.
BURNETT, W. J. 1853. The cell; its physiology, pathology, and philosophy, as

deduced from original investigations. Trans. Am. Med. Assn. 6.
CHUBB, G. C. 1910. Article on Cytology in Encycl. Brit., llth ed.
DELAGE, Y. 1895. La structure du protoplasme et les theories sur I'hereditc.

Paris.

DONCASTER, L. 1920. An Introduction to the Study of Cytology. London.
FLEMMING, W. 1882. Zellsubstanz, Kern und Kerntheilung. Leipzig.

1981-1897. Referate iiber Zelle. Ergeb. d. Anat. u. Entw. 1-7.
GURWITSCH, A. 1904. Morphologie und Biologie der Zelle. Jena.
HAECKER, V. 1899. Praxis und Theorie der Zellen- und Befruchtungslehre.
HEIDENHAIN, M. 1907. Plasma und Zelle. Jena.

HISTORICAL SKETCH 17

HENNEGUY, L. F. 1896. Legons sur la Cellule. Paris.

HERTWIG, O. 1893. Die Zelle und die Gewebe. Jena. (Engl. Transl. by H. J.

Campbell?)

1900. Die Entwicklung der Biologic im 19. Jahrhundert. Jena.
HOFMEISTER, W. 1867. Die Lehre von der Pflanzenzelle.
HUXLEY, T. H. 1853. The Cell Theory. Brit, and For. Med.-Chir. Rev. 12.

Also in "Scientific Memoirs" 1. 1898.
JOHNSON, D. S. 1914. The evolution of a botanical problem. The history of the

discovery of sex in plants. Science 39 : 2S9-319.
KELLOGG, V. L. 1907. Darwinism Today. New York.

LILLIE, F. R. 1916. The history of the fertilization problem. Science 43: 39-53.
KOERNICKE, M. 1903. Der heutige Stand der pflanzlichen Zellforschung. Ber.

deu. Bot. Ges. 21: (66)-(134).
LOCY, W. A. 1901. Malpighi, Swammerdam, and Leeuwenhoek. Pop. Sci. Mo.

68 : 561-584.

1905. Von Baer and the rise of embryology. Ibid. 67 : 97-126.
1915. Biology and its Makers. 3d ed. New York.
MARK, E. L. 1881. Maturation, fecundation, and segmentation in Limax campes-

tris. Bull. Mus. Comp. Zool. Harvard Coll. 6 : 173-625. pis. 5. (Early litera-
ture of mitosis and fertilization.)

MEVES, FR. 1896, 1898. Referate iiber Zelltheilung. Ergeb. d. Anat. u. Entw. 6, 8.
VON MOHL, H. 1851. The Vegetable Cell. (Engl. transl. by Henfrey.)
OSBORN, H. F. 1894. From the Greeks to Darwin.
RUCKERT, J. 1893. Die Chromatinreduktion bei der Reifung der Sexualzellen.

Ergeb. d. Anat. u. Entw. 3: 517-583.

VON SACHS, J. 1875. History of Botany. (Engl. transl. 1889.)
STRASBURGER, E. 1907. Die Ontogenie der Zelle siet 1875. Prog. Rei Bot. 1.
1910. The minute structure of cells in relation to inheritance. In " Darwin and

Modern Science" (Seward, editor).

THOMSON, J. A. 1899. The Science of Life. Chapters 9 and 10.
TURNER, W. 1890. The cell theory, past and present. Nature 43 : 10-15.
TYSON, J. 1878. The Cell Doctrine: its History and Present State. Phila.
WAGER, H. 1911. Article on Plants: Cytology, in Encycl. Brit., llth ed.
WALDEYER, W. 1888. Ueber Karyokinese und ihre Beziehung zu den Befruchtnngs-

vorgangen. Arch. Mikr. Anat. 32 : 1-122. (Engl. transl. in Quar. Jour. Micr.

Sci. 30: 159-281. 1889.) (Early cell literature.)
WHITMAN, C. O. 1878. The embryology of Clepsine. Quar. Jour. Micr. Sci. 18:

215-315. pis. 12-15. (Early literature of mitosis and fertilization.)
1894. (1) Evolution and Epigenesis. (2) Bonnet's theory of evolution. Woods

Hole Biol. Lectures 1894.
WHEELER, W. M. 1898. Caspar Friedrich Wolff and the Theoria Generationis.

Ibid. 1898.

WILSON, E. B. 1900. The Cell in Development and Inheritance. 2d ed.
•ZiMMERMANN, A. 1893-1894. Sammel-Referate aus dem Gesammtgebiete der

Zellenlehre. Beih. Bot. Centr. 3 and 4. (Reviews of early literature).
For other reviews of early botanical cell literature see Jahresbericht uber die Fort-
schritte der Anatomic und Physiologic, 15-20. 1886-1892; and Neue Folge, Vols.
1-13. 1892-1907.

B. Special works referred to in historical sketch:

AMICI, G. B. 1830. Note sur le mode d'action du pollen sur le stigmate. (Extrait

d'une lettre de M. Amici a M. Mirbel.) Ann. Sci. Nat. Bot. I. 21: 329-332.

Account of Amici's work in Atti della quarta Riunione degli scientiati Italiani
2

18 INTRODUCTION TO CYTOLOGY

tenuta in Padova nel Settembre del 1842. Padova 1843. See FACCHINI 1845.

See also Giorn. Bot. Ital. Anno 2.

AUERBACH, L. 1874. Organologische Studien. Breslau.
VON BAER, K. E. 1828, 1837. Uber Entwickelungsgeschichte der Thiere.
BARRY, M. 1838-1841. Embryological memoirs in Phil. Trans. Roy. Soc. London

128-131. See also: On the first changes consequent on fecundation in the mam-

miferous ovum. Rep. Brit. Assn. Adv. Sci. 1840.
DE BARY, H. A. 1862. Uber den Bau und das Wesen der Zelle. Flora 20.

1864. Die Mycetozoen. 2d ed. Leipzig.
VAN BENEDEN, E. 1875. La maturation de 1'oeuf, la fecondation et les premieres

phases du developpement embryonnaire des mamiferes d'apres des recherches

faites chez le lapin. Bull. Acad. Roy. Belg. 40.
1876. Contribution a 1'histoire de la vesicule germinative et du premier noyau

embryonnaire. Ibid. 41.
1883. Recherches sur la niaturation de 1'oeuf, la fecondation et la division cellu-

laire. Arch, de Biol. 4.

BERNHARDI, J. J. 1805. Beobachtungen iiber Pflanzengefasse.
BOVERI, TH. 1887a. Ueber die Befruchtung der Eier von Ascaris megalocephala.

Sitzungsber. Gesell. Morph. Fhys. Miinchen 3.
18876. Ueber Differenzierung der Zellkerne wahrend der Furchung des Eies von

Ascaris megalocephala. Anat. Anz. 2 : 688—693.
1887-1890. Zellenstudien 1, II, III. Jenaische Zeitsch. 21-24.
BRAUN, ALEX. 1850. Betrachtungen liber die Erscheinung der Verjlingung in der

Natur, inbesondere in der Lebens- und Bildungsgeschichte der Pflanze. (English

transl., Ray Society 1853.)
BROWN, R. 1833. Observations on the organs and mode of fecundation in Orchideae

and AsclepiadesD. Trans. Linn. Soc. (Paper read and privately printed in 1831.)

Also in: Misc. Bot. Works, Ray Society, 1866.
1866. A brief account of microscopical observations on the particles contained in

the pollen of plants; and on the general existence of active molecules in organic

and inorganic bodies. Misc. Bot. Works, Ray Society. (Observations made in

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BUTSCHLI, O. 1875a. Vorl. Mitteilung iiber Untersuchungen betreffend die ersten

Entwicklungsvorgange im befruchteten Ei von Nematoden und Schnecken.

Zeit. Wiss. Zool. 26: 201.
18756. Vorl. Mitteilung einiger Resultate von Studien iiber Conjugation der In-

fusorien und die Zelltheilung. Zeit. Wiss. Zool. 25 : 426.
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Gesell. 10.

CAMERARIUS, R. J. 1691. De Sexu Plantarum. 1694.
CARDS, C. 1824. Von den ausseren Lebensbedingungen der Weiss- und Kalt-

bliitigen Thiere, etc. Leipzig.
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Wiss. Bot. 3 : 325-337.
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Caes. Leop. Carol. Nat. Cur. Bonn 22 : 605-764. (English abst. by Busk, Ray

Soc. 1853.)
CORTI, B. 1772. Observationi misc. sulla Tremella e sulla circolazione del fluid in

una pianta acquaiola. Lucca, 1774.
DUJARDIN, F. 1835. Sur les pretendus estomacs des animalcules infusories et sur

une substance appelee Sarcode. Ann. Sci. Nat. Zool. II 4 : 364-377.

HISTORICAL SKETCH 19

DUTROCHET, H. J. 1837. Memoires pour servir a 1'histoire anatomique et physio-

logique des vegetaux et des animaux.
FACCHINI. 1845. Ueber die Amici'sche Ansicht von der Befruchtung der Pflanzen,

ein Beitrag. Flora 28: 193-198. See also 27: 359-360.
FLEMMING, W. 1879o, 1880, 1881. Beitrage zur Kenntniss der Zelle und ihre

Lebenserscheinungen. I, II, III. Arch. Mikr. Anat. 16, 19, 20.
1879b. Ueber das Verhalten des Kernes bei der Zelltheilung, usw. Virchow's

Archiv. 77.

1882. Zellsubstanz, Kern und Zelltheilung. Leipzig.
1887. Neue Beitrage zur Kenntniss der Zelle. Arch. Mikr. Anat. 29.
FOL, H. 1873. Die erste Entwicklung des Geryoniden-Eies. Jen. Zeitsch. 7.

1875. Sur le developpement des Pteropodes. Arch, de Zool. 4.

1876. Sur les phenomenes de la division cellulaire. Comptes Rend. Acad. Sci.
Paris 83 : 667-669.

1877. Sur le commencement de I'h6nogenie chez divers animaux. Arch. Sci. Nat.
et Phys. Geneve 58.

1879. Recherches sur la fecondation et la commencement de 1'henogenie. Mem.

Soc. Phys. et Nat. Geneve 26: 89-397.
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venin de la vipere. Florence.

GAERTNEB, J. 1788. De fructibus et seminibus plantarum.
GAERTNER, K. F. 1849. Vereuche und Beobachtungen iiber die Bastardzeugung.

Stuttgart.
GEGENBAUR, K. 1861. Ueber den Bau und die Entwicklung der Wirbelthiere.

Mliller's Archiv f. Anat. u. Physiol. p. 451-529.

GIARD, A. 1877. Sur la signification morphologique des globules polaires.
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1842. On Peyer's glands. Lond. and Edinb. Mo. Jour.
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1868. Anatomical Memoirs. (Ed. by Turner.) Edinburgh.
GREW, N. 1682. The Anatomy of Plants. London.
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vegetaux angiospermes. Comptes Rend. Acad. Sci. Paris 128: 864-871. figs.

19.

HAECKEL, E. 1866. Generelle Morphologic. Jena.
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Lebensverrichtungen. Heidelberg.
HARVEY, WM. 1651. Exercitationes de Generatione Animalium. (English transl.,

Sydenham Society, 1847.)
HENLE, J. 1837. Symbolae ad anatomiam villorum intestinalium.

1841. Allgemeine Anatomic. Leipzig.
HERTWIG, O. 1875. Beitrage zur Kenntniss der Bildung, Befruchtung, und Theil-

ung des tierischen Eies, I. Morph. Jahrb. 1. See also 2-4.
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Vererbung. Jenaische Zeitschr. 18 : 276-318.

HOFMEISTER, W. 1849. Die Entstehung des Embryos der Phanerogamen.
1858. Neuere Beobachtungen liber Embryobildung der Phanerogamen. Jahrb.

Wiss. Bot, 1: 82-188. pis. 7-10.
HOOKE, R. 1665. Micrographia, or some physiological descriptions of minute

bodies made by magnifying glasses. London.

HUXLEY, T. H. 1868. The Physical Basis of Life. (Collected Essays.)
KIESER. 1812. Memoire sur 1'organisation des plantes.

20 INTRODUCTION TO CYTOLOGY

VON KOLLIKER, A. 1845. Die Lehre von der thierischen Zelle. Zeit. Wiss. Bot. 2.
1885. Die Bedeutung der Zellkerne fur die Vorgange der Vererbung. Zeit. Wiss.

Zool. 42.
KOELRETJTER, J. G. 1761-1766. Vorlaufige Nachricht von einigen Geschlecht der

Pflanzen betreffenden Versuchen und Beobachtungen.
KOWALEVSKY, A. 1871. Embryologische Studien an Wiirmern und Arthropoden.

Mem. Acad. Imp. Sci. de St. Petersburg VII 16 : 13. pi. 4. figs. 24.
KtJHNE, W. 1864. Untersuchungen iiber das Protoplasma. Leipzig.
LA VALETTE ST. GEORGE. 1865. Ueber die Genese der Samenkorper. Arch. Mikr.

Anat. 1. See also Vols. 2 and 3.

VAN LEEXJWENHOEK, A. 1673-1723. Brieven. Leiden, Delft.
LINK, H. F. 1807. Grundlehren der Anatomic und Physiologic der Pflanzen.
MALPIGHI, M. 1675. Anatome Plantarum.
MARK, E. L. 1881. Maturation, fecundation, and segmentation of Limax

campestris. Bull. Mus. Comp. Zool. Harvard 6: 173-625. pis. 5.
MEYEN, F. J. F. 1826. De Primis Vitse Phenomenis in Fluidis.
1830. Lehrbuch der Phytotomie. Berlin.
1837-1839. Neues System der Pflanzenphysiologie.
MEYER, A. 1881. Ueber die Struktur der Starkekorner. Bot. Zeit. 39: 841-846,

857-864. pi. 9.
1883a. Ueber Krystalloide der Trophoplasten und liber die Chromoplasten der

Angiospermen. Ibid 41 : 489, 505, 525.
18836. Das Chlorophyllkorn. pp. 91. pis. 3. Leipzig.
MIRBEL, C. F. 1801. Traite d'anatomie et de physiologic vegetale.
1808. Exposition et defense de ma theorie de 1'organisation vegetale.
1833. Recherches sur la Marchantia. Mem. French Inst. 1835.
VON MOHL, H. 1835, 1837. Ueber die Vermehrung der Pflanzenzelle durch Theilung.
Dissert. Tubingen 1835. Flora 46. 1837.

1844. Einige Betrachtungen iiber den Bau der vegetabilische Zelle. Bot. Zeit.
2 : 273-277, 289-294, 305-310, 321-326, 337-342. pi. 2.

1845. Vermischte Schriften.

1846. Ueber die Saftbewegung im Inneren der Zellen. Bot. Zeit. 4 : 73-78, 89-94.
1851. Grundziige der Anatomic und Physiologic der vegetabilische Zelle. (Engl.

transl. by Henfrey, London, 1852.)

MOLDENHAWER, J. J. P. 1812. Bcitrage zur Anatomic der Pflanzen.
VON NAGELI, C. 1844, 1846. Zellkerne, Zellbildung, und Zellwachsthum. Zeitschr.
Wiss. Bot. 1, 3. (Engl. transl. by Henfrey, Ray Society; London, 1846, 1849.)
1846. On the utricular structures in the contents of cells. Ray Society, 1849.

(Engl. Transl. by Henfrey.)
1858. Die Starkekorner. Zurich.
NAWASCHIN, S. 1899. Neue Beobachtungen iiber Befruchtung bei Fritillaria und

Lilium. Bot. Centr. 77 : 62. (Account in Russian, 1898.)

NEWPORT, G. 1851, 1853, 1854. On the impregnation of the ovum in the amphi-
bia. Phil. Trans. Roy. Soc. London.
PAYEN, A. 1839. Memoire sur I'amidon, etc. Paris.

1846. Memoire sur les developpements des vegetaux, etc. Mem. Acad. Paris 9.
PREVOST and DUMAS. 1824. Nouvelle Theorie de la generation. Ann. Sci. Nat. 1 :

1, 167; 2 : 100, 129.
PRINGSHEIM, N. 1855. Ueber die Befruchtung und Keimung der Algen und das

Wesen des Zeugungsaktes. Monatsber. K. Akad. Wiss. Berlin 1.
1856. Ueber die Befruchtung der Algen. Ibid.
1858. Morphologic der Oedogonieen. Jahrb. Wiss. Bot. 1: 11-81.

HISTORICAL SKETCH 21

RKMAK, R. 1841. Ueber Theilung rother Blutzellen beim Embryo. Med. Vcr.

Zeit. Miiller's Archiv f. Anat. u. Physiol. 1858; 177-188. pi. 8.
1852. Ueber extracellulare Entstehung thierische Zellen und liber Vermehrung

derselben durch Theilung. Muller's Archiv f. Anat. u. Physiol. 1852; 47-57.
RUDOLPHI, K. A. 1807. Anatomie der Pflanzen.
SCHACHT, H. 1850 Entwicklungsgeschichte des Pflanzenembryon. Amsterdam

1850. In Ann. Sci. Nat. Bot. Ill 15: 80-109. 1851.

1858. Ueber Pflanzenbefruchtung. Jahrb. Wiss. Bot. 1: 193-232. pis. 11-15.
SCHIMPER, A. F. W. 1880-1881. Untersuchungen iiber die Entstehung der Starke-
korner. Bot. Zeit. 38: 881; 39: 185, 201, 217. pis. 13, 2.

1883. Ueber die Entwicklung der Chlorophyllkorner und Farbkorper. Bot. Zeit.
41: 105, 121, 137, 153. pi. 1.

1885. Untersuchungen iiber die Chlorophyllkorper und die ihnen homologen

Gebilde.
SCHLEIDEN, M. J. 1837. Einige Blick auf die Entwicklungsgeschichte des vege-

tabilische Organismus bei den Phanerogamen. Wiegmann's Archiv. 1: 289.
1838. Beitrage zur Phylogenesis. Miiller's Archiv f. Anat. u. Phys. (English

transl., Sydenham Society, 1847.J
1842. Grundziige zur Wissenschaftlichen Botanik. Zweite Aufl. (English

transl. by Lankester, 1849.)

1844. Bemerkung zur Bildungsgeschichte des Veg. Embryo. Flora 27: 787-789.
SCHNEIDER, A. 1873. Untersuchungen iiber Platelminthen. Jahrb. d. Oberhess.

Gesell. Natur-Heilkunde 14. Giessen.
SCHMITZ, FR. 1879. Untersuchungen iiber die Zellkerne der Thallophyten. Ver-

handl. Naturhist. Ver. Preuss. Rheinl. u. Westf. p. 346.
SCHNEIDER, A. 1883. Das Ei und seine Befruchtung. Breslau.
SCHULTZE, M. 1861. Uber Muskelkorperchen und das was man eine Zelle zu nennen

hat. Arch. Anat. u. Physiol. 1-27.
SCHWANN, TH. 1839. Mikroskopische Untersuchungen iiber die Uebereinstimmung

in der Struktur und dem Wachsthum der Thiere. und Pflanzen. (English

transl., Sydenham Soc., 1847). Preliminary statement in Froriep's Notizen,

No. 91: 103, 112. 1838.
SCHWEIGGER-SEIDEL, O. 1865. Ueber die Samenkorperchen und ihre Entwicklung.

Arch. Mikr. Anat. 1: 309-335. pi. 19.
SPALLANZANI, L. 1786. Experiences pour servir a Phistoire de la ge'ne'ration des ani-

maux et des plantes. Geneva.
SPRENGEL, C. K. 1793. Das neu entdekfce Geheimniss der Natur in Bait und

Befruchtung der Blumen. Berlin.

SPRENGEL, K. 1802. Anleitung zur Kenntniss der Gewachse.
STRABTJRGER, E. 1875. Ueber Zellbildung und Zelltheilung. Jena.
1877. Ueber Befruchtung und Zelltheilung. Jenaische Zeitschr. 11.
1879. Die Angiospermen und die Gymnospermen. Jena.

1884. Neue Untersuchungen iiber die Befruchtungsvorgang bei den Phanero-
gamen, als Grundlage fur eine Theorie der Zeugung. Jena.

1888. Ueber Kern- und Zelltheilung im Pflanzenreich, nebst ein Anhang iiber

Befruchtung. Hist. Beitr. 1.
THURET, G. 1854-1855. Recherches sur la fecondation des Fucees, suivies des

observations sur les antheridies des algues. Ann. Sci. Nat. Bot. IV, 2 and 3.
TREVIRANUS, L. C. 1806. Vom inwendigen Bau der Gewachse.

1811. Beitrage zur Pflanzenphysiologie.

TULASNE, L. R. 1849. Etudes d'embryogenie vegetale. Ann. Sci. Nat. Bot. Ill,
12: 21-137. pis. 3-7.

22 INTRODUCTION TO CYTOLOGY

UNGER, F. 1844. Ueber das Wachsthum der Internodien, von anatomischer Seite
betrachtet. Bot. Zeit. Meristematische Zellbildung. Ibid.

VIRCHOW, R. 1855. Cellular Pathologie. Arch. Path. Anat. Physiol. 8.
1858. Die Cellularpathologie, usw. (Transl. by Chance, 1860.)

WEISMANN, A. 1885. Die Kontinuitat des Keimplasmas als Grunglage einer
Theorie der Vererbung. Jena.

WHITMAN, C. O. 1878. The embryology of Clepsine. Quar. Jour. Micr. Sci.
18: 215-315. pis. 12-15.

WOLFF, C. F 1759. Theoria Genera tionis.

CHAPTER II

PRELIMINARY DESCRIPTION OF THE CELL

In a survey of the evolution of biological science it is noticeable that,
while diverging lines of inquiry have broadened the field of view, the
attention of investigators, speaking generally, has been directed in turn
to successively smaller constituent parts of the organism. For many
years plants and animals were studied chiefly as wholes. But very
early there were made many scattered observations on the various or-
gans and tissues composing the body, and from these relatively crude
beginnings morphology and histology later arose. Again, when the
protoplasmic mass which we know as the cell came to be recognized as
the unit of structure and of function, it was evident that the problems
it presents should be investigated to a certain extent by themselves, and
such investigation is the task of modern cytology.

Within the field of cytology itself the focus of attention has gradu-
ally shortened. While many workers occupied themselves with a study
of the general behavior of the cell nucleus, others devoted their efforts
entirely to an investigation of its important constituent elements, the
chromosomes. Furthermore, cytologists at present are much interested
in knowing whether or not any smaller units, corresponding to the
"genes" of the geneticist, can be directly demonstrated, and whether
or not the chromatic granules or " chromomeres " are of significance in
this respect.

In the course of all such studies there are encountered questions
which must be referred ultimately to the chemical molecules and atoms
and their interactions within the cell, so that biochemistry may in a
measure be looked upon as a department of cytology, just as it is to be
regarded in other respects as a subdivision of chemistry. The subject of
cytology thus occupies an important position in the system of natural
sciences. It stands with chemistry and physics on the one hand and the
complex phenomena peculiar to living organisms on the other; and the
steady mutual approach of the physico-chemical and biological fields
is due in large measure to the results of morphological and physiological
studies on the cell.

For the term cell we are indebted to Robert Hooke and the other
microscopists of the seventeenth century, who applied it to the small
cavities in the honeycomb-like structure which they discovered in plant
tissues. Today the term denotes primarily the protoplasmic "cell
contents," which, strangely enough, the early workers regarded as an

23

24

INTRODUCTION TO CYTOLOGY

unimportant fluid product. The term protoplast, proposed by Hanstein
(1880), is more appropriate and is coming into more general use, but long
usage and brevity have probably insured the permanence of the older
term.

FIG. 1. — Diagram of the cell, showing its principal constituent parts.
A, centrosphere, with centrosome and aster. B, nucleolus or plasmosome. C, nuclear
membrane. D, nucleus, filled with karyolymph. E, nuclear reticulum, composed of
linin and chromatin. F, plastid. G, metaplasmic inclusion. H, chondriosomes. /,
vacuole. /, tonoplast or vacuolar membrane. K, cytoplasm. L, ectoplast.

Description of the Cell. — The morphology of the cell will here be
sketched only in its barest outlines, by way of introduction to the detailed
descriptions presented in the subsequent chapters.

The two most constant components of the cell (Fig. 1) are the
cytoplasm, in which the other cell organs are imbedded, and the

PRELIMINARY DESCRIPTION OF THE CELL

25

nucleus, which at least in many respects is the most important of these
organs.1

The cytoplasm, a more or less transparent, viscous, granular fluid,
may, with its inclusions, occupy the whole volume of the cell. This is
generally true of animal cells and the younger cells of plants. If the
cell is vacuolate, as is usually the case in the mature plant cell, the
cytoplasm may constitute only a thin layer lining the wall, the central
vacuole with its cell sap often far exceeding it in volume (Fig. 2, C).

FIG. 2.

A-C, diagram of a plant cell in three stages of development: the vacuoles increase in
volume and the protoplasm becomes limited to the parietal region. D, cell of stamen hair
of Tradescantia, showing streaming movements in the cytoplasmic strands. E, paren-
chyma cell from cortex of Polygonella, showing nucleus, plastids, and scanty cytoplasm.

In many cases the cytoplasm forms a system of anastomosing strands that
often show active streaming (Fig. 2, D}. Externally the cytoplasm is
limited by a layer of different consistency, the plasma membrane, or
ectoplast. Where it comes in contact with the enclosed vacuole it is also
limited by a membrane, the vacuole membrane, or tonoplast.

The nucleus is bounded by a nuclear membrane and contains an
extremely clear fluid, the nuclear sap, or karyolymph. In the karyo-
lymph is imbedded the nuclear reticulum, composed usually of linin, an
achromatic supporting material, and chromatin, the " nuclear substance
par excellence." One or more true nucleoli, or plasmosomes, are commonly
present in the nucleus, and may or may not be closely associated with
the reticulum. There are often present also chromatin nucleoli, or karyo-
somes, which represent accumulations of chromatin at certain points on
the reticulum, and should not be confused with the true nucleoli.

1 According to the older usage the extra-nuclear portion of the protoplast was
called "protoplasm," which was unfortunate because we now know that the nucleus
also is composed of protoplasm, or living substance in its broader sense. It is now
the general custom to avoid this ambiguity by employing Strasburger's terms cytoplasm
and nucleoplasm (karyoplasm, Flemming). The older usage, however, has not been
entirely superseded.

26 INTRODUCTION TO CYTOLOGY

There are usually plastids of one or more kinds in the cytoplasm,
the most conspicuous in plant cells being the green chloroplasts.

A centrosome is present in the majority of animal cells and in those of
certain lower plants. It may occupy the center of a visibly differentiated
region, the centrosphere or attraction sphere, and at the time of cell-
division is the focus of a conspicuous system of radiating astral rays,
collectively known as the aster.

Chondriosomes have now been demonstrated in the cells of nearly all
plant and animal groups. These are minute bodies having the form of
granules, rods, or threads, and apparently constitute a group of materials
having various functions.

Metaplasmic inclusions are accumulations of food materials and
differentiation products that are relatively passive. These non-proto-
plasmic bodies may exist in the form of droplets or crystals, and those
which are not transitory or reserve food materials apparently play a very
minor role in the life of the cell.

Strictly speaking, the cell wall as at present understood is not a part
of the cell proper, or protoplast, but is rather regarded by many as a
secretion of the latter. In many cells, particularly those of animals and
the motile cells of algae and flagellates, it may be absent.

The foregoing is a bare sketch of the general structure of a "typical"
cell. It is scarcely necessary to point out that the cell should not be
thought of as a static thing with a permanent physical structure: it is
rather a dynamic system in a constantly changing state of molecular
flux, its constitution at any given moment being dependent upon ante-
cedent states and upon environmental conditions. As stated by Moore
(1912), "the living cell may be regarded, from the physico-chemical
point of view, as a peculiar energy transformer, through which a continu-
ally varying flux of energy ceaselessly goes on, and the whole life of the
cell is an expression of variations and alterations in rates of flow of
energy, and of swings in the balance between various forms of energy."
In the words of Harper (1919), the cell is a colloidal system in which the
various processes have become progressively localized in certain regions,
with the resulting formation of organs, which, with the increasing con-
stancy of the processes involved, have come to possess a permanence
and individuality of their own. In view of the spatial relationship and
definite physiological integration of the various components of the cell,
we are to look upon the cell not as a mere mixture of complex substances,
but as a definitely organized system.

The Differentiation of Cells. — It is a striking fact that in spite of
many minor variations the fundamental structure of the cell is essentially
similar in nearly all living organisms, and in all the kinds of tissues
which go to make up the body of any one of them. As Harper (1919)
remarks, "evolution as we know it has not consisted in the production

PRELIMINARY DESCRIPTION OF THE CELL 27

of new types of protoplasmic structure or cellular organization, but in
the development of constantly greater specialization and division of
labor between larger and larger groups of cells." One obvious reason for
the fundamental similarity of the cells of widely different tissues is found
in the fact that all of them are derived by progressive modification from
relatively undifferentiated "embryonic" or "meristematic" cells during
the course of the ontogeny. In a young vascular plant, for example,
definite regions (meristems) consisting of such cells are present in the
root tip and stem tip, and, at a later stage of development in many
cases, in the cambium also. As a general rule these meristematic cells
are without large vacuoles or other conspicuous products of differentiation,
and are separated by no intercellular spaces. They undergo successive
divisions very rapidly (hence the use of root tips for the study of mitosis) ;
and while some of the products of division become greatly modified in
structure in connection with their specialization in function, others
retain their embryonic or meristematic character and continue to produce
new cells from which new tissues are built up throughout the life of the
plant.

In the bryophytes and pteridophytes the meristematic activity of the
apex (root tip or stem tip, or apical region of thallus) usually centers in
a single "apical cell" of definite shape, which cuts
off segments (daughter cells) from its various faces
with great geometrical regularity. In the Mar-
chantiales and Anthoceros the apical cell is cuneate
(wedge-shaped) and forms segments from four of
its faces; in the anacrogynous Jungermanniales
it is sometimes cuneate but more often dolabrate
(ax-shaped) and produces segments from its two
lateral faces ; and in the acrogynous Jungermanniales
and mosses it has the form of a triangular pyramid,
cutting off segments from its three lateral faces.
This last type is found also in the pteridophytes :
in the stem tip it produces segments from its three section of the root tip of
lateral faces, whereas in the root tip, in addition Osmunda, showing the

triangular pyramidal

to these three series of segments, it cuts off from its apical cell, x 144.
distally directed face a fourth series, which becomes

the tissue of the root cap (Fig. 3). In the higher vascular plants there
is no single cell characteristically different from the others of the apical
meristematic group.

Most of the visible characters which ordinarily serve to distinguish
the various kinds of differentiated cells of the vascular plant are found
in the cell wall rather than in the protoplast itself; strictly speaking,
such characters are histological rather than cytological. Thus we have,
besides meristematic and little modified parenchymatous cells (Fig. 2,

28

E), a number of other types, such as tracheids, vessels, wood fibers,
sclerenchyma fibers, and sieve tubes (Fig. 4), all of which are characterized
by the peculiar ways in which their walls become modified through sec-
ondary and tertiary thickenings, and by the form and arrangement as-
sumed by the pits. (See p. 191.) The protoplasts may finally disappear
completely from wood cells, leaving a tissue or framework composed of
lifeless cell walls.

FIG. 4. — Differentiated cells from vascular plants.

A, wood fiber with thickened wall. B, C, portions of tracheids with spiral and annular
thickenings. D, pitted tracheid. E, portion of sieve tube with adjacent companion cells.
F, face view of sieve plate shown in section in E.

All of this variety of form and structure is conditioned by varied
functional activity on the part of different protoplasts : in the process of
cell differentiation morphological and physiological changes stand in the
closest relationship. All functional differences are accompanied by
chemical or physical differences of some sort in the protoplasm, but it
is mainly in the non-protoplasmic inclusions and secretions (including
the wall) rather than in any conspicuous structural changes in the
protoplast itself that cell differentiation is rendered visible in the case
of plants. Apart from differences in shape, amount of vacuolar material,
accumulated food, and other products of differentiation (see p. 133),
protoplasts performing widely different functions may appear much alike.

Structural differentiation in connection with division of labor is very
striking in animal cells, which are destitute of such walls as plant cells
possess. The muscle cell shows many fine longitudinal fibrillae which
have to do with the cell's power of contractility. In certain muscles
these fibrillse are so segmented that the whole cell, or muscle, fiber, has a
transversely striped appearance (Fig. 5, F). The nerve cell (Fig. 5,

PRELIMINARY DESCRIPTION OF THE CELL

29

G

FIG. 5. — Nerve and muscle cells of animals.

A, diagram of a typical neuron: a, axis cylinder process or axon, ending in arborescent
system; d, dendrites. (After Obersteiner and Hill.) B, cell from human spinal cord,
X 75. (After Obersteiner and Hill.) C, nerve cell from the eye. (After Lenhossek.) D.
Nerve cell from the earthworm. (After Kowalski.) E, young voluntary muscle cell.
F, portion of mature voluntary muscle cell, showing striations. G, Involuntary muscle
cell from intestine. (E-G after Piersol.)

I

FIG. 6.

A, connective tissue from the jelly fish, showing branching cells and elastic fibers
imbedded in gelatinous matrix. (After Lang.) B, cells of hyaline cartilage imbedded
in their secretion. C, blood cell from chick embryo.

30

INTRODUCTION TO CYTOLOGY

A-D) typically possesses a single unbranched prolongation (axon) and
one or more others (dendrites) which often become very elaborately
branched, especially in the ganglion cells of the spinal cord and brain.
The cytoplasm of the nerve cell contains fine fibrils, and also granules
of chromatic "Nissl substance." Cells specialized in connection with
motility. such as spermatozoa (Fig. 103) and the cells of certain epithelial
tissues (Fig. 36), show complex structural modifications not only in the
flagelke, cilia, and cirri which they bear (p. 45), but also in the other
cell organs with which the activities of these motile structures are closely

Fia. 7. — ParamcBcium caudatum. Semidiagrammatic figure showing principal parts.
C. V., contractile vacuoles. T, trichocyst. N, n, mega- and micronuclei. P, peri-
stomial groove. M, mouth. O, oesophagus, with undulating membrane, U. M. F. V.,
food vacuoles. (After Lang.) '

connected. (See Chapter IV.) Secretory cells are often distinguishable
not only by the accumulations of secretion products in their cytoplasm,
but also by the peculiar forms assumed by their nuclei (Fig. 17, A, C).
The cells of connective tissue (Fig. 6. A) form many long interlacing pro-
cesses and lie in a supporting matrix which represents their secretions.
Cartilage and bone cells (Fig. 6, B} are likewise imbedded in their secre-
tions, which are here produced in relatively enormous amounts and, where
present, constitute the main supporting framework of the body.

We thus see that the life of the complex multicellular organism is
dependent upon the correlated activities of a multitude of cells per-
forming many diverse special functions. It is a remarkable fact, that

PRELIMINARY DESCRIPTION OF THE CELL 31

all of the functions delegated, as it were, to different cells (contractility,
motility, mechanical support, the reception and conduction of stimuli,
secretion, and excretion), as well as those general functions common to
all cells (nutrition and reproduction by division), may in the protozoa
and protophyta be carried on within the limits of a single cell. Such a
cell as, for example, the body of a Paramcecium (Fig. 7), exhibits a cor-
responding regional differentiation in structure, certain functions being
localized in definitely developed organs. Differentiation is therefore
something which, fundamentally, does not require multicellular struc-
ture for its expression; in fact the most important single step ever taken
in differentiation was that which set apart nucleus and cytoplasm, giving
the type of organic unit common to all subsequently evolved organisms.
It is further evident, however, that the evolution of the higher organisms
has unquestionably been very largely conditioned by the multicellular
state, and has involved a progressive division of labor in a very real sense.
The many functions of a single cell have become distributed among a
number of cells in such a way that there has been produced a harmonious
whole which is efficient, adaptable, and progressive to a degree not other-
wise attainable.

Bibliography 2

See Bibliography I A for general works on the cell and for reviews of early cell
literature. For the latter see especially the works of Boveri, Flemming, Koernicke,
Mark, Meves, Riickert, Waldeyer, Whitman, and Zimmermann. Other works
referred to in Chapter II:
HANSTEIN, J. 1880. Das Protoplasma als Trager der pflanzlichen und thierischen

Lebensverrichtungen. Heidelberg.

HARPER, R. A. 1919. The structure of protoplasm. Am. Jour. Bot. 6 : 273-300.
MOORE, B. 1912. The Origin and Nature of Life. N. Y. and London.

CHAPTER III

PROTOPLASM

In his famous essay on protoplasm in 1868 Huxley very fittingly
referred to it as "the physical basis of life." With a realization of the
full significance of this phrase there comes the conviction that protoplasm
is the most interesting and important substance to which we can turn our
attention, for with it the phenomena of life, in so far as we know them,
are invariably associated.

In spite of the enormous amount of work which has been done upon
protoplasm during many years, our knowledge of it must still be regarded
as very superficial and fragmentary. We can scarcely yet say definitely
that a given kind of protoplasm is not a single complex chemical com-
pound, as is held by one prominent school of biochemists: all ordinary
analysis seems to indicate that it represents a somewhat looser combina-
tion of substances, many of which are in turn very elaborate in composi-
tion; and further that these substances probably differ from those found
elsewhere not in any fundamental manner, but only in the degree of their
complexity. Proteins, fats, crystalloids, water, and other compounds
make up protoplasm, but protoplasm is not a mere mixture of these
materials; it is organized — it is a system of complex substances, the
activities of which are fully coordinated. Only if we recognize in pro-
toplasm an organization can we conceive of it as a physico-chemical
substratum for those peculiar orderly activities characterizing living
substance, namely, synthetic metabolism, reproduction, irritability, and
adaptive response.

Physical Properties. — Certain early ideas regarding the physical
nature of protoplasm may be briefly reviewed at this point.

Protoplasm appeared to its earliest observers merely as a colorless,
viscid substance containing minute granules. Two general opinions
soon developed : some held that protoplasm consists of but a single fluid,
whereas others regarded it as a combination of two fluids. Briicke
(1861), who was one of the first to lay emphasis on the fact that pro-
toplasm is an organized substance, looked upon the cell body as a con-
tractile, semi-solid material through which there streams a fluid carrying
granules. Similar to this was the idea of Cienkowski (1863), who be-
lieved he saw in the protoplasm of myxomycetes two fluids, one of them
hyaline and only semi-fluid (the "ground substance"), and the other
a more limpid fluid with granules suspended in it. De Bary (1859,

32

PROTOPLASM 33

1864), on the other hand, regarded protoplasm as a single semi-fluid sub-
stance, contractile throughout, but showing many local differences due to
varying water content. To this general view the work of Hanstein
(1870, 1880, 1882) lent support.

Much more prominent have been the structural theories associated
with the names of Klein, Flemming, Altman, and Biitschli, and known
respectively as the "reticular," "fibrillar," "granular," and "alveolar"
theories.

The reticular theory, which was formulated by Fromman (1865,
1876, 1884), was developed especially by Klein (1878-9) and supported
by van Beneden, Carnoy, Leydig, and others. These workers saw in
protoplasm a reticulum or fine network of a rather solid substance
(spongioplasm) , which holds a fluid and granules in its meshes. This
view was adopted for a time by Strasburger.

The fibrillar, or filar, theory, announced by Velten (1873-6) as a
result of his observations on Tradescantia and other forms, stated that
protoplasm is composed of fine fibrils, which, though often branched,
do not form a continuous network. This idea was developed mainly by
Flemming (1882), who called the substance of the fibrils mitome and the
fluid bathing them paramitome. Some observers asserted that the fibrils
are in reality minute canals filled with a liquid, the granules seen by
others being merely sections of these canals. An extreme view was that
of Schneider (1891), who thought the entire cell might consist of but a
single greatly convoluted filament.

To the followers of the reticular and fibrillar theories the fluid held
between the fibers was known variously as ground substance, enchylema
(Hanstein 1880), hyaloplasma (Hanstein), paramitome, and inter-filar
substance. The granules were known generally as microsomes (Hanstein).

According to the granular theory protoplasm is a compound of in-
numerable minute granules which alone form the essential active basis for
the phenomena exhibited; the observed fibrillar and alveolar structures
are of secondary importance. Martin (1881) held that fibrils and net-
works are due entirely to certain arrangements of these granules, or
microsomes. Pfitzner (1883) pointed out that the granules are semi-
solid and float in a more fluid ground substance. For Altman (1886, etc.),
who was the most prominent exponent of the theory, the granules were
actual elementary living units, or bioplasts, the liquid containing them
being a non-living hyaloplasm. The cell was therefore looked upon not
as a unit, but as an assemblage of bioplasts, "like bacteria in a zooglcea,"
and the bioplasts were believed to arise only by division of others of their
kind (omne granulum e granulo!).

The alveolar theory, also known as the emulsion, or foam, theory, was
elaborated principally by Biitschli (1882, etc.), and is of special interest
in view of our present-day notions of protoplasmic structure. According

3

34 INTRODUCTION TO CYTOLOGY

to Biitschli protoplasm consists of minute droplets (averaging I/A in
diameter) of a liquid "alveolar substance" (enchylema) suspended in
another continuous liquid " interalveolar substance." The structure
is therefore that of an extremely fine emulsion, and the appearances
described by other workers are due to optical effects encountered in
examining the minute alveolar structure. Biitschli supported his theory
by making artificial emulsions with soaps and oils which showed amoeboid
movement and other striking resemblances to living protoplasm.

The above four theories have been termed " monomorphic theories,"
for the reason that each of them stated that protoplasm has a single
characteristic physical structure. Strasburger in 1892 and thereafter
maintained that the protoplast is regularly composed of two portions;
an active fibrillar kinoplasm, concerned primarily with the motor work of
the cell, and a less active alveolar trophoplasm, chiefly nutritive in func-
tion. It was shown by von Kolliker, Unna (1895), and others, moreover,
that one type of structure may be transformed into another. Flemming
later adopted the view that no single type characterizes protoplasm, but
that the latter may be homogeneous, alveolar, fibrillar, or granular —
i.e., it is "polymorphic." Wilson (1899) found that all four states are
successively passed through in the echinoderm egg. This observation,
which was made upon both living and fixed material, showed in a striking
manner the colloidal nature of protoplasm (see below), since it is now
known that colloids may assume very diverse structures under the in-
fluence of changing environmental conditions. The work of A. Fischer
(1899), who treated non-living proteins with cytological fixing reagents
and so produced artifacts similar to alveolae, reticula, and granules, should
make one cautious in drawing conclusions regarding protoplasmic
structure from fixed material. It should be understood that the only
trustworthy observations are those which are made at least in part on
living material, for it is not difficult to discover all four types of struc-
ture in prepared slides: the protoplasm has there been coagulated by
fixing reagents, and we know that in the coagulation of such substances
as compose protoplasm an entirely new structure may be assumed.

Protoplasm as a Colloidal System. — For adequate reasons it is now
customary to speak of protoplasm in terms of the physics and chemistry
of colloids. Colloids are those glue-like substances which are uncrystal-
line, semi-solid, and very slightly or not at all osmotic. They have a
high surface tension, coagulate readily, and conduct the electric current
very poorly. They are "disperse heterogeneous sytems, i.e., they consist
essentially of particles larger than molecules of a substance or substances
in a medium of dispersion which may be water or some other fluid"
(Child 19151). The particles range in size from those visible to the naked

1 These paragraphs on colloids are based largely upon the convenient summary
given by Child (1915, pp. 20 ff.). See also Czapek (19116), Bayliss (1915), Hatschek
(1916), Bechhold (1919), and Robertson (1920).

PROTOPLASM 35

eye down to single molecules and ions. In the latter case we have a true
solution: between colloid and crystalloid the line of demarcation is thus a
purely arbitrary one. In many cases the suspended particles are too
small to be seen with the ordinary microscope, which will not render
visible a body with a diameter less than about 0.20 to 0.25 jw; but with
the ultramicroscope, which will reveal particles about one-fortieth of
this size, they may be clearly seen. Again, the ultramicroscope is insuffi-
cient in the case of certain colloids, in which the presence of suspended
particles can still be shown, however, by the Tyndall effect (a milky
appearance when a beam of light is passed through them). Most proto-
plasmic colloids are of this last type.

In a colloidal solution the particles are separate from one another,
(sol), whereas in the denser "set" condition (gel) they are more closely
aggregated and hence not free to move upon one another. A colloid
may be made to pass from the sol to the gel state or vice versa; in some
cases this change is reversible, but in others it is not.

Colloids are usually classified as suspensoids and emulsoids. Sus-
pensoids, in which the particles are solid, are comparatively unstable;
are readily precipitated or coagulated by salts; carry a constant electric
charge of definite sign; are not viscous; do not show a lower surface
tension than that of the medium of dispersion alone; and are mostly
only slightly reversible. Emulsoids, in which the suspended particles
are fluid, are comparatively stable; are less readily coagulated by salts;
are either positively or negatively charged; are usually viscous; have a
lower surface tension than the medium of dispersion; form surface mem-
branes; and are highly reversible. Most organic colloids are emulsoids,
and there can be no doubt that many of the characteristics of living or-
ganisms are due to their presence.

In an emulsion each physically homogenous constituent is known as
a phase. In mayonnaise dressing, to cite a familiar example, there are
three phases: a water phase, consisting of water and substances dissolved
in it; an oil phase; and a protein phase (egg). These three physically
diverse substances are brought into the emulsified state by beating; one
of them is the medium of dispersion (external phase) and the others
(internal phases) are suspended in it as liquid particles or droplets. In
such an emulsion a given phase usually consists of more than one chemi-
cal substance: the water phase, for example, is not pure water, butjan
aqueous solution of salts and other water-soluble substances. These dif-
ferent chemical substances, including the solvent, which make up a single
phase, are known as components.

It is shown by certain investigators (Bancroft, Clowes) that the drop-
lets of a suspended phase in a stable emulsion are bounded by films of
different constitution: between the phases of an alkaline water-oil emul-
sion, for example, there appear to be delicate films of a soapy nature.

36 INTRODUCTION TO CYTOLOGY

These films not only prevent the coalescence of the droplets, but also,
through alterations in surface tension, influence the transposition or
inversion of phases which occurs under certain conditions, whereby the
suspended phase becomes the medium of dispersion and vice versa (Fig.
8). Such inversion probably plays an important role in many cases of
transformation of sol into gel and of gel into sol.

FIG. 8. — Diagram of a colloidal emulsion, illustrating transformation of emulsion of oil

in water to emulsion of water in oil.

A, aqueous phase. B, oil or other non-aqueous phase. C, surface film of soap or other
dispersing agent. (After Clowes, 1916.)

The properties and behavior of colloidal substances in general appear
to be due primarily to the enormous extent of the reacting surface be-
tween the constituent phases which results from the finely divided state
of one or more of them. In the accompanying table is shown the amount
of surface which a given mass of matter may expose when subdivided
into successively smaller particles.

TABLE SHOWING THE INCREASE OF SURFACE WITH THE SUBDIVISION OF 1 c.c. OF
MATTER IN THE FORM OF A CUBE (DATA PARTLY FROM HATSCHEK, 1919.)

Length of edge of cube

Number of cubes

Total surface exposed

1 cm.

1

6 sq. cm.

1 mm.

1,000

60 sq. cm.

100M

10"

600 sq. cm.

10u

10"

6,000 sq. cm.

IM

1012

6 sq. m.

1(XW

1015

60 sq. m.

1<W

101*

600 sq. m.

W

1021

6,000 sq. m.

The evidence at hand supports the view that protoplasm is essentially
a colloidal solution of the emulsion type. It consists of at least three
principal phases: a water phase, containing a number of dissolved compo-

PROTOPLASM 37

nents; a fat phase, consisting of fats and fat-soluble components; and a
complex protein phase. Since the water phase is here the medium of
dispersion protoplasm is classed as a "hydrosol" in its ordinary state,
or as a "hydrogel" in the set condition. There are doubtless additional
minor phases present, protoplasm being in reality a "complex polyphase
colloidal system."

The presence of water in protoplasm is a matter of fundamental im-
portance. As emphatically stated by Henderson (1913), "... the
physiologist has found that water is invariably the principal constitu-
ent of active living organisms. Water is ingested in greater amounts
than all other substances combined, and it is no less the chief excretion.
It is the vehicle of the principal foods and excretion products, for most
of these are dissolved as they enter or leave the body [across the wall of
the intestine and across the epithelia of kidneys, lungs, and sweat glands].
Indeed, as clearer ideas of the physico-chemical organization of proto-
plasm have developed it has become evident that the organism itself is
essentially an aqueous solution in which are spread out colloidal sub-
stances of great complexity [Bechhold 1912]. As a result of these condi-
tions there is hardly a physiological process in which water is not of
fundamental importance" (pp. 75-77).

The amount of water in protoplasm varies greatly under different
conditions, but normally it is present in large proportions. It makes up
85 to 95 per cent of the weight of actively streaming protoplasm such as
is seen in Elodea and Tradescantia, and in actively functioning cells it
rarely drops below 70 per cent. In dry spores, however, it may be
reduced to 10 or 15 per cent, in which case the protoplasm becomes very
viscous. The percentage differs constantly in different parts of the cell;
nucleus, cytoplasm, and plastids, though all are composed primarily of
protoplasm, contain very different amounts of water. Since active pro-
toplasm is a liquid, the phenomena of surface tension and other properties
of liquids must enter largely into explanations of its behavior.

The colloidal nature of protoplasm is manifested in many of its prop-
erties. Its power of adsorption, which lies at the basis of many cell
reactions and certain staining processes, is similar to that of other
colloids. Protoplasm, like other colloids, is semi-permeable : a semi-per-
meable region is probably present wherever protoplasm comes in contact
with other substances, such as water; and the permeability of a vacuolate
cell is in general the resultant of the permeabilities of the ectoplast,
cytoplasm, and tonoplast.

Protoplasm shows most strikingly its colloidal character in the changes
of physical state which it undergoes as the effects of variations in the
external conditions. The alterations due to changes in temperature will
serve for illustration. Above a certain temperature the colloid gelatin
exists in the sol state — it is a hydrosol. If the temperature is sufficiently

38 INTRODUCTION TO CYTOLOGY

lowered the gelatin "sets" — it becomes a hydrogel. This setting is a
reversible process: if the temperature is again raised the sol state is
resumed. On the contrary, egg albumen is a hydrosol at ordinary
temperatures and becomes a hydrogel when heated; in this case the
change is an actual coagulation and is not reversible. Many of the
colloids of the cell are of this non-reversible type. "The evidence that
in this colloidal condition the transition from liquid to solid, from
sol to gel, tends especially to pass into an indefinite series of gradations
gave a basis for the explanation of that mixture of the properties of solids
and liquids which has puzzled students of protoplasm" (Harper 1919).
Protoplasm is thus easily coagulated, not only by a high temperature,
but by a variety of chemical substances. The "fixation" of the cell
structures by the reagents employed in cytological technique is primarily
a coagulation phenomenon, and in the act of coagulation a substance,
especially one as complex as protoplasm, undergoes an alteration in
physical structure. Although such fixing fluids preserve very well the
general structure of the cell, the effects of coagulation should always be
borne in mind in interpreting finer details in preparations of fixed cells,
and in evaluating the results of those who have made special studies on
the ultimate structure of protoplasm.

Microdissection. — Much has been added to our knowledge of the
physical nature of protoplasm in recent years through microdissection.
Certain workers, notably Barber (1911, 1914), Kite (1913), Chambers
(1914, 1915, 1917, 1918), and Seifriz (1918, 1920) have developed a
technique (fully described by Barber 1914, and Chambers 1918) whereby
they have been able to dissect living cells under the high powers of the
microscope, thus opening a most promising field for investigation. Kite,
working on the cells of several plants and animals, found that protoplasm
exists in the form of sols and gels of varying consistency, that of plant
cells being as a rule more dilute and less rigid than that of animals. The
cytoplasm is commonly somewhat more viscous than is usually thought,
having the consistence of a "soft gel," while the nucleus may often be
surprisingly firm. (See Chapter IV.)

Chambers (1917), who gives a convenient bibliography of the subject,
states that in the early germ cells and eggs of certain animals the proto-
plasm is in the sol state with a surface layer in the gel state, whereas
adult cells are usually gels. He further asserts that the surface gel is
readily regenerated after injury, a new gel film being formed over the
injured area. As regards the structure of protoplasm, he finds it to
consist of a hyaline fluid carrying microsomes and macrosomes, which
measure less than I/* and from 2-4// in diameter respectively. Upon dis-
organization the macrosomes, which are more sensitive to injury than
are the microsomes, swell and go into solution, while the hyaline fluid
flows out and mixes with water or coagulates, forming a reticular or
granular structure.

PROTOPLASM 39

Seifriz (1920) has investigated with care the viscosity of the proto-
plasms of a number of myxomycetes, algae, pollen tubes, protozoans,
and echinoderm eggs. He finds the degree of viscosity to vary widely,
from a very watery to a fairly rigid gel condition, not only in the different
organs of the cell but also in the protoplast as a whole at different stages
of its development. He warns against accepting viscosity alone as an
index of the gel or sol state of the protoplasm, since it is physical structure
and not viscosity which determines these states in an emulsion.

It is to be hoped that the methods employed by the above investi-
gators will be further developed and applied more widely, for through
them many misconceptions will undoubtedly be corrected.

It should be evident from all these considerations that there is prob-
ably no sinlge visible structure characteristic of protoplasm at all times.
Any fundamental structure which it may have remains to be discovered
in the ultramicroscopic constitution of the colloids and other materials
of which it is composed, and in the physical relations which these bear
to one another. It should be pointed out, however, that in the idea of
protoplasm as a complex colloidal emulsion we have the best hypothesis
yet offered as a basis for the interpretation of the behavior of living
substance.

Chemical Nature of Protoplasm. — Chemically, as well as physically,
protoplasm is exceedingly complex, and the study of its constitution has
opened a field of research which is continually broadening. Only a brief
summary of some of the more important chemical facts can be presented
here; for more detailed accounts special works on the subject must be
consulted.1

As already pointed out, the substances of which protoplasm is com-
posed are probably not fundamentally different from those found else-
where, but show rather a greater complexity and a high degree of
organization. Protoplasm is an intricately organized system of water,
proteins, enzymes, fatty substances, carbohydrates, salts, and other
minor constituents. The often cited analysis by Reinke and Rodewald
(1881) of the myxomycete JEihalium septicum (Fuligo) showed the proto-
plasm of this form to have the following composition :

PER CENT DRY WEIGHT PER CENT DRY WEIGHT

Proteins 40 Cholesterin (lipoid) 2.0

Albumins and enzymes 15 Ca salts (except CaCO3). . 0.5

Other N compounds 2 Other salts .65

Carbohydrates 12 Resins ... 1 .2

p'ats 12 Undetermined 6.5

The protein matter of protoplasm exists in relative complex forms.
"The chief mass of the protein substances of the cells does not consist of

1 See especially the books of Hammarsten (1909), Wells (1914), Czapek (1915),
Bayliss (1915), Mathews (1916), Palladin (1918), Robertson (1920), and the review by
Zacharias (1909).

40

INTRODUCTION TO CYTOLOGY

proteids in the ordinary sense, but consists of more complex phosphorized
bodies . . ." (Hammarsten). Such "phosphorized bodies" are the
nucleo-proteins, which are "probably the most important constituents
of the cell, both in quantity and in relation to cell activity" (Wells).
A long series of chemical investigations beginning with the pioneer work
of Miescher, Hoppe-Seyler, and Reinke, have shown that these nucleo-
proteins are essentially combinations of nucleic acid with proteins, or
sometimes with the simpler histones or the still simpler protamines.

The nucleus as a rule is free from or very poor in uncombined carbohy-
drates, fats, and salts, but is characterized rather by the abundance of a
nucleo-protein called nuclein, isolated in 1871 by Miescher, who gave it the
formula C29H49N9P3O22. It was shown by Altman (1889) that nuclein,
like the other nucleo-proteins, could be split into two substances: nucleic
acid and a form of albumin (protein), the two existing in chemical com-
bination like an ordinary salt. Nucleic acid from yeast was given the gen-
eral formula CjoHsgNuC^ — 2P2O5, and that from fish sperm C^HseNuO
le — 2P2O5. Nucleic acid was further analysed into phosphoric acid and
certain bases. The relation of these simple substances to nuclein, and
also the relation of nuclein to more complex nucleo-proteins, are shown in
the following scheme (mainly from Wells) :

Higher
nucleo-
proteins

Proteins

Nuclein

Proteins

(albumins, etc.)

f Phosphoric acid
Nucleic acid

Levulinic acid
Purin bases
Pyrimidins
Pentoses

Xanthin

Guanin

Adenin

etc.

The nucleo-proteins of the nucleus (chromatin) contain very little of
the protein constituent and are thus relatively rich in phosphorus.
Glaser (1916) accordingly speaks of chromatin as "a conjugated phospho-
protein group with a nucleic acid group, the latter group being a complex
of phosphoric acid and a nuclein base." Kossel (1889, 1891, 1893)
even concluded that in certain instances (during mitosis) chromatin
might be simply nucleic acid.

In the cytoplasm, on the contrary, the proportion of the protein
constituents is relatively high. The cytoplasm probably has no true
nuclein, but is rich in nucleo-albumins, albumins, globulins, and pep-
tones, which, unlike nuclein, have little or no phosphorus. As a result
its reaction is alkaline, in contrast to the acidity of the nucleus. Accord-
ing to Hammarsten (1909), "the globulins and albumins are to be con-
sidered as nutritive materials for the cell or as destructive products in

PROTOPLASM 41

the chemical transformation of the protoplasm." Granules of " volutin "
formed in the cytoplasm are also looked upon as a food substance used
by the nucleus in the elaboration of chromatin.

The fatty components of the cel^ comprise both ordinary fats and
lipoids (fat-like bodies not decomposed by alkalis); among the latter
lecithin and cholesterin are of great importance, particularly in the cells
of animals.

The carbohydrates found in protoplasm are chiefly pentoses and
hexoses, which are as a rule combined with proteins and with lipoids.
Glycogen exists free in the cells of many tissues and serves as a source of
heat and energy. The important role played by pentosans in the activity
of the plant cell is strongly emphasized by Spoehr (1919) and Macdougal
(1920); in fact these authors speak of protoplasm as "an intermeshed
pentosan-protein colloid."

Inorganic salts are present in considerable variety, as shown by the
presence of the following elements in the ash of Fuligo protoplasm: Cl,
S, P, K, Mg, Na, Ca, Fe.

Because of their failure to find any new types of chemical compounds
in their analysis of protoplasm Reinke and Rodewald (1881) thought it
probable that the peculiarities of protoplasm are due to its structure
rather than to its chemical composition. It has since been found, how-
ever, that certain of the life processes continue for a time after the pro-
toplasm has been ground up mechanically. Moreover, more refined
analytical methods have enabled chemists to isolate from protoplasm
certain extremely complex and unstable proteins (the " protoplasmids "
of Etard), which differ greatly in degree of complexity, if not otherwise,
from proteins encountered elsewhere.

Varieties of Protoplasm. — From the foregoing resume it is plain that
in protoplasm, because of the many combinations possible among con-
stituents present in such great variety, we have a substance which may
exist in a vast number of different forms. When it is further recalled
that many of the constituents exhibit singly the phenomenon of stereo-
isomerism this number is seen to be incalculable. For example, it was
shown by Miescher that an albumin molecule with 40 carbon atoms
could have about one billion stereoisomers, and some albumins probably
have more than 700 carbon atoms. Albumin, moreover, is only one of
many complex substances present in protoplasm. Hence, the state-
ment that all living cells are composed of the same substance, proto-
plasm, is true only in a general sense. Although they are made up of
the same categories of substances existing in the same general type of
organization — the hydrocolloidal state — the protoplasms of different
organisms vary widely in the relative amounts of these leading con-
stituents. For example, the lipoids are much more abundant in the
protoplasm of animals than in that of plants, and the carbohydrate-

42 INTRODUCTION TO CYTOLOGY

protein ratio also shows notable differences in the two kingdoms. Ana-
logous differences also exist between the smaller plant and animal groups,
and with these differences in chemical constitution are associated many
characteristic diversities in metabolic activity. Thus it is not simply
with protoplasm but with protoplasms that the working biologist has
to deal.

Special emphasis has been placed upon the relation of this great
diversity in the constitution of protoplasms to the amazing variety
observed among living organisms by Kossel, Reichert, and a number of
other writers. As Reichert states, the evidence seems to indicate that
"in different organisms corresponding complex organic substances that
constitute the supreme structural components of protoplasm and the
major synthetic products of protoplasmic activity are not in any case
absolutely identical in chemical constitution, and that each substance
may exist in countless modifications, each modification being character-
istic of the form of protoplasm, the organ, the individual, the sex, the
species, and the genus." With regard to the integration of the various
protoplasmic constituents, Mathews (1916) says: "Protoplasm, that
is the real living protoplast, consists of a gel, or sol, which is composed
of the colloids of an unknown nature which include protein, lipin and
carbohydrate. Whether these colloidal particles consist of one large
colloidal compound in which enzymes, protein, phospholipin and car-
bohydrate are united to make a molecule which may be called a biogen
[Verworn 1895, 1903], cannot be definitely stated, but it seems probable
that something of the sort is the case."

The Plasma Membrane. — It was recognized very early that there is
at the surface of the protoplast a thin layer of relatively resistant, hya-
line protoplasm which Hanstein called ectoplasm, distinguishing it thus
from the granular endoplasm within. Pfeffer (1890) employed the cor-
responding terms hyaloplasm and polioplasm. The ectoplasmic envelope,
which is best seen on "naked" masses of protoplasm, such as amoeba,
myxomycetes, and the zoospores and gametes of alga3, has been variously
referred to by different writers as the ectoplast, plasma membrane, Haut-
schicht, and Plasmahaut.1

The proponents of the reticular and fibrillar theories of the structure
of protoplasm looked upon this external layer as a region in which the
fibrils are more closely compacted or interwoven, whereas Biitschli re-
garded its relative .firmness as due to a compact radial arrangement of
alveolae. Pfeffer (1890) held that such a limiting membrane, which
living protoplasm always produces on an exposed surface and which
consists mostly or entirely of protein substances, is not itself truly proto-
plasmic, whereas the majority of cytologists have thought it to be a

1 A discussion of ectoplasm and endoplasm based upon a large number of ob-;
servations on Amoeba is given in a new work by Schaeffer (1920).

PROTOPLASM 43

special protoplasmic layer: Strasburger, for instance, believed it to be
composed of kinoplasm.

The microdissection studies of Kite (1913) and Chambers (1917)
mentioned above have extended our knowledge of the physical nature
of the plasma membrane. Both of these observers describe the ecto-
plast of an Amceba as a concentrated gel. Seifriz (1918), as a result of
such studies on the Fucus egg and myxomycetes, states that the mem-
brane is a definite morphological structure, very elastic and glutinous,
and capable of constant repair. He further asserts that membrane form-
ation is a physical process dependent upon the physical state of the
protoplasm and not upon that of the medium, and that it does not occur
after death.

That the formation of such a limiting membrane at the surface of
protoplasm is the result of the tendency of colloidal particles to accumu-
late on any interface has been pointed out by physical chemists. Citing,
by way of illustration, the film which forms on the surface of cooling
milk, Moore (1912) says: "The chief colloid of the milk, on account of
its affinities, accumulates on the surface, the accumulation gives increased
concentration, the presence of the increased concentration causes the
multi-molecules to build together, the larger molecules fall out of solu-
tion as particles, and these join to form a close network or film." In a
similar manner the unicellular organism or other mass of naked proto-
plasm develops its resistant envelope, and the enclosed protoplast of
the higher plant its ectoplasmic layer and tonoplast.

Permeability. — The physico-chemical nature of the plasma mem-
brane has been a subject of much discussion among physiologists. On
the assumption that the permeability of the cell is a case of solubility in
the ectoplasm, E. Overton (1895, 1899, 1900) developed a theory of the
constitution of the ectoplast. It was pointed out first, that the ectoplast
is not miscible with water; second, that in plant and animal cells the only
bodies which are not miscible with water in the ordinary state are fats and
oils; third, that the ectoplast is more or less permeable to substances ac-
cording as the latter are more or less soluble in fats and oils; and fourth,
that any substance insoluble in another substance will not pass through a
membrane composed of the latter. It was therefore concluded that the
ectoplast is made up of some lipoid compound, such as lecithin, which
acts as a semi-permeable membrane. This theory, though very sug-
gestive, was effectively opposed by Ruhland (1909, 1915) and a number
of other investigators, who called attention to many substances which
do not behave according to the requirements of the theory stated in
so simple a form. A more nearly adequate conception of the constitution
of the ectoplast has thus been souhgt.

Of the more recent theories which have been offered in connection
with the problem of permeability the most promising are those which

44 INTRODUCTION TO CYTOLOGY

interpret the ectoplast as an emulsion. According to Czapek (1910,
1911, 1915) the ectoplast is an emulsion of lipoids, proteins, and other
substances, the lipoids forming a suspended phase. "Protoplasm is a
colloidal emulsion of lipoids in hydrocolloidal media, the latter containing
proteins and mineral salts." Lepeschkin (1910, 1911) advanced the
contrary view that the lipoids form the medium of dispersion. In at-
tempting to account for changes in permeability Clowes (1916) points
out that inversion of phases probably plays an important role, while
Spaeth (1916) ascribes changes in permeability to alterations in the
degree of dispersion of the colloids, with resulting changes in the vis-
cosity of the membrane. A more definitely stated hypothesis of the
latter type is that tentatively suggested by Lloyd (1915)'and Free (1918).
Colloids are known that "have two liquid phases which differ in composi-
tion only in the relative proportion of water and of the substance of the
colloid" (Free). It is accordingly possible that
alterations in permeability may be due to changes
in the distribution of water between two such
phases present in the plasma membrane. When
water passes from the internal (suspended) to the
external (continuous) phase, the droplets of the
former would become very small; when the move-
ment is in the opposite direction they would be-
come very large and closely packed. As a result
NAVV\ there would be such changes in the physical
ec t. -§t) nature of the membrane as would aid in interpret-
ing the behavior of the latter toward substances

*IG. 9. — Amoeba, show-
ing ectoplasm, endo- entering or leaving the cell. It is held that such a
plasm, and contractile hypothesis accounts more readily for the gradual

vacuole. .

changes in permeability observed than does the

inversion theory of Clowes, according to which the change might be ex-
pected to occur suddenly. It is pointed out, however, that both
processes are probably involved.

Whatever the degree of correspondence between the above inter-
pretations and reality may be, it is scarcely open to doubt, especially
since the work of Bancroft (1913) and Clowes (1916) on colloids, that in
such theories we have our best prospect of reaching an adequate knowl-
edge of the plasma membrane, which, because of its great importance in
the life of the cell, is to be regarded as a definite "osmotic organ."

Protozoa. — It is in the Protozoa that the ectoplast shows its most
elaborate structural differentiations. (See Minchin, 1912, Chapter V.)
Here the ectoplast clearly has several functions : protective, motor, excre-
tory, and sensory. In most forms other than the Sarcodina there is a
resistant envelope of some sort. This may represent (a) the entire ecto-
plast modified (the "periplast" of Flagellata); (6) a superficial modified

PROTOPLASM

45

layer of the ectoplast (the "pellicle " of Infusoria and some Amcebae) ; (c) a
secreted layer ("cell membrane") rather than a modification of the
ectoplast. In certain cases definite actively protective organs, the tri-
chocysts, are differentiated in the ectoplasm.

Among the ectoplasmic structures with a motor function the simplest
are the pseudopodia; in the larger ones there is a core of endoplasm (Fig.
9), but the more delicate "filose" ones consist entirely of ectoplasm (Fig.
10). The flagellum of Euglena was reported by Biitschli to have an elas-

FIG. 10. — Gromia oviformis,
showing filose-reticulate pseu-
dopodia composed of ectoplasm.

(From Minchin, after Schultze.)

FIG. 11.

A, flagellum of Euglena, showing endoplasmic core
and ectoplasmic sheath. (After Biltschli.) B, Trypano-
soma tineas, with undulating membrane. (After Min-
chin.) C, Trypanosoma percce, showing myonemes. (After
Minchin.) D, flagellum of Euglena. (After Dellinger.)

tic endoplasmic core with a contractile ectoplasmic sheath (Fig. 11, A),
but the later figure of Dellinger (1909) represents it as composed of four
twisted filaments ending within the animal as a system of branching
rootlets (Fig. 11, D). Cilia, which are short and numerous and show
rythmic pulsation; cirri, which are formed of tufts of cilia; membranellce,
representing fused rows of cilia; and undulating membranes, which are
sheet-like extensions of the ectoplasm (Fig. 11, B}, are all essentially
ectoplasmic organs. A further motor differentiation is seen in the
minute contractile fibrils known as myonemes, which are analogous to a

46

INTRODUCTION TO CYTOLOGY

system of muscle fibers (Fig. 11, C). In ciliated forms they run beneath
the rows of cilia.

Contractile vacuoles, which exercise an excretory function, originate
in the ectoplasm, although later they may lie much deeper.

A sensory function is performed by the "eyespot," which is sensitive
to light, and also by the flagellae and cilia, which are often receptors of
tactile stimuli. The eyespot seems in some instances to be plastid-like
in character, and will be discussed in Chapter VI.

Protoplasmic Connections.— The fine protoplasmic strands (Plasmo-
desmen) connecting many plant cells through pores in the intervening
walls are extensions of the ectoplasm (Fig. 12). Several early workers

A B C

FIG. 12. — Protoplasmic connections in vascular plants.

A, B, Pinus pinea: cells of cotyledon. X 375. (After Gardiner and Hill, 1901.) C,
Phytelephas ("vegetable ivory"): endosperm cells with greatly thickened walls (w),
showing spindle-shaped bundles of connecting strands, m, middle lamella. Semidia-
grammatic.

suspected the presence of such connections before they were able to see
them, and even the coarse strands passing through the sieve plates of
sieve tubes, though often observed, were not well known until the time
of Hanstein's work in 1864. The finer strands of other plant tissues
where described in a large number of researches between 1880 and 1900.
Among these may be mentioned those of Wille (1883) and Borzi (1886) on
the Cyanophycese; Kohl (1891), Overton (1889), and Meyer (1896) on
the Chlorophyceae ; Hick (1885) on the Fucaceae; Hick (1883), Massee
(1884), and Rosenvinge (1888) on Florideae; and, on vascular plants,
those of Tangl (1879), Russow (1882), Strasburger (1882, 1901), Goros-
chankin (1883), Terletzki (1884), Wortman (1887, 1889), Haberlandt
(1890), Kienitz-Gerloff (1891), Jonsson (1892), Kuhla (1900), Gardiner
(1884, 1897, 1900), Hill (1900, 1901), and Gardiner and Hill (1901).

With respect to the origin and development of these connecting
strands very little is accurately known. Some observers have claimed
that the pores through which they pass are present from the time the
primary wall is first formed, no wall substance being laid down at these
points. Gardiner (1900) believed them to arise directly from the median

PROTOPLASM 47

portion of the fibers of the achromatic figure at the close of mitosis. His
observations were made on the endosperm of Lilium and Tamus. Others,
on the contrary, have regarded them as secondarily developed structures.
Their absence from the walls between Cuscuta and Viscum and their
hosts (Kienitz-Gerloff, Kuhla, Strasburger 1901), and also from many
cells which glide over one another during growth, is a fact opposed to the
latter interpretation. Although they have been demonstrated in a
number of kinds of tissue they probably do not occur so widely as some
have supposed; but it may nevertheless be true that in many cases their
apparent absence is due to the fact that the special methods often neces-
sary to their demonstration have not been widely employed.

As to their function, it can scarcely be doubted that they may serve
to transmit stimuli of one kind or another from cell to cell (Pf eff er 1896) .
Noteworthy in this connection is their presence in tissues of plant parts
known to be particularly responsive to external stimuli, such as the leaves
of Mimosa (Gardiner 1884) and Dioncea (Gardiner 1884; Macfarlane
1892), the stamens of Berberis (Gardiner 1884), and the sensitive labellum
of the orchid, Masderallia muscosa (Oliver 1888). Their extensive
development in storage tissues, such as the endosperm of seeds (Tangl
1879; Gardiner 1897), would also indicate that they are in part responsible
for the readiness with which nutritive materials are translocated in such
specialized tissues.

Vacuoles. — Vacuoles in the cytoplasm are more characteristic of
plant than of animal cells. They are usually absent in the very young
cell, but appear as growth and differentiation progress. In case they are
very small and numerous the cytoplasm takes on an alveolar appearance,
but more commonly they coalesce to form one large vacuole which
may occupy a volume greater than that of the protoplast itself (Fig. 2).
This condition is characteristic of many mature cells of plants, but is
comparatively rare in animals.

The ordinary vacuole is essentially a droplet of fluid, consisting of
water with differentiation products in solution, surrounded by a delicate
limiting membrane. DeVries (1885) developed the theory that vacuoles
are derived from "tonoplasts." The tonoplasts were believed to be
small bodies imbedded in the cytoplasm and multiplying by fission.
Through the absorption of water they swell and become vacuoles, the
vacuole wall thus being made up of the material of the tonoplast body.
We still refer to the vacuole wall as the tonoplast. De Vries looked upon
the vacuole as a body with an individuality somewhat similar to that
of a nucleus, since the tonoplast from which it develops was supposed to
arise from a preexisting tonoplast by division. The theory was supported
by certain other workers, but it does not enjoy wide acceptance today

It has been found by Bensley (1910) and others that there is in the
cytoplasm of certain comparatively young cells a system of fine canals

48

INTRODUCTION TO CYTOLOGY

canaliculae.
berlain.)

which later open up to form vacuoles (Fig. 13). The fixing reagents
commonly employed in cytological technique destroy these canaliculce;
and since Bensley, by using special reagents, demonstrated such canals
in the familiar cells of the onion root tip, it is highly probable that they
occur very widely. It seems more reasonable oo suppose that the fluid
differentiation producos, when they are first
forming, gradually come to move along certain
paths, forming canals, and later accumulate in
the form of vacuoles, than to suppose that the
vacuoles originate in such individualized units
as the tonoplasts of deVries.

Fluids other than water may also occur in
the form of vacuoles; oil vacuoles, for example,
are not uncommon in certain cells. If fats, oils,
and other products of metabolism take their
FIG 13 —Cell from root or^Sm m chondriosomes, as some suppose (see
tip of Aiiium cepa, showing Chapter VI), it is not improbable that some-
' am~ thing at least analogous to the above mentioned
tonoplast behavior may occur in the case of
certain substances appearing in the cell. The cell sap and other
differentiation products in the cytoplasm will be discussed further in
Chapter VII.

PROTOPLASM AS THE SUBSTRATUM OF LIFE

Since the true significance of protoplasm was first recognized in the
middle of the last century many suggestions have been ventured regard-
ing the nature of the relation existing between life and its physical basis.
A full discussion of this subject obviously cannot be entered upon here,
but theories of two types, the micromeric and the chemical, may be
cited by way of illustration.

Micromeric Theories. — Many years ago there were developed certain
speculative "micromeric theories" of the constitution of protoplasm;
.these became particularly prominent during the latter half of the nine-
teenth century. According to these "atomic theories of biology" the
principle of life was held to reside in ultimate fundamental particles.
The particles were supposed to be for the most part of ultramicroscopic
size, capable of independent growth and reproduction, and associated
like members of a vast colony in protoplasm. Such vital units were
compared by some to chemical molecules, but they were generally
regarded as something much more complex. Examples of such units
were the "organic molecules" of Buffon, the "microzymes" of Bechamp,
the "physiological units" of Spencer, the " plastidules " of Maggi and
Haeckel, the "bioplasts" of Altman, the "vital particles" of Wiesner,

PROTOPLASM 49

the "gemmules" of Darwin, the "biophores" of Weismann, the "pan-
gens" of de Vries, and the "ergatules" and "generatules" of Hatschek.

In a somewhat similar manner a number of the later investigators
occupied with the study of the ultimate structure of protoplasm have
often been led to inquire which of the constituents of protoplasm are the
actually living elements. Among those who viewed protoplasm as a
reticular structure some held the material of the reticulum to be the true
living substance, the liquid ground substance being lifeless, whereas
others held the reverse to be true. Many of those who saw in protoplasm
a granular structure regarded the granules as the ultimate living units,
and more recently there has even been a tendency on the part of some
investigators (Beijerinck, Lepeschkin), who have emphasized the emul-
sion nature of protoplasm, to view the droplets of the suspended phase in
a similar light. To Butschli the continuous phase was the essential
substance.

By most modern biologists such attempts to assign the principle of
life to any particular constituent unit of protoplasm or of the cell, whether
this unit be an observed structural component or a purely imaginary one,
are regarded as not in harmony with an adequate modern conception
of the term "living." It has been repeatedly emphasized that life should
be thought of not as a property of any particular cell constituent, but
as an attribute of the cell system as a whole (Wilson 1899) ; or, as Brooks
(1899) put it, not merely as a property but as a relation or adjustment
between the properties of the organism and those of its environment.
This recalls Herbert Spencer's characterization of life as a "continu-
ous adjustment of internal relations to external relations." As Sachs
(1892, 1895) and others urged, the various elements in the cell should be
referred to as active and passive rather than living and lifeless. These
elements play various roles in the cell's activity: each contributes to the
orderly operation of the whole. When any part fails to function properly,
or when the proper adjustment is not maintained, the whole system of
correlated reactions, the resultant of which we call life, must become
disorganized. As Child (1915) remarks, the theories postulating vital
units only transfer the problems of life from the organism to something
smaller; the fundamental problem of coordination is no nearer solution
than before, and the whole question is placed outside the field of experi-
mentation. Harper (1919) also points out that modern cytology no
longer looks upon protoplasm as a substance with a single specific struc-
ture, or as one made up of ultimate fundamental units of some kind,
but rather as a colloidal system or group of systems of varying structure
and composition. "The fundamental organization of living material is
expressed in the structure of the cell." The cell itself, and not some
hypothetical corpuscle, is the unit of organic structure. Protoplasm is
accordingly not made up of structural units arranged in various ways to

50 INTRODUCTION TO CYTOLOGY

form the cell organs, but is rather a colloidal system in which special
processes and functions have become localized and fixed in certain regions ;
and this in turn has resulted in the evolution of organs possessing more
or less permanence.

Chemical Theories. — Much more suggestive, if not conclusive, have
been certain attempts to place the phenomena of the organism upon a
purely chemical basis. With the development of organic chemistry from
the time of Wohler's (1828) synthesis of urea onward there has grown
up the idea that life processes and chemical reaction not only resemble
each other but are actually the same fundamentally. When protoplasm
was subjected to chemical analysis and found to consist chiefly of water
and proteins, and when these substances became more intimately known,
the task of explaining the activity of protoplasm in terms of the chemis-
try of proteins was undertaken. One group of workers developed the
hypothesis that peculiarly labile protein molecules are responsible for
the organism's reactions, "death" being primarily a change from the
labile to the stable condition on the part of these molecules. Such
molecules were called "biogens" t\y Verworn (1903). The molecule
itself was not thought of as alive, but its constitution was held to be the
basis of life, which "results from the chemical transformations which its
lability makes possible." Accordingly, "life itself consists in chemical
change, not in chemical constitution" (Child 1915).

Adami (1908, 1918) contends that life is thus "the function, or sum of
functions, of a special order of molecules." These ultimate molecules of
living matter he calls biophores (not to be confused with the biophores
of Weismann, which were molecular complexes), and he locates them in
the nucleus, the cytoplasm having merely "sub vital" functions. They
are proteidogenous in nature, i.e., they compose an active substance
which takes the form of relatively inert proteins when subjected to
chemical analysis. The biophore is conceived by Adami to have the
form of a ring or a ring of rings of the benzene type — a ring of amino acid
radicles with many unsatisfied affinities or bonds. The biophore grows
in a manner analogous to that of the inorganic crystal : ions and radicles
from the surrounding medium become attached as side chains to the free
bonds of the central ring and take on a grouping similar to that of the
latter; in this way the biophoric molecules are multiplied. Since side
chains can be detached and new ones of other kinds added, the biophore
is changeable and may exist in many different forms. Although the
central ring is thought to be relatively stable and fixed, the variety of side
chains and their many possible arrangements probably give to each
species a distinct kind of biophore. On this hypothesis the molecule of
living matter (biophore) is one "of extraordinary complexity, and in a
state of constant unsatisfaction, built up by linking on other simple
molecules, and as constantly, in the performance of function, giving up

PROTOPLASM 51

or discharging into the surrounding medium these and other molecular
complexes which it has elaborated" (Adami 1918, pp. 251-2). "All
vital manifestations are manifestations of chemical change in proteidogen-
ous matter, are, in short, the outcome of arrangement of that matter
with the necessary liberation or storing up of energy" (p. 225). Accord-
ingly, life is "a state of persistent and incomplete recurrent satisfaction
and dissatisfaction of ... certain proteidogenous molecules" (1908.
Vol. I, p. 55).

Pictet (1918) also associates the phenomena of life with a special
structure of the organic molecule. Only the arrangement of the atoms in
open chains, he asserts, permits the manifestation of life and its main-
tenance; the cyclic structure is that of substances which have lost this
faculty; and death results, from the chemical point of view, from a
cyclization of the elements of the protoplasm.

To the theory that the vital processes are bound up with a special
form of protein or protein-like molecule many have objected. For
example, Hober (1911) has contended that there are present in the
organism only those kinds of proteins which may be formed in the
laboratory. He urges that life should not be thought of as a single
process, or as dependent upon any particular kind of molecule, but rather
that it should be looked upon as the result of many correlated processes
occurring between many substances under certain conditions. "If we
accept this idea," says Child (1915, p. 19), "we must abandon the
assumption of a living substance in the sense of a definite chemical
compound. Life is a complex of dynamic processes occurring in a certain
field or substratum. Protoplasm, instead of being a peculiar living
substance with a peculiar complex morphological structure necessary for
life, is on the one hand a colloidal product of the chemical reactions, and
on the other hand a substratum in which the reactions occur and which
influences their course and character both physically and chemically.
In short, the organism is a physico-chemical system of a certain kind."

Harper (1919) is also opposed to theories based upon the conception
of protoplasm as a single complex chemical substance, as well as to those
which hold protoplasm to be a relatively simple two-phase colloidal
system — the alveolar and granular theories, for example. "The crude
simplicity and general inadequacy of these . . . conceptions . . .
have done much to bring the whole subject of protoplasmic organization
into disrepute. On the other hand the conception of protoplasm as an
aggregate of complex compounds, a polyphase colloidal system or system
of systems, seems to do much more adequate justice to the observed
facts."

Conclusion. — As stated at the opening of the present chapter, it is
with protoplasm that the phenomena of life, in so far as we know them,
are invariably associated. The complex behavior of the living organism

52 INTRODUCTION TO CYTOLOGY

can receive scientific explanation (i.e., be fitted into an orderly scheme
of antecedents and consequents), if at all, only on the basis of the
constitution and properties of the materials composing protoplasm; the
structural organization of protoplasm; the relation of the reactions and
responses of protoplasm in the form of organized units or cells to the
environmental conditions; the chain of energy changes occurring in
connection with all of the organism's activities; and the correlation of
all these conditions and events. It is largely the effort to account for
organization and regulatory correlation, and the consequent behavior of
the complex organism as a versatile and consistent unit or individual — as
something more than a cell aggregate — that has led to certain present
day vitalistic theories, as opposed to those which would hold life to be
dependent upon "nothing but" the correlated physico-chemical reac-
tions and interactions occurring in protoplasm.

Whatever our ultimate judgment in this matter shall be — for any
decision at present is premature — it is scarcely to be denied that the
hypotheses that have thus far been most stimulating to research in
biological science and most valuable in analysing the data afforded by
this research are those which seek to formulate vital activity in terms of
what the physicist for convenience calls matter and energy; and which
hold life to be not the manifestation of a super-organic, non-perceptual
entity, or even of a distinct perceptual but hypothetical vital energy,
but rather the resultant of the many correlated interactions involving
only energies of known kinds. The way must not be closed, however,
against possible new categories of energy. The description (reduction
to order) of our perceptual experience of organic nature, which is the
primary task of biological science and which has been scarcely more than
begun, must for the present be made as far as possible in terms applicable
also to inorganic nature. It is here that achieved results would seem to
justify the judicious use of a "mechanistic" working hypothesis, whereby
the attempt is made to "describe the changes in organic phenomena by
the same conceptual shorthand of notation as suffices to describe inor-
ganic phenomena" (Pearson). To what extent our ultimate biological
theory is to show the need of non-mechanical energies or principles will
depend very largely upon what this scientific description (orderly formu-
lation) turns out to be like as investigation proceeds, and also upon the
degree of success with which the physicist will resume the phenomena of
inorganic nature in mechanical formulae. Thus, as Professor D'Arcy W.
Thompson forcefully says :

"While we keep an open mind on this question of vitalism, or while we lean,
as so many of us now do, or even cling with a great yearning, to the belief that
something other than the physical forces animates the dust of which we are made,
it is rather the business of the philosopher than of the biologist, or of the biologist
only when he has served his humble and severe apprenticeship to philosophy,

PROTOPLASM 53

to deal with the ultimate problem. It is the plain bounden duty of the bioloigst
to pursue his course unprejudiced by vitalistic hypotheses, along the road of
observation and experiment, according to the accepted discipline of the natural
and physical sciences. . . It is an elementary scientific duty, it is a rule that
Kant himself laid down, that we should explain, just as far as we possibly can,
all that is capable of such explanation, in the light of the properties of matter and
of the forms of energy with which we are already acquainted." (Presidential
address before the Zoological Section of the British Association for the Advance-
ment of Science, 1911.)

Bibliography 3

Protoplasm
ADAMI, J. G. 1908. Principles of Pathology. 1st ed.

1918. Medical Contributions to the Study of Evolution. London.
ALTMANN, R. 1886. Studien liber die Zelle. I. Leipzig.
1887. Die Genese der Zellen. Leipzig.

1889. Die Struktur des Zellkerns. Arch. Anat. u. Physiol. p. 409-411.

1890, 1894. Die Elementarorganismen und ihre Beziehung zu den Zellen. Leipzig.
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223-230.

.1893. Die Granulartheorie und ihre Kritik. Ibid. p. 55-66.
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501-520.

1914. The theory of colloid chemistry. Ibid. 18: 549-558.

BARBER, M. A. 191 la. A technic for the inoculation of bacetria and other sub-
stances into living cells. Jour. Infect. Diseases 8: 348-360.
191 Ib. The effect on the protoplasm of Nitella of various chemical substances and
of microorganisms introduced into the cavity of the living cell. Ibid. 9 : 117-129.

1914. The pipette method in the isolation of single microorganisms and in the
inoculation of substances into living cells. Philip. Jour. Sci. 9.

DE BARY, H. A. 1859. Myxomyceten. Zeitschr. Wiss. Zool. 10 : 88-175. pis. 5.

1864. Die Mycetozoen. 2 Aufl. Leipzig.

BAYLISS, W. M. 1915. Principles of General Physiology. London.
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transl. by J. G. M. Bullowa, N. Y., 1919.)
BENSLEY, R. R. 1910. On the nature of the canalicular apparatus of animal

cells. Biol. Bull. 19: 174-194. figs. 3.
BLACKMAN, F. F. 1912. The plasmatic membrane and its organization New

Phytol. 11: 180-195.
BoRzl, A. 1886. Le communicazioni intracellulari delle Nostochineae. Malpighia

1:74,97,145,197. pi. 3.

BROOKS, W. K. 1899. The Foundations of Zoology. New York.
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381-406.
BUTSCHLI, O. 1892. Untersuchungen iiber mikroskopische Schaume und das

Protoplasma. Leipzig. (Engl. transl. by E. A. Minchin, 1894.)
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Arch. Entwmech. Org. 11: 499-584. pi. 20.
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40: 824-827.

1915. Microdissection studies on the germ cell. Ibid. 41 : 290-293.

54 INTRODUCTION TO CYTOLOGY

1917a. Microdissection Studies. I. The visible structure of the cell protoplasm
and death changes. Am. Jour. Physiol. 43 : 1-12. figs. 2.

19176. Microdissection Studies. II. The cell aster. A reversible gelation phe-
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1918. The Microvivisection Method. Biol. Bull. 34: 121-136.

1919. Changes in protoplasmic consistency and their relation to cell division.
Jour. Gen. Physiol. 2 : 49-68.

CHILD, C. M. 1915. Senescence and Rejuvenescence. Chicago.

CIENKOWSKI, L. 1863. Zur Entwicklungsgeschichte der Myxomyceten. Jahrb.

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der Plasmahaut von Pflanzenzellen. Jena.
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BELLINGER, O. P. 1909. The ciliura as a key to the structure of contractile proto-
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plants. Oxford.

FISCHER, A. 1899. Fixierung, Farbung und' Bau des Protoplasmas. Jena.
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1884. Untersuchdngen iiber Struktur, Lebenserscheinungen und Reaktionen

thierischen und pflanzlicher Zellen. Ibid. 17: 1-349. pis. 3.

GARDINER, W. 1884. On the continuity of the protoplasm through the walls of vege-
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1897. The histology of the cell wall, with special reference to the mode of con-
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plant cell (Prelim. Comm.) Ibid 66. 186-188.

GARDINER, W. and HILL, A. W. 1901. The histology of the cell wall with special
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GATENBY, J. B. 1919. Identification of intra-cellular elements. Jour. Roy. Micr.
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1920. The cytoplasmic inclusions of the germ-cells. VII. The modern technique
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GLASER, O. 1916. The basis of individuality in organisms. Science 44: 219-224.

PROTOPLASM 55

GOHOSCHANKIN, J. 1883. Zur Kenntniss der Corpuscula bei den Gymnospermen.

Bot. Zeit. 41: 825-831. pi. 7.
HABERLANDT, G. 1890. Das Reizleitende Gewebesysteiu der Sinnpfianzen.

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HAMMARSTEN, O. 1909. A Textbook of Physical Chemistry. 5th ed. N. Y.
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18806. Einige Ziige aus der Biologie des Protoplasmas. Hanstein's Bot. Abhandl.

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HARPER, R. A. 1919. The structure of protoplasm. Am. Jour. Bot. 6: 273-300.
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HENDERSON, L. J. 1913. The Fitness of the Environment. New York.
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1885. Protoplasmic continuity in the Fucacese. Jour. Bot. 23 : 97-102, 354-357.

pi. 255.
HILL, A. W. 1900. Distribution and character of connecting threads in the tissues of

Pinus sylveslris and other allied species. Proc. Roy. Soc. London 67 : 437-439.
1901. The histology of the sieve tubes of Pinus. Ann. Bot. 15: 575-611. pis.

31-33.

HOBER. 1911. Physikalische Chemie der Zelle und der Gewebe. 3d ed. Leipzig.
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Phanerogamen, hauptsachtlich der Leguminosen. Ber. Deu. Bot. Ges. 10 :

494-513. pi. 27.

KASSOWITZ, M. 1899. Allgemeine Biologie. Vienna.
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ten Gewebselementen in der Pflanzen. Bot. Zeit. 49: 1, 17, 33, 49, 65. pis. 2.

(Bibliography.)
KITE, G. L. 1913. Studies on the physical properties of protoplasm. I. Am. Jour.

Physiol. 32 : 146-164.
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Jour. Micr. Sci. 18: 315-339, pi. 16; 19: 125-175, pi. 7.
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Berlin.
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KOSSEL, A. 1880. Ueber das Nuclein der Hefe. Zeit. Physiol. Chem. 3, 4.
1889. Ueber die Nucleine. Centr. Med. Wiss. pp. 417, 593.
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KUHLA, F. 1900. Die Plasmaverbindungen bei Viscum album. Bot. Zeit. 58 :

29-58. pi. 3.
KUHNE, W. 1864. Untersuchungen tiber das Protoplasma und die Contractilitat.

Leipzig.
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Nat. Bot. VII 10: 193-324. pis. 21-24.
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28: 91-103, 383-393.
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Ibid. 29: 247-260. See also pp. 349-351.

56 INTRODUCTION TO CYTOLOGY

LIVINGSTON, B. E. 1903. The role of diffusion and osmotic pressure in plants.

Chicago.
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Carnegie Inst. Washington 14: 66-69.
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Publ. 297. pp. 176.

MACDOUGAL, D. T. and SPCEHR, H. A. 1920. The components and colloidal be-
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Contrib. Bot. Lab. Univ. Pa. 1: 7-44. pi. 4.
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Innsbruck 20.

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MASSEE, G. 1884. On the formation and growth of cells in the genus Polysiphonia.

Jour. Roy. Micr. Soc. II 4 : 198-200. pi. 6.
MATHEWS, A. P. 1916. Physiological Chemistry.
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usw. Bot. Zeit. 54: 187-215. pi. 8.
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Gesell. in Basel 6.

MINCHIN, E. A. 1912. An Introduction to the Study of the Protozoa. London.
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Vierteljahrsschr. Nat. Ges. Zurich 40: 159-184.

1899. Ueber die allgemeinen osmotischen Eigenschaften der Zelle, ihre vermuth-
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1900. Studien iiber die Aufnahme der Anilinfarben durch die lebende Zelle.
Jahrb. Wiss. Bot. 34: 669-701. See Pfliiger's Archiv 92.

PALLADIN, V. I. 1918. Plant Physiology. (Engl. transl., ed. by Livingston.)

PEARSON, K. 1892. The Grammar of Science.

PICTET, A. 1919. Life and the structure of molecules. (Geneva Arch. Phys. et

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Abh. Math.-Phys. kl., Sachs. Ges. Wiss. 16.

1896. Ueber den Einfluss des Zellkerns auf die Bildung der Zellhaut.
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Berlin.
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649-661.

PROTOPLASM 57

HEINKE und KODKWALD. 1881. Die chcmische Ziisammeiisctzung des Protoplasmas

von Mthal/ivan septicum. Unters. Bot. Lab. Gottiugen.
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1883. Ein Beitrag zur physiologischen Chemie von JElhalium septicum. Ibid.
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Biochem. Zeitschr. 54: 59-77.

Russow. 1883. Ueber die Perforation der Zellwand und den Zusammenhang der
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1895. Physiologische Notizen. IX. Weitere Betrachtungen iiber Energiden und

Zellen. Ibid. 81: 405-434. (See Wilson 1900, p. 30.)
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section. Biol. Bull. 34 : 307-424. figs. 3. (Bibliography.)
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STRASBURGER, E. 1882. Ueber den Bail und das Wachsthum der Zellhaute. Jena.
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Befruchtung. Histol. Beitr. 4.
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493-610. pis. 14, 15.

TANGL, E. 1879. Ueber offene Communicationen zwischen den Zellen des Endo-
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Deu. Med. Zeit. 1895. p. 98.
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Wiss. Bot. 16: 465-598.

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chromaceen. Ber. Deu. Bot. Ges. 1 : 243-246.

58 INTRODUCTION TO CYTOLOdY

WILSON, E. B. 1899. On protoplasmic structure in the eggs of echinoderius and

some other animals. Jour. Morph. 15 : Suppl. 1-23.
WORTMANN, J. 1887. Zur Kenntniss der Reizbewegungen. Bot. Zeit. 45 : 785,

801, 817, 833.
1889. Ueber die Beziehungen der Reizbewegungen wachsender Organe zu den

normalen Wachsthumerscheinungen. Ibid. 47: 453, 469, 485.
ZACHARIAS, E. 1881-1893. Ueber die chemisohe Beschaffenheit des Zellkerns.

Bot. Zeit. 39 : 169. Ueber den Zellkern. Ibid. 40 : 611. Ueber Eiweiss, Nuclein,

und Ilastin. Ibid. 41 : 209. Ueber den Nukleolus. Ibid. 43 : 257. Beitrage

zur Kenntniss des Zellkerns und der Sexualzellen. Ibid. 45 : 281. Ueber Chroma-

tophilie. Ber. Deu. Bot. Ges. 11 : 188-195.
ZIMMERMANN, A. 1893. Sammel-Referate 2, 3. Beih. Bot. Centr. 3: 209-217

321-328.

CHAPTER IV
THE NUCLEUS

It is now half a century since the modern period of cytology was
ushered in by a series of researches revealing the remarkable behavior
of the nucleus during the critical stages of the life cycle. Because of
the peculiarly intimate relation which this behavior has been shown to
have to many outstanding biological problems, including that of heredity,
it is largely in nuclear phenomena that cytological interest has con-
tinued to center throughout the period. The most striking of these
phenomena form the subjects of several subsequent chapters: at this
point we shall consider the nucleus only as it appears in the "resting"
cell, i.e., in the cell not undergoing division.

Occurrence. — The most conspicuous, and in some respects the most
important of the cell organs is the nucleus. Whether or not we shall say
that every living cell contains a nucleus will depend upon what we are to
include under the term. If the chromatin or chromatin-like substances,
no matter whether distributed throughout the cell in the form of granules
or aggregated to form a well defined organ, be regarded as constituting a
nucleus, then it follows that all plant and animal cells normally have
nuclei. If, however, as certain protozoologists prefer, the term nucleus
be employed only with reference to a distinctly delimited organ, we must
regard those lowly organized cells with scattered chromatic material as
devoid of nuclei, although they possess, as all cells apparently do, material
which performs at least the nutritive functions of a nucleus. This, latter
type of organization, which is found in certain members of the Protozoa
and Bacteria, and also some Cyanophycese, will be discussed later on in
connection with nuclear structure (p. 66) and cell-division (Chapter X).
In myxomycetes, where simple and primitive conditions might be expected,
Jahn (1908, 1911) and Olive (1907) have demonstrated the presence of
definite nuclei showing mitosis and the phenomenon of chromosome
reduction.

General Characters. — The vast majority of cells have one nucleus
each. A few exceptions may be noted. In tapetal cells, laticiferous ves-
sels, the internodal cells of Characese, and certain other cells there are
often several nuclei arising by the division of one. In the Siphonese among
green algae (Fig. 14, V) and the Phycomycetes among fungi there are no
cross walls in the filamentous and much branched vegetative body, so that

59

60

INTRODUCTION TO CYTOLOGY

large numbers of nuclei are associated in one extensive mass of cyto-
plasm. Such a body is called a coenocyte, and the coenocytic condition is
found in a number of the lower organisms. In the Uredinese (rusts) the
typical life history is made up of two phases, with uninucleate and
binucleate cells respectively. In certain Infusoria two kinds of nuclei
are regularly present. Thus in Paramoecium caudatum (Fig. 15) there is
one small micronucleus which divides by a peculiar form of mitosis, and
one large meganucleus (macronucleus) which divides amitotically. In
P. aurelia there are two micronuclei and one meganucleus, whereas in

FIG. 14.

V, portion of body of Vaucheria, showing
ccenocytic condition; nuclei dark and
plastids in outline. C, portion of body of
Cladophora, showing semi-ccenocytic
condition.

FIG. 15. — Paramcecium caudatum un-
dergoing fission; mega- and micronuclei
dividing. (From Minchin, after Biitscfili
and Schewiakoff.)

Stentor there may be one meganucleus and several micronuclei. In
general the meganucleus seems to be a storage organ of the cell; it may
disappear and be replaced by a new one. The micronucleus performs
the usual nuclear functions. The mammalian red blood corpuscle begins
its life as a nucleated cell, but later on the nucleus is lost.

The position of the nucleus in the cell is determined largely by physical
causes, such as surface tension, the position of the vacuoles, and the
relative density of the cytoplasm in different portions of the cell. In a
non-vacuolated cell it ordinarily occupies the center of the cytoplasmic
mass, whereas in a cell with vacuoles it is imbedded in the cytoplasm even
when the latter is reduced to a thin parietal layer; it never lies free in the
vacuole. In the Cladophoracese (Carter 1919) it is regularly imbedded,

THE NUCLEUS

61

FIG. 16.

A, The thickening of
the inner wall of an epi- —

at least partially, in the chloroplast , and this is true even in cells possess-
ing a considerable amount of colorless cytoplasm. Its position is also
related to the functions of the cell: in
general it lies in the region character-
ized by the most active metabolism.
For example, in young growing root
hairs (Fig. 16, J3) and pollen tubes it
is commonly found where elongation is
taking place, and in thickening epidermal
cells (Fig. 16, A) it frequently, though
not always, lies near the wall upon
which the thickening material is being
deposited. This relation of position to
function was emphasized in the works

of Haberlandt (1887) and Gerassimow (1890, 1899, 1901).
In form the nucleus is typically spherical or ellipsoidal, its
shape being determined by a number of physical factors.
Under comparatively uniform conditions, as obtain where a small
nucleus lies in a relatively large amount of non-vacuolated cytoplasm,
a spherical, shape is assumed because of the phenomena of surface
tension. Exceptions are often seen in cells with specialized functions.

root hairs in Pisum sativum.
(After Haberlandt.)

FIG. 17. — Unusual forms of nuclei.

A, portion of nucleus from spinning gland of Vanessa urticce, showing irregular form
and finely divided state of the chromatin. (After Korschelt, 1896.) B, Spirostomum
ambiguum, with moniliform nucleus. (After Stein.) C, Nucleus from salivary gland of
Chironomus: the chromatic material exists as a series of discs in a convoluted thread, which
ends in two nucleoli. (After Balbiani, 1881.) D, Chcenia teres, with chromatic granules
scattered throughout the body. (After Gruber, 1884.) E, Nucleus from root tip of Mar-
silia, showing concentration of chromatic material in the nucleolus. (After Berghs, 1909.)

In the cells of the spinning glands of Pieris and Vanessa (butterflies)
the physiological conditions result in the assumption of very irregular
forms whereby the nuclear surface is considerably increased (Fig. 17, A).
Nuclei seem rather commonly to undergo amo?boid changes in shape;

62 INTRODUCTION TO CYTOLOGY

such active movement can be directly observed in the nucleus of Un-
living cycad spermatozoid. In the long, narrow cells of vascular bundles
the nuclei, which are not free to grow in all dimensions, come to be
correspondingly elongated. The nucleus may also, be passively forced
into very irregular shapes by the dense accumulation of starch grains
and the diminution in the amount of cytoplasm, as in the endosperm
cells of maize. In Stentor and Spirostomum the nucleus has the form
of a string of beads (Fig. 17, B).

In size the nucleus shows a wide variation, ranging in plants from
the extremely minute nucleus of Mucor, lju or less in diameter, to the
relatively gigantic nucleus of the Dioon egg, with a diameter of 600^.
A similar range is seen in animal nuclei. Although the nuclei of the
fungi are characterized by small size, most of them being less than OM
in diameter, they may grow to a large size at certain stages. The primary
nucleus of Synchytrium, for instance, reaches a diameter of over 60ju-
The majority of nuclei, however, fall between SM and 25^- In spite of
the wide range in the size of nuclei of different organisms, in a given
tissue it is comparatively uniform.

With respect to the physical nature of the nucleus as a whole, the
researches of Kite (1913) and Chambers (1914, 1917) have shown that it
ordinarily consists at least in part of a gel of higher viscosity than the
cytoplasm, often being so firm that it can easily be handled without
injury by means of the dissecting instrument. This obviously would be
impossible were the nucleus merely a watery droplet or vesicle in the cyto-
plasm. The germinal vesicle (nucleus) of the animal egg Chambers
(1917) finds to be a sol droplet with a gel membrane; if it is pinched in two
by the dissecting instrument the two halves will reunite if they come in
contact.

The chemical nature of the nucleus has been dealt with in the preced-
ing chapter. With regard to its electrical properties, the nucleus is
apparently negative to the cytoplasm. R. S. Lillie (1903) found that
free nuclei and the heads of spermatozoa, which are almost entirely
nuclear material, pass to the anode in an isotonic cane sugar solution,
whereas cells rich in cytoplasm, such as large leucocytes, pass to the
cathode. These results have been confirmed by Hardy (1913).

Nucleoplasmic Ratio. — Of more importance than the absolute size
of the nucleus is the relation of its volume to that of the cytoplasm — the
so-called nucleoplasmic or Kernplasma relation. Many years ago it was
held by Sachs (1892, 1893, 1895) and by Strasburger (1893) that the size
of a meristematic cell in a plant, owing to a supposed limitation of the
sphere of influence of the nucleus, maintains a very definite relation to
the size of its nucleus. This conception has recently been emphasized
anew by Winkler (1916), and parallel views have been expressed by
several zoologists (e.g., Hegner on Arcella, 1919). In the case of certain

THE NUCLEUS 63

terminal meristems of plants such a rule may well hold true within
limits, but the condition reported by Bailey (1920) in the lateral meristem
(cambium) shows clearly that it cannot have universal application.
The cambial initials may vary enormously in size with no corresponding
variation in the size of their nuclei: two such initials, one of them having
many hundreds of times the volume of the other, may possess nuclei
of approximately equal size.

The nucleoplasmic ratio has figured prominently in discussions of
the problem of senescence. R. Hertwig in 1889 advanced the theory
that senescence and natural death are associated with an increase in the
relative size of the nucleus. He later asserted (1903, 1908) that the nucleo-
plasmic relation is self-regulatory within certain limits for each kind of
cell, exercising thereby a control over many cell activities, including cell-
division. Minot (1891, 1908, 1913), on the contrary, believed that the
increase in the relative volume of the cytoplasm, in addition to its differ-
entiation, is a fundamental factor in senescence and death. Conklin
(1912), as a result of his work on Crepidula, denied the existence of a
constant and self-regulatory nucleoplasmic relation, holding rather that
changes in this relation are not causes of such cell activities as cell-
division, but are results of the metabolic processes by which such cell
activities are brought about. Child (1915) points out that in most
animal tissues there is an increase in the relative amount of cytoplasm
during senescence, whereas in plants, although the cell enlarges through
vacuolation, the relative volume of cytoplasm often does not increase.
He therefore concludes that the nucleoplasmic relation cannot be regarded
as a universal factor in senescence ; it is rather an indication of the kind
and rate of metabolism. The differentiation of the cytoplasm, apart
from its mere change in volume, Child, with many other workers (Minot.
Delage, Jennings, etc.), regards as a matter of the greatest importance in
senescence. Further discussion of this subject is deferred to Chapter VII.

Not only has it been held that there is a certain relation between the
mass of the nucleus and that of the cytoplasm, whatever the significance
of this relation may be, but there also seems to be a size relationship
between the nucleus and its contained chromosomes. In 1896 Boveri
showed that the size of the nuclei in merogonic echinoderm larvae (see
p. 325) is dependent upon the number of chromosomes each contains.
In a more extended study (1905) he demonstrated that it is the surface
of the nucleus that is proportional to the chromosome number, and also
that the size of the cell is proportional to both. Gates (1909), however,
adduced evidence to show that this rule is by no means universal.

Structure. — Having reviewed the general features of the nucleus as
a whole, we may next give attention to its structure, as seen in typical
cases.

The nucleus is bounded by a distinct nuclear membrane. The nature

64 INTRODUCTION TO CYTOLOGY

of this membrane has been a subject of much controversy. Some have
regarded it as a precipitation membrane laid down when the newly
formed karyolymph comes in contact with the cytoplasm at the time the
daughter nuclei are reconstructed during the closing phases of mitosis,
while others (Lawson 1903) have interpreted it as merely a denser limit-
ing layer of the cytoplasm. The above cited work of Kite and Chambers,
however, leaves no doubt that the membrane is a definite morphological
structure belonging to the nucleus: although it is at times very delicate,
it remains intact when the nucleus is pushed and pulled about by the
dissecting instrument, and is thrown into folds when the karyolymph is
withdrawn with a pipette.

Within the nuclear membrane is a series of gels of varying consistency.
The nuclear sap, or karyolymph, is a highly transparent substance which
is generally looked upon as homogeneous, although it has been thought
by some workers (Reinke 1894) to be made up of large, pale "cedamatin
granules." It may be in the sol or gel state. Imbedded in the karyo-
lymph is a network or reticulum, which may be relatively uniform through-
out the nucleus or only fragmentary and incomplete. It is usually said
to be composed of a gel substance known as achromatin (Flemming 1879)
or linin (Schwarz 1887). Supported on the linin reticulum is the chroma-
tin (Flemming 1879). This highly stainable substance may exist in the
form of small granules or droplets at the nodes of the reticulum, or
apparently in many nuclei as a fluid thin enough to distribute itself more
or less uniformly throughout the achromatic substance. In the latter
case the whole reticulum appears to be composed of a single unevenly
stained material, careful examination showing the "chromatic granules"
and " achromatic support " to be its thicker and finer portions respectively
(Fig. 51) (Gre"goire and Wyagerts 1903; Gregoire 1906; Sharp 1913,
1920). According to Kite (1913) the granules in the living nucleus con-
sist of a very concentrated gel, the supporting reticulum of a somewhat
more dilute but not at all fibrous gel, and the karyolymph of a gel which
is the most dilute of all.

Heidenhain (1894) found imbedded in the colorless linin net two sorts
of chromatin in the form of granules : oxychromatin, consisting largely of
plastin, poor in phosphorus, and staining with the acid dyes; and basi-
chromatin, composed mainly of nuclein, rich in phosphorus, and staining
with the basic dyes. These two forms of chromatin apparently may
change into each other by the addition or loss of phosphorus. The peri-
odic changes in the staining reactions of many nuclei therefore indicate
changes in the chemical composition of the chromatin, and these in turn
point to the intimate association of the nucleus with the periodic physi-
ological processes of the cell. As used by many writers the term oxy-
chromatin includes also the linin, so that in much cytological literature
linin and oxychromatin are more or less interchangeable terms, while

THE NUCLEUS 65

"chromatin" refers to the basichromatin. Oxychromatin appears to be
closely similar in composition to the achromatic structures in the cyto-
plasm, such as spindle fibers and centrosomes. The prominent place
occupied by the nucleus in cytology is due in large measure to the" con-
spicuous behavior of its chromatic substance at the time of cell-division
and fertilization, topics which are to receive detailed consideration in
subsequent chapters.

In many nuclei basichromatin accumulates at certain points in the
reticulum, forming karyosomes, also called "net knots" and chromatin
nucleoli. These seem to be masses of surplus chromatin elaborated by
the nucleus during the resting phase or in some cases chromatin which
has flowed to these points from the other parts of the reticulum. During
the next mitosis they are distributed with the rest of the chromatin. As
Rosen (1892) long ago showed by his studies of their staining reactions,
they differ decidedly in composition from true nucleoli, although they
may closely resemble the latter after treatment with certain stains (iron-
alum-ha3matoxylin) .

One or more true nucleoli, or plasmosomes (Ogata 1883), are usually
present in the nucleus. A single nucleolus is probably characteristic of
most nuclei ; there are rarely many, and in some cases there is none. The
nucleolus may be in close organic connection with the nuclear reticulum
or it may lie entirely apart from it. In composition it consists largely of
such oxychromatic substances as plastin and pyrenin, or of nuclein well
saturated with protein (Zacharias). It usually stains with the acid
dyes: by a proper selection of stains it may, therefore, be distinguished
from the karyosomes, which, being composed of basichromatin, take the
basic dyes as a general rule. In structure the nucleolus may appear to
be homogeneous throughout, like an oil globule; in other cases it has an
outer envelope of different consistency and staining reaction. Very often
vacuoles, occasionally containing granules, are present in the interior.
Crystalloid bodies are also frequently observed in the nucleolus (Digby
on Galtonia, 1910; Reed on Allium, 1914; Kuwada on Zea, 1919). In
the epithelial cells of the frog intestine Carleton (1920) finds one or more
intranucleolar bodies which he calls "nucleolini." These appear to
divide and pass to the daughter cells at the time of mitosis, and may
possibly initiate the formation of new nucleoli in the daughter nuclei.
Montgomery (1899) * concluded that the nucleolus grows in size by the
apposition of smaller particles of nucleolar material on its surface, and
by the intussusception of vacuolar substance arising outside the nucleolus.

Function of Nucleolus. — Various opinions have been entertained re-
garding the function of the nucleolus. By many workers it has been
looked upon as chiefly a passive by-product of no further use in the life

1 An exhaustive review of the literature dealing with the nucleolus up to 1899
is given in this paper.
5

66 INTRODUCTION TO CYTOLOGY

of the cell (Haecker). Strasburger (1895, 1897), who observed the dis-
appearance of the nucleolus at about the time the spindle fibers appear
during the prophases of mitosis, concluded that it is a mass of reserve
kinoplasm which gives rise indirectly to the achromatic figure. While
some have agreed in the main with this conclusion, many have denied
the relationship of nucleolus and spindle, contending that the former is
rather a reserve constituent for the linin reticulum (Eisen 1900) or the
chromatin (Schiirhoff 1918). Frequently the bulk of the basichrom-
atic material of the nucleus is lodged in the nucleolus at certain stages.
In the somatic nuclei of Marsilia (Fig. 17, E), for example, Berghs (1909)
shows that it is transferred to the nucleolus during the telophases of
mitosis, and returned to the reticulum in the following prophases. This
phenomenon, which has an important bearing on the role of the chrom-
atin and the individuality of the chromosomes, will be referred to again
in Chapter VIII.

In many cells, as shown by the work of the zoologists the nucleolus
appears to be concerned in the elaboration of secretion and storage prod-
ucts. In the eggs of certain animals Macallum (1890) showed that
the nucleolar material, which appears to differentiate from the chrom-
atin, passes into the cytoplasm and there combines with another
substance to form the yolk globules. In the cells of the pancreas he
further found that material often present in the form of nucleoli func-
tions in a similar manner in the production of zymogen. Many other
observations of this general nature have -been reported. In the silk-
gland cells of certain insects it has recently been shown by Nakahara
(1917) that some of the nucleoli, which may originally be passive by-
products, later pass into the cytoplasm and contribute to the formation
of the secretion products. An extreme view of the importance of the
nucleolus is that of Derschau (1914), who regards the nucleolus as the
real center of the life of the cell. Granules of oxychromatin, he asserts,
pass out from the nucleolus through the cytoplasm in the form of chon-
driosomes, carrying basichromatin as a building material to the places
where it is required.

It is highly probable that the nucleolus has various functions in dif-
ferent cells, but in general we may conclude that it is a mass of accumu-
lated material which is usually, though not always, utilized in the
metabolic processes of the nucleus

The Nuclei of Bacteria and Other Protista. — The question of the
nucleus in bacteria is one that it appears to be particularly difficult
to settle satisfactorily. This is due not only to the minute size of these
organisms, which makes special methods necessary and observation very
difficult, but also to the fact that a variety of conditions seems to be
present in the group. That the bacterial cell is devoid of a nucleus has
been held by several investigators including Fischer (1894, 1897, 1899,

THE NUCLEUS 67

1903), who looked upon the observed granules as reserve materials
rather than nuclear substance. Migula (1894, 1897, 1904) regarded the
existence of nuclei in bacteria as very doubtful. The majority of workers,
on the contrary, have held that a nucleus or at least nuclear material is
present in some form. The most striking view is that which regards the
whole cell in some cases as a naked nucleus (Hiippe 1886; Zettnow 1891,
1897, 1899; Ruzicka 1908, 1909; and, in the case of small bacteria,
Biitschli.1890, 1892, 1896, 1902). The evidence advanced in support of
this hypothesis, however, is of very doubtful value.

In many bacteria, particularly the larger forms, there is present a
granular substance which has certain characteristics of chromatin, and
which in some species exists as a single well defined mass. The "central
body" of the sulphur bacterium Biitschli regarded as the homologue of a
nucleus, the peripheral portion of the cell being cytoplasm. In a careful
study of the entire life cycle of Bacillus Butschlii Schaudin (1902) found
that the chromatic material present during most of the cycle as chromidia
unites at certain stages to form peculiar spiral figures; in the spores it
takes the form of dense masses. Such scattered chromidia and "spiral
filament nuclei" were also observed by Guilliermond (1908, 1909), who
has given a review of the subject (1907). Nakanishi (1901), who employed
both intra-vitam methods and fixed material, reported the presence of
nuclei in the vegetative cells and spores of a number of species.

The nucleus of the large Bacterium gammari was studied by Vejdow-
sky (1900), who in 1904 described its division by mitosis. Mencl (1904,
1905, 1907, 1909) demonstrated by careful methods the nuclei in many
species and also reported mitotic division in Bacterium gammari. Doubt
concerning the systematic position of this form, however, has been raised
by some investigators, who think it not improbable that it is a yeast-
like fungus rather than a bacterium.

Dobell (1908, 1909, 1911), whose review of the subject has been of
service in the preparation of this summary, has studied with much care
many species of bacteria in their natural culture media. His conclusions
are summarized in the following quotation (1911):

"All bacteria which have been adequately investigated are — like all
other Protista — nucleate cells.

"The form of the nucleus is variable, not only in different bacteria,
but also at different periods' in the life cycle of the same species.

"The nucleus may be in the form of a discrete system of granules
(chromidia); in the form of a filament of various configuration; in the
form of one or more relatively large aggregated masses of nuclear sub-
stance; in the form of a system of irregularly branched or bent short
strands, rods, or networks; and probably also in the vesicular form char-
acteristic of the nuclei of many animals, plants, and protists.

"There is no evidence that enucleate bacteria exist."

68 INTRODUCTION TO CYTOLOGY

The apparent discrepancy between this view of bacterial organization
and that of Minchin, stated below, will be seen to be largely a matter of
terminology.

It is therefore among the Proitsta that the widest departures from
the usual type of nuclear structure are found, certain of them in all prob-
ability representing relatively primitive stages in the evolution of the
true nucleus. Such an interpretation is evidently to be placed upon the
"distributed nuclei" seen in certain bacteria, protozoans, flagellates,
and Cyanophycese (p. 202), which consist of granules of a material akin
to chromatin scattered throughout the cell, sometimes with a limiting
membrane of some sort but often with none. It is doubtful if granules
scattered with no definite limitations throughout the cell, as in Chcenia
teres (Fig. 17, D) or Chroococcus turgidus (Fig. 72, A), should be spoken
of collectively as a nucleus. As pointed out at the beginning of this
chapter, it seems preferable to certain workers to limit the term to those
chromatic aggregations which actually have the characters of a definitely
localized organ. In discussing the advisability of so restricting the appli-
cation of the term Minchin (1912, Chapter VI) points out that "the word
'chromatin' connotes an essentially physiological and biological con-
ception . . . of a substance, far from uniform in its chemical nature,
which has certain definite relations to the life history and vital activities
of the cell. The word 'nucleus,' on the other hand . . . is essentially
a morphological conception, as of a body, contained in the cell, which
exhibits a structure and organization of a certain complexity, and in
which the essential constituents, the chromatin particles, are distributed,
lodged, and maintained, in the midst of achromatinic elements which
exhibit an organized arranegment, variable in different species, but more
or less constant in the corresponding phases of the same species." Ac-
cording to this interpretation the term "nucleus" would not be applicable
to a mass of granules (chromidia) scattered throughout the cell. Minchin
states further that " . . .as soon as a mass or a number of particles of
chromatin begin to concentrate and separate themselves from the sur-
rounding protoplasm, with formation of distinct nuclear sap and ap-
pearance of achromatinic supporting elements, we have the beginning
at least of that definite organization and structural complexity which is
the criterion of a nucleus as distinguished from a chromidial mass."

Those Protista of the lower (bacterial) grade, in which there are only
scattered grains of chromatic material, are looked upon as "non-cellular"
in organization by Minchin, who believes that from such a primitive
state the "strictly cellular grade of organization has been evolved by
concentration of some or all of the chromatin to form a nucleus." In
its simplest condition such a nucleus consists of one or more chromatin
granules in a sort of vacuole, and is known as a " protokaryon." In other
cases the chromatin forms a single large mass at the center of the nucleus

THE NUCLEUS 69

("vesicular nucleus"). Since "the chromatic particles are the only con-
stituents of the cell which maintain persistently and uninterruptedly
their existence throughout the whole life cycle of living organisms uni-
versally," Minchin (1916) believes that the earliest living things, which
he calls " Biococci," were minute particles of a chromatin-like substance.
These were the ancestors of the present chromatin grains and find their
nearest modern representatives in certain pathogenic Chlamydozoa. Ac-
cording to this view the cytoplasm was differentiated later in the evolu-
tion of the cell, whereas the more general view probably is that chromatin
and cytoplasm were coexistent as two substances in cells from the earliest
known stages (Wilson).1

The Function of the Nucleus. — It may be said without reservation
that the nucleus dominates the morphological and physiological changes
in the cell. Although the type of organization formed by a nucleus in
combination with cytoplasm is required for the carrying on of cell activity,
it is nevertheless evident from a huge mass of accumulated observations
that in the nucleus is to be found the center of control for both the func-
tional activities and for cell reproduction (cell-division). Many years
ago Claude Bernard (1878) pointed out that while the cytoplasm is the
seat of destructive metabolism, the nucleus is the seat of constructive
metabolism, this physiological role offering "the key to its significance as
the organ of development, regeneration, and inheritance " (Wilson). The
inability of a cell deprived of its nucleus to carry on synthetic metabolism
in any complete manner has often been noted, though such a cell may not
perish for some time. The mammalian erythrocyte, for example, loses
its nucleus at an early stage and may continue to exist in the enucleate
state for from 15 to 30 days (Hunter, Quincke). Klebs found that
enucleate cells of Spirogyra may continue for some time to form starch.
But such cells are apparently unable to divide or to increase their bulk
by the elaboration of new cell substance. Many ordinary activities, such
as cell wall formation (Townsend 1897; Gerassimow 1899, 1901), fail to
occur. From such observations it is concluded that the nucleus is nec-
essary for the synthetic processes associated with growth and reproduc-
tion. This conclusion is supported by the facts of regeneration.

The role of the nucleus in regeneration was strikingly shown by the
well known experiments of Gruber (1885) and F. R. Lillie (1896) on Sten-
tor. This unicellular organism, which has a nucleus like a string of beads,
may be cut into fragments: any fragment containing a portion of the
nucleus has the power of regenerating a complete new animal, whereas
enucleate fragments, although they may live for a little time, undergo no
regeneration and eventually perish.

1 For more complete descriptions of the nuclei of Protista the works of Wilson
(1900) and Minchin (1912) should be consulted. The behavior of such nuclei at the
time of cell-division is briefly described in Chapter X of this book.

70 INTRODUCTION TO CYTOLOGY

A number of biologists (Gruber 1886, Hertwig 1898, Heidenhain 1894,
Henneguy 1896, Conklin 1902) concluded that in general the chromo-
somes (basichromatin) are concerned chiefly with differentiation and
regulation, while the achromatin (oxychromatin) has to do with metabo-
lism (Conklin 1917). Metabolism is in reality a great complex of
reactions : the reactions are not independent of one another but are closely
correlated, and thus constitute an intricately adjusted reaction system.
Among these many reactions, according to modern physiology, the most
important is oxidation, for the energy utilized by the organism is derived
immediately from the union of protoplasm or of its constituent elements
with oxygen. Oxidation has been called the "independent variable"
(Loeb and Wasteneys 1911) upon which the other reactions largely
depend : oxidation is the dominant factor in cell activity, and it is there-
fore of the greatest importance to understand as well as possible the rela-
tion of the parts of the cell to this process.

Following the experiments of Spitzer (1897), who observed that nucleo-
proteins extracted from certain animal tissues have the same oxidizing
power as the tissues themselves, it was advocated by Loeb (1899) that the
nucleus is the center of oxidation in the cell. Loeb pointed out that this
would explain the inability of enucleated cell-fragments to undergo
regeneration. This conclusion was supported by R. S. Lillie (1903), who
later (1913) showed that rapid oxidation occurs both at the surface of the
cell and at the surface of the nucleus, and also by Mat hews (1915). Other
workers (Wherry 1913, Schultze 1913, Reed 1915) however, have failed
to agree. Osterhout (1917), who briefly summarizes the subject, found
that "injury produces in the leaf-cells of the Indian Pipe (Monotropa
uniflora) a darkening which is due to oxidation. The oxidation is much
more rapid in the nucleus than in the cytoplasm and the facts indicate
that this is also the case with the oxidation of the uninjured cell."

The role of the nucleus in development and inheritance, which has
been a subject of so much discussion in recent years, will be dealt with
in later special chapters (XIV-XVIII), after the behavior of the nucleus
in somatic cell-division, maturation, and fertilization has been described.

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1908. Ueber neue Probleme der Zellenlehre. Arch. Zellf . 1 : 1-32. figs. 9.
HUPPE, F. 1886. Die Formen der Bakterien und ihre Beziehungen zu Gattungen

und Arten. Wiesbaden.
JAHN, E. 1904. Myxomycetenstudien. 3. Kernteilung und Geisselbildung bei den

Schwarmern von Stemonitis flaccida Lister. Ber. Deu. Bot. Gesell. 22 : 84-92.

pi. 6.

1908. Myxomycetenstudien. 7. Ceratiomyxa Ibid. 26a: 342-352.
1911. Myxomycetenstudien. 8. Der Sexualakt. Ibid. 29: 231-247. pi. 11.
JENNINGS, H. S. 1912. Age, death and conjugation in the light of work on lower

organisms. Harvey Lectures 1911-1912. Pop. Sci. Mo. 80: 563-577.
1913. The effect of conjugation in Paramcecium. Jour. Exp. Zool. 14: 279-392.

figs. 2.

KITE, G. L. 1913. See Bibliography 3.

KORSCHELT, E. 1896. Ueber die Structur der Kerne in den Spinndriisen der
Raupen. Arch. Mikr. Anat. 47 : 500-549. pis. 26-28.

THE NUCLEUS 73

KUWAUA, Y. 11)19. Die Chromosomenzahl von Zca Mays L. Jour. Coll. Sci.
Tokyo 39: 1-148. pis. 2.

LAWSON, A. A. 1903. On the relationship of the nuclear membrane to the proto-
plast. Bot. Gaz. 35: 305-319. pi. 15.

LILLIE, F. R. 1896. On the smallest parts of the Stentor capable of regeneration.
Jour. Morph. 12 : 239-249.

LILLIE, R. S. 1902. On the oxidative properties of the cell nucleus. Am. Jour.
Physiol. 7:412-421. fig. 1.

1903. On differences in the direction of the electrical connection of certain free
cells and nuclei. Ibid. 8 : 273-283.

1913. The formation of indophenol at the nuclear and plasma membranes of
frogs' blood corpuscles and its acceleration by induction shocks. Jour. Biol.
Chem. 15: 237-248. pi. 1.
LOEB, J. 1899. Warum ist die Regeneration kernloser Protoplasmastiicken unmog-

lich, usw? Arch. En tw. 8 : 689-693.
LOEB, J. and WASTENEYS. 1911. Sind die Oxidationsvorgange die unabhangige

Variable in den Lebenserscheinungen ? Biochem. Zeitschr. 36: 345-356.
MATHEWS, A. P. 1915. Physiological Chemistry, p. 180.
MENCL, E. 1904. Einige Beobachtungen liber die Struktur und Sporenbildung bei

symbiotischen Bakterien. Centralbl. Bakt. II 12: 559.
1905. Cytologisches iiber die Bakterien der Prager Wasserleitung. Ibid. 16:

544.
1907a. Eine Bemerkung zur Organisation der Periplaneta-Symbionten. Arch. f.

Protistenk. 10: 188.
19076. Nachtrage zu den Strukturverhaltnissen von Bakterium gammari Vejd.

Ibid. 8:259.
1909. Die Bakterienkerne und die "cloisons transversales " Guilliermonds.

Ibid. 16:62.
MIGTJLA, W. 1894. Ueber den Zellinhalt von Bacillus oxalaticus Zopf. Arb

Bakt. Inst. Karlsruhe 1.
1897, 1900. System der Bakterien. Jena.

1904. Der Bau der Bakterienzelle. Lafar's Handb. Techn. Mykol. 1 : 48.
MINCHIN, E. A. 1912. An Introduction to the Study of the Protozoa.

1916. The evolution of the cell. Am. Nat. 50: 106-118.
MINOT, C. S. Senescence and Rejuvenation. Jour. Physiol. 12 : 97-153.
1908. The problem of age, growth, and death. New York.
1913. Moderne Probleme der Biologic. Jena.

MONTGOMERY, T. H. 1899. Comparative cytological studies, with special regard
to the morphology of the nucleolus. Jour. Morph. 15 : 265-582. (Bibliography
of about 450 titles.)
NAKAHARA, W. 1917. On the physiology of the nucleoli as seen in the silk-gland

of certain insects. Jour. Morph. 29: 55-74. pis. 2.

1918. Some observations on the growing oocytes of the stonefly, Perla immarginata,
Say, with special regard to the origin and function of the nucleolar structures.
Anat. Rec. 15: 203-216. figs. 9.

NAKANISHI, K. 1901. Ueber den Bau der Bakterien. Centr. Bakt. 1 30: 97.
OGATA, M. 1883. Die Veranderung der Pancreaszellen bei der secretion. Arch.

Anat. Physiol. (Physiol. Abt.) 405-437. pi. 6.
OLIVE, E. W. 1907. Cytological Studies on Ceratiomyxa. Trans. Wis. Acad.

Sci. 16:754-773. pi. 47.

OSTERHOUT, W. J. V. 1917. The role of the nucleus in oxidation. Science 46:
367-369.

74 INTRODUCTION TO CYTOLOGY

RKKD, G. B. 1915. The role of oxidases in respiration. Jour. Biol. Chein. 22:

99-111. pi. 1.
REED, T. 1914. The nature of the double spirem in Allium cepa. Ann. Bot.

28:271-281. pis. 18, 19.
REINKE, FR. 1894. Zellenstudien. I. Arch. Mikr. Anat. 43: 377-422. pis.

22-24.
ROSEN, F. 1892. Beitrage zur Kenntniss der Pflanzenzelle. 1. Ueber tinctionelle

Unterscheidung verschiedener Kernbestandteile und der Sexualkerne. Cohn's

Beitr. z. Biol. d. Pfl. 5 : 443-459 pi. 16.
RUZICKA, V. 1908. Sporenbildung und andere biologische Vorgange bei dem

Bact. anthracis. Arch. f. Hyg. 64: 219-294. pis. 1-3.
1909. Die Cytologie der Sporenbildenden Bakterien und ihr Verhaltnis zur

Chromidienlehre. Centr. Bakt. 11 23 : 289-300. figs. 8.
SACHS, J. 1892. Physiologische Notizeri II. Beitrage zur Zellentheorie. Flora

75: 57-67.
1893. Physiologische Notizen VI. Ueber einige Beziehungen der specifischen

Grosse der Pflanzen zu ihrer Organisation. Ibid. 77 : 49-81.
1895. Physiologische Notizen IX. Weitere Betrachtungen iiber Energiden und

Zellen. Flora (Ergiinzungsband) 81 : 405-434.
SCHAUDINN, F. 1902. Beitrage zur Kenntniss der Bakterien und verwandten

Organismen. 1. Bacillus Biitschlii, n. sp. Arch. Protistenk. 1 : 306.

1903. II. Bacillus sporonema, n. sp. Ibid. 2:421.

SCHULTZE, W. H. 1913. Die Sauerstofforte der Zelle. Verh. Deu. Path. Ges. 16:

161-168.
SCHURHOFF, P. N. 1918. Die Beziehungen des Kernkorperchens zu den Chromo-

somen und Spindelfiisern. Flora 110: 52-66. figs. 3.
SCHWARZ, FR. 1887. Die morphologische und chemische Zusammensetzung des

Protoplasmas. Breslau.
SPITZER. 1897. Die Bedeutung gewisser Nukleoproteide ftir die oxidative Leistung

der Zelle. Arch. f. Ges. Physiol. 67 : 615-656.
STRASBURGER, E; 1893. Ueber die Wirkungssphare den Kerne und die Zellgrosse.

Histol. Beitr. 5: 97-124. Jena.

1895. Karyokinetische Probleme. Jahrb. Wiss. Bot. 28: 151-204. pis. 2, 3.
1897. Ueber Cytoplasmastrukturen, Kern- und Zelltheilung. Ibid. 30: 375-405.

figs. 2.
TOWNSEND, C. O. 1897. Der Einfluss des Zellkerns auf die Bildung der Zellhaut.

Jahrb. Wiss. Bot. 30: 484-510. pis. 20, 21.
VEJDOWSKY, F. 1900. Bemerkungen iiber den Bau und Entwicklung der Bakterien.

Centr. Bakt. 11 6 : 577-589. 1 pi.

1904. Ueber den Kern der Bakterien und seine Teilung. Ibid. 11 : 481-496.
IpL

WHERRY, E. T. 1913. On the metamorphosis of an amoeba, Vahlkampfia sp., into

flagellates and vice versa. Science 37 : 494-496.

WILSON, E. B. 1900. The Cell in Development and Inheritance. 2d ed.
WILSON, E. B. and MATHEWS, A. P. 1895. Maturation, fertilization, and polarity

in the echinoderm egg. New light on the "Quadrille of the Centers." Jour.

Morph. 10: 319-342. figs. 8.
WINKLER, H. 1916. Ueber die experimentelle Erzeugung von Pflanzen mit abwei-

chenden Chromosomenzahlen. Zeitschr. f. Bot. 8: 417-531.
YATSU, N. 1905. The formation of centrosomes in enucleated egg-fragments.

Jour. Exp. Zool. 2: 287-312. figs. 8.
ZACHARIAS, E. 1881-1893. See Bibliography 3.

THE NUCLEUS 75

ZETTNOW, E. 1891. Uebcr den Bau den Bakterien. Cenlr. Bakt. 10: (590.
1897. Ueber den Bau der Grossen Spirillum. Zeit. Hyg. 24: 72.
1899, 1900. Romanowski's Farbung bei Bakterien. Ibid. 30: 1, and Centr.

Bakt. 1 27 : 803.
1908. Ueber Schwellengrebel's Chromatinbander in Spirillum volulans. Centr.

Bakt. 146: 193.

ZIMMERMANN, A. 1893-1894. Sammel-Referate. 5, 7, 8. Beih. Bot. Centr. 3:
333-342, 401-436; 4: 81-89. (No. 7 reviews 153 papers on nuclei of various
plant groups.)

CHAPTER V
THE CENTROSOME AND THE BLEPHAROPLAST

THE CENTROSOME

For a full description of the morphology and behavior of the centro-
some, based upon the large amount of work done on animal cells prior to
1900, reference should be made to Wilson's book on the cell. In the
present account attention will be devoted mainly to centrosome structures
in plants. The centrosomes of animal cells will be described only in
general terms, their role in cell-division being dealt with in later chapters.
Occurrence and General Characters. — The centrosome is an organ
which is characteristic chiefly of the cells of animals: in the great majority
of these cells it has been found, at least during certain stages. In plants
centrosomes are limited to the cells of algae and fungi and the spermato-
genous cells of certain bryophytes and pteridophytes. If the blepharo-
plast be regarded as a modified centrosome, a question which will be
discussed further on, all motile cells (spermatozoids) of bryophytes,
pteridophytes, and gymnosperms must be looked upon as possessing
centrosomes. During the last decade of the nineteenth century several
botanists reported the presence of centrosomes in the cells of a number of
angiosperms, but these cases have all failed to stand the test of subsequent
more critical research.1

It is scarcely possible to give a description which will apply to all
centrosomes, since to any rule there are apparently exceptions. The
"typical" centrosome, as seen in animal cells, is a very minute granule,
which stains intensely with certain dyes. It is usually situated in the
cytoplasm, but in some cases it is found within the nucleus (Fig. 18).
It commonly lies in a more or less hyaline mass of material, called the
centrosphere (Strasburger 1892), attraction sphere (van Beneden 1883),
astrosphere (Fol 1891), or hyaloplasm sphere (Wilson 1901). 2 This centro-
sphere may often show two or more concentric zones differing somewhat
in structure and appearance (Fig. 60). At certain stages, especially
during nuclear division, the centrosome becomes the focus of a system of
delicate rays known collectively as the aster (Fol 1877). The aster will

1 For a review of these cases see Koernicke (1903, 1906).

2 There has been much confusion in the application these terms. (See Wilson
1900, p. 324.)

76

THE CENTROSOME AND THE BLEPH AROPLAST

77

receive consideration in the chapter on the achromatic figure.1 Very
often there are two centrosomes lying side by side in the centrosphere,

I

FIG. 18. — Centrosomes in animal cells.

A, attraction sphere above nucleus in spermatocyte of Salamandra. (After Rawitz;
see also Fig. 59.) B-F, intranuclear centrosome in spermatocyte of Ascaris megalocephala
and its behavior during the prophases of mitosis; c, centrosphere; chr, chromosome tetrad.
(After Brauer, 1893.)

the two having arisen by the division of one, apparently in preparation
for the next cell-division (Fig. 19). Von Winiwarter (1912) noticed
that in interstitial testicular cells, which may
have one, two, or four nuclei, there are re-
spectively two, four, and eight rod-like cen-
trosomes lying in midst of a granular mass
("idiosome"). In some cells there may be a
larger number of smaller "centrioles" rather
then one centrosome, and occasionally there are
one or more concentric series of granules about
the central centrosome. Several such types
described by various writers are shown in
Wilson's Fig. 152. It is questionable how far
these are normal appearances, for Chambers
(1917) asserts that several of them may be pro-
duced in the animal egg by subjecting the latter
to abnormal environmental conditions.

Individuality. — The centrosome was dis-
covered and described by Flemming (1875)
and independently by van Beneden (1876). In
1887 van Beneden and Boveri, as a result of
their researches on the thread-worm, Ascaris
megalocephala, independently concluded that the
centrosome, like the nucleus, is a permanent cell
organ maintaining its individuality throughout

1 Because of the relation of the centrosome to the achromatic figure it will be
necessary to make constant reference to the latter. Chapter IX should be consulted
in this connection.

FIG. 19. — Centrosomes in

epithelial cells.
A, from cornea of mon-
key. B, from gastric gland
of man. (After Zimmer-
mann.)

78

INTRODUCTION TO CYTOLOGY

successive cell generations. They observed that, prior to cell-division,
the centrosome divides to form two daughter centrosomes, which movo
apart to opposite sides of the cell and form the poles between which the
mitotic figure is established; and further, that after cell-division is
completed the centrosome included in each daughter cell does not
disappear, but remains visible in the cytoplasm through the ensuing
resting stage. Because of this striking behavior at the time of cell-
division (see further p. 177) the centrosome soon came to be known as
"the dynamic center of the cell."

The above facts seemed to constitute ample ground for the conception
of the centrosome as a permanent cell organ, but many obstacles have
been found in the way of its acceptance as a theory of universal applica-
tion. At certain stages in the history of many animal cells its presence

FIG. 20. — Artificial cytasters in the egg of Arbacia. (After M organ, 1899.)

cannot be demonstrated, and it is entirely absent from the cells of higher
plants. Furthermore, Mead (1898) and Morgan (1896. 1898) found that
the formation of centrosomes with asters may be induced in the eggs of
certain animals by artifical means (treatment with NaCl and MgCl2
solutions) (Fig. 20), and it has been claimed that centrosomes so formed
may function normally in the ensuing division (cleavage) of the egg.
Contrary to the opinion of Boveri (1901), Wilson (1901) regarded such
"artificial cytasters" as true asters with true centrosomes. Conklin
(1912), however, contends that they do not function in mitosis. It is
probable that no single conclusion can be drawn concerning this matter
which will apply to all cases. There seems to be good evidence for the
view that the centrosome in some tissues exists as a permanent cell organ,
dividing at each mitosis and remaining visible through the resting stages,

THE CENTROSOME AND THE BLEPHAROPLAST

79

at least for a number of cell generations. In other cases it disappears at
the close of mitosis, a new one being apparently formed just before the
next mitosis. The fact that the formation of centrosomes may be
brought about by artificial means suggests that the regular appearance of
the centrosome in successive mitoses is closely associated with regularly
recurring physiological conditions in the cell; and that its presence in
successive cell-divisions does not require an uninterrupted morphological
continuity through the intervening stages. Its constant presence in some
tissues probably indicates the continuity of some physiological function.
Centrosomes in Algae.1 — One of the earliest known centrosomes in
plants was that of the diatom Surirella, discovered by Smith (1886-7)
and Blitschli, and fully described by Lauterborn (1896) and Karsten
(1900). It lies near the nucleus, becomes surrounded by radiations, and
divides to form the central spindle of the mitotic figure in a very
peculiar manner.

FIG. 21. — Centrosomes in algse.
A, Stypocaulon. (After Swingle, 1897.) B, Stypocaulon.

(After Escoyez, 1909.) C,

eentrosphere-like bodies in Polysiphonia.
dichotoma. (After Mottier, 1900.)-

(After Yamanouchi, 1906.) D, E, Dictyota

Centrosomes in the Sphacelariaceae have been described by Humphrey
(1894), Swingle (1897), Strasburger (1900), and Escoyez (1909). In the
vegetative cells of Sphacelaria, according to Strasburger, the centrosome
is situated in a centrosphere at the focus of an aster. Previous to mitosis
it divides into two which take up positions at opposite poles of the
spindle. In Stypocaulon (Swingle) essentially the same condition exists
(Fig. 21). Escoyez later concluded, however, that the asters of Stypocau-
lon are formed independently rather than by division, and that the
central corpusclse are probably not true centrosomes, but cytoplasmic
microsomes.

1 This review of plant centrosomes and also that of the blepharoplast in subsequent
pages are based upon similar reviews given by the author in his paper on Spermato-
genesis in Equisetum (1912).

80 INTRODUCTION TO CYTOLOGY

In the oogonium and segmenting oospore of Fucus Farmer and
Williams (1896, 1898) described two centrospheres containing granules
and arising independently at opposite sides of the nucleus. Strasburger
(1897) reported definite centrosomes with asters all through mitosis.
In the sporeling he observed appearances indicating the division of the
centrosome, and concluded that the latter represents a permanent cell
organ. In a very detailed investigation Yamanouchi (1909) demon-
strated in the antheridium and oogonium two very definite centrosomes,
which appear independently of each other, become surrounded by con-
spicuous asters, and occupy the spindle poles during mitosis (Fig. 61, C).
He further showed that when the sperm reaches the egg nucleus a new
centrosome appears on the nuclear membrane at the point where the
sperm enters.

In Dictyota dichotoma Mottier (1898, 1900) states that in the two
divisions in the tetrasporocyte, in at least the first three or four cell
generations of the sporeling, and in all the vegetative cells of the tetra-
sporic plant curved rod-shaped centrosomes with asters occur at the
spindle poles, the two having arisen by the division of one during the
early phases of mitosis (Fig. 21, D, E). Williams (1904) further reports
that the entrance of the sperm causes a centrosome to appear in the egg
cytoplasm. Centrosomes in Nemalion were described by Wolfe (1904).

In Polysiphonia violacea (Yamanouchi 1906) there are present during
the prophases of every mitosis two centrosome-like bodies in the kino-
plasm at opposite sides of the nucleus. A little later the small bodies
disappear, while the kinoplasm takes the form of two large centrosphere-
like structures (Fig. 21, C); during the later stages of mitosis these fade
from view. Yamanouchi believes that these structures do not represent
permanent cell organs, but are formed de novo at the beginning of each
mitosis. Somewhat similar temporary centrospheres, with radiations
but no centrosomes, are present in the tetrasporocyte of Corallina (Davis
1898; Yamanouchi).

Fungi. — Among the fungi the best known centrosomes are those of the
Ascomycetes (Fig. 22). Harper (1895, 1897, 1899, 1905) described granu-
lar disc-shaped centrospheres surrounded by asters at the poles of the
spindle in the asci of Peziza, Ascobolus, Erysiphe, Lachnea, Phyllactinia,
and other genera. He regarded them as permanent organs of the cell.
In a recent paper (1919) he speaks of the ascomycete centrosome as a
structure differentiated "as a region of connection between nucleus and
cytoplasm and for the formation of fibrillar kinoplasm." Harper be-
lieved the ascospore walls to be formed by the lateral fusion of the curved
astral rays focussing upon the centrosome, a point disputed by Faull
(1905) and others. Centrosomes in additional genera were figured by
Guilliermond (1904, 1905). InGallactinia succosa (Marie 1905; Guillier-
mond 1911) a single centrosome, which arises within the nucleus with a

THE CENTROSOME AND THE BLEPHAROPLAST

81

cone of fibrils extending toward the chromatin, divides into two which
take up positions opposite each other at the nuclear membrane, at which
time asters develop in the cytoplasm. Faull (1905) found centrosomes in
Hydnobolites, Neotiella, and Sordaria; in the last named genus they
appear to be discoid while the cell is in the resting condition but round
and smaller during mitosis. In Humaria rutilans Miss Fraser (1908)
observed two centrosomes lying near each other, each at the apex of a
cone of fibers and surrounded by a faint aster. These move apart and

FIG. 22. — Centrosomes in ascomycetes.

A-C, Phyllactinia corylea: division of nucleus in ascus, showing behavior of centro-
somes. D, Erisiphe cichoracearum: formation of ascopore wall. (After Harper, 1905.)

establish the spindle in the usual manner. Centrosomes are also figured
in Ascobolus and Lachnea by Fraser and Brooks (1909); in Otidea and
Peziza by Fraser and Welsford (1908); in Microsphcera by Sands (1907);
and in Pyronema by Claussen (1912).

In the Basidiomycete Boletus (Levine 1913) the centrosomes present
during the last mitosis in the basidium attach themselves to the basidium
wall, and in close connection with them the daughter nuclei are recon-
structed. They mark the points of origin of the sterigmata and eventu-
ally pass into the spores.

82 INTRODUCTION TO CYTOLOGY

Bryophytes. — The first centrosome known in the liverworts was that
of Marchantia described by Schottlander (1893), according to whom the
centrosome in the spermatogenous cells divides during the anaphases of
mitosis, so that each daughter nucleus is accompanied by two (Fig. 27).
In the gametophytic cells certain minute bodies with radiations at the
poles of the elongated nucleus and of the spindle were believed by Van
Hook (1900) to represent centrosomes. Centrospheres with conspicuous
radiations but without true centrosomes were described in the mitoses of
the germinating spore of Pellia by Farmer and Reeves (1894), Davis
(1901), and Chamberlain (1903). Gregoire and Berghs (1904), however,
pointed out that the centrospheres observed by the foregoing writers in
Pellia are in reality only appearances due to the intersection of numerous
astral rays, and are not distinct bodies.

Wftw

^^/tasasy

B ""^— . J^' • "

FIG. 23. — Centrosomes in Preissia quadrata.

A, in fertilized egg just prior to nuclear fusion, B, in cells of young embryo. (After
Graham, 1918.)

In the cells of Preissia quadrata Miss Graham (1918) has more recently
made some observations of much interest. She describes and figures two
distinct centrosomes with a few astral rays in the cytoplasm of the fer-
tilized egg, at the time when the sexual nuclei are approaching each other
and in contact (Fig. 23, A). This, together with Yamanouchi's observa-
tion on Fucus and that of Williams on Dictyota, cited above, suggests
that in certain plants, as in animals, the formation of centrosomes and
asters in the egg cytoplasm is in some way induced by the entrance of the
sperm. Similar appearances have been noted by Meyer (1911) in Cor-
sinia and by Florin (1918) in Riccardia (Aneura). Centrosomes were
also observed by Miss Graham in the four-celled embryo of Preissia
(Fig. 23, B), this being one of the only cases in which centrosomes have
been seen in non-spermatogenous cells in plants above the algae.

Conclusion. — With regard to centrosomes in plants, it may bo con-
cluded from the above review that although there is no adequate evidence
for their existence in the cells of angiosperms, they are clearly present in
many algae, fungi, and probably certain bryophytes, where they perform

THE CENTROSOME AND THE BLEPHAROPLAST 83

definite functions in the life of the cell. The question of centrosomes in
the spermatogenous cells of bryophytes, pteridophytes, and gymnosperms
is dealt with in the following discussion of the blepharoplast.

THE BLEPHAROPLAST

Occurrence. — The blepharoplast, as indicated by the name given to
it by Webber (1897), is the cilia-bearing organ of the cell. Blepharo-
plasts of one kind or another are found generally in the motile cells of
plants and animals, such as motile unicellular organisms (Flagellata,
Ciliata, etc.), swarm spores, spermatozoa, and spermatozoids; and also
in cells which, though not freely motile themselves, have motile organs
performing other functions, as in the case of ciliated epithelium. In
plants blepharoplasts are most conspicuously displayed in the sperma-
togenous cells of bryophytes, pteridophytes, and gymnosperms (cycads
and Ginkgo), where their striking resemblance to ordinary centrosomes
has led to much controversy over their nature. Some cytologists have
regarded the blepharoplast as a more or less modified centrosome, where-
as others have contended that it is a special kinoplasmic or cytoplasmic
organ distinct from the centrosome. In recent years the evidence has
tended strongly to support the former view.

In the following pages the blepharoplasts of various organisms and
the manner in which they function in the development of the motor
apparatus will be described in some detail. Attention will be given
chiefly to the situations found in plants; the corresponding phenomena
in animals will be more briefly considered.

Flagellates. — In the flagellates several types of flagellar apparatus
are found (see Minchin 1912, pp. 82 ff ., 262-3) : in one series of forms
the cell contains a single nucleus and "centriole," the latter functioning
both as a centrosome and as a blepharoplast. The centriole may lie
either against or within the nucleus, so that the flagellum which grows
from it appears to arise directly from the nucleus (Mastigind); in other
forms (Mastigella) the centriole is quite independent of the nucleus.

In a second series of forms a single nucleus and centrosome are present,
and in addition one or more blepharoplasts. * Three conditions have been
distinguished here: (a) at the time of cell-division the blepharoplasts
and flagella are lost, new blepharoplasts arising from the centrosomes
during or after mitosis; (6) the blepharoplasts may persist, dividing to
form daughter blepharoplasts from which new flagella arise (Polytomella) ;
(c) the centrosome divides to furnish a blepharoplast which subdivides to
two: a distal blepharoplast or basal granule of the flagellum, and a
proximal blepharoplast or "anchoring granule" at the surface of the
nucleus, the two being connected by a delicate strand known as the
rhizoplast, rhizonema, or centrodesmose (Peranema trichophorum) . Entz
(1918) has recently reinvestigated the structure of Polytoma uvella, first

84

INTRODUCTION TO CYTOLOGY

described by Dangeard (1901), and finds the elaborate organization

shown in Fig. 24.

In a third series of forms two nuclei are present : a principal or trophic

nucleus and an accessory or kinetic
nucleus. Here there are apparently
three conditions : (a) a single centrosome,
associated with the kinetonucleus, acts
both as a blepharoplast and as a division
center; (6) usually both nuclei have
centrosomes associated with them, the

FIG. 24. FIG. 25.

FIG. 24.— Diagram of structure of Polytoma uvella. (After Entz, 1918.)
a, end-piece of flagellum. b, uniform portion of flagellum. c, lateronema. d, baso-
plast or basal granule, e, contractile vacuole. /, cell envelope, g, eyespot. h, rhizonema.
i, karyoplast or anchoring granule, j, centronema. k, nucleolus. I, nuclear membrane.
m, starch, n, surface of protoplast.

FIG. 25. — Trypanosoma theileri.

A, flagellum inserted on basal granule. B, formation of new flagellum from daughter
basal granule after division; nucleus dividing. (After Hartmann and Noller, 1918.)

one lying near or within the kinetonucleus acting as the blepharoplast;
(c) it is possible that in some cases there is a blepharoplast distinct from
the centrosomes accompanying the two nuclei.

In the trypanosomes (Fig. 25) the recent researches of Kuczynski
(1917) and Hartmann and Noller (1918) have shown that the flagellum

THE CENTROSOME AND THE BLEPHAROPLAST

85

is inserted on a "basal granule" (centrosome?) very near the "blepharo-
plast" (kinetonucleus?). At the time of cell-division the trophic nucleus,
blepharoplast, and basal granule all divide, the division of the blepharo-
plast showing certain features suggesting mitosis. Although earlier
investigators thought the flagellum also split, the above named workers
find that the old flagellum remains attached to one of the daughter basal
granules while a new flagellum grows out from the other daughter granule.

In flagellate organisms, therefore, the centrosome and the blepharo-
plast clearly stand in very intimate relationship with one another: in
some of the forms they are one and the same organ.

Thallophytes. — Among the earliest investigations of the blepharoplast
in algae were those of Strasburger (1892, 1900). During the development
of the zoospores of (Edogonium, Cladophora, and Vaucheria Strasburger

FIG. 26. — Blepharoplasts in Thallophytes.

A-D, formation of the cilia-bearing ring in the zoospore of Derbesia. (After Davis,
1908.) E, Stemonitis flaccida: cilia growing from centrosomes during late stage of division
in the formation of swarmers. (After Jahn, 1904.)

found that the nucleus approaches the plasma membrane, which at that
point forms a lens-shaped thickening. From this structure grow out the
cilia, and at the base of each a small refractive granule is present. The
blepharoplasts of the higher groups were believed by Strasburger to have
been derived from such swollen ectoplasmic organs of the algae, and that
all of them are morphologically distinct from centrosomes. Dangeard
(1898) likewise found a deeply staining granule at the base of the cilia
in Chlorogonium.

In Hydrodictyon (Timberlake 1902) the cilia are inserted on a small
body lying in contact with the plasma membrane and joined with the
nucleus by a delicate protoplasmic strand. The possible relationship
of this body with the granules seen occupying the spindle poles during
the formation of the spore cells was not determined. In the young

86

INTRODUCTION TO CYTOLOGY

zoospore cell of Derbesia (Davis 1908) the nucleus migrates toward the
plasma membrane, and from it many granules, which are not centrosomes,
move out along radiating strands of cytoplasm to the surface of the cell,
where by fusion they form a ring-shaped structure from which the cilia
develop (Fig. 26, A-D). In the developing spermatozoid of 'Chara
(Belajeff 1894; Mottier 1904) the blepharoplast arises as a differentiation
of the plasma membrane and bears two cilia. No centrosomes or other
granules were seen at the base of the cilia, although Schottlander (1893)
had previously reported centrosomes in the cells of the spermatogenous
filament.

In the zoospore of the fungus Rhodochytrium (Griggs 1904) there is a
deeply staining body at the insertion point of the cilia; this is connected
by fine cytoplasmic fibers with the nucleus. In the myxomycete Stemo-
nitis Jahn (1904) made an observation that is highly suggestive in con-
nection with the question of the relationship of the centrosome and the
blepharoplast. At the last mitosis in the formation of the swarmers the
spindle poles are occupied by centrosomes. and during the anaphases
the flagella of the resulting swarmers grow out directly from these cen-
trosomes (Fig. 26, E), just as in the spermatocytes of certain insects
(p. 95).

FIG. 27. — Spermatogenesis in Marchantia.

b, blepharoplast; c, centrosome; c. n., " chromatoider Nebenkorper ; " n, nucleus.
(After Ikeno, 1903.)

Bryophytes. — Among the bryophytes the blepharoplasts of Mar-
chantia and Fegatella (Conocephalus) have received much attention.
According to Ikeno (1903) a centrosome comes out of the nucleus at
each spermatogenous division in Marchantia and divides to form two
which separate to opposite sides of the cell, occupy the spindle poles,
and disappear at the close of mitosis : it is possible that they are included
in the daughter nuclei. After the last (diagonal) division, however, they
remain in the cytoplasm as the blepharoplasts, elongating and bearing
two cilia (Fig. 27). Another body, the chromatoider Nebenkorper, is

THE CENTROSOME AND THE BLEPHABOPLAST

87

also present in the cytoplasm. Similar in most points is the account
of Schaffner (1908). Miyake (1905), as the result of his studies on
Marchantia, Fegatella, Pellia, Aneura, and Makinoa, believes that such
liverwort centrosomes are merely centers of cytoplasmic radiation, and
inclines toward the view of Strasburger that the blepharoplast and the
centrosome are not homologous structures. Escoyez (1907) finds two
''corpuscles" appearing in contact with the plasma membrane in each
cell of the penultimate generation in the antheridia of Marchantia and
Fegatella; they occupy the spindle poles and function as blepharoplasts
in the spermatids (the cells which transform directly into spermatozoids).
Bolleter (1905) believes the cent rosome-1 ike body in Fegatella to arise
within the nucleus.

In the antheridium of Riccia Lewis (1906) reported centrosome-like
bodies in both the early and diagonal divisions. They apparently arise
de novo in the cytoplasm prior to each mitosis, showing no continuity
through succeeding cell generations except after the last mitosis, when
they persist and become the blepharoplasts.

..«r-;:r^:v „ X'.^^v.K

- •

b

FIG. 28. — Spermatogenesis in Blasia.
b, blepharoplast; n, nucleus. X 4200. (After Sharp, 1920.)

Woodburn (1911, 1913, 1915) has given accounts of spermatogenesis
in Porella, Asterella, Marchantia, Fegatella, Blasia, and Mnium. He
finds that the blepharoplast is first distinguishable as a special granule in
the cytoplasm of the spermatid, and holds that it represents, as Mottier
(1904) had formerly suggested, an individualized portion of the kinoplasm
arising de novo in certain spermatogenous cells. In a more recent con-
tribution (Sharp 1920) it has been shown that in Blasia (Fig. 28) a
blepharoplast is present at each spindle pole during all stages of the last
spermatogenous mitosis, and that in the spermatid it fragments as it

88 INTRODUCTION TO CYTOLOGY

becomes transformed into the cilia-bearing thread after the manner of
the blepharoplast of Equisetum, described below.

Spermatogenesis in Pellia, Atrichum, and Mnium has been described
by M. Wilson (1911). In Mnium and Atrichum the spermatogenous
divisions show no centrosomes, whereas in Pellia centrospheres, and
probably centrosomes, are present during the later mitoses. The origin
of the blepharoplast as here described is very peculiar. In the spermatid
of Mnium a number of bodies are said to separate from the nucleolus and
pass out into the cytoplasm where they coalesce to form a limosphere,
The nucleolus then divides into two masses, both of which pass into the
cj'toplasm; one functions as the blepharoplast and the other gives rise
to an accessory body. In Atrichum the first body separated from the
nucleolus becomes the blepharoplast, a second forms the limosphere, and
a third the accessory body. In all three plants the blepharoplast goes
to the periphery of the cell and grows out into a thread-like structure
along the plasma membrane. The nucleus then moves against this
( hread and the two grow together to form the spirally coiled spermatozoid.
Two cilia grow out from the anterior end of the blepharoplast.

The most detailed and critical of all researches on the motile cells
of bryophytes are those of C. E. Allen (1912, 1917) on Polytrichum
(Fig. 29). The first of these papers contains a description of the cyto-
logical phenomena accompanying the multiplication of the spermato-
genous cells (androgones} up through the last mitosis, which differentiates
the spermatids (androcytes) . In the cytoplasm of all the androgones
there is a deeply staining kinoplasmic mass; in the early androgones this
has the form of a flat plate, while in the later ones it consists of a group
of granules (kinetosomes). Prior to each mitosis the plate or group
divides to daughter plates or groups which pass to the daugher cells.
In the cells of the penultimate generation (androcyte mother-cells) there
are no kinetosomes, but instead a spherical "central body" with radia-
tions. This body divides into two which move apart and occupy the
spindle poles during the last mitosis. Each resulting androcyte therefore
has one such body, which functions as the blepharoplast. Allen does not
regard the kinetosomes as definite morphological entities, but rather as
masses of reserve kinoplasm. The blepharoplast, however, is a definite
cell organ, and although Allen inclines toward the view that it is the
homologue of a centrosome he regards the question as an open one.
Sapehin (1913) looks upon these bodies as plastids.

Allen's second paper deals with the transformation of the androcyte
(spermatid) into the spermatozoid. The blepharoplast elongates to
form a uniform rod and develops two cilia from near its anterior end.
The nucleus moves against the middle portion of the blepharoplast and
the two elongate together in close union to form the body of the sperma-
tozoid, the blepharoplast projecting beyond the anterior end of the

THE CENTROSOME AND THE BLEPHAROPLAST

89

nucleus. At about the time the blepharoplast begins to elongate a
limosphere appears in the cytoplasm and takes up a position near the
anterior end of the blepharoplast. Here it divides, giving rise to a small
apical body that remains visible at the end of the blepharoplast until a
comparatively late stage. The remaining portion of the limosphere may
be seen lying against the nucleus until the maturity of the spermatozoid

FIG. 29. — Spermatogenesis in Polytrichum.

A-D, androgones, showing behavior of kinoplasmic plates and kinetosomes (fc) during
imitoss. E-G, androcyte mother-cells, showing division of central body. H, telophase
of last mitosis; each androcyte has a blepharoplast (6). I-K, stages in the transformation
of the androcyte into the spermatozoid: a, apical body; /, limosphere; n, nucleus; p,
percnosome. L, mature spermatozoid. X 2535. (After Allen, 1912, 1917.)

Another body, the percnosome, is also seen in the cytoplasm at certain
stages. In the opinion of Allen the limosphere is probably identical with
the chromatoider Nebenkorper described by Ikeno in Marchantia, and the
percnosome with what M. Wilson (1911) terms the accessory body. The
apical body is here described for the first time by Allen.

Pteridophytes. — The early papers dealing with the spermatozoid of
pteridophytes, such as those of Buchtien (1887), Campbell (1887), Bela-

90

INTRODUCTION TO CYTOLOGY

Jeff (1888), Guignard (1889), and Schottlander (1893), give but little
information concerning the development of the blepharoplast. Our more
definite knowledge of this subject dates from 1897, when Belajeff published
three short papers. In the first of these (1897a) it was stated that the
fern spermatozoid consists of a thread-shaped nucleus and a plasma band,
with a great many cilia growing out from the latter. In the plasma band
is enclosed a thin thread which arises by the elongation of a small body
seen in the spermatid. In the second paper (18976) the blepharoplast of
Equisetum was first described as a crescent-shaped body lying against the
nucleus of the spermatid; this body stretches out to form the cilia-bearing
thread. The third contribution (1897c) is a short account of the trans-
formation of the spermatid into the spermatozoid in Chara, Equisetum,

FIG. 30. — Spermatogenesis in Equisetum arvense, showing behavior of blepharoplast
(eentrosome) in last spermatogenous mitosis and in transformation of spermatid into sper-
matozoid. X 1900. (After Sharp, 1912.)

and ferns. In all these forms a small body elongates to form a thread
upon which small swellings arise and grow out into cilia. In a comparison
with animal spermatogenesis Belajeff here homologized the blepharoplast,
the thread to which it elongates, and the cilia of the plant, with the
eentrosome, middle piece, and tail (perhaps only the axial filament),
respectively, of the animal. In the following year (1898) he figured the
details made out in Gymnogramme and Equisetum. In Gymnogramme the
two blepharoplasts appear at opposite sides of the nucleus in the spermatid
mother-cell, whereas in Equisetum a single blepharoplast is first figured
lying close to the nucleus of the spermatid. More recently it has been
shown (Sharp 1912) that the blepharoplast of Equisetum (Fig. 30) appears
first in the cells of the penultimate generation; there it divides to two
which separate and establish between them the achromatic figure after
the manner of animal centrosomes. At the close of mitosis the blepharp-

77/7? CENTROSOME AND THE. BLEPHAROPLAST

91

plast in each spermatid fragments into a number of pieces; these later
join to form a continuous beaded thread from which the cilia grow out.
In Equisetum the elongating nucleus and blepharoplast do not become
closely joined, but are held together only by the rather abundant cyto-
plasm.. The spermatozoid is multiciliate like that of all other pterido-
phytes with the exception of Lycopodium, Phylloglossum, and Selaginella:
in these three genera the spermatozoids are biciliate like those of the
bryophytes.

The most careful work on the blepharoplast of homosporous ferns is
that of Yamanouchi (1908) on Nephrodium (Fig. 31). In this form no
centrosomes are found. The two blepharoplasts,
which arise de novo in the cytoplasm of the sperm- f '"• ...

atid mother-cell, take no active part in nuclear
division, merely lying near the poles of the spindle.
In the spermatid the blepharoplast elongates spirally
in close union with the nucleus to form the body of
the spermatozoid. In Adiantum and Aspidium Miss
R. F. Allen (1911) and Thorn (1899) see the blepharo-
plast first in the spermatid.

One of the most interesting blepharoplasts is that
of Marsilia (Fig. 32), first described by Shaw (1898).
According to Shaw a small granule, or "blepharo-
plastoid," appears near each daughter nucleus of the
mitosis which differentiates the grandmother-cell of
the spermatid (the second of the four spermatogenous
mitoses). During the next (third) division the
blepharoplastoid divides but soon disappears, and a
blepharoplast appears near each spindle pole. In the
next cell generation (spermatid mother-cell) the
blepharoplast divides into two which are situated at
the spindle poles during the final mitosis. In the
spermatid the blepharoplast gives rise to several
granules by a sort of fragmentation; these together
form a thread which elongates spirally in close union
with the nucleus and bears many cilia. The spermatozoid is of the usual
fern type, with several coils and a cytoplasmic vesicle. Shaw saw in the
foregoing facts no ground for the homology of the blepharoplast and the
centrosome. Belajeff (1899) found that centrosomes occur at the spindle
poles during all, excepting possibly the first, of the four divisions which
result in the 16 spermatids. He reported that after each mitosis the
centrosome divides into two which occupy the spindle poles during the
succeeding mitosis, and in the .spermatids perform the usual functions of
blepharoplasts. Belajeff regarded this as a strong confirmation of his
theory that the blepharoplast and centrosome are homologous organs.

FIG. 31 . — Two
stages in the sperma-
togenesis of Nephro-
dium.

A, blepharoplasts
near poles of spindle
in last spermatogen-
ous mitosis. B, elon-
gation of blepharo-
plast near nucleus.
Nebenkern at left.
(After Yamanouchi,
1908.)

92

INTRODUCTION TO CYTOLOGY

The results of Shaw were in the main confirmed by the later work of
Sharp (1914), who, however, saw in the achromatic structures accom-
panying the blepharoplast striking evidence in favor of BelajefF s view
of its homology. In line with this conclusion the suggestion has recently
been made (Sharp 1920) that the fragmentation of the blepharoplast
in Blasia, Equisetum, Marsilia, and the cycads may be homologized with
the normal division exhibited by ordinary centrosomes.

FIG. 32. — Spermatogenesis in Marsilia quadrifolia.

A, first spermatogenous mitosis; no centrosomes. B, second mitosis, centrosomes
present. C, third mitosis; centrosomes present; old centrosome divided and degenerating
in cytoplasm. D, penultimate spermatogenous cell; daughter centrosomes separating.
E, last spermatogenous mitosis; blepharoplasts (centrosomes) becoming vacuolate. F, frag-
mentation of blepharoplast in spermatid. G, transformation of spermatid into spermato-
zoid. H, free swimming spermatozoid. X 1400. (After Sharp, 1914.)

Gymnosperms. — The first known blepharoplast in plants above the
algae was discovered in Ginkgo by Hirase in 1894. He observed two, one
on either side of the body cell nucleus, and because of their great simi-
larity to certain structures in animal cells he believed them to be attrac-
tion spheres. In 1897 Webber observed the same bodies, and noted their
cytoplasmic origin. On account of certain differences between these
organs and ordinary centrosomes he expressed the opinion that they are
not true centrosomes, but distinct organs of spermatogenous cells.
The blepharoplast of Ginkgo was later investigated by Fujii (1898, 1899,
1900) and Miyake (1906).

THE CENTROSOME AND THE BLEPHAROPLAST

93

In 1897 and 1901 Webber described the blepharoplast of Zamia (Fig.
33). Up to the time of the division of the body cell the two blepharo-
plasts, which arise de novo in the cytoplasm, are surrounded by radiations,
but they have no part in the formation of the spindle, which is entirely
intranuclear. During mitosis they lie opposite the poles, increase greatly
in size, become vacuolate, and break up to many granules : these in the
spermatid coalesce to form a spirally coiled cilia-bearign band lying just
inside the cell membrane. In his full account (1901) Webber gives an
extensive discussion of the homology of the blepharoplast.

•

& ' --*<&

tyF*"^**^,'...-

FIG. 33. — Spermatogenesis in Zamia.

A-E, five stages in the vacuolation and fragmentation of the blepharoplast during the
mitosis differentiating the spermatids. F, the two spermatozoids in the end of the pollen
tube; prothallial and stalk cells below. Compare Fig. 34. A-D, X 350; E, X 1200.
(After Webber, 1901.)

In Ikeno's (1898) account of gametogenesis and fertilization in Cycas
it was shown that the blepharoplasts appear in the body cell, lie opposite
the spindle poles during mitosis, and break up to granules which fuse to
form the spiral band in a manner similar to that described by Webber for
Zamia. The behavior of the blepharoplast in Microcycas (Caldwell
1907) is essentially the same.

Chamberlain (1909) observed in the cytoplasm of the body cell of
Dioon (Fig. 34) a number of very minute "black granules" which he was
inclined to believe originate within the nucleus. Very soon two undoubted
blepharoplasts are present, and are apparently formed by the enlarge-

94

INTRODUCTION TO CYTOLOGY

tnent of two of the black granules. Very conspicuous radiations develop
about them, and after mitosis they form ribbon-like cilia-bearing bands
in the spermatids as in the other cycads.

*-;,'•-

. .-' ". v^fepSi*stt^

*•-•' •"••'•?••''••••• 'i:''^%^%$$3&&

FIG. 34. — Spermatogenesis in Dioon edule.

A, "body cell," with black granules in cytoplasm. X 1890. B, two blepharoplasts
differentiated. X 1890. C, body cell with two blepharoplasts; prothallial and stalk cells
below. X 237. D, fragmentation of blepharoplast in spermatid as spiral band begins to
form. X 1890. E, portion of edge of spermatozoid, showing spiral band cut at two points
and cilia growing from it. X 945. (After Chamberlain, 1909.)

Ikeno in 1898 expressed the opinion that the blepharoplast of Ginkgo
and the cycads is a true centrosome, a view shared by Chamberlain (1898)
and Guignard (1898). Two additional papers dealing with this subject
were published by Ikeno (1904, 1906). In the first of these Ahe made
comparisons with analogous phenomena in animals which he believed to

THE CENTROSOME AND THE BLEPHAROPLAST 95

sustain the homologies suggested by Belajeff. He pointed out that in
Marchantia centrosomes are present in all the spermatogenous divisions,
whereas in other liverworts they appear much later, and from this he
argued that the bryophytes show various stages in the elimination of the
centrosome. He strongly reasserted his belief that blepharoplasts are
centrosomes, and spoke of the "transformation of a centrosome into a
blepharoplast " in the development of a spermatic! into a spermatozoid.
The ectoplasmic blepharoplasts of the algae were also held to be derived
from centrosomes. In the second paper he insisted less strongly upon the
morphological identity of all blepharoplasts, separating them into three
categories : (1) centrosomatic blepharoplasts, including those of the myxo-
mycetes, bryophytes, pteridophytes, and gymnosperms; (2) plasmoder-
mal blepharoplasts, including those of Cham and some Chlorophyceae;
(3) nuclear blepharoplasts, found only in a few flagellates.

For a further discussion of this question the student is referred to
the present author's papers on Equisetum and Marsilia. The main
conclusions reached may be stated in two extracts from the former paper :

Although limited to a single mitosis in the antheridium, the blepharoplast
[of Equisetum} retains in its activities the most unmistakable evidences of a
centrosome nature, and at the same time shows a metamorphosis strikingly like
that in the cycads. In thus combining the main characteristics of true centro-
somes with the peculiar features of the most advanced blepharoplasts, it reveals
in its ntogeny an outline of the phylogeny of the blepharoplast as it is seen
developing through bryophytes, pteridophytes, and gymnosperms, from a func-
tional centrosome to a highly differentiated cilia-bearing organ with very few
centrosome resemblances.

The activities of the blepharoplast in Equisetum [Marsilia, and Blasia],
taken together with the behavior of recognized true centrosomes in plants and
analogous p henomena in animals, are believed to constitute conclusive evidence
in favor of the theory that the blepharoplasts of bryophytes, pteridophytes, and
gymnosperms are derived ontogenetically or phylogenetically from centrosomes.

Animals.— The early researches of Moore (1895), Meves (1897, 1899),
Korff (1899), Paulmier (1899), and many other more recent investigators
have established the fact that the centrosome (or centrosomes) of the
animal spermatid plays an important role in the formation of the motor
apparatus of the spermatozoon, the axial filament of the tail growing out
directly from it (Fig. 35). Henneguy (1898) even saw flagella attached
to the centrosomes of the mitotic figure in the spermatocyte of an insect,
an observation which has been often repeated. Wilson (1900, p. 175)
concludes that "the facts give the strongest ground for the conclusion
that the formation of the spermatozoids agrees in its essential features
with that of the spermatozoa . . . " and that the blepharoplast is
without doubt to be identified with the centrosome.

96

INTRODUCTION TO CYTOLOGY

Although there is comparatively little question that the granule at the
base of the flagellum in the flagellates, like the body from which the axial
filament of the spermatozoon grows, is of centrosomic nature, the nature
of the basal granules in Ciliata and in the ciliated epithelial cells of higher
animals is much more difficult to determine. It was held by Henneguy
(1897), Lenhossek (1898), Hertwig (1902), and others that these granules,
like the basal granules of flagella, are modified centrosomes; whereas
certain other investigators (Maier 1903, Studnicka 1899, Schuberg 1905)
have found evidence in favor of a contrary interpretation. An extensive

FIG. 35. — Spermatogenesis in Helix pomatia,
showing growth of flagellum from outer centro-
some, and elongation of inner centrosome to
form axial filament of middle piece. (After
Korff, 1899.)

FIG. 36. — Diagram of a ciliated epi-
thelial cell. (Constructed from figures of
Saguchi, 1917.)

discussion of this question is given by Erhard (1911), who concludes that
although the basal corpuscles arise from the nucleus in a manner similar
to that of the centrosomes of such cells, the evidence is on the whole
unfavorable to the theory of Henneguy and Lenhossek.

Still more recent are the researches of Saguchi (1917), who describes
in great detail the insertion of the cilia in epithelial cells. At the base
of each cilium, which itself shows no internal structural differentiation,
there is always a basal corpuscle (Fig. 36) . These corpuscles, and hence
the cilia, are in parallel rows ; and beneath each row there is a transparent
zone in which the rootlets of the cilia are anchored, and through which
they pass and become continuous with strands of the cytoplasmic recti-
culum. Cilium, corpuscle, rootlet, and cytoplasmic strand form one
continuous structure. Saguchi believes that neither the cilium nor the
rootlet causes the ciliary movement, but that the kinetic center of this
movement is in the basal corpuscles, as Henneguy and Lenhossek thought.
Contrary to the opinion of those authors, however, he regards the ciliary

THE CENTROSOME AND THE BLEPHAROPLAST 97

apparatus as entirely independent of centrosomes, holding rather that it
is produced by the differentiation of chondriosomes, and that the resem-
blance of ciliated cells to spermatids, in which centrosomes do produce
the motor apparatus, is an accidental one.

Conclusion. — In conclusion it may be said that it is highly probable
that cilia-bearing structures are not homologous in all plant and animal
groups. It is beyond question that in animals the centrosomes of the
spermatid produce the motor apparatus of the spermatozoon. That a
similar interpretation is to be placed upon the blepharoplasts in the sper-
matids (androcytes) of bryophytes, pteridophytes, and gymnosperms
appears to be equally well demonstrated. The blepharoplasts of the
flagellates are also probably centrosomic in nature, at least in certain
cases. In the " plasmodermal blepharoplasts" of motile alga cells we
have organs which, in the light of our present knowledge, do not appear
to belong to the centrosomic category, but final disposition of them must
await further information concerning those algae which possess both cen-
trosomes and blepharoplasts. It can scarcely be doubted that the basal
corpuscles of ciliated cells represent organs belonging to various categories.
It must be left for further research to determine just how far these
structures, which are functionally analogous, are homologous with each
other and with other cell organs.

Bibliography 5

Centrosome and Blepharoplast

ALLEN, C. E. 1912. Cell structure, growth and division in the antheridia of Poly-

irichum juniperinum Willd. Arch. Zellforsch. 8: 121-188. pis. 6-9.
1917. The spermatogenesis of Polylrichum juniperinum. Ann. Bot. 31: 269-292.

pis. 15, 16.
ALLEN, R. F. 1911. Studies in spermatogenesis and apogamy in ferns. Trans. Wis.

Acad. Sci. 17: 1-56. pis. 1-6.
BELAJEFF, W. 1888. Ueber Bau and Entwicklung der Spermatozoiden der Gefass-

kryptogamen. Ber. Deu. Bot. Gesell. 7: 122-125.
1894. Ueber Bau und Entwicklung der Spermatozoiden der Pflanzen. Flora 79 :

Erganzb.: 1-48. pi. 1.
1897a. Ueber den Nebenkern in spermatogenen Zellen und die Spermatogenese

bei den Farnkrautern. Ber. Dea. Bot. Ges. 15 : 337-339.

18976. Ueber die Spermatogenese bei den Schachtelhalmen. Ibid. 15: 339-342.
1897c. Ueber die Aehnlichkeit einiger Erscheinungen in der Spermatogenese bei
Thieren und Pflanzen. Ibid. 15 : 342-345.

1898. Ueber die Cilienbildner in spermatogenen Zellen. Ibid. 16: 140-144. pi. 7.

1899. Ueber die Centrosome in den spermatogenen Zellen. Ibid. 17: 199-205.
pi. 15.

VAN BENEDEN, E. 1876. Recherches sur les Dicyemides, survivants actuelles d'un
embranchement des Mesozoaires. Bull. Acad. Roy. Belg. 41: 1160-1205; 42:
35-97. pis. 3.

1883. Recherches sur la maturation de 1'oeuf, la fecondation et la division cellu-
laire. Arch. d. Biol. 4.

98 INTRODUCTION TO CYTOLOGY

1887. (van Beneden et Neyt.) Nouvelles recherches sur la fecundation et la
division mitosique chez i'Ascaride megalocephale. Bull. Acad. Roy. Belg. Ill 14:
215-295. pi. 1.

BOLLETER, E. 1905. Fegatelia (L.> corda. Einc morphologisch-physiologische
Monographie. Beih. Bot. Centr. 18: 327-408. pis. 12, 13.

BOVERI, TH. 1887a. Ueher die Befruchtung der Eier von Ascaris megalocephala.

Sitz-ber. Ges. Morph. Phys. Miinchen 3.
18876. Ueber den Anteil des Spermatozoon an der Teilung des Eies. Ibid.

1888. Zellenstudien. 11. Jenaische Zeitschrift 22: 685-882. pis. 19-23

1901. Zellenstudien. IV. Ueber die Natur der Centrosomen. Jen. Zeitschr.

36 : 1-220. pis. 1-8.
BRATJER, A. 1893. Zur Kenntniss der Spermatogenese von Ascaris megalocephala.

Arch. Mikr. Anat. 42: 153-212. pis. 11-13.
BUCHTIEN, O. 1887. Entwicklungsgeschichte des Prothalliums von Equiselum.

Bibliotheca Botanica 8 : 1-49. pis. 1-6.
BUTSCHLI, O. 1891. Ueber die sogenannten Centralkorper der Zelle und ihre

Bedeutang. Verh. Naturhist.-Med. Ver. Heidelb. 4: 535-538.

CALDWELL, O. W. 1907. Microcycas calocoma. Bot. Gaz. 44: 118-141. pis. 10-13.
CAMPBELL, D. H. 1887. Zur Entwicklungsgeschichte der Spermatozoiden. Ber.

Deu. Bot. Gesell. 5: 120-127. pi. 6.
CHAMBERLAIN, C. J. 1898. The homology of the blepharoplast. Bot. Gaz. 26 :

431-435.

1903. Mitosis in Pellia. Decenn. Pub. Univ. Chicago 10: 329-345. pis. 25-27.
1909. Spermatogenesis in Dioon edule. Bot. Gaz. 47: 215-236. pis. 15-18.
CHAMBERS, R. 1917. Microdissection studies. II. The cell aster. A reversible

gelation phenomenon. Joar. Exp. Zool. 23: 483-504.
CLAUSSEN, P. 1912. Zur Entwicklungsgeschichte der Ascomyceten. Zeitschr.

Bot. 4: 1-64. pis. 1-6. figs. 13.
CONKLIN, E. G. 1912. Experimental studies on nuclear and cell division in the

eggs of Crepidula. Jour. Acad. Nat. Sci. Phila. 15: 503-591. pis. 43-49.
DANGEARD, P. A. 1896. Memoire sur les Chlamydomonadinees on 1'histoire d'une

cellule. Le Botaniste 6: 65-290. figs. 19.
1901. Etude sur la structure de la cellule et ses fonctions. Le Polytoma uvella.

Ibid. 8 : 5-58. figs. 4.
DAVIS, B. M. 1898. Kerntheilung in der Tetrasporemnutterzelle bei Corallina

officinalis L. var. mediterranea. Ber. Deu. Bot. Gesell. 16: 266-272. pis. 16, 17.

1908. Spore formation in Derbesia. Ibid. 22 : 1-20. pis. 1, 2.

ENTZ, G. 1918. Ueber die mitotische Teilung von Polytoma uvella. Arch. f.

Protistenk. 38: 324-354. pis. 12, 13. figs. 5.
ERHARD, H. 1911. Die Henneguy-Lenhosseksche Theorie. Ergeb. Anat. u. Entw.

19: 893-929. (List of 146 papers.)
ESCOYEZ, E. 1907. Blepharoplaste et centrosome dans le Marchantia polymorpha.

La Cellule 24: 247-256. pi. 1.

1909. Caryocinese, centrosome et kinoplasme dans le Stypocaulon scoparium. La
Cellule 25: 181-201. 1 pi.

.FARMER, J. B. and REEVES, J. 1894. On the occurrence of centrospheres in Pellia

epiphytta, Nees. Ann. Bot. 8: 219-224. pi. 14.
FARMER, J. B. and WILLIAMS, J. L. 1896. On fertilization, and the segmentation

of the spore in Fucm. Ibid. 10: 479-487.

1898. Contributions to our knowledge of the Fucaceae; their life history and cyt-
ology. Phil. Trans. Roy. Soc. London B 190: 623- 645. pis. 19-24.
FAULL, J. H. 1905. Development of ascus and spore formation in ascomycetes.
Proc. Boston Soc. Nat. Hist. 32: 77-113. pis. 7-11.

THE CENTROSOME AND THE BLEPHAROPLAST 9(J

FLEMMING, W. 1875. Studien in der Entwicklungsgeschichte der Najaden. Sitz-

ber. Akad. Wiss. Wien 71.
FLORIN, R. 1918. Das Archegoniurn der Riccardia pinguis (L) B. Gr. Svensk. Bot

Tidskr. 12 : 464-470. figs. 4.
FOL, H. 1877. Sur le commencement de 1'henogenie chez divers animaux. Arch.

Sci. Nat. u. Phys. Geneve 68.
1891. Die " Centrenquadrille," ein neue Episode aus der Befruchtungsgeschichto.

Anat. Anz. 6 : 266-274. figs. 10.
FRASER, H. C. 1. 1908. Contributions to the cytology of Humaria rutilans. Ann.

Bot. 22 : 35-55. pis. 4, 5.
FRASER, H. C. I. and WELSFORD, E. J. 1908. Further contributions to the cytology

of the ascomycetes. Ibid. 22: 465-477. pis. 26, 27.
FRASER, H. C. I. and BROOKS, W. E. ST. J. 190S. Further studies on the cytology

of the ascus. Ibid. 23 : 538-549.
FUJII, K. 1898. (Has the spermatozoid of Ginkgo a tail or not?) Bot. Mag.

Tokyo 12 : 287-290. (Japanese.)

1899. (On the morphology of the spermatozoid of Ginkgo biloba.) Ibid. 13: 260-
266. pi. 7. (Japanese.)

1900. (Account of a sperm with two spiral bands.) Ibid. 14: 16-17. (Japanese.)
GRAHAM, M. 1918. Centrosomes in fertilization stages of Preissia quadrata (Scop.)

Nees. Ann. Bot. 32 : 415-420. pi. 10.
GREGOIRE, V. et BERGHS, J. 1904. La figure achromatique dans le Pellia epiphylld.

La Cellule 21 : 193-238. pis. 1, 2.
GRIGGS, R. F. 1912. The development and cytology of Rhodochylrium. Bot. Gaz.

63: 127-173. pis. 11-16.
GUIGNARD, L. 1889. Developpement et constitution des antherozoides. Rev.

G<5n. Bot. 1: 11-27, 63-78, 136-145, 175-194. pis. 2-6.

1898. Centrosomes in Plants. Bot. Gaz. 25: 158-164.

GUILLIERMOND, A. 1904. Recherches sur la karyokinese chez les ascomycetes.

Rev. Gen. Bot. 16: 129-143. pis. 14, 15.
1905. Remarques sur la karyokine&e des ascomycetes. Ann. Mycol. 3: 344-361.

pis. 10-12.
1911. Apercu sur 1'evolution nucleaire des ascomycetes et nouvelles observations

sur les mitoses des asques. Rev. Gen. Bofc. 23: 89-121. figs. 8. pis. 4, 5.
HARPER, R. A. 1895. Beitrag zur Kenntniss der Kernteilung und Sporenbildung

im Ascus. Ber. Deu. Bot. Ges. 13: (67)-(68). pi 27.
1897. Kerntheilung und freie Zellbildung im Ascus. Jahrb. Wiss. Bot. 30 : 249-

284. pis. 11, 12.

1899. Cell division in sporangia and asci. Ann. Bot. 13: 467-525. pis. 24-26.
1905. Sexual reproduction and the organization of the nucleus in certain mildews.

Carnegie. Inst. Publ. 37. Washington.

1919. The structure of protoplasm. Am. Jour. Bot. 6: 273-300.
HARTMANN, M. und NOLLER, W. 1918. Untersuchungen iiber die Cytologie von

Trypanosoma theileri. Arch. Protistenk. 38: 355-374. pis. 14, 15. figs. 6.
HENNEGUY, L. F. 1897. Sur les rapports des cils vibratiles avec les centrosomes.

Arch. d'Anat Micr. 1 : 481-496. figs. 5.
HIRASE, S. 1894. Notes on the attraction spheres in the pollen cells of Ginkgo biloba.

Bot. Mag. Tokyo 8 : 359.
HUMPHREY, J. E. 1894. Nucleolen und Centrosomen. Ber. Deu. Bot. Ges. 12 :

108-117. pi. 6.
HUMPHREY, H. B. 1906. The development of Fossombronia longiseta Austr

Ann. Bot. 20: 83-108. pis. 5, 6. figs. 8.

100 INTRODUCTION TO CYTOLOGY

IKENO, S. 1898. Untersuchungen iiber die Entwicklung der Geschlechtsorgane und
den Vorgang der Befruchtung bei Cycas revoluta. Jahrb. Wiss. Bot. 32 : 557-602.
pis. 8-10.

1903. Die Sperm atogenese von Marchantia polymorpha. Beih. Bot. Centralbl.
15:65-88. pi. 3.

1904. Blepharoplasten im Pflanzenreich. Ibid. 24: 211-221. figs. 1-3.

1906. Zur Frage nach der Homologie der Blepharoplasten. Flora 96 : 538-542.
JAHN, E. Myxomycetenstudien. 3. Kernteilung und Geisselbildung bei den

Schwarmern von Stemonitis flaccida Lister. Ber. Deu. Bot. Gesell. 22 : 84-92.

pi. 6
KARSTEN, G. 1900. Die Auxosporenbildung der Gattungen Cocconeis, Surirella,

und Cymatopleura. Flora 87 : 253-283. pis. 8-10.
KOERNICKE, M. 1903. Der heutige Stand der pflanzlichen Zellforschung. Ber.

Deu. Bot. Gesell. 21: (66)-(134).

1906. Zentrosomen bei Angiospermen? Flora 96: 501-522. pi. 5.
VON KORFP, K. 1899. Zur Histogenese der Spermien von Helix pomalia. Arch.

Mikr. Anat. 54: 291-296. pi. 16.
KTJCZYNSKI, M. H. 1917. Ueber die Teilung der Trypanosomenzelle nebst Bemer-

kungen zur Organization einiger nahestehender Flagellaten. Arch. f. Protistenk.

38:94-112. pis. 3, 4.
LAUTERBORN, R. 1896. Untersuchungen iiber Bau, Kernteilung, und Bewegung

der Diatomen. Leipzg.

VON LENHOSSEK, M. 1898. Ueber Flimmerzellen. Verh. Anat. Ges. Kiel 12: 106.
LEVINE, M. 1913. The cytology of Hymenomycetes, especially the Boleti. Bull.

Torr. Bot. Club 40: 137-181. pis. 4-8.
LEWIS, C. E. 1906. Embryology and development of Riccia lutescens and Riccia

crystailina. Bot. Gaz. 41 : 109-138. pis. 5-9.
MAIER, H. N. 1903. Ueber die feineren Bau der Wimperapparate der Infusorien.

Arch. f. Protistenk. 2.
MAIRE, R. 1905. Recherches cytologiques sur quelques ascomycetes. Ann. Mycol.

3:123-154. pis. 3-5.
MEAD, A. D. 1898. The origin and behavior of the centrosomes in the annelid egg.

Jour. Morph. 14: 181-218.
MEVES, F. 1897. Ueber Struktur und Histogenese der Samenfaden von Salamandra

maculosa. Arch. Mikr. Anat. 50: 110-141. pis. 7, 8.
1899. Ueber Struktur und Histogenese der Samenfaden des Meerschweinchens.

Ibid. 64: 329-402. pis. 19-21. figs. 16.
MEYER, K. 1911. Untersuchungen tiber den Sporophyt der Lebermoose. I. Ent-

wicklungsgeschichte des Sporogons der Corsinia marchantioides. Bull. Soc. Imp.

Moscou 236-286.

MINCHIN, E. A. 1912. An Introduction to the Study of the Protozoa. London.
MIYAKE, K. 1905. On the centrosome of Hepaticse. Bot. Mag. Tokyo 19: 98-

101.
1906. The spermatozoid of Ginkgo. Jour. Appl. Micr. and Lab. Methods 5 :

1773-1780. figs. 10.
MOORE, J. E. S. 1895. Structural changes in the reproductive cells during spermato-

genesis of elasmobranchs. Quar. Jour. Micr. Sci. 38: 275-313. pis. 13-16.
MORGAN, T. H. 1896. The production of artificial astrospheres. Arch. Entw. 3:

339-361. pi. 19.
1899. The action of salt solutions on the unfertilized and fertilized eggs of Arbacia

and other animals. Ibid. 8: 448-539. pis. 7-10. figs. 21.

MOTHER, D. M. 1898. Das Centrosom bei Dictyota. Ber. Deu. Bot. Ges. 16:
123-128. figs. 5.

THE CENTROSOME AND THE BLEPHAROPLAST 101

1900. Nuclear and cell division in Dictyota dichotoma. Ann. Bot. 14: 166-192.

pi. 11.

1904. The development of the spermatozoid of Chora. Ibid. 18 : 245-254. pi. 17.
PAULMIER, F. C. 1899. The spermatogenesis of Anasa Iristis. Jour. Morph. 16 :

Suppl. 223-272. pis. 13, 14.
RAWITZ, B. 1896. Untersuchungen iiber Zelltheilung. I. Arch. Mikr. Anat. 47:

159-180, pi. 11.

SAGUCHI, S. 1917. Studies on ciliated cells. Jour. Morph. 29: 217-279. pis. 1-4.
SANDS, M. C. 1907. Nuclear structure and spore formation in Microsphcera.

Trans. Wis. Acad. Sci. 16 : 733-752. pi. 46.
SAPEHIN, A. A. 1913. Untersuchungen iiber die Individuality der Plastide. Ber.

Deu. Bot. Ges. 31 : 14-66. fig. 1.
SCHAFFNER, J. H. 1908. The centrosomes of Marchantia potymorpha. Ohio Nat.

9. 363-388.
SCHOTTLANDER, P. 1893. Beitragc /ur Kenntniss des Zellkerns und der Sexual-

zellen bei Kryptogamen. Cohn's Beitr. Biol. Pflanzen 6 : 267-304. pis. 4, 5.
SHARP, L. W. 1912. Spermatogenesis in Equisetum. Bot. Gaz. 64: 89-119. pis.

7,8.

1914. Spermatogenesis in Marsilia. Ibid. 58: 419-431. pis. 33, 34.
1920. Spermatogenesis in Blasia. Ibid. 69: 258-268. pi. 15.
SHAW, W. R. 1898. Ueber die Blepharoplasten bei Onoclea und Marsilia. Ber.

Dea. Bot. Ges. 16: 177-184. pi. 11.
SMITH, H. L. 1886-1887. A contribution to the life history of the Diatomacese.

Proc. Am. Soc. Micr. Pts. I and II.
STRASBTJRGER, E. 1892. Schwarmsporen, Gameten, pflanzliche Spermatozoiden,

und das Wesen der Befruchtung. Hist. Beitr. 4 : 49-158. pi. 3.
1897. Kerntheilung und Befruchtung bei Fucus. Jahrb. Wiss. Bot. 30 : 351-374.

pis. 27, 28.

1900. Ueber Reduktionstheilung, Spindelbildung, Centrosomen, und Cilienbildner
im Pflanzenreich. Hist. Beitr. 6: 1-224. pis. 1-4.

STUDNICKA, F. K. 1899. Ueber Flimmer- und Cuticularzellen mit besonderer

Beriicksichtigung der Centrosomenfrage. Sitz-Ber. K. Bohmisch. Ges. Wiss.

Math.-Naturwiss. Classe, 36.
SWINGLE, W. T. 1897. Zur Kenntniss der Kern- und Zelltheilung bei den Sphacela-

riaceen. Jahrb. Wiss. Bot. 30: 296-350. pis. 15, 16.
THOM, C. 1899. The process of fertilization in Aspidium and Adiantum. Trans

Acad. Sci. St. Louis 9: 285-314. pis. 36-38.
TIMBERLAKE, H. G. 1902. Development and structure of the swarm spores of

Hydrodictyon. Trans. Wis. Acad. Sci. 13: 486-522. pis. 29, 30.
VAN HOOK, J. M. 1900. Notes on the division of the cell and nucleus in liverworts,

Bot. Gaz. 30 : 394-399. pis. 2, 3.
WEBBER, H. J. 1897a. Peculiar structures occurring in the pollen tube of Zamia.

Bot. Gaz. 23: 453-459. pi. 40.

18976. The development of the antherozoid of Zamia. Ibid. 24: 16-22. figs. 5.
1897c. Notes on the fecundation of Zamia and the pollen tube apparatus of

Ginkgo. Ibid. 24: 225-235. pi. 10.

1901. Spermatogenesis and fecundation in Zamia. U. S. Dept. Agr. Pit. Ind
Bull. 2. pp. 100. pis. 7.

WILLIAMS, J. L. 1904. Studies in the Dictyotaceae. II. The cytology of the game-

tophyte generation. Ann. Bot. 18: 183-204. pis. 12-14.
WILSON, E. B. 1900. The Cell in Development and Inheritance, (p. 175.)

1901. Experimental Studies in Cytology. I. A cytological study of partheno

genesis in sea urchin eggs. Arch. Entw. 12: 529-596. pis. 11-17.

102 INTRODUCTION TO CYTOLOGY

WILSON, M. 1911. Sperm atogenesis in the Bryophyta. Ann. Bot. 25: 415-457.

pis. 37, 38. figs. 3.
VON WINIWARTER, H. 1912. Observations cytologiques sur les cellules interstiti-

elles du testicule humaine. Anat. Anz. 41 : 309-320. pis. 1-2.
WOLFE, J. J. 1904. Cytological studies on Nemalion. Ann. Bot. 18: 607-630.

pis. 40, 41. fig. 1.
WOODBURN, W. L. 1911. Spermatogenesis in certain Hepaticse. Ann. Bot. 25:

299-313. pi. 25.

1913. Spermatogenesis in Blasia pusilla. Ibid. 27: 93-101. pi. 11.
1915. Spermatogenesis in Mnium affine, var. ciliaris (Grev.), C.M. Ibid. 29:

441-456. pi. 21.
YAMANOUCHI, S. 1906. The life history of Polysiphonia violacea. Bot. Gaz. 42 :

401-448. pis. 19-28.

1908. Spermatogenesis, oogenesis, and fertilization in Nephr odium. Ibid. 45 :
145-175. pis. 6-8.

1909. Mitosis in Fucus. Ibid. 47: 173-197. pis. 8-11.

ZIMMERMANN, A. 1893-1894. Sammel-Referate. 6. Die Centralkorper und die
Kerntheilung. 12. Die Cilien und Pseudocilien. Beih. Bot. Centralbl. 3 :
342-354; 4: 169-171.

CHAPTER VI
PLASTIDS AND CHONDRIOSOMES

PLASTIDS

Next to the nucleus, the most conspicuous organ held within the
cytoplasm of the plant cell is the plastid. Cytologists have long been
aware of the important physiological roles played by plastids of various
types in the life of the cell, but it is only recently that an added inter-
est has been given these organs by the discovery that certain peculiar
characters showing definite modes of inheritance are closely bound up
with their behavior. Such problems are complicated by the relation
apparently borne by plastids to chondriosomes. In the present chap-
ter will be set forth some of the more important facts regarding these
two classes of cell elements.

General Nature and Occurrence. — Plastids are differentiated portions
of the protoplasm, as von Mohl long ago pointed out, ' and represent
regions in which certain processes have become localized (Harper 1919).
In view of their power of growth and division and their definite relation to
certain important physiological functions they are to be regarded as
distinct cell organs.

Although plastids can be found in the cells of both animals and plants
they are chiefly characteristic of the latter, where they are present in
one form or another in all groups with the possible exception of bacteria,
myxomycetes, and certain fungi. They are abundant only in those plant
parts which have to do with specialized physiological functions. Within
a single cell there may be regularly but one plastid, as in many algse,
Anthoceros, and the meristematic cells of Selaginella (Haberlandt 1888,
1905) ; or two, as in Zygnema; or a /figher number, as in the green tissues/
of most higher plants. They lie imbedded in the cytoplasm and are
often closely associated with the nucleus; they are never found normally
in the vacuole. The positions which they assume within the cell are fre-
quently related in a definite manner to certain external conditions: in
the palisade cells of green leaves, for example, the chloroplasts are found
near the upper surface if the incident light is weak, whereas they react to
strong illumination by taking up less exposed positions along the lateral
walls.

Plastids may be conveniently classified on the basis of their contained
coloring matters. This difference in color, however, is secondary in
importance; the fundamental distinction is that based upon the kind of

103

104

INTRODUCTION TO CYTOLOGY

physiological work being done, the various pigments being associated in
an intimate manner with different reactions occurring within the plastids.

Leucoplasts. — Leucoplasts are relatively small and colorless. They
are found commonly in the cells of meristematic tissue, and may be
retained in some kinds of differentiated cells, such as the glandular hairs of
Pelargonium. K lister (1911) states that the leucoplasts of Orchis are
very fluid in consistency, undergoing amoeboid changes of shape and
multiplying by irregular fission. The smaller leucoplasts appear to
represent juvenile stages in the development of plastids of more highly
differentiated types, for under certain conditions they develop into the
larger and more highly specialized leucoplasts known as amyloplasts, and
into the various kinds of chromatophores mentioned below.

Chromatophores. — Chromatophores, or chromoplasts, are plastids bear-
ing one or more pigments, and having thus a more or less decided
color. In green plants the most important of these pigments are chlo-

FIG. 37. — Various forms of plastids.

A, Draparnaldia. B, Spirogyra. C, Anthoceros. D, chromoplasts of Arisoema. E,
cell of Selaginella, showing position assumed by plastid in response to light (direction shown
by arrow). A, B, and C show pyrenoids. (E After Haberlandt.)

rophyll, carotin, and xanthophyll. Chlorophyll is apparently a combi-
nation of two simpler pigments, chlorophyll a and chlorophyll 6. The
cells of the Phaeophyceae, Cyanophyceae, and Rhodophyceae are character-
ized'respectively by the presence of yellow carotin, blue phycocyanin,
and red phycoerythrin, in addition to chlorophyll. The Cyanophyceae
exhibit an especially rich variety of pigments, which in many cases do not
appear to be held within definite chromatophores.1

Chromatophores are usually spherical, ovoid, or discoid in shape, but
many peculiar forms are known, particularly among the green algae. In
Ulothrix the chloroplast has the form of a complete or incomplete hollow
cylinder; in Draparnaldia (Fig. 37, A), a hollow cylinder with very

1 For the literature pertaining to plant pigments see Palladin 1918. See also
Haas and Hill (1913), Willstatter and Stoll (1913), Jorgensen and Stiles (1917),
Wheldale (1916), and Beauverie (1919). The distribution of carotin is discussed in
an earlier paper by Tammes (1900).

PLASTIDS AND CHONDRIOSOMES 105

irregular ends; in dEdogonium, an irregular parietal net; in Spirogyra
(Fig. 37, B), a spirally coiled ribbon; and in the desrnids, a series of
radiating plates (Carter 1919, 1920). The chromatophore of Antho-
ceros (Fig. 37, C) is spindle-shaped, becoming chain-like in the elongated
columella cells (Scherrer 1914). The chromatophore of Selaginella
may also assume this form (Haberlandt 1888). The chromoplasts of
Arisoema (Fig. 37, D) are frequently sharply angular. In the Clado-
phoraceae (Carter 1919) the cell is completely lined by a thin chromato-
phore which may be entire or fenestrated. In many cells irregular strands
pass inward through the cell cavity. Indeed it seems not improbable
that in some such cases the plastid may be not at all sharply distinct
from the rest of the cytoplasm, the two grading one into the other, and
the chlorophyll at certain stages permeating all parts of the cytoplasm.
The observations of Timberlake and Harper appear to show that such is
the condition in the young cells of Hydrodictyon. Thus the physiological
processes show various degrees of localization in the cell, causing manifold
degrees of structural transformation and delimitation of the cytoplasmic
regions involved (Harper).

Of all chromatophores the chloroplasts stand first in importance, for
they bear the green pigment, chlorophyll, which, in the presence of
light, enables them to combine water from the soil or other surrounding
medium with carbon dioxide from the atmosphere to form carbohydrates,
the first visible product being starch. The chloroplasts are therefore
the world's ultimate food producers. In addition to chlorophyll other
pigments, notably xanthophyll, are usually present. Although the body
of the chloroplast can be developed in darkness, the chlorophyll will
usually not be elaborated unless light is present. Most young seedlings
grown in the absence of light show a pale yellowish color, which is due to a
substance known as chlorophyllogen, contained in the plastids. When
such "etiolated" plants are placed in the light the plastids become green,
apparently through an alteration of the chlorophyllogen to chlorophyll
( Monte verde and Lubimenko 1911). Other conditions necessary for the
development of chlorophyll are a favorable temperature and the presence
of iron, oxygen, and certain carbohydrates.

The structure of the chloroplast is an extremely difficult matter to
determine, and has been the subject of some controversy. It is generally
thought that the body of the plastid is composed of a finely fibrillar
meshwork, the stroma, which may be somewhat denser at the periphery,
and that the coloring matters are held in the meshes of the stroma in the
form of minute droplets. No limiting membrane is definitely known.
The included droplets are apparently not composed of the pigments alone :
it is probable that they are rather globules of some oily or fatty material
containing the pigments in solution. The pigments may easily be dis-
solved out with alcohol and other reagents. On the other hand, it has

106

been held by some observers that the stroma is a homogeneous body in
which tlie droplets of chlorophyll solution are imbedded, and that the
reticular structure so often reported is an artifact due to the reagent
employed in removing the chlorophyll. By others the pigment has been
thought to form a layer about the plastid. In any case it seems evident
that the chlorophyll is not uniformly distributed throughout the stroma.
In chromatophores other than chloroplasts the pigments may at times
take the form of solid granules or crystals.

Starch. — After a period of photosynthetic
activity the chloroplast contains starch, the first
visible product of that activity, in the form of
minute granules. This "assimilation starch" is
formed within the body of the chloroplast, as
Meyer originally showed (Fig. 38, A, B). It is
later transformed through the agency of enzymes
into some soluble compound, usually a sugar; in
this form it may be carried to growing regions,
where, after further changes, it is built into the
structure of the plant. Or, it may pass to storage
organs where it is transformed into the ordinary
"reserve starch," or "storage starch." This de-
position of reserve starch is brought about through
the agency of amyloplasts, which are leucoplasts
capable of changing already elaborated organic
materials, such as glucose, into starch (Fig. 38, C).
Reserve starch, upon which we depend so
largely for food, is a carbohydrate with a composi-
tion expressed by the general formula (C6Hi0O5)n,
and exists in the form of granules ranging in size
approximately from 2//. to 200/j in different plants.
Potato-starch grains are usually about 90^ in
diameter. The reserve starch grain is formed
within the body of the arnyloplast, and is made
up of a series of concentric layers successively
laid down about a center, or "hilum" (Fig. 39, A).1 In case the grain
starts to form near the middle of the amyloplast it may develop sym-
metrically, but commonly the developing grain lies near the periphery
of the amyloplast, which becomes greatly distended as the grain grows.
Material is thus deposited unevenly upon the grain so that the latter
becomes very eccentric; in extreme cases the grain ruptures the amylo-
plast and remains in contact with it only at one side, where all new
material is then deposited. Several grains may start to develop simul-

1 For the structure of the starch grain see the papers of Nageli, Schimper, Meyer,
Binz, Dodel, Salter, and Kramer.

FIG. 38. — Formation of
starch by plastids.
A, dividing chloro-
plasts of Funaria, with
grains of assimilation
starch. X 940. (After
Strasburger.) B, chloro-
plast of Zygnema, with
several large starch
grains about a central
pyrenoid. (After Bour-
quin, 1917.) C, leuco-
plast (amyloplast) in
aerial tuber of Phajus
grandifolius with grain
of reserve starch. (After
Strasburger.)

PLASTIDS AND CHONDRIOSOMES

107

taneously in a single amyloplast, later growing together to form :i
"compound grain " with more than one hilum. In case the parts making
up the compound grain are enveloped in one or more common outer
layers the grain is said to be "half-compound." Potato starch is made
up of simple, compound, and half-compound grains, whereas in oats
and rice all or nearly all of the grains are said to be of the compound type.
The successively deposited layers making up the grain differ mainly in
water content, the innermost layers being richest and the outermost
poorest in water. As a result of this non-uniform composition the grain
often splits radially when dried.

'•[

FIG. 39. — Reserve starch grains from various plants.
A, potato; simple and half-compound grains. B, Colombo starch. C, arrowroot.

D, pea. E, maize; intact and partially digested grains. F, rye. G, maize.
/, bean. J, rice. K, wheat. (After Tschirch.)

H, Euphorbia.

As a result of his classic researches Nageli (1858) advanced the theory
that the starch grain is made up of ultramicroscopic crystalline particles
which he called " micellae/' these being surrounded by water films of
varying thickness. It was similarly held by A. Meyer (1883, 1895)
that the grain is composed of radially arranged needle-shaped crystals
known as "trichites;" these are composed of a- and /3-amylose which
turn blue with iodine. In some starch amylodextrin and dextrin are
also present, such grains turning red with iodine. Both Nageli and Meyer
held the stratification of the grain to be due to the varying numbers of
the crystalline units in the successive layers, and Meyer showed that in
certain cases it is correlated with the alternation of day and night, and
therefore with a periodic activity on the part of the plastid. This con-
clusion was confirmed by Salter (1898).

The statement made by Schimper (1880) and Meyer (1883, 1895) that
starch is always formed by plastids still holds good in its essential feature :
so far as is certainly known no primary product of photosynthesis is
formed in the cytoplasm apart from plastids, although in some cases,
such as the young cells of Hydrodictyon, according to Harper, it is very
difficult or even impossible to distinguish the limits of these organs. The

108 INTRODUCTION TO CYTOLOGY

granules of paramylum in Euglena and those of "Floridean starch" in
the red alga? first appear in the cytoplasm; but, although they arc the
first substances which are visible, it is highly probable that they arise
through the transformation of a non-visible product (sugar?) of the
photosynthetic activity of the plastids, and are not immediately built
up from water and carbon dioxide. A similar interpretation may be
placed upon corresponding appearances reported in the case of higher
plants. Owing to the great difficulty of determining the true cell struc-
ture of the Cyanophyceae (see p. 202) it is possible to speak of plastid
activity in such forms only with great reserve. If, as Olive (1904) and
Gardner (1906) hold, these cells are without plastids, the product of
photosynthetic activity, commonly glycogen, must be elaborated in the
cytoplasm without their aid. If, on the other hand, the peripheral
portion of the protoplast represents a large chromatophore (Fischer 1898),
or cytoplasm containing a large number of minute chromatophores
(Hegler 1901, Kohl 1903, Wager 1903), the photosynthetic process,
although it may result in the production of a different substance, is
dependent upon the powers of definite protoplasmic organs much the
same as in higher plants. Among bacteria and other low forms in which
it seems more certain that plastids and the ordinary pigments are absent,
widely different types of metabolism are met with. For further discus-
sion of this subject, which lies outside the scope of the present book,
more special physiological works should be consulted.

The Pyrenoid. — The term pyrenoid was applied by Schmitz (1882)
to the refractive kernel-like bodies imbedded in the chromatophores of the
algae. Pyrenoids are characteristic of the Chlorophycese especially,
being present almost universally in the members of this group. They
are known in a few representatives of the IJiodophyceae (Nemalion
and the Bangiacese), but apparently do not occur in the cells of the
Cyanophyceae, Phaeophyceae, and Characeae. Very rarely they are
present in forms above the algae : a conspicuous example is the liverwort
Anthoceros. The chromatophore may contain but one pyrenoid, as in
Zygnema, or a larger number, as in Spirogyra, Draparnaldia, and many
other forms (Fig. 37).

As held by de Bary (1858), Schmitz (1884), and Schimper (1885),
the pyrenoid appears to be composed of a protein substance with a thick
gelatinous consistency. When a single pyrenoid is present in the chroma-
tophore it may multiply by fission along with the latter when the cell
divides, while in those forms possessing several pyrenoids this multiplica-
tion may be much more extensive. Also, as pointed out by Schmitz
and Schimper, and more recently by Smith (1914), the pyrenoid may
disappear and arise de novo from the cytoplasm or from the plastid
protoplasm.

With regard to its function, the early workers referred to above ob-

PLASTIDS AND CHONDRIOSOMES 109

served that under certain conditions the pyrenoid is closely surrounded
by a mass of starch grains, and concluded that it is an organ, or portion of
an organ (chromatophore), intimately concerned in the process of starch
formation, its action being somewhat similar to that of the amyloplast.
The pyrenoid, in fact, has often been likened to a leucoplast imbedded in
the chromatophore; Wiesner, for instance, believed the pyrenoid to
contain several leucoplast bodies, each of which gave rise to a starch
grain. In general, more recent researches have emphasized the close
association of the pyrenoid with the starch forming process, although
the precise nature of this process remains very much in doubt. Accord-
ing to Timberlake (1901) the pyrenoid in Hydrodictyon is differentiated
from the cytoplasm and is very active in starch production, segments
splitting off from its periphery and forming starch within them. In
this way the entire pyrenoid may become a mass of "pyrenoid starch,"
as distinguished from ordinary, or "stroma starch." McAllister (1913)
describes a similar splitting up of the pyrenoid to form several starch
grains in Tetraspora. Yamanouchi (1913), however, in his description
of a new species of Hydrodictyon, states that some of the chloroplasts
give rise to starch while others give rise to pyrenoids, and that the
latter have nothing to do with starch formation.

A similar diversity of opinion exists with respect to the role of the
pyrenoid in Zygnema. Chmielewskij (1896), who looked upon the
pyrenoid as a permanent cell organ multiplying only by division, held
that starch grains arise wholly from the substance of the pyrenoid,
plate-like extensions of the latter being present between and in intimate
contact with the developing grains. More recently Miss Bourquin
(1917) asserts that the pyrenoid has nothing to do with the appearance
of starch, the body of the chromatophore alone being concerned. She
observes the starch grains appearing first near the periphery of the
chromatophore entirely apart from the pyrenoid, the later formed grains
differentiating in positions progressively nearer the pyrenoid (Fig. 38, B).

The pyrenoid of Anthoceros (Fig. 37, C) as described by McAllister
(1914) is in reality a group of about 25-300 small "pyrenoid bodies"
which are probably composed of a protein substance. The outermost
bodies become starch, new ones apparently being formed by the fission
of those lying on the interior of the group. McAllister states that no
pyrenoid is visible in the young sporogenous tissue, starch being formed
without its aid. Somewhat later several small bodies appear and
aggregate to form the pyrenoid.

Cleland (1919) recently reports a close association of the pyrenoid
of Nemalion with the formation of Floridean starch.

Elaioplasts and Oil Bodies. — In 1888 Wakker discovered in the cells
of Vanilla planifolia and V. aromatica certain plastid-like bodies to
which he gave the name elaioplasts, since they seemed to be concerned in

110

INTRODUCTION TO CYTOLOGY

the elaboration of oil (Fig. 40, A}. They were soon observed in a number
of monocotyledons by Zimmermann (1893), Raciborski (1893), and Kiis-
ter (1894); and some time later in the flower parts of a dicotyledon,
Gaillardia, by Beer (1909). Politis (1914) has found them in monocoty-
ledonous plants belonging to 19 different genera, and in five genera of
dicotyledons (Malvaceae).

There is a considerable lack of agreement in the opinions expressed
on the subject of the origin and significance of elaioplasts. Wakker
thought it probable that they represent meta-
morphosed chloroplasts, which they often closely
resemble in structure (Kiister), whereas Raciborski
asserted that they arise as small refractive gran-
ules in the cytoplasm and multiply by budding.
In the zygospores of Sporodinia grandis and
Phycomyces nitens Miss Keene (1914, 1919) reports
the presence of a number of globular structures
with which oil is associated from their earliest
stages. These unite to form one or two large
reticulate bodies which Miss Keene believes are
related to the elaioplasts of higher plants. All of
these investigators, with Politis, agree that elaio-
plasts are normal cell organs with a special func-
tion, namely, the formation of oily substances
having a role in nutrition. Beer, on the contrary,
states that in Gaillardia they are formed secon-
darily by the aggregation of many small degen-
erating plastids and their products at one or
more points in the cell, all stages of the process
being observed. Although the bodies so formed
may, if green, produce starch, or, if colorless, an
oily yellow pigment, Beer thinks it probable that
they have no important special function in the life
of the plant.

Closely associated with investigations on elaioplasts have been those
concerned with the oil bodies found in the cells of many liverworts (Fig.
40, B). These bodies, discovered by Gottsche in 1843, were first carefully
described by Pfeffer (1874). Pfeft'er stated that they arise by the fusion
of many minute droplets of fatty oil appearing in the cytoplasm of very
young cells, and later come to lie in the cell sap; he further believed them
to possess a special membrane. Wakker (1888) held them to be analo-
gous to leucoplasts and chloroplasts, multiplying by fission at each
cell-division, and pointed out that they lie in the cytoplasm rather than in
the cell sap. He was inclined to view them as products of elaioplasts,
which Ktister (1894) supposed them to resemble in having a spongy
stroma containing oil in the form of minute droplets.

FIG. 40.

A, elaioplast forming
oil droplets in epidermal
cell of perianth of Poli-
anthes tuberosa; nucleus
with small plastids at
right. (After Politis,
1914.) B, oil bodies in
various stages of develop-
ment in a cell of Caly-
pogeia. (After Gargeanne,
1903.)

PL AST IDS AND CHONDRIOSOMES 111

Quite different were the views of Gargeanne (1903). According to
him they arise from vacuoles, their limiting membranes thus being the
original tonoplasts. While in the juvenile vacuole stage they may multi-
ply by division, but when once fully formed they remain unchanged and
divide no further. Gargeanne observed small oil droplets moving about
freely within the oil body, and hence concluded that the latter has a
fluid consistency rather than a spongy stroma as Kiister thought.

The most noteworthy recent observations on oil bodies are those of
Rivett (1918), who finds them to be very conspicuous in the cells of
Alicularia scolaris. Rivett holds that they are in reality only oil vacuoles
—that they originate by the coalescence of numerous minute oil droplets
secreted by the protoplasm in a manner entirely similar to that in which
the ordinary sap vacuole arises (cf. Pfeffer). Although they become very
large and project well into the sap vacuole, they continue to be surrounded
by a thin film of cytoplasm. The oil body, in the opinion of Rivett, is
therefore in no sense a plastid, nor is it formed by any special elaioplast :
it is simply an accumulation of ethereal and fatty oils together with some
protein substance. The "membrane" observed by Pfeffer is the limiting
layer of the surrounding cytoplasm, which may be slightly changed by
contact with the oil.

Accumulations of oil apparently quite similar to those in liverwort
cells have been described in the cells of various angiosperms by a number
of writers. To these the term elaiospheres was applied by Lidforss (1893).

The published figures of elaioplasts and oil bodies in many cases
bear striking resemblance to those of fat- and oil-secreting chondrio-
somes (see below), and it is not improbable that the problem of their
origin and significance will be brought nearer solution by further studies
of the latter class of bodies.

The Eyespot. — The so-called eyespot present in the flagellate cell and
in the zoospores and gametes of many algae has certain characteristics
in common with plastids, and may therefore receive consideration here.
This body, which nearly all workers agree is a light-sensitive organ, is an
elongated or circular and flattened structure lying in the anterior region
of the cell (flagellates) or near its lateral margin, usually in close associa-
tion with the chromatophore and the plasma membrane. (Overtoil
1889; Klebs 1883, 1892; Johnson 1893; Strasburger 1900; Wollenweber
1907, 1908). With respect to its mode of origin, it has been variously
reported to arise de novo in each newly formed zoospore in several green
algae (Overton); to develop from a colorless plastid in the young
antheridial cell in the case of the spermatozoid of Fucus (Guignard 1889) ;
to arise as a differentiated portion of the plastid in the zoospores and
gametes of Zanardinia ( Yamanouchi) ; and finally to multiply by fission
at the time of cell-division in flagellates (Klebs 1892).

It is generally agreed that the eyespot in many instances consists of

112

INTRODUCTION TO CYTOLOGY

a finely reticulate stroma in which an oily red pigment with many of the
characteristics of hsematochrom is held in the form of minute droplets or
granules (Schilling 1891; Klebs 1883; Franze" 1893; Wager 1900; Wollen-
weber 1907, 1908) (Fig. 41, D). As shown by the careful researches of
Franze", the stroma may also bear one or more refractive inclusions,
which in the Chlamydomonadaceae and Volvocacese consist of starch, and

in the Euglenoidese of paramylum (Fig. 41,
E). These inclusions were thought by
Franze to increase the sensitivity of the eye-
spot by concentrating the light at certain
points.

The eyespot of the zoospore of Cladophora
(Strasburger 1900) appears to arise as a
swelling of the plasma membrane, and consists
of an external pigmented layer beneath which
is a lens-shaped mass of hyaline substance
(Fig. 41, B}. InGonium and Eudorina (Mast
1916) the lens-shaped portion lies outside with
the cup-shaped opaque portion beneath it
(Fig. 41, A), an arrangement strongly sug-
gesting the primitive eyes of certain higher
organisms. In neither portion could any finer
structure be detected. Mast has shown that
the orientation of the colony is brought about
through changes in the intensity of the light

As

the unoriented swimming colony rotates on
its axis, those zooids turning away from the
light have the hyaline portion of their eye-

-Eyespots of various

types.

A, zooid of Eudorina; e, eye-
spot. (From Mast, After Grave.)

B, zoospore of Cladophora, falling upon the light-sensitive substance.

(After Strasburger, 1900.) C,
anterior end of Euglena viridis,
showing eyespot at surface of
oesophagus, and in front of it a
swelling on one root of the

flageiium;face view of eyespot spots shaded by the opaque cup; this sudden

reduction in the amount of light energy
received brings about an increase in the
activity of the flagellae of those zooids, with

and crystalloid body.
Frame'.)

(After

at right, showing pigment gran-
ules. (After Wager, 1900.) D,
eyespot of Euglena velata.
(After Franze, 1893.) E, eye-
spot of Trachelomonas volvo-

dna, with pigment granules the result that the colony as a whole turns

more directly toward the source of light.

In Euglena viridis the morphological con-
nection between the eyespot and the motor apparatus is particularly
close. Here Wager (1900) has shown that the eyespot, which is a
discoid protoplasmic body containing a layer of large pigment droplets,
is situated at the surface bounding the oesophagus in close contact with a
swelling on one of the basal branches of the flagellum (Fig. 41, C).

In general it may be concluded that the eyespot in some cases bears in
its structure, and to a certain extent in its evident function, such a close
resemblance to the ordinary plastid that a relationship of some sort

PLASTIDS AND CHONDRIOSOMES 113

between the two seems highly probable ; whereas in other cases (Gonium,
Cladophora) it appears to represent a differentiation of the ectoplast.
It is more than likely that light-sensitive organs have arisen more than
once in the evolution of the lower organisms, and that they cannot all
be placed in the same category.

The Individuality of the Plastid. — It was believed by the early
observers, notably Schimper (1883) and Meyer (1883), that plastids
never originate de novo but always arise from preexisting plastids
by division. Fully differentiated plastids, such as chloroplasts, can
readily be seen multiplying in this manner in growing tissues with a
frequency sufficient to account for the large number of plastids present
in mature plant parts. Since it is known, however, that chloroplasts
and other differentiated chromoplasts may arise from leucoplasts through
the development of pigments and other characters in the latter, and also
that the individual plant arises from sex cells or a spore in which the
plastids are usually in a colorless and relatively undifferentiated state, the
problem of the individuality of the plastid is mainly one of determining
whether these undifferentiated plastids, leucoplasts, or "plastid primor-
dia" later developing into chloroplasts and other types are continuous
through the critical stages of the life cycle, multiplying only by division,
or arise de novo as new differentiations of the cytoplasm. At this point
we may review certain cases in which the plastid has been followed
through gametogenesis and fertilization.

In Zygnema (Kurssanow 1911) each vegetative cell contains one
nucleus and two plastids, all of which divide at each vegetative cell-
division. In sexual reproduction the entire protoplast, with its nucleus
and two plastids, passes through the conjugating tube as a "male"
gamete and unites with a similar complete protoplast ("female" gamete)
of another filament. The two nuclei fuse, giving the primary nucleus of
the new individual (zygospore nucleus), while the two plastids contrib-
uted by the "male" gamete degenerate, leaving the two furnished by
the "female" gamete as the plastids of the new individual.

In Coleochcete (Allen 1905) each vegetative cell and gamete has one
nucleus and one plastid: after the sexual union of the gamete nuclei the
fertilized egg therefore contains one nucleus and two plastids. These two
plastids divide at the first division of the fertilized egg but not at the
second, the four resulting cells consequently having one plastid each.
In the third cell-division the plastids also divide, so that each cell of
the several-celled structure developing from the fertilized egg has its
single plastid. Each of the several cells eventually becomes a zoospore
which germinates to produce a new Coleochcete body with a single plastid
in each cell, the plastid dividing with the nucleus at each cell-division.

A somewhat similar regularity in the behavior of the plastid is shown
in Anthoceros (Davis 1899; Scherrer 1914). Each gametophytic cell

114

INTRODUCTION TO CYTOLOGY

contains a single plastid which divides with the nucleus at each cell-
division. The egg likewise contains a plastid, but the spermatozoid has
none: the fertilized egg and sporophyte cells which it later forms are
therefore characterized, like the cells of the gametophyte, by the presence
of one plastid. Although it is difficult to demonstrate the plastid in the
young sporogenous cells, every sporocyte shows one very clearly. As
shown by Davis (Fig. 42), the sporocyte plastid divides twice during the
prophases of the first (heterotypic) division of the sporocyte nucleus, so
that each spore of the resulting tetrad receives one. Upon germination
the spore produces a gametophyte with one plastid in each cell, and the
cycle is complete.

FIG. 42. — The behavior of the plastid in the sporocyte of Anthoceroa.
A, sporocyte with single nucleus and plastid. B, plastid divided; nucleus in prophase
of mitosis. C, plastids divided to four; two nuclei present. D, three of the four spore
cells, each of which has a single nucleus and plastid. (After Davis, 1899.)

In all of the foregoing examples it is evident that the plastids, as
stated by Scherrer for Anthoceros, remain as morphological individuals
throughout the whole life cycle, multiplying exclusively by division. A
similar claim is made for the plastids of mosses by Sapehin (1915), who
has also studied the behavior of the plastids in Selaginella and Isoetes
(1911, 1913). In such cases the plastids each possess an individuality com-
parable to that of nuclei, from which they differ conspicuously, however,
in undergoing no fusion at the time of fertilization. The constancy in
number is nevertheless maintained : by the degeneration of the plastids of
one gamete in Zygnema; by their failure to divide at one cell-division in
Coleochcete; and because of the fact that the male gamete carries no
plastid in Anthoceros. It appears to be generally true that while the eggs
in all plant groups contain plastids (usually leucoplasts), the latter are
present in male gametes in the algae only. Sapehin (1913), however,
believes that the blepharoplasts of the higher groups represent plastids.

It should be said that only in a comparatively few forms has such a
regularity in the behavior of the plastid as that outlined above been
demonstrated. A number of investigators, working on a great variety of
cells, have been forced to conclude that plastids are either formed de novo
as well as by division, or are carried through certain stages of the life

PLASTIDS AND CHONDRIOSOMES 115

cycle in some less conspicuous form. If they represent regional trans-
formations of the cytoplasm resulting from the localization of certain
processes, they might well be expected to differentiate anew as these
processes begin in the life of the cell, and to preserve varying degrees
of permanence depending upon the processes carried on (Harper).
Their individual continuity through certain life cycles would accordingly
be interpreted to mean that in such forms there is a persistence of certain
types of physiological activity through all stages.

In recent years a number of cytologists have described the develop-
ment of plastids from minute granular primordia in the cytoplasm, and
have attempted to show that these primordia are members of the class of
cell inclusions known as chondriosomes. A general theory of the indi-
viduality of the plastid must therefore involve the question of the relation
of plastids to chondriosomes, and the further question of the origin of the
chondriosomes themselves. These matters will be taken up in the fol-
lowing pages.

CHONDRIOSOMES

Notwithstanding the large amount of work which has been done upon
chondriosomes during recent years, the condition of opinion as to their
origin, behavior, and significance is still so unsettled that little more than
a review and partial classification of the more prominent views will
here be attempted.

Chondriosomes were probably first observed many years ago by
Flemming and Altman in the course of their studies on protoplasm.
They were first clearly described by La Vallette St. George (1886), who
observed them in the male cells of animals and called them "cytomicro-
somes." In plants they were first described by Meves (1904) in the
tapetal cells of the anthers of Nymphcea (Fig. 43, B). Benda in 1897
and the following years discovered them in cells of many types, notably
in the spermatogenous cells of animals, and applied to them the term
"mitochondria." It was not until a decade later, through the researches
of Meves, Regaud, Faure^Fremiet, Lewitski, Guilliermond, and others
that they came into prominence. Since that time they have been very
intensively studied by both zoologists and botanists, and a special
literature of considerable bulk has developed.1 It now seems evident
that the filaments ("fila") of Flemming, the "bioplasts" of Altman, the
" plastidules " of Maggi, the " archoplasmic granules" of Boveri, and the
"mitochondria" of Benda are all one and the same thing — chondriosomes
(Duesberg 1919).

General Nature and Occurrence. — Chondriosomes occur in the cyto-
plasm of the cell, commonly in the form of minute granules, rods, and

1 Reviews of the subject are given by Duesberg (1911, 1919), Schmidt (1912),
Cavers (1914). and Guilliermond (1919). See also Meves (1918).

116

INTRODUCTION TO CYTOLOGY

threads, but also in a great variety of irregular shapes (Fig. 43). At
present it is customary with the majority of workers to refer to all types
as chondrio somes or mitochondria. For those which are definitely rod-
and thread-shaped the terms chondriokonts and chondriomites are also
used. It is not to be thought that the various forms constitute distinct
Masses, for several investigators (N. H. Cowdry; M. and W. Lewis 1915)
have observed the chondriosomes undergoing marked changes in shape
in living cells, granular ones becoming rod-shaped and filamentous, and
vice versa. Schaxel (1911) and Kingery (1917) state, moreover, that in
fixed material the shape of the chondriosomes is to a certain extent
dependent upon the character of the microtechnical methods employed.

FIG. 43. — Chondriosomes in plant and animal cells.

A, nerve cell from guinea pig. X 480. (After E. V. Cowdry, 1914.) B, tapetal cell
of Nymphaea alba. (After Meves, 1904.) C, living epidermal cell of tulip petal. D, ascus
of Pustularia vesiculosa. E, hypha of Rhizopus nigricans. F, portion of embryo sac of
Lilium; chondriosomes clustered about nucleus. G, cell of root tip of Allium — (C-F.
After Guilliermond, 1918.)

Although when first discovered chondriosomes were believed to be
rather limited in distribution, they have now been reported in the cells of
plants and animals belonging to nearly all of the larger natural groups.
It is asserted by N. H. Cowdry (1917) that "in all forms of animals,
from amoeba to man, which have been investigated with adequate
methods of technique, they occur without exception." They are present,
furthermore, in the cells of all tissues. In plants it is probable that they
are no less universally present, although it has not yet been possible to
demonstrate them with certainty in bacteria, Cyanophycese, and certain
Chlorophycese, such as the Conjugate and Confervales (Guilliermond
1915). They are abundant in myxomycetes (N. H. Cowdry 1918),
Charales (Mirande 1919), brown and red algae, fungi, and all the higher
groups.

A critical comparison of the chondriosomes of plants with those of
animals has been made by N. H. Cowdry (1917), who concludes, contrary

PLASTIDS AND CHONDRIOSOMES 117

to the opinion of Pensa (1914), that there is every reason to regard them
as homologous in the two kingdoms. He finds plant and animal chon-
driosomes to be practically identical in morphology, reaction to fixatives
and dyes, and distribution in resting and dividing cells : any conspicuous
differences in arrangement seem to be due to the more pronounced
polarity of the animal cell. In both cases they are most abundant in
the active stages in the life of the cell. As the cell ages and becomes
fully differentiated, i.e., as cytomorphosis proceeds, they diminish in
number and may completely disappear.

Physico-chemical Nature. — With regard to the chemical and physical
nature of chondriosomes, Regaud (1908), Faure-Fremiet (1910), and
Lowschin (1913), working respectively on mammals, protozoa, and plants,
agree that they are chemically a combination of phospholipin and albu-
min. They closely resemble phosphatids, which are combinations of
phosphoric and fatty acids, glycerol, and nitrogen bases. Lecithin is
such a compound. Since chondriosomes are soluble in alcohol, ether,
chloroform, and dilute acetic acid, many of the fixing reagents commonly
employed in microtechnique destroy them: this accounts in part for the
fact that they were not observed in many familiar tissues until a compara-
tively recent date. They are well fixed by neutral formalin, potassium
bichromate, osmium tetroxid, and chromium trioxid (chromic acid) ; and
these, therefore, are the principal ingredients of the fixing reagents em-
ployed in researches upon chondriosomes. Examples of such fluids are
those of Altman, Benda, Bensley, Helley, Kopsch, Regaud, and Zenker.1
Besides staining with hsematoxylin and several other dyes commonly
employed with fixed material, the chondriosomes show a characteristic
affinity for certain intra-vitam stains, such as Janus green B, Janus blue,
Janus black I, and diethylsafranin, the reaction with the first of these
being especially strong. After certain treatments the chondriosomes may
closely resemble the "chromidial substance," or granules of nucleo-
protein distributed throughout the cytoplasm in some cells. That the
two are not to be confused has been emphasized by Duesberg and by
E. V. Cowdry. According to the latter author (1916) chondriosomes are
"a concrete class of cell granulations," and may be provisionally defined
as "substances which occur in the form of granules, rods and filaments in
almost all living cells, which react positively to Janus green and which,
by their solubilities and staining reactions, resemble phospholipins and
to a lesser extent, albumins."

Origin and Multiplication. — The questions of the origin and multipli-
cation of chondriosomes are much debated ones. Certain cytologists

1 For convenient summaries of the effects of various reagents upon chondriosomes
the student may refer to Kingsbury's (1912) paper on cytoplasmic fixation, E. V.
Cowdry's (1914) on vital staining, and N. H. Cowdry's (1917) on plant and animal
chondriosomes.

118 INTRODUCTION TO CYTOLOGY

believe that they have found good evidence for the view that chondrio-
somes may multiply by division, and some (Guilliermond ; Moreau 1914;
Terni 1914) have held this to be their sole mode of origin— that they arise
only from preexisting chondriosomes and are therefore permanent cell
organs. Others are convinced that they may arise de novo in the cyto-
plasm, and that the evidence for their division is unsatisfactory (Orman
1913; Lowschin 1913; Scherrer 1914; Miss Beckwith 1914; Chambers
1915; M. and W. Lewis 1915: Twiss 1919; and others). The investiga-
tors of the foregoing group, together with Meves (1900), Lewitski (1910),
and Forenbacher (1911), hold that the chondriosomes arise from the cyto-
plasm, but certain others believe they take their origin from the nucleus.
Tischler (1906) and Wassilief (1907), for example, state that they arise
from surplus chromatin. Alexieff (1917) thinks that although cyto-

FIG. 44. — Examples of regular behavior of chondriosomes in cell-division.
A-C, spermatocyte of Gryllotalpa vulgaris, (After Vo'inov, 1916) : A, chondriosomal
material in cytoplasm about nucleus; B, heterotypic mitosis, showing chondriosomes (at
sides) occupying the spindle with the chromosomes (at center) ; C, stages in the division
of a chondriosome. D, Dividing cell of Geotriton fuscus, showing division of individual
chondriosomes as cell constricts at equator. (After Terni, 1914.)

iplasmic dfferentiation is due to them, they are at least in some cases
of nuclear origin; and further that they are not fundamentally different
from chromosomes and chromidia, a conclusion contradictory to that of
Duesberg and E. V. Cowdry, cited above. Shaffer (1920) believes them
to arise as the result of a chemical action of the nucleus upon products of
assimilation in the adjacent cytoplasm. Wildman (1913) classifies the
cytoplasmic inclusions present throughout spermatogenesis in Ascaris
into two main types, both of nuclear origin: "karyochondria," equiva-
lent to the mitochondria of other writers, and "plastochondria," which
pass into the cytoplasm, form yolk within them, and fuse to form the
food supply ("refractive body") of the spermatozoon.

That the behavior of the chondriosomes at the time of cell-division
is a matter of considerable importance has been generally recognized. In
many cases their distribution to the two daughter cells seems to be quite
fortuitous, whereas in some tissues more or less definite modes of distribu-
tion have been described. According to Faure-Fremict (1910), Terni
(1914), Korotneff (1909), and others, the individual chondriosomes divide
at the time of mitosis (Fig. 44, D), a conclusion with which many others
fail to agree (Orman 1913; Miss Beckwith 1914; etc.). In the cells of

PL AST IDS AND CHONDRIOSOMES 1 19

the grasshopper, Dissosteiro, Carolina, Chambers (1915) finds that the
chondriosomal material forms a granular net work surrounding the nucleus
during the resting stages and the mitotic figure during division. During
the later phases of mitosis the strands and granules of this network
lengthen into delicate filaments between the two daughter chromosome
groups, and finally separate into two granular masses which gradually
invest the daughter nuclei.

In the mole cricket, Gryllotalpa borealis, the distribution of the chon-
driosomes to the daughter cells is accomplished with even greater defin-
iteness. According to Payne (1916) they become thread-like and break
near the middle, the halves passing to the daughter cells. Voi'nov (1916)
states that the "mitochondria" in the spermatocyte of G. vulgaris fuse to
form a thread which then segments into a number (70 or more) "chpndrio-
somes." These are arranged on the spindle along with the chromo-
somes, which they may closely resemble, and divide to form daughter
bodies at both maturation divisions, so that they are equally distributed
to the four resulting spermatozoa (Fig. 44, A-C}.

In certain scorpions also the chondriosomal material is distributed
with surprising precision. In a species from Arizona (Wilson 1916) this
material in the spermatocyte takes the form of a single ring-shaped body.
This ring divides accurately, much like a chromosome, at both maturation
divisions, each of the four spermatids, and hence each spermatozoon of
the tetrad, receives a quarter of its substance. In a California species
(Wilson) there is no ring formed, but instead about 24 hollow spherical
bodies. At the two maturation divisions these show no evidence of
division, but are passively separated into four approximately equal groups,
each spermatid receiving six (occasionally five or seven) . A European
species described by Sokolow (1913) agrees essentially with this.

Function. — Our knowledge of chondriosomes is yet too incomplete to
warrant any categorical statements regarding their functions, but a
number of opinions have been expressed, some of them based upon ob-
servational evidence and others upon conjecture. Certain of the more
prominent opinions may here be reviewed.

It was in 1897 that Benda suggested that chondriosomes might be
distinct cell organs with a special function. In a series of papers which
began to appear ten years later Meves (1907 etc.) put forth and empha-
sized the theory that they play an important role in heredity — that they
carry the hereditary characters of the cytoplasm. Evidence supporting
this view was seen by Meves and Benda in certain experiments of God-
lewski which seemed to show that the appearance of certain hereditary
characters is dependent upon something present in the cytoplasm rather
than in the nucleus. (See Chapter XIV.) This theory has had the
support of a number of investigators, among whom are the botanists
Cavers (1914) and Mottier (1916). Vomov (1916) also believes that the

120

INTRODUCTION TO CYTOLOGY

regular distribution of the chondriosomal substance in Gryllotalpa
strongly favors the view that this substance is of some significance in
heredity. It is probable, however, that the majority of cytologists regard
the evidence brought forward in support of the view as very inadequate.
Wildman (1913) points out that the chondriosomes may be largely lost
during spermatogenesis, and others have recalled cases in which the
nucleus is the only portion of the male gamete which can be seen to enter
the egg at fertilization. Meves (1911, 1915) and Benda, on the other
hand, show that chondriosomes also enter, at least in the forms studied
by them (Fig. 45). In the animal spermatid the chondriosomes appear
most commonly to contribute to the formation of the Nebenkern of the

spermatozoon (La Vallette St. George 1886;
Popoff 1907; Chambers 1915; Shaffer 1920; and
others), in some cases later elongating into a
sheath around the axial filament of the tail
(Shaffer on Cicada). Duesberg (1919) states
that although the fate of the chondriosomes of
the spermatid varies in different animals, they
are nevertheless always present in the sperma-
tozoon, and that it has not been clearly shown
in any case that they do not enter the egg at
fertilization. In many eggs which they do enter,
however, they behave with great irregularity
during the subsequent cleavage stages (Van der
Stricht, etc.). It is not at all improbable that
they are in some way concerned in the reactions
through which hereditary characters are de-
veloped in the individual, but the general
opinion is that their apparent variability and
indefiniteness in behavior in so many cases are
against the view that they take any part in the transmission of factors
upon whose presence the development of the characters depends
(Gatenby 1918, 1919). The equal distribution of chondriosomes at
the time of cell-division is thought to be without any significance in
this connection by Harper (1919).

It is obvious that much work remains to be done before the possible
relation of chondriosomes to heredity and development can be made
clear. For the present it is safest to assume, as will be emphasized in
later chapters, that hereditary transmission is the function of the nucleus,
chiefly if not entirely, since the chromosomes afford a mechanism of
precisely the kind required to account for the observed distribution of
hereditary characters.

Meves (1907a6, 1909) and Duesberg (1909) have also called attention
to the close relation of chondriosomes to muscle fibers in the developing

FIG. 45. — Fertilization in
Filaria papillosa, showing
chondriosomes of sperma-
tozoon (at top) distributing
themselves in the cytoplasm
of the egg. (After Meves,
1915.)

PLASTIDS AND CHONDRIOSOMES

121

chick embryo. They believe that the chondriosome elongates and
directly becomes the young fiber. Gaudissart (1913), on the contrary,
shows that the fiber does not arise exclusively from the chondriosome, but
that the primary basis is furnished by the plasmatic reticulum with
which the chondriosomes cooperate in building up the fiber. Although
the chondriosomes thus have a part in the genesis of the muscle fiber,
the latter is not a "modified filamentous chondriosome," as Duesberg
believed.

Hoven (1910a) and Meves have similarly attempted to show that
chondriosomes are concerned in the differentiation of neurofibrils and the
collagenous fibers of cartilage. Regaud (1911), Guilliermond (1914),

FIG. 46.

A, formation of fat in cell of rabbit by granular and rod-shaped chondriosomes. (From
Guilliermond, after Dubreuil, 1913.) B, formation of needle-shaped crystals of carotin in
chromoplasts derived from chondriosomes in epidermal cell of Iris petal. (After Guillier-
mond, 1918.) C, chondriosomes and chloroplasts in young cell of Pinus banksiana. X 750.
(After Mottier, 1918.) D, transformation of plastid primordia into leucoplasts in root
cell of Pisum; some of the leucoplasts contain starch. (After Mottier.)

Hoven (19106, 1911), and Lewitski (1914) have thought that the chon-
driosomes may in some cases perform a secretory function, and Dubreuil
(1913) has associated them with the production of fat (Fig. 46, A). In
the oocyte of Cicada Shaffer (1920) finds them transforming into yolk
spherules. The activity of bodies called " plastochondria " by Wildman
(1913) in the elaboration of the food supply in the spermatozoon of
A scam has already been mentioned.

Relation of Chondriosomes to Plastids. — One of the most conspicuous
views regarding the significance of chondriosomes is that which holds
some of them to be the primordia of plastids. After studying the cells of
Pisum and Asparagus Lewitski (1910) concluded that the chondriosomes
are essential constituents of the cytoplasm, and that they develop into
chloroplasts and leucoplasts in the cells of the stem and root respectively.

122 INTRODUCTION TO CYTOLOGY

Evidence in support of this conception was contributed by Forenbaelx-r
(1911), Pensa (1914), Cavers (1914), and others. Guilliermond (1911-
1920) in particular was led by the results of his extensive researches on
the subject to the view that the chondriosomes, arising only from preexist-
ing ones by division, persist through the egg and embryonic cells
and later become amyloplasts, chloroplasts, and chromoplasts. In
this he saw strong evidence for the individuality of the plastid. In
1915 he advanced the opinion that in fungi the chondriosomes function
like the amyloplasts of higher plants, forming reserve products as the
latter form starch. In this development of chondriosomes into plastids
Guilliermond (1913-1915) and Moreau (1914) were able to show that the
chondriosomes produce within them certain phenolic compounds which
either appear at once as anthocyanin pigments, or as colorless products
which may acquire color later through chemical alteration (Fig. 46, J5).

Among the most recent researches in this field are those of Mottier
(1916, 1918) on the cells of Zea, Pisum, Elodea, Pinus, Adiantum, Antho-
ceros, Pallavicinia, Marchantia, and several algae. He finds that leuco-
plasts and chloroplasts are derived from small rod-shaped primordia
(Fig. 46, C, D) which he regards as permanent cell organs of the same
rank as the nucleus. Both primordia and mature chloroplasts multiply
by fission. In the cells of Marchantia, Anthoceros, and the seed plants
he finds also a second series of bodies, which he calls chondriosomes:
these like the plastid primordia, are permanent cell organs multiplying
by division, but they do not become chloroplasts or leucoplasts. Further-
more, both chondriosomes and primordia are thought by Mottier to
be concerned in the transmission of certain hereditary characters.

It is also reported by Emberger (1920a6) that in the roots and spor-
angia of ferns two kinds of granular elements may be recognized at all
times, one of them representing the initial stage of plastid development.
Contrary to Mottier's opinion, however, he regards both kinds as true
mitochondria. Guilliermond (1920) likewise distinguishes two such
types in Iris germanica.

P. A. and P. Dangeard (1919, 1920), as a result of their researches
on the cells of barley, Selaginella, Larix, Taxus, and Ginkgo, distinguish
three classes of cytoplasmic structures differing in .reaction to reagents
and in function. In their initial stages all have the granular form. The
plastidomes first appear as minute "mitoplasts," which gradually enlarge
and develop into plastids. The spheromes are at first recognizable as
"microsomes," some of which may be seen to give rise to fat and oil
globules while others appear to undergo no change. The vacuomes
begin their history as "metachromes;" these elongate and form a peculiar
network which later develops into a system of vacuoles. Guilliermond
(1920) denies the metachromatic nature of this third class of bodies, and
holds them to be quite distinct from mitochondria.

PLASTJDS AND CHONDRIOSOMES 123

The existence of such a genetic relationship between chondriosomes
and plastids as that described above has been denied by many writers,
among whom may be mentioned Lundegardh (1910), Meyer (1911),
Rudolph (1912), Lowschin (1913, 1914), Scherrer (1914), Miss Beckwith
(1914), Derschau (1914), von Winiwarter (1914), Sa'pehin (1915),
Chambers (1915), M. and W. Lewis (1915), and Harper (1919). These
workers for the most part hold that chondriosomes are not distinct cell
organs at all, but regard them rather as more or less transient visible
products of protoplasmic activity. Derschau asserts that they arise
neither de novo nor by fission, but that they are merely small masses
of plastin and nuclein concerned in nutrition, arising from basichromatin
at the surface of the nucleus. Miss Beckwith speaks of them as differ-
entiation products of the cytoplasm. Lowschin, who made some ex-
periments in the production of artificial chondriosomes, believes them
to be due to the emulsified state of the protoplasm and in some instances
to the action of fixing agents upon it. To Chambers they appear in
living cells not as persistent structures but as temporary physical states
of the colloidal substances composing protoplasm. M. and W. Lewis
have studied them in tissue cultures and observe that they are continually
being formed and used up, and that they show no sharply distinct types.
Faure-Fremiet (1910a) distinguishes "mitochondria," which have an
individuality of their own and are permanent cell organs, from "lipo-
somes," which are temporary accumulations of reserve substance.

The almost universal occurrence of chondriosomes in the cells of
living organisms, and their frequent alterations in number and appear-
ance, suggest a connection with some fundamental process going on
almost constantly and common to all living matter. That this process
may be oxidation, the chondriosomes being a "structural expression of
the reducing substances concerned in cellular respiration" (Kingsbury),
has been regarded as highly probable by Kingsbury (1912), Mayer,
Rathery, and Schaeffer (1914), N. H. Cowdry (1917, 1918), and others.
Evidence favoring this interpretation is seen in the fact that the chondrio-
somes occur so widely in the cytoplasm, which acts as a reducing sub-
stance; and also in the close similarity between their chemical composition
and that of phosphatids, which appear to be capable of auto-oxidation.

Conclusion. — From the foregoing review it should be more than plain
that the state of our knowledge of chondriosomes is such that almost no
definite final statements can be made regarding their origin and function.
The evidence at hand apparently indicates that the class of cell inclusions
known as chondriosomes comprises a variety of bodies which play differ-
ent roles in the life of the cell. It is scarcely open to doubt that some of
them are temporary accumulations of substances involved in metabolism,
appearing and disappearing in the cell in a manner somewhat analogous
to that of starch. The most plausible hypothesis concerning the specific

124 INTRODUCTION TO CYTOLOGY

physiological role of such changeable types of chondriosomes is that thoy
have to do with the processes of oxidation and reduction — with cellular
respiration. It is also becoming increasingly apparent that other chon-
driosomes represent the juvenile stages in the development of plastids
of various kinds, and that they are in some way concerned in the forma-
tion of chlorophyll and other pigments. If this is true they are clearly
of the highest importance.

Whether or not any of the chondriosomes are to be considered as
permanent cell organs is a question to which, in view of the conflicting
testimony of competent observers, no final answer can at present be
given. To determine whether these minute bodies arise de novo or
always multiply by division is a matter of extreme practical difficulty.
Until this question is settled it is obviously impossible to come to a deci-
sion regarding the individuality of those plastids which appear to take
their origin from chondriosomes, or to know what may be the possible
relation of chondriosomes to inheritance. With respect to the latter
point, the chondriosomes, like all other structures concerned in meta-
bolism, may be indirectly associated with the development of hereditary
characters, but the view that they transmit or represent differential
factors for such characters is as yet unsupported by adequate evidence.

From the fact that the chondriosomes may not preserve their indi-
viduality at all times, however, it does not follow that they must be denied
the rank of cell organs. Their great variability, indifferent behavior at
the time of cell-division in so many cases, and their unknown mode of
origin are, as Kingsbury (1912) states, against the view that they are cell
organs; and it is doubtless true that many chondriosomes should for such
reasons be denied such rank. On the other hand, those chondriosomes
which seem clearly to perform important and specific functions in the life
of the cell should, like centrosomes appearing de novo at each cell-division,
be looked upon as cell organs, though not as permanent ones with an
uninterrupted continuity.

In spite of the fact that the study of chondriosomes has so far raised
more problems than it has solved, it has already proved of much value,
for it has turned to the cytoplasm some of the attention so long directed
almost exclusively to the nucleus, and it appears that many problems of
much importance to cytology pertain to the cytoplasm. It has also
been of great service in bringing about a closer scrutiny of the effects
of fixation and a renewed emphasis upon the importance of the study of
living protoplasm. Much has already been learned as the result of this
study, but the solution of the principal problems involving chondriosomes
must await the results of further research.

PL AST IDS AND CHONDRIOSOMES 125

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1'LASTWS AND CHONDRIOSOMES 127

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Nouvelles observations sur le choudriome de 1'asque de Pustularia vesiculosa.
Ibid. 646-649. (d) Nouvelles remarques sur la signification des plastes de W.
Schimper par rapport aux mitochondries actuelles. Ibid. 437-440.

1914a. Etat actuel de la question de 1'evolution et du r61e physiologique des

mitochondries. Rev. Gen. Bot. 26: 129-149, 182-210. figs. 16.
19146. Bemerkungen iiber die Mitochondrien der vegetativen Zellen und ihre

Verwandlung in Plastiden. Ber. Deu. Bot. Ges. 32: 282-301. figs. 2.
1915a. Nouvelles observations vitales sur le chondriome des cellules epidermiques

de la fleur d'Iris germanica. Comp. Rend. Soc. Biol. Paris 67 : 241-249.
19156. Recherches sur le chondriome chez les champignons et les algues. Rev.

G£n. Bot. 27: 193, 236, 271, 297, 315. pis. 12.
1917a. Sur la nature et le role des mitochondries des cellules v6g6tales. Comp.

Rend. Soc. Biol. Paris 69: 916-924.
19176. Observations vitales sur le chondriome de la fleur de Tulipe. Comp.

Rend. Acad. Sci. Paris 164: 407-409.

1917c. Contributions a 1'etude de la fixation da cytoplasme. Ibid. 643-646.
1917rf. Recherches sur 1'origine des chromoplastes et le mode de formation do

pigments du groupe des xanthophylles et des carotins. Ibid. 232-234.
1917e. Sur les alterations et les caracteres du chondriome dans les cellules epi-

dermique de la fleur de Tulipe. Ibid. 609-612.

1917/. Sur les ph6nomenes cytologiques de la degenerescence de cellules epi-
dermiques pendant la fanaison des fleurs. Compt. Rend. Soc. Biol. Paris 69:

726-729.

1918. Sur 1'origine mitochondriale des plastids. Compt. Rend. Acad. Sci. Paris
167 : 430-433.

1919. Observations vitales sur le chondriome des vegetaux et recherches sur
1'origine des chromoplastides et le mode de formation des pigments xantho-
phylliens et carotiniens. Rev. Gen. Bot. 31 : 372-413, 446-508, 532-603, 635-
770. pis. 60. figs. 35.

1920a. Sur 1'evolution du chondriome dans la cellule veg4tale. Compt. Rend.
Acad. Sci. Paris 170 : 194-197. figs. 4.

19206. Sur les elements figures du cytoplasme. Ibid. 170: 612-615. figs. 5.

1920c. Nouvelles recherches sur 1'appareil vacuolaire dans les ve'getaux. Ibid.

171:1071-1074. figs. 25.

HAAS,- P. and HILL, T. G. 1913. An introduction to the chemistry of plant prod-
ucts. London.

HABERLANDT, G. 1888. Die Chlorophyllkorper dcr Selaginellen. Flora 71:
291-308. pi. 5.

1905. Ueber die Plasmahaut der Chloroplasten in den Assimilationzellen von
Selagindla Martensii Spring. Ber. Deu. Bot. Ges. 23 : 441-452. pi. 20.

128 INTRODUCTION TO CYTOLOGY

HARPER, R. A. 1919. The structure of protoplasm. Am. Jour. Bot. 6 : 273-300.
HEGLER, R. 1901. Untersuchungen tiber die Organization der Phycochromzelle.

Jahrb. Wiss. Bot. 36: 229-354. pis. 5, 6. figs. 5.
HOVEN, H. 1910a. Sur 1'histogenese du systeme nerveux peripherique et sur le

role des chondriosomes dans la neuro fibrillation. Arch, de Biol. 25: 427-494.

pis. 15, 16.
19106. Contribution a 1'etude du fonctionnement des cellules glandulaires. Du

role du chondriome dans la secretion. Anat. Anz. 37: 343-351. figs. 7. (Note

prelim.)
1911. Du role du chondriome dans 1'elaboration des produits de la glande mam-

maire. Anat. Anz. 39: 321-326. figs. 4.
JANSSENS, F. A., VAN DE PUTTE, E., et HELSMORTEL, J. 1913. Le chondriosome

dans les champignons. (Notes prelim.) La Cellule 28: 447-452. pis. 1, 2.
JOHNSON, L. M. 1893. Observations on the zoospores of Draparnaldia. Bot. Gaz.

18 : 294-298. pi. 32.
JORGENSEN, I. and STILES, W. 1917. Carbon assimilation. New Phytol. Reprint

No. 10. London.
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Ann. Bot. 28:455-470. pis. 35, 36.
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KINGSBTJRY, B. F. 1912. Cytoplasmic fixation. Anat. Record 6: 39-52.
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Beziehungen zu Algen und Infusorien. Unters. Bot. Inst. Tubingen 1 : 233-262.

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1892. Flagellaten-Studien. 1,11. Zeit. Wiss. Zool. 65 : 265-445. pis. 13-18.
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Ber. Deu. Bot. Ges. 29 : 362-369. figs. 4.
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Ipl.
LIDFORSS, B. 1893. Studien ofver elaiosferer i ortbladens mesophyll och epidermis.

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PL AST IDS AND CHONDRIOSOMES 129

LOWSCHIN, A. M. 1913. " Myelinformen " und Chondriosomen. Ber. Deu. Bot.

Ges. 31: 203-209.

1914. Vergleichende experimental-cytologische Untersuchungen iiber Mito-
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LUNDEGARDH, H. 1910. Bin Beitrag zur Kritik zweier Vererbungshypothesen.

Jahrb. Wiss. Bot. 48: 285-378.
MAST, S. O. 1911. Light and the behavior of organisms, pp. 410. N. Y.

1916. The process of orientation in the colonial organism, Gonium pectorale,
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MAYER, RATHERY and SCHAEFFER. 1914. Les granulations ou mitochondries

de la cellule hepatique. Jour. Physiol. Path. Gen. 16: 607-622.
MCALLISTER, F. 1913. Nuclear division in Tetraspora lubrica. Ann. Bot. 27 :

681-696. pi. 56.

1914. The pyrenoid of Anthoceros. Am. Jour. Bot. 1 : 79-95. pi. 8.
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19076. Die Chondriokonten in -ihrem Verhaltnis zur Filarmasse Flemmings.
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1908. Die Chondriosomen als Trager erblicher Anlagen. Cytologische Studien
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1909. Ueber Neubildung quergestreifter Muskelfasern nach Beobachtungen am
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1914. Was sind die Plastosomen? Ibid. 86: 279-302. figs. 17.

1915. Ueber Mitwirkung der Plastosomen bei der Befruchtung des Eies von
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9

130 INTRODUCTION TO CYTOLOGY

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OLIVE, E. W. • 1904. Mitotic division of the nuclei of the Cyanophycete. Beih. Bot.

Centr. 18: 9-44. pis. 1, 2.
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vegetaux. 1. Le sac embryonnaire des Liliacees. La Cellule 28: 365-441. pis.

1-4.
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RIVETT, M. F. 1918. The structure of the cytoplasm in the cells of Alicularia

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PLASTIDS AND CHONDRIOSOMES 131

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132 INTRODUCTION TO CYTOLOGY

WILLSTATTER, R. und Stoll, A. 1913. Untersuchungen liber Chlorophyll. Berlin.
WILSON, E. B. 1916. The distribution of the chondriosomes to the spermatozoa of

scorpions. Science 43 : 539. (Refers also to work of Sokolow.)
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du testicule humain. Anat. Anz. 41 : 309-320. pis. 1, 2.
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26:316-320. pi. 11.
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pis. 12-16. figs. 12.
YAMANOUCHI, S. 1913. Hydrodictyon Africanum, a new species. Bot. Gaz. 55:

74-79.
ZIMMERMANN, A. 1893. Ueber die Elaioplasten. Beitr. z. Morph. u. Physiol. der

Zelle 1 : 185-197.
1894. Sammel-Referate. 9. Die Chromatophoren. 10. Die Augenfleck. 11.

Elaioplasten, Elaiospharen und verwandte Korper. 15. Die Starkekorner und

verwandten Korper. Beih. Bot. Centr. 4: 90-101, 161-165, 165-169, 329-335.

CHAPTER VII
METAPLASM ; POLARITY

In the foregoing chapters we have described successively the various
organs of the cell. Our account of the resting cell will now be com-
pleted by passing in brief review some of its more conspicuous non-
protoplasmic inclusions. We shall also call attention to another
characteristic but imperfectly understood attribute of the protoplast,
namely, its polarity.

Metaplasm. — In addition to their definite cell organs — nucleus,
cytoplasm, centrosomes, plastids, and possibly chondriosomes — cells
which have undergone any amount of differentiation usually contain a
variety of other materials representing products of metabolism. Many
of these substances are held in solution in the cell sap, itself a differentia-
tion product, while others are present in insoluble form in the cytoplasm,
often in special vacuoles. All such non-protoplasmic inclusions, partic-
ularly those existing in some visible form, are referred to as metaplasm,
a term introduced by Hanstein. Although it has been held by some
(Kassowitz 1899) that metaplasm is always inactive and to be sharply
set apart from active protoplasm, it is more probable, as Child (1915)
contends, that no absolute distinction can be made between the two.
Most of the products of differentiation, however, are clearly non-proto-
plasmic and relatively inactive.

In cells of many types, even in the comparatively undifferentiated
cells of the root meristem, there often occur accumulations of chemically
complex substances in the form of small globules or irregular masses in
the cytoplasm. In many cases these more or less transient bodies, which
often stain intensely with the nuclear dyes and are therefore referred to
as "chromatic bodies," show reactions indicating a composition closely
approaching that of the extra-nuclear granules of nucleo-protein (chro-
midia) which R. Hertwig and Goldschmidt interpret as granules of escaped
chromatin concerned in cell differentiation. Others resemble the fatty
chondriosomes in form and composition. It is therefore a matter of some
difficulty to distinguish between these various substances, which, as a
matter of fact, probably do not represent sharply distinct classes.

The most conspicuous non-protoplasmic inclusions represent food
materials in transitory form or in the storage condition; they are conse-
quently abundant in cells carrying a supply of reserve foods, such as
spores and eggs, and in storage organs, such as many roots and the endo-

133

134

INTRODUCTION TO CYTOLOGY

sperm and cotyledons of seeds. In the animal egg the storage material
commonly exists in the form of "yolk globules," or "deutoplasm spheres,"
which consist for the most part of relatively complex protein compounds .
Fat or oil globules are usually present with them. In plants the most
characteristic storage product is starch, the origin and characters of which
were described in Chapter VI along with the plastids by which they are
formed. In some organisms, including the fungi, glycogen appears to
carry on the function performed by starch and sugar in the higher plants.
Fats and oils, usually in the form of droplets but sometimes of soft grains
or even crystals (nutmeg), comprise another important class of storage
substances: these are especially prevalent in seeds and spores, where light
weight is of advantage. In many cases oil may be produced anywhere in
the cell, but in certain forms it has been found that special elaioplasts, and

FIG. 47. — Crystalline and other inclusions in the cells of various plants.
A, cystolith in subepidermal cell of Ficus leaf. B, crystal cells in Arctostaphylos. C,
druse in cell of Rheum palmatum. D-K, aleurone grains: D, E, from Myristica; F, from-
Datura stramonium; G, from Ricinus communis; H, from Amygdalus communis; I, from
Bertholletia excelsa; J, from Faeniculum; K, from Elceis guiniensis. L, raphides in leaf
of Agave. M, inulin crystals in preserved cells of artichoke. (B—K after Tschirch.)

possibly also chondriosomes, are concerned in this process. The peculiar,
oil bodies found in the cells of certain liverworts appear to represent oil
vacuoles: these also have been discussed, together with elaioplasts, in
Chapter VI. Large masses of intranuclear metaplasm are found charac-
teristically in the eggs of gymnosperms.

Aleurone grains occur in small vacuoles in the cells of many seeds,
particularly in such oily ones as those of Ricinus, Juglans, and Bertholl-
etia. In maize and wheat grains they are limited to a single layer of cells,
the "aleurone layer." The aleurone grain varies much in. structure and
form, several types being described by Pfeffer in 1872. The grain con-
sists primarily of an amorphous protein substance, often with an outer,
somewhat more opaque shell. Some examples show no greater differen-
tiation than this, but many are much more elaborate (Fig. 47, D-K).
Those of Ricinus contain within them a single angular crystal of protein
(albumen), often referred to as the "crystalloid," and a globule of a double
phosphate of calcium and magnesium with certain organic substances

METAPLASM; POLARITY 135

called the "globoid" (Fig. 47, (7). The crystalline inclusions sometimes
grow to be very large. It has been thought by certain workers that
aleurone grains are self-perpetuating bodies with an individuality com-
parable to that of nuclei and certain plastids. That this view is correct
has been rendered very improbable by the researches of East and Hayes
(1911, 1915) and Emerson (1914, 1917) on the inheritance of aleurone
characters in maize, and also by the work of Thompson (1912), who suc-
ceeded in producing artificial aleurone grains in all essential respects
similar to those elaborated by the plant.

Crystals occur in great variety in the differentiated cells of plants.
They may lie in the cytoplasm, in vacuoles, attached to or imbedded in
the cell wall, and even in special cells. They are usually salts of calcium,
calcium oxalate being especially prevalent. The bundles of needle-
shaped crystals known as "raphides" (Fig. 47, L) found in the leaves of a
number of plants are composed of the latter salt, as are also the spherical
aggregations called "druses," or " sphserraphides " (Fig. 47, C). The
curious clustered " cystoliths " of the Ficus leaf (Fig. 47, A) are made up of
cellulose and calcium carbonate. Crystals of silica are very abundant in
the thickened walls of wood cells and in many other tissues, such as the
outer cells of the Equisetum stem. Crystals of albumen, aside from those
found in aleurone grains, are frequently present in the cytoplasm of cells
poor in starch, as in the outer portion of the potato tuber. The leucoplast
of Phajus often contains a rod-shaped albumen crystal. Protein crystals
of various shapes are occasionally observed within the nucleus (Stock
1892; Zimmermann 1893).

Cellulose is a common storage material, existing as a rule in the form
of laminae deposited upon the original cell wall.

As already pointed out, the sap of vacuolated cells may contain a
number of differentiation products in solution. The cell sap is usually
slightly acid in reaction, owing to the presence or organic acids (malic,
formic, acetic, oxalic) and their salts. Inogranic salts are probably always
present. Amides, such as glutamin and asparagin, glucosides, sugars,
proteins, tannin, and many other substances are of frequent occurrence
in the cell sap of various plants. The carbohydrate inulin may be pre-
cipitated out of the sap by alcohol: this accounts for the presence of
nodules of radiating inulin crystals frequently encountered in preserved
material (Fig. 47, M}. Rubber is present in the form of a suspension of
minute droplets in the cell sap of Ficus elastica and several other plants.
Gutta-percha occurs in a similar state in Isonandra gutta. The cell sap
in such cases has a characteristic milky appearance. The cell sap is
often colored by red, blue, and yellow anthocyanin pigments (Wheldale
1916; Palladin 1918; Beauverie 1919), some of which change color when
the reaction of the sap is altered from acid to basic and vice versa. The
striking colors of flowers are due to "(1) the varying color of the sap, (2)

136 INTRODUCTION TO CYTOLOGY

the distribution of the cells containing it, and (3) combinations of colored
sap with chloro- and chromoplasts." Autumnal coloring is due to the
formation of pigments as disorganization products : when cytoplasm and
chlorophyll are the main disorganizing substances a yellowish color results,
whereas if sugars are present in considerable amounts in the cell sap the
brighter pigments are formed.

Extruded Chromatin.1 — The actual extrusion of chromatin from the
nucleus into the cytoplasm has been reported in a number of instances:
in the microsporocytes of various angiosperms by Digby (1909, 1911,
1914), Derschau (1908, 1914), West and Lechmere (1915), and others;
in ferns by Farmer and Digby (1910); and in the Ascomycete Helvella
crispa by Carruthers (1911). The extruded chromatin commonly takes
the form of deeply staining globules or irregular masses in the cytoplasm ;
often a clear area suggesting a nuclear vesicle is present about them. In
some cases, such as Gallonia candicans (Digby 1909) and Lilium candidum
(West and Lechmere 1915), the chromatin may pass through the wall
into an adjacent cell, where it forms a rounded mass connected by a
chromatic strand with the nucleus from which it originated.

The significance of this phenomenon is by no means apparent. It is
not at all unlikely that nutritive materials passing from nucleus to cyto-
plasm during the normal metabolism of the cell occur at times as visible
globules at the nuclear surface. The extrusion of chromatin into neigh-
boring cells, on the other hand, in many cases has every appearance of
a phenomenon associated with degeneration or some other abnormal
physiological condition. West and Lechmere, however, view the process
as one which occurs normally at certain stages, and which will probably
be found to be more general in plants. Sakamura's (1920) extensive
researches on chloralized cells have led him to regard the extrusion of
large masses of chromatin as an abnormal phenomenon which occurs as a
result of a disturbance of the metabolism of the cell. Its more frequent
occurrence in sporocytes than in other cells is attributed to the unusual
sensitiveness of the former to disturbing influences.

The Senescence of the Cell. — The accumulation of products of
metabolism ("differentiation products") has a direct bearing on the
problem of protoplasmic senility. As its life progresses the cell gradually
"ages," and if nothing occurs to prevent it the process eventually term-
inates in death. What shall be taken as an index of the degree of senes-
cence has been the subject of much discussion. We have already called
attention to the attempts which have been made to correlate senescence
with a progressive change in the nucleoplasmic relation, concluding
that no constant correlation of the kind has been shown to exist (p. 63).

Child (1915) has brought forward much evidence to show that the

1 Extruded chromatin is not metaplasm, but it has been found convenient to
treat it at this point along with other inclusions of the cytoplasm.

METAPLASM; POLARITY 137

relative rate of metabolism is the main criterion of the cell's physiological
age, "young" cells having a high rate and "old" cells a relatively low
rate, and a gradual decline in this rate occurring throughout the life of
the cell. In embryonic (physiologically young) cells the cytoplasm ap-
pears to be comparatively homogeneous and undifferentiated. Older
cells, on the contrary, are ordinarily marked by the presence of products
of differentiation in the cytoplasm. The true measure of age is there-
fore not time, but physiological differentiation.

In many cells a rejuvenating process may occur, whereby a high meta-
bolic rate is restored and the products of differentiation lost: this is
regarded as a "return to the embryonic state" — a real physiological
rejuvenescence. "Senescence is primarily a decrease in rate of the
dynamic processes conditioned by the accumulation, differentiation, and
other associated changes of the material of the colloid substratum.
Rejuvenescence is an increase in rate of dynamic processes conditioned
by changes in the colloid substratum in reduction and dedifferentiation "
(Child, p. 58). Such a rejuvenescence occurs in connection with
regeneration, vegetative and other asexual reproduction, and sexual
reproduction. In each case the cell which begins the new life cycle — the
meristematic regenerating cell, the zoospore, or the zygote — has a high
metabolic rate and is comparatively free from the products of differentia-
tion.

In the lower organisms cell differentiation in this sense is not so great
but that almost any cell may retain the power to " dedifferentiate " and
begin the development of a new individual vegetatively. In these forms
asexual reproduction may occur repeatedly and keep the organism as a
whole (in protozoa and protophyta) or the protoplasm of the race (in
lower metazoa and metaphyta) physiologically young. Only when the
metabolic rate falls very low does sexual reproduction, the most effective
of all the rejuvenating agencies, ensue.

In the higher plants the retention of the power of dedifferentiation
is strikingly shown in the well known cases of Begonia and Bryophyllum,
which can regenerate complete new individuals from a few leaf cells.
In the higher animals cell differentiation is usually so great that the
somatic cells can no longer dedifferentiate and reproduce the organism
asexually. Here rejuvenation occurs only after the union of two gametes,
which are themselves, unlike the'zoospores of algae, physiologically old.
Although local rejuvenescence may occur, as in secretory cells which are
"younger" after secretion, and also in wound tissue, the differentiation
of the body cells is carried so far that their metabolic rate falls low enough
to make a recovery or rejuvenescence no longer possible. Thus it is
only the functioning reproductive cells that endure: the ultimate cessation
of all life processes in the body cells is the price which is inevitably paid
by the complex multicellular organism for the advantages conferred by
its high degree of differentiation.

138 INTRODUCTION TO CYTOLOGY

Of the highest importance in this connection are the results of at-
tempts to maintain the cells and tissues of higher animals in the living
condition in artificial culture media outside the body. It has been shown
by the remarkable experiments of Carrel, Leo Loeb, Burrows, H. V.
Wilson and others that cells may be isolated from any of the highly
differentiated essential tissues of the body and kept actively growing and
multiplying in vitro for a length of time frequently far exceeding that to
which they would have lived in the body. They do not appear to grow
old: indeed it is not improbable that in such a constantly favorable
environment somatic cells are as "potentially immortal" as the germ
cells (see p. 403). In the words of Pearl (1921), "It is the differentiation
and specialization of function of the mutually dependent aggregate of
cells and tissues which constitutes the metazoan body which brings about
death, and not any inherent or inevitable mortal process in the indivi-
dual cells themselves."

POLARITY

Polarity is a feature which is exhibited in some form by the cells of
all higher organisms, and in at least many of the simpler ones, as shown by
Tobler (1902, 1904) for certain algae; indeed it is probable that it is
possessed in some form and degree by ah1 cells. Harper (1919) calls
attention to the fact that "in the presence of polarity and the various
symmetry relations we have a fundamental distinction between cell
organization and that of polyphase colloidal systems as they are com-
monly produced in vitro."

This polarity has two aspects, the morphological and the physiological.
In the first place, the various constituents of the cell may be arranged
symmetrically about one or more ideal axes, so that the cell has more
or less distinctly differentiate.d anterior and posterior ends. This
structural aspect of polarity has been the one chiefly emphasized by
certain workers: van Beneden (1883), for instance, looked upon polarity
as "a primary morphological attribute of the cell," the axis passing
through the nucleus and the centrosome. Later writers, among them
Heidenhain (1894, 1895), made this conception of morphological polarity
the basis for interpretations of many of the phenomena of cell behavior.
(See Wilson 1900, pp. 55-56.) However, as Harper (1919) points out,
polarity "is apparently independent of the uni- or multinucleated condi-
tion of the cell, which shows that it is in some cases at least a more
generalized characteristic of the cell as a whole rather than a mere ex-
pression of the space relations of the nucleus and cytoplasm ..." Other
investigators (Hatschek 1888; Rabl 1889, 1892) early laid emphasis upon
the physiological expression of polarity. The cell shows 'a polar differ-
entiation in physiological labor: the processes in one portion of the cell
differ from those in another, this difference in the case of tissue cells

METAPLASM; POLARITY 139

l>eing due to different environments in the tissue. For these workers
this physiological differentiation is the essential element of polarity;
any morphological polarity is due secondarily to it.

Metabolic Gradient. — The most suggestive physiological conception
recently developed in this connection is that of Child (1911-1916).
Child has shown in the case of Planaria and other lower animals, as well
as in certain algae, that along each of the axes of symmetry there exists a
"metabolic gradient," or "axial gradient:" the rate of the physiological
processes is highest at one end of the axis and diminishes progressively
toward the other end. The anterior end of a planarian, for example,
has a higher metabolic rate than the posterior portions. Furthermore,
the portions of higher rate dominate and control the development of
those portions having a lower rate, with the result that the young indivi-
dual soon develops and maintains a definite physiological correlation of
anterior and posterior parts. Similarly in individuals with more than one
axis of symmetry, there may be a corresponding dorsal-ventral, as well
as an axial-marginal, correlation. That polarity is here primarily a
physiological matter is indicated by the fact that experimental altera-
tions in the metabolic rate in different parts is followed by abnormalities
in structural development.

As to the means by which the dominance of certain regions over others
is exercised, correlating the activities of the various parts of the or-
ganism, there are two principal theories in the field. According to one
theory chemical substances (hormones) are produced at certain places
and transmitted through the body. Although the circulation of such
hormones clearly has much to do with correlation in higher complex
organisms, Child adduces good evidence in support of the second theory,
namely, that the fundamental relations of polarity "depend primarily
upon impulses or changes of some sort transmitted from the dominant
region, rather than upon the transportation of chemical substances"
(p. 224).

It cannot at present be said to what extent this conception of polarity
is applicable to the single cell. The work of Child shows in a very
definite manner the coincidence of the morphological and physiological
axes of polarity, which indicates that the two are but different aspects of
one and the same polar differentiation. A similar coincidence exists very
generally in the case of the single cell. In the cell, as in the organism as a
whole, functional and structural differentiation are inseparably connected.
In the present state of our knowledge the attempt to determine the real
essence of polarity raises questions which cannot yet be answered. Does
physiological polarity depend upon a polarized structure which is a
fundamental attribute of the cell's ultimate organization? Or does a
polarized morphological arrangement follow and depend upon a physio-
logical division of labor arising as a difference in intensity or rate in proc-

140 INTRODUCTION TO CYTOLOGY

esses originally common to all parts of the cell? If so, to what internal
or external factors is the establishment of this difference due in cells having
no initial polarity? Analogies with electrical polarity have been resorted
to in this connection, concerning which Harper (1919) says: "To pro-
vide an adequate basis for understanding the observed facts of polarity,
however, it seems to me that the conception of compound aggregate
polyphase systems is more suggestive than these attempted analogies . . .
In the spatial arrangement and interactions of these systems polar dif-
ferences of the most diversified types are bound to arise in the mass as a
whole and express themselves in the form and relative rigidity and surface
tension of different parts, as well as in the interrelations between the cells
of a group in contact."

The polarity of the multicellular organism as a whole is closely bound
up with the polarities of its constituent cells. Harper has clearly shown
(1918) that in Pediastrum the position of the swarm-spores in the colony
which they unite to form is directly dependent upon their polarity.
This does not mean, however, that the polarity of the multicellular organ-
ism is nothing more than the sum of the polarities of its constituent
cells, unless we return to Schwann's simple conception of the organism as
merely an aggregate of independent cells. (See p. 12.) The higher
individuality, the colony, has its own polarity, which may be related to,
but is not the same as, that of its individual cells. In the ordinary multi-
cellular organism the polarity is an outgrowth of the polarity of the
fertilized egg cell rather than of the polarities of the many adult tissue
cells.

In polarity, then, we encounter another problem which must be
brought nearer a solution before we can have any adequate understanding
of the relation of the cell to the multicellular organism as a whole, and of
the perplexing matter of organic individuality.

Bibliography 7

Metaplasm — Senescence — Polarity
VAN BENEDEN, E. 1883. Recherches sur la maturation de 1'oeuf, la fecondation et

la division cellulaire. Arch, de Biol. 4.
BOVERI, TH. 1901a. Ueber die Polaritat des Seeigeleies. Verh. Phys.-Med. Ges.

Wiirzburg 34.
19015. Die Polaritat von Ovocyte, Ei und Larve des Strongylocentrotus lividus.

Zool. Jahrb. (Anat. Abt.) 14: 630-653. pis. 48-50.
CARRUTHERS, D. 1911. Contributions to the cytology of Helvetia crispa Fries.

Ann. Bot. 26: 243-252. pis. 18, 19.
CHILD, C. M. 1911. Studies on the dynamics of morphogenesis and inheritance in

experimental reproduction : I. The axial gradient in Planaria dorotocephala as a

limiting factor in regulation. Jour. Exp. Zool. 10: 265-320. figs. 7.
1912. Studies, etc. IV. Certain dynamic factors in the regulatory morphogenesis

of Planaria dorotocephala in relation to the axial gradient. Ibid. 13 : 103-152.

figs. 46.

METAPLASM; POLARITY 141

1913. Studies, etc. VI. The nature of the axial gradients in Planana and their
relation to antero-posterior dominance, polarity and symmetry. Arch. Entw.
37: 108-158. figs. 13.

1915. Senescence and Rejuvenescence. Chicago.

1916. Axial susceptibility gradients in alga?. Bot. Gaz. 62: 89-114.

VON DERSCHAU, M. 1908. Beitrage zur pflanzlichen mitose: Centern, Blepharo-
plasten. Jahrb Wiss. Bot. 46: 103-118. pi. 6.

1914. Zum Chromatindualismus der Pflanzenzelle. Arch. Zellf. 12: 220-240.
pi. 17.

DIGBY, L. 1909. Observations on "chromatin bodies" and their relation to the

nucleolus in Gallonia candicans Decsne. Ann. Bot. 23: 491-502. pis. 33, 34.
1910. The somatic, premeiotic, and meiotic nuclear divisions in Galtonia candicans.
Ann. Bot. 24: 727-757. pis. 59-63.

1914. A critical study of the cytology of Crepis virens. Arch. Zellf. 12: 97-146.
pis. 8-10.

DUESBERG, J. 1911. Plastosomen, "Apparato reticolaro interno," und Chromi-
dialapparat. Ergebn. Anat. Entw. 20: 567-916. (Review.)

EAST, E. M. and HAYES, H. K. 1911. Inheritance in maize. Conn. Exp. Sta.
Bull. No. 167.

1915. Further experiments in inheritance in maize. Ibid No. 188.

EMERSON, R. A. 1914. The inheritance of a recurring somatic variation in varie-
gated ears of maize. Am. Nat. 48: 87-115.

1917. Genetical analysis of variegated pericarp in maize. Genetics 2.
HARPER, R. A. 1918a. Organization, reproduction and inheritance in Pediastrum.

Proc. Am. Phil. Soc. 67 : 375-439. pis. 2. figs. 35.

19186. The evolution of cell types and contact and pressure responses in Pedi-
astrum. Mem. Torr. Bot. Club 17: 210-240. figs. 27.
1919. The structure of protoplasm. Am. Jour. Bot. 6: 273-300.
HATSCHEK, B. 1888. Lehrbuch der Zoologie.
HEIDENHAIN, M. 1894. Neue Untersuchungen iiber die Centralkorper und ihre

Beziehungen zum Kern und Zellprotoplasma. Arch. Mikr. Anat. 43 : 423-758.

pis. 25-31.

1895. Cytomechanische Studien. Arch. Entw. 1 : 473-577. pi. 20. figs. 17.
KASSOWITZ, M. 1899. Allgemeine Biologie. Vienna.
PEARL, R. 1921. The biology of death. II-The conditions of cellular immortality.

Sci. Mo. 12: 321-335; figs. 6.
PFEFFER, W. 1872. Untersuchungen liber die Proteinkorner und die Bedeutung

des Asparagins beim Keimen der Samen. Jahrb. Wiss. Bot. 8: 429-574. pis.

36-38.
RABL, C. 1885. Ueber Zelltheilung. Morph. Jahrb. 10: 214-330. pis. 7-13.

figs. 5.

1889. Ueber Zelltheilung. Anat. Anz. 4: 21-30. figs. 2.
SAKAMURA, T. 1920. Experimentelle Studien iiber die Zell- und Kernteilung mit

besonderer Riicksicht auf Form, Grosse und Zahl der Chromosomen. Jour. Coll.

Sci. Imp. Univ. Tokyo 39: pp. 221. pis. 7.
STOCK, G. 1892. Ein Beitrag zur Kenntniss der Proteinkrystalle. Cohn's Beitr.

Biol. Pflanzen 6: 213-235. pi. 1.
THOMPSON, W. P. 1912. Artificial production of aleurone grains. Bot. Gaz.

54:336-338. 1 fig.
TOBLER, F. 1902. Zerfall und Reproduktionsvermogen des Thallus einer Rhodo-

melacese. Ber. Deu. Bot. Ges. 20 : 357-365. pi. 18.
1904. Ueber Eigenwachstum der Zelle und Pflanzenform. Jahrb. Wiss. Bot.

39 : 527-580. pi. 10.

142 INTRODUCTION TO CYTOLOGY

TSCHIRCH, A. 1889. Angewandte Pflanzenanatomie. Wien u. Leipzig.

WEST, C. and LECHMERE, A. E. 1915. On chromatin extrusion in pollen mother-
cells of Lilium candidum, Linn. Ann. Bot. 29: 285-291. pi. 15.

WHELDALE, M. 1916. The anthocyanin pigments of plants. Cambridge.

WILSON, E. B. 1900. The Cell in Development and Inheritance.

ZIMMERMANN, A. 1893a. Ueber die Proteinkrystalloide. Beitr. z. Morph. u.

Physiol. d. Pflanzenzelle 1 : 54-79. pis. 2.
18936. Ueber Proteinkrystalloide. Ibid. 2: 112-158.

1894. Sammel-Referate. 11. Elaioplasten, Elaiospharen und verwandte Korper.
13. Die Aleurone- oder Proteinkorner, My rosin- und Emulsionkorner. 14.
Die Proteinkrystalloide, Rhabdoiden und Stachelkugeln. 15. Die Starkekornrr
und verwandten Korper. Beih. Bot. Centr. 4: 165-169, 321-335.

CHAPTER VIII

SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY

SOMATIC MITOSIS

Since the time when the cell was pointed out as the unit of structure
and function it has been recognized that the mode of origin of new cells
is a matter of fundamental importance. We have seen in our historical
sketch that cells were believed by the founders of the Cell Theory to
arise de now from a mother liquor, or "cytoblastema," a misconception
removed by later investigations in which it was shown beyond question
that cells arise only by the division of preexisting cells. By several early
observers the nucleus was seen to have a more or less prominent part in
the process, its division preceding that of the cell, but "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 complicated trans-
formation of the nucleus to which Schleicher (1878) afterward gave the
name karyokinesis. It soon appeared, however, that this mode of divi-
sion was not of universal occurrence; and that cell-division is of two widely
different types, which Van Beneden (1876) distinguished as fragmenta-
tion, corresponding nearly to the simple process described by Remak,
and division, involving the more complicated process of karyokinesis.
Three years later Flemming (1879) proposed to substitute for these
terms direct and indirect division, which are still used. Still later (1882)
the same author suggested the terms mitosis (indirect or karyokinetic
division) and amitosis (direct or akinetic division), which have rapidly
made their way into general use, though the earlier terms are often
employed. Modern research has demonstrated the fact that amitosis
or direct division, regarded by Remak and his followers as of universal
occurrence, is in reality a rare and exceptional process;. . . 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" (Wilson 1900,
pp. 64-65). 1

1 The following additional historical data arc of interest. The chromosomes,
though they appeared in the figures of Schneider (1873), were first adequately drawn
by Strasburger in 1875. Longitudinal splitting was described by Flemming in 1882.
The terms prophase, mctaphase, and anaphase were introduced by Strasburger in

143

144

INTRODUCTION TO CYTOLOGY

In view of the fact that the phenomena of growth, differentiation,
reproduction, and inheritance are now known to be intimately bound up
with the process of cell-division, it is obvious that a detailed knowledge
of this process is an absolute prerequisite to a solution of many of the
problems which confront us. In the present chapter the essential fea-
tures of vegetative or somatic nuclear division will be described. After a
preliminary sketch of the process of mitosis we shall take up in some
detail the behavior of the chromosomes and the question of their individ-
uality. In the following chapter attention will be devoted to other
features of cell- division: the achromatic figure, the mechanism of mitosis,
cytokinesis (the division of the extra-nuclear portion of the cell), and the
formation of the cell wall.

SOMATIC MITOSIS

FIG. 48. — Diagram of a typical case of somatic mitosis in plants.

Preliminary Sketch of Mitosis. — The main steps in a typical case of
somatic mitosis in plants may be very briefly outlined as follows (Fig.
48):

The chromatic material of the "resting" nucleus, as described in
Chapter IV, exists in the form of a more or less irregular reticulum. As
the process of mitosis begins this reticulum resolves itself into a definite
number of slender threads which represent chromosomes. These in

w-

1884, and Heidenhain in 1894 first used the term telokinesis (telophase). Lundegardh
(19126) added interphase. The chromosome was named by Waldeyer in 1888.
Hermann in 1891 distinguished connecting fibers (central spindle) and mantle fibers.
That the halves of each split chromosome go to opposite poles was shown by van
Beneden for animals and by Heuser for plants in 1884. The achromatic spindle was
first figured by Kowalevsky (1871) and Fol (1873), and first carefully described by
Butschli (187506).

SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 145

many nuclei are distinct from each other from the first, whereas in other
cases they may be arranged end-to-end in a more or less continuous
thread, or spireme, which later segments transversely into independent
chromosomes. The slender threads (chromosomes) now split longitu-
dinally throughout their entire length. A progressive shortening and
thickening of the split threads ensues, so that the nucleus is eventually
seen to have a certain number of chromosomes which have become
double through a longitudinal cleavage.

While the above changes are occurring, fine fibrils are differentiated
in the cytoplasm near the nucleus and become arranged in two opposed
groups. The nuclear membrane now disappears and the fibers extend
into the nuclear region, where some of them (the "mantle fibers")
attach themselves to the double chromosomes, while others (the "con-
necting fibers") pass through from one pole to the other. The double
chromosomes quickly become arranged in a single plane at the equator
of the cell, the fibers meanwhile forming the achromatic figure, or spindle.
This stage is known as the metaphase; all the steps leading up to it, be-
ginning with the initial changes in the resting reticulum, constitute the
prophase.

The daughter chromosomes (the halves of the longitudinally split
chromosomes) now move apart toward the poles of the achromatic
figure, where they soon form two closely packed groups with the central
spindle of connecting fibers extending between them. The period during
which the daughter chromosomes are thus moving apart is known as the
anaphase. The two groups of daughter chromosomes now reorganize
the daughter nuclei, in each of which the chromosomes again form a
reticulum like that of the original mother nucleus. This reorganization
period is called the telophase. During the telophase there is formed
upon the connecting fibers (central spindle) a separating wall, which
completes the division of the cell. The nucleolus as a rule plays no con-
spicuous part in mitosis: it usually disappears during the late stages of
the prophase, new nucleoli being formed in the daughter nuclei in the
telophase. In rapidly dividing cells the period between two successive
mitoses is called the interphase.

Mitosis in animals (Fig. 49) is closely similar to that in plants as
regards the behavior of the chromosomes. It normally differs in two
conspicuous features, namely, the presence of centrosomes and the
mode of cytokinesis following the division of the nucleus.

During the prophases the centrosome with its aster, if not dlready
double, divides. The two daughter centrosomes, each with its own
aster, move apart, and a small bundle of fibers extends between them;
all these structures together form the amphiaster. The rays on the side
toward the nucleus extend into the latter when the membrane dissolves
and become attached to the chromosomes, often before the two centro-
10

146

INTRODUCTION TO CYTOLOGY

somes have reached polar positions. The centrosomes, surrounded by
asters, remain at the poles of the achromatic figure during metaphase,
anaphase, and telophase, and after mitosis has been completed they may
disappear or remain through the resting stage to function in the next
mitosis. (See p. 78.)

The division of the cell following nuclear division is commonly
brought about in animals by the formation of a cleavage furrow, which
grows inward from the periphery as described in the following chapter,
rather than by the formation of a wall on the spindle fibers as in
plants.

FIG. 49. — Diagram of a typical case of somatic mitosis in animals.

Although the two above points serve in general to distinguish mitosis
in animals from that in plants, the distinction is not a sharp one: cen-
trosomes are regularly present in the cells of many lower plants, while
cytokinesis by furrowing also occurs in certain cases, as will later be
shown. The essential point to be borne in mind is that the significant
feature of mitosis — the division of the chromatin and its distribution to
the daughter nuclei — is fundamentally the same in both plants and
animals.

The relative duration of the various phases of mitosis has been studied
in a few cases. As an example may be taken the observations of M. and
W. Lewis (1917) on the mesenchyme cells of the chick growing in tissue
cultures. These investigators summarize the researches of others upon
the subject and give the following figures for the chick cells: prophase,
5 to 50 minutes, usually more than 30; metaphase, 1 to 15, usually 2
to 10; anaphase 1 to 5, usually 2 to 3; telophase up to cytokinesis, 2 to 13,
usually 3 to 6; telophasic reconstruction of daughter nuclei, 30 to 120;
total, 70 to 180 minutes.

SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 147

Detailed Description of the Behavior of the Chromosomes in Somatic
Mitosis.1 — In the present account we shall depart from the order usually
followed in descriptions of mitosis. Instead of commencing with the
resting nucleus and tracing the steps leading to the formation of two
daughter resting nuclei, we shall begin the description with the fully
formed chromosomes as they appear at the metaphase and follow them
through anaphase, telophase, resting stage, and prophase to the next
metaphase, when they are again clearly seen. This is done in order that
the account of the telophasic transformation of the chromosomes to
form a resting reticulum, and the prophasic condensation of the latter
to form chromosomes, may be given without interruption, which seems
advisable in view of the nature of certain questions which are later to
be discussed in the light of chromosome behavior.

Metaphase (Fig. 50, A}. — As the chromosomes arrange themselves
upon the spindle preparatory to their anaphasic separation their double
nature is clearly evident. The two halves may lie very close together
and in the case of long chromosomes may be somewhat twisted about
each other. When they lie a little apart they may often show small
connecting strands or anastomoses; in the immediately preceding stages
(late prophase) the halves are usually pressed tightly together, so that
these anastomoses appear to be due to mutual coherence at certain
points when the halves move slightly apart after the disappearance of
the nuclear membrane. As the double chromosomes take their places
on the spindle, the spindle fibers become attached to them, not to all parts
but to a particular portion of each. In the case of long chromosomes
the point of attachment is often at about the middle, whereas in shorter
ones it is commonly near one end. At their points of attachment to
the spindle the double chromosomes all lie with their halves superposed
(one half toward each spindle pole) and in a single plane; those portions
to which no fibers are attached may extend in various directions with no
regular arrangement.

Anaphase (Fig. 50, B-D}. — The daughter chromosomes (the halves
of the double chromosomes seen at metaphase) now begin to separate,
first at the point of insertion, and gradually move away from the equa-
torial plane. Owing to the different locations of the points of fiber attach-
ment, and also to the fact that the free ends of the chromosomes occupy
various positions, the chromosomes, unless they are very short, may now

1 This description is based on the author's accounts of somatic mitosis in Vicia
faba (1913) and Tradescanlia virginiana (1920). In these papers, especially in the
first, there is presented a more extensive comparison of the results of other investi-
gators than can be given here. Comparative studies have shown that in general
the present description is widely applicable to mitotic phenomena in plants and ani-
mals, although many modifications in detail are known, particularly in forms with
small chromosomes. A useful list of works on mitosis in angiosperms is given by
Picard (1913).

148

INTRODUCTION TO CYTOLOGY

be drawn into a number of peculiar shapes. In the case of long chromo-
somes the portions to which the fibers are attached may have reached
the poles of the spindle while the other portions are not yet separated
at the equatorial plane. As soon as the daughter chromosomes become
entirely free from one another they quickly draw apart and contract
into two dense masses, which are often actually farther apart than were

•0

FIG. 50. — Somatic mitosis in Tradescantia virginiana: metaphase (A), anaphase (B-D),
and telophase (E-G). At F are shown cross sections of chromosomes in the stage shown
at E. X 1900. (After Sharp, 1920.)

the poles of the spindle at metaphase. In these masses the individual
chromosomes can be distinguished only with great difficulty or not at all.
With this stage, which has been referred to by Gregoire and Wygaerts
(1903) as the tassement polaire, the anaphase ends and the telophase

begins.

Telophase (Fig. 50, E— Fig. 51, 7). — After remaining tightly pressed
together for a short time the chromosomes of each daughter group begin
to separate, their individual boundaries again becoming visible. As they
do so they cohere at various points where their substance becomes
drawn out to form anastomoses. It seems clear that the main connec-

SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 149

tions between the chromosomes of the reorganizing telophase nucleus
are formed in this way, at least in mitoses showing a tassement polaire
stage; but it is also probable, as several investigators have pointed out,
that other anastomoses may grow out from one chromosome to another
in the manner of pseudopodia (Boveri 1904; Gates 1912; Strasburger
1905; Dehorne 1911; Muller 1912; Lundegardh).

Reactions taking place between the various chromosomes and espec-
ially between them and the cytoplasm now result in the production of
the nuclear sap, or karyolymph. Between the outermost chromosomes
and the cytoplasm and also within the chromosome group droplets of
clear karyolymph appear, and where these come in contact with the
cytoplasm a nuclear membrane is formed. As the karyolymph .increases
in amount the nucleus enlarges and the chromosomes become more
widely separated.

The telophasic alveolation of the chromosomes, although it may in
exceptional cases begin much earlier, usually commences at about the
time the chromosomes first separate from one another in the early
reorganization stages of the daughter nucleus. Within each chromosome
vacuoles appear, first as obscure though rather sharply delimited circular
or elongated areas. They lie not only along the axis but also near
and against the periphery. This point is of importance in evaluating
the claim advanced by certain investigators (Lundegardh 1910, 1912;
Fraser and Snell 1911; Fraser 1914; Digby 1919) that the vacuolation
is median and results in a splitting of the chromosomes during the telo-
phase, rather than in the prophase. While the vacuoles develop into
open spaces through the breaking down of the thin portions bounding
them the nucleus increases rapidly in volume, so that each chromosome
appears as an irregular net-like band joined to its neighbors by fine anas-
tomoses. Careful study of the details of these telophasic changes (see
cross sections of chromosomes in Fig. 50, F) shows that the alveolation
proceeds with little regularity, and that each chromosome becomes an
alveolar and then reticulate body with nothing which can properly be
called a longitudinal split.

In certain cases these internal changes, which result in the trans-
formation of the chromosomes into a reticulum, and which as a rule do not
begin until the telophase, may be initiated during the anaphase. In
Allium, for instance, Miss Merriman (1904), Lundegardh (1910, 19126),
and Nemec (1910) all report that the vacuolation of the chromosomes
begins at this time. Even more striking is the case of Trillium (Gre"goire
and Wygaerts 1903), in which the unusually large chromosomes may
show vacuoles as early as the metaphase (Fig. 54, A~). Internal changes
of other types have also been described in anaphase chromosomes, but
not with sufficient clearness to warrant their use in general interpre-
tations.

150

INTRODUCTION TO CYTOLOGY

According to Bonnevie (1908, 1911) the chromosomes of Allium,
Ascaris, and Amphiuma each give rise to an endogenous spiral thread
during the telophases, this spiral thread persisting through the resting
stages until the next prophase, when it again condenses to form a chromo-
some (Fig. 54, J5). In his work on Salamandra Dehorne (1911) asserted
that each chromosome is represented at telophase by two interlaced
spirals arising from an anaphasic split, and further that these double
structures are associated in pairs and persist in this condition through
the resting stages. These two conceptions have been criticized by
Gregoire (1912), Sharp (1913), and de Smet (1914), who have inter-
preted such appearances as occasional aspects of the alveolized chromo-
somes without the significance attributed to them by Bonnevie and
Dehorne.

FIG. 51. — Somatic mitosis in Tradescantia virginiana: late telophase (H, I), inter-
phase and resting stage (-7, K), and early prophase (L, M). X 1900. (After Sharp,
1920.)

In the young telophass nucleus the chromosomes may become ar-
ranged in the form of a more or less continuous daughter spireme which
is then transformed into the resting reticulum. This spireme stage,
however, is not a necessary one; its absence is being reported with
sufficient frequency to throw much doubt upon the view that it is a
phenomenon of even general occurrence.

The nucleolus usually makes its appearance during the early telophase
as a small droplet or as several such droplets which may later flow
together. It seems to have little direct connection with the chromo-
somes, but there can be no doubt that its appearance is closely associated
with their physiological activities.

As the telophasic changes proceed the chromosomes with their
anastomoses gradually form a more and more uniform reticulum, in

SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 151

which, however, tho limits of the component chromosomes can !><•
distinguished until a very late stage.

Inter phase (Fig. 51, «/).— It often happens that in rapidly growing
tissue, such as the meristem of the root tip, the mitoses succeed one
another so rapidly that the telophasic changes may not proceed far
enough to obscure the limits of the chromosomes in the reticulum
before the changes of the ensuing prophase begin. In such tissue it is
not always possible to tell whether a given nucleus will undergo further
telophasic change or will at once enter upon the prophases. Such
interphasic nuclei develop nucleoli, but karyosomes (in species which
have these bodies) are usually not formed until a more advanced stage.

Resting Stage (Fig. 51, J, K). — In slowly growing tissue the successive-
mitoses do not follow one another with very great rapidity, and the
telophasic changes are carried on until the condition characteristic of
the typical resting nucleus is reached: the interphase here becomes the
prolonged resting stage. The structure of the resting nucleus has been
fully described in Chapter IV. In the reticulum the limits of the con-
stituent chromosomes usually become indistinguishable, although it is
known that in certain cases such nuclei, if properly sectioned and stained,
may reveal heavier and lighter areas in the reticulum which represent
respectively the chromosomes and the regions of anastomosis between
them. The importance of these facts will be apparent in our treatment
of the individuality of the chromosomes.

Prophase (Fig. 51, L-Fig. 52). — The first indication that the prophasic
changes have begun is seen in the breaking down of the reticulum in
certain regions. In the case of nuclei which show heavier and lighter
areas in their reticula this breaking down occurs along the light portions.
In view of what has been said concerning the origin of the reticulum at
telophase it is apparent that the breaking up of the reticulum in the
prophase represents in such cases the separation of the constituent
chromosomes from each other along the lines of their telophasic union,
and it has been inferred that a similar interpretation applies to those
nuclei in which the reticulum is perfectly uniform or in which the nuclear
material assumes more irregular forms. In this way there are developed
from the resting reticulum a number of more or less distinct reticulate
units, which, in view of their subsequent behavior, we know to be the
chromosomes (Fig. 51, L, M). That these units are essentially the same
as those which went to make up the reticulum at the preceding telophase
seems highly probable; there can be little doubt on this point when the
interphase is short.

The material of each reticulate unit (chromosome) now gradually con-
denses in a very irregular fashion about its open spaces and cavities.
The thinner regions bounding these spaces and cavities become broken
down, and the thicker portions remain as a very irregular zigzag thread of

152

INTRODUCTION TO CYTOLOGY

uneven thickness, which soon begins to straighten out (Fig. 52, P). At
the same time the material composing the thread becomes more evenly
arranged throughout its length, so that the chromosome eventually takes
the form of a single slender thread. All of these changes — condensa-
tion, straightening, and equalization in thickness — may be seen going
on simultaneously in different chromosomes of the same nucleus, or even
in different portions of a single chromosome.

The formation of the slender prophase chromosomes from the retic-
ulum in the above manner was first described in detail by Gre"goire and
Wygaerts (1903) and Gregoire (1906), and new cases have since been
added. The above writers, together with Nemec (1910), Digby (1910),

**

.s

FIG. 52. — Somatic mitosis in Tradescantia virginiana: prophases.

At N are shown cross sections of chromosomes in the stage shown in Fig. 51, M.
X 1900. (After Sharp, 1920.)

and Miiller (1912), believe that the separated portions of the reticulum
may also condense directly into the slender threads without passing
through the very irregular zigzag stage above described. It is probable
that both methods are followed in different cases, direct condensation
possibly being the rule in small nuclei. The view of Bonnevie (1908,
1911) concerning the origin of the zigzag threads has already been
mentioned in the paragraphs on the telophase. According to this worker
and to certain others (Wilson 1912a6) the chromatic material forms a
spiral thread within the chromosome during the telophase, this thread
uncoiling and emerging from the chromosome in the following prophase.
It is true that the zigzag threads occasionally have a strikingly regular
spiral aspect, but in view of the many other aspects observed and the
process which is known to give rise to them, it is probable that the

SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 153

formation of spirals in the manner described by Bonnevie is at least
very exceptional.

The true longitudinal splitting of the chromosomes is initiated in
the slender threads of the early prophase (Figs. 52 and 53, Q). As soon
as a thread becomes sufficiently equalized in diameter small vacuoles
appear along its axis and rapidly develop into a more or less continuous
split. Not all the threads, nor even all portions of the same thread,
undergo the change at the same time. If we consider the whole nucleus
at once, the processes of condensation, straightening, equalization, and

FIG. 53. — Somatic mitosis in Vicia faba: prophases.
Stages P, Q, and S correspond with P, Q, and S of Fig. 52. X 1650.
1913.)

(After Sharp,

splitting are all going on simultaneously; only in a given small portion of
a thread do they follow in definite sequence. Furthermore, as soon as
the threads become equalized they at once begin to shorten and thicken,
so that when vacuolation and splitting are a little delayed they occur in
somewhat heavier threads.

The manner in which the vacuoles develop into the complete split
should be carefully noted. The vacuoles quickly form openings which
extend completely through the chromosome, so that the latter soon takes
the form of two parallel strands connected by heavy cross pieces repre-
senting the portions between the original vacuoles (Figs. 52 and 53, S).

154 INTRODUCTION TO CYTOLOGY

The material constituting tin1 cross pieces gradually moves to the two
side strands, the center portion of the cross piece becoming progressively
thinner and the material accumulating on the side strands as a pair of
chromatic lumps. Although some of the cross pieces may persist until a
relatively late stage most of them soon disappear completely, and the
material in the two chromatic lumps is gradually distributed more or
less evenly along the parallel strands, which represent the daughter
chromosomes resulting from the split.

The double chromosomes now shorten and thicken, forming the
"thick spireme" so conspicuous in prophase nuclei (Fig. 53, T, U). As
pointed out in the preliminary sketch of mitosis, the chromosomes in the
prophase may form a more or less continuous spireme, but it is becoming
increasingly apparent that this is not a universal phenomenon. It is
certain that in many cases the chromosomes are separate from the first,
and it seems therefore that any association in the form of a continuous
spireme is a matter of secondary importance. As the shortening and
thickening proceed the split may become obscured by the close
association of the halves, but suitable methods reveal its presence.

While indications of spindle formation are appearing in the cytoplasm
the nucleolus disappears and the nucleus begins to contract, so that the
thick double chromosomes become very closely packed together. While
the* contraction is at its height the nuclear membrane disappears, after
which the chromosomes loosen up as an irregularly arranged group.
This contraction stage evidently does not occur in many mitoses: the
membrane may disappear while the nucleus has its full size. However,
when it does occur it is of very short duration, so that it may take place
in more cases than has been supposed. After the disappearance of the
nuclear membrane the spindle fibers establish connection with the chro-
mosomes, which quickly become arranged with their halves in superposi-
tion at the equatorial plane, as described in the paragraph on the
metaphase. This brings us to the point with which our description
began.

It should be added that in many descriptions of mitosis, notably
those presented in general text books, the chromosomes are said to split
during the metaphase, after they have become arranged upon the spindle.
Such a late development of the split may indeed occur in some cases, but
it is not improbable that closer examination would often reveal the
inception of the process at a much earlier stage. As has been pointed
out in the foregoing description, the early formed split frequently be-
comes obscured during the later prophases owing to the shortening and
thickening of the chromatin threads, and becomes conspicuous again
only after the metaphase figure has been established.

Chromomeres. — One matter which should receive special attention is
that of the chromomeres. It was held by Roux (1883) that the compli-

SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 155

rated process of mitosis is meaningless unless the chronudin is quali-
tatively different in the various regions of the nucleus, and that the
arrangement of the material of the chromosome in the form of a long
thread prior to its splitting is a means whereby all these qualities, ar-
ranged in a linear series in the thread, are equationally divided and
distributed to the daughter nuclei. The theory of Balbiani (1876) and
Pfitzner (1881), that the chromatin granules visible in the nuclear reticu-
lum arrange themselves in a series in the chromosome and by their
division initiate its splitting, had much to do with the formulation of this
hypothesis. That the chromatic granules, or chromomeres (Fol 1891),
represent the qualities of Roux is a theory which has been widely accepted
by cytologists. It was the opinion of Brauer (1893) and many later
workers that the granules or chromomeres, rather than the chromosomes
themselves, are the significant units in the nucleus, and that their division
is an act of reproduction. The division and separation of chromosomes
was accordingly regarded as a means of distributing the daughter granules
to the daughter cells. That the chromomere is made up of still smaller
"chromioles" was held by Eisen (1899, 1900). Strasburger, Allen (1905),
and Mottier (1907) also found the chromomere to be composed of smaller
chromatic granules.

FIG. 54.

A, vacuoles in chromosomes at metaphase in Trillium. X 1800. (After Gregoire and
Wygaerts, 1903.) B, spiral arrangement of chromatin material within the chromosomes of
Allium. (After Bonnevie, 1911.) C, D, stages of chromosome splitting in Najas marina,
showing chromomeres. X 2250. (After Midler, 1912.)

Although a large number of investigators, particularly those interested
in the hereditary role of the chromatin, have placed much confidence in
the importance of the chromomeres (Strasburger 1884, 1888), others
have raised serious objections to the theory that they are significant units
or individuals. Gregoire and Wygaerts (1903), Martins Mano (1904),
Gregoire (1906, 1907), Marechal (1907), Bonnevie (1908), Stomps
(1910), Lundegardh (1912), Sharp (1913, 1920), and others have found
no such definite behavior on the part of the chromatin granules in the
dividing chromosomes studied by them, and have suggested other ex-
planations for the appearances observed. According to a modification
of the chromomere theory adopted by Miiller (1912) the portions of the
thread between the chromomeres split first, the division of the chromo-

156 INTRODUCTION TO CYTOLOGY

meres then following. It has been pointed out (Sharp 1913) that Miiller's
figures (Fig. 54, C, D), which are very similar to the later ones of Stras-
burger (1907), may be interpreted as steps in the division of a homogene-
ous chromatic thread by the formation of vacuoles, and that the
chromomeres in this case are merely the cross pieces between the halves
of the incompletely split chromosome, as described in the foregoing account
of the prophase (Fig. 53, $).

It is becoming increasingly apparent that the distinction between
chromatin granules and supporting thread is not so sharp as has been
supposed, since the chromatic substance is often very fluid -in consist-
ency; and many have felt that the granules when present are far too
inconstant in number and behavior to serve as the ultimate units which
students of heredity hope to find. On the other hand, it should be said
that the constancy in size and position of the chromomeres described
by Wenrich (1916) for the grasshopper, Phrynotettix (Fig. 155), argues
strongly for the hereditary significance of these bodies, some of which
can be seen to retain their identity through the resting stages. But
whatever their importance may be, the arrangement of the chromatic
material in the form of a long slender thread and its accurate splitting
into exactly similar halves are very suggestive in connection with the
theory of Roux that many qualities are arranged in a row and all
divided at the time of nuclear and cell division. This subject will
receive further attention in the chapters dealing with heredity.

Summary. — The chromosomes, after having arrived at the poles of the
achromatic figure, become irregularly alveolized during the telophase and
form ragged net-like structures. These are joined to each other by fine
anastomoses and so make up the continuous reticulum of the resting
stage. In the next prophase this reticulum breaks up into separate
small nets or alveolar units, each of which represents a chromosome.
The units condense in a peculiar manner and become long slender threads.
These threads undergo a longitudinal splitting. The double threads so
formed shorten and thicken, and become the double chromosomes which
are arranged on the spindle at metaphase. The two halves (daughter
chromosomes) making up each double chromosome separate and pass to
opposite poles during the anaphase.

The outstanding and significant feature of somatic mitosis is this:
each chromosome is accurately divided into two exactly equal longitudinal
halves which are distributed to the two daughter nuclei. The two daughter
cells thus receive exactly similar halves of the chromatin of the mother cell.
Furthermore, as will be shown below, there is good evidence for the view
that the chromosomes maintain an individuality of some sort, so that,
since all the nuclei of the body arise by the repeated equational division
of a single nucleus, all the somatic (body) cells are qualitatively similar in
chromatin content: they contain representatives or descendants of each and

SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 157

every chromosome present in the first cell of the series. The great theoretical
importance of these facts will be apparent when we take up the subject
of chromosome reduction, and the application of cytological phenomena
to the problems of heredity.

THE INDIVIDUALITY OF THE CHROMOSOME

In later chapters the question of the significance of the nuclear struc-
tures in heredity is to be considered. In connection with this question
it is of the highest importance to determine whether or not the chromo-
somes to which the reticulum gives rise in the prophase are in any real
sense the same as those which went to make up the reticulum at the pre-
ceding telophase. That they do preserve their identity as individuals
through the resting stage, arise only by division, and maintain therefore
a genetic continuity throughout the life cycle, was held by van Beneden
(1883), Rabl (1885) and Boveri (1887, 1888, 1891) many years ago, and
since that time the idea has received the support of a large number of
investigators. We shall now briefly review some of the evidences which
have led the majority of cytologists to the view that the chromosomes,
"if . . . not actually persistent individuals, as Rabl and Boveri have
maintained, . . . must at least be regarded as genetic homologues that
are connected by some definite bond of individual continuity from gen-
eration to generation of cells" (Wilson 1909).

The Frequent Persistence of Visible Chromosome Limits in the
Resting Reticulum. — In the foregoing description of the behavior of the
chromosomes in mitosis it was pointed out that in rapidly dividing tissue
the telophasic alveolation of the chromosomes and their anastomosis to
form the reticulum often do not proceed far enough during the interphase
to obliterate the boundaries between the chromosomes, which separate
again in the ensuing prophase without having lost their visible identity.
In such nuclei there can be little doubt that the autonomy of the chromo-
some is preserved. In other cases, however, the telophasic transforma-
tion of the chromosomes is more complete and the resulting reticulum
reacts very weakly to the stains, so that the limits of the constituent
chromosomes disappear from view completely. Many workers have
therefore objected to the statement that here also the chromosomes are
present as individuals, although invisible. Haecker (1902) and Boveri
(1904) pointed out that this objection may be met by assuming that it is
the achromatic framework of the alveolized chromosome, and not neces-
sarily the basichromatic fluid held within it, that maintains a structural
independence. This view had the support of the earlier observation
made by Boveri (1887o, 1888a, 1891; also 1909) and confirmed by Herla
(1893), that the chromosomes in the segmenting egg of Ascaris have a
certain arrangement when they build up the nuclear reticulum in the
telophase and reappear from the reticulum in the same position at the
next prophase.

1 .I

INTRODUCTION TO CYTOLOGY

Special emphasis was laid upon this interpretation by Marechal (1904,
1907) as a result of his studies on the growth stage of animal oocytes.
At this period in the development of the ovum the chromosomes assume
a finely branched form (Fig. 86, C, D) and their ordinary staining capacity
is lost completely. Although the chromatic fluid may flow from the
reticulum to. the nucleolus and vice versa, and may periodically undergo
chemical changes which radically alter its staining reactions, the achro-
matic chromosomal substratum nevertheless maintains an uninterrupted
structural continuity. Such a transfer of the basichromatic material
from the persistent reticulum to the nucleolus during the telophase, and
to the reticulum again during the succeeding prophase, has also been

FIG. 55. — Some evidences for chromosome individuality.

A, chromosomal vesicles in Brachystola magna; x-chromosome in vesicle at right.
(After Sutton, 1902.) B, chromosomal vesicles in Fundulus embryo. X c. 1800. (After
Richards, 1917.) C, chromomere vesicle (c) on chromosome of Chorthippus. X 1500.
(After Wenrich, 1917.) D, prochromosomes in Pinguicula. X 4200.

observed by Strasburger (1907) and Berghs (1909) in the somatic nuclei
of Marsilia (Fig. 17, E}. The chromosome, as Marechal urges, is not
simply a mass of chromatin, but rather "a structure periodically chro-
matic;" hence the disappearance of stainable substance does not signify
the loss of structural continuity on the part of the chromosome.

The "chromosomal vesicles" (Fig. 55, A, B) observed by certain
investigators constitute valuable evidence in this connection. In the
spermatogonia of the grasshopper, Phrynotettix, for example, Wenrich
1916) has shown that each of the alveolizing chromosomes forms its
own vesicle about it at telophase, the several vesicles joining to form a
common nucleus. In some cases the boundaries between the vesicles do
not entirely disappear during the resting stages, and at the next prophase

SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 159

the chromatic material of each vesicle organizes in the form of a chromo-
some. The same condition is found in the nuclei of Fundulus (Richards
1917), Crepidula (Conklin 1902), and certain fish hybrids (Pinney 1918).
From this it is evident that the morphological identity of the chromo-
somes has not been lost between mitoses, although a very different type
of organization has been assumed.

In Carex aquatilis Stout (1912) has found a peculiar condition. Here
the very small spherical chromosomes, which maintain a serial arrange-
ment, are visible in the resting state, and can be traced continuously
through all stages of the somatic and germ cell divisions with the excep-
tion of synizesis.

The interpretations of Bonnevie (1908, 1911) and Dehorne (1911),
according to whom the chromosomes persist through the resting stage as
spirals or double spirals, have been mentioned in the description of
mitosis.

Prochromosomes. — Bodies known as prochromosomes have been
described in the nuclei of a number of plants : in Thalictrum, Calycanthus,
Campanula, Helleborus, Podophyllum, and Richardia by Overton (1905,
1909) ; in the Cruciferse by Laibach (1907) ; in Drosera and other forms by
Rosenberg (1909); in Acer platanoides by Darling (1914); in Musa by
Tischler (1910) ; and in a number of other forms. These prochromosomes
appear as small chromatic masses in the reticulum (Fig. 55, D), and
correspond approximately in number to the chromosomes of the species.
They are generally looked upon as portions of chromosomes which have
not undergone complete alveolation, and as centers about which the
chromosomes again condense at the next prophase. This interpretation is
in all probability a valid one in many of the described cases, but in others
the significance of such chromatic masses is questionable. In Crepis
virens de Smet (1914), in harmony with the conclusions of Miss Digby
(1914), finds them to be accumulations of material formed during the
resting stages. If such is the case they are to be regarded as karyosomes.

Persistence of Parental Chromosome Groups After Fertilization. — In
Chapter XII it will be shown that at fertilization there are brought
together two sets of chromosomes, one set from each parent; and that in
every nucleus of the resulting individual the chromosomes furnished by
the two parents are present together, all of them dividing at every mitosis.
When the chromosomes of the male parent are similar to those of the
female parent it is usually impossible to distinguish them in the nuclei
of the offspring. In a number of cases, however, such as Crepidula
(Conklin 1897, 1901), Cyclops (Hsecker 1895; Ruckert 1895), and Crypto-
branchus (Smith 1919) (Fig. 109), the two parental groups are distinguish-
able on the mitotic spindle, and often at other stages, through several
embryonal cell generations. It is in hybrids that this phenomenon is
shown most strikingly. In hybrid fishes obtained by crossing Fundulus

160 INTRODUCTION TO CYTOLOGY

with Menidia Moenkhaus (1904) was able to distinguish easily between
the long (2.18 /*) chromosomes of Fundulus and the short (1 ju) ones of
Menidia. Here, as in Crepidula and Cyclops, the paternal and maternal
chromosomes form separate groups in the mitotic figure. A similar
condition was seen by Tennent (1912) in hybrid echinoderms obtained by
crossing in various ways Moira, Toxopneustes, and Arbacia. In the
later cell-divisions the parental chromosomes mingle more or less, but are
nevertheless distinguishable. In Fundulus X Ctenolabrus hybrids (Morris
1914; Richards 1916), as well as in the normally fertilized Cryptobranchus
(Smith 1919), the chromatin contributions of the two parents are dis-
tinguishable even in the resting nuclei.

Size and Shape of Chromosomes.- — One of the most striking evidences
favoring the theory of individuality has been found in those plants and
animals which show constant differences in size and shape among the
various members of each parental chromosome group, so that particular
chromosomes are recognizable in the group appearing at each mitosis.
Since each parent furnishes a set of chromosomes to the new individual,
each kind of chromosome is present in duplicate in the nuclei of this indi-
vidual: it is therefore customary to speak of them as being present in
pairs, although at most stages of the life history there is ordinarily no
actual spatial pairing.

Since the description of the chromosomes of Brachystola by Sutton in
1902 (Fig. 101) the reported cases in which the different pairs of the
chromosome complement possess different characteristic sizes and shapes
have become increasingly numerous. This is notably true of insect
cytology, as is evident in a review of the extensive researches of McClung
(1905, 1914, 1917), Robertson (1916), Harman (1915), Carothers (1917),
and many others. In the sea urchin, Echinus, Baltzer (1909) found that
the 36 chromosomes have constant differences in length and shape,
some being hooked and some horseshoe-shaped. In the flatworm,
Gyrodactylus, (Gille 1914) there are six pairs, all different in length. In
Ambystoma tigrinum Parmenter (1919) finds 14 pairs of graded sizes.
In plants may be cited the cases of Crepis virens (Rosenberg 1909; de
Smet 1914; M. Nawaschin 1915) (Fig. 56 bis, A), which has three pairs of
different size; Vicia faba (Sharp 1914; Sakamura 1915), with five short
pairs and one long pair (Fig. 56); and Najas (Tschernoyarow 1914), in
which there are seven distinguishable pairs (Fig. 56 bis, B) . In Najas
the smallest pair is attached to one of the larger pairs: Sakamura (1920)
thinks that these together are really a single pair with pronounced
constrictions.

Not only may certain chromosomes be distinguished on the basis of
comparative length, but in some cases there may be other characteristics
which serve as marks of identification. In the chromosomes of many
plants and animals there are pronounced constrictions in some of the

SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 161

D

FIG. 56. — The chromosome complement of Vicia faba.

A, B, two successive sections of a mitotic figure in the root tip, showing together the
12 split chromosomes, 2 of them about twice as long as the other 10. C, cross section of
the group of chromosomes at anaphase: each of the long chromosomes, being drawn pole-
wards by the middle, shows both ends, making the number apparently 14. D, E, two
successive sections of a heterotypic figure in the microsporocyte, showing the 6 bivalents;
the large one is at the left. F, polar view of heterotypic mitosis at metaphase, showing the
6 bivalents. X 1400. (Original.)

FIG. 56 bis.

A, anaphase of somatic mitosis in Crepis virens, showing 2 long, 2 medium sized, and 2
short chromosomes passing to each pole. (After Rosenberg, 1920.) B, the chromosome
complement in a somatic cell of Najas major, showing the 7 homologous pairs. (After
Tschernoyarow, 1914.)

11

162 INTRODUCTION TO CYTOLOGY

members of the group. It has been shown in certain instances that these
constrictions have constant positions in the chromosome. A careful
study of this phenomenon has been made by Sakamura (1915, 1920).
In Viciafaba, for example; he finds that each of the two long chromosomes
("M-chromosomes") of the somatic group has two constant constrictions,
one at the middle and one near the end ("m-constriction" and "e-con-
striction") (Fig. 56, A). The m-constriction marks the point of attach-
ment of the spindle fibers. There are also end-constrictions in 8 of the 10
short chromosomes. On the basis of the widespread occurrence of con-
strictions in the chromosomes of both plants and animals Sakamura has
interpreted a number of puzzling phenomena, such as the apparent vari-
ation in chromosome number within the species (see below) and certain
features of the reduction process (Chapter XI)

Such regularly situated constrictions have also been demonstrated in
Fritillaria tenella by S. Nawaschin (1914). Here they are present at the
middle of the largest chromosomes, nearer one end in the medium-sized
chromosomes, and close to the end of the smallest ones. In Crepis virens
(M. Nawaschin 1915) there are constrictions near one end in two of the
three chromosomes of the haploid group in the pollen grain, in four of the
six chromosomes of the diploid group in the somatic cells, and in six of
the nine chromosomes of the triploid group in the endosperm cells. Such
a definiteness in the location of constrictions was also seen earlier by Agar
(1912) in the chromosomes of the fish, Lepidosiren.

Somewhat similar evidence has been brought forward by Wenrich
(1916), who finds that the chromatic lumps, or chromomeres, have a
striking constancy in position as well as in size in the chromosomes of
Phrynotettix (Fig. 155). Wenrich (1917) also reports that the small
"chromomere vesicles" attached to the chromosomes of- certain orthop-
terans always appear at definite points along the chromosome (Fig. 55, C).
It therefore appears that the chromosomes of a given group or comple-
ment not only maintain a genetic continuity from cell to cell, but are also
in some way qualitatively different from one another. They are conse-
quently said to have a specificity as well as an individuality, or continuity.
The relatively constant positions of the constrictions, chromatic lumps,
and chromomere vesicles afford further visible evidences that the chrom-
osome may possess some kind of lengthwise differentiation, a fact which,
if clearly demonstrated, would be of the highest importance in connection
with current views of the role of the chromosomes in heredity. (See
Chapter XVII.) The significance of chromosome constrictions in this
respect has been emphasized by Janssens (1909), S. Nawaschin (1915),
and Sakamura (1920).

Chromosome Number.1 — It was long ago noticed by Boveri, van

1 For lists of chromosome numbers in plants see Ishikawa (1916) and Tischler
(1916). For the numbers in animals see Harvey (1916, 1920).

SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 163

Beneden, and Strasburger that the number of chromosomes in any given
species is relatively constant. It was largely upon this fact that the
theory of chromosome individuality was originally based: the fact that
the number of chromosomes appearing at every mitosis is almost invari-
ably the same was taken to mean that the structural identity of the
chromosomes is never lost. Certain observers (Fick 1905, 1909) have
held that the apparent constancy in number is not due to a structural
continuity or individuality of any sort, but rather to the fact that the
successive nuclei have a relatively uniform amount of nuclear material,
the chromosomes "crystallizing out" of this material in each prophase
and going into solution at the close of mitosis. This idea was especially
developed by Delia Valle (1909, 1912a&), who described the formation
of chromosomes by the aggregation of fluid crystals during the prophase.
These chromosomes he held to be in no sense morphologically continuous
individuals, but only temporary chromatic accumulations which are in-
constant in number and lose their identity in the telophase. Delia Valle's
interpretation of chromosome formation has been criticized by a number
of writers and his position shown to be untenable by Montgomery (1910),
McClung (1917), and Parmenter (1919).

Some of the experiments on echinoderm eggs with which Boveri (1895,
1902, 1903, 19046, 1905, 1907) and others supported the theory of chromo-
some individuality may be briefly reviewed.

Boveri found that if the number of chromosomes is increased or de-
creased by artificial means the altered number appears at every mitosis
thereafter, (a) An enucleate egg fragment may be entered by a sperma-
tozoon, and may then develop into a larva with half the normal number
of chromosomes in every cell. (&) In another experiment the unfertilized
egg of a sea urchin was caused to undergo division by artificial means,
after which a spermatozoon was allowed to enter one of the blastomeres
(daughter cells) . A larva resulted in which one-half of the cells had regu-
larly 18 chromosomes (half the normal number) while the other half had
the normal 36. (c) Two spermatozoa occasionally fertilized one egg:
the cells of the resulting larvae had 54 chromosomes, the triploid number.
Abnormal mitotic figures were often formed in such dispermic eggs,
bringing about an irregular distribution of the chromosomes. For ex-
ample, a quadripolar spindle was produced, separating the 54 split chromo-
somes (108 daughter chromosomes) into four groups, with 18, 22, 32, and
36 chromosomes respectively (Fig. 127 bis). The resulting abnormal
larva ("pluteus") showed these four chromosome numbers in the cells
of four different regions of its body. Boveri (1914) later suggested that
malignant tumors might be due to such abnormal chromosome distri-
bution, (d) The number of chromosomes was doubled by shaking the
eggs while the chromosomes were split during the early stages of cell-
division. In this manner larvae wore produced with 72 chromosomes, the

164 INTRODUCTION TO CYTOLOGY

tetraploid number, in all of their cells, (e) In the threadworm, Ascaris
megalocephala, fertilization of an egg of the variety bivalens (two chromo-
somes) by a spermatozoon of the variety univalens (one chromosome)
resulted in a larva with three chromosomes in all its cells, the chromosome
contributed by the male parent being distinguishable from the other two
(Boveri 1888a; Herla 1893; Zoja 1895).

Results such as the above led Boveri to the conclusion that the number
of chromosomes arising from the reticulum in prophase is directly and
exclusively dependent upon the number that went to make it up in the
preceding telophase. If a nucleus is reconstructed in the telophase by an
abnormal number of chromosomes as the result of a disturbance of the

10 9

FIG. 57.— The chromosome complement of Hesperotettix viridis.

A, the 12 bivalent chromosomes of the spermatocyte, including the accessory chromo-
some (No. 4.) B, complement from another individual, showing two "multiple chromo-
somes." Nos. 11 and 12 have united temporarily, as have also Nos. 4 and 9. X 1800.
(After McClung, 1917.)

mitotic process, the altered number invariably appears in the succeeding
prophase: if extra chromosomes are present they are not eliminated in
any way during the resting stages, and if chromosomes have been lost
during abnormal mitosis they are not replaced. These conclusions have
been strikingly confirmed by Sakamura's (1920) work on cells subjected
to the influence of chloral hydrate and other agencies causing aberrant
chromosome behavior.

Variations in Number. — Although the number of chromosomes in a
given species is on the whole remarkably constant, departures from nor-
mal numbers are occasionally observed. Strasburger (1905) believed
that the number, though determined by heredity, is not so rigidly fixed
that all variation in the vegetative cells is excluded ; only in the reproduc-
tive cells did he hold constancy in number to be necessary. Much light

SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 165

has been recently thrown upon such apparent variations in number by
McClung (1917) and Miss Holt (1917) in their researches on multiple
chromosomes and chromosome complexes.

McClung finds in his analysis of the chromosome groups of the
orthopterans Hesperotettix and Mermiria that temporary associations
often occur between various members of a group, with the resulting forma-
tion of "multiple chromosomes" and a consequent decrease in the appar-
ent number. In Hesperotettix, for instance, the cells normally have 12
pairs of chromosomes, but because of the formation of such multiple
chromosomes individuals with apparently 11, 10, or 9 pairs are frequently
found (Fig. 57). For a given individual the number so formed is exactly
constant, since the members of a multiple remain together in all the cells
of the body; but for the species it is variable within certain limits, owing
to the varying numbers of chromosomes which may become involved in
such multiple combinations. In all cases the full number of chromosome
pairs is present, but some of them are so combined that there is an appar-
ent, though not actual, variation in the number. A similar condition is
found in other forms by Robertson (1916).

In Culex there are three pairs of chromosomes in the somatic cells.
During a certain stage in the insect's metamorphosis it has been shown
by Miss Holt (1917) that the chromosomes may split repeatedly, giving
cells with much larger numbers — up to 72 in some cases. These larger
numbers, however, are nearly always multiples of three, indicating that
the subdivision of the chromosomes is an orderly process. The daughter
chromosomes, moreover, that are formed by the subdivision of each of
the original six, remain more or less closely associated as a "multiple
complex," which behaves as a single individual in mitosis. It therefore
appears that the three pairs of chromosomes "are made up of quite
distinct individuals differing from each other to such a degree that
chromatin split from one cannot associate itself with that from another
pair. . . . Chromosome individuality, alone, can account for these
conditions."

Somewhat similar evidence has been brought forward by Hance (1917,
1918a6). Hance finds that the chromosome number in the spermatogo-
nia of the pig is regularly 40, whereas in the somatic cells it varies from
40 to 57. Similarly in (Enothera scintillans, which has 15 chromosomes
in its microsporocytes, there may be from 15 to 21 chromosomes in the
somatic cells. Measurements of the members of the various chromosome
groups show that the larger numbers are due to a fragmentation, prob-
ably of the larger chromosomes, in the somatic cells. Such fragments
divide normally, and it appears probable that the fragments of a single
original chromosome are held together by colorless portions and behave
as a unit, much as do the multiple complexes of Culex.

Sakamura (1920) believes that the chief reason for frequently reported

166 INTRODUCTION TO CYTOLOGY

inconstancies in chromosome number is to be found in the chromosome
constrictions, which under certain conditions become especially pro-
nounced and temporarily divide one or more of the chromosomes of the
group into loosely connected smaller parts. This suggestion, which
Sakamura supports with much direct evidence, is probably one of the
most fruitful which has been made in this connection.

The theory of chromosome individuality is believed by McClung
and Hance to be strengthened, rather than weakened, by such instances
of numerical variation as those described above. McClung emphasizes
the point that the composition of a given chromosome can be fully under-
stood only if something is known of its genetic history, for what appears
as a chromosome may often be either an aggregation of two or three
chromosomes, or, on the other hand, only a portion of the true chromo-
some individual. How widely this interpretation may be applicable to
other reported cases of numerical variation and to chromosome structure
in general cannot at present be stated, but it promises to lead to signifi-
cant results.

Discussion and Conclusions. — The author's views on the subject of
the individuality of the chromosomes can be most effectively stated in
the words of McClung (1917):

" . . . the practical matter before us is to decide whether the metaphase chromo-
somes of two cells are individually identical organic members of a series because
they were produced by the observed reproduction of a similar series of the parent
cell, or whether the resemblance is independent of this genetic relation and due to
chance association of indifferent materials, or to a reconstituting action of the cell
as a whole."

"If it were possible for chromosomes to reproduce themselves and still pre-
serve their physical configuration unchanged, there would probably be little
question of their continuity and individuality — the demonstration would be self-
evident. But it happens that the necessities of the case require that each newly
produced chromosome should take part in the formation of a new nucleus, through
whose activities the cell as a whole and each chromosome, individually, is enabled
to restore the volume diminished by the act of division. During this process the
outlines of the chromosomes become materially changed and in their extreme
diffusion can no longer be traced in many cases. Because of our limitations in
observational power they appear to be lost as separate individuals and we are
thus deprived of the simple test of observed continuity. Later, in the same cell,
there reappears a series of chromosomes severally like those which seemed to
disappear during the period of metabolic activity. We confront two alternative
explanations for this reintegration of the chromosomes; either they actually
persist as discrete units of extremely variable form, or they are entirely lost as
individual entities and are reconstituted by some extrinsic agency. There is no
other possible explanation and we must weigh the facts for one or the other of the
alternatives.

All the facts which indicate order and system in chromosome features speak
for the former, those which demonstrate variability and indefiniteness, for the

SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 167

hitter. The case for discontinuity is strongest in the absence of any chromosome
order, and becomes progressively weaker with the establishment of definiteness
and precision in form and behavior."

"So far as I can see there is no half way ground between the assumption that
the chromosomes are definite, self-perpetuating organic structures and the other
which presents them as mere incidental products of cellular action. According
to one view individual chromosomes are descendents of like elements and possess
certain qualities and behavior because of their material descent, the visible
mechanism for which is the process of mitosis : according to the other any similari-
ties that may exist in the complexes are the result of chance aggregations of non-
specific materials. It is a choice between organization and non-organization in
the last analysis, at least in terms of cellular structures. To attempt the sub-
stitution of a conception of molecular organization, which is beyond the ex-
perience of the biologist and which exceeds the present powers of the chemist to
analyse, is to cast aside all hope of solving the problem of cellular action, because
it is necessary to understand, not only the physical and chemical phenomena
involved, but also their different forms in the various parts of the cell."

"That the chromosomes do not maintain a compact and easily recognizable
form in the interval between mitoses is accepted by many . . . biologists as proof
that they no longer exist as entities. All the other manifold indications of char-
acter and continuity do not weigh against this apparent loss of identity. Doubt-
less it would be more satisfying if we could at all times perceive the chromosomes
in unchanging form in all stages of cellular activity, but why we should demand
this condition as a test of individuality in the chromosomes when we unhesitat-
ingly admit the unity of the organism in all the varied changes of its develop-
ment from a single cell, through such complexities of change and metamorphosis
as to give rise to doubts of even the phyletic position of some stages, it is difficult
to see. Being organic, the chromosomes must change their form, they must suffer
division of their substance and they are obliged to restore this loss through meta-
bolic changes. Since these changes of substance take place at surface contacts
there is an obvious advantage in increased superficies and, in common with other,
larger structural elements, the chromosomes become extended and their sub-
stances are diffused. In this state their boundaries may not be well defined and
this circumstance has been seized upon as a disproof of their continuity."

"Since it is not possible to observe directly the action of the chromosome we
are obliged to make use of indirect evidence, seeking parallels between elements
of structure and action in the chromosomes, and the mass effect of cellular action
as exhibited in the so-called body characters. Such a method is justified by all
other experience in tracing relations between structure and function in organisms,
and while it apparently resolves the organism into parts of greater or less in-
dependence, has given us our best conceptions of it as a whole."

"What is postulated ... is that the chromosomes are self-perpetuating
entities with individual peculiarities of form and function to identify them.
Characteristics of form and behavior we see; certain very definite parallels be-
tween these and the manifestations of somatic characters exist beyond question ;
provision for the perpetuation of the organic unity of the individual chromosomes
is found in the process of mitosis; the actual direct result of its operation appears
in the uniform conditions of the complex in the individual animal; the extension

168 INTRODUCTION TO CYTOLOGY

of this beyond the organism to the group and the means for it in the phenomena
of maturation and fertilization are easily established by observation; the age
old existence of all these circumstances is revealed by the near approach to
uniformity in the chromosome complex of the multitude of species of unnumbered
individuals constituting a family. And yet, in the face of this overwhelming
mass of evidence indicative of order, system and specific chromosome organiza-
tion, some conceive only the action of ordinary chemical forces, or the chance
association of indifferent substances, while others, over impressed with the
thought of a general coordinating force in the organism, deny significance to the
orderly play of its cellular parts."

"It is my belief that the observed act of reproduction, by which the organiza-
tion of the chromosomes is materially transmitted in each mitosis, together
with all facts indicating extensive distribution of given conditions, definiteness
of organization, uniformity of behavior and consistence of deviation from the
normal, are so many clear indications of the individual character of the chromo-
somes. Transmutation of form, even to an extreme degree, can not be held as a
valid argument against a persistent individuality. A consideration of the criteria
applied to larger organic aggregates well supports this view. Such objects are
said to possess individuality when they exhibit a more or less definite unity which
is persistent and characterized by peculiarities of form and function. Most
clearly defined is this individuality when it may be perpetuated through some
form of reproduction to find expression in new units of similar character. The
term does not connote unchangeability, and there may be fusions with more or
less loss of physical delimitations, followed by separation, even after exchange
of substances. The test of individuality is material continuity, but it does not
necessarily involve complete or entirely persistent contiguity.1 An organism may
bud off new individuals similar to itself, the substance of its body differs from
time to time, movements of parts take place, fragmentation occurs, extreme
attenuation or extension of substance is found, even separation and recombina-
tion of parts may happen and yet the individual maintains itself. What it may
have been in the past, what its possibilities of future development are, what
potentialities of multiplied individuality it suppresses do not affect the reality of
its individuality. It is, as Huxley says, 'a single thing of a given kind.' If
one such thing divides into two, there are two individuals; if two unite into one
indistinguishably there is a single individual; if a fusion of two things occurs in
part, without loss of physical configuration, there are still two individuals in
existence. Only when the substance of one thing disappears or becomes in-
corporated integrally into the organization of another does its individuality
depart.

If all these variations of physical state may occur in the history of an organism
without sacrifice of individuality, there can be no reason for urging them against
a conception of the individuality of the self-perpetuating chromosomes."

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Kernbestandteile wahrend der Embryonalentwicklung von Cyclops. Arch.
Mikr. Anat. 46: 579-617. pis. 28-30.

SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 171

l'.K)2. Ueber das Schicksal dor elterlichen und grosselterlichen Kernanteile.

Jcti. Zeitschr. 37: 299-400. pis. 17-20. figs. 16.
HANCE, It. T. 1917. The diploid chromosome complexes of the pig (»S'«.« scrofa)

and their variation. Jour. Morph. 30.
1918a. Variations in the number of somatic chromosomes of (Enothera scinlillans

de Vries. Genetics 3 : 225-275. pis. 7.
19186. Variations in somatic chromosomes. Biol. Bull. 35.
HARMAN, M. T. 1915. Spermatogenesis in Paratettix. Science 41 : 440.
HARPER, R. A. 1919. The structure of protoplasm. Am. Jour. Bot. 6: 273-300.
HARVEY, E. B. 1916, 1920. A review of the chromosome numbers in the Metazoa.

Jour. Morph. 28: 1-63; 34: 1-68.
HEIDENHAIN, M. 1894. Neue Untersuchungen iiber die Centralkorper und ihre

Beziehungen zum Kern und Zellprotoplasma. Arch. Mikr. Anat. 43: 423-758.

pis. 25-31.
HERLA, V. 1893. fitude des variations de la mitose chez Y Ascaride megalocephale.

Arch, de Biol. 13 : 423-520. pis. 15-19.
HERMANN, J. 1891. Beitrage zur Lehre von der Entstehung der karyokinetischen

Spindel. Arch. Mikr. Anat. 37: 569-586. pi. 31. 2 figs.
HERTWIG, O. 1875. Beitrage zur Kenntniss der Bildung, Befruchtung, und Thei-

lung des tierischen Eies. I. Morph. Jahrb. 1; see also 2-4.
HEUSER, E. 1884. Beobachtungen iiber Zellkernteilung. Bot. Centr. 17: 27-32,

57-59, 85-95, 117-128, 154-157. pis. 2.
HOLT, C. M. 1917. Multiple complexes in the alimentary tract of Culex pipiens.

Jour. Morph. 29: 607-627. pis. 4.
ISHIKAWA, M. 1916. A list of the number of chromosomes. Bot. Mag. Tokyo

30:404-448. (Plants.)
JANSSENS, F. A. et WILLEMS, J. 1909. Spermatogenese dans les Batrachiens. IV.

La Cellule 26: 151-178. pis. 2.
KOWALEVSKY, A. 1871. Embryologischen Studien an Wurmern und Arthropoden.

Mem. Acad. Imp. Sci. de St. Petersburg VII 16 : 13.
LAIBACH, F. 1907. Zur Frage nach der Individualitat der Chromosomen in Pflan-

zenreich. Beih. Bot. Centralbl. 22 : 191-210. pi. 8.
LEWIS, W. H. and LEWIS, M. R. 1917. The duration of the various phases of mitosis

in the mesenchyme cells of tissue cultures. Anat. Record 13 : 359-368.
LUNDEGARDH, H. 1910. Ueber Kernteilung in den Wurzelspitzen von Allium

cepa und Vicia faba. Svensk. Bot. Tidskr. 4: 174-196. figs. 11.
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19126. Das Karyotin im Ruhekern und sein Verhalten bei der Bildung und Auflo-

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Karyokinese. Beitr. Biol. Pflanzen 11: 373-542. pis. 11-14.
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Keimblaschen des Selachiereies. Anat. Anz. 25 : 383-398. figs. 25.
1907. Sur 1'ovogenese des Selachiens. La Cellule 24 : 1-239. pis. 10.
MARTINS MANO, TH. 1904. Nucleole et Chromosomes. La Cellule 22: 57-76.

pis. 1-4.
McCLUNG, C. E. 1905. The chromosome complex of orthopteran sperm atocytes.

Biol. Bull. 9 : 304-340. figs. 21.
1914. A comparative study of the chromosomes in orthopteran Spermatogenesis.

Jour. Morph. 25: 651-749. pis. 1-10.
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Ibid. 29 : 519-604. pis. 8.

172 INTRODUCTION TO CYTOLOGY

MERKIMAN, M. L. 1904. Vegetative cell division in Allium. Bot. Gaz. 37: 178-

207. pis. 11-13.
MEVES, FR. 1896, 1898. Zelltheihing. Ergeb. Anat. Entw. 6: 285-390; 8: 430-

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1911. Chromosomenlangen bei Salamandra, nebst Bemerkungen zur Individual-

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MOENKHAUS, W. J. 1904. The development of the hybrids between Fundulus

heteroditus and Menidia notatus, with especial reference to the behavior of

maternal and paternal chromosomes. Jour. Anat. 3: 29-65.
MONTGOMERY, T. H. 1910. On the dimegalous sperm and chromosomal variation

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145. pis. 9, 10.
MORRIS, M. 1914. The behavior of the chromatin in hybrids between Fundidus

and CtenoLabrus. Jour. Exp. Zool. 16: 501-522. pis. 5.
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NAWASCHIN, M. 1915. " Haploide, diploide, und triploide Kerne von Crepis virens,

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NAWASCHIN, S. 1914. "Sur quelques indices de 1'organisation du chromosome."

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NEMEC, B. 1910. Das Problem der Befruchtungsvorgange und andere Zytologische

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1909. On the organization of the nuclei in the pollen mother cells of certain plants,

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23:19-61. pis. 3.
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angiosperms. Bull. Torr. Bot. Club 40 : 575-590.
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heredity and development in teleost hybrids. Jour. Morph. 31: 225-292.

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figs. 2.

RABL, C. 1885. Ueber Zelltheilung. Ibid. 10: 214-330. pis. 7-13. figs. 5.
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Physiol. 1858.
1852. Ueber extrazellulare Entstehung thierischer Zellen und iiber Vermehrung

derselben durch Theilung. Ibid.
RICHARDS, A. 1916. Chromosome individuality in fish eggs. Science 43 : 178.

1917. The history of the chromosomal vesicles in Fundulus and the theory of
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ROBERTSON, W. R. B. 1916. Chromosome studies. I. Jour. Morph. 27: 179-

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Svensk. Bot. Tidskr. 3: 64-77. pi. 1.
19096. Ueber den Bau des Ruhekerns. Ibid. 3: 163-173. pi. 5. fig. 1.

1918. Chromosomenzahlen und Chromosomendimensionen in der Gattung
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1920. Weitere Untersuchungen iiber die Chromosomenverhaltnisse in Crepis.
Svensk Bot. Tidskr. 14 : 320-326. figs. 5.

SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 173

Roux, W. 1883. Ueber die Bedeutung der Kernteilungsfiguren. Leipzig.
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Kernsubstanz wahrend der ersten Entwicklung des befruchteten Cyciops-Eies.

Arch. Mikr. Anat. 46 : 339-369. pis. 21, 22.
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28: 131-147. pi. 2.

1915. Ueber die Einschniirung der Chromosomen bei Vicia faba L. (Vorl. Mitt.).
Ibid 29 : 287-300. pi. 13. figs. 12.

1920. Experimentelle Studien iiber die Zell- und Kernteilung mit besonderer

Riicksicht auf Form, Grosse, und Zahl der Chromosomen. Jour. Coll. Sci. Imp.

Univ. Tokyo 39: pp. 221. pis. 7.
SCHLEICHER, W. 1878. Die Knorpelzelltheilung. Arch. Mikr. Anat. 16: 248-

300. pis. 12-14. 1879. (Centr. Med. Wiss., Berlin, 1878.)
SCHNEIDER, A. 1873. Untersuchungen iiber Platelminthen. Jahrb. Oberhess. Gesell.

Natur-Heilk. 14. Giessen.

1883. Das Ei und seine Befruchtung. Breslau.

SHARP, L. W. 1913. Somatic chromosomes in Vicia. La Cellule 29: 297-331.

pis. 2.

1914. Maturation in Vicia. (Prelim, note.) Bot. Gaz. 57: 531.
1920. Somatic chromosomes in Tradescantia. Am. Jour. Bot. 7: 341-354. pis.
22, 23.

DE SMET, E 1914. Chromosomes, prochromosomes, et nucleole dans quelques
Dicotylees. La Cellule 29: 335-377. pis. 3.

SMITH, B. G. 1919. The individuality of the germ nuclei during the cleavage
of the egg of Cryptobranchus alleghanwnsis. Biol. Bull. 37 : 246-287. pis. 9.

STOMPS, T. J. 1910. Kerndeeling en synapsis bij Spinacea oleracea. (German
transl. in Biol. Centralbl. 31 : 257-320. pis. 3.)

STOUT, A. B. 1912. The individuality of the chromosomes and their serial arrange-
ment in Carex aquatilis. Arch. Zellf. 9: 114-140. pis. 11, 12.

STRASBURGER, E. 1875. Ueber Zellbildung und Zelltheilung. Jena.

1884. Die Controversen der indirekten Kernteilung. Arch. Mikr. Anat. 23 :
246-304. pis. 13, 14.

1888. Ueber Kern- und Zellteilung im Pflanzenreich, nebst einem Anhang tiber

Befruchtung. Hist. Beitr. 1. pp. 258. pis. 3.
1900. Ueber Reduktionsteilung, Spindelbildung, Centrosomen und Cilienbildner

im Pflanzenreich. Hist. Beitr. 6. pp. 124. pis. 4.
1905. Typische und allotypische Kernteilung. Jahrb. Wiss. Bot. 42: 1-71.

pi. 1.

1907o. Apogamie bei Marsilia. Flora 97: 123-191. pis. 3-8.
19076. Ueber die Individualitat der Chromosomen und die Fropfhybriden-

Frage. Jahrb. wiss. Bot. 44: 482-555. pis. 5-7.
SUTTON, W. S. 1902. On the morphology of the chromosome group in Brachystola

magna. Biol. Bull. 4: 24-39. figs. 11.
TENNENT, D. H. 1912. Studies in cytology. 1, 11. Jour. Exp. Zool. 12: 391-411.

•figs. 21.
TISCHLER, G. 1910. Untersuchungen iiber die Entwicklung des Bananen-Pollens.

I. Arch. f. Zellf. 6.

1916. Chromosomenzahl, -Form und -Individualitat im Pflanzenreich. Prog.
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TSCHERNOYAROW, M. 1914. Ueber die Chromosomenzahl in besonders beschaffene
Chromosomen im Zellkerne von Najas major. Ber. Deu. Bot. Ges. 32: 411-416.
pi. 10.

174 INTRODUCTION TO CYTOLOGY

WALDEYER, W. 1888. Ueber Karyokinese und ihre Beziehung zu den Befrucht-

ungsvorgangen. Arch. Mikr. Anat. 32: 1-122. figs. 14. (English transl. in

Quar. Jour. Micr. Sci. 30: 159-281. 1889.)
WENBICH, D. H. 1916. The spermatogenesis of Phrynotellix magnus with special

reterence to synapsis and the individuality of the chromosomes. Bull. Mus.

Comp. Zool. Harvard Coll. 60 : 55-136. 10 pis.
1917. Synapsis and chromosome organization in Chorthippus (Stendbothrus)

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pis. 1-3.
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1909. The cell in relation to heredity and evolution. In "Fifty Years of Darwin-
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1912a. Some aspects of cytology in relation to the study of genetics. Am. Nat

46:57-67.

19126. Studies on chromosomes. VIII. Jour. Exp. Zool. 13 : 345-449. pis. 9.
ZIMMERMANN, A. 1893. Sammel-Referate 6, 7. Beih. Bot. Cent. 3: 342-354,

401 -436.
ZOJA, R. 1895. Sulla indipendenza della cromatina paterna e materna nel nucleo

delle cellule embrionali. Anat. Anz. 11: 289-293. figs. 3.
ZUE STRASSEN, O. L. 1898. Ueber Riesenbildung bei Ascam-Eieren. Arch.

Entw. 7: 642-676. pis. 16, 17. figs. 9.

CHAPTER IX

THE ACHROMATIC FIGURE, CYTOKINESIS, AND THE CELL

WALL

THE ACHROMATIC FIGURE

The spindle fibers and asters about the centrosomes (when these are
present) are collectively termed the achromatic figure, in contradistinction
to the chromatic figure, or chromosomes. Compared with the chromo-
somes the achromatic figure is relatively little understood, which makes
it a very unsatisfactory subject for discussion. We shall first describe
the achromatic figure in its more common forms, and after mentioning
certain theories which have been propounded to explain its origin and
nature we shall briefly review a few of the suggestions which have been
made on the subject of the mechanism of mitosis.

In Higher Plants. — In somatic mitosis in higher plants the achromatic
figure is devoid of centrosomes and asters. Ordinarily it arises and be-
haves as follows: While the prophasic changes are taking place within
the nucleus the first indications of spindle formation appear in the cyto-
plasm in the immediate vicinity of the nucleus. At the two sides of the
latter, in the general position of the future spindle poles, there are de-
veloped two masses of more or less hyaline material, usually called "kino-
plasmic caps." In these two polar caps delicate fibrils soon appear, as
if by a process of condensation (Fig. 58, A, B). The nucleus commonly
shrinks at this time, while the fibrous areas increase in size and together
form a more definitely spindle-shaped figure. After the nuclear mem-
brane has shrunken more closely about the chromosomes it goes into
solution and the ingrowing fibers attach themselves to the longitudinally
split chromosomes. In many cases the membrane disappears without
shrinking, the fibers growing considerably in length to reach the chromo-
somes. The latter quickly become regularly arranged in the equatorial
plane preparatory to their separation (Chapter VIII). The mitotic
figure is now established (Fig. 48). The many fibers composing the
spindle may focus at a single sharp point at each pole, or they may end
indefinitely without converging to a point, forming in the latter case a
broad-poled figure which in extreme cases may be as wide at the poles
as at the equator (Fig. 74, D). Some of the fibers extend from the poles
to the chromosomes, to which they are attached, while others pass
through from one pole to the other without being so attached: these

175

176

INTRODUCTION TO CYTOLOGY

two sets of fibers are known respectively as mantle fibers and connecting
fibers. The latter are also collectively termed the central spindle.

It is during the anaphases and telophases that the connecting fibers
become most evident; in mitotic figures with many chromosomes it may
be impossible to see them at metaphase. At the beginning of the telo-
phase they may form a bundle no greater in diameter than the daughter
chromosome groups, but as the daughter nuclei reorganize the fibers
commonly bend outward at the middle, forming a barrel-shaped phrag-
moplast (Fig. 58, C) which in plants usually continues to widen by the
addition of new fibers until it comes in contact with the lateral walls of
the cell.

FIG. 58.

A, spindle beginning to differentiate in kinoplasmic caps at poles of nucleus in Ncphro-
dium. (After Yamanouchi, 1908.) B, same in Marsilia. (After Berghs, 1909.) C, D,
the origin of the cell wall in Pinus: C, connecting fibers between daughter nuclei at telo-
phase ; D, thickenings appearing on fibers. E, the continued extension of the cell wall
after the completion of mitosis in the endosperm of Physostegia virginiana. X 215. (After
Sharp, 1911.) F, multipolar stage of spindle development in microsporocyte of Acer
Negundo. X 1125. (After Taylor, 1920.)

While the above changes are occurring the new cell wall which is to
be formed between the daughter nuclei begins to differentiate. As the
central spindle widens the fibers become fainter near the nuclei and more
prominent at the equatorial region: this appearance seems to be due to
the flow of the material composing the fibers toward the latter region.
On the thickened fibers there now appear small swellings (Fig. 58, D)
which increase in size until they fuse to form a continuous plate across
the equator of the mother cell, thus dividing the latter into two daughter
cells. As this cell plate undergoes further changes (see p. 190) the fibers
disappear completely, first near the two nuclei and ultimately at the
equatorial region near the new wall. If the cell undergoing division
is very broad it often happens that wall formation begins near the center
of the phragmoplast while the latter is still extending laterally. In

THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 177

extreme cases wall formation may still be seen in progress at the periphery
after the fibers have completely disappeared at the central region (Fig.
58, E}. Such is notably the case in the tangential divisions of elongated
cambium cells (Bailey 1919, 1920).

The spindle in many cases has an origin somewhat -different from that
described above. The first indication of its differentiation is here the
appearance of a weft of fine fibrils in the cytoplasm all around the nucleus
(Fig. 84, E}. As these fibrils increase in size and number they may
form several distinct groups extending in various directions, thus giving
a multipolar spindle (Fig. 58, F). Some of the groups then gradually
disappear, while others alter their positions and coalesce, so that a bipolar
spindle eventually results. This, in general, is the manner in which the
spindle arises in the microsporocytes of angiosperms. For example,
Lawson (1898, 1900, 1903) finds that in Cobcea, Gladiolus, and Iris a
zone of granular " perikaryoplasm " collects about the nucleus during
the prophases of mitosis. When the nuclear membrane dissolves, this
substance together with the linin of the nucleus forms a fibrous network
which grows out into several cones of fibers, and these later become
arranged in two opposed groups.

In Animals. — In the majority of animal cells, and in certain cells of
lower plants also, the achromatic figure is a much more elaborate struc-
ture than that of the higher plants described above. This is due to the
presence of centrosomes, which with their asters are very conspicuous at
the time of mitosis. Commonly the aster is not present during the resting
stages of the cell, but cases are known in which both centrosome and
aster are visible, forming with other materials an "attraction sphere" in
the cytoplasm. As the process of mitosis begins (Fig. 59), an aster, if
not already present, develops about the centrosome. The centrosome
divides, and as the daughter centrosomes move apart each is seen to be
surrounded by its own aster, and a small group of fibers ("central spin-
dle") extends between them. The achromatic figure, made up of the
asters and the spindle connecting them, is known both at this stage and
later as the amphiaster. As the daughter centrosomes continue to sepa-
rate the astral rays increase in prominence. Some of the rays grow into
the nucleus when its membrane disappears and become attached as
mantle fibers to the chromosomes, while the lengthening central spindle
between the asters becomes the central spindle portion (connecting fibers)
of the completed mitotic figure (Fig. 49). All the fibers focus upon the
centrosomes.

During the anaphase the asters remain very conspicuous, but as the
telophases progress they gradually fade from view, except in those forms
which have a more or less permanent attraction sphere. Aside from the
presence of centrosomes and asters the achromatic figure in animal cells

differs most conspicuously from that of higher plant cells in its behavior
12

178

INTRODUCTION TO CYTOLOGY

II

III

IV

FIG. 59. — Mitosis in the spermatocyte of Salamandra.

I, prophase, centrosomes in astrosphere substance; latter spread out on nucleus. //,
prophase; bivalent chromosomes 'formed; centrosomes beginning to diverge; central spin-
dle and asters developed. ///, late prophase: nuclear membrane dissolved; spindle
fibers attaching to chromosomes, centrosomes moving apart. IV, anaphase: connecting
fibers prominent. V, telophase: constriction of cell nearly complete; mid-body forming
on central spindle or interzonal fibers. (After Meves, 1907.)

FIG. 60. — Elaborate achromatic figure in oocyte of Piscinln. X 1000.
(After Jnrgcnscn, 1913.)

THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 179

during the telophases. Instead of forming thickenings which become an
equatorial cell plate, the connecting fibers play relatively little part in
cytokinesis. One or more granules may be differentiated on the fibers
at the equatorial region, forming the so-called "mid-body," but the
actual division of the cell is brought about by the development of a
cleavage furrow, as will be described in the section on cytokinesis.

Intranuclear Figures. — In the above described cases of mitosis in
plants and animals the achromatic figure is derived mainly from the cyto-
plasmic region of the cell, the nuclear materials playing a relatively
minor part. In a number of forms, both among animals and plants (fungi,
for example), the spindle arises entirely in the nuclear region, forming
an intranuclear figure which may be completely established before the

FIG. 61.

A, B, anaphase and telophase of mitosis in ascus of Laboulbenia chcetophora. X 1350.
(After Faull, 1912.) (See also Figt 22.) C, intranuclear mitotic figure in oogonium of
Fucus. (After Yamanouchi, 1909.)

nuclear membrane disappears. Cases are known in which the centro-
somes themselves are also intranuclear, but usually these bodies lie in
the cytoplasm against the nuclear membrane, so that although the spindle
portion of the figure is within the nucleus the asters lie in the cytoplasm.

In the division of the nucleus in the ascus of an ascomycete,1 to take
a single example, the process is as follows (Figs. 22; 61 A, B): The
centrosome, which in ascomycetes is often discoid in shape, lies against
the nuclear membrane. As mitosis begins an aster develops in the cyto-
plasm about the centrosome, and the latter divides to form two daughter
centrosomes. The central spindle, if formed at all, does not persist.
From each of the daughter centrosomes, which begin to move apart
along the nuclear membrane, a group of fibers extends into the nucleus
where the chromosomes are being formed from the reticulum. The
centrosomes finally reach opposite sides of the nucleus, and their two

1 For references to the literature of mitosis in ascomycetes see page 290.

180 INTRODUCTION TO CYTOLOGY

groups of fibers become arranged in the form of a sharp poled spindle
extending through the nucleus with the chromosomes at the equator.
The nuclear membrane commonly remains intact until the chromosomes
approach the poles at anaphase; it then disappears, allowing the
nucleolus, which has remained unchanged, to escape into the cytoplasm
nearby. Between the two densely packed daughter chromosome groups
there extends a long strand of chromatic material: this soon disappears
and the two daughter chromosome groups reorganize two daughter
nuclei not separated by a wall. In those cases in which the division of the
fungus nucleus is followed by the development of a separating wall the
latter is formed by a cleavage furrow independently of the achromatic
figure.

Origin of the Figure. — Having before us the above examples of the
achromatic figure, we may now refer very briefly to some of the ideas
which have been advanced regarding the details of its origin in the cell.1

Early observers looked upon the whole mitotic figure — chromosomes,
spindle, and all — as a transformed nucleus, all the structures being formed
from the nuclear material at each mitosis. Strasburger, who first held
this view, later (1888), with Hermann (1891), believed the spindle to arise
wholly from the cytoplasm, whereas 0. Hertwig pointed out cases in
which the astral rays arise from the cytoplasm and the spindle from the
linin reticulum of the nucleus. Flemming (1891) derived the fibers from
the linin and the nuclear membrane. It soon became -evident that the
spindle, although in some cases arising entirely within the nucleus or
wholly from the cytoplasm, is commonly made up of materials derived
from both regions, as is evident from the examples described in the fore-
going paragraphs.

When van Beneden and Boveri announced their view that the centro-
some is a permanent cell organ, transmitted by division to daughter cells
and directly concerned in the formation of the asters, the theory was
adopted that the figure arises from the cytoplasm as a result of the
influence of the centrosome. The centrosome therefore came to be
known as "the dynamic center of the cell." Although this organ does
play a conspicuous role when present, its importance in connection with
the achromatic figure was somewhat diminished when it became evi-
dent that many centrosomes do not persist from one cell generation
to the next, and that such bodies are entirely absent from the cells of
higher plants.

Rearrangement Theories. — Many attempts have been made to account
for the formation of the achromatic fibrils in the cytoplasm. According
to some the fibers and astral rays arise as the result of a morphological
rearrangement of the preexistent protoplasmic structure, chiefly under

1 Extensive reviews of the early theories are given by Wilson (1900, pp. 72-86 and
316-329)

THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 181

the influence of the centrosome. Biitschli (1876), who looked upon
protoplasm as alveolar in nature, held that the rays are not really fibers,
but only the lamellae between radially elongated alveolae about the cen-
trosome. It was the opinion of Wilson (1899) on the other hand, that
the rays are actual fibers, though their material is derived from the
alveolar walls. Klein (1878) and others who believed protoplasm to be
ultimately fibrillar or reticular in structure, regarded the rays as radially
arranged fibrillse. Van Beneden (1883) supposed these fibrillae to be
derived partly from the intranuclear reticulum, and Rabl (1889)
pointed out that they are continuous with the unaltered cytoplasmic
meshwork and arise by a direct transformation of the latter. In Passi-
flora Williams (1899) found that the nuclear membrane forms a meshwork
connecting the linin reticulum with the cytoplasmic reticulum, all three
together organizing the spindle.

Special Substance Theories. — According to another group of theories
the spindle and asters are not formed merely by the rearrangement of a
structure already present, but arise from a special substance in the cell.
This substance was held by some to be a constantly present constituent
of the cell, forming the achromatic figure at the time of mitosis and re-
maining in reserve through the resting stages. Boveri's archoplasm
hypothesis in its earlier form (1888) was a prominent development of
this idea. According to this hypothesis the attraction sphere is composed
of a distinct substance called archoplasm, which consists in turn of fine
granules or microsomes aggregated about the centrosome as a result of
the centrosome's attractive force. The entire achromatic figure was
held to arise from this mass of archoplasm, the fibers and astral rays
growing out from it like roots, to be withdrawn again into the daughter
masses of archoplasm at the two poles during the closing phases of mitosis.
In this way each daughter cell was thought to receive half of the archo-
plasm. Although other workers (Watase 1894) also held that the fibers
are outgrowths of the centrosome or centrosphere substance, it was made
evident later that the material composing the fiber comes from the cyto-
plasm, being added to the growing fiber at its end. This was the view of
Driiner (1894, 1895). Boveri later (1895) modified his archoplasm hypo-
thesis, adopting the view that the fiber is formed from the substance of
the cytoplasm and not necessarily from a constantly present archoplasm.

Another theory based on the idea of a special substance in the cell
was that of Strasburger (1892, 1897, 1898). Strasburger held that the
cell has two kinds of protoplasm: an active fibrillar kinoplasm and a less
active alveolar trophoplasm. The former constitutes the ectoplast, centro-
somes, the mitotic fibers, and the contractile substance of cilia and
allied structures. The kinoplasm is thus concerned with the motor
work of the cell, whereas the trophoplasm has to do chiefly with
nutrition.

182 INTRODUCTION TO CYTOLOGY

The nucleolus has been thought by some observers to furnish material
for the formation of the spindle, because of the fact that it very commonly
disappears from view at about the time the spindle begins to differentiate.
It is possible that in some cases there may be a connection of this sort
between nucleolus and spindle, but it is clear that this cannot serve as a
general interpretation of spindle origin.

That the achromatic figure may arise from a special substance not
constantly present in the cell, but formed anew at each mitosis, is a
theory which several workers have advanced. The researches of Devise
(1914) and Miss Nothnagel (1916) may be cited for illustration. Devise,
as the result of a careful study of the development of the spindle in the
microsporocytes of Larix, concluded that the spindle is not formed by the
rearrangement of any preexistent nuclear or cytoplasmic structures, but
arises from a substance which develops in the nuclear region during the
late prophases (after diakinesis). He was not able to decide whether this
substance is of purely nuclear origin or is formed when the karyolymph
comes in contact with the cytoplasm. The interaction of karyolymph and
cytoplasm is emphasized by Miss Nothnagel in her work on Allium.
She points out that the contact of newly formed karyolymph with the
cytoplasm at telophase brings about the precipitation of the nuclear
membrane, and that in an analogous manner an exosmosis of karyo-
lymph through the nuclear membrane into the cytoplasm during prophase
causes the precipitation of fine fibrils around the nucleus, these fibrils then
developing into the spindle. The achromatic figure therefore arises
from a special substance, but this substance, as in the case of Larix,
is newly formed at each mitosis.

Conclusion. — In general it may be said that although the spindle
fibers and the motor and contractile elements of the cell appear to have
a substantial relationship with one another, the substance common to
them is probably "not to be regarded as being necessarily a permanent
constituent of the cell, but only as a phase, more or less persistent, in
the general metabolic transformation of the cell substance" (Wilson).
Indeed the conspicuous tendency on the part of cytologists at present
is to regard the achromatic figure neither as a mere rearrangement of a
structure previously present, nor as a form assumed by a special spindle
substance, but rather as the result of streaming, gelation, and other
temporary alterations in the colloidal substratum. This interpretation
is strongly supported by the microdissection studies to be cited in a
subsequent paragraph.

The Mechanism of Mitosis. — Since the phenomenon of mitosis was
first described there have been put forward a number of theories to ac-
count for the operation of the achromatic structures in bringing about the
separation of the daughter chromosomes and for the division of the cell.
Many of the suggestions undoubtedly contain elements of truth, but it

THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 183

must be admitted that there is no immediate prospect of a satisfactory
solution of these problems.

Contractility. — One of the simplest and most widely accepted theories
was that of fibrillar contractility suggested by Klein (1878) and van
Beneden (1883, 1887), according to which the chromosomes are simply
dragged apart by the contraction of two opposed groups of spindle fibers.
This theory and its modifications are fully reviewed by Wilson: it will
be sufficient here to point out that, whereas many facts were cited in its
favor, and elastic models made which simulated the supposed contraction
and its results (Heidenhain), the further evidence brought forward by
Hermann (1891), Driiner (1894, 1895), Calkins (1898), and others led to
the general restriction of the role of contractility, until it became appa-
rent that this factor, although it may contribute to the general result,
must be of minor importance. The contractility factor appeared again
in the more elaborate theory proposed by Rhumbler, which may be
briefly stated as follows: The centrosome arises as a local solidification
of the walls of the alveolae; the denser constituents of the protoplasm
collect at this point and form an attraction sphere, driving the less dense
constituents to the other parts of the cell where the pressure is lower; this
migration of fluid affects particularly those strands of the protoplasmic
reticulum which radiate more directly from the centrosomes; these
strands or rays, in giving up their fluid, shorten, and thus exert a trac-
tive force which draws the daughter chromosomes apart. In this theory,
therefore, the main factors are streaming and contractility.

Streaming. — The phenomena of streaming and surface tension have
been prominent factors in several attempts to explain both karyokinesis
and cytokinesis. The role of streaming in karyokinesis has been held to
be especially important since Butschli, Hertwig, and Fol showed many
years ago that currents exist in the protoplasm. Rhumbler (1896, 1899),
Morgan (1899), Wilson (1901), and Conklin (1902) all held that the
astral rays are due at least in part to centripetal currents. This inter-
pretation has recently been confirmed by Chambers (1917) in his micro-
dissection studies on the living cell. With regard to the aster Chambers
says: "The formation of the aster consists in the gelation of the hyalo-
plasm which comes under the influence of the astral center. A hyaline
liquid separates out during the gelation and flows in innumerable centri-
petal paths toward the center where it accumulates to form a sphere.
This centripetal flow brings about an arrangement of the gelled hyalo-
plasm containing the cell-granules into radial strands separated by the
hyaline-liquid paths. This produces the astral figure. The strands of
gelatinized cytoplasm merge peripherally into the surrounding liquid
cytoplasm or reach and anchor themselves in the substance of the gelled
surface when the aster is fully formed. The liquid rays merge centrally
into the substance of the sphere, the liquid of the rays and of the sphere
being thus identical."

184 INTRODUCTION TO CYTOLOGY

Sakamura (1920), although holding the fibers to be important agents
in the normal separation of the daughter chromosomes, observes that in
abnormal nuclear divisions where no fibers are present the chromosomes
still show movements which are probably due to streaming of the cyto-
plasm and to surface tension phenomena.

The relation of streaming and surface tension to cytokinesis will be
discussed in the section dealing more particularly with cytokinesis.

Osmosis. — In a theory of the mechanics of karyokinesis proposed by
Lawson (1911) the principal factor involved is osmosis. Lawson's ex-
planation is essentially as follows. During the late prophase karyolymph
passes outward through the nuclear membrane by osmosis, this loss of
fluid resulting in a contraction of the nucleus. Owing to the fact that
the cytoplasmic reticulum is continuous with the nuclear membrane this
contraction sets up radial lines of tension in this reticulum on all sides of
the nucleus. As the process continues these lines or "fibers" gradually
become arranged in two opposed groups, while the nuclear membrane to
which they are attached continues to contract until it actually enwraps
each double chromosome. To each double chromosome there are thus
attached fibers which represent stretched and distorted regions of the
cytoplasmic reticulum extending to the two sides of the cell. When the
chromosomes become properly arranged at the equatorial plane the
fibers, which are under considerable tension, are able to pull the daughter
chromosomes apart and draw them to the poles. As the fibers relax they
resume their true reticular state. Although the chromosomes are thus
drawn apart by the shortening of "fibers" attached to them, Lawson
points out that this is not to be regarded as a case of true active contrac-
tility, but only as a release of tension set up in the passive but elastic
cytoplasmic reticulum as the result of the exosmosis of karyolymph from
the nucleus. This theory has been severely criticized by a number of
writers, chiefly on the grounds that such an enwrapping of the chromo-
somes by the nuclear membrane as Lawson describes cannot be demon-
strated in many objects subsequently examined, and that the membrane
frequently goes into solution when both it and the growing fibers are
still some distance from the chromosomes.

Electrical Theories. — The striking resemblance between the achromatic
figure and the lines of force in an electromagnetic field early led to at-
tempts to account for mitosis on the basis of electrical principles. Several
investigators, working with various chemical substances, succeeded in
modelling fields of force that illustrated graphically the changes supposed
to take place in the dividing cell. In later years the electromagnetic
interpretation was again brought into prominence by Gallardo, Hartog,
and Prenant. At first Gallardo (1896) believed the two spindle poles
to be of unlike sign, but later (1906), as the result of the researches of
Lillie (1903) (see p. 62) on the behavior of nucleus and cytoplasm in

THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 185

the electromagnetic field, he concluded that the chromosomes and the
cytoplasm carry charges of unlike sign: the daughter centrosomes repel
each other and move apart because of their like sign, the spindle poles
being of like sign also. The movement of the chromosomes to the poles
he held to be due to the combined action of two forces: the mutual repul-
sion of the similarly charged daughter chromosomes, and the attraction
between the oppositely charged centrosomes and chromosomes.

The fact that the two centrosomes and hence the two spindle poles
are electrically homopolar (Lillie) and alike osmotically at once makes it
apparent that the mitotic figure does not represent an ordinary electro-
magnetic field, for in the latter the poles are of unlike sign — the field is
heteropolar. It has consequently been suggested by Prenant (1910) and
Hartog (1905, 1914) that the mitotic figure is the seat of a special force,
analogous to electrostatic force but not identical with it, which is peculiar
to living organisms. This new force they call "mitokinetism."

A large amount of discussion has centered about the possible role of
electrical forces in mitosis, and many kinds of normal and abnormal
mitotic phenomena have been cited as evidence for various views. So
far as conclusive statements are concerned, there is disappointingly little
of a definite nature that can be said. Meek (1913) asserts that the only
generalization which is at present possible is the negative one that
"the mitotic spindle is not a figure formed entirely by the action of
forces at its poles."

Conclusion. — In conclusion we may emphasize the fact that the
achromatic figure depends for its operation upon a variety of interacting
factors. Certain investigators have doubtless done good service in em-
phasizing the importance of one or another of these factors — streaming,
surface tension, contractility, gelation, electrical phenomena, and the
like — but it has become increasingly evident that in no one of them alone
is the key to the problem of mitosis to be found. In spite of the confi-
dence that some progress has been made, at least in the elucidation of
certain phenomena which must have a part in any ultimate explanation,
it is nevertheless true that the statements made twenty years ago by
Wilson (1900, p. Ill) may be taken as an essentially accurate expression
of the condition of the subject: "When all is said, we must admit that
the mechanism of mitosis in every phase still awaits adequate physio-
logical analysis. The suggestive experiments of Biitschli and Heidenhain
lead us to hope that a partial solution of the problem may be reached
along the lines of physical and chemical experiment. At present we can
only admit that none of the conclusions thus far reached, whether by
observation or by experiment, are more than the first naive attempts to
analyse a group of most complex phenomena of which we have little real
understanding."

186

INTRODUCTION TO CYTOLOGY

CYTOKINESIS

In the foregoing pages discussion has been limited largely to karyo-
kinesis. In the present section attention will be directed to cytokinesis,
or the division of the extra-nuclear portion of the cell.

In plants the wall separating the two daughter cells is formed by
two general methods: cell plate formation and furrowing. The first and
more common of these methods, by which a wall is formed in close
association with the spindle fibers at the close of mitosis, has been briefly
described in the foregoing section on the achromatic figure (p. 176) and
will be taken up in greater detail in the following section on the cell wall
(p. 190). At this point we shall therefore describe the second method,
that of furrowing, which in plants is seen most conspicuously in the
thallophytes and in the microsporocytes of the higher plants. The
review of the subject given by Farr (1916) will be followed.

Thallophytes. — In Spirogyra Strasburger (1875) showed that the
wall between the two daughter cells appears as a "girdle" or ring-like
ingrowth from the side wall of the parent cell. This wall continues to

,\

I*! /•"* — S1'

gill

FIG. 62.

FIG. 63.

FIG. 62. — Cytokinesis by furrowing in Closterium. Only the central part of the cell
is shown. X 700. (After Lutman, 1911.)

FIG. 63.

A, Cleavage furrows beginning to form at periphery of sporangium of Rhizopus nigricans.
X 1500. B, Cleavage in the sporangium of Phycomyces nitens: intersporal substance in the
angular furrows. X 500. (Both after D. -B. Swingle, 1903.)

grow centripetally by the addition of new material at its inner edge while
the protoplast develops a deep cleavage furrow, the process continuing
until the separating wall is completed at the center of the cell. A
somewhat similar process occurs in Closterium (Lutman 1911) (Fig. 62).
In the brown algae Sphacelaria (Strasburger 1892; W. T. Swingle 1897)
and Dictyota (Mottier 1900) the wall develops uniformly across the

THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 187

whole equatorial plane at the same time, and not as a progressive in-
growth from the periphery.

In the fungi Harper and others showed that the two daughter cells are
separated by the development of a cleavage furrow in which the new wall
is laid down. In large multinucleate masses that become broken up into
spores this progressive cleavage is a very complicated process. The
manner in which the furrows develop is shown in the studies of Timberlake
(1902) on Hydrodictyon, D. B. Swingle (1903) and Moreau (1913) on
Rhizopus and Phy corny ces, Davis (1903) on Saprolegnia, Rytz (1907)
on Synchytrium, and Harper (1899, 1900, 1914) on Synchytrium, Pilobolus,
Sporodinia, Fuligo, and Didymium. In Rhizopus (Fig. 63, A) the
cleavage furrows begin to form both at the peripheral membrane of the
sporangium and at the columella and work gradually into the multi-
nucleate protoplasm, eventually cutting out multinucleate blocks which
become the spores. In Phycomyces (Fig. 63, B) small vacuoles appear in
the midst of the multinucleate protoplasm, enlarge and become stellate,
and cut out spore masses with from 1 to 12 nuclei
each. In the myxomycete, Fuligo, the cleavage
is from the surface inward, and the multinucleate
blocks are subdivided by further furrowing into
uninucleate spores. In Didymium the spores are
delimited in a similar way by furrows which
begin to form along the young capillitium fila-
ments in the interior of the multinucleate mass
as well as at its periphery.

Microsporocytes. — In the microsporocytes of
the higher plants it has been shown with great
clearness by Farr (1916, 1918) that the quadri-
partition to form spore tetrads of the tetrahedral
type is brought about by furrowing, previous ac-
counts having generally stated that the walls
are formed by the cell plate method. Farr finds
that after the four microspore nuclei are formed
they all become connected by a series of six
spindles, or sets of connecting fibers. The two
spindles of the second maturation mitosis may
persist, four new ones being added, or the two
may disappear, six new ones being developed.
Although some sporadic thickenings may ap-
pear on these fibers they have nothing to do with the formation of the
separating walls, there being no centrifugally growing cell plates such as
are seen in cells dividing by the cell plate method. Constriction fur-
rows appear at the periphery of the cell (Fig. 64) and grow inward until
they meet at the center, dividing the protoplast simultaneously into four

FIG. 64. — Cytokinesis by
furrowing in the micro-
sporocyte of Nicotiana.
X 1400. (After Farr, 1916.)

188 INTRODUCTION TO CYTOLOGY

spores. Any fibers which these furrows encounter as they grow inward
are probably incorporated in the new wall, but they play no prominent
part in wall formation: the development of the furrows appears to be
entirely independent of the fibers present.

• In his first paper (1916) Farr states that the microspore tetrads of
the bilateral type are usually formed by the cell plate method, a wall
being formed across the diameter of the microsporocyte on the connecting
fibers after the first maturation mitosis, and the two daughter cells
being divided in a similar way after the second mitosis. In his second
contribution (1918) he shows that in Magnolia such tetrads also are
formed by furrowing. After the first mitosis a cleavage furrow starts
to form, but its development is arrested until after the second mitosis,
when it resumes its growth toward the center and forms a wall across the
diameter of the spherical protoplast. At the same time other new
furrows subdivide each hemisphere, so that four uninucleate microspores
result. Farr states that no case of bipartition by furrowing is known in
the higher plants; bipartition begins in Magnolia, but the furrow ceases to
grow until other furrows are formed after the second mitosis, the eventual
division occurring by quadripartition. In the lower plants, however,
bilateral tetrads may be formed by the cell plate method. It is the opinion
of Farr that furrowing in microsporocytes is due to conditions similar to
those which bring it about in animal eggs (see below), since both float
freely in a liquid.

Animals. — In animals there is found nothing corresponding to the
formation of a cell plate on the spindle fibers and its development into a
thick wall such as is seen in plants. As noted in the section on the
achromatic figure, there is often a slight differentiation at this region
(the "mid-body"), but it has nothing to do with cytokinesis, which is
brought about by simple constriction or furrowing. This process is
most easily followed in the segmenting egg. In small eggs, such as those
of worms, the daughter cells (blastomeres) round up and become more or
less spherical, whereas in larger eggs, such as that of the frog, a cleavage
furrow appears at one pole and develops through the egg without altering
the shape of the latter, so that the first two blastomeres have the form of
hemispheres. It is with animal eggs that most of the researches on the
mechanism of cytokinesis by furrowing have been carried out.

Mechanism of Furrowing. — Attempts to explain furrowing and the
separation of the daughter cells on physico-chemical grounds have been
rather numerous. Many years ago Biitschli (1876) advanced the view
that as a result of a specific activity on the part of the centrosomes cyto-
plasmic currents are set up which flow toward the centrosomes and
produce a higher surface tension at the equator of the cell, this in turn
bringing about furrowing and cell-division. McClendon (1910, 1913)
also reported an increase in surface tension at the region of furrowing.

THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 189

On the contrary, Robertson (1911, 1913) and others attribute furrowing
rather to a decrease in the equatorial surface tension, this decrease being
due to a diffusion of materials toward that region from the daughter
nuclei. Evidence favoring Butschli's interpretation has been afforded
by the studies of Spek (1918). Spek imitated furrowing and division
with oil and mercury droplets in water, and showed that by lowering the
surface tension at two poles of the droplet the relatively higher surface
tension at the equatorial region could be made to bring about the con-
striction and fission of the droplet. In both droplet and dividing egg
he found streamings such as Erlangen (1897) had described in the nema-
tode egg: an axial movement polewards to the regions of low surface
tension and a superficial streaming toward the equatorial region of higher
surface tension, the streams turning inward at the furrow (Fig. 65).

FIG. 65. — Diagram showing streaming and furrowing in the egg of Rhabditis
(A) and an oil droplet (B). (After Spek, 1918.)

Although the causes of the initial changes in surface tension in the case
of the cell are relatively obscure, these experiments of Spek show beyond
question that alteration in surface tension and streaming are very im-
portant factors in cell-division of this type.

The relation of periodic changes in the viscosity of the egg substance
to cytokinesis by furrowing has recently been discussed by Chambers
(1919). Immediately after the entrance of the spermatozoon into the
echinoderm egg the sperm aster begins to differentiate as a semi-solid
region near the sperm head. (See p. 279.) When the aster is most fully
developed the egg has its maximum viscosity (Heilbrunn 1915). As
the aster disappears the egg again becomes more fluid. Then a second
solidification begins at two centers forming the amphiaster, or bipolar
figure. The growth of these two semi-solid masses results in the elonga-
tion of the egg, and eventually in the development of a cleavage furrow
in the more fluid portion of the egg substance separating them. After
cleavage is complete the semi-solid masses (asters) revert to a more fluid
state. The formation of the cleavage furrow, moreover, may be pre-
vented by mechanical means. At the second mitosis in eggs so treated
(binucleate eggs) there are four centers of semi-solidification rather than
two, and the egg cleaves simultaneously into four blastomeres. An egg
cut into two pieces during the amphiaster stage will, provided it does not

190 INTRODUCTION TO CYTOLOGY

return to the fluid state, continue to cleave along the normal plane
through the equator of the cell as if nothing unusual had happened. All
of these observations indicate a close dependence of cytokinesis upon the
temporary differentiation of semi-solid masses in the egg cytoplasm, and
throw much light upon the question of the true nature of the achromatic
figure.

THE CELL WALL

Probably the most striking difference which meets the eye in a com-
parison of animal and plant tissues lies in the relative degree of dis-
tinctness with which the limits of the individual cells may be made out.
Animal cells as a rule are separated only by very thin limiting membranes
which in many tissues are so delicate as to be scarcely discernible,
whereas the cells of plants usually possess conspicuous firm walls, which
in the case of woody plants become greatly thickened and afford
mechanical support to large bodies.

The Primary Wall Layer. — Since the time when mitotic cell-division
was first carefully studied with the aid of modern methods it has been
known that in the cell wall of plants the primary layer, or middle lamella
(the "intercellular substance" and "cement" of early writers), is formed
in most cases in close connection with the spindle fibers at the close of
mitosis.1 The exact manner of its origin, however, has proved to be a
very difficult point to determine, and has formed the subject of a Idng
continued controversy. (See papers of Timberlake and Allen, 1900
and 1901.) During the telophases of mitosis the spindle fibers con-
necting the two daughter nuclei develop thickenings (Fig. 58, D), enlarge
until they come in contact with one another and fuse to form a cell plate,
or partition, between the daughter cells. For some time it was thought
(Strasburger 1875, 1882, 1884) that the cell plate so formed became at
once the middle lamella, upon which secondary and frequently tertiary
layers were subsequently deposited by the protoplasts on either side.
Strasburger here found support for his theory that the cell wall is essen-
tially a transformed layer of the protoplast, in opposition to Nageli and
von Mohl, who regarded it as primarily a secretion product. As a
result of further researches, however, he later (1898) abandoned this
view and adopted an interpretation that had been suggested by Treub
(1878), namely, that the cell plate formed by the consolidation of the
swellings ("microsomes") on the spindle fibers very soon splits to form
the plasma membranes of the two daughter cells, and that there is then
secreted between these membranes by the protoplasts a substance
which becomes the primary layer, or middle lamella. The correctness
of this view was confirmed by the careful researches of Timberlake
(1900) and Allen (1901). Timberlake pointed out that in the micro-

1 Discussion is here limited to the walls of higher plant tissues. The ectoplast of
naked cells has been dealt with in Chapter III.

THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 191

sporocytes of Larix and the root cells of Allium the connecting fibers
first thicken near the nuclei, then become uniform throughout their
length, and finally become swollen at the equatorial region, indicating a
transfer toward that region of the material that is to compose the cell
plate. Allen was able to show not only that the middle lamella itself
may increase in thickness by the addition of new material before the
secondary layers begin to be laid down, but also
that it consists in reality of two layers representing
the secretions contributed by the two daughter
protoplasts. Where these two masses of secreted
material meet there is developed a median plane of
weakness which is ordinarily invisible but along
which the lamella invariably splits when inter-
cellular spaces are developed by the rounding up
of the cells. By the use of proper staining methods
it has been found possible to differentiate this
" primary cleavage plane." The continuity of the
middle lamella is interrupted, if at all, only by
the fine pores through which pass the protoplasmic
strands connecting adjacent cells. (See p. 46.)

Secondary and Tertiary Wall Layers (Fig. 66).—
It is probable that the deposition of the secondary
layer begins after the cell has reached nearly or
quite [its full size, though to this there are ap-
parently certain exceptions. The secondary layer,
which seems to be formed with considerable
rapidity, differs from the primary layer not only
chemically (see below) but also in structure, being
interrupted by circular or elongated areas in
which no secondary substance is deposited, so
that the cells at these places are separated only by
the delicate primary membrane. Such a wall is
said to be "pitted," the primary lamella extending
across the pit being termed the closing membrane.
The central portion of this membrane sometimes
(vascular cells of gymnosperms chiefly) has a more or less conspicuous
thickening known as the torus. The portion of the membrane
around the torus is pierced by fine pores: in some cases these may
become so large and numerous that the torus appears to be suspended
on a meshwork (Fig. 67), while extreme cases are known in which it is
held in place only by a few strands. In bordered pits (Fig. 68) the second-
ary wall overarches the margins of the closing membrane. In this type
of pit, characteristic chiefly of water-conducting cells of the gymnosperms,
the closing membrane is of such a nature that its position in the center of

j

r

FIG. 66. — Longitudinal
and transverse sections
of a gymnosperm tra-
cheid; p, primary wall
or middle lamella; s,
secondary layer; t, spiral
tertiary thickening.

192

INTRODUCTION TO CYTOLOGY

the pit is readily altered. Probably because of changes in pressure it
swings to the side of the pit; the torus then lies against the pit opening,
or "mouth," and the pit is blocked except for slow diffusion through the
rather thick torus. The latter may even be forced tightly into the pit
mouth.

The secondary wall layer may be even more limited in extent, only a
small portion of the primary wall being covered. Such is the case in
protoxylem cells, in which the secondary layer is deposited in the form of
rings and spirals (Fig. 4). This form of thickening, together with the

FIG. 67. FIG. 68.

FIG. 67. — Pits in the wood of Larix, showing perforations in pit membrane. X 800.
(After Bailey.)

FIG. 68. — Diagram of bordered pit of coniferous wood.

A, section of pit showing closing membrane supporting the torus, and secondary layers
on each side of middle lamella. B, face view of same. C, section- showing torus forced
against mouth of pit. (After Bailey.)

peculiarly extensible character of their primary walls, allows for the great
increase in length of these cells necessitated by the continued growth of
the young organs in which they chiefly function. In some cells, notably
the tracheids of certain gymnosperms and the vessels of many angio-
sperms, a tertiary layer is deposited upon the secondary wall. This ter-
tiary layer takes the form of slender spirals, rings, and other figures
resembling the secondary thickenings of protoxylem cells.

The Physical Nature of the Cell Wall.— Hugo von Mohl (1853, 1858)
first expressed the idea that the cell wall grows by apposition, i.e., by.the
deposition of material in successive laminae. Although certain other
workers (Wigand 1856) supported this view, it became over-shadowed for
a time by the theory of Nageli. This investigator, as a result of his classic
researches on the wall and on starch grains (1858, 1862, 1863), concluded

THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 193

that the wall is made up of ultramicroscopic crystalline micellae, sur-
rounded by water films. Growth of the wall in thickness and in area he
believed to be due to the intercalation of new micellae between the old ones,
a process termed intussusception. Contrasted with this was Strasburger's
development of the apposition theory (1882, 1889). Although Stras-
burger agreed that the wall had both solid and liquid constituents, he
held that the latter were not complex micellae, but only molecules linked
together in the form of a reticular framework by their chemical affinities.
Growth in area he thought was merely a matter of stretching without the
intercalation of additional particles, while increase in thickness was
supposed to be accomplished by apposition, or the deposition of layers
of new material in the form of small particles, or microsomes. The
striations which both he and von Mohl observed in the wall substance
were regarded by Strasburger as due to the linear arrangement of these
microsomes.

That the cell wall is not merely a lifeless secretion of the protoplast,
but contains protoplasm in some form, is a view which has often been
upheld, and involves problems which are still far from being solved.
Prominence was given to the view by Wiesner (1886), who looked upon
the growing cell membrane as a living part of the cell. Following Stras-
burger's early view, he held the primary layer- to be wholly protoplasmic,
and supposed the growing wall to be made up of regularly arranged
particles, which he called dermatosomes, connected by fine fibrils of
protoplasm. Growth was accomplished by the intussusception of new
dermatosomes. Evidence in support of Wiesner's interpretation was
brought forward by Molisch (1888), who showed that when tyloses come
into contact pits are formed exactly opposite each other in the two
abutting walls, a phenomenon which it would be difficult to explain were
the walls without living substance.

The new intussusception theory of Wiesner was accepted by a number
of workers including Haberlandt and Zacharias (1891). The apposition,
or lamination, theory of Strasburger also had many supporters, among
them being Noll (1887), Klebs (1886), Zimmermann (1887), and Askenasy
(1890). According to Pfeffer (1892) both processes, the intussusception
of new particles or molecules and the apposition of new material in layers,
are concerned in the development of the wall. This view was later
adopted by Strasburger (1898), and has received general acceptance.
But much work must be done before any final conclusion can be drawn re-
garding many points. Especially obscure is the exact relationship of
the protoplasm and the wall. The solution of this difficult problem
must await the results of further inquiries by both the cytologist and the
biochemist.

The Chemical Nature of the Cell Wall. — Through the researches of
Payen (1842), Fre"my (1859), Kabsch (1863), Wiesner (1864, 1878), and

13

194 INTRODUCTION TO CYTOLOGY

particularly Mangin (1888-1893) it has been found that the chief constitu-
ents of the newly formed cell walls of plants are pectose and cellulose —
that the primary wall or middle lamella consists of pectose, the secondary
layer of pectose and cellulose, and the tertiary layer of cellulose. These
substances however, rarely exist in the wall in pure and unmodified form.
The pectose of the primary layer changes later to insoluble pectates,
especially the pectate of calcium, while the secondary and tertiary layers
very soon become greatly changed in composition, not alone through the
addition of a variety of new substances, but also through an actual trans-
formation which in some cases appears to be complete. For example, the
secondary and tertiary layers of xylem cells, although at first containing
much cellulose, may later become so completely transformed into or re-
placed by lignin that they show no reaction whatever to cellulose stains.
In some cases the primary wall may undergo a certain amount of lignifi-
cation also. The walls of many cells become heavily impregnated with
cutin or suberin, the latter substance being responsible for the peculiar
character of corky tissues. Infiltration by cutin, or "cutinization," is
to be distinguished from "cuticularization," by which is meant the secre-
tion of a layer of cutin (cuticle) on the outside of the cell. A variety of
mineral substances, such as silica, calcium oxalate, and calcium carbonate,
as well as more complex organic compounds, such as tannin, oils, and
resins, are often deposited in the walls of old cells. The heart woods
of trees owe their qualities largely to the presence of these additional
materials.

In spite of these modifications, however, it is still true that Cellulose
is the substance chiefly characteristic of plant cell walls in general. Al-
though cellulose has been identified in certain animals, the membranes of
practically all animal cells are composed of other substances, such as
keratin, elastin, gelatin, and chitin. In the fungi also the role of cellulose
appears to be played in part by chitin.

The Walls of Spores. — Special attention has been given to the develop-
ment of the elaborate walls, or coats, of the spores of various plants in a
number of investigations. Strasburger (1882, 1889, 1898, 1907) con-
cluded that such coats arise by two general methods: (1) by the growth in
thickness (by apposition) of the original wall of the spore cell through the
activity of the protoplast, as in the pollen grains of Malva and other angio-
sperms, and (2) by a deposition of material upon the original wall by the
tapetal fluid in which the young spores lie, as in the case of the megaspore
of Marsilia.

The highly specialized coats of the megaspore of Selaginella have been
most intensively studied, particularly by Fitting (1900, 1906) and Miss
Lyon (1905), whose accounts disagree in several points. At the close of
the tetrad division there is formed about each young spore a thick gela-
tinous "special wall," at the inner surface of which, according to Fitting,

195

the spore coats begin to differentiate. The exospore first appears, and
just outside of it the rough perispore soon begins to develop. Then a
second layer, the mesospore, is formed within the exospore. Between the
protoplast, which is at this time very small, and the mesospore, and
between the exospore and the mesospore, there are developed two cavities
filled with a sporangial fluid which furnishes material to the growing
coats. Emphasis is placed on the fact that the coats are able to increase
in thickness while they have no immediate contact with the protoplast.
The protoplast now expands, after which a third coat, the endospore, is
formed at its surface. The mature spore thus has three coats according
to Fitting's interpretation, which Denke (1902) and Campbell (1902)
confirmed.

e x. s.m, e n.

FIG. 69. — The developing megaspore coat of Selaginella rupestris:
p, protoplast with nucleus; en, endospore; s.m., undifferentiated portion of "spore
membrane;" ex, exospore: the outer denser portion is the "perinium." (After Lyon, 1905.)

Miss Lyon found that the spore coats (S. rupestris) begin to differ-
entiate in the midst of the "spore membrane" ("special wall:" Fitting),
rather than at its inner surface as Fitting thought. The exospore
first appears as a double zone, the outer part of which becomes the per-
inium (perispore: Fitting) (Fig. 69). The small protoplast gradually
expands and pushes back the undifferentiated inner portion of the spore
membrane; and while it does so a second coat is formed at its surface
and becomes the endospore (mesospore: Fitting) which increases in thick-
ness by lamination. In another species (S. emiliana) the exospore and
endospore form simultaneously. Miss Lyon thus finds two coats rather

196 INTRODUCTION TO CYTOLOGY

than three, but points out that a portion of the spore membrane which
may remain in an undifferentiated condition until a late stage may easily
be mistaken for a third coat. The two "spaces" in the immature spore
wall she holds to be undifferentiated regions in the spore membrane, and
not cavities filled with a foreign fluid; and further urges that the proto-
plast is at all times in contact with the gelatinous spore membrane in
which the coats are differentiating, opposing the view that the latter
have the power of independent growth in thickness.

Evidence favoring the view that the spore coats can grow while not
in contact with the protoplast has been brought forward by Beer (1905,
1911) and Tischler (1908). Beer asserts that although both the primary
wall and the secondary thickening layer of the pollen grain (in certain
members of the Onagraceae) originate in intimate connection with the
plasma membrane, most of their subsequent growth occurs by intussus-
ception while they are completely separated from the protoplast, which
secretes the material used. The development of the pollen wall in
Ipomoea purpurea has been described in great detail by Beer. Around
each young spore immediately after its formation there appears a tem-
porary gelatinous "special wall," upon the inner surface of which the
protoplast deposits the exine, or outer spore coat. This is at first homo-
geneous, but soon differentiates into a thin outer lamella and an inner
zone made up of a network of thickenings with the rudiments of spines
at its nodes. Both the spines and the small rodlets, which develop in a
clear space appearing between the outer lamella and the network of
thickenings (mesospore) , undergo most of their development after they
are separated from the protoplast. Tischler (1908) reports that the
exine of the pollen of sterile Mirdbilis hybrids may continue to increase
in thickness after the protoplast begins to degenerate.

As an example of the formation of spore coats through the activity
of a tapetal plasmodium may be taken the case of Equisetum, described
by Beer (1909) and Hannig (1911). The spores of this form have three
coats: an endospore, an exospore, and a perispore consisting of several
layers including the one which splits to form the "elaters." The young
spore cell at first has a simple membrane, the rudiment of the exospore.
The walls of the tapetal cells dissolve, allowing the cell contents to flow
freely among the spores as a tapetal plasmodium. Upon the spore
membrane the plasmodium deposits successively (1) an inner gelatinous
layer, (2) the "middle coat," (3) an outer gelatinous layer, and (4) the
elater layer. The exospore develops from the original membrane after the
middle coat is formed, and the endospore, or innermost coat, is developed
last of all.

From this brief review, to which other examples might be added, it is
evident that spore coats may develop in a variety of ways, but too little
is known to warrant any statement as to which method may be the most

THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 197

general one. Although cytological interest centers chiefly in other
problems, further studies on spore coats would not only contribute to
our understanding of cell wall formation, but would also aid in solving the
problem of the possible existence of protoplasm in the wall.

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200 INTRODUCTION TO CYTOLOGY

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THE A CHROMA TIC FIG URE, C YT 0 KINESIS, A ND CELL WALL 201

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WILSON, E. B. 1899. On protoplasmic structure in the eggs of echinoderms and

some other animals. Jour. Morph. 16 : Suppl. 1-23.

1900. The Cell in Development and Inheritance.

1901. Experimental studies in cytology. I. A cytological study of parthenogene-
sis in sea urchin eggs. Arch. Entw. 12: 529-596. pis. 11-17. figs. 12.

ZACHARIAS, E. 1888. Ueber Kern und Zellteilung. Bot. Zeit. 46: 33-40, 51-62.

pi. 2.
1891. Ueber das Wachstum der Zellhaut bei Wurzelhaaren. Flora 74: 466-491.

pis. 16, 17.
ZIMMERMANN, A. 1887. Die Pflanzenzelle.

1893. Sammel-Referate. 6. Beih. Bot. Centr. 3: 342-354

CHAPTER X
OTHER MODES OF NUCLEAR DIVISION

In accordance with the well established principle which states that
only through the simpler organisms can an adequate understanding of
those higher in the scale of complexity be approached, search has been
made for primitive modes of nuclear division with the hope that light
may thereby be thrown upon the origin and significance of the elaborate
karyokinetic process which is so universally found in the cells of higher
animals and plants. It is to be acknowledged that such a phylogenetic
explanation of mitosis is very far from being reached, but many of the
observations recorded are nevertheless of a very suggestive nature. To
botanists the most interesting of these have been made upon the Cyano-
phyceae, which have long been a subject of controversy in this connection.

Cyanophyceae. — For many years the nature of the "central body"
of the cells of such blue-green algae as Oscillatoria remained very obscure.
Biitschli (1890), Dangeard (1892), Scott (1888), and others believed it
to be a nucleus of a somewhat primitive type, whereas other investigators,
among them Zacharias (1892) and Chodat (1894), denied its nuclear
nature. Zukal (1892) held that the peripheral portion of the cell repre-
sents a chromatophore, the central body consisting of cytoplasm with a
number of minute nuclei imbedded in it.

One of the first critical accounts based partly on the study of sections
was that of Fischer in 1897. Fischer concluded that the central body,
in which he found no chromatin, is the main portion of the cytoplasm,
and not to be regarded as the forerunner of the nucleus or indeed as an
independent organ at all. He also investigated the nature of the periph-
eral portion of the protoplast. By treating the plants with 10 per
cent hydrofluoric acid he dissolved away the other parts of the cell, leav-
ing this portion intact; and as the result of comparative studies on other
plants he concluded, in harmony with Zukal, that it is a single large
chromatophore.

Since Fischer's work the most important contributions are tKose of
Hegler, Kohl, Olive, Phillips, Gardner, and Miss Acton. Contrary to
the view of Fischer, all of these cytologists interpret the central body
as a nucleus, and the first three regard its division as essentially mitotic.
The opinions of these workers with respect to the organization of the cell
of the Cyanophyceae and the behavior of its nucleus are summarized

below.

202

OTHER MODES OF NUCLEAR DIVISION

203

According to Hegler (1901) the nucleus contains granules of chromatin
but no nucleolus or nuclear membrane, division occurring by a simple
form of mitosis. The coloring matter exists in the form of minute
granules or cyanoplasts. Two other kinds of bodies are also present:
albuminous slime globules and albuminous crystals (cyanophycin granules)
representing reserve food.

Kohl's (1904) description of the cell of Tolypothrix (Fig. 70) is one of
the most detailed which has been given in this group of researches. Kohl
shows that the nucleus of this form has extensions reaching outward

FIG. 70. — Structure and division of the cell of Tolypothrix lanata.
A, cell in the vegetative state: c, cytoplasm; n, nucleus; /, fat droplets; p, phycocyanin
and chlorophyll granules; s, slime globules; g, granules of cyanophycin. B, four stages of
cell-division in Tolypothrix, showing transverse division of chromosomes. C, diagram
showing 6 stages of cell-division. (After Kohl, 1903.)

toward the cell wall, and that they are withdrawn at the time of nuclear
division. The nucleus, which contains chromatin, also includes a num-
ber of large Zentralkorner, or slime globules, while in the cytoplasm are
fat droplets, cyanophycin granules of reserve albumen, and granules of
chlorophyll and phycocyanin. The nucleus, which is very rarely in the
resting state, divides as follows : the chromatic material forms a spireme
which segments into a definite number of chromosomes; these lie in the
direction of the long axis of the cell and break transversely as the separat-
ing wall grows inward from the periphery. Their halves are thus in-
cluded in the two daughter cells, where they form daughter nuclei without
membranes.

204 INTRODUCTION TO CYTOLOGY

Olive (1904) l finds that the nucleus of Oscillatoria (Fig. 71) consists
of a fibrous achromatic framework with a number of very small chroma-
tin granules, and is nearly always in some stage of division. A spireme
is formed carrying 16 chromatin granules (8 in Glceocapsa and 32 in one
species of Oscillatoria), each representing a chromosome. The spireme
and its chromatin granules are split longitudinally, and the daughter
spiremes with the daughter granules separate, a distinct central spindle
extending between them. The dividing wall is formed as a centripetally

growing partition. In Glceocapsa the cell di-

F- ';A,Li * , U*4l-:-;^ vides by simple constriction. The vegetative
)'•:'•.'• f\n^^^- — 4^4 nuclei of Oscillatoria very rarely approach

the resting condition, but in spores and
heterocysts they soon pass into this state, a
nuclear membrane and vacuole being de-
veloped. In the heterocyst the protoplast
disorganizes. Olive regards the central body
of the Cyanophycese as not essentially different
FIG. 71.— Nuclear division from the nucleus of the higher plants, although
in Oscillatoria Froeiichia. 1,2, ft [s relatively primitive in several features.

3, 4, four successive stages. .

(After Olive, 1904.)] -In the cytoplasm he rinds both cyanophycm

granules and slime globules, but no cyano-

plasts, the coloring matters being diffused in the peripheral portion
of the protoplast.

Fischer (1905), in reply to the claims of Kohl and Olive, reasserted
his view that the central body is not a nucleus, but rather an accumulation
of carbohydrate materials. The glycogen formed as a result of assimila-
tory activity gathers in the central body where it is transformed into
another carbohydrate, anabcenin, which assumes the form of sausage-
shaped structures. At the time of cell-division these masses of reserve
material are distributed by a process of " pseudomitosis " to the daughter
cells. Fischer therefore regards the mitotic figures observed by others
as significant in connection with nutrition rather than with the functions
usually attributed to nuclei.

Gardner (1906), investigating a number of species, found nuclei of
three kinds, which he called the diffuse type, the net karyosome type,
and the primitive mitosis type respectively. The "diffuse type" of
nucleus, which has no very definite delimitation from the peripheral
portion of the protoplast, contains an indefinite number of chromatin
masses. As the cell divides this central aggregation of chromatic material
divides into approximately equal portions. In the "net karyosome"
type, found in Dermocarpa, the distinction between the nucleus and the
surrounding cytoplasm is much clearer. The nucleus has an achromatic

1 A very convenient tabulation of the results of researches on cell structure in the
Cyanophyceae up to 1904 is given by Olive.

OTHER MODES OF NUCLEAR DIVISION

205

network with chromatin granules at its nodes, and constricts simultane-
ously into a large number of daughter nuclei which pass to the conidia.
In Synochocystis aquatilis occurs the "primitive mitosis" type: here
Gardner found the only case of anything approaching mitotic behavior.
A spireme develops and segments into three pieces which arrange them-
selves parallel to the long axis of the cell and divide transversely; the
daughter pieces then separate and a centripetally growing cell wall
completes the division of the cell. Gardner thus finds in the Cyano-
phyceae "a series of nuclear structures, beginning with a very simple

m

P

7)

FIG. 72. — The nuclei of various members of the Chroococcacese.
A, cell of Chroococcus turgidus with scattered metachromatin granules (m) and plasma-
tic microsomes (p) ; division beginning. X 2500. B, cell of Gloeocapso. with chromatic
granules. C, Merismopedia elegans, showing two stages of nuclear division. X 1500.
D. Chroococcus macrococcus: n, nucleus; m, metachromatin; v, vacuole. X 2500. E,
dividing nucleus of Chroococcus macrococcus. X 2500. (After Acton. 1914.)

form of nucleus scarcely differentiated from the surrounding cytoplasm
and dividing by simple direct division" and passing "by very gradual
steps to a highly differentiated form of nucleus which in dividing shows
a primitive type of mitosis, and in structure approximates the nucleus of
the Chlorophycese and the higher plants."

In a more recent investigation of the Chroococcaceae Miss Acton
(1914) finds that the nucleus is in general much simpler than that of the
higher plants. Like Gardner, however, she points out a series beginning
with a form in which definite organization is almost entirely lacking and
ending with one in which the structure of the higher plant nucleus is
closely approached (Fig. 72). In Chroococcus turgidus the protoplast is

206 INTRODUCTION TO CYTOLOGY

made up of a ground substance with a reticulum bearing bodies of two
sorts: granules of metachromatin closely similar to chromatin in reaction,
and cyanophycin granules, or plasmatic microsomes. Although there is
no definitely delimited central region in the cell the metachromatin is
found mostly at the center and the cyanophycin mostly nearer the pe-
riphery. When the metachromatin granules become numerous division
sets in, a centripetally growing wall cleaving the protoplast into two
daughter cells. In Glceocapsa the central region is somewhat more definite
and may often show a spireme-like appearance such as Olive describes;
but this may possibly be an artifact. In Merismopedia elegans there is a
definitely delimited nucleus, not like that of the higher plants but merely
an accumulation of chromatin or chromatin-like material which divides
just before the cell constricts into two portions. In Chroococcus macro-
coccus, finally, the nucleus and cytoplasm are sharply distinct, the former
having a reticulum with chromatin granules at its nodes and dividing
by a sort of constriction at the time of cell-division.

As a result of these observations Miss Acton advances a theory of the
evolution of nucleus and cytoplasm, which is briefly as follows. The
excess food elaborated by the protoplast with its pigments was first
stored as plasmatic microsomes composed of a carbohydrate, cyanophy-
cin. As the reserve material became more complex in nature the nucleo-
protein metachromatin was elaborated; this became aggregated at the
center of the cell, insuring its equal distribution in cell-division, as in
Merismopedia. There thus arose in the cell a physiological and mor-
phological differentiation, the nucleo-protein with its portion of the
supporting reticulum becoming a stable nucleus, as in Chroococcus
macrococcus, and the ground substance remaining as the cytoplasm.

Summary. — In the Cyanophyceae, therefore, although these forms in
all probability had nothing directly to do with the evolution of the higher
plants, we see a series of stages such as may well have occurred in the
evolution of the nucleus and its complicated mitotic division. In the
simplest forms the material concerned with those cell activities which
in higher organisms are associated with the nucleus, is scattered through-
out the cell without the morphological distinctness characteristic of an
organ in the strict sense. It is passively distributed to the daughter
cells when the cleavage wall is formed at the time of cell-division. In
other cases this material reacts more strongly like true chromatin and
may form a more or less definite aggregation separating into two masses
as the cell divides. This metachromatin, which is a nucleic acid com-
pound, has also been observed in other algse, in Protozoa, and in fungi,
including the yeasts. It appears to represent a reserve material, though
it may also have other functions. Finally, definite and well organized
nuclei are present in certain of the forms described in the foregoing
pages, and although these nuclei may lack some of the features exhibited

OTHER MODES OF NUCLEAR DIVISION

207

by the nuclei of higher organisms, they show in the division and distribu-
tion of their chromatic elements many of the characteristics of true
mitosis. In the evolution of the nucleus through such a series of stages
we have an illustration of "the conception of cell structure which im-

FIG. 73. — Nuclear division in Protozoa.

A, one form of mitosis in Amoeba diplomilotica. (From Minchin, after Aragao.) B,
Nuclear division in Coccidium schubergi. X 2250. (From Minchin, after Schaudinn.)
C, mitotic division of micronucleus of Paramcecium (horizontal figure on smaller scale
than others.) (From Minchin, after Hertwig.)

plies differentiated regions of a colloidal system in which special processes
have become localized and tend to remain fixed" (Harper 1919).

Protozoa. — Such an apparent derivation of mitosis from a simpler,
more indefinite division of a less sharply delimited mass of special

208 INTRODUCTION TO CYTOLOGY

chromatic substance is seen also in the protozoa. Here the chromatic
material in a number of species is held together in loose granular aggrega-
tions, and the achromatic figure is seen in curious, relatively simple
manifestations. In many cases, however, even among the Rhizopoda,
there occurs a very advanced type of mitosis, with spindle, centrosomes,
asters, and a definite number of chromosomes.

One of the simplest types of nuclear division found among the pro-
tozoa is known as "chromidial fragmentation." Here the nucleus is
resolved into a large number of chromatin granules, or chromidia, which
reassemble in two or more groups and form new nuclei. In nuclei of the
"vesicular type," found commonly among the Microsporidia, the chroma-
tin is concentrated in a single large body, or karyosome, which becomes
dumbbell-shaped and divides, the rest of the nucleus then dividing also.
In other forms, such as Coccidium (Fig. 73, B], the nucleus contains in
addition a second kind of chromatin which is approximately halved in
nuclear division. It is but a short step from such non-mitotic division
as this to the simplest types of mitosis ("promitosis") seen in many
protozoa. Within the group are found all gradations in complexity
from such primitive modes of division up to the advanced types showing
a complete achromatic figure with chromosomes which are regular in
form, number, arrangement, and division, just as in the higher animals.1

Both Metcalf and Kofoid (1915) have emphasized the fundamental
similarity of protozoan and metazoan nuclei. The process of mitosis
has the same succession of phases in the two cases, though many minor
variations occur. In some representatives of all the main groups of
protozoa are found elongated chromosomes which Metcalf regards as
linear aggregates of chromatin granules, and which split longitudinally,
giving exact equivalence to the daughter-nuclei. Although the cell
mechanism of Mendelian inheritance is thus held to be present in members
of each great protozoan group and to operate as in the metazoa at the
sexual stages, Metcalf believes this mechanism is not kept intact through
the vegetative phases as it is in the higher groups.

Other Cases in Plants. — With regard to the myxomycetes, the
researches of Strasburger (1884), Harper (1900), Jahn (1904, 1911),
Olive (1907), and Winge (1912) have shown that nuclear division is
essentially mitotic, and that in some cases the chromosomes are not
only definite in number but undergo a reduction prior to spore formation.
As an example of an exceptional condition may be taken Sorodiscus
(Fig. 74, A), in which Winge describes two sorts of chromatin: vegetative
trophochromatin and generative idiochromatin, the two forming a single
mass at the center of the nucleus. As nuclear division begins this mass
takes the form of three or four bodies very similar to chromosomes.

1 For a description of mitotic phenomena in protozoa see Minchin (1912, Chapter
VII).

OTHER MODES OF NUCLEAR DIVISION

209

The two kinds of chromatin now separate, the "trophochromatin placing
itself in the center and the generative or idiochromatin lying like a thin
equatorial plate around it." As the nucleus elongates the tropho-
chromatin body becomes dumbbell-shaped and breaks into two, while
the idiochromatin plate splits into daughter plates which apparently
move to the poles and cooperate with the trophochromatin in the forma-
tion of the daughter nuclei.

FIG. 74.

A, two stages of mitosis in Sorodiscus. (After Winge, 1912.) B, anaphase of nuclear
division in Euglena. Chromosomes grouped about dividing "nucleolo-centrosome."
(After Keuten, 1895.) C, chromosomes developing from nucleolus in Spirogyra. X 1335.
(After Berghs, 1906.) D, Mitosis in Spirogyra crassa. (After Merriman, 1913.)

A process with much the same appearance at certain stages is seen in
the flagellate, Euglena (Keuten 1895) (Fig. 74, B). Here the chromo-
somes group themselves about the large nucleolus which soon takes the
form of a dumbbell-shaped "central spindle" or "centrodesmose."
The nucleolus completes its division, the chromosomes meanwhile
separating into two groups which pass to the poles and reorganize the
daughter nuclei. In certain other flagellates Kofoid (1915) reports a
split spireme and a definite number of chromosomes which differ markedly
in size and shape.

In Cladophora (Carter 1919) nearly all the chromatin is contained in
one or more large chromatin nucleoli, or karyosomes. After the numer-
ous chromosomes have arrived at the two poles at the close of the ana-
phase the spindle connecting the two groups constricts and completes
the division of the nucleus.

Another unusual condition is found in Spirogyra (Fig. 74, C, D).
In this form nearly all of the chromatic material is lodged in the large
nucleolus, the nuclear reticulum being very delicate and almost invisible
in many preparations. According to Berghs (1906), Karsten (1908),
and Trondle (1912) all the chromosomes which appear in the prophase
and split as usual are derived from this nucleolus, most of its material

14

210 INTRODUCTION TO CYTOLOGY

being used in their formation. In the opinion of Miss Merriman(1913)
the chromatic bodies observed by the above workers are not true chromo-
somes, but are rather more indefinite chromatic aggregations which are
variable in number and appearance, and which are irregularly pulled
apart as mitosis proceeds. She finds here "no evidence throughout the
karyokinesis of an equational division of autonomous bodies."

In Zygnema both Escoyez (1907) and van Wisselingh (1914) find that
the reticulum, and not the nucleolus, gives rise to all the chromosomes.
Although the nucleolus furnishes no morphological element, chromatic
material may flow from it to the chromosomes as they develop from the
reticulum. Much the same condition is found in Marsilia (Strasburger
1907; Berghs 1909). Strasburger points out that in the somatic nuclei
(in the cells of the root and the young prothallium) most of the chromatic
substance is held in the nucleolus during the resting stages (Fig. 17, E),
and that the material of the reticular framework, which is very delicate,
is to be regarded as the substance of importance in heredity. Berghs
shows that the nucleolus consists of an achromatic substratum which
appears independently of the reticulum in the telophase and soon becomes
impregnated with chromatic material transferred to it from the chromo-
somes. In the next prophase the chromatic material flows back to the
delicate reticulum, from which the chromosomes gradually develop. As
the chromosomes increase in distinctness the nucleolus becomes paler,
and when the nuclear membrane breaks down the nucleolus dissolves in
the protoplasmic liquid. It is therefore clear that in Marsilia the nu-
cleolus is not a mere aggregation of the chromosomes of the telophase, as
might at first be supposed. The chromosomes arise from the reticulum
as usual, and not from the nucleolus as reported for Spirogyra. In these
observations we have additional evidence favoring the view of Haecker,
Boveri, Mare'chal, and others (see Chapter VIII) that it is the achromatic
substratum of the chromosome, and not the chromatic substance which
it carries, that should be regarded as. the persistent structural unity
representing the basis of inheritance.

Amitosis. — In amitotic or direct nuclear division the nucleus simply
constricts and separates into two portions while in the "resting" condi-
tion, no condensed chromosomes, centrosomes, spindle, or asters being
formed. As a general rule such a division of the nucleus is not followed
by a division of the cell; cells with two or more nuclei therefore commonly
result. As examples may be cited the tapetal cells in the anthers of
angiosperms, the internodal cells of Chara (Fig. 75) (Johow 1881), and
certain glandular cells of animals. The presence of more than one
nucleus cannot by itself be regarded as evidence that amitosis has
occurred, however. Amitosis appears to be of rather frequent occurrence
among the lower organisms, some of which show other methods of divi-
sion also. For example, amitosis occurs regularly in budding yeasts,

OTHER MODES OF NUCLEAR DIVISION

211

though the divisions giving rise to the ascospore nuclei have been shown
to be mitotic in certain cases. (See Guilliermond 1920.) Amitosis
was once believed to be the normal mode of nuclear division, mitosis
being looked upon as very exceptional. The true condition, so far as
higher organisms are concerned, has turned out to be quite the reverse:
it is evident that amitosis occurs frequently in certain kinds of cells, but
the mitotic method of division has been found to be
almost universal.

What the physiological significance of amitosis
may be is not well known. It was once suggested
(Chun 1890) that it aids the processes of metabolism
by increasing the nuclear surface in the cell, since
it is of such frequent occurrence in cells with a dis-
tinctively nutritive function. This view has recently
been restated by Nakahara (1917) as a result of his
work on the larva of Pieris. l The most generally held
opinion regarding amitosis in the higher organisms
was for many years that expressed by Flemming
(1891), namely, that it represents a degeneration
phenomenon or aberration of some kind, which would
explain why it is so often found in degenerating and
pathological tissues. In the words of vom Rath
(1891), "when once a cell has undergone amitotic
division it has received its death-warrant; it may indeed continue to
divide for a time by amitosis, but inevitably perishes in the end."

That the view of vom Rath must be modified has been indicated by
the results of a number of investigations. For instance, Pfeffer (1899)
and Nathansohn (1900) found that if Spirogyra filaments are placed in a
^ to 1 per cent solution of ether the nuclei divide by amitosis only, and
that when the filaments are returned to pure water the mitotic method
of division is resumed, with no evidence of degeneration. Haecker, how-
ever, working on the eggs of Cyclops, came to view such artificially in-
duced behavior not as true amitosis but rather as a much modified
mitotic division, which he termed "pseudoamitosis." Other cytolpgists
observed nuclear divisions that seemed intermediate in character between
mitosis and amitosis (Dixon in the endosperm of Fritillaria, 1895; Sargant
in the embryo sac of Lilium, 1896; R. Hertwig in Actinosphcerium, 1898;
Buscalioni in the endosperm of Corydalis, 1898; and Wasielewski in the
roots of Vicia faba, 1902, 1903). Hertwig accordingly concluded that
mitosis and amitosis are separated by no sharp boundary line, but are
connected by an unbroken series of transition stages.

1 In a second paper (1918) Nakahara gives a convenient review of the literature
of the subject.

FIG. 75. — Amitosis
in internodal cell of
Chara. X 413.

212

INTRODUCTION TO CYTOLOGY

As a result of his recent researches on chloralized cells (Fig. 76)
Sakamura (1920) interprets all such unusual types of nuclear division as
those described by Hertwig and Wasielewski as the effect of disturbed
mitotic division, but denies the claim of those authors that such types
of division represent actual transition stages between amitosis and
mitosis. True amitosis he regards as a fundamentally different process,
and as essentially a degeneration phenomenon.

FIG. 76. — Abnormal mitosis in chloralized root cells of Vicia.

A, chromosomes distributed irregularly in cell. B, scattered chromosomes beginning to
assume nuclear form. C, nucleus reconstructed by scattered chromosomes. D, scattered
chromosomes reconstructing 3 separate nuclei. E, chromosomes reconstructing 2 nuclei
connected by bridge. F, amitosis-like appearance resulting from condition shown in E.
(After Sakamura, 1920.)

On the contrary, Des Cilleuls (1914) reports that in the rabbit
periods of amitosis and mitosis succeed each other regularly in the same
cell lineage without affecting the vitality of the cells. In his opinion,
therefore, amitosis does not necessarily place the stigma of senescence
upon the cell. A similar conclusion is reached by Arber (1914), who finds
amitosis supplementing mitosis in the early growth stages of the leaves
and adventitious roots of Stratiotes aloides; and by McLean (1914), who
asserts that it is the sole method of nuclear division in the cortical
parenchyma of several aquatic angiosperms. Saguchi (1917) likewise
states that the nuclei in the ciliated cells of vertebrates divide by amitosis
only.

Amitosis and Heredity. — One of the most important theoretical ques-
tions raised by the phenomenon of amitosis is that of the effect which
the process may have upon the hereditary mechanism of the cell. Ac-
cording to the chromosome theory of heredity and development in its
usual form it has been thought that, although amitosis may occur in
connection with an altered metabolism in cells not to undergo further
differentiation, mitosis must occur exclusively in the germ cell lineage,
in order that the chromosomes and the hereditary elements they con-

OTHER MODES OF NUCLEAR DIVISION 213

tain shall be properly distributed to the reproductive cells; and also in
developing tissues and organs, so that differentiation may proceed nor-
mally. On the other hand, several workers (Meves; Flemming in his
later papers) admit that amitosis may not affect any hereditary powers
which the nuclei concerned may possess. Child (1907, 1911), who re-
ports amitosis in both the somatic and germ cells of certain animals,
where it appears to play an important role in the developmental cycle,
strongly urges that such facts render the hypothesis of chromosome in-
dividuality highly improbable, and that our conceptions of the r61e of
the cell organs in heredity must be greatly altered.

The hopelessly unsettled state of opinion on this question may be
illustrated by the list of authors and their views cited by Conklin (1917).
That amitosis frequently occurs in the process of normal cell differ-
entiation, and therefore constitutes evidence against the chromosome
theory, has been held by Nathansohn (1900), Wasielewski (1902, 1903),
Gurwitsch (1905), Hargitt (1904, 1911), Child (1907, 1911), Patterson
(1908), Glaser (1908), Jordan (1908), Jorgensen (1908), Maximow (1908),
Moroff (1909), Knoche (1910), Nowikoff (1910), and Foot and Strobell
(1911). Several of these investigators, together with R. Hertwig (1898),
Lang (1901), Calkins (1901), Herbst (1909), Godlewski (1909), and
Konopacki (1911), see no principal distinction between amitosis and
mitosis, believing that both may occur without interfering with normal
differentiation.

Haecker (1900), Nemec (1903), and Schiller (1909) dissented from the
above view, which was also strongly contested by Boveri (1907) and
Strasburger (1908). Richards (1909, 1911) and Harman (1913) failed to
confirm the results of Child on amitosis in cestodes, but Child (1911)
reasserted his view, which was supported by Young (1913). Schurhoff
(1919), working on Podocarpus, emphatically states that a nucleus which
has once undergone true amitosis is incapable of dividing mitotically.
Sakamura (1920) is of the same opinion.

In a careful study of maturation and cleavage in Crepidula plana
Conklin (1917) finds that the nuclei divide only by mitosis. There are
many apparent cases of amitosis, but upon careful examination they all
prove to be only various modifications of the regular mitotic process.
Such modifications are these : the scattering of the chromosomes and their
failure to unite into a single nucleus; mitosis without cytokinesis, giving
cells with two or more nuclei; the failure of certain daughter chromosomes
to pull apart, leaving a chromatic bridge between the daughter nuclei;
the persistence of the nuclear membrane, with a division of the chromo-
somes by mitosis and of the nuclear vesicle by constriction. Conklin
concludes as a result of his many observations and an examination of
the evidence offered by others, that there is not known a single conclusive
case of true amitosis in a normally differentiating cell, and that all attacks

214 INTRODUCTION TO CYTOLOGY

upon the chromosome theory on the ground of amitosis have signally
failed. The results obtained by Sakamura (1920) in his study of modi-
fied mitosis in chloralized plant cells are strikingly similar to those of
Conklin, and his conclusions regarding the chromosome theory are es-
sentially the same.

From the foregoing it is evident that the problem of the effect of
amitosis upon the differentiation of the tissues in which it occurs and
upon the hereditary powers of the nucleus is by no means easy of solution,
and that much care must be used in interpreting supposed amitotic phe-
nomena in fixed preparations. The work of Conklin and Sakamura has
shown clearly that many of the phenomena reported as amitosis are in
reality aberrations of the mitotic process, and that the opinions of many
writers are undoubtedly due to a failure to recognize this fact. Should it
be proved, however, that true amitosis may occur in the lineage of
normally functioning germ cells a serious obstacle would be placed in the
way of the chromosome theory of inheritance in its current form, for this
theory requires that, no matter what happens in cells not in the direct line
of the germ cells, nuclear division in this line must be exclusively mitotic
in order that the hereditary mechanism in the nucleus shall be preserved.
This mechanism, as we shall see in later chapters, is supposed to be of
such a nature that amitosis would seriously derange its organization.,
In each daughter nucleus of an amitotic division some of the elements
necessary for normal functional activity would presumably be lacking,
owing to the simple mass division of the chromatin. With reference to
this point it has been contended by Child that the nucleus is a dynamic
system capable of regenerating its lost parts and "producing a whole"
after amitosis. But it is a well established fact that when chromosomes
are lost in abnormal mitotic division they are not regenerated by the
daughter nuclei (non-disjunction; Chapter XVII).

In this connection an experiment performed by Chambers (1917) is
of interest. This investigator succeeded in pinching the nucleus of an
animal egg into two pieces. The two "amitotic" nuclei so produced
reunited upon touching, after which the egg was fertilized and passed
through the early cleavage stages in the normal manner. It is known
that the character of these early stages is largely independent of the
nuclei present, being the outgrowth of an organization already present
in the egg cytoplasm. (See Chapter XIV.) The later stages, in which
the effects of the hereditary constitution of the nucleus appear, were not
reached in the present experiment. Moreover, the entire chromatin
outfit was present in the reunited nucleus, which is not supposed to be
true of a daughter nucleus of an amitotic division. From this experiment,
therefore, it can only be concluded that whatever disturbance of the
spatial arrangement of the nuclear elements . may have been caused by
the temporary separation of the nucleus into two parts, it had no serious

OTHER MODES OF NUCLEAR DIVISION 215

effect on the nutritive functions performed by the nucleus during the
early cleavage stages. Development did not proceed far enough to
warrant any conclusion regarding the effect upon the role of the nucleus
in differentiation and inheritance.

Although what probably represents amitosis has been observed in
young germ cells, it has not been shown with certainty in any case that
descendants of these amitotically dividing nuclei become the nuclei of
normally functioning gametes. To gain conclusive evidence for such an
occurrence it would be necessary to trace the descendants of the amitotic-
ally dividing nuclei through to particular gametes or spores and then to
note the effect upon the individuals produced by them. This would be a
matter of extreme experimental difficulty, and not at all possible in
most organisms. If it were successfully accomplished and the individuals
were found to be normal in every respect, not only in the cleavage stages
but throughout development, the revision of the chromosome theory
which various workers have advised would at once become necessary.

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1919. The structure of protoplasm. Am. Jour. Bot. 6: 273-300.
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HERBST, C. 1909. Vererbungsstudien VI. Arch. Entw. 27: 266-308. pis. 7-10.
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OTHER MODES OF NUCLEAR DIVISION 217

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pi. 6.

1908. Myxomycetenstudien. 7. Ceratiomyxa. Ibid. 26a: 342-352.
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LANG, A. 1901. Lehrbuch der vergleichenden Anatomic. II Aufl. Jena.
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METCALF, M. M. 1915. Chromosomes in protozoa. Science 42: 658.
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218 INTRODUCTION TO CYTOLOGY

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CHAPTER XI
THE REDUCTION OF THE CHROMOSOMES

The subject of chromosome reduction is one of the most important to
be met with in the study of cytology. Many of the problems, both theo-
retical and practical, upon which biological investigators are expending
their most intense efforts seem to be bound up directly or indirectly with
the reduction of the chromosomes. The essential feature of reduction is
relatively simple in nature, and must be thoroughly grasped in order that
the discussions in the following chapters may be intelligible. The entire
process by which reduction is accomplished, on the other hand, is very
complicated and extremely difficult to observe and interpret with any
degree of confidence. In spite of the enormous amount of work already
done there still exists much difference of opinion regarding some of the
significant steps in the series of changes undergone by the nuclear material.
In the present chapter a number of these opinions will be reviewed, but
our main purpose will be to make clear the fundamental feature of chro-
mosome reduction.

We have seen that all the cells of the body in a given species are char-
acterized by the presence of a certain number of chromosomes in their
nuclei, and that this number is held constant throughout development by
an equational division of every chromosome at every somatic mitosis.
When we speak of "reduction" we ordinarily refer to the fact that at a
certain stage in the life history of the organism the number of chromosomes
is reduced one-half. This mere change in the number of chromosomes,
though very important, is not in itself the essential feature of the reducing
process, as will be seen further on. The whole number is restored at the
time of fertilization, when two nuclei, each with the reduced number,
unite. In all organisms reproducing sexually reduction and fertilization
thus represent the two most critical stages in the life cycle so far as the
chromosomes are concerned ; hence the exhaustive researches on these two
processes.

Discovery. — The discovery of reduction was made by van Beneden,
who in 1883 announced that the nuclei of the egg and spermatozoon of
Ascaris each contain one-half the number of chromosomes found in the
body cells. Although van Beneden and other early workers believed that
the change in number was brought about by the simple casting out of half
the chromosomes during the growth of the germ cells, it was soon shown
that this view was incorrect, and that "reduction is effected by a rearrange-

219

220 INTRODUCTION TO CYTOLOGY

ment and redistribution of the nuclear substance without loss of any of its
essential constituents" (Wilson 1900, p. 233).

In plants the discovery of reduction came somewhat later. Stras-
burger in 1888 showed that in angiosperms the number of chromosomes in
the egg and male nuclei is fixed by a reduction occurring in the mother-
cells of the embryo sac and pollen respectively. This was at once con-
firmed by Guignard (1889, 1891). E. Overton (1893) found that the
female gametophyte cells in the cycad, Ceratozamia, have half the number
of chromosomes found in the cells of the sporophyte. He further sug-
gested that reduction probably occurs in the sporocytes in mosses and ferns.
In the liverwort, Pallavicinia, Farmer (1894) found the gametophyte cells
to have four chromosomes and the sporophyte cells eight. That Overton's
theory of a reduction in the sporocytes of bryophytes and pteridophytes
was correct was demonstrated by Strasburger (1894), who postulated
the occurrence of a periodic reduction of the chromosomes in all organ-
isms reproducing sexually.

The Stage in the Life Cycle at which Reduction Occurs. — The reduc-
tion of the chromosomes is accomplished during the course of two nuclear
divisions which, since in animals they have to do with the maturing of the
gametes, early came to be known as the maturation divisions. Because
of its peculiar character the first of these divisions was termed the hetero-
typic by Flemming (1887), while the second, which is essentially like a
somatic division, was called the homceotypic (sometimes written homo-
typic) . Although the essential act of reduction usually occurs at the first
division, the entire process, to which the name meiosis has been applied,
is of such a nature that the second division is normally necessary for its
completion. As a result of the two divisions the "reduced " nuclei or cells
are formed in groups of four, or tetrads, though all members of a tetrad may
not function. The point in the life cycle at which these divisions take
place in various organisms will now be noted.

In animals, almost without exception, reduction occurs at gameto-
genesis (Fig. 77). In the male those cells (spermatogonia) in the testes
whose ultimate descendants are to become spermatozoa multiply by
divisions of the ordinary equational type until a certain number are
produced. These cells, now called primary spermatocytes, enlarge a little
and quickly undergo two successive divisions: the first division in each
is heterotypic and results in two cells called secondary spermatocytes; the
second is homceotypic and divides the two secondary spermatocytes into
four spermatids, each of which becomes transformed into a spermatozoon.
The four spermatozoa are therefore the immediate result of the two
maturation divisions. In the female the situation is somewhat different:
here nearly all of the differentiation of the gamete is accomplished before
the nuclear divisions bringing about reduction actually occur. The
primary oocytes (ovocytes) are the descendants of a number of generations

THE REDUCTION OF THE CHROMOSOMES

221

of oogonia (ovogonia). The oocyte, usually while its nucleus is in the
prophases of the first maturation division, enlarges greatly ("growth
period"), becomes filled with stored food, and develops the general fea-
tures characterizing the egg. The oocyte is now called the "ovarian egg,"
and it actually is an egg in all respects save one of much importance : its
nucleus still has the full number of chromosomes. At a comparatively
late stage, in many cases even after the spermatozoon has entered the egg
at fertilization, the oocyte nucleus (germinal vesicle), having passed
through some of the prophasic changes characteristic of the heterotypic

ANIMAL

FIG.

77. — Diagram showing the history of the chromosomes in the ordinary
life cycles of animals and plants.

mitosis before and during the growth period, gives rise to a mitotic figure
which is often surprisingly small for the volume of the nucleus. The
spindle takes up a position perpendicular to the surface of the cell, and at
telophase the chromosomes passing to the outer pole are included in the
first polar body, a small cell budded off at this point. (See Fig. 106.) A
second spindle is rapidly formed about the chromosomes remaining in
the egg (called at this stage the secondary oocyte) and the second matura-
tion mitosis occurs, one daughter nucleus being included in the second
polar body. In the course of these two divisions chromosome reduction
is accomplished. The first polar body may divide to form two, thus
completing the tetrad of cells corresponding to the tetrad of spermatozoa
in the male. Although the polar bodies are normally functionless they are

222 INTRODUCTION TO CYTOLOGY

generally looked upon as eggs historically: the maturation divisions
probably resulted formerly in a tetrad of eggs, whereas now only one
relatively large and highly differentiated egg is produced at the expense
of the other three cells, which remain small and functionless.

Among the protozoa (see Minchin 1912) it has been found that in
those forms which appear to have their chromatin aggregated into no
definite number of chromosomes, there often occur two successive nuclear
divisions suggestive in certain respects of maturation divisions, a part of
the products then degenerating. Some have regarded this as a "casting
out of effete vegetative chromatin," an interpretation which was at one
time placed upon the maturation process generally. In many cases this
"reduction" of the chromatin occurs immediately prior to syngamy
(sexual union),1 and so agrees with reduction in higher forms in taking
place at gametogenesis ; but in other cases it immediately follows syngamy,
as in certain alga3 mentioned below. Other protozoa have been shown to
have a definite chromosome number which is regularly reduced in a man-
ner essentially comparable to that in the metazoa.

In plants it is among the members of the lower groups (thallophytes)
that a striking diversity is shown in the stage of the life cycle at which
reduction takes place : in the groups above the thallophytes it is regularly
accomplished at sporogenesis. In the myxomycete, Ceratiomyxa, it
has been shown by Olive (1907) and Jahn (1908) that spore formation is
accompanied by a chromosome reduction. In the green alga3 it is in the
first two divisions of the zygote (either a zygospore or a fertilized egg)
that reduction occurs: this has been definitely established in Spirogyra
(Karsten 1908; Trondle 1911), Zygnema (Kurssanow 1911), Coleochcete
(Allen 1905c), and Chara (Oelkers 1916). In a number of other forms,
such as Uloihrix, (Edogonium, Sphceroplea, and Closterium, in which the
chromosomes are not well known, it is probable that the same condition
holds, since the zygote upon germination gives rise with considerable
regularity to four cells; in some cases ((Edogonium) these four cells are
zoospores.

In the BROWN ALG.E Cutleria (Yamanouchi 1912), Zanardinia (Yama-
nouchi), and Ectocarpus (Kylin 1918a) reduction occurs in connection
with zoospore formation. In Fucus, however, an exceptional condition
is found: here reduction takes place in the antheridium and oogonium
initials, in the first two divisions following the one delimiting the stalk
cell (Fig. 78, B). Since there are only three divisions in the oogonium,
which thus produces eight eggs, the eggs are but one division removed
from the four products of the maturation mitoses, a condition closely
approaching that in animals. That reduction in Fucus is associated
with gametogenesis was inferred by Strasburger (1897) and Farmer and
Williams (1898) and demonstrated by Yamanouchi (1909).

1 See the cases of Actinophrys sot and Amoeba albida, Chapter XII.

THE REDUCTION OF THE CHROMOSOMES

223

In the RED ALG^E reduction occurs in the two divisions differentiating
the nuclei of the tetraspores when the latter are present in the life history.
Such is the case in Polysiphonia (Yamanouchi 1906) (Fig. 78, A), Gri-
ffithsia (Lewis 1909), and Corallina (Yamanouchi). The brown algae
Dictyota (Williams 1904) and Padina (Wolfe 1918) also conform to this
scheme. In Nemalion, which has no tetraspores, it was long supposed
(Wolfe 1904) that reduction occurs in connection with carpospore forma-
tion, .but Cleland (1919) has recently shown that
it takes place at the time the zygote germinates, as
in so many green algse.1

In the ASCOMYCETES reduction occurs in the
course of the first two of the three mitoses initiated
by the primary ascus nucleus (Figs. 22, 61) and re-
sulting in the eight ascospore nuclei. It was for a
long time generally thought that there were two
nuclear fusions in the life history — one in the archicarp
and one in the ascus (see p. 290), and the three di-
visions in the ascus were accordingly regarded as a
process whose function was to reduce the "quadri-
valent" chromosomes to the univalent condition
(Harper 1905; Overton 1906). Such a double reduc-
tion was described by Miss Fraser (1907, 1908) for
Humaria rutilans: the first mitosis she found to be
heterotypic, the second homceotypic, and the third
"brachymeiotic," the last bringing about a further
reduction by the separation of the chromosomes
into two smaller groups. This was also reported
for Otidea aurantia and Peziza vesiculosa (Fraser and
Welsford 1908), Lachnea stercorea, Ascobolus furfura-
ceus, and Humaria granulata (Fraser and Brooks 1909),
and Helvella crispa (Carruthers 1911). Harper (1900,
1905), although he thought two fusions occurred, found
no double reduction, holding rather that the fusion of the two ascus
nuclei and their chromsomes is so complete as to render the quadrivalent
character of the latter entirely invisible. Other investigators also find
no double reduction in the ascus. They show rather that the first two
mitoses correspond to the heterotypic and homosotypic mitoses of other
organisms, and that the third division is purely vegetative or equational
in character. As instances may be cited the work of Faull (1905, 1912)

1 For a review of sexual reproduction and alternation of generations in the algae
see Bonnet (1914). Davis (1916) gives a convenient summary of the life histories
of the red algse. Dodge (1914) summarizes and compares the life histories of red
algae and ascomycetes. See Atkinson (1915) for a complete review of researches on
ascomycetes. For the cytology of the yeasts see Guilliermond 1920.

B

FIG. 78.

A, prophase of
heterotypic division
in the tetrasporocyte
of Polysiphonia.
(After Yamanouchi,
1906.) B

in oogonium of Fucus.
(After Yamanouchi,
1909.)

224

INTRODUCTION TO CYTOLOGY

on Hydnobolites, Neotiella, and Laboulbenia, and that of Claussen (1912)
on Pyronema. Furthermore, it is becoming increasingly apparent (see p.
291) that there is but one fusion in the life cycle — that in the ascus, so
that the necessity for a second reduction is removed.

In the BASIDIOMYCETES it has been shown by the researches of Juel
(1898), Maire (1905), Guilliermond (1910), Kniep (1911, 1913), Levine
(1913), and others on the hymenomycetes, and by those of V. H. Black-
man (1904), Dietel (1911), Fitzpatrick (1918), and others on the rusts,1
that reduction occurs in the two mitoses giving rise to the four basidio-

6 I?

FIG. 79. — Sexual fusion and maturation divisions in the basidium of

Nidularia pisiformis.

a, two sexual nuclei about to unite. 6, prophase of heterotypic division in fusion
nucleus, c, heterotypic mitosis, d, homceotypic mitosis, e, the four basidiospore
nuclei. X 1800. (After Fries, 1911.)

spore nuclei (Fig. 79). As in the ascomycetes, it thus follows immediately
upon the nuclear fusion: in the basidium in hymenomycetes and in the
teleutospore in rusts. An exception is reported in the case of Hygro-
phorus conicus, in which Fries (1911) finds in the basidium neither a
nuclear fusion nor a reduction.

In the BRYOPHYTES reduction, so far as known, is universally brought
about by the two mitoses which differentiate the four nuclei of each spore
tetrad. It was at one time reported (van Leeuwen-Reijnvaan 1907)
that in Polytrichum there is a second reduction at spermatogenesis and
oogenesis: the sporophyte was said to have 12 chromosomes, the spore
and gametophyte six, and the gametes three. This double reduction
was thought to be compensated for by the fusion of the ventral canal
cell with the egg, raising the number in the latter to six, in combination
with the entrance of two sperms into the egg at fertilization, making the
sporophytic number 12. This interpretation has been shown to be
false by both Vandendries (1913) and Walker (1913), who find the life
cycle normal in every respect: a reduction from 12 to 6 occurs at sporo-
genesis but no second reduction follows at gametogenesis.

1 A summary of researches on rusts is given by Maire (1911).
nuclei in the cells of basidiomycetes is given by Levine (1913).

A list of numbers of

THE REDUCTION OF THE CHROMOSOMES

225

In VASCULAR PLANTS reduction in all normal life cycles in both homo-
sporous and heterosporous forms occurs uniformly in the divisions differ-
entiating the spore tetrads (Fig. 77). The sporocytes, particularly the
microsporocytes ("pollen mother-cells"), of the higher plants have long
been favorite objects for the study of reduction. Since the gametophyte
generation in the higher plants is so abbreviated, reduction closely pre-
cedes fertilization in these forms. In the ordinary angiosperm embryo
sac in which the eight nuclei are derived from a single megaspore of the
tetrad, the egg nucleus is removed from the product of reduction (mega-
spore nucleus) by only three mitoses. In some cases, of which Lilium
is the best known example, walls fail to form be-
tween the four megaspore nuclei (Fig. 80, B],
leaving them in a common cavity (embryo sac)
where they undergo but one further division to
produce the eight nuclei of the female gameto-
phyte. The egg here is consequently removed
from the product of reduction by a single
mitosis. In one known case, Plumbagella
(Dahlgren 1915), the four reduced nuclei,
formed as in Lilium, divide no further, one of
them functioning directly as the egg nucleus.
Here, therefore, the condition characteristic of
animals has been reached : the gamete nucleus is
itself the direct product of reduction, and the
haploid generation usually produced by the
spore is eliminated. The male gametophyte
also has undergone much abbreviation in higher

plants, but the male nucleus is still removed from the reduction product
(microspore nucleus) by two mitoses. In no known case does the micro-
spore nucleus function directly as a gamete nucleus.

The term gonotokont was introduced by Lotsy (1904) to designate
any cell, whatever its origin or position in the life cycle, in which the
reduction process is initiated. In animals the gonotokonts are therefore
the primary spermatocyte and the primary oocyte. In most green algs&
the gonotokont is the zygote; in the red algae it is usually the tetrasporo-
cyte; in the ascomycetes it is the ascus; in the basidiomycetes it is the
basidium ; and in the bryophytes and vascular plants it is the sporocyte —
the microsporocyte and megasporocyte in the case of heterosporous forms.

The Meaning of Reduction. — In order that the true meaning of reduc-
tion may be appreciated it will be necessary to indicate the main points
of a theory first suggested by Roux (1883) and later developed particu-
larly by Weismann (1887, 1891, 1892). It had been believed by the earlier
workers that reduction was merely a process whose function was "to
prevent a summation through fertilization of the nuclear mass and of

15

A

FIG. 80. — Megaspore

tetrads in Angiosperms.

A, tetrad of walled cells

in Physostegia virginiana;

formation of two upper

ones just being completed.

X462. (After Sharp, 1911.)

B, tetrad of megaspore

nuclei in Lilium canadense.

226 INTRODUCTION TO CYTOLOGY

the chromatic elements" (Hertwig 1890). But the chromatic mass is
actually quartered at reduction, whereas the number of chromosomes is
halved. Moreover, great changes in nuclear volume occur with no
change in the number of chromosomes. This careful guarding, so to
speak, of the chromosome number was siezed upon as a most significant
fact by Roux, who "argued that the facts of mitosis are only explicable
under the assumption that the chromatin is not a homogeneous substance,
but differs qualitatively in different 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 dis-
tributed 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."1 Upon this conception
Weismann based his remarkable theory, the starting point of which was
"the hypothesis of De Vries that the chromatin is a congeries or colony of
invisible self-propagating vital units or biophores, somewhat like Darwin's
'gemmules,' 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 'ids,' each of which ... is assumed to possess the
complete architecture of the germ-plasm characteristic of the species.
The 'ids' finally, which are identified with the visible chromatin-granules,
are arranged in linear series to form 'idants' or chromosomes. It is
assumed further that the 'ids' differ slightly in a manner corresponding
with the individual variations of the species, each chromosome therefore
being a particular group of slightly different germ-plasms and differing
qualitatively from all the others.

"We come now to the essence of Weismann's interpretation. The
end of fertilization is to produce new combinations of variations by the
mixture of different ids. Since, however, their number, like that of the
chromosomes which they form, is doubled by the union of two germ-
nuclei, an infinite complexity of the chromatin would soon arise did not
a periodic reduction occur. Assuming, then, that the 'ancestral germ-
plasms' (ids) are arranged in a linear series in the spireme thread or the
chromosomes derived from it, Weismann ventured the prediction (1887)
that two kinds of mitosis would be found to occur. The first of these is
characterized by a longitudinal splitting of the thread, as in ordinary
cell-division, 'by means of which all the ancestral germ-plasms are
equally distributed in each of the daughter-nuclei after having been
divided into halves.' This form of division, which he called equal
division (Aequationstheilung), was then a known fact. The second
'This and the following quotations are from Wilson (1900, pp. 245-246).

THE REDUCTION OF THE CHROMOSOMES 227

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 (Reduktionstheilung), and suggested
that this might be effected either by a transverse division of the chromo-
somes, or by the elimination 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."

Reduction in Weismann's sense, then, is a reduction of the number of
kinds of germ-plasm or ancestral hereditary qualities present, this
reduction being brought about by means of a redistribution, half of the
qualities to one daughter nucleus and the remainder to the other daughter
nucleus. The change in the number of chromosomes is a consequence of
the manner in which this redistribution is accomplished, as we shall see.

Interpretations Based on Weismann's Theory. — As would be ex-
pected, there were announced certain interpretations of chromosome be-
havior based on Weismann's idea. Several cytologists thought that
they found the chromsomes actually dividing transversely at one or the
other of the two maturation mitoses. This interpretation, however,
proved to be incorrect. Much light was thrown on the problem when
Henking (1891), Ruckert (1891, etc.), Haecker (1890-9), vom Rath
(1892-3), and others showed that the double chromosomes appearing in
the reduced number on the spindle at the first maturation mitosis are not
split chromosomes like those seen in somatic divisions, but are pairs of
chromosomes, or bivalent chromosomes, each arising by an end-to-end
conjugation (synapsis) of two somatic chromosomes. The two parts of
each bivalent then separate at the first or second maturation division,
the entire chromosomes thus being segregated into two groups, each
with the reduced number. Thus it appeared unnecessary that a single
chromosome, representing a linear series of different qualities, should be
transversely divided in order for Weismannian reduction to occur: it
was only necessary to assume that the whole chromosomes differ qualita-
tively from one another, so that when the two members of a bivalent
pair separate there would be a segregation of different qualities. It is in
the light of this bivalent chromosome conception that we are to interpret
the many early reports of a transverse division of the chromosome during
maturation. What was called a transverse division was merely the
separation of two entire chromsomes placed end-to-end.

A number of workers soon found that in many cases there is nothing
even simulating a transverse division, either of single chromosomes or of
bivalent pairs, but that both maturation divisions are apparently longi-
tudinal (Flemming, Brauer 1893, Moore 1896, Meves 1896, Gregoire,
etc.). How, then, is there any Weismannian reduction if there is neither

228

INTRODUCTION TO CYTOLOGY

a transverse division of the chromosome nor a transverse separation of
bivalents? Montgomery (1901), von Winiwarter (1900), Sutton (1902),
Boveri (1904), and a number of others showed that here the chromo-
somes conjugate side-by-side rather than end-to-end. Thus when they
separate there is an appearance of a longitudinal division, but reduction
is nevertheless accomplished, since entire somatic chromosomes suppos-
edly qualitatively different, and not the longitudinal halves of split
chromosomes, are separating. The appearance of a longitudinal division
may also be present after an end-to-end conjugation, for the two members
may bend around to a side-by-side position before finally separating.

FIG. 81. — Diagram showing essential difference between somatic and heterotypic

mitoses.

As a matter of fact, the maturation divisions in nearly all cases, especially
those studied by botanists, are both longitudinal in appearance. End-
to-end conjugation later came to be called telosynapsis or metasyndese,
and side-by-side conjugation parasynapsis or parasyndese.

Somatic and Heterotypic Mitoses Compared. — Before taking up a
more detailed account of the process of reduction as it has been described
by various investigators it is of the utmost importance to fix clearly in
mind the essential difference between somatic and heterotypic mitosis
in order to realize what constitutes the cardinal feature of reduction, and
thereby to detect the significant points of the various theories. This
essential difference is illustrated in Figs. 81 and 82. In a somatic or
vegetative mitosis every chromosome is split into two exactly similar longi-
tudinal halves which are distributed to the two daughter nuclei. The daughter
nuclei are therefore like each other and like the mother nucleus in the quality

THE REDUCTION OF THE CHROMOSOMES

229

of their substance. In the heterotypic mitosis, the first of the two matura-
tion mitoses, the chromosomes conjugate two by two during the prophase to
form the reduced number of bivalent chromosomes, which take their place
on the spindle. The members of each pair, which are supposed to differ
qualitatively from each other, separate and pass to the two daughter nuclei.
These nuclei are therefore qualitatively unlike each other, having different
members of the full chromosome group; and also unlike the mother nucleus,
since each of them has only half as many chromosomes as the latter. The
second maturation mitosis (not shown in the diagrams) is essentially a
vegetative mitosis in most cases: each chromosome splits longitudinally

SOMATIC MITOSIS

FIG. 82. — Diagram showing essential difference between somatic and hetero-
typic mitoses.

and the halves are distributed to the daughter nuclei. The four nuclei,
and consequently the four cells, resulting from the two maturation divi-
sions are therefore of two kinds : two of them have half of the chromosomes
of the original nucleus and the other two have the remaining ones.

Assuming, then, that the chromatin of the nucleus represents the
principal physical basis of inheritance (see Chapter XIV), reduction is
essentially this: a reduction in the number of kinds of hereditary units by
the separation and distribution of qualitatively different masses of chromatin
to different cells and eventually into different hereditary lines, rather than an
equational division and distribution of all the qualities as in somatic mitosis.
As has already been stated, the change in the number of chromosomes
("numerical reduction") is a consequence of the method by which this
qualitative reduction is brought about, this method being the distribution
of entire chromosomes, each representing one or more particular qualities,
to different cells.

230 INTRODUCTION TO CYTOLOGY

Another important feature of the reduction process should be noted
before proceeding further. In many cases, chiefly among animals, the
chromosomes appearing on the spindle of the first maturation mitosis are
not merely double, but quadruple. This is due to the fact that each of the
conjugating chromosomes is already longitudinally split, giving the
bivalent chromosome the form of a chromosome tetrad (not to be confused
with tetrads of cells or nuclei) . The four constituents of the chromosome
tetrad are known as chromatids, and are distributed by the two matura-
tion mitoses to the four resulting cells. It is thus seen that in the case of
chromosome tetrads the lines of separation for both maturation mitoses
are marked out in the prophase of the first. It should be borne in mind
that one of these lines represents a plane of chromosome conjugation,
and the other a plane of true longitudinal splitting. When the chroma-
tids separate along the conjugation plane reduction occurs, whether this
be at the first or second mitosis, and when separation along the plane of
splitting occurs the mitosis is equational, as in somatic division. .
MODES OF CHROMOSOME REDUCTION

In all cytology there is scarcely a subject upon which there has been
entertained so great a variety of opinion as upon the question of the
exact behavior of the chromosomes during the meiotic phases. Entirely
aside from the theoretical interpretations placed upon the process of
maturation, cytologists have yet failed to arrive at any universally
accepted conclusion regarding all the structural changes which occur.
This diversity of opinion is due in part to the complexity of the process
and the difficulty of interpreting its various stages, some of which fail
to stand out clearly in preparations made by our available methods.
On the other hand, a great variety of organisms have been studied, and
these undoubtedly differ considerably in the details of the reduction
process, so that agreement in all particulars is not to be expected. The
attempt has too often been made to apply universally an interpretation
founded upon a study of one or two organisms. Certain essential fea-
tures of meiosis may be expected to show close agreement in all organisms
reproducing sexually, as Strasburger pointed out, but it is evident that
there is no full correspondence as regards the exact manner in which the
essential changes are accomplished. In the following pages are given
brief descriptions of a few representative interpretations advanced by
various cytologists.1

The two interpretations of reduction which have been most conspicu-
ous in the literature of recent years are diagrammed in Figs. 83 and 89.

1 No attempt can be made in a work of this scope to give a complete summary
and classification of all the interpretations that have been put upon the maturation
phenomena. Only enough will be presented to afford a starting point for a study of
this complex subject. For a review and criticism of all views expressed up to 1910
see Gr6goire's two invaluable works (1905, 1910). A useful list of works on somatic
and heterotypic mitosis in angiosperms is given by Picard (1913).

THE REDUCTION OF THE CHROMOSOMES

231

Nearly all of the accounts of reduction now appearing, especially those
given by botanists, conform in general to one or the other of these two
schemes, though they vary greatly in detail. Both theories have been
upheld by competent observers, and it may be possible that both modes
of reduction actually occur; but the same objects have been so differently
described by the two opposing schools that it seems very probable that
interpretation is chiefly responsible for the persistent diversity of opinion.
For convenience the two theories will be referred to as Scheme A and
Scheme B.

UL7TONEMA

STfltPSINLMA

FIG. 83. — The method of chromosome reduction according to Scheme A.
Explanation in text.

Scheme A. — The first of the two main interpretations of reduction
came into prominence in 1900 and shortly after, when von Winiwarter
(1900), Gregoire (1904, 1907, 1909), A. and K. E. Schreiner (1904-1908),
and Berghs (1904, 1905) applied it to the phenomena observed by them
in several animals and plants. Its essential points are as follows (Figs.
83-88) :

At the beginning of the heterotypic prophase the nuclear reticulum,
without breaking down into such distinct elementary nets or alveolar
units as are seen in the somatic prophase, takes the form of long slender
threads (leptotene or leptonema stage).1 During the very early prophase

1 The terms leplotene, synaptene, pachylene, and diplotene were proposed by von
Winiwarter (1900); 'leptonema, zygotene, pachynema, and strepsinema by Gre'goire
(1907); amphitene by Janssens (1905); strepsitene by Dixon (1900); diakinesis by
Haecker (1897); synapsis by Moore (1896); synizesis by McClung (1905); and meiosis
by Farmer and Moore (1905). The terms ending in -tene are ordinarily used as
adjectives.

232

INTRODUCTION TO CYTOLOGY

these threads conjugate in pairs side-by-side (parasynaptically ; para-
syndetically) . The association does not take place at all regions of the
threads at once: it begins at one or two points, commonly at one end,

FIG. 84. — Reduction in sporocyte of Nephrodium.

A, leptonema. B, recovery from synizesis; parallel conjugation consummated during
synizesis. C, pachynema. D, second contraction of bivalent spireme. E, diakinesis.
F, anaphase of heterotypic mitosis. G, interkinesis. H, homoaotypic mitosis. /, two
of the four spore cells. (After Yamanouchi, 1908.)

and gradually involves all portions, so that at stages when the process is
yet incomplete the two threads may be closely paired at some points and
widely divergent at others, giving an appearance very unlike that of the
halves of a longitudinally split chromosome in a somatic cell. This is

THE REDUCTION OF THE CHROMOSOMES

233

known as the zygotene, zygonema, synaptene, or amphitene stage. Usually
before the union is complete the nucleus enlarges somewhat and the
threads contract, forming a tight knot at one side of the nucleus. This
stage, formerly called synapsis (see p. 255), is now more properly known as
synizesis. The pairing threads come into very close association during
synizesis, which, though variable in time, usually ensues at about this
stage. When the closely paired threads recover from the synizesis con-
traction they extend more uniformly throughout the nucleus ("open
spireme"), and are now seen to be much thicker t(pachytene; pachynema).
In many cases they may again contract into a
loose knot with loops extending from it ("second
contraction"). As they continue to decrease in
length and increase in diameter the members of
each pair twist more or less tightly about each
other for a short time (strepsitene; strepsinema;
diplotene). Eventually they become very short
and thick, and the various pairs (gemini;
bivalent chromosomes), present in the haploid
or reduced number, lie scattered throughout
the nucleus (diakinesis) . The two components
of each geminus may now separate slightly at
one or both ends or at the middle, which gives
them the form of Ys, Vs, Xs, and Os. The
bivalent chromosomes are now fully formed and
ready to take their places on the spindle, which
soon forms.

In the case of the animal egg the "growth
period" introduces a complication. In the
sporocytes of plants and the sperm atocytes of
animals there is some enlargement of the cell
and nucleus during the stages just described,
but the chromosomes pass directly from the
strepsinema stage to diakinesis. During the
relatively enormous growth of the oocyte on the other hand,
the chromosomes, which have usually reached the strepsinema stage
when the enlargement begins, become greatly modified in form.
Their achromatic framework takes the form of fine threads extending
out in all directions, giving the chromosome an irregular brush-
like form (Fig. 86, C, D), while the chromatic substance either may
flow into the nucleolus, leaving the chromosome framework uncolored and
very difficult to observe, or by loss of its staining capacity through chem-
ical change it may disappear from view completely. As the growth
period comes to an end, however, the original staining capacity returns
and the chromosomes again assume the compact form and pass into the
diakinesis stage.

FIG. 85. — Parasynapsis in
Phrynotettix magnus.
A, leptonema; x, sex-
chromosome. B, conjuga-
tion of "chromosome A."
Portions of other uncon-
jugated threads and one
other bivalent also present
in section. X 2000.
(After Wenrich, 1916.)

234

INTRODUCTION TO CYTOLOGY

In the case of most animals, and apparently in certain plants also,
the split which is to function in the homceotypic mitosis may develop dur-
ing diakinesis or even much earlier, the result being the formation of
chromosome tetrads. This introduces another element of complication
which will be touched upon later (p. 243).

B

FIG. 86.

A, parasynapsis in Allium fistula sum. B,pa,rsisynapsismOsmundaregalis. C, nucleus
of oocyte of Scyllium canicula (Selachian) in "growth stage." D, single chromosome in
growth stage, showing the fine subdivision of its substance. (A and B after Gregoire, 1907,
C and D after Marechal 1907.)

The diakinesis stage is terminated by the dissolution of the nuclear
membrane and the formation of the spindle, upon which the bivalent
chromosomes, whether secondarily split or not, now become arranged.
Because of the peculiar form and consistency of the heterotype chromo-
somes the mitotic figure presents a striking contrast in appearance to the
ordinary figure of somatic cells. This is especially true as the chromo-
somes are drawn into various curious shapes as their anaphasic separation
begins. The two univalent components of each bivalent chromosome
eventually become free from each other and pass to the two daughter
nuclei, bringing about reduction. During the anaphase the separating

THE REDUCTION OF THE CHROMOSOMES

235

FIG. 87. — Nuclei from microsporocytes of Vicia faba, showing parasynapsis.
Synigesis beginning in No. 6. X 1900.

Fia. 88. — Heterotypic prophases in spermatocyte of Tomopteris onisciformis.
A, pairing of leptotene threads beginning. B, pairing complete in some threads and
only beginning in others. C, conjugation complete; pachynema stage. D, resplittirig
of pachytene threads (separation of conjugated chromosomes.) (After A. and K. E.
Schreiner, 1905.)

236

INTRODUCTION TO CYTOLOGY

univalents, if not already double, rapidly develop a longitudinal split, in
some cases even before they are entirely free from each other. The
resulting halves tend to open out along this split; chromosomes being
drawn endwise to the poles thus take the form of simple Vs, while those
to which the fibers are attached at the middle appear as double Vs. After
reaching the poles the split chromosomes begin the reconstruction of the
daughter nuclei. As a rule this does not proceed very far, since the
horn ceoty pic mitosis follows very quickly upon the heterotypic. Well
organized daughter nuclei are often formed, whereas in the animal egg
there may be no reconstruction whatever, the daughter chromosomes of
the first mitosis at once taking their places on a newly formed spindle for
the second mitosis.

In the homceotypic mitosis the chromosomes, if there has been an inter-
vening interkinesis of any length, usually appear much longer and thin-
ner than in the heterotypic mitosis, and separate along the longitudinal
line of fission seen in the preceding anaphase. The homceotypic mitosis
is therefore equational in character, and differs from an ordinary somatic
mitosis only in the number of its chromosomes and the precocity of their
splitting. In each of the four nuclei resulting from the two maturation
mitoses there is now the haploid number of univalent chromosomes, and
meiosis is complete.

The foregoing interpretation of reduction has been widely accepted
from the first by both botanists and zoologists. The following is a partial
list of works in which it has been described.

PLANTS

Gr6goire 1904, '07 Lilium, Allium, Osmunda

Berghs 1904, '05 Allium, Drosera, Hetteborus, etc.

Rosenberg 1905, '07, '08, '09 Drosera, Composite

Allen 19056c Lilium, Coleochcete

J. B. Overton 1905, '09 Thalictrum, Calycanthus, Richardia

Strasburger 1905, '07, '08, '09 Lilium, Galtonia, etc.

Miyake 1905 Lilium, Funkia, Iris, Allium, Trades-

cantia, Galtonia

Tischler 1906 Ribes

Cardiff 1906 Acer, Salomonia, Botrychium, Ginkgo

Lagerberg 1906, '09 Adoxa

Yamanouchi 1906, '08, '10 Polysiphonia, Nephrodium, Osmunda

Martins Mano 1909 Funkia

Lundegardh 1909, '14 Trollius

Frisendahl 1912 Myricaria

McAllister 1913 Smilacina

Schneider 1913, '14 Thelygonium

Weinzieher 1914 Xyris

Sakamura 1914 Vida

de Litardiere 1917 Poly-podium

237

ANIMALS

von Winiwarter
Marechal

1900

1904, '05, '07

A. and K. E. Schreiner 1906, '07, '08

Lerat 1905

Deton 1908

Gregoire 1909

Janssens 1905, '09
Janssens et Willems 1909

Schleip 1906, '07

Debaisieux 1909

Montgomery 1911

Kornhauser 1914, '15

Wenrich 1916, '17

Fasten 1914, '18

Malone 1918

Pratt and Long 1917

Robertson 1916

Rabbit, Man

Tunicates, Selachians, Teleosts, Amphi-

oxus
Tomopleris, Ophryotrocha, Zoogonus,

Enteroxenos, Myxine, Salamandra,

Spinax
Cyclops
Thysanozoon
Zoogontis
Batracoseps
Alytes
Planaria
Dytiscus

Hersilia, Enchenopa

Phrynotettix, Chorthippus

Cambarus, Cancer

Cants

Mus

Insects

"^

FIG. 89. — The method of chromosome reduction according to Scheme B.
Explanation in text.

Scheme B. — The second of the two conspicuous interpretations was
advanced by Farmer and Moore (1903, 1905), and is essentially as
follows (Figs. 89-92) : In the early heterotypic prophase the reticulum
becomes more thready in structure and contracts into a tight knot
(synizesis). When this knot loosens up the chromatic material has

238

INTRODUCTION TO CYTOLOGY

assumed the form of a continuous spireme which is double. This double-
ness is believed to represent a true longitudinal split, and although it
usually disappears from view during the later prophases it is thought to

A B c

FIG. 90. — The heterotypic prophases in Lilium, according to Mottier (1907.)
A, synizesis knot loosening up; threads splitting; note chromomeres. B, hollow spireme.
C, second contraction. D, diakinesis. X 900.

FIG. 91.— Maturation mitoses in microsporocyte of Vicia faba.
A, anaphase of heterotypic mitosis; split for second mitosis evident in separating
daughter chromosomes. B, one daughter nucleus in early telophase of heterotypic mitosis.
C, later telophase. D, metaphase of homoeotypic mitosis. E, anaphase of same, showing
portions of both spindles. F, three of the four microspore nuclei. X 1335. (After
Fraser, 1914.)

persist and reappear at a much later stage. After extending loosely
throughout the nucleus ("open spireme"), the double spireme, now con-
siderably thickened and twisted (strepsinema) , contracts again and is

THE REDUCTION OF THE CHROMOSOMES 239

thrown into loops (" second contraction"). These loops then break
apart from one another through a segmentation of the spireme; each of
them is composed of two split chromosomes arranged end-to-end. Chro-
mosome conjugation has thus occurred telosynaptically (metasyndetic-
ally] either while the spireme was being formed or when the daughter
spiremes were formed in the preceding telophase. The two members of
each pair are brought around to a side-by-side position by the looping
at the second contraction, usually but not always remaining closely
connected at the original point of conjugation. The resulting bivalent
chromosomes, with their split obscured, become much shortened and
thickened (diakinesis) and take up their positions on the first maturation
spindle. In case the original split, instead of being wholly obscured, is
visible at this time or earlier, chromosome tetrads are evident. In the
heterotypic anaphase the bivalents are separated into their component
univalents, bringing about reduction. During the anaphase the uni-
valents often widen out along the line of fission which had been tempo-
rarily obscured, giving them the form of simple or double Vs as described
for Scheme A. They remain through interkinesis in the double condi-
tion, and in the homoeotypic mitosis separate along this line of fission.

The following is a list of the principal works in which this theory of
eduction has been advocated.

PLANTS

Farmer and Moore 1903, '05 Lilium, Osmunda, Psilotum, Aneura

Farmer and Digby 1910 Gallonia

Farmer and Shove 1905 Tradescantia

Mottier 1907, '09, '14 Lilium, Acer, Allium, Podophyllum,

Tradescantia, Staphylea

Gregory 1904 Ferns

Lewis 1908 Finns, Thuja

Schaffner 1906, '09 Agave

Digby 1910, '12, '14, '19 Galtonia, Primula, Crepis, Osmunda

Fraser 1914 Vicia

Lawson 1912 Smilacina

McAvoy 1912 Fuchsia

Beer 1912, '13 Equisetum, Crepis, Tragopogoh

Woolery 1915 Smilacina

Nothnagel 1916 Allium

ANIMALS

Farmer and Moore 1905 Periplanela, Elasmobranchs

Montgomery 1903, '04, '05, '06, Hemiptera, Amphibia

'10

Moore and Embleton 1906 Amphibia

Griggs 1906 A scans

Zweiger 1907 Forficula

H. S. Davis 1908 Insects

Nakahara 1920 Perla

240

INTRODUCTION TO CYTOLOGY

Some of the above named investigators, notably Miss Digby (1910,
1912, 1914, 1919), Miss Fraser (1914), and Miss Nothnagel (1916),
have laid emphasis upon the view that the split seen in the early hetero-
typic prophase has its origin in the telophase of the last premeiotic divi-
sion, each chromosome persisting through the intervening resting stage
in the double condition. It is consequently held, as fully stated by Miss
Digby (1919) in her account of the archesporial and meiotic phases of
Osmunda (see Fig. 92), that the lateral pairing of thin threads in the

LAST PRtHtlOTlC

IC MITOSIS

FIG. 92. — Diagram showing behavior of chromosomes in premeiotic and

meiotic phases in Osmunda, according to Digby (1919).

a, split which originates in telophase of premeiotic mitosis, persists (though obscured at
times) through heterotypic prophases, reappears in heterotypic anaphase, and becomes
effective in homceotypic mitosis, b, split which originates in heterotypic telophase,
persists obscured through homceotypic prophases, reappears in homceotypic anaphase,
and becomes effective in post-homceotypic division, x, plane of conjugation.

heterotypic prophase which the advocates of Scheme A have regarded
as a conjugation of entire chromosomes is in reality only the reassocia-
tion of the two halves of one chromosome which had been split in the
preceding telophase. Such a reassociation is thought to occur in every
prophase, somatic and heterotypic, since these workers regard chromo-
some splitting as regularly a thlophasic phenomenon. The split which
forms in the last premeiotic telophase functions in the homceotypic
mitosis : the homceotypic division is therefore looked upon as the continua-
tion of the premeiotic division, the heterotypic mitosis being an inter-
polated process bringing about numerical reduction. Not only does this
premeiotic split reappear in the anaphase of the heterotypic mitosis to
function in the homceotypic, but a new split develops in the heterotypic

THE REDUCTION OF THE CHROMOSOMES

241

telophase, and after being temporarily obscured functions in the post-
homceotypic division.1

A variation of Scheme B has been observed in (Enothera (Gates 1908,
1909, 1911; Geerts 1908; B. M. Davis 1909, 1910, 1911); in Fucus and
Cutleria (Yamanouchi 1909, 1912) ; in Bufo (King 1907) ; and in a few
other forms. Here the spireme in the heterotypic prophase does not
become double, the split for the second division appearing first in the
heterotypic anaphase.

Comparison of Schemes A and B. — According to both of the fore-
going prominent theories of reduction the conjugated chromosomes
separate at the first maturation mitosis, thus causing reduction, and

v@o

v

A (!S
V (il*.

FIG. 93. — -Diagram showing distinction between Schemes A and B. See text.

divide longitudinally (equationally) at the second mitosis, so that the
final result is essentially the same: two of the resulting four nuclei differ
qualitatively from the other two in their chromatin content (Fig. 93).
The distinction between the two interpretations is nevertheless an im-
portant one, and may be emphasized in the following summary.

According to Scheme A the double character of the chromatin spiremes
of the early heterotypic prophase is due to a lateral pairing of simple
threads each representing an entire somatic chromosome, the second con-
traction not being significant as regards pairing. The bivalent chromo-
somes so formed, after much shortening and thickening, are separated in
the heterotypic mitosis, during the anaphase of which (or earlier in the

1 A more detailed summary of this view may be found in a review of Miss Digby's
paper on Osmunda by the present author (1920a).

16

242 INTRODUCTION TO CYTOLOGY

case of chromosome tetrads) the split that is to function in the homoeotypic
mitosis makes its appearance. The double ness in the heterotypic pro-
phase is therefore not homologous with that in the somatic prophase:
in the former it is due to a conjugation and in the latter to a split.

According to Scheme B the doubleness of the heterotypic prophase is
due to a true splitting as in the case of somatic division. In both cases,
moreover, the split may have its origin in the preceding telophase. The
bivalent chromosome is formed by the association in pairs (often at first
end-to-end in the spireme but later side-by-side) of segments of this split
spireme at the time of the second contraction. The two split univalents
composing the bivalent are separated in the heterotypic mitosis, while
in the homceotypic mitosis the separation is along the line of the split
originating in the last premeiotic telophase and seen in the spireme of the
early heterotypic prophase. The doubleness of the early heterotypic
prophase is therefore regarded as homologous with that of the somatic
prophase: in both cases it represents a true split.

It cannot yet be said what the outcome of this controversy is to be.
The advocates of Scheme A believe that those of Scheme B have mis-
interpreted the changes occurring in the early heterotypic prophase and
in all telophases, while the latter charge the former with a neglect of the
second contraction stage. Scheme B as fully elaborated by Miss Digby
has certain advantages: it allows one interpretation to be placed upon
the double spireme in both somatic and heterotypic prophases, irrespective
of the exact time at which the split originates, and it also helps to explain
the sudden appearance of the split for the second maturation mitosis in
the anaphase of the first. Scheme A, on the other hand, is preferred by
geneticists because of the earlier and much longer continued association
of the conjugating chromosomes, which allows a greater opportunity for
"crossing-over" to occur. The significance of this point will be brought
out in Chapter XVII.

This question, however, must be settled primarily by direct evidence.
It is obvious that its solution depends upon the exact manner in which the
telophasic transformation of the chromosomes and the derivation of the
latter from the reticulum in the prophase are accomplished. It is granted
by both schools that the alveolar or reticulate condition in which the
chromosomes are found in late telophase is continuous with the similar
condition seen in the succeeding prophase. If, then, it is true (1) that
the telophasic transformation (alveolation) represents a true splitting,
and (2) that the early prophasic reticulate condition passes directly
into the double spireme, it follows that this doubleness in every prophase
is due to the split originating in the preceding telophase. But workers on
mitosis are not at all agreed that the evolution of the chromosomes is
that stated in (1) and (2). It has been shown in Viciafaba (Sharp 1913),
Tradescantia (Sharp 19206), and a number of other instances (see Chapter

THE REDUCTION OF THE CHROMOSOMES 243

VIII) not only that the telophasic alveolation is too irregular to be
regarded as a splitting, but also that the reticulate condition of the pro-
phase, instead of developing directly into the definitive split, gives rise
to simple thin threads in which a new split is developed. From this it
cannot be concluded that in no form does the split develop directly from
the early reticulate condition, or that the telophasic alveolation, though
irregular, may not later become so equalized as to constitute the first
stages of the split; but it does follow that it is quite unsafe to use the
principle of telophasic splitting as a premise from which to draw the
conclusion that the approximation of thin threads in the early heterotypic
prophase represents the reassociation of the halves of a single split
chromosome. It is well to emphasize the possible importance of the
premeiotic telophase, but any ultimate solution of this perplexing prob-
lem must be reached mainly through a more refined analysis of those

FIG. 94. — Chromosome pair "B" in Phrynotettix magnus, showing condensation of
bivalent pair during the heterotypic prophases to form the compact chromosomes appearing
on the spindle at metaphase. X 1734. (After Wenrich, 1916.)

prophasic changes which have led a long list of investigators to the con-
clusion that the early heterotypic association of slender threads represents
a conjugation of entire chromosomes which separate in the first matura-
tion mitosis.

One of the most convincing pieces of direct evidence favoring Scheme
A is found in Wenrich's recent work on Phrynotettix (1916). Wenrich is
able to trace a single pair of chromosomes, distinguishable by their
peculiar form and the arrangement of their chromatic accumulations or
chromomeres, through every stage from the spermatogonia to the sperma-
tids. During the heterotypic prophase the two members of the pair
conjugate parasynaptically while in the form of slender filaments. Simi-
larly strong arguments are advanced by Robertson (1916) as the result
of his detailed analysis of the chromosome groups in other Tettigidse
and Acrididse, in which the homologous members can be followed with
much certainty because of their frequent inequality in size.

Reduction With Chromosome Tetrads. — As already pointed out, the
marking out of the lines of separation for both maturation divisions
during the heterotypic prophase, with the resulting formation of chromo-
some tetrads, increases in no inconsiderable manner the difficulty of
interpreting the essential changes at these stages. The four chromatids
composing the tetrad represent two conjugated chromosomes each of
which is longitudinally split. Because of the variety of ways in which

244

INTRODUCTION TO CYTOLOGY

these may arrange themselves with reference to one another — in the form
of simple or compound rods, crosses, and rings — their distribution to the
daughter nuclei, as well as the manner of their origin, is very difficult to
follow with certainty. The accompanying diagrams will serve to illus-
trate the more common modes of behavior described for chromosome
tetrads, which are found chiefly in the cells of animals.

Figure 95, D represents an exceptional method of tetrad formation
described by Henking (1891) f or Pyrrochoris and by Korschelt (1895) for
Ophryotrocha. The continuous spireme segments to form the diploid
number of chromosomes,1 which then split longitudinally and shorten.

FIG. 95. — Reduction with chromosome tetrads.

D, in Pyrrochoris (Henking) and Ophryotrocha (Korschelt.) E, in certain copepods
(Riickert, Haecker, and vom Rath.) F, in Anasa and Allolobophora (Paulmier; Foot and
Strobell).

No conjugation occurs until the metaphase, when the split chromosomes
come together end-to-end, forming tetrads. They at once separate in
the anaphase, bringing about reduction. In the second mitosis they
divide along the original split, so that each of the four resulting nuclei
receives the haploid number of chromosomes, two of the nuclei thus
differing from the other two as the result of the separation of entire
(though secondarily split) chromosomes at the first mitosis. According
to Goldschmidt (1905), the chromosomes of Zoogonus mirus, after thus
undergoing 'no prophasic conjugation, divide longitudinally at the first

1 For the sake of uniformity and clearness the diploid number is represented as 6
in all of these diagrams.

THE REDUCTION OF THE CHROMOSOMES 245

mitosis and separate into two haploid groups at the second. To this
simple form of reduction Goldschmidt applied the term "Primsertypus."
Gregoire (1909a), on the contrary, found parasynapsis and the usual
mode of reduction in Zoogonus.

The interpretation at one time given by Riickert (1893, 1894),
Haecker (1895), and vom Rath (1895) for certain copepods is shown* in
Fig. 95, E. The continuous spireme splits throughout its length and then
breaks into the haploid number of segments. These again break trans-
versely, forming chromosome tetrads, each composed of two split chromo-
somes arranged end-to-end. In some species the chromatids open out to
form four-parted rings, whereas in others they maintain the rod form.
A separation occurs along the line of the original split at the first mitosis,
which is therefore equational, and along the plane of conjugation at the
second mitosis, which is therefore reductional. In Dicrocodium Gold-
schmidt (1908) reported that such tetrads divide reductionally at the
first mitosis. Lerat (1905), moreover, has found that in Cyclops strenuus,
one of the forms used by the earlier workers, the tetrads arise by a parallel
conjugation of thin threads which later split.

A third mode of tetrad behavior is that reported by Paulmier (1899)
for Anasa tristis and by Foot and Strobell (1905, 1907) for Anasa and
Allolobophora foetida (Fig. 95, F). Here the chromosomes conjugate
end-to-end, the bivalents so formed then splitting longitudinally, giving
tetrads which take on a cross or ring form. At the first mitosis the sepa-
ration is along the plane of conjugation, effecting reduction, and at the
second it is along the plane of splitting. According to McClung (1902),
Button (1902, 1905), Robertson (1908), and others, such tetrads separate
reductionally at the second mitosis (postreduction) rather than at the
first (prereduction) in certain orthopterans studied by them.

Figure 96 illustrates the origin of chromosome tetrads of five charac-
teristic types by the two prominent modes of reduction described in
detail in foregoing pages. According to Scheme A (Ai-Ci), two chromo-
somes conjugate parasynaptically while in the form of slender threads.
Instead of remaining unsplit as in most plants, each member then splits
longitudinally in a plane at right angles to the conjugation plane, thus
giving a tetrad composed of four parallel strands (chromatids) (D).
According to Scheme B (Az-Cz), the two chromosomes are at first ar-
ranged telosynaptically in the spireme and the latter splits throughout its
length. The two conjugating members then take up a side-by-side
position, and their split, instead of becoming obscured as usually occurs
in plants, remains open, giving the tetrad of parallel strands (D).

The tetrad, by whichever method it has arisen, may now undergo a
variety of alterations, some of which are shown at E and F. The chroma-
tids may simply shorten and thicken, the tetrad at diakinesis maintaining
the form of parallel rods (Ei, FI). They may open out alongjhe plane of

INTRODUCTION TO CYTOLOGY

conjugation (E£) and take the form of rod tetrads (F2) like those described
by Riickert and Haecker. While opening out in this manner the longi-

FIG. 96. — Diagram showing the origin of the tetrad of chromatids (D) according to
Scheme A (A\-C\) and Scheme B (Az-C*), and the further transformation of this tetrad into
tetrads of five types (Fi-F$).

tudinal halves of each chromosome may diverge where the two chromo-
somes remain in contact (£"3), the tetrad eventually taking the form of a
cross (F3) as in the cases described by Paulmier and by Foot and StrobelL

THE REDUCTION OF THE CHROMOSOMES

247

If the conjugated chromosomes remain in contact at both ends (#4) a
complete ring results (F4). In certain orthopterans the four chromatids
open out along the conjugation plane in some regions and along the plane
of splitting in other regions; this results in the curious compound rings
(Fig. 156) found in the cells of these insects. Finally, the chromatids
may open out from one end along the conjugation plane and from the
other end along the splitting plane (EB), the tetrad then assuming the
form of a ring composed of four parts (F5). In all cases the tetrads usu-
ally condense into compact quadruple bodies by the time they take their
places on the spindle of the heterotypic mitosis.

FIG. 97. — Reduction with chromosome tetrads in Fasciola hepatica, according
to Schellenberg (1911). Explanation in text.

The four chromatids composing the completed tetrad are in most cases
exactly similar in appearance, so that it is a matter of much difficulty to
determine along which plane they are separated at the first maturation
mitosis. According to the two theories of tetrad origin illustrated in
the foregoing diagram, however, the chromatids are supposed in almost
all cases to separate along the plane of conjugation at the first mitosis,
and this conclusion is supported by the behavior of those bivalent chromo-
somes which are not divided into tetrads of chromatids.

A further interpretation of reduction involving chromosome tetrads
has been given by Schellenberg (1911) for the parasitic flatworm, Fasciola
hepatica (Fig. 97) . The chromatin in the heterotypic prophase takes the
form of a long slender filament which splits longitudinally soon after
synizesis. This double thread then segments into the haploid number of

248 INTRODUCTION TO CYTOLOGY

pieces, each representing two chromosomes end-to-end; these have the
form of loops with a definite orientation ("first boquet stage"). Each
segments again, giving the diploid number of split chromosomes, which
again assume the form of oriented loops ("second boquet stage"). The
halves twist tightly about each other, shorten to form the double bodies
seen at diakinesis in the diploid rather than the haploid number, and then
conjugate to form the haploid number of chromosome tetrads. The
conjugating members (each split) separate at the first mitosis, bringing
about reduction; at the second mitosis the separation is along the line of
the original split. According to this interpretation, therefore, the double-
ness of the early heterotypic prophase is due to a split, as in Scheme B,
but the chromosomes arranged end-to-end in the spireme soon become
separated and do not conjugate again until diakinesis.

For a number of years it was thought (Carnoy 1886; Boveri 1887;
Hertwig 1890; Brauer 1893) that the chromosome tetrad in Ascaris
megalocephala was exceptional in being formed by two longitudinal fis-
sions of a primary chromatin rod, there being as a consequence no quali-
tative reduction in the two maturation divisions unless the organization
of the chromatin were different from that of other organisms. But it has
since been shown that they arise as in other organisms by the conjugation
of two split chromosomes (Sabaschnikoff 1897; Tretjakoff 1904; Griggs
1906). In the oogenesis Griggs reports telosynapsis with prereduction,
whereas in the spermatogenesis Tretjakoff describes parasynapsis followed
by postreduction. In Ascaris canis (Marcus 1908; Walton 1918) the
four chromatids each show a transverse constriction, the chromosomes on
the first maturation spindle having the form of octads.

Although the formation of well differentiated chromosome tetrads
occurs very commonly in animals, it appears to be very rare in plants.
Farmer (1895) described tetrads in Fossombronia, and they have since
been reported in at least three other bryophytes: Pallavicinia (Moore
1905), Sphagnum (Melin 1915), and Chiloscyphus (Florin 1918). They
have also been described in a few vascular plants : Equisetum (Osterhout
1897), Pteris (Calkins 1897), Ariscema (Atkinson 1899), Tricyrtis (Ikeda
1902), Thalictrum, Calycanthus, and Richardia (Overton 1909) (Fig. 98),
Spinacia (Stomps 1911), Primula (Digby 1912), and Lopezia (Tackholm
1914).

According to Gregoire (1905) such structures in plants are not true
tetrads, but resemble them because the chromosomes are often bent and
have their material accumulated largely at their ends. Sakamura
(1920) interprets them as conjugated constricted chromosomes, and denies
that the quadripartite condition has anything to do with reduction in
such cases. He likewise accounts for the metasynaptic rod tetrads (Fig.
95, E) described by several investigators of maturation in animals,
holding that they represent two constricted chromosomes conjugated

THE REDUCTION OF THE CHROMOSOMES

249

parasynaptically rather than two split ones placed end-to-end. In
support of this contention he cites the following observations: such
"tetrads" are seen not only in the oocytes and spermatocytes but also in
oogonia, spermatogonia, and somatic cells; the supposed telosynaptically
conjugated members are often very unequal in size; such tetrads are
sometimes divided in the transverse plane at neither maturation mitosis ;
not only tetrads, but also octads and hexads are often observed, even in

FIG. 98. — Chromosome tetrads.

A, five stages in the development of the tetrad in the spermatocyte of Anasa tristis.
X 3000. From a preparation by Dr. H. E. Stork. B, tetrads in sporocyte of Chiloscyphus.
Enlarged; X 2800. (After Florin, 1918.) C, tetrads in Richardia africana. (After
Overton, 1909.) D, false tetrads in somatic cells of Pisum due to action of chloral hydrate
on constricted chromosomes. (After Sakamura, 1920.)

the same cell, and these are plainly due to the presence of additional
accentuated constrictions. Robertson (1916) also interprets such telo-
synaptic rod tetrads as those observed by Haecker in the copepods as
constricted chromosomes. The constrictions, according to this writer,
represent points of temporary union between non-homologous elements.
From these considerations it is evident that constrictions have much to
do with the appearances assumed by chromosomes, and that they should
be taken into account in interpreting the chromosome tetrad.

Numerical Reduction Without Qualitative Reduction. — Figure 99
illustrates the behavior of the chromosomes in maturation according to
three not widely accepted views. A few workers, including Fick (1907,

250

INTRODUCTION TO CYTOLOGY

1908), Meves (1907, 1908, 1911), Giglio-Tos (1908), and Granata (1910),
reject the theory of chromosome individuality and specificity, and
therefore do not regard the chromosomes which are distributed to the
four cells at maturation as at all identical with those of the divisions im-
mediately preceding, except in so far as they are composed of the same
nuclear material. Accordingly they, recognize no qualitative reduction,
but only a numerical one. This reduction in number results from the
fact that the spireme formed in the heterotypic prophase (Fig. 99, A)
segments into the haploid number of pieces instead of the diploid number,
these pieces being simply divided longitudinally at both maturation
divisions, and the four resulting nuclei being qualitatively similar.

FIG. 99. — Diagram showing three reported modes of numerical reduction with-
out qualitative reduction.

A, according to Fick et al. B, according to Vejdowsky; complete fusion of conjugating
members. C, according to Bonne vie; bivalents arranged on spindles in juxtaposition;
fusion of conjugating members eventually becomes complete.

According to Vejdowsky (1907) (Fig. 99, B} the chromosomes appear
in diploid number in the heterotypic prophase and conjugate parasynapti-
cally. The members of the pair fuse completely and lose their individual
identity, so that the chromosomes appearing on the first maturation
spindle in haploid number are new entities, and not merely temporary
pairs of somatic chromosome individuals. At both divisions these bodies
split longitudinally, giving equivalence to the four resulting nuclei.
Here, as in the foregoing example, there is no definite qualitative reduc-
tion in Weismann's sense, though a numerical reduction is brought about
by means of a complete fusion at the time of chromosome conjugation.

THE REDUCTION OF THE CHROMOSOMES 251

An interpretation put forward by Bonnevie (1906, 1908) is shown in
Fig. 99, C. Here the chromosomes conjugate parasynaptically and
come into very intimate union: although they appear to undergo a real
fusion their identity is maintained for a time. Owing to the fact that
these bivalent chromosomes are inserted upon the spindle with their
halves in juxtaposition (side-by-side with respect to the poles) rather than
in superposition (one toward each pole), the members of a conjugated
pair separate neither at the first division nor at the second. As a result
each of the four cells receives the haploid number of chromosomes, all
of which are bivalent, and no qualitative reduction occurs. Bonnevie
believes that the conjugating members of each pair finally fuse completely
in the subsequent stages. In this case, therefore, as in the preceding one,
numerical reduction is supposed to result from a complete fusion of the
chromosomes in pairs.

Whether any confidence is to be placed in such interpretations or not
— and according to most cytologists none should be — they at least serve
to show how it is possible that numerical reduction may occur without
effecting any qualitative reduction, and that the essential feature of the
reduction of the chromosomes is something other than the mere change
in their number, as pointed out at the beginning of the chapter.

SYNAPSIS, OR CHROMOSOME CONJUGATION

The phenomenon of chromosome conjugation, or synapsis, which we
have seen above is such an important feature of the reduction process,
must now be somewhat more closely examined. Attention will be
directed to three points: the relationship of the conjugating members
(the "synaptic mates"); the stage at which the synaptic union takes
place, and the exact nature of this union.

Relationship of the Synaptic Mates. — We may first inquire into the
relationship which may exist between the two chromosomes pairing to
form a given bivalent chromosome: is any chromosome of the duplex
group (the two intermingled parental chromosome sets in the individual's
nuclei) present in the gonotokont free to pair with any other chromosome,
or does the pairing take place according to more restricting rules?

It was suggested by Henking (1891) that the two synaptic mates are
ultimately derived from the two parents at the previous fertilization,
one from the father and the other from the mother: the chromosomes of
one parental set pair with those of the other parental set to form the
haploid number of bivalent chromosomes appearing on the first matura-
tion spindle. This idea was later emphasized and developed by Mont-
gomery (1900-4), Sutton (1902), Boveri (1901), and others, who found
for it much supporting evidence in organisms with chromosomes differ-
ing in size and shape. An observation made by Rosenberg (1909) on
Drosera hybrids is significant in this connection. When Drosera rotundi-

252 INTRODUCTION TO CYTOLOGY

folia (20 chromosomes) ' is crossed with D. longifolia (40 chromosomes)
there results a hybrid with 30 chromosomes, of which 10 are contributed
by rotundifolia and 20 by longifolia. When synapsis occurs preparatory
to reduction in this hybrid only 10 bivalents are formed, 10 chromosomes
remaining unpaired. This was taken by Rosenberg to mean that the 10
rotundifolia chromosomes pair with 10 of the longifolia ones, leaving the
other 10 of longifolia without synaptic mates. Had any chromosome of
the duplex group of 30 been free to pair with any other, 15 bivalents
would have been produced.

Other instances of this phenomenon may be mentioned. By crossing
(Enothera Lamarckiana (seven chromosomes in gamete) with (E. gigas (14
in gamete) individuals with 21 chromosomes are obtained. Geerts
(1911) found that, preparatory to reduction, the seven Lamarckiana chro-
mosomes pair with seven of the gigas chromosomes, leaving the other
seven of gigas unpaired. On the contrary, however, Gates (1909) found
that the 21 chromosomes in a lata-gigas hybrid simply separate into two
approximately equal groups, usually of 10 and 11 chromosomes re-
spectively. Kihara (1919) reports that in some 35-chromosome wheat
hybrids formed by crossing Triticum polonicum (14 chromosomes in
gamete) with T. spelta (21 in gamete) there are present in the heterotypic
prophase 14 bivalents (polonicum conjugated with spelta) and seven
univalents (spelta). The 14 bivalents are arranged on the spindle and
separate as usual, whereas the seven unpaired spelta chromosomes split
longitudinally at the first mitosis and distribute themselves irregularly
at the second (Fig. 100). An analogous condition is found in Pigcera
hybrids by Federley (1913).

A very significant additional suggestion with respect to synapsis was
made by McClung (1900) and Sutton (1902): not only are the two chromo-
somes which conjugate derived from the two parents, but they are hom-
ologous— each chromosome of one parental set pairs with a particular
chromosome of the other parental set, the two members of the resulting
bivalent being presumably of corresponding hereditary value, as will be
shown in Chapter XV. The evidence for this important hypothesis was
found chiefly in Brachystola (Fig. 101) and a number of other insects
having chromosome complements made up of members with constant
characteristic differences in size and shape. Many such cases have been
subsequently discovered, especially by McClung and his eoworkers in
their extensive researches on insect spermatocytes. As examples among
plants may be cited Crepis virens, Najas major, N. marina, and Vicia
faba.

Crepis virens (Rosenberg 1909) (Fig. 102) has six chromosomes: two
long, two medium sized, and two short. When synapsis occurs the like
chromosomes pair, forming bivalents of three sizes. The members of
each pair separate and pass to the daughter cells at the first maturation

THE REDUCTION OF THE CHROMOSOMES

253

mitosis, each microspore (after the second mitosis) having as a result
three chromosomes: one long, one medium sized, and one short. Since
the gamete receives such a simplex group of three chromosomes, and the

Fio. 100. — Heterotypic mitosis in Triticum potonicum X T. spelta.
A, the 21 chromosomes (polar view). B, 14 bivalents separated into component
univalents; 7 unpaired spelta chromosomes have split and are about to be distributed.
(After Kihara, 1919.)

somatic cells of the new individual show six (two of each length), it is
evident that the other gamete furnishes a similar simplex group of three.

FIG. 101. — The chromosome complement in the spermatocyte of Brachystola
magna. (After Sutton, 1902.)

In Najas marina and Najas major (Miiller 1911; Tschernoyarow
1914) the duplex group of 14 chromosomes is made up of seven visibly
different pairs (Fig. 56 bis). In the heterotypic prophase these conjugate
selectively to form seven bivalents, the reduced nuclei therefore receiving

254

INTRODUCTION TO CYTOLOGY

a set of seven visibly different chromosomes. Sakamura (1920) holds
the number here to be six rather than seven. (See p. 160.)

In Vicia faba (Sharp 1914; Sakamura 1915, 1920) there are in the
somatic cells 12 chromosomes, two of them being about twice as long as
the other 10 (Figs. 56 and 102). At synapsis in the microsporocyte there
are formed six bivalents, one of them having about twice the length of
the other five. Hence it is clear that the two long chromosomes pair
with each other. In the heterotypic mitosis the synaptic mates separate

SIMILAR PROCESS IN <? A X

A -(/

CREPIS V1RLNS S.MH.AR PROCESS .N

FIG. 102. — Chromosome cycles in Vicia faba and Crepis virens, showing
homologous pairing.

and pass to the daughter nuclei, bringing about reduction. At the close
of the homceotypic mitosis the microspore, and hence the male gamete
to which it later gives rise, receives a simplex group of six chromosomes:
one long and five short. Since the somatic cells contain each of these in
duplicate it is evident that a similar set is contributed by the female
gamete.

Summing up, we may draw from the above facts certain very impor-
tant conclusions: (1) Each parent furnishes the offspring with a set of
chromosomes, the members of the two sets being intermingled in all the
nuclei of the new indiiidual. This point will receive further attention in
the following chapter on fertilization. (2) The two members of each
bivalent chromosome formed at synapsis are derived one from each parental
set. (3) Each chromosome of the paternal set conjugates with a particular
chromosome of the maternal set: the two are in some sense homologous.

THE REDUCTION OF THE CHROMOSOMES 255

It should be pointed out that cytologists and geneticists have generally
assumed that each synaptic pair is independent of all the others as
regards the manner in which it is oriented on the heterotypic spindle.
In some pairs the paternal members are directed toward one pole and in
other pairs toward the other pole. It is conceivable that in some cases
all the paternal members might go to one pole and all the maternal
members to the other. Direct evidence that the assortment of the
various chromosome pairs is in this respect a random one as originally
assumed has been furnished by Miss Carothers (1913, 1917). In the
grasshopper, Trimerotropis, she finds that the components of some of the
bivalents are visibly different in size, in the mode of attachment to the
spindle fibers, and in the presence of constrictions; and that these differ-
ences make it possible to show beyond question that the several pairs are
entirely independent of one another as regards their orientation on the
spindle and their consequent distribution to the daughter cells.

From the precise manner in which the distribution of chromosomes
at the time of reduction and at other stages of the life cycle parallels the
distribution of the hereditary characters it is inferred that such hom-
ologous chromosome pairs represent the material basis for the allelo-
morphic pairs of Mendelian characters exhibited by the organism. This
subject is to be taken up in Chapter XV.

The Stage at Which Conjugation Occurs. — In the great majority of
observed cases chromosome conjugation occurs during the prophase
of the first maturation division. Since the chromatin threads at some
time during these prophases usually take the form of a tightly contracted
knot out of which they emerge in an obviously double condition, it was
suggested (Moore 1896) that the contraction is an important factor in
bringing about the conjugation, and the contraction itself came to be
called " synapsis." But an examination of the various modes of reduction
shows that the conjugation may begin very early, before the contraction
(Fig. 83) or, on the other hand, not until the spindle is established
(Fig. 95, /)).• The conjugation of the chromosomes is therefore to be
distinguished from the contraction. It has now become customary to
refer to the former, at whatever stage it occurs, as synapsis, and to the
latter as synizesis.

In an increasing number of reported cases the paired association
apparently begins even before the heterotypic prophase. The chromo-
somes have been observed in several instances to undergo pairing during
the anaphase and telophase of the last premeiotic division. Such is the
condition in certain Hemiptera (Montgomery 1900, 1901), Oniscus
(Nichols 1902), Brachystola (Sutton 1902), Scolopendra (Blackman
1903, 1905), Pedicellina (Dublin 1905), and a number of more recent
cases. Furthermore, the pairing has been stated to begin in the sperm-
atogonia several cell generations before maturation in certain Hemiptera

256 INTRODUCTION TO CYTOLOGY

«

and Ascaris (Montgomery 1904, 1905, 1908, 1910), Alytes (Janssens
and Willems 1909), Helix and Sagitta (Stevens 1903; Ancel 1903), certain
Diptera (Stevens 1908, 1911), and Pediculus (Doncaster 1920).

More recently it has been shown that the homologous chromosomes
may begin to show a paired arrangement even earlier in the cycle, in
some cases directly after the parental groups are brought together at
fertilization. In the Diptera, for example, Metz (1916a) has shown that
the association, which at certain stages is so close as to constitute a
synapsis, begins before the cleavage of the fertilized egg, and that the
paired condition is maintained in all cells, somatic and germinal, through-
out the life cycle. Metz examined 80 species and in all of them found
such a somatic pairing. In Culex (Stevens 1910, 1911; Taylor 1914,
1917), which has six chromosomes, the association can be seen in the
nuclei of the segmenting egg, and in the early larval stages there follows
an actual parasynaptic fusion, so that the somatic cells thereafter show
three bivalent chromosomes rather than six univalents. In the matura-
tion divisions the members of each pair separate, the gametes receiving
three chromosomes each, just as they would had conjugation begun in
the heterotypic prophase as usual.

A loosely paired arrangement of the chromosomes in the somatic
cells of plants has been reported by Strasburger (1905, 1907, 1910) for
Galtonia candicans, Funkia Sieboldiana, Pisum sativum, Melandrium,
Mercurialis, and Cannabis; by Sykes (1908) for Hydrocharis, Lychnis,
Begonia, Funkia, and Pisum; by Overton (1909) for Calycanthus; by
Miiller (1909, 1911) for Yucca and other forms; by Stomps (1910, 1911)
for Spinacia; by Kuwada (1910) for Oryza; by Tahara (1910) for Morus;
and by Ishikawa (1911) for Dahlia. This is another matter that will be
considered further in Chapter XII.

The Nature of the Synaptic Union. — Because of the manner in which
chromosome behavior is at present being applied to the solution of the
problems of inheritance, no question concerning chromosome conjugation
is more important than that of the exact nature of the synaptic union.
In reviewing some of the opinions of this subject it will be convenient
to list separately the views of the telosynaptists and the parasynaptists.

In such cases as those described by Henking and by Goldschmidt
Fig. 95, D) there is only a momentary end-to-end association of the fully
formed chromosomes on the spindle of the heterotypic mitosis, there
being no real fusion and almost no opportunity for an "interchange of
influences." In the other tetrad chromosomes formed by telosynapsis
(Fig. 95, E and F; Fig. 90) there is only slightly greater opportunity
for such interchange. According to Scheme B (Fig. 89) the synaptic
mates are at first arranged end-to-end, and only later, when partially
condensed, do they take up a side-by-side position, allowing a more
intimate and extensive union for a short time.

THE REDUCTION OF THE CHROMOSOMES 257

Generally speaking, the parasynaptists have given more attention
to the details of the synaptic union than have the telosynaptists. Al-
though cases are on record in which there is only a momentary para-
synaptic association of fully formed chromosomes (von Voss 1914),
the association usually extends over considerable time. Most para-
synaptists hold that the conjugation begins with the association of the
leptotene threads before or during synizesis, continues through the
remainder of the prophase, and ends with the anaphasic separation
(Scheme A). The association of the synaptic mates is thus long and
intimate. Concerning the closeness of the union, however, opinions
differ widely.

A few investigators (Vejdowsky 1907; Bonnevie 1906, 1908, 1911;
Winiwarter and Sainmont 1909: Schneider 1914) have thought that the
conjugating members fuse completely and lose their individual identity,
the "mixochromosome" so formed then undergoing two true longitudinal
splits along new planes at the two maturation divisions. In some cases
(Bonnevie; see p. 251) the fusion may not be fully consummated until
during the post-meiotic divisions. Others believe the split for the
heterotypic mitosis to be along the plane of conjugation (Cardiff 1906,
Fasten 1914, and others). Probably the most widely advocated view is
that there is no actual fusion of the synaptic threads, the latter main-
taining their identity completely. Although their association may at
times be so intimate that they seem to constitute a single thick thread,
the doubleness, if thus lost to view, reappears during later stages (Berghs

1904, 1905; A. and K. E. Schreiner 1905, 1906; Mare"chal 1907; Overton

1905, 1909; Robertson 1915, 1916) (Fig. 88). Several careful observers
have reported that the doubleness can be seen at all stages (Gregoire
1907, 1910; Schleip 1906, 1907; Montgomery 1911; Kornhauser 1914,
1915; Wenrich 1915, 1917). Gregoire, who has argued strongly for this
interpretation, has emphasized the ease with which the closely appressed
threads may be mistaken for a single thick structure.

One of the most important suggestions which has been made concern-
ing chromosome conjugation is embodied in the "Chiasmatype Hypo-
thesis" of Janssens (1909). According to Janssens, the pairing threads,
though remaining separate throughout the greater part of their length,
fuse at one or more points as they twist about each other. When they
again separate a break occurs at each of these fusion points, but along a
new plane, so that each of the two resulting chromosomes is composed
of portions of both conjugating members (Fig. 149). This interpreta-
tion, which has been admitted as possible by several of the investigators
named in the preceding paragraph, is significant in that it shows how an
orderly evolution of chromosomes with new constitutions mayjoccur, a
point of great importance in connection with current conceptions of the

17

258 INTRODUCTION TO CYTOLOGY

physical basis of heredity. Special attention will be devoted to this
question in Chapter XVII.

Chromomeres. — An important role has been attributed to the chro-
momeres by many students of synapsis. Allen (1905), for example,
maintained that the fusion of the leptotene threads in Lilium involves a
fusion of their chromomeres, the subsequent division of the fused chro-
momeres initiating the resplitting of the pachytene thread. Allen found
the chromomeres to be composed of still smaller chromatic elements,
and offered various suggestions concerning the manner in which the re-
splitting of the pachytene thread might be supposed to effect a redistri-
bution of the "idioplasms." That chromosome conjugation is primarily
a conjugation of small chromatic elements within the chromosome was
held by Strasburger (1904, 1905) and the Schreiners (1906). The visible
chromatin granules, or "pangenosomes," were conceived by Strasburger
to represent complexes of "pangens" such as were postulated by de Vries,
conjugation involving an interchange of these latter units.

The chromomere interpretation has been adversely criticised by
Gre*goire (1907, 1910) on the basis of further evidence obtained from a
study of the chromatic structures themselves. This author points out
several serious objections to the view that the chromomeres are auton-
omous bodies, and concludes that they are rather to be regarded simply
as swellings or thicker portions of the chromatin thread, such thick por-
tions, remaining as the thread undergoes a stretching which is not uni-
formly resisted at all points. Their frequently striking correspondence
or paired arrangement in the synaptic threads is explained as the result
of the response of the two closely associated threads to the same stretch-
ing force. This interpretation is also shown to account for the variability
in the dimensions of the chromomeres, their tapering form, the often
reported absence of correspondence between the chromomeres of the
two threads, and various other aspects. Wenrich (1916, 1917), on the
other hand, has found that in Phrynotettix (Fig. 155) the chromomeres
show a remarkable individual constancy in size and position in a given
member of the chromosome complement, not only in the various cells of
a given individual, but also in those of different individuals. These
facts strongly suggest an autonomy of the bodies in question.

Because of their great theoretical importance (see Chapter XVII) it
is to be regretted that after such a large amount of research so many
points regarding the process of synapsis should remain in such an un-
settled state. It is hoped that further refinements in microtechnique
may remove some of the obscurity which at present surrounds them.

OTHER OPINIONS ON THE HETEROTYPIC PROPHASE

Although the phenomena of the heterotypic prophase, particularly
synizesis and synapsis, are generally looked upon as normal occurrences

THE REDUCTION OF THE CHROMOSOMES 259

of considerable significance, not all investigators concur in this opinion.
That synizesis is an artifact due to faulty fixation is an interpretation
which, though it may be justified for certain cases in which the contrac-
tion may be very slight or absent, is not of general application. Fixa-
tion often serves to accentuate the appearance of contraction, but the
characteristic synizesis figure has been observed widely enough in faith-
fully preserved, and even in living material to make it evident that we
are dealing here with a normal feature of the heterotypic prophase. It
may not, however, be of universal occurrence.

R. Hertwig (1908) came to the conclusion, as a deduction from his
theory of the nucleoplasmic relation, that the phenomena of the hetero-
typic prophase represent an abortive mitosis : the disturbed nucleoplasmic
balance is restored to the normal by a multiplication of chromatin without
an actual mitosis, the process taking the form of the changes peculiar to
the heterotypic prophase. This view of Hertwig, which was denied by
Gregoire (19096), is supported by Kingsbury and Hirsch (1912), who
state :

"According to this view, on the one hand, synizesis represents 'an attempt
on the part of the spermatogonia to divide again — which fails; while on the other
hand, the reputed conjugation of chromosomes occurring at about this time is
but the imperfect fission and subsequent fusion of daughter chromosomes of such
abortive division."

The above quoted authors regard synizesis and synapsis as indications
of the onset of degeneration. In this conclusion they are supported by
Kingery (1917), who, in his investigation of the white mouse, finds
synizesis in the primitive germ cells which degenerate, but not in the
definitive germ cells. Observations of a similar nature were made by
Wodsedalek (1916) in the mule. If, as Kingery (1917) and Popoff
(1908) point out, the "heterotypic" changes are due to degeneration,
they should be found in abnormal somatic cells. Marcus (1907), in
fact, had observed a contraction similar to that of synizesis preceding
degeneration in the cells of the thymus gland. Nemec (1903) and Kemp
(1910) also found that in the cells of roots treated with chloral hydrate
the nuclei come to have an abnormally high number of chromosomes
("syndiploid nuclei"), this number, according to Nemec, being gradually
restored to the normal during the subsequent mitoses, which show
phenomena of a heterotypic nature. Strasburger (1911), while agreeing
with Nemec that the syndiploid condition gradually disappears, denied
that any truly heterotypic phenomena are concerned. The "hetero-
typic" changes observed by Nemec he held to be only peculiar vegetative
mitoses with a superficial resemblance to genuine reduction divisions.

Nemec's conclusion regarding a reduction in chloralized vegetative
cells is also contradicted by Sakamura (1920), who has made a particu-
larly exhaustive study of these phenomena. Sakamura finds that a

260 INTRODUCTION TO CYTOLOGY

variety of agencies, including chloral hydrate, benzene vapor, ether, chlo-
roform, and the gall-producing secretions of Heterodera, may be employed
to bring about aberrations of the mitotic process. After the chromosomes
are divided and partially distributed they may be reorganized in a single
"didiploid" nucleus. In other cases the chromosomes may reorganize
as two or more nuclei with various chromosome numbers, and these may
often fuse to form " syndiploid ' ' nuclei. To all the kinds of stimuli applied
the chromosomes react by becoming shorter and thicker, and thus appear
like heterotypic chromosomes. Furthermore, latent or obscure constric-
tions are rendered more conspicuous, so that some of the split chromo-
somes appear like chromosome tetrads. Sakamura shows that these false
tetrads do not represent heterotypic phenomena in any true sense : they are
merely the result of the response of split and constricted chromosomes to
the abnormal conditions induced in the cell, and have nothing to do with
any reducing process. No such autoregulative reduction occurs in these
didiploid cells, their gradual decrease in relative number being due to
their lowered capacity for, and rate of, division.

Child (1915) emphasizes the physiological significance of maturation,
and shows that the heterotypic phenomena are associated with a low
metabolic rate in the cells, that they may occur occasionally in other cells
having a low rate, and that they can be induced artificially with narcotics
as Nemec stated.

All of these observations are interesting in that they indicate the
nature of some of the physiological changes occurring at the time of matur-
ation. The description of the heterotypic phenomena upon which will
be based our ultimate interpretation of its significance, will not be com-
plete until the physiological as well as the morphological changes have
been exhaustively examined and correlated. But because it has been
found that the onset of the meiotic process is associated with a lowering of
the rate of metabolism which, if continued, may result in degeneration;
or because appearances similar to those of the heterotypic prophase may
occur in other cells with disturbed metabolism; it does not at all follow that
the heterotypic phenomena are at bottom phases of a degeneration
process, or that they have no other significance in the normal life cycle.
These phenomena occur almost universally throughout the whole world
of living organisms at a very critical stage in the life cycle and lead to
significant results with a high degree of regularity. The lowered rate of
metabolism accompanying them offers in the vast majority of cases no
check to the normal functioning of the products of the maturation di-
visions. It therefore seems more reasonable to regard the observed
degeneration as a secondary effect that may occasionally set in during the
normal heterotypic prophases because the metabolic rate is already at a
relatively low level at that time, than to look upon the heterotypic
changes as a part of a degeneration process which is only exceptionally

THE REDUCTION OF THE CHROMOSOMES 261

completed — unless, indeed, all changes in the organism which are accom-
panied by a fall in the metabolic rate be regarded as degenerative in
character.

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262 INTRODUCTION TO CYTOLOGY

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THE REDUCTION OF THE CHROMOSOMES 263

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Biol. Bull. 34: 277-306. pis. 1-4.
FAULL, J. H. 1905. Development of ascus and spore formation in ascomycetes.

Proc. Boston Soc. Nat. Hist. 32; 77-113. pis. 7-11.
1912. The cytology of Laboulbenia chcetophora and L. Gyrinidarum. Ann. Bot.

26 : 325-355. pis. 37-40.
FEDERLEY, H. 1913. Das Verhalten der Chromosomen bei der Spermatogenese der

Schmetterlinge Pygaera anachoreta, curtula und pigra sowie einiger ihrer Bastarde.

Zeitschr. Ind. Abst. Vererb. 9: 1-110. pis. 1-4. figs. 5.
FICK, R. 1905. Betrachtungen iiber die Chromosomen, ihre Individuality, Reduk-

tion und Vererbung. Arch. Anat. u. Physiol.. Anat. Abt., Suppl. 179-228.

1907. Vererbungsfragen, Reduktions- und Chromosomenhypothesen, Bastard-
regeln. Ergeb. Anat. u. Entw. 16: 1-140. (Review.)

1908. Zur Konjugation der Chromosomen. Arch. Zellf. 1: 604-611.
FITZPATRICK, H. M. 1918. The cytology of Eucronarlium muscicola. Am. Jour.

Bot. 6 : 397-419. pis. 30-32.

FLEMMING, W. 1887. Neue Beitrage zur Kenntniss der Zelle. Arch. Mikr. Anat.
29 : 389-463. pis. 23-26.

FLORIN, R. 1918. Cytologische Bryophytenstudien. I. Ueber Sporenbildung bei
Chiloscyphus polyanthus (L) Corda. Ark. f. Bot. 15: 1-10. 1 pi.

FOOT, K. and STROBELL, E. C. 1905. Prophases and metaphase of the first matura-
tion spindle of Allolobophora Jcetida. Am. Jour. Anat. 4: 199-243. pis. 1-9.

1909. The nucleoli in the spermatocytes and germinal vesicles of Euschistus vario-
larius. Biol. Bull. 16: 215-238. 1 pi.

1910. Pseudo-reduction in the oogenesis of Allolobophora fcelida. Arch. Zellf. 5:
149-165. pis. 11, 12. 1 fig.

1912. A study of chromosomes and chromatin nucleoli in Euschistus crassus.

Ibid. 9 : 47-62. pis. 2-4.
FRASER, H. C. I. 1907. On the sexuality and development of the ascocarp of

Lachnea stercorea Pers. Ann. Bot. 21 : 349-360. pis. 29, 30
1908. Contributions to the cytology of Humaria rutilans. Ibid. 22 : 35-55. pis.

4,5.
1914. The behavior of the chromatin in the meiotic divisions of Vicia Faba. Ibid.

28 : 633-642. pis. 43, 44.

FRASER, H. C. I. and WELSFORD, E. J. 1908. Further contributions to the cytology
of the ascomycetes. Ann. Bot. 22 : 465-477. pis. 26, 27.

264 INTRODUCTION TO CYTOLOGY

FRASER, H. C. I. and BROOKS, W. E. 1909. Further studies in the cytology of the

ascus. Ann. Bot. 23 : 538-549.
FRASER, H. C. 1. and SNELL, J. 1911. The vegetative divisions in Vicia /aba.

Ann. Bot. 25: 845-855. pis. 62, 63.
FRIES, R. E. 1911. Zur Kenntniss der Cytologie von Hygrophorus conicus. Svensk.

Bot. Tids. 5: 241-251. pi. 1.
FRISENDAHL, A. 1912. Cytologische und entwicklungsgeschichtliche Studien an

Myricaria germanica Dear. Kgl. Svensk. Vet. Handl. 48.
GATES, R. R. 1908. A study of reduction in (Enolhera rubrinervis. Bot. Gaz. 48 :

1-34. pis. 1-3.

1909. The behavior of chromosomes in (Enothera lata X 0. gigas. Ibid. 48 . 179-199.
pis. 12-14.

1911. The mode of chromosome reduction. Ibid. 51: 321-344.

GIGLIO-TOS, E. e GRANATA, L. 1908. I mitocondrii nelle cellule seminali di Pam-

phagus marmoratus, Burm. Biologica 2 : No. 4.
GOLDSCHMIDT, R. 1905. Eireifung, Befruchtung und Embryonalentwicklung des

Zoogonus mirus Lss. Zool. Jahrb. 21: 607-654. pis. 36-38.
1908c. Die Chromatinreifung der Geschlechtszellen des Zoogonus mirus Lss. und

die Primartypus der Reduktion. Arch. Zellf. 2 : 348-370. pis. 24, 25. figs. 6.
1908o. Ueber das Verhalten des Chromatins bei der Eireifung und Befruchtung

des Dicrocalium lanceolatum Stil. et Has. (Distomum lanceolatum.) Ibid. 1 :

232-244. pi. 7.

19086. 1st eine parallele Chromosomenkonjugation bewiesen? Ibid. 1: 620-622.
GOLDSMITH, W. M. 1919. A comparative study of the chromosomes of tiger beetles

(Cicindelidae). Jour. Morph. 32 : 437-487. pis. 1-10.
GRANATA, L. 1910. Le cinesi sperm atogenetische di Pamphagus marmoratus, Burm.

Arch. Zellf. 5.
GREGOIRE, V. 1899. Les cineses polliniques chez les Liliace'es. La Cellule 16:

235-297. pis. 2.

1904. La reduction numerique des chromosomes et les cineses de maturation.
Ibid. 21 : 297-314.

1905. Les resultats acquis sur les cineses de maturation dans les deux regnes.
Ibid. 22:221-376. (Review.)

1907. La formation des gemini heterotypiques dans les ve'ge'taux. Ibid. 24: 369-

420. pis. 2.
1909a. La reduction dans le Zoogonus mirus Lss. et le "Primartypus." Ibid. 25:

245-285. pis. 2.
1909&. Les ph6nomenes de I'e'tape synaptique represent-ils une caryocinese

avort6e? Ibid. 25: 87-99.

1910. Les cineses de maturation dans les deux regnes. L' unite essentielle du pro-
cessus meiotique. Ibid. 26: 223-422. figs. 145.

1912. La verit6 du schema heterohomeotypique. Compt. Rend. Acad. Sci.
Paris 155: 1098-1100.

GREGORY, R. P. 1904. Spore formation in leptosporangiate ferns. Ann. Bot. 18:

445-458. pi. 31. 1 fig.
GRIGGS, R. F. 1906. A reduction division in Ascaris. Ohio Nat. 6: 519-528. pi.

33.
GUIGNARD, L. 1891. Nouvelles eludes sur la fecondation. Ann. Sci. Nat. Bot.

Vll 14: 163-296. pis. 9-18.
1899. Le developpement du pollen et la reduction dans le Naias minor. Arch.

d'Anat. Micr. 2 : 455-509. pis. 19, 20.
GUILLIERMOND, A. 1910. La sexualite chez les champignons. Bull. Sci. France et

Belg. 44: 109-196.

THE REDUCTION OF THE CHROMOSOMES 265

HAECKER, V. 1890. Ueber die Reifungsvorgange bei Cyclops. Zool. Anz. 13 : 551-

558. 1 fig.
1892. Die Eibildung bei Cyclops und Canthocamptus. Zool. Jahrb. 6: 211-248.

pi. 19.
1895a. The reduction of the chromosomes in the sexual cells. Ann. Bot. 9:

95-102.
18956. Die Vorstadien der Eireifung. Arch. Mikr. Anat. 46: 200-272. pis.

14-17.
1897. Weitere Uebereinstimmungen zwischen den Fortpflanzungsvorgange der

Thiere und Pflanzen. Biol. Centralbl. 17 : 689-705, 721-745. figs. 36.
1899. Die Reifungserscheinungen. Ergebn. Anat. u. Entw. 8 : 847-922. figs. 22.
HARMAN, M. T. 1920. Chromosome studies in Tettigidse. II. Biol. Bull. 38:

213-230.

HARPER, R. A. 1900. Sexual reproduction in Pyronema confluens and the morphol-
ogy of the ascocarp. Ann. Bot. 14: 321-396. pis. 19-21.
1905. Sexual reproduction and the organization of the nucleus in certain mildews.

Carnegie Inst. Wash. Publ. 37.
VON HENKING, H. 1891. Untersuchungen iiber der ersten Entwicklungsvorgange

in den Eiern der Insekten. Zeit. Wiss. Zool. 51 : 685-736. pis. 35-37.
1892. Same title. 111. Ibid. 64: 1-274. pis. 1-12. figs. 12.
HERTWIG, R. 1908. Ueber neue Probleme der Zellenlehre. Arch. Zellf. 1: 1-32.

figs. 9.
HEUSER, F. 1884. Beobachtungen tiber Zellkerntheilung. Bot. Centralbl. 17 : 27,

57, 85, 117, 154. pis. 1, 2.
IKEDA, T. 1902. Studies on the physiological functions of antipodals and related

phenomena of fertilization in Liliacese. 1. Tricyrtis hirta. Bull. Coll. Sci. Agr.

Tokyo 5.
ISHIKAWA, M. 1911. Cytologische Studien iiber Dahlien. Bot. Mag. Tokyo 25:

1-8. 1 pi.
JAHN, E. 1908. Myxomycetenstudien. 7. Ceratiomyxa. Ber. Deu. Bot. Ges. 26a :

342-352.
JANSSENS, F. A. 1905. Spermatoge'nese dans les batrachiens. III. Evolution des

auxocytes males du Batracoseps attenuatus. La Cellule 22: 379-425. pis. 7.
1909. La theorie de la chiasmatypie. Ibid. 25: 389-411. pis. 2.
JANSSENS, F. A. et WILLEMS, J. 1909. Spermatogenese dans les batraciens. IV.

Ibid. 25: 151-178. pis. 2.
JUEL, H. O. 1898. Die Kerntheilungen in den Basidien und die Phylogenie der

Basidiomyceten. Jahrb. Wiss. Bot. 32: 361-388. pi. 4.
KARSTEN, G. 1908. Die Entwicklung der Zygoten von Spirogyra jugalis, Ktzg

Flora 99: 1-11. pi. 1.
KEMP, H. P. 1910. On the question of the occurrence of " heterotypical reduction"

in somatic cells. Ann. Bot. 24 : 775-803. pis. 66, 67.
KIHARA, H. 1919. Ueber cytologische Studien bei einigen Getreidearten. Bot.

Mag. Tokyo 32: 17-38. figs. 21.
KING, H. D. 1907. The spermatogenesis of Bufo lentiginosus. Am. Jour. Anat. 7 :

345-388. pis. 3.

1908. The oogenesis of Bufo lentiginosus. Jour. Morph. 19: 369-438. pis. 4.
KINGERY, H. M. 1917. Oogenesis in the white mouse. Ibid. 30: 261-316. 5 pis.
KINGSBURY, B. F. and HIRSCH, P. E. 1912. The degeneration of the secondary

spermatogonia of Desmognathus fusca. Jour. Morph. 23: 231-253. pis. 3.
KNIEP, H. 1911. Ueber das Auftreten von Basidien im einkernigen Mycel von

ArmiUaria mellea Fl. Don. Zeit. f . Bot. 3 : 529-553. pis. 3, 4.

266 INTRODUCTION TO CYTOLOGY

KOERNICKE, M. 1904, 1905. Die neueren Arbeiten iiber die Chromosomenreduktion

im Pflanzenreich und daran anschliessende karyokinetische Probleme. Bot.

Zeit. 62: II, 305-314; 63: 11, 289-307.
KORNHATJSER, S. I. 1914. A comparative study of the chromosomes in the

spermatogenesis of Enchenopa binotata (Say) and Enchenopa (Campyleuchia

Stal) curvata (Fabr.). Arch. Zellf. 12: 241-298. pis. 18-22.
1915. A cytological study of the semi-parasitic copepod, Hersilia apodiformis

(Phil.), with some general considerations of copepod chromosomes. Ibid. 13:

B99-445. pis. 17-19. figs. 9.
KORSCHELT, E. 1895. Ueber Kerntheilung, Eireifung, und Befruchtung bei Ophryo-

trocha puerilis. Zeitschr. Wiss. Zool. 60 : 543-688. pis. 28-34.
KURSSANOW, L. 1911. Ueber Befruchtung, Reifung, und Keimung bei Zygnema.

Flora 104: 65-84. pis. 1-4.
KUWADA, Y. 1910. A cytological study of Oryza sativa L. Bot. Mag. Tokyo 24:

267-281. pi. 8.
1919. Die Chromosomenzahl von Zea Mays L. Jour. Coll. Sci. Tokyo 39 : 1-248.

pis. 2. figs. 4.
KYLIN, H. 1918. Studien tiber die Entwicklungsgeschichte der Phaeophyceen.

Svensk. Bot. Tidskr. 12 : 1-64.
LAGERBERG, T. 1906. Ueber die prasynaptische und synaptische Entwicklung der

Kerne in den Embryosackmutterzellen von Adoxa moschatellina. Bot. Stud.

Kjelmann, Uppsala.
1909. Studien liber die Entwicklungsgeschichte und systematische Stellung von

Adoxa moschatellina, L. Kgl. Svensk. Vet. Akad. Handl. 44.
LAWSON, A. A. 1911. The phase of the nucleus known as synapsis. Trans. Roy.

Soc. Edinburgh 47 : III, 591-604. pis. 1, 2.

1912. A study in chromosome reduction. Ibid. 48 : 601-627. pis. 3.
LERAT, P. 1905. Les ph6nomenes de maturation dans I'ovoge'nese et la spermato-

g6nese du Cyclops strenuits. La Cellule 22 : 163-198: pis. 4.
LEVINE, M. 1913. The cytology of Hymenomycetes, especially the Boleti. Bull.

Torr. Bot. Club 40: 137-181. pis. 4-8.
LEVY, F. 1914-1915. Studien zur Zeugungslehre. IV., Ueber die Chromatin-

verhaltnisse in der Spermatogenese von Rana esculenta. Arch. Mikr. Anat. 86':

85-177. pis. 3-5. figs. 15. (Bibliography.)

LEWIS, I. F. 1909. The life history of Griffithsia bornetiana. Ann. Bot. 23 : 639-689.
LEWIS, I. M. 1908. The behavior of the chromosomes in Pinus and Thuja. Ibid.

22 : 529-556. pis. 29, 30.
DE LITARDIERE, R. 1912. Formation des chromosomes h6terotypiques chez le

Polypodium vulgare L. Compt. Rend. Acad. Sci. Paris 155 : 1023-1026.
LOTSY, J. P. 1904. Die Wendung der Dyaden beim Reifung der Tiereier als

Stiitze fur die Bivalenz der Chromosomen nach der numerische Reduktion.

Flora 93: 65-86. figs. 19.
LUNDEGARDH, H. 1909. Ueber Reduktionsteilung in den Pollenmutterzellen

einiger dikotylen Pflanzen. Svensk. Bot. Tidskr. 3: 78-124. pis. 2, 3.
1914. Zur Kenntniss der heterotypischen Kernteilung. Arch. Zellf. 13: 145-157.

pi. 4.
MAIRE, R. 1905. La mitose he'te'rotypique et la signification des protochromosomes

chez les Basidiomyce'tes. Compt. Rend. Soc. Biol. Paris 571 : 726-728.
1911. La biologic des Uredinales. Prog. Rei. Bot. 4: 110-162.
MALONE, J. Y. 1918. Spermatogenesis of the dog. Trans. Am. Micr. Soc. 37 :

97-110. pis. 9, 10.
MARCUS, H. 1906. Ei und Samenreife bei Ascaris canis (Werner) (Ascaris mystax).

Arch. Mikr. Anat. 68: 441-490. pis. 29, 30. figs. 10.
1907. Ueber die Thymus. Verh. Anat. Gesell. Wiirzburg 21.

THE REDUCTION OF THE CHROMOSOMES 267

MARECHAL, J. 1904. Ueber die morphologische Entwicklung der Chromosomen im

Keimblaschen des Selachiereies. Anat. Anz. 25 : 383-398. figs. 25.
1905. Ueber die morphologische Entwicklung der Chromosomen im Teleostierei,

usw. Ibid. 26: 641-652. figs. 27.

1907. Sur 1'ovogenese des Selachiens. La Cellule 24: 1-239. pis. 10.
MARTINS MANO, T. 1909. La microsporogenese dans le Funkia ovata. Broteria,

ser. bot., 8.
MCALLISTER, F. 1913. On the cytology and embryology of Smilacina racemosa.

Trans. Wis. Acad. Sci. 17 » : 599-660. pis. 56-58.
McAvoY, B. 1912. The reduction divisions in Fuchsia. Ohio Nat. 13: 1-18.

pis. 1, 2.
McCLUNG, C. E. 1900. The spermatocyte divisions of the Acrididae. Kans.

Univ. Quarterly 9 : 73-100. pis. 15-17.
MELIN, E. 1915. Die Sporogenese von Sphagnum squarrosum Pers., nebst einigen

Bemerkungen tiber das Antheridium von Sphagnum acutifolium Ehrh. Svensk.

Bot. Tidskr. 9: 261-293.
METZ, C. W. 1914. Chromosome studies in the Diptera. I. A preliminary survey

of five different types of chromosome groups in the genus Drosophila. Jour.

Exp. Zool. 17 : 45-56. 1 pi.
1916a. Chromosome studies in the Diptera. II. The paired association of the

chromosomes in the Diptera, and its significance. Ibid. 21 : 213-280. pis. 1-8.
19166. Chromosome studies in the Diptera. 111. Additional types of chromo-
some groups in the Drosophilidae. Am. Nat. 50: 587-599. pi. 1. fig. 1.
MEVES, FR. 1896. Ueber die Entwicklung der mannlichen Gesechlechtszellen von

Salamandra maculosa. Arch. Mikr. Anat. 48 : 1-83. pis. 1-5.

1907. Die Spermatocytenteilungen bei der Honigbiene (Apis mellifica) nebst
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1908. Es gibt keine parallele Konjugation der Chromosomen. Arch. Zellf. 1:
612-619. 1 fig.

1911. Chromosomenlangen bei Salamandra, nebst Bemerkungen zur Individual-
itatstheorie der Chromosomen. Arch. Mikr. Anat. 77: II, 273-300. pis. 11, 12.
MINCHIN, E. A. 1912. An Introduction to the Study of the Protozoa. London.
MIYAKE, K. 1905. Ueber Reduktionsteilung in den Pollenmutterzellen einigen

Monokotylen. Jahrb. Wiss. Bot. 42 : 83-120. pis. 3-5.
MONTGOMERY, T. H. 1901a. A study of the germ cells of Metazoa. Trans. Am.

Phil. Soc. 20: 154-236. pis. 4-8.

19016. Further studies on the chromosomes of the Hemiptera heteroptera. Proc.
Acad. Nat. Sci. Phila. 53: 261-271. pi. 10.

1903. The heterotype maturation mitoses in Amphibia and its general significance.
Biol. Bull. 4: 259-269. figs. 8.

1904. Some observations and considerations upon the maturation phenomena
of the germ cells. Ibid. 6 : 137-158. pis. 3.

1905. The spermatogenesis of Syrbula and Lycosa with general considerations
upon chromosome reduction and the heterochromosomes. Proc. Acad. Nat.
Sci. Phila. 57 : 162-205. pis. 9, 10.

1906. Chromosomes in the spermatogenesis of the Hemiptera heteroptera.
Trans. Am. Phil. Soc. 21 ; 97-174. pis. 9-13.

1908. On the morphological difference of the chromosomes in Ascaris megalo-
cephala. Arch. Zellf. 2 : 66-75. pis. 6, 7.

1910. On the dimegalous sperm and chromosomal variation in Euschistus, with
reference to chromosomal continuity. Ibid. 5 : 121-145. pis. 9, 10. 1 fig.

1911. The spermatogenesis of an Hemipteran, Euschislus. Jour. Morph. 22;
731-798. pis. 1-5.

268 INTRODUCTION TO CYTOLOGY

MOORE, A. C. 1905. Sporogenesis in Pallavicinia. Bot. Gaz. 40: 81-96. pis.

3, 4.
MOORE, J. E. S. 1896. On the structural changes in the reproductive cells during

the spermatogenesis of the Elasmobranchs. Quar. Jour. Micr. Sci. 38 : 275-314.

pis. 13-16.
MOORE, J. E. S. and EMBLETON, A. L. 1906. On the synapsis in Amphibia. Proc.

Roy. Soc. London 77 : 555-562. pis. 20-23.
MOTTIER, D. M. 1897. Beitrage zur Kenntniss der Kerntheilung in den Pollen-

mutterzellen einiger Dikotylen und Monokotylen. Jahrb. Wiss. Bot. 30:

169-204. pis. 3-5.
1903. The behavior of the chromosomes in the spore mother-cells of higher plants

and the homology of the pollen and embryo-sac mother-cells. Bot. Gaz. 36 :

250-292. pis. 11-14.

1905. The development of the heterotypic chromosomes in pollen mother-cells.
Bot. Gaz. 40: 171-177.

1907. The development of the heterotypic chromosomes in pollen mother-cells.

Ann. Bot. 21 : 309-347. pis. 27, 28.
1909. Prophases of heterotypic mitosis in the embryo-sac mother-cells of LUium.

Ibid. 23: 343-352. pi. 23.
1914. Mitosis in the pollen mother-cells of Acer negundo, L., and Staphylea

trifolia, L. Ibid. 28: 115-133. pis. 9, 10.
MOTTIER, D. M. and NOTHNAGEL, M. 1913. The development and behavior of

the chromosomes in the first or heterotypic mitosis of the pollen mother-cells

of Allium cernuum Roth. Bull. Torr. Bot. Club 40: 555-565. pis. 23, 24.
MULLER, H. A. CL. 1909. Ueber karyokinetische Bilder in den Wurzelspitzen von

Yucca. Jahrb. Wiss. Bot. 47: 99-117. pis. 1-3.

1911. Kernstudien an Pflanzen. I u. II. Arch. Zellf. 8: 1-51. pis. 1, 2.
NAKAHARA, W. 1919. A study of the chromosomes in the spermatogenesis of

the stonefly, Perla immarginata Say, with special reference to the question of

synapsis. Jour. Morph. 32 : 509-530. pis. 3.
1920. Side-to-side versus end-to-end conjugation of chromosomes in relation to

crossing over. Science 62 : 82-84.
NEMEC, B. 1903. Ueber die Einwirkung des Chloralhydrats auf die Kern- und

Zell-teilung. Jahrb. Wiss. Bot. 39: 645-730. figs. 157.
NICHOLS, G. E. 1908. The development of the pollen of Sarracenia. Bot. Gaz.

46 : 31-37. pi. 5.
NOTHNAGEL, M. 1916. Reduction divisions in the pollen mother-cells of AUium

tricoccum. Bot. Gaz. 61: 453-476. pis. 28-30. fig. 1.
OEHLKERS, F. 1916. Beitrag zur Kenntniss der Kernteilung bei den Characeen.

Ber. Deu. Bot. Ges. 34: 223-227.
OLIVE, E. W. 1907. Cytological studies on Ceratiomyxa. Trans. Wis. Acad. Sci.

15 : 754-773. pi. 47.
OSTERHOTJT, W J. V. 1897. Ueber Entstehung der karyokinetischen Spindel bei

Equisetum. Jahrb. Wiss. Bot. 30: 159-168. pis. 1, 2.
OVERTON, E. 1893. On the reduction of the chromosomes in the nuclei of plants.

Ann. Bot. 7 : 139-143.
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Dikotylen. Jahrb. Wiss. Bot. 42 : 121-153. pis. 6, 7.

1906. The morphology of the ascocarp and spore formation in the many-spored
asci of Theocalheus pelletieri. Bot. Gaz. 42: 450-492. pis. 29, 30.

1909. On the organization of the nuclei in the pollen mother cells of certain
plants, with special reference to the permanence of the chromosomes. Ann.
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THE REDUCTION OF THE CHROMOSOMES 269

PAULMIER, F. C. 1898. Chromosome reduction in the Hemiptera. Anat. Anz. 14 :

514-520. figs. 19.
1899. The sperm atogenesis of Anasa tristis. Jour. Morph. 15: Suppl. 223-272.

pis. 13, 14.
PICARD, M. 1913. A bibliography of works on meiosis and somatic mitosis in the

angiosperms. Bull. Torr. Bot. Club 40: 575-590.

POPOFF, M. 1908. Experimentelle Zellstudien. Arch. Zellf. 1 : 245-380. figs. 18.
PRATT, B. H. and LONG, J. A. 1917. The period of synapsis in the egg of the white

rat, Mus norvegicus albinus. Jour. Morph. 29 : 441-458. pi. 1. figs. 2.
VOM RATH, O. 1892. Zur Kenntniss der Spermatogenese von Gryllotalpa vulgaris

Latr. Arch. Mikr. Anat. 40: 102-132. pi. 5.
1893. Beitrage zur Kenntniss der Spermatogenese von Salamandra maculosa.

Zeitschr. Wiss. Zool. 57 : 97-185. pis. 7-9.
1895. Neue Beitrage zur Frage der Chromatinreduktion in der Samen- und

Eireife, Arch. Mikr. Anat. 46 : 168-238. pis. 6-8.
ROBERTSON, W. R. B. 1916. Chromosome studies. I. Jour. Morph. 27: 179-332.

pis. 1-26.
ROSENBERG, O. 1907. Zur Kenntniss der praesynaptischen Entwicklungsphasen

der Reduktionsteilung. Svensk. Bot. Tidskr. 1.
1909o. Zur Kenntniss von den Tetradenteilungen der Compositen. Ibid. 3:

64-77. pi. 1.
19096. Cytologische und morphologische Studien an Drosera longifolia X rotundi-

folia. Kgl. Svensk. Vet. Handl. 43 : 3-64. pis. 4. figs. 33.
1918. Chromosomenzahlen und Chromosomendimensionen in der Gattung

Crepis. Ark. f. Bot. 15: 1-16. figs. 6.

Roux, W. 1883. Ueber die Bedeutung der Kerntheilungsfiguren. Leipzig.
RUCKERT, J. 1892. Zur Entwicklungsgeschichte des Ovarialeies bei Selachiern.
Anat. Anz. 7 : 107-158. figs. 6.

1893. Die Chromatinreduktion bei der Reifung der Sexualzellen. Ergeb. d. Anat.
u. Entw. 3: 517-583. (Review.)

1894. Zur Eireif ung der Copepoden. Ibid., Anat. Hefte. 4: 261-352. pis. 21-25.
SABASCHNIKOFF, M. 1897. Beitrage zur Kenntnis der Chromatinreduktion in der

Ovogenesis von Ascaris. Bull. Soc. Nat. Moscow 1.
SAKAMURA, T. 1914. Ueber die Kernteilung bei Vicia cracca. Bot. Mag. Tokyo 28 :

131-147. pi. 2.
1915. Ueber die Einschniirung der Chromosomen bei Vicia Fabalj. Ibid. 29: 287-

300. pi. 13. figs. 12.
1920. Experimentelle Studien liber die Zell- und Kernteilung mit besonderer

Riicksicht auf Form, Grosse und Zahl der Chromosomen. Jour. Coll. Sci. Imp.

Univ. Tokyo 39 : pp. 221. pis. 7.
SCHAFFNER, J. H. 1909. The reduction division in the microsporocytes of Agave

virginica. Bot. Gaz. 47: 198-214. pis. 12-14.
SCHLEIP, W. 1906. Die Entwicklung der Chromosomen im Ei von Planaria gono-

cephala Dug. Zool. Jahrb. (Anat. Abt.) 23: 357-380. pis. 23, 24.
1907. Die Samenreifung bei Planarien. Ibid. 21: 129-174. pis. 14, 15.
SCHELLENBERG, A. 1911. Ovogenese, Eireifung und Befruchtung von Fasciola

hepatica,!,. Arch. Zellf. 6: 443-484. pis. 24-26.
SCHNEIDER, H. 1914. Ueber die Prophasen der ersten Reifeteilung in Pollenmutter-

zellen, inbesondere bei Thelygonium Cynocrambe L. Arch. Zellf. 12 : 359-371. pi.

28.
SCHREINER, A. u. K. E. 1904. Die Reifungsteilungen bei den Wirbeltieren. Anat

Anz. 24: 561-578. figs. 24.

270 INTRODUCTION TO CYTOLOGY

1905. Ueber die Entwicklung der mannlichen Geschlechtszellen von Myxine glu-
tinosa (L.). Arch. d. Biol. 21: 183-355. pis. 5-14.

1906. Neue Studien liber die Chromatinreifung der Geschlechtszellen. Arch. d.
Biol. 22: 1-70, pis. 1-3; 419-492, pis. 23-26; Anat. Anz. 29: 465-479. figs. 17.

SHARP, L. W. 1913. Somatic chromosomes in Vicia. La Cellule 29: 297-331.

pis. 2.

1914. Maturation in Vicia. (Prelim. Note). Bot. Gaz. 57: 531.
1920a. Mitosis in Osmunda. (Review). Ibid. 69: 88-91.
1920&. Somatic chromosomes in Tradescanlia. Am. Jour. Bot. 7: 341-354. pis.

22, 23.
STEVENS, N. M. 1903. On the ovogenesis and spermatogenesis of Sagitta bipunctata.

Zool. Jahrb. 18: 227-240. pis. 20, 21.
1908. A study of the germ cells of certain Diptera, with reference to the hetero-

chromosomes and the phenomena of synapsis. Jour. Exp. Zool. 5: 359-374.

pis. 4.
1911. Further studies on the heterochromosomes of the mosquitoes. Biol. Bull.

20 : 109-120. figs. 38.
STOMPS, T. J. 1910. Kerndeeling en synapsis bij Spinacia oleracea. pp. 162. pis.

2. See Biol. Centr. 31 : 257-320. pis. 1-3. 1911.
STRASBURGER, E. 1888. Ueber Kern- und Zellteilung im Pflanzenreich, nebst

einem Anhang liber Befruchtung. Hist. Beitr. 1. pp. 258. pis. 3.
1894. The periodic reduction of chromosomes in living organisms. Ann. Bot. 8:

281-316.
1897. Kerntheilung und Befruchtung bei Fucus. Jahrb. Wiss. Bot. 30: 351-374.

pis. 27, 28.
1900. Ueber Reduktionsteilung, Spindelbildung, Centrosomen und Cilienbildner

im Pflanzenreich. Histol. Beitr. 6. pp. 224. pis. 4.
1904a. Anlage des Embryosacks und Prothalliumbildung bei der Eibe nebst

anschliessenden Erorterungen. Festschr. f. Haeckel. Jena.
19046. Ueber Reduktionsteilung. Sitzber. Berlin. Acad. Wiss., phys.-math.Kl.

18:587-614. figs. 9.
1905. Typische und allotypische Kernteilung. Jahrb. Wiss. Bot. 42: 1-71. pi. 1.

1907. Ueber die Individ ualitat der Chromosomen und die Propfhybriden-Frage.
Ibid. 44:472-555. pis. 5-7.

1908. Chromosomenzahlen, Plasmastrukturen, Vererbungstrager und Reduktions-
teilung. Ibid. 46 : 479-568. pis. 1-3.

1910. Ueber geschlechtsbestimmende Ursachen. Ibid. 47: 427-520. pis. 9, 10.

1911. Kernteilungsbilder bei der Erbse. Flora 102: 1-23. pi. 1.

SUTTON, W. S. 1902. On the morphology of the chromosome group in Brachystola

magna. Biol. Bull. 4: 24-39. figs. 11.
SVEDEUTJS, N. 1914a. Ueber die Tetradenteilung in den vielkernigen Tetraspor-

angiumanlagen bei NitophyUum punctamm. Ber. Deu. Bot. Ges. 32: 48-57.

pi. 1. 1 fig.
1914f>. Ueber Sporen an Geschlechtspflanzen von NitophyUum punctatum, usw.

Ibid. 32: 106-116. pi. 2. 1 fig.
SYKES, M. G. 1908. Nuclear division in Funkia. Arch. Zellf. 1: 381-398. pis.

8, 9. 1 fig.
TACKHOLM, G. 1914. Zar Kenntniss der Embryosackentwicklung von Lopezia

coronala Andr. Svensk. Bot. Tidskr. 8.
TAHARA, M. 1910. Ueber die Kernteilung bei Morus. Bot. Mag. Tokyo 24:

281-289. pi. 9.
TAYLOR, M. 1914, 1917. The chromosome complex of Culex pipiens. Quar. Jour.

Micr. Sci. 60: 377-398. pis. 27, 28; 62: 287-302. pi. 20.

THE REDUCTION OF THE CHROMOSOMES 271

TISCHLBR, G. 1906. Ueber die Entwicklung des Pollens und der Tapetenzellen bei

flibes-Hybriden. Jahrb. Wiss. Bot. 42 : 545-578. pi. 15.
1916. Chromosomenzahl, -Form und -Individualitat in Pflanzenreich. Prog. Rei.

Bot. 6: 164-284. (Bibliography).
TRETJAKOFF, D. 1904. Die Spermatogenese bei Ascaris megalocephala. Arch.

Mikr. Anat. 65; 383-438. pis. 22-24. 1 fig.
TRONDLE, A. 1911. Ueber die Reduktionsteilung in den Zygoten von Spirogyra

und iiber die Bedeutung der Synapsis. Zeitschr. f. Bot. 3 : 593-619. ' 1 pi. 20

figs. 4
TSCHERNOYAROW, M. 1914. Ueber die Chromosomenzahl und besonders beschaf-

fene. Chromosomen im Zellkerne von Najas major. Ber. Deu. Bot. Ges. 32 :

411-416. pi. 10.
VANDENDRIES, R. 1913. Le nombre des Chromosomes dans la spermatoge'nese des

Polytrichum. La Cellule 28 : 257-261. figs. 11.
VAN LEEUWEN-REIJNVAAN, D. 1907. Ueber eine zweifache Reduktion bei einigen

Polytrichum-Arten.. Rec. Trav. Bot. Neerl. 4.
1908. Ueber die Spermatogenese der Moose. Ber. Deu. Bot. Ges. 26a: 301-308.

pi. 5.
VEJDOWSKY F. 1907. Neue Untersuchungen iiber Reifung und Befruchtung.

Kgl. Bohm. Ges. Wiss. Prag.

1912. Zum Problem der Vererbungstrager. pp. 184. pis. 12. Prag. 1911-2.
VON Voss, H. 1914. Cytologische Studien an Mesostoma Ehrenbergi. Arch. Zellf.

12:159-194. pis. 12-14. figs. 5.

DEVRIES, H. 1889. Intracellulare Pangenesis. Jena.
WALKER, N. 1913. On abnormal cell-fusion in the archegonium; and on spermato-

genesis in Polytrichum. Ann. Bot. 27: 115-132. pis. 13, 14.
WALTON, A. C. 1918. The oogenesis and early embryology of Ascaris cams Werner.

Jour. Morph. 30: 527-604. pis. 9. 1 fig.
WEINZIEHER, S. 1914. Beitrage zur Entwicklungsgeschichte von Xyris indica L.

Flora 106 : 393-432. pis. 6, 7. figs. 10.
WEISMANN, A. 1887-1902. Ueber die Zahl der Richtungskorper und liber ihre

Bedeutung fur die Vererbung. Jena 1887. Essays upon Heredity. Oxford

1891-2. Das Keimplasma. 1892. (Engl. 1893.) The Evolution Theory. 1902.
WENRICH, D. H. 1915. Synapsis and the individuality of the chromosomes.

Science 41 : 440.

1916. The spermatogenesis of Phrynotetlix magnus, with special reference to syn-
apsis and the individuality of the chromosomes. Bull. Mus. Comp. Zool.
Harvard Coll. 60: 55-136. 10 pis.

1917. Synapsis and chromosome organization in Chorthippus (Stenobothrus)
curtipennis and Trimerotropis suffusa (Orthoptera). Jour. Morph. 29: 471-516.
pis. 1-3.

WILLIAMS, J. L. 1904. Studies in the Die tyotacese. II. The cytology of the game-

tophyte generation. Ann. Bot. 18 : 183-204. pis. 12-14.
WILSON, E. B. 1900. The Cell in Development and Inheritance. 2d ed.
VON WINIWARTER, H. 1900. Recherches sur I'ovog6nese et I'organoge'nese de

1'ovaire des mammiferes (Lapin et Homme). Arch. d. Biol. 17: 33-199. pis.

3G
— O.

VON WINIWARTER, H. et SAINMONT. 1909. Nouvelles recherches sur I'ovogSnese et
1'organogenese de 1'ovaire des Mammiferes (Chat). Chapter IV. Ibid. 24:
1-142, 165-276, 373-432, 627-652. pis. 11.

WODSEDALEK, J. E. 1916. Causes of sterility in the mule. Biol. Bull. 30: 1-56.

WOLFE, J. J. 1904. Cytological studies on Nemalion. Ann. Bot. 18: 607-630.
pis. 40, 41. 1 fig.

272 INTRODUCTION TO CYTOLOGY

1918. Alternation rnd parthenogenesis in Padina. Jour. Elisha Mitchell Sci. Soc.

34:78-109.
WOOLERY, R. 1915. Meiotic divisions in the microspore mother-cells of Smilacina

racemosa (L) Desf. Ann. Bot. 29 : 471-482. pi. 22. 1 fig.
YAMANOUCHI S. 1906. The life history of Polysiphonia violacea. Bot. Gaz. 42:

401-449. pis. 19-28.

1908. Sporogenesis in Nephrodium. Ibid. 46 : 1-30. pis. 1-4.

1909. Mitosis in Fucus. Ibid. 47: 173-197. pis. 8-11.

1910. Chromosomes in Osmunda. Ibid. 49 : 1-12. pi. 1.

1912. The life history of Cutleria. Ibid. 54 : 441-502. pis. 26-35. figs. 15.
ZWEIGER. 1907. Die Sperm atogenese von Forficula auricularia. Jen Zeitschr. 42 :
143-172. pis. 11-14.

CHAPTER XII
FERTILIZATION

We have already pointed out that reduction and fertilization con-
stitute the two principal cytological crises in the life cycles of all organisms
reproducing sexually. Although the first of these processes was not dis-
covered until 1883, some of the grosser features of fertilization had
been made out many years previously (Chapter I). But the central
feature of this process — the union of the two parental nuclei — was not
known until 1875, when O. Hertwig discovered it in animals, Strasburger's
parallel discoveries in plants following in 1877 (Spirogyra) and 1884
(angiosperms). As the finer details of fertilization and the significance
of its results become better understood, the aptness of Huxley's (1878)
often quoted simile, in which he compares the organism to "a web of
which the warp is derived from the female and the woof from the male,"
becomes increasingly striking.

We shall first describe the morphology of the fertilization process
as it is typically shown in many animals, after which attention will be
given to some of its physiological aspects. The second half of the chapter
will be devoted to a review of fertilization in the various groups of the
plant kingdom.

FERTILIZATION IN ANIMALS1

The Gametes. — The spermatozoa of different animals exhibit a
surprising variety of form and structure (Fig. 103). What may be
referred to as the "typical" spermatozoon consists of three fairly distinct
parts: head, middle piece, and tail or flagellum (Fig. 104). The head
represents the nuclear portion of the sperm cell : it consists almost wholly
of an extremely compact mass of chromatin. It has an envelope of
cytoplasm which in few forms is very conspicuous and in many cases is
scarcely distinguishable. Anterior to the nucleus there may be an
acrosome, and the end of the head often has the form of a sharp point,
the perferatorium. Posterior to the head is the middle piece; this is
made up of cytoplasm in which are located the centrosomal structures,
together with chondriosomes and other inclusions, such as the "Golgi
bodies." The flagellum, or tail, consists of a slender axial filament,

1 In the preparation of this portion of the chapter the author has drawn very freely
upon Professor F. R. Lillie's admirable and concise presentation of the subject,
Problems of Fertilization (1919).

18 273

274

INTRODUCTION TO CYTOLOGY

which grows out from the centrosome in the middle piece or in some
cases apparently from the base of the nucleus, and a cytoplasmic sheath
which usually extends not quite to its end. The sheath sometimes has
the form of an undulating membrane. The spermatozoa of crustaceans
and nematodes are non-flagellate, and in other groups various departures
from the "typical" form and structure are known. A few of the many
known types are shown in Fig, 103.

FIG. 103. — Various types of spermatozoa.

A, Triton (salamander). (After Ballowitz.) B, Nereis (annelid). (After Lillie, 1912.)
C, guinea pig. (After Meves.) D, Phyllopneuste (bird). (After Ballowitz.) E, sturgeon
(After Ballowitz.) F, Vesperugo (bat). (After Ballowitz.) G, Castrada hofmanni (turbel-
larian). (After Luther.) H, Pinnotheres veterum (crustacean) . (After Koltzoff.) I,Homa-
rus (lobster). (After Herrick). J, Ascaris (nematode) ; a, apical body; n, nucleus; r, "re-
fractive body." (After Scheben.)

The ovum undergoes nearly or quite all of its elaborate differentiation
before the maturation divisions occur. Certain cells in the ovary gradu-
ally become greatly enlarged (Fig. 105), and during this "growth period"
the cytoplasm may not only differentiate into visibly distinct regions
but may also become stored with energy-containing materials ("food"),
which in the case of some animals, such as birds, is present in relatively
enormous amounts. The "ovarian egg" or primary oocyte, as the egg
cell is called before the maturation mitoses take place, may have a definite
limiting membrane at its surface, but in many forms this cannot be
demonstrated. The nucleus of the primary oocyte is known as the

FERTILIZATION

275

germinal vesicle: it is very large and contains in addition to its chromatin
a considerable amount of material which appears to take no part in the
formation of the chromosomes when division ensues. After the cyto-
plasmic differentiation is complete and the oocyte has reached its full
size — even after the spermatozoon has entered in many
cases — the oocyte nucleus undergoes two divisions in rapid
succession at the periphery of the egg, which at this point
buds off two small nucleated cells, the polar bodies (Fig. 106).
The first polar body may or may not divide again. The
details of chromosome behavior in these two mitoses have been
described in the preceding chapter. The reduced or haploid
number of chromosomes left in the egg organize the egg
nucleus ("female pronucleus"), rendering the egg ready for
the sexual fusion.

FIG. 104. FIG. 105.

FIG. 104. — ^Diagram of typical flagellate spermatozoon.

P, perferatorium; A, acrosome; N, nucleus; M, middle piece; F, axial filament; S,
cytoplasmic sheath; E, end piece. (After Wilson.)

FIG. 105. — The differentiation of the oocyte in Hydra.

A, very young oocyte lying between ectodermal cells at right. B, oocyte after growth
period, with yolk globules, X 500. (After Downing, 1909.)

The time relation of the maturation of the egg and the entrance of the
spermatozoon varies considerably in different animals. In echinoderms
and some other forms maturation is completed before the spermatozoon
penetrates. In some other animals it proceeds as far as the metaphase
of the heterotypic mitosis (Chcetopterus, Cerebratulus) or the prophase of
the homceotypic mitosis (many vertebrates), but does not go further
unless penetration occurs. In the marine annelid, Nereis, finally, the

276

INTRODUCTION TO CYTOLOGY

germinal vesicle undergoes no change unless the spermatozoon has
entered the egg.

FIG. 106. — Maturation and fertilization in Ascaris.

A, spermatozoon about to enter egg. B, spermatozoon inside; first maturation mitosis
in progress. C, first maturation mitosis completed; first polar body budded off. D,
second maturation mitosis, forming second polar body; sperm nucleus below. E, male and
female pronuclei, each with 2 chromosomes, meeting. F, first cleavage mitosis, showing 2
paternal and 2 maternal chromosomes. (After Hertwig.)

FIG. 107. — Fertilization in Physa (snail.) Sperm head and amphiaster at right, with
long flagellum extending toward left. Second maturation mitosis in progress. (After
Kostanecki and Wierzyski, 1896.)

The Fusion of the Gametes. — In most cases the whole spermatozoon
enters the egg (Fig. 107). In some sea urchins only the head and middle
piece enter, while in Nereis the head alone passes in, the middle piece

FERTILIZATION 211

and tail being left on the egg surface. The process in Nereis as de-
scribed by Lillie (1912, 1919) is as follows. The egg of this worm has a
tough vitelline membrane, an alveolar cortical layer, many yolk and oil
droplets, and a large central germinal vesicle (nucleus). If many
spermatozoa are present in the vicinity a large number attach themselves
to the egg, but usually all but one are carried away by an outflow of jelly
from the alveolae of the cortical layer. This layer now takes the form of
a zone traversed by radial protoplasmic plates representing the walls of
the alveola?. A transparent "fertilization cone" extends from the inner
part of the egg across this zone and touches the membrane at the point
where the spermatozoon is beginning to penetrate. The perferatorium
pierces the egg membrane and becomes attached to the transparent cone.
The latter is now withdrawn, carrying the head of the spermatozoon
into the egg with it. Thus it appears that the initiative for the final act
of penetration lies with the egg rather than with the spermatozoon.
Since only the head enters the egg in Nereis it seems clear that the only
necessary portion of the spermatozoon in the actual union is the nucleus :
the middle piece and tail are accessory and function only as locomotor
organs.

The immediate visible effects of the entrance of the sperm are seen
chiefly in changes in the appearance of the cortical region of the egg. If
a vitelline membrane is present, as in vertebrates, a " perivitelline
space" usually appears between the membrane and the egg; and this
space may in some cases (frog) be great enough to permit the rotation
of the egg within the membrane. In the sea urchin a fertilization mem-
brane is formed as the result of fertilization : it first appears at the point
where the spermatozoon is attached and spreads over the egg with great
rapidity. It seems probable that a delicate membrane already present is
raised and thus made more conspicuous. In Ascaris, which is parasitic
in the intestine of the horse, this membrane becomes very thick and later
acts as a protection against the digestive juices of the host. These
cortical changes do not depend upon the actual entrance of the sperma-
tozoon into the egg: in Nereis they occur before the slow penetration
can be completed, or even if the spermatozoon is shaken loose shortly
after penetration has begun.

In describing the remarkable transformation undergone by the
spermatozoon within the egg the behavior of its different organs will for
the sake of clearness be considered separately.

The Nucleus. — Immediately after gaining entrance to the egg (Fig.
108) the sperm head begins to enlarge and assumes the usual form and
structure of a nucleus. Meanwhile it advances toward the egg nucleus.
As Lillie points out, both male and female pronuclei pass toward a posi-
tion of equilibrium in a cell preparing to divide and consequently meet :
the assumption of an attractive force between them is unnecessary. By

278

INTRODUCTION TO CYTOLOGY

the time they meet the male pronucleus has usually, but not always,
become equal in size and appearance to the female pronucleus. The
union of the two pronuclei to form a fusion nucleus, or synkaryon, usually

FIG. 108. — Diagram of fertilization and cleavage in an animal. It is assumed that
in this case the egg has undergone maturation before the penetration of the
spermatozoon.

FIG. 109. — -Independence of parental chromatin contributions in the cleavage

of -the egg of Cryptobranchus.

A, first cleavage mitosis. B, C, prophase and metaphase of fourth cleavage mitosis
(After Smith, 1919.)

occurs at once after they meet. In a great many cases there may be no
actual fusion of the pronuclei at all: as they come close to one another
each passes through the prophase stages and gives rise independently to

FERTILIZATION 279

its group of chromosomes, the two groups arranging themselves on a
common spindle which organizes when the nuclear membranes dissolve.
The first cleavage mitosis (first embryonal division) then ensues, and
the two daughter nuclei receive longitudinal halves of each and every
chromosome. Thus in the act of fertilization, in both animals and plants,
each parent furnishes the offspring with a haploid set of chromosomes,
the two intermingled sets constituting the diploid set of the new individual.
Since every chromosome divides equationally at every subsequent somatic
mitosis, every cell of the body receives half of its chromosome complement
from each parent. The cardinal importance of this fact in connection
with current theories of heredity will be apparent in subsequent chapters.

The two groups of chromoosmes, paternal and maternal, can often be
distinguished not only on the spindle of the first cleavage division, but
in several divisions thereafter. As examples may be cited Cyclops
(Riickert 1895; Hsecker 1895), Crepidula (Conklin 1901), and Crypto-
branchus (Smith 1919) (Fig. 109). This phenomenon is especially evident
in hybrids (p. 160). There is much reason to believe that the chro-
matins of the two parents, although intermingled in the nuclei of the
offspring, never actually fuse, unless it is at the time of synapsis in the
next maturation; and it has already been pointed out (Chapter XI) that
they may not fuse even then. This fact also has an important bearing on
the chromosome theory of heredity.

The Centrosome.— (See Wilson 1900, pp. 208 ff.) Shortly after the
entrance of the spermatozoon into the egg (Figs. 106-108) an aster devel-
ops at the base of the sperm head, and in the aster a centrosome appears.
Since the centrosome thus arises in the position of the middle piece, and
since the centrosome of the spermatid is included in the middle piece dur-
ing spermatogenesis, a widely accepted theory has been that the newly
appearing centrosome is in reality that of the spermatid. Whatever its
origin, it soon divides to form the two which function in the first cleavage
mitosis. These facts had much to do with the formulation of a theory of
fertilization set forth by Boveri (1887, 1891), who was much impressed by
the conspicuous part played by the centrosomes in cell-division. Accord-
ing to Boveri's theory the egg is not able to undergo division because of
the lack of any centrosome to initiate the process, while the spermatozoon
has a centrosome but not sufficient cytoplasm in which to act. Through
the union of the gametes all the organs necessary for division are brought
together and cleavage proceeds. This theory has recently been recalled
by Walton (1918) in his work on Ascaris canis.

Another early view of the origin of the cleavage centrosomes was that
of van Beneden (1887) and Wheeler (1895, 1897), who believed them to be
the centrosomes of the egg cell.

The theory that the cleavage centrosomes arise from both egg and
spermatozoon is of some historic interest. It was suggested by Rabl

280 INTRODUCTION TO CYTOLOGY

(1889) that if the centrosome is a permanent cell organ the conjugation of
the gametes must involve not only a union of nuclei but also a union of
centrosomes (Wilson, p. 210). Fol (1891), in his work on echinoderm
eggs, thought that he observed just such a process, which he termed "The
Quadrille of the Centers." The egg centrosome and the centrosome
brought in by the spermatozoon were both supposed to divide, the prod-
ucts then fusing in pairs to form the two cleavage centrosomes. A simi-
lar thing was reported by certain other investigators, but none of the cases
stood the test of later work. Another theory now abandoned was that
advanced by Carnoy and Lebrun (1897), who also attempted to derive one
centrosome from each gamete. The cleavage centrosomes were thought
to arise de novo and separately, one inside each pronucleus, to migrate
thence into the cytoplasm.

Much less confidence is now placed in the persistence of the spermatid
or egg centrosomes through the fertilization stages. Since the middle
piece, which is thought to contain the centrosome, does not enter the egg
at all in Nereis, it seems probable that the male nucleus in some way
induces the formation of asters and centrosomes by the egg cytoplasm.
Lillie found that even a portion of the sperm head will bring about this
effect. In Unio (Lillie 1897, 1898) and Crepidula (Conklin 1897) it seems
not unlikely that each pronucleus causes the formation of one cleavage
centrosome. In the sea urchin Wilson (1901) concluded that the cleav-
age centrosomes in all probability arise by the division of one which orig-
inates de novo at the nuclear membrane. In almost every case there are
gaps in the known history of the centrosome in fertilization, and it seems
very doubtful whether the cleavage centrosomes are continuous with
those of either gamete. This conclusion is supported by the fact that the
formation of asters with centrosomes in the egg cytoplasm can be arti-
ficially induced by treating the eggs with certain chemicals, such as weak
MgCl2. It is possible that the spermatozoon carries a substance which
brings about centrosome formation in a similar way. However this may
be, the importance of the centrosome undoubtedly lies in its relation to
cleavage rather than to fertilization.

Cytoplasm and Chondriosomes. — In some cases (Nereis} no cytoplasm
can be shown to enter the egg with the spermatozoon, whereas in others
(A scam) a relatively large amount is brought in. Its great indefiniteness
in behavior makes it seem probable that it has no special significance in
the fertilization process.

The importance of the chondriosomes in fertilization has been empha-
sized by Meves (1911, 1915), who finds that many of these bodies are
present in the large cytoplasmic mass accompanying the sperm nucleus
in Ascaris, and that they mingle with the chondriosomes of the egg.
Meves (1908, 1915, 1918), together with other writers, accordingly thinks
that they are concerned in the transmission of certain hereditary char-

FERTILIZATION

281

acters. Several observations, however, fail to harmonize with this view.
In some echinoderms the middle piece, which contains the chondriosomal
material, is not distributed to the daughter cells during the cleavage
divisions, but remains in one of them. It is unlikely that hereditary
material would behave in this manner. Furthermore, in the worm,
Nereis, the middle piece does not even enter the egg. In Peripatus
(Montgomery 1912) the chondriosomal mass is thrown out of the
spermatid. Wildman (1913) points out that in Ascaris also the chon-
driosomes may be largely lost during spermatogenesis. Although their
almost constant presence in the spermatozoon indicates that they have
a special physiological role comparable to that in other cells, there is as
yet no adequate reason to regard them as important in the transmission
of the factors of inheritance.

FIG. 110. — Chromidiogamy in Arcella. (From Minchin, after Swarczewsky.)
Somewhat modified.

Fertilization in Protozoa. — Among the protozoa there are found
several modes of sexual union very different from that described above for
the higher animals. Four illustrative cases may be cited (see Minchin
1912).

In Arcella, a rhizopod with a hemispherical shell, the protoplast has
two "primary nuclei" and many scattered chromidia. Two individuals
come together with their shell openings opposite one another (Fig. 110),
and the protoplasm of one flows almost entirely into the other shell,
where it mingles with that of the other individual. The primary nuclei
degenerate, while the chromidia become divided up into fine granules.
The protoplasm with the fine chromidia now becomes evenly distributed
in the two shells, after which the latter separate. In each individual the
chromidia now form "secondary nuclei;" around these are organized
uninucleate amcebulse which escape from the shell and give rise to new
Arcella individuals. This process, in which the chromatin chiefly con-

282

INTRODUCTION TO CYTOLOGY

cerned is that of the chromidia and not that of the larger nuclei, is quite
rare and is known as chromidiogamy. Arcella also has the other form of
sexual union, karyogamy.

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^ /" t \ 4i>55fcJ^t \ TJ

x _/ i r/ i \ jf

FIG. 111. — Copulation \n-Actinophrys sol. X 850. (From Minchin, after

Schaudinn.)

In Actinophrys sol (Schaudinn 1896) (Fig. Ill) two individuals, each
-vith a single nucleus, approach each other and become enclosed in a
common cyst. In each of them the nucleus now undergoes two pre-
liminary mitotic divisions, at each of which a small "reduction nucleus"
is expelled from the body in a manner very similar to the expulsion of
chromatin into the polar bodies of higher animals. The two individuals,

FIG. 112. — Autogamy in Amoeba albida. (From Minchin, after Ndgler.)

or gametes, as they may now be called, fuse completely, their nuclei
uniting to form a synkaryon. Soon the synkaryon divides mitotically,
this being followed by the division of the cell to form two individuals
which escape from the cyst and resume the vegetative state.

In Amoeba albida (Nagler 1909) (Fig. 112) a peculiar process known as
autogamy occurs while the organism is in the encysted state. The single

FERTILIZATION

283

nucleus divides, forming a large vegetative nucleus and a smaller gener-
ative nucleus. The former moves to the surface of the cell and degener-
ates, while the latter gives rise to the gamete nuclei in the following
manner. After becoming elongated and incompletely divided it buds
off four "reduction nuclei" — two from one end and then two from the

FIG. 113. Conjugation in Paramcecium. (After Morgan, 1913.)

other. It then completes its division to form the two gamete nuclei,
or pronuclei, while the four reduction nuclei degenerate. After lying
apart for some time the two pronuclei approach each other and fuse ;
sexual union thus takes place between sister nuclei.

The complicated nuclear behavior occurring at the time of conjugation
in Paramcecium caudatum (Fig. 113) may best be described in the words

284 INTRODUCTION TO CYTOLOGY

of Wilson (1900, pp. 224-226), In "Paramcecium co.udatum, which
possesses a single macronucleus and micronucleus, . . . conjugation is
temporary and fertilization mutual. The two animals become united
by their ventral sides and the macronucleus of each begins to degenerate,
while the micronucleus divides twice to form four spindle-shaped bodies.
Three of these degenerate, forming the 'corpuscles de rebut,' which
play no further part. The fourth divides into two, one of which, the
'female pronucleus,' remains in the body, while the other, or 'male
pronucleus,' passes into the other animal and fuses with the female
pronucleus. 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.
By two succeeding fissions the four macronuclei are then distributed,
one to each of the four resulting individuals. In some other species
the micronuclei are equally distributed in like manner, but in P. caudatum
the process is more complicated, since three of them degenerate, and
the fourth divides twice to produce four new micronuclei. In either
case at the close of the process each of the conjugating individuals
has given rise to four descendants, each containing a macronucleus and a
micronucleus derived from the cleavage-nucleus. From this time forward
fission follows fission in the usual manner, both nuclei dividing at each
fission, until, after many generations, conjugation recurs."

The Physiology of Fertilization. — The principal results of fertiliza-
tion are two: the activation of the egg, and, in dioecious organisms,
biparental heredity. Both of these have their physiological as well as
their morphological aspects, and in the present section the first of them
will be considered with special reference to its physiology. What is the
nature of the physiological reactions through which the development of
the egg is initiated? In the terms used by Child (1915), how does
fertilization bring about the rejuvenation of the egg, which is a physio-
logically old cell? The attack upon this problem has been carried out
along two main lines: by a study of artificial parthenogenesis and by a
direct analysis of the chemical constitution of the gametes at these stages.
Artificial Parthenogenesis. — This line of attack has been followed
particularly by Loeb, who has found a number of methods by which the
parthenogenetic development of unfertilized eggs may be artificially
induced.1 As stated in the foregoing pages, the first externally visible
effect of fertilization is in many animals the formation of a fertilization
membrane. The formation of this membrane, which seems to be a
condition necessary to the future development of the egg, Loeb was able
to induce in the California sea urchin by placing the eggs for 2 minutes
in a solution made up of 50 c.c. of sea water and 3 c.c. of a tenth-normal
1 For a convenient summary of such methods see Harvey (1910).

FERTILIZATION 285

fatty acid (butyric, propionic, or valerianic), and then back into pure
sea water: the membrane then forms by a cytolysis of the cortical layer
of the egg. Although in some forms (starfish) this one treatment is
sufficient to bring about successful development, in most cases (sea
urchin) the eggs become sickly and die. Loeb found that this sickli-
ness may be prevented, allowing normal development, by either of two
second treatments. If, after membrane formation, the eggs are placed
for 20 minutes in hypertonic sea water or other solution with an osmotic
pressure 50 per cent above that of ordinary sea water, they will develop
normally when returned to pure sea water. The same effect may be
brought about, though not always so successfully, by placing the eggs
for 3 hours in sea water free from oxygen, or into sea water with a trace
of KCN. It is therefore concluded by Loeb that the stimulus to such
parthenogenetic development has two phases: the inducement of mem-
brane formation by cytolysis, and the subsequent effect of the hyper-
tonic solution. In rare cases the first treatment alone is sufficient for
normal development, but in all cases it at least starts the egg into activity.
As a result of these experiments Loeb has interpreted the action of the
spermatozoon in normal fertilization on the assumption that it carries
two substances: first, a lysin which brings about membrane formation
by cytolysing the cortical layer of the egg, and which can act even if
the spermatozoon does not enter the egg; and second, a substance which
produces an effect similar to that of the hypertonic sea water employed
in the experiments. The quite different explanation offered by Lillie
will be mentioned further on.

How it is that cytolysis of the cortical layer of the egg brings about
activation Loeb attempts to explain in the following manner. A calcium
lipoid compound forms a continuous layer just beneath the surface of the
egg, and the solution of this layer would probably result in the destruc-
tion of the cortical emulsion. It is assumed that in this cortical region
there is a catalytic agent which increases the metabolism (rate of oxida-
tion, etc.) of the egg. Following Warburg (1914) Loeb suggests that the
cytolysis releases the catalyzer by breaking down the cortical emulsion;
this results in an increase in the rate of oxidation and other reactions, and
development proceeds.

That the process of activation is bound up primarily with reactions
occurring in the cortical region of the egg is shown further by the experi-
ments of Guyer (1907), Herlant (1913, 1917), McClendon (1912), Loeb
and Bancroft (1913), and particularly Bataillon (1910), who have shown
that the egg of the frog may be made to develop by pricking it with a
needle, especially if some blood enters the egg with it; and also by the
researches of R. S. Lillie (1908, 1915), who finds that starfish eggs may
be made to develop parthenogenetically by exposing them to high
temperatures for definite periods. (See F. R. Lillie, 1919, Chapter VII.)

286 INTRODUCTION TO CYTOLOGY

Heilbrunn (1920) shows that the egg of Cumingia can be induced to
undergo maturation by agencies which release the fluid cytoplasm from
the restraint of the tough vitelline membrane. If the membrane be
swollen, elevated above the egg surface, ruptured, or removed the
maturation changes begin at once.

The sickliness and death of those eggs given only the first treatment
Loeb thought to be due to the continued action of the cytolytic agent.
Against this conception it is urged by F. R. Lillie that since any activated
egg not developing normally cytolyzes sooner or later from internal causes,
it is more probable that the sickliness and death are due to some internal
cause resulting from activation, and points out that such a conclusion is
supported by the cytological phenomena in eggs activated by Loeb's
method. To these phenomena we may turn for a moment.

Eggs which have been given the first treatment alone do not begin to
disorganize for many (12 to 24) hours. During this period Herlant (1917)
has observed the following events. After the formation of the mem-
brane and a hyaline zone, alterations cease, and the nucleus becomes the
seat of a series of conspicuous changes. The nuclear membrane dissolves,
and around the chromosomes there is formed a monaster (one-poled
group of achromatic fibers), but no amphiaster develops. The chromo-
somes divide but do not separate, and although the cytoplasm becomes
active no cytokinesis ensues. The chromosomes then return to the
resting condition. This process is repeated several times, the nucleus
increasing in bulk each time, but it soon becomes very irregular and the
egg ultimately breaks down by general cytolysis. The second treatment
(Loeb's method) in some way gives the egg the capacity to divide regu-
larly. Morgan (1899) and Wilson had long before shown that such
treatment with hypertonic sea water causes aster formation in the
unfertilized sea urchin egg. Herlant shows that one of these asters and a
second aster formed in connection with the egg nucleus together form an
amphiaster, normal division then ensuing.

In the light of these facts it seems evident that the death of the egg
after the first treatment alone is not due to the continued action of the
cytolytic agent employed, but rather to irregularities in the activation
processes aroused by the cortical changes in the absence of a proper
coordination of nuclear and cell division. The second treatment pro-
duces a regulatory effect, partly through aster formation, resulting in
normal development. This recalls Boveri's morphological theory of
normal fertilization.

Direct Analysis of the Fertilization Process. — In contrast to the theory
that the spermatozoon contributes organs (Boveri) or substances (Loeb)
necessary for the activation, Lillie (1919, Chapter VII) regards the egg
itself as an "independently activable system." "The egg possesses all
substances needed for activation; the spermatozoon is an inciting cause

FERTILIZATION 287

of those reactions within the egg system upon which development
depends;" As a result of his direct analysis of the gametes durirg the
fertilization period Lillie has identified a substance in the egg which he
calls fertilizin. This substance is present in the egg for a short time
only; its formation usually begins at about the time the germinal vesicle
begins to break down, and immediately after fertilization its production
ceases, possibly through the neutralizing action of a second substance,
called "anti-fertilizin." As a rule it is only during the period at which
fertilizin is present that spermatozoa will enter the egg; the egg re-
mains fertilizable for but a short time. Hence it seems clear that it is
not the fertilization membrane that prevents the entrance of other
spermatozoa, as Fol thought, but rather the physiological state of the egg.
That the protection is thus a physiological rather than a mechanical
one is indicated by the fact that membraneless egg fragments without
fertilizin are not entered by spermatozoa.

Fertilizin has two effects: it first acts by causing an agglutination
of the spermatozoa at the surface of the egg, and later causes the activa-
tion of the egg. It may thus be said to stand between the spermatozoon
and the activation reactions in the egg. Being present in the egg secre-
tion at a certain period it binds the spermatozoon to the surface of the egg,
and the spermatozoon, without necessarily penetrating the egg at all,
by means of a substance which it bears releases the activity of the
fertilizin within the egg, which results in development. In brief, the
activating substance is already present in the egg and is not brought
to it by the spermatozoon. It may be incited to activity by the sperm-
atozoon, but by other agencies as well.

In concluding this sketch of the physiological features of fertiliza-
tion we may state briefly the immediate physiological consequences of the
process as summarized by Lillie (1919, Chapter V). The rate of oxida-
tion increases in most cases in which it has been investigated. In the sea
urchin egg (Warburg 1908-1914) this rate increases as much as six- or
seven-fold ; in Strongylocentrotus, four- or five-fold (Loeb and Wasteneys,
1912, 1913); in the starfish, apparently not at all. The egg membrane
becomes more permeable to oxygen, CO2, pigment, water, alkalis, intra-
vitam stains, and a number of other substances. The protoplasm be-
comes less fluid after fertilization (Heilbrunn 1915). This gelation effect
Chambers (1917) believes to center upon the sperm aster. The volume
of the egg decreases and its electrical conductivity rises. The most
conspicuous chemical change is seen in the loss of the fertilizin, and with
it the loss of capacity for further fertilization reaction.

FERTILIZATION IN PLANTS

Although the central act of the process of fertilization is regularly
the union of two sexually differentiated nuclei, the morphological

288

INTRODUCTION TO CYTOLOGY

features associated with this fusion are more varied in plants than in
animals. This is especially true of the algae and fungi.

Algae. — In Ulothrix fertilization consists in the complete union of
two morphologically similar, motile biciliate gametes (Fig. 114, A).
In Fucus the two gametes are very dissimilar: the male (spermatozoid) is
small, laterally biciliate, and actively motile (Fig. 114, 5), while the
female (egg), though discharged from the oogonium, is large and passive,
as in all higher plants and animals. In (Edogonium (Fig. 114, D, E) the

FIG. 114.- — Spermatozoids of plants.

A, Ulothrix: , 1, gamete; 2, gametes fusing (isogamy); 3, zygospore. B, Fucus. (After
Guignard.) C, Zamia. (After Webber.) D, bit of filament of (Edogonium; spermatozoids
escaping from antheridial cells below; spermatozoid about to enter egg above. (After
Coulter.) E, spermatozoid of (Edogonium. F, Chara. (After Belajeff.) G, Onoclea.
(After Steil.) For figures of spermatozoids of Blasia, Polytrichum, Equisetum, and Marsilia,
see Figs. 28, 29, 30, and 32.

egg is not shed from the cell which produces it, but is fertilized in situ,
a condition which is retained in all the higher plant groups. The sperm-
atozoid in this genus has a crown-like ring of cilia. In Spirogyra (and
other Conjugates) certain vegetative cells, without further morphological
differentiation, function as gametes. The entire contents of such a cell
pass through a conjugation tube to a similar cell in an adjacent filament,
where the two unite to form the zygospore. The two nuclei fuse, but the
chloroplasts furnished by the contributing ("male") gamete may event-
ually degenerate (Zygnema). In Polysiphonia a non-motile male gamete

FERTILIZATION

289

(spermatium) comes in contact with a prolongation (trichogyne) of the
female sex organ (carpogonium). Solution of the intervening walls allows
the nucleus of the spermatium to pass into the trichogyne and down to the
female nucleus in the base of the carpogonium. In Polysiphonia we
have one of the few cases among lower plants in which the fusion of the
sexual nuclei has been minutely described. According to Yamanouchi
(1906) the male. nucleus, by the time it has reached the female nucleus,
has resolved itself into a group of 20 chromosomes (Fig. 115, A). In this

FIG. 115.

A, fertilization in Polysiphonia. Group of male chromosomes about to enter female
nucleus. (After Yamanouchi, 1906.) B, fertilization in Albugo Candida. Female nucleus
lying in center of ooplasm near the "coenocentrum" (larger dark body.) Antheridial tube
about to discharge a male nucleus; another male nucleus in neck of tube. Additional nuclei
in periplasm surrounding the ooplasm. (After Davis, 1900.)

condition it enters the female nucleus while the latter is yet in the reticu-
late state. Soon the female reticulum becomes transformed into 20
chromosomes, which arrange themselves with the 20 paternal chromo-
somes upon the spindle as the fusion nucleus divides.

Fungi. — In the PHYCOMYCETES sexual reproduction occurs in two princi-
pal forms, which serve to divide the group into two main divisions:
Oomycetes and Zygomycetes.

In the Oomycetes the cytological phenomena are best known in the
Peronosporales and Saprolegniales. In the former there is differentiated
in the oogonium a single large egg into which the contents of an antheri-
dium are discharged through a penetrating tube. In Albugo bliti and
A. portulaccce (Stevens 1899, 1901) the egg has a large number of nuclei,

19

290 INTRODUCTION TO CYTOLOGY

and after the entrance of the antheridial nuclei about 100 sexual fusions
occur. In Albugo Candida (Cystopus candidus) (Wager 1896; Davis
1900), Peronospora parasitica (Wager 1900), Albugo tragopogonis and
A. ipomceae (Stevens 1901) the mature egg has but one nucleus, which
fuses with a single male nucleus discharged into the egg by an antheridium
(Fig. 115, .B). In all cases an oospore results.

In the Saprolegniales, as shown by the researches of Davis (1903,
1905), Miyake (1901), Trow (1895-1905), and Claussen (1908), there are
two general conditions. In Saprolegnia (Trow, Davis, Claussen) from
10 to 15 uninucleate eggs are formed within an oogonium. One or more
antheridia send in conjugating tubes and deliver a male nucleus to each
egg, in which a single sexual fusion then occurs. In Pyihium (Trow 1901 ;
Miyake 1901) a single uninucleate egg is produced, the fertilization
process closely resembling that in Albugo Candida.

In the Zygomycetes, represented chiefly by the Mucoraceae, the sexual
process consists in the union of the contents of two similar (except oc
casionally in size) multinucleate gametangia, the result of the fusion
being a zygospore. As shown by Blakeslee (1904) these two gametangia
are borne on the same mycelium in some species ("homothallic" species),
whereas in other species ("heterothallic" species) they are regularly
borne on different mycelia, no zygospores being formed in the latter spe-
cies on a mycelium arising from a single spore. Owing to the extremely
minute size of the nuclei their behavior at these stages is not well known.
By some investigators (Macormick on Rhizopus nigricans, 1912) it is
held that only one fusion occurs, the remaining nuclei degenerating.
Others (Keene on Sporodinia grandis, 1914) think it probable that al-
though some degeneration occurs, the nuclei nevertheless fuse in pairs
in considerable numbers. Until further researches have been carried out
very little of a definite nature can be said concerning the nuclear history
of the Zygomycetes.

In the ASCOMYCETES (see Atkinson 1915) the fusion of two nuclei
in the ascus was first described for several species by Dangeard (1894)
(Fig. 116, A), who regarded it as a sexual fusion and the ascus as an oogo-
nium. The matter soon became complicated when a number of cytolo-
gists, beginning with Harper (1895 etc.), found what they believed to be
a nuclear fusion at an earlier stage in the life cycle. This fusion was
described as occurring (a) in the archicarp when fertilized by the contents
of an antheridium (Harper on Sphcerotheca castagnei, 1895, 1896, Erisiphe
1896, Pyronema confluens 1900, and Phyllactinia 1905; Blackman and Fra-
ser on Sphcerotheca 1905; Claussen on Boudiera 1905) ; (6) in the archicarp
when the antheridium is functionless or absent (Blackman and Fraser
on Humaria granulata 1906; Fraser on Lachnea stercorea 1907; Welsford
on A scobolus furfur aceus 1907; Dale on Aspergillus repens 1909); or (c)
in the vegetative cells when the archicarp is functionless or absent (Fraser

FERTILIZATION

291

on Humaria rutilans 1907, 1908; Carruthers on Helvella crispa 1911;
Blackman and Welsford on Polystigma rubrum 1912). By the above
investigators this early fusion was regarded as a sexual one, that in the
ascus being vegetative in nature; and some described a "double reduc-
tion" in the ascus to compensate for the two nuclear fusions. (See
p. 223.)

In a series of somewhat later researches another group of observers
found the evidence for an early fusion to be very unsatisfactory, and'
concluded that the only nuclear union in the life cycle is that occurring
in the ascus: with Dangeard they saw in this union the sexual act. Fur-
thermore, no "double reduction" was found in the ascus. Among the
researches supporting this view, which now appears to be the more
probable, may be cited the following: Claussen on Pyronema confluens

FIG. 116.

A, nuclear fusion in the ascus of Peziza vesiculosa. (After Dangeard, 1894.) B, cell
fusion in seciospore sorus of Phragmidium speciosum. After Christman, 1905.)

1907, 1912; Schikorra on Monascus 1909; W. H. Brown on Pyronema
confluens 1909, Lachnea scutellata 1911, and Leotia 1910; Faull on Lab-
oulbenia 1911, 1912; Blackman on Collema pulposum 1913; Nienburg on
Polystigma rubrum 1914; Ramlow on Ascophanus carneus and Ascobolus
immersus 1914; Brooks on Gnomonia erythrostoma 1910; McCubbin on
Helvella elastica 1910; H. B. Brown on Xylaria tentaculata 1913; and Fitz-
patrick on Rhizina undulata 1918a.

As the two nuclei fuse in the young ascus Harper (1905) observed in
the case of Phyllactinia ccfrylea that not only the chromatin systems but
also the nucleoli and "central bodies" (centrosomes), upon which the
chromatin strands converge, unite. In the Ascomycetes generally the
fusion nucleus, or "primary ascus nucleus," undergoes three successive
mitoses to form the eight ascospore nuclei, the spore walls in each case
being formed in association with the curving astral rays which focus upon
the centrosome. (See p. 80.)

292 INTRODUCTION TO CYTOLOGY

In certain yeasts it has been shown (see Guilliermond 1920) that the
production of ascospores is preceded by a copulation of two cells with a
fusion of their nuclei, the fusion nucleus dividing to form the spore nuclei.
A somewhat similar copulation of the ascospores themselves has also
been observed in a few cases.

Among the BASIDIOMYCETES the nuclear phenomena are best known
in the case of the rusts, owing to the researches of Blackman (1904),
Christman (1905), and a number of later writers. In the typical rust
life cycle there is a fusion of uninucleate cells at the base of the aecial
sorus (Fig. 116, B}. The binucleate cells thus arising produce the binu-
cleate aeciospores; and these upon germination form a mycelium with
binucleate cells, the two nuclei dividing in unison ("conjugately") at
each cell-division. After producing a series of crops of binucleate
uredospores this mycelium eventually bears teliospores which may con-
sist of one or more cells. In each cell of the teliospore the two nuclei
delivered to it as the result of the conjugate divisions throughout the
binucleate mycelium finally unite, initiating the uninucleate phase of
the life cycle. Here the fusion of sexual cells and the fusion of their
nnclei — two events which in most organisms occur very near each other
in time — are widely separated in the cycle. The two nuclei dividing
conjugately constitute together a synkaryon in many respects equivaleng
to a diploid nucleus. Since there is as yet no evidence to show in what
degree the two effects of fertilization (the stimulus to development and the
mixing of hereditary lines) are brought about in the rusts by the fusion
of the sexual cells on the one hand and by the final union of their nuclei
on the other, it seems best to regard the two fusions as two phases of the
fertilization process in spite of their wide separation in the life history.

In the Hymenomycetes it has been known for some time that a fusion
of two nuclei occurs in the basidium, itself the terminal cell of a binucleate
hypha, prior to the formation of the four basidiospore nuclei (Fig. 79).
The origin of the binucleate condition in the mycelium which has ap-
parently arisen from a uninucleate spore has long been an obscure point.
It has recently been shown by Miss Bensaude (1918) in the case of
Coprinus fimetarius that the binucleate hyphae arise as the result of cell
fusions ("plasmogamy;" "pseudogamy") between uninucleate hyphsB
arising from different spores, and that no carpophores are produced upon
a uninucleate mycelium arising from a single spore. Thus it appears
that in at least some hymenomycetes the sexual process is initiated by a
fusion of two cells of different strains ("plus" and "minus"), as in the
heterothallic molds.

Bryophytes and Pteridophytes. — In bryophytes and pteridophytes
the details of the union of the motile spermatozoid with the egg in the
archegonium have been described in very few cases. In the former
group may be cited the works of Garber (1904) and Black (1913) on

FERTILIZATION

293

Riccia, Meyer (1911) on Corsinia, Graham (1918) on Preissia, and
Woodburn (1920) on Reboulia. It appears that in bryophytes the body
of the biciliate spermatozoid, which consists mainly of nuclear material,
undergoes in the egg cytoplasm a transformation into a reticulate nucleus
before fusing with the egg nucleus (Fig. 117). The fate of the non-
nuclear structures (cytoplasm, blepharoplast, and cilia) is not known
with certainty, but it is probable that they are absorbed in the egg cyto-
plasm. In the liverwort, Preissia
quadrata, Miss Graham has found
two centrosomes with weakly de-
veloped asters in the cytoplasm of
the egg at the time the two pronuclei
are about to fuse (Fig. 23, A). It is
not known what relation their ap-
pearance may have to the entrance
of the spermatozoid.

The most detailed account of fer-
tilization in a pteridophyte is that
given by Yamanouchi (1908) for

FIG. 117. FIG. 118.

Fia. 117. — Fertilization in Anthoceros. Male and female pronuclei about to fuse in
lower part of egg in venter of archegonium; elongated plastid above them. Gametophyte
cells show one nucleus and one plastid each. X 1050.

FIG. 118. — Fertilization in Nephrodium.

A, spermatozoid entering egg nucleus. B, spermatozoid becoming reticulate in midst
of female reticulum. (After Yamanouchi, 1908.)

Nephrodium (Fig. 118). In Nephrodium the multiciliate spermatozoid
enters bodily into the egg nucleus with no previous alteration into the
reticulate state. Here it gradually becomes reticulate and irregular in
shape, until finally its limits are indistinguishable, the chromatic material
contributed by the two gametes apparently forming a single fine-meshed
network.

Gymnosperms. — Among living gymnosperms the Cycadales and
Ginkgoales are characterized by the possession of motile spermatozoids.

294

INTRODUCTION TO CYTOLOGY

These spermatozoids are very much alike in structure and behavior in
the two groups, and are unusually large, being easily visible to the naked
eye. The body is made up of a large nucleus surrounded by a thin
cytoplasmic layer in which is imbedded a long, spirally coiled blepharo-
plast bearing many cilia (Fig. 114, C). The behavior of the spermatozoid
in fertilization has been studied in Ginkgo by Hirase (1895, 1918) and
Ikeno (1901); in Cycas revoluta by Ikeno (1898); in Zamiafloridanaby
Webber (1901); and in Dioon edule, Ceratozamia mexicana, and Stangeria
paradoxa by Chamberlain (1910, 1912, 1916).
In all cases the entire spermatozoid penetrates
into the egg cytoplasm, where the nucleus frees
itself from the cytoplasmic sheath with its
blepharoplast and cilia and advances alone to
the egg nucleus, with which it fuses (Fig. 119).
The behavior of the chromatin during the fusion
is not well known in either Ginkgo or the cycads.
In the Coniferales and Gnetales the male
cells have no motile apparatus. Each consists
of a nucleus surrounded by a more or less
sharply delimited mass of cytoplasm. In most
cases this cytoplasm remains intact until after
the male cell has entered the egg, but in other
forms, such as Pinus, it mingles with the cyto-
plasm of the pollen tube, so that only male
nuclei, rather than completely organized male
cells, are delivered to the egg. All the nuclei
present in the pollen tube — stalk nucleus, tube
nucleus, the two male nuclei, and in certain
species free prothallial nuclei — may be dis-
charged into the egg. All but the functioning
male nucleus usually degenerate at once, but in
some cases they have been observed to undergo
division.

When a complete male cell enters the egg the cytoplasm of the former
shows two general modes of behavior. In some species it may be left
behind in the peripheral region of the egg as the male nucleus frees itself
and advances alone to the female nucleus. This type of behavior has
been reported in Pinus (Ferguson 1901, 1904), Thuja (Land 1902),
Juniperus (Noren 1904), Cryptomeria (Lawson 1904), and Libocedrus
(Lawson 1907). In Sequoia (Lawson 1904) the male nuclei escape from
their cytoplasm before their discharge from the pollen tube, and enter
the egg alone. In a second group of species the male cytoplasm remains
intact and invests the fusing sexual nuclei, being clearly distinguishable
from the cytoplasm of the egg. The pollen tube cytoplasm often plays

FIG. 119. — Fertilization
in Zamia. Male nucleus
uniting with egg nucleus at
center; cytoplasmic sheath
with spiral blepharoplast
above. Another sperm
outside egg. X 25. (After
Webber, 1901.)

FERTILIZATION

295

a conspicuous part in the formation of this "mantle." This phenomenon,
the significance of which can only be conjectured, is found in Taxodium
(Coker 1903), Torreya calif ornica (Robertson 1904), Torreya taxifolia
(Coulter and Land 1905), Cephalotaxus Fortunei (Coker 1907), Ephedra
(Berridge and Sanday 1907; Land 1907), Phyllocladus (Kildahl 1908),
Juniperus (Nichols 1910), Agathis (Eames 1913), and Taxus (Dupler
1917).

Chromosome Behavior. — The behavior of the chromosomes during
the fusion of the sexual nuclei and the first embryonal division has
been described in a number of conifers. As a general rule, to judge from
the data at hand, the chromatin contributions of the two pronuclei do
not become intimately associated in the fusion nucleus, but remain
distinguishable until the first embryonal mitosis occurs. Each of the
pronuclei then gives rise to its complement of chromosomes which

B f

FIG. 120 — Fertilization in Pinus.

A, male nucleus pressing into female nucleus. X 140. B, first embryonal mitosis,
showing separate paternal and maternal chromosome groups. X 472. (After Ferguson,
1904.)

become arranged, often as two separate groups, upon a common
spindle. Such an independent formation of the male and female
chromosome groups has been observed in Pinus (Blackman 1898 ; Cham-
berlain 1899; Ferguson 1909, 1904) (Fig. 120), Larix (Woyciki 1899),
Tsuga candensis (Murrill 1900), Juniperus communis (Noren 1907),
Cunninghamia (Miyake 1910), and Abies (Hutchinson 1915). In
Sequoia, on the other hand, Lawson (1904) reports that the two nuclei
form a common reticulum in which the male and female constituents
cannot be distinguished. With regard to the first embryonal mitosis
the general opinion has been that all the chromosomes, paternal and
maternal, split longitudinally, the daughter chromosomes being distri-
buted to the daughter nuclei as in any other somatic mitosis. This
type of behavior was described for the chromosomes of Pinus by Miss
Ferguson (1904) and at once came to be regarded as general for coni-
fers, as it had been for other organisms.

A new interpretation differing in certain fundamental points from the
above has been more recently suggested by Hutchinson (1915), as a result

296

INTRODUCTION TO CYTOLOGY

of his work on Abies balsamea. According to Hutchinson (Fig. 121)
there appear in the fusion nucleus two groups of chromosomes, each
containing the haploid number (16). A spindle is differentiated about
each group; and the two spindles soon unite to form one, thus bringing
the two chromosome groups, representing the two parental contributions,
into closer association. The chromosomes now approximate two by two
to form 16 pairs. The members of each pair twist about each other and
become looped; each of them becomes transversely segmented at the
apex of the loop, forming 32 (2x) pairs of segments; these pairs separate
to form 64 (4x) chromosomes; a new spindle is formed and 32 (2x)
chromosomes pass to each pole.

TWUTINC LMNN

IMMMMTMD

tnnto KUCUI
(»*•»)

FIG. 121. — The behavior of the chromosomes in fertilization and the first embryonal mitosis
in Abies, according to Hutchinson. (1915.)

This interpretation of chromosome behavior at fertilization is remark-
able not only because it indicates features resembling those of the hetero-
typic prophase, but chiefly because it actually calls for a qualitative
reduction of the chromatin at the first embryonal mitosis if the chroma-
tin is not qualitatively the same throughout the nucleus. This impli-
cation has not been discussed by the advocates of the new theory. The
chromosomes pair and twist about one another in a way that parallels
closely their behavior during the prophase of a reduction division. That
the doubleness seen is due to a pairing and not to a splitting as has
heretofore been held is supported by the assertion that the pairs are
present in the haploid number, rather than in the diploid number as
would be the case were a splitting of all the chromosomes occurring. If
the two members of each pair were to separate at the first embryonal

FERTILIZATION

297

mitosis, a reduction, qualitative as well as numerical, in all respects
similar to that accomplished in the regular heterotypic mitosis, would
be brought about if the pairing members are qualitatively different.
But instead of such a separation, each member of each pair segments
transversely, giving 4x segments which are equally distributed to the two
daughter nuclei, each of the latter receiving the diploid number. Since
the 4x segments become more or less intermingled before their distri-
bution it is probably impossible to determine just which ones pass to
each pole. If both halves of one transversely divided chromosome pass
to one pole (see Fig. 122), that daughter nucleus only, and not the other,
will receive the kind of chromatin carried by that chromosome, so that

CLUWAfrE MITOSIS EQUftTIONAL

FUSION NUCLEUS

V

SEGMENTATION

FIG. 122. — Diagram showing the behavior of the chromosomes in fertilization and the
first embryonal mitosis as usually interpreted (upper part) and according to Hutchinson's
interpretation (lower part).

the two nuclei will be qualitatively different. A qualitative reduction
will have occurred, but without a change in the number of chromosomes,
since each old chromosome has become two new ones. If, on the other
hand, the two halves of the transversely segmented chromosome regularly
pass to opposite poles, each daughter nucleus will receive a half of each
and every parental chromosome: thus if there are just as many kinds of
chromatin as there are chromosomes, these nuclei will be qualitatively
alike, just as they would be had the division been longitudinal instead of
transverse. But, as has been stated in the chapter on reduction and will
be developed at greater length in Chapter XVII, there is a considerable
body of evidence which indicates that each chromosome is not only
qualitatively different from its fellows, but possesses a linear differen-

298 INTRODUCTION TO CYTOLOGY

tiation of some sort; so that the separation of the two halves of a trans-
versely divided chromosme would constitute a qualitative reduction.
If such actually is the condition of the chromatin, and if the chromosomes
do behave as Hutchinson supposes, a qualitative reduction must immedi-
ately follow each fertilization, and half of the resulting body cells must
have a constitution differing from that of the other half. Since there are
known no chromosome fusions in which a restoration in the number of
qualities is known to occur, the number of these qualities in a single
chromosome would in a few generations be reduced to one: in view of
the large number of past generations this must have already occurred.

This new interpretation of chromosome behavior at fertilization and
the ensuing mitosis is thus seen to offer a direct challenge to those
theories of heredity that are based upon the idea of chromosomes carry-
ing linear series of differentiated units. It has now been put forward by
Hutchinson (1915) for Abies balsamea, by Chamberlain (1916) for
Stangeria paradoxa, and by Miss Weniger (1918) for Lilium philadel-
phicum and L. longiflorum. Consequently several investigators have
renewed the study of fertilization, and evidence contradictory to the
new theory has been found by Miss Nothnagel (1918) and Sax (1918),
whose researches are summarized in the following section on the
angiosperms.

Angiosperms. — The angiosperms are characterized by the occurrence
of "double fertilization," a phenomenon discovered independently by
Nawaschin (1898) and Guignard (1899). One .of the two male nuclei
formed by the male gametophyte and brought into the embryo sac by
the pollen tube, enters the egg and fuses with its nucleus, thus forming
the primary nucleus of the embryo, while the other male nucleus fuses
with the two polar nuclei to form the primary endosperm nucleus (Fig.
123, 5). As the male nuclei pass down the pollen tube they are usually
unaccompanied by any specially differentiated .cytoplasm: the male
gametes are naked nuclei and not complete cells. In some cases, how-
ever, male cells have been reported (Fig. 123, A). When they are
liberated in the embryo sac by the rupture of the end of the pollen tube
any such cytoplasm is indistinguishable from that of the sac and that
discharged from the pollen tube. The male nuclei may appear in all
respects similar to other nuclei, or they may be distinctly vermiform,
as was observed by Mottier (1898) and later by many other workers
(Fig. 123, E). That such vermiform nuclei have the power of inde-
pendent movement has been held by Nawaschin (1899, 1900, 1909,
1910) for Lilium and Fritillaria, by Guignard (1900) for Tulipa, and by
Blackman and Welsford (1913) and Miss Welsford (1914) for Lilium
Martagon and L. auratum. The vermiform condition may persist until
the time of fusion, but in other cases, such as Fritillaria (Sax 1916), it
gives way to the ordinary shape. This change may occur more rapidly

FERTILIZATION

299

in one male nucleus than in the other, so that the two may appear quite
unlike during the later stages. Miss Welsford also sees in the male
cytoplasm certain granules which she thinks may represent the vestiges
of blepharoplasts.

B

FIG. 123. — Fertilization in angiosperms.

A, end of pollen tube from basal portion of style of Lilium auratum, showing two male
cells and tube nucleus. X 250. (After Welsford, 1914.) B, double fertilization in Lilium
canadense: male and female nuclei about to fuse in egg; second male and two polar nuclei
fusing at center of embryo sac; s, synergids, one degenerated; a, antipodals X 250.
C, fusion of sexual nuclei in egg of Lilium philadelphicum. X 1000. (After Weniger,
1918.) D, the second male and two polar nuclei in Lilium Martagon. X 750. (After
Nothnagel, 1918.) E, vermiform male nucleus in contact with egg nucleus in Triticum
durum. X 600. (After Sax, 1918.) F, spireme stage of triple fusion nucleus in Triticum
durum, showing distinctness of three chromatin contributions. X 750. (After Sax, 1918.)
G, inclusion of cytoplasm in fusing sexual nuclei of Peperomia sintenesii. (After Brown,
1910.)

Fusion in Egg. — As already stated, one male nucleus passes into
the egg and fuses with the egg nucleus. So far as observations enable
one to say, only the male nucleus, and no cytoplasm, enters the egg, a
point of much importance in connection with the transmission of heredi-
tary characters from the male parent. It would be a matter of extreme
difficulty, however, to demonstrate conclusively that in passing through
the egg membrane the male nucleus is absolutely freed of all adhering
cytoplasm or chondriosomes; and it must be admitted that such a
demonstration has not yet been given in any case. The fusion of the two
sexual nuclei probably occurs in most cases very soon after they come
in contact, though in certain forms the actual fusion is known to be

300 INTRODUCTION TO CYTOLOGY

considerably delayed. The chromatin of the two nuclei at the time
these unite may be in the reticulate (resting) condition, the male and
female chromatins being indistinguishable in the fusion nucleus. This
situation was described by most of the earlier workers, including Stras-
burger (1900, 1901), Mottier (1904), Nawaschin (1898, 1899), and Ernst
(1902). It has also been reported by Sax (1916) in his recent work on
Fritillaria. In other cases, as early reported by Guignard (1891), the
chromatin has already reached the spireme stage characteristic of the
prophase, the male and female elements being distinguishable on the
spindle in the ensuing division of the fertilized egg. Such is the condi-
tion, for instance, in Calopogon (Pace 1909), Trillium (Nothnagel 1918),
and Lilium (Weniger 1918). That the same species may show con-
siderable variation in this respect is indicated by the situation in
Fritillaria, in which Sax (1916, 1918) finds that fusion, though it usually
occurs in the resting stage, sometimes takes place after the spiremes
have been developed. Miss Weniger (1918) reports that in Lilium
philadelphicum and L. longiflorum the egg nucleus is in the resting con-
dition and the male nucleus in the spireme stage at the time of union.

Chromosome Behavior. — With regard to the behavior of the two par-
ental groups of chromosomes, it has been generally held that, whatever
their condition at the time of the nuclear union, all of them, both pater-
nal and maternal, split longitudinally at the first division of the fer-
tilized egg, the daughter chromosomes so formed being distributed to
the two resulting nuclei, just as in all the subsequent somatic divisions.
Recently, however, Miss Weniger (1918) has reported a condition in
Lilium similar to that described by Hutchinson (1915) for Abies: the
maternal and paternal chromosomes form pairs and divide transversely
into daughter segments which pass to the poles. (See p. 296.) With
this conclusion other recent investigations of fertilization in angiosperms
are not in agreement. Miss Nothnagel (1918) finds in Trillium no such
pairing and cross segmentation as Hutchinson and Miss Weniger de-
scribe, and states that each chromosome splits longitudinally as held
by Miss Ferguson for Pinus and by cytologists in general. Sax (1918)
also shows that each paternal and maternal chromosome in Fritillaria
divides longitudinally, the diploid number (24) passing to each pole.
In Trillium he reports an essentially similar state of affairs, the diploid
number here being about 28. He therefore holds that the first mitosis
in the fertilized egg is like any other somatic mitosis, and that no mech-
anism for the segregation of factors of inheritance, such as occurs at
reduction, exists here. The outcome of this controversy is awaited with
much interest because of its great theoretical importance.

Endosperm Fusion. — The fusion of the second male nucleus with
the two polar nuclei of the embryo sac to form the primary endosperm
nucleus may be carried out in a variety of ways. The most commonly

FERTILIZATION 301

reported method is that by which the two polars fuse to form an "em-
bryo sac nucleus" before the entrance of the pollen tube, the male
nucleus later being added. Ernst (1902), for example, found this to be
the method in Pan's quadrifolia. Less frequently the male nucleus
meets and fuses with the polar nucleus of the micropylar end of the sac,
the other polar then fusing with the product. This is the method de-
scribed by Nawaschin (1898, 1899) in his account of the discovery of
double fertilization in Lilium Martagon and Fritillaria tenella. The
simultaneous fusion of all three nuclei appears to be a common
occurrence; it has recently been described in some detail by Miss Noth-
nagel (1918) for Trillium and Lilium. Just as in the case of the union
of the first male nucleus with the egg nucleus, the chromatin of the
second male and two polar nuclei may be either in the reticulate or the
spireme condition as they come together. In Lilium Martagon (Noth-
nagel 1918) (Fig. 123, Z>) it is in the form of fine strands, intermediate
between the resting and spireme stages. Although the three nuclei often
appear exactly alike, it is frequently possible to distinguish the male
from the polars, not only by its shape and smaller size, but by the
condition of its chromatin: in Lilium longiflorum (Weniger 1918), for
example, the male nucleus is in the spireme stage while the polar nuclei
are still in the resting condition. The membranes of the three nuclei
may persist for some time after they come into intimate contact, and
even after they have dissolved the chromatic elements of the three con-
stituent nuclei may in many cases be distinguished if the section has
been made in a favorable plane. When fusion occurs in the resting
stage this is not so apparent, but when it occurs in the spireme stage
the three chromatic groups are made out with little difficulty.

Endosperm. — As the division of the endosperm nucleus approaches
the spiremes of its three constituent nuclei become increasingly dis-
tinct, even' if one or more of the nuclei have fused in the resting stage.
Nothnagel (1918), Weniger (1918), and Sax (1918) in their recent studies
all report this condition (Fig. 123, F). As the spiremes are being
developed into completed chromosomes all of them (3x in number) split
longitudinally, no observer reporting such a pairing as some have thought
to occur between the chromosomes of the egg and first male nuclei.
Miss Nothnagel describes the formation of a tripolar spindle about the
chromosomes; the bipolar condition soon develops from this. How
frequently this may occur is not known. Eventually in any case the
mitosis proceeds along the usual lines and the two daughter nuclei receive
3x chromosomes each. This number is characteristic of all the cells of
the endosperm formed by the repeated division of these nuclei. An
exceptional condition has been noted by Sax (1918) in Fritillaria.
Here the lower polar nucleus, because of an irregularity in the mitosis
giving rise to it, has 24 (2x) chromosomes instead of the normal 12

302 INTRODUCTION TO CYTOLOGY

(x). Consequently the female parent contributes 3C chromosomes (24 in
one polar nucleus and 12 in the other) to the endosperm, while the male
parent contributes only 12; thus the endosperm has 48 (4x) chromo-
somes instead of the normal 36.

Although in the great majority of known examples endosperm is
formed by the repeated division of a triple fusion nucleus, cases are known
in which it is produced by the polar fusion nucleus (embryo sac nucleus)
alone without the male, or by the fusion product of the male and one
polar, or by one polar alone. Combinations of these three methods may
be found in the same embryo sac. The development of the endosperm
may be initiated by the formation of a number of free nuclei which are
parietally placed and in later mitoses become separated by walls, or
by the formation of walled cells from the start. (See Coulter and Cham-
berlain, 1903.)

The term xenia was applied by Focke (1881) to the effect of foreign
pollen on the endosperm of the resulting seed in angiosperms. Thus if
maize of a certain strain which produces seeds with white endosperm when
self-pollinated, is pollinated with pollen from a plant whose seeds have
red endosperm, the endosperm of the resulting hybrid seeds is red like
that of the pollen parent. No satisfactory explanation of this phenom-
enon was at hand until the discovery of double fertilization by Nawas-
chin and Guignard in 1898-9. It then became clear that the endosperm,
which was formerly supposed to contain only maternal nuclear material,
may show endosperm characters of the parent furnishing the pollen for the
reason that the latter contributes a nucleus to the primary endosperm
nucleus, so that every endosperm cell contains some nuclear material
from the pollen parent.

Normally the endosperm cells are all alike in containing two chromo-
some sets from the female parent and one set from the male, and the nor-
mal inheritance of endosperm characters as well as all ordinary cases of
xenia can be understood on this basis. Mottled or mosaic effects in the
endosperm of maize hybrids were attributed by Webber (1900) to such
abnormal modes of endosperm origin as were referred to in a foregoing
paragraph: some of the cells may have been formed by polar nuclei
of purely maternal constitution while other cells were the result of the
independent division of the second male nucleus. Although this explana-
tion may fit some cases, it is becoming apparent from the work of Emerson
and others that most of them can be better accounted for on the basis of
aberrant chromosome behavior, such behavior having been observed in
certain other organisms.

Additional evidence is found in all these phenomena for the theory
that the nuclear substance in some way represents the physical basis of
inheritance. The second male nucleus not only is concerned in the
initiation of the development of the endosperm, but xenia shows that it

FERTILIZATION 303

also transmits parental characters. Here, therefore, as in the sexual
fusion in the egg, the two principal effects of fertilization may be
recognized.

Bibliography 12

Fertilization

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304 INTRODUCTION TO CYTOLOGY

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FERTILIZATION 305

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FITZPATRICK, H. M. 1918o. Sexuality in Rhizina undutata Fries. Bot. Gaz. 65:

201-226 pis. 3 4.
19186. The cytology of Eucronarlium muscicola. Am. Jour. Bot. 6: 397-419.

pis. 30-32.

FOCKE, W. O. 1881. Die Pflanzen-Mischlinge. Berlin.
FOL, H. 1891. Die "Centrenquadrille," ein neue Episode aus der Befruchtungs-

geschichte. Anat. Anz. 6 : 266-274. figs. 10.
FRASER, H. C. I. 1907. On the sexuality and development of the ascocarp of

Lachnea stercorea Pers. Ann. Bot. 21 : 349-360. pis. 29, 30.
1908. Contributions to the cytology of Humaria rutilans. Ibid. 22: 35-55.

pis. 4, 5.
FRIES, R. E. 1911. Zur Kenntniss der Cytologie von Hygrophorus conicus. Svensk.

Bot. Tids. 5: 241-251. pi. 1.
GARBER, J. F. 1904. The life history of Ricciocarpus natans. Bot. Gaz. 37: 161-

177. pis. 9, 10.
GRAHAM, M. 1918. Centrosomes in fertilization stages of Preissia quadrata (Scop.)

Nees. Ann. Bot. 32 : 415-420. pi. 10.
GUIGNARD, L. 1899., Sur les antherozoids et la double copulation sexuelle chez les

ve'ge'taux angiospermes. Comp. Rend. Acad. Sci. Paris 128: 864-871. figs. 19.

1900. L'appareil sexuel et la double fe"condation dans les Tulipes. Ann. Sci.
Nat. Bot. V1I1 11: 365-387. pis. 9-11.

1901. La double f4condation chez les Renonculace'es. Jour. Botanique 16:
394-408. figs. 16.

1902. La double f^condation chez les Solanees. Ibid. 16 : 145-167. figs. 45.
20

306 INTRODUCTION TO CYTOLOGY

GUILLIERMOND, A. 1910. La sexualite chez les champignons. Bull. Sci. France et

Belg. 44: 109-196.

1920. The Yeasts. (Engl. transl. by F. W. Turner.) N. Y.
GUYER, M. F. 1907. The development of unfertilized frogs' eggs injected with

blood. Science N. S. 25: 910-911.
HAECKER, V. 1895. Ueber die Selbstandigkeit der vaterlichen und miitterlichen

Kernbestandtheile wahrend der Embryonalentwicklung von Cyclops. Arch.

Mikr. Anat. 46 : 579-617. pis. 28-30.

1899. Praxis und Theorie der Zellen- und Befruchtungslehre.

HARPER, R. A. 1895. Beitrag zur Kenntniss der Kerntheilung und Sporenbildung

im ASCU&. Ber. Deu. Bot. Ges. 13: (67)- (78). pi. 27.

1896. Ueber das Verhalten der Kerne bei den FruchteR. ntwicklung einiger Asco-
myceten. Jahrb. Wiss. Bot. 29: 655-685. pis. 11, 12.

1900. Sexual reproduction in Pyronema confluens and the morphology of the
ascocarp. Ann. Bot. 14: 321-396. pis. 19-21.

1905. Sexual reproduction and the organization of the nucleus in certain mildews.

Carnegie Inst. Publ. 37.
HARVEY, E. N. 1910. Methods of artificial parthenogenesis. Biol. Bull. 18:

269-180.
HEILBRUNN, L. V. 1915. Studies in artificial parthenogenesis. 11. Physical

changes in the egg of Arbacia. Biol. Bull. 29: 149-203.
1920. Studies in artificial parthenogenesis. 111. Cortical changes and the inia-

tion of maturation in the egg of Cumingia. Biol. Bull. 38: 317-339.
HERLANT, M. 1913. Le me'chanisme de la parthenoge"nese esperimentelle. Bull.

Sci. France et Belg. VII 50 : 381-404.

1917. Etude sur les bases cytologiques du mecanisme de la parthenogenese experi-
mentelle chez les amphibiens. Arch. d. Biol. 28: 505-608.

HERTWIG, O. 1875. Beitrage zur Kenntniss der Bildung, Befruchtung, und Thei-

lung des tierischen Eies, I. Morph. Jahrb. 1.
HIRASE, S. 1895. Etudes sur la fecondation et 1'embryogenie du Ginkgo biloba.

Jour. Imp. Coll. Sci. Tokyo 8: 307-322. pis. 31, 32.
1898. Etudes sur la fecondation et 1'embryogenie du Ginkgo biloba. (Second

memoire.) Ibid. 12: 103-149. pis. 7-9.

1918. Further studies on the fertilization and embryogeny in Ginkgo biloba.
Bot. Mag. Tokyo 32: No. 378.

HOYT, W. D. 1910. Physiological aspects of fertilization and hybridization in

ferns. Bot. Gaz. 49: 340-370. figs. 12.
HUTCHINSON, A. H. 1915. Fertilization in Abies balsamea. Bot. Gaz. 60: 457-

472. pis. 16-20. fig. 1.
HUXLEY, T. H. 1878. Evolution in Biology.
IKENO, S. 1898. Untersuchungen ilber die Entwicklung der Geschlechtsorgane

und der Vorgang der Befruchtung bei Cycas revoluta. Jahrb. Wiss. Bot. 32 :

557-602. pis. 8-10.

1901. Contributions a 1'etude de la fecondation chez le Ginkgo biloba. Ann. Sci.
Nat. Bot. VIII 13: 305-318. pis. 2, 3.

JONES, W. N. 1918. On the nature of fertilization and sex. New Phytol. 17 : 167-188.
KEENE, M. L. 1914. Cytological studies of the zygospores of Sporodinia grandis.

Ann. Bot. 28: 455-470. pis. 25, 26.
KILDAHL, N. J. 1908. The morphology of Phyllocladiis alpina. Bot. Gaz. 46:

339-348. pis. 20-22.
KOLTZOFF, N. K. 1906. Studien liber die Gestalt der Zelle: I, Untersuchungen iiber

die Spermien der Decapoden. Arch. Mikr. Anat. 67: 364-572. pis. 25-29.

figs. 37.

FERTILIZATION 307

LAND, W. J. G. 1900. Double fertilization in Corapositse. Bot. Gaz. 30: 252-

260. pis. 15, 16.

1902. A morphological study of Thuja. Ibid. 34: 249-259. pis. 6-8.
1907. Fertilization and embryogeny of Ephedra trifurca. Ibid. 44: 273-292.

pis. 20-22.
LAWSON, A. A. 1904a. The gametophyte, archegonia, fertilization and embryo

of Sequoia sempervirens. Ann. Bot. 18: 1-28. pis. 1-4.
19046. The gametophyte, fertilization and embryo of Cryplomeria japonica.

Ibid. 18: 417-444. pis. 27-30.
1907. The gametophytes, fertilization and embryo of Cephalotaxus drupacea.

Ibid. 21 ; 1-23. pis. 1-4.
LEVINE, M. 1913. The cytology of Hymenomycetes, especially fche Boleti. Bull.

Torr. Bot. Club 40: 137-181. pis. 4-8.

LILLIE, F. R. 1901. The organization of the egg in Unio based on a study of its
maturation, fertilization and cleavage. Jour. Morph. 17:' 227-292. pis. 24-27.
1912. Studies of fertilization in Nereis. III. The morphology of normal fertiliza-
tion in Nereis. IV. The fertilizing power of portions of the spermatozoon.
Jour. Exp. Zool. 12 r 413-476. pis. 1-11.

1914. Studies of fertilization. VI. The mechanism of fertilization in Arbacia.
Ibid. 16: 523-590.

1919. Problems of Fertilization. Chicago.

LILLIE. R. S. 1908. Momentary elevation of temperature as a means of producing
artificial parthenogenesis in starfish eggs and the conditions of its- action. Jour.
Exp. Zool. 6 : 375-428.

1915. On the conditions of activation of unfertilized starfish eggs under the
influence of high temperatures and fatty acid solution. Biol. Bull. 28. 260-303.

LOEB, J. 1910. Die Hemmung verschiedener Giftwirkungen auf das befruchtete
Seeigelei durch Hemmung der Oxidationen in demselben. Biochem. Zeitschr. 29.

1911. Auf welche Weise rettet die Befruchtung das Leben des Eies? Arch. Entw.
31:658-668.

1912. The Mechanistic Conception of Life. Chicago.

1913. Artificial Parthenogenesis and Fertilization. Chicago.

LOEB, J. and BANCROFT, F. W. 1913. The sex of a parthenogenetic tadpole and frog.

Jour. Exp. Zool. 14: 275-277.
LOEB, J. and WASTENEYS, H. 1910. Warum hemmt Natrium cyanide die Gift-

wirkung einer Chlornatriumlosung fur das Seeigelei? Biochem. Zeitsch. 2.

1911. Sind die Oxidationsvorgange die unabhangige Variable in den Lebenser-
scheinungen? Ibid. 36: 345-356.

1912. Die Oxidationsvorgange im befruchteten und unbefruchteten Seesternei.
Arch. Entw. 36 : 555-557.

LUTHER, A. 1904. Die Eumesostominen. Zeit. Wiss. Zool. 77: 1-273. pis. 9:

figs. 16.
MAcCtTRDY, H. M. 1919. Division, nuclear organization and conjugation in

Arcella vulgaris. Mich. Acad. Sci. Rep. 21: 111-113.
MACORMICK, F. A. 1912. Development of zygospore in Rhizopvs nigricans. Bot.

Gaz. 63: 67-68.
McCLENDON, J. J. 1912. Dynamics of cell division. Artificial parthenogenesis in

Vertebrates. Am. Jour. Physiol. 29: 228-301.
McCuBBiN, W. A. 1910. Development of the Helvellineae. Bot. Gaz. 49: 195-

206. pis. 14- 16
MEVES, FR. 1899. Ueber Struktur und Histogenese der Samenfaden des Meersch-

weinchens. Arch. Mikr. Anat. 64: 329-402. pis. 19-21. figs. 16.

308 INTRODUCTION TO CYTOLOGY

1908. Die Chondriosomen als Trager erblicher Anlagcn. Cytologische Studien
am Hiihnerembryo. Ibid 72: 816-867. pis. 39-42.

1915. Ueber Mitwirkung der Plastosomen bei der Befruchtung des Eies von

Filaria papillosa. Ibid. 37 : 11 12-46. pis. 1-4.
1918. Die Plastosomentheorie der Vererbung. Ibid. 92: II 41-136. figs. 18.

(Bibliography).
MEYER, K. 1911. Untersuchungen iiber die Sporophyt der Lebermoose. 1.

Entwicklungsgeschichte des Sporogons der Corsinia marchantioides. Bull. Soc.

Imp. Moscow 236-286.

MINCHIN, E. A. 1912. An Introduction to the Study of the Protozoa. London.
MIYAKE, K. 1901. The fertilization of Pylhium deBaryanum. Ann. Bot. 15:

653-667. pi. 36.
1910. The development of the gametophytes and embryogeny in Cunninghamia

sinensis. Beih. Bot. Centr. 27: 1-25. pis. 5.
MORGAN, T. H. 1899. The action of salt solutions on the unfertilized and fertilized

eggs of Arbacia, and of other animals. Arch. Entw. 8: 448-539. pis. 7-10.

figs. 21.

1913. Heredity and Sex. New York.
MOTTIER, D. M. 1898. Ueber das Verhalten der Kerne bei der Entwicklung des

Embryosacks und die Vorgange bei der Befruchtung. Jahrb. Wiss. Bot. 31 :

125-158 pis. 2, 3.

1904. Fecundation in Plants. Carnegie Inst. Publ. 15.
MURRILL, W. A. 1900. The development of the archegonium and fertilization in

the hemlock spruce (Tsuga canadensis, Carr.) Ann. Bot. 14: 583-607. pis. 31,

32.
NAGLER, K. 1909. Entwicklungsgeschichtliche Studien iiber Amoben. Arch. f.

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1909. Ueber das selbstandige Bewegungsvermogen der Spermakerne bei einigen
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1910. Naheres iiber die Bildung der Spermakerne bei Lilium Martagon. Ann. Jard.
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NEMEC, B. 1910. Das Problem der Befruchtungsvorgange. Jena.

1912. Ueber die Befruchtung bei Gagea. Bull. Internat. Acad. Sci. Boheme, 1-17.

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NICHOLS, G. E. 1910. A morphological study of Juniperus communis var. depressa.

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nucleus in certain Liliacese. Bot. Gaz. 66: 143-161. pis. 3-5.
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7-9.
RABL, C. 1889. Ueber ZeUtheibuig. Anat. Anz. 4: 21-30. figs. 2.

FERTILIZATION 309

RAMLOW, G. 1914. Beitrage zur Entwicklungsgeschichte der Ascoboleen. Mycol.

Centralbl. 5: 177-198. pis. 1, 2. figs. 20.
ROBERTSON, A. 1904. Studies in the morphology of Torreya californica. 11. The

sexual organs and fertilization. New Phytol. 3: 205-216. pis. 7-9.
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6: 254-325. pis. 14-17.
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Arch. Mikr. Anat. 45: 339-369. pis. 21, 22.
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Oogenesis. Ann. Bot. 10 : 445-477. pis. 22, 23.

1897. Same title. II. Spermatogenesis. Ibid. 11: 187-224. pis. 12, 13.
SAWYER, M. L. 1917. Pollen tube and spermatogenesis in Iris. Bot. Gaz. 64:

159-164. figs. 18.
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505-522. pis. 27-29.
1918. The behavior of the chromosomes in fertilization. Genetics 3. 309-327.

pis. 1, 2.
SCHAUDINN, F. 1896. Ueber die Copulation von Actinophrys sol. Sitzber. Acad.

Wiss. Berlin.

SCHEBEN, L. 1905. Beitrage zur Kenntniss des Spermatozoons von Ascaris megalo-
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SCHIKORRA, W. 1910. Ueber "die Entwicklungsgescnichte von Monascus. Zeitschr.

f . Bot. 1 : 379-410.
SHARP, L. W. 1912. Spermatogenesis in Eguisetum. Bot. Gaz. 64: 89-119.

pis. 7, 8.

1914. Spermatogenesis in Marsilia. Ibid. 68: 419-431. pis. 33, 34.
1920. Spermatogenesis in Blasia. Ibid. 69 : 258-268. pi. 15.
SMITH, B. G. 1919. The individuality of the germ-nuclei during the cleavage of the

egg of Cryptobranchus alleghaniensis. Biol. Bull. 37 . 246-287.
STEIL, W. N. 1918. Method for staining antherozoid of fern. Bot. Gaz. 66 :

562-563. 1 fig.
STEVENS, F. L. 1899. The compound oosphere of Albugo blili. Bot. Gaz. 28:

149-176. pis. 11-15.

1901. Gametogenesis and fertilization in Albugo. Ibid. 32: 77-98. pis. 1-4.
STRASBURGER, E. 1877. Ueber Befruchtung und Zelltheilung. Jen. Zeitschr. 11.
1884. Neue Untersuchungen tiber die Befruchtungsvorgang bei den Phanero-

gamen, als Grundlage fur eine Theorie der Zeugung. Jena.
1892. Schwarmsporen, Gameten, pflanzlichen Spermatozoiden, und das Wesen der

Befruchtung. Histol. Beitr. 4: 49-158. pi. 3.
1897. Kerntheilung und Befruchtung bei Fucus. Jahrb. Wiss. Bot. 30 : 351-374.

pis. 27, 28.

1900. Einige Bemerkungen zur Frage nach der "doppelten Befruchtung" bei
den Angiospermen. Bot. Zeit. 68: 293-316.

1901. Ueber Befruchtung. Ibid. 59: 11, 353-368.

TROW, A. H. 1895. The karyology of Saprolegnia. Ann. Bot. 9: 609-652. pis.

24, 25.
1899. Observations on the biology and cytology of a new variety of Achlya ameri-

cana. Ibid. 13: 131-179. pis. 8-10.
1901. Biology and cytology of Pylhium ultimum, n. sp. Ann. Bot. 15: 269-

312. pis. 15, 16.
1904. On fertilization in the Saprolegniacese. Ibid. 18: 541-569. pis. 34-36.

310 INTRODUCTION TO CYTOLOGY

WAGER, H. 1896. On the structure and reproduction of Cystopus candidus Lev.

Ann. Bot. 10: 295-342. pis. 15,16. See also pp. 89-91.
1899. The sexuality of fungi. Ibid. 13: 575-597.
WALDEYER, W. 1888. Ueber Karyokinese und ihre Beziehung zu den Befrucht-

ungsvorgangen. Arch. Mikr. Anat. 32: 1-122. figs. 14. (Engl. transl. in

Quar. Jour. Micr. Sci. 30: 159-281. pi. 14. 1889.)
WALTON, A. C. 1918. The oogenesis and early embryology of Ascaris canis Werner

Jour. Morph. 30: 527-604. pis. 9. fig. 1.
WARBURG, O. 1908. Beobachtungen tiber die Oxidationsprozesse im Seeigelei.

Zeitschr. Physiol. Chem. 57. .

1910. Ueber die Oxidationen im lebenden Zellen nach Versuchen am Seeigelei.
Ibid. 66.

1911. Untersuchungen tiber die Oxidationsprozesse im Zellen. Mtinchener Med.
Wochenschr. 57.

1914. Beitrage zur Physiologie der Zelle, inbesondere tiber die Oxidationsgesch-

windigkeit in Zellen. Ergeb. d. Physiol. 14.
WEBBER, H. J. 1900. Xenia, or the immediate effect of pollen in maize. U. S.

Dept. Agr., Div. Veg. Path, and Physiol., Bull. 22: pis. 4.
1901. Spermatogenesis and fecundation in Zamia. U. S. Dept. Agr. Bur. Pit.

Ind. Bull. 2. pp. 100. pis. 7.
\VELSFORD, E. J. 1907. Fertilization in Ascobolus furfuraceus. New Phytol. 6:

156.
1914. The genesis of the male nuclei in Lilium. Ann. Bot. 28: 265-270. pis. 16,

17.

WENIGER, W. 1918. Fertilization in Lilium. Bot. Gaz. 66: 259-268. pis. 11-13.
WHEELER, W. M. 1895. The behavior of the centrosomes in the fertilized egg of

Myzostoma glabrum Leukart. Jour. Morph. 10: 305-311. figs. 10.
1897. The maturation, fecundation and early cleavage of Myzostoma glabrum

Leukart. Arch. d. Biol. 15: 1-77. pis. 1-3.
WILDMAN, E. E. 1913. The spermatogenesis of Ascaris megalocephaia with special

reference to the two cytoplasmic inclusions, the refractive body and the "mito-
chondria": their origin, nature and role in fertilization. Jour. Morph. 24:

421-157. pis. 3.
WILSON, E. B. 1900. The Cell in Development and Inheritance. 2d ed.

1901. Experimental studies in cytology. I. A cytological study of artificial

parthenogenesis in sea urchin eggs. Arch. Entw. 12: 529-596. pis. 11-17.

figs. 12.
WINGE, O. 1914. The pollination and fertilization process in Humulus lupulus L.

and H. Japonicus. Comp. Rend. Trav. Lab. Carlsberg 11.
WOODBURN, W. L. 1920. Preliminary notes on the embryology of Reboulia hemis-

phcerica. Bull. Torr. Bot. Club. 46: 461-464. pi. 19.

WOYCICKI, Z. 1899. (On fertilization in Coniferse.) pp. 57. pis. 2. (Russian.)
YAMANOTTCHI, S. 1906. The life history of Polysiphonia violacea. Bofc. Gaz. 42:

401-449. pis. 19-28.
1908. Spermatogenesis, oogenesis, and fertilization in Nephrodium. Ibid. 45:

145-175. pis. 6-8.

CHAPTER XIII

APOGAMY, APOSPORY, AND PARTHENOGENESIS
APOGAMY AND APOSPORY

The life cycle in all bryophytes and vascular plants is characterized
by a regular alternation of two well marked phases or generations: the
gametophyte, which arises from the spore and produces gametes; and the
sporophyte, which arises from the fusion product of two gametes and
produces spores. In such a normal life cycle the number of chromosomes
in the nuclei is doubled at the union of the gametes and reduced to the
original number at sporogenesis ; the gametophyte is therefore the haploid
generation and the sporophyte the diploid generation, their limits being
marked by the two cytological crises, fertilization and reduction. Such
an alternation of haploid and diploid phases has been discovered in the
life cycles of many alga? and fungi also, so that the general conception
of alternation of generations has been extended to these lower groups.
This, however, is not the place for a discussion of the homologies implied.
It should be added that gametophyte and sporophyte may arise not only
from each other, but either generation may also multiply by vegetative
means.

Many instances in which the above typical life cycle is departed from,
and in which the correlation between the alternation of two generations
and periodic changes in chromosome number is broken, are now known,
the conspicuous examples being found among the ferns and certain
angiosperms. The very convenient classification of such abnormalities
drawn up by Vines (1911) .is given as the basis for the present portion
of the chapter. All dates and the matter included within square brackets
have been added by the present author.

"In the first place, the sporophyte may be developed either after an
abnormal sexual act, or without any preceding sexual act at all, a con-
dition known as apogamy. In the second, the gametophyte may be
developed otherwise than from a post-meiotic spore, a condition known
as apospory.1

1 [Apogamy in ferns was discovered by Farlow in 1874. Apospory was discovered
in mosses by Pringsheim in 1876 and in ferns by Druery in 1884. General discussions
of these phenomena are given by Winkler (1908) and Strasburger (19096).]

311

312

INTRODUCTION TO CYTOLOGY

Apogamy. — The cases to be considered under this head may be
arranged in two groups:

1. Pseudapogamy: sexual act abnormal. — The following abnormalities
have been observed:

(a) Fusion of two female organs: observed (Christman 1905) in
certain Uredinese (Cceoma nitens, Phragmidium speciosum,
Uromyces Caladii) where adjacent archicarps fuse: male cells
(spermatia) are present but functionless.

(6) Fusion between nuclei of the same female organ: observed in the
ascogonium of certain ascomycetes, Humaria granulata (Black-

FIG. 124.- — Apogamy in ferns.

A, nuclear migration in gametophyte cells of Lastrcea pseudo-mas var. polydactyla.
X 500. (After Farmer and Digby, 1907.) B, section through gametophyte, showing
young sporophytic tissue (s) "engrafted" into surrounding gametophy tic tissue (g).
(After Farmer and Digby.) C, sporophyte arising apogamously from gametophyte in
Pteris cretica: bl, first leaf; v, stem apex; w, root. (After de Bary.)

man 1906), where there is no male organ; Lachnea stercorea
(Fraser 1907), where the male organ (pollinodium) is present
but apparently functionless. [A similar condition has been
reported in Ascobolus furfuraceus (Welsford 1907), Aspergillus
repens (Dale 1909), and Ascophanus carneus (Cutting 1909).]
(c) Fusion of a female organ with an adjacent tissue-cell: observed
(Blackman 19046) [Blackman and Fraser 1906] in the archicarp
of some Uredineae (Phragmidium violaceum, Uromyces Poce,
Puccinia Poarum) : male cells (spermatia) present but function-
less.

APOGAMY, APOSPORY, AND PARTHENOGENESIS

313

(d) There is no female organ: fusion takes place between two adjacent
tissue-cells of the gametophyte ; the sporophyte is developed from
diploid cells [''grafted tissue"] thus produced, but there is no
proper zygote as there is in a, b, and c: observed (Farmer [and
Digby] 1907) in the prothallium of certain ferns (Lastrcea pseudo-
mas, var. polydactyla} [Fig. 124, A] : male organs (and sometimes
female) present but functionless. Another such case is that
of Humaria rutilans (ascomycete), in which nuclear fusion

••• v--«*v*x /wi. & * • •"

'•

i >Mm** Jf- •••

FIG. 125.

A, cell fusion in the sporangium of Aspidium falcatum. X 1950. (After R. F.Allen
1911.) B, incomplete nuclear division in sporangium of Nephrodium hirtipes. X 1250.
(After Steil, 1919.) C, apogamy and sporophytic budding in the embryo sac of Alchemilla
pastoralis: egg developing apogamously below; cell of nucellus forming an embryo above;
two polar nuclei and one synergid nucleus at center. (After Murbeck, 1902.)

has been observed (Fraser 1908) in hyphse of the hypothecium :
the asci are developed from these hyphae, and in them meiosis
takes place; there are no sexual organs. [A similar condition
has been reported in Helvella crispa (Carruthers 1911) and
Polystigma rubrum (Blackman and Welsford 1912). It has
already been pointed out (p. 291) that many students of the
ascomycetes deny the existence of a nuclear fusion in the
archicarp or vegetative cells, holding rather that the only

314 INTRODUCTION TO CYTOLOGY

fusion in the life cycle is that observed in the ascus, and that
this fusion is the real sexual act.]

[(e) Fusion of two haploid sporocytes: In Aspidium falcatum (R. F.
Allen 1911) a haploid sporophyte arises by vegetative apogamy
from a haploid gametophyte. In the sporangium the 16
haploid sporocytes fuse in pairs, producing eight diploid cells
(Fig. 125, A}. In these cells reduction occurs, 32 haploid
spores resulting.]

2. Eu-apogamy: no kind of sexual act.
(a)" The gametophyte is haploid:

(a) The sporophyte is developed from the unfertilized haploid
oosphere : no such case of true parthenogenesis has yet been
observed. [Kusano (1915) has observed the division of
the haploid nucleus of an unfertilized egg in a few excep-
tional cases in the orchid, Gastrodia data. Parthenogeneti c
development proceeds no further. The unfertilized egg of
Fucus has been made to begin development by artificial
means (Overton 1913), but the cytological facts are not
known here. Motile gametes of certain other algae have
been observed to develop without conjugation, as in
Ectocarpus tomentosus (Kylin 1918).]

(/3) The sporophyte is developed vegetatively from the gameto-
phyte and is haploid : observed in the prothallia of certain
ferns, Lastrcea pseudo-mas, var. cristata-apospora (Farmer
and Digby 1907), and Nephrodium molle (Yamanouchi
1908). [In the gametophytes of Nephrodium molle, which
has antheridia but no functional archegonia, Yamanouchi
found no nuclear migrations such as Farmer described
in Lastrcea (see Id} ; but there was haploid grafted tissue,
from which a haploid sporophyte developed. In Nephro-
dium hirtipes (Steil 1919) a haploid sporophyte arises by
vegetative apogamy from a haploid gametophyte. When
there are eight sporogenous cells in the sporangium there
is an incomplete nuclear and cell division (Fig. 125, B),
each nucleus coming to have the diploid number of chromo-
somes. These eight diploid cells function as sporocytes
and produce 32 haploid spores. Steil at first (1915)
adopted Allen's interpretation (le) for his material, but
later decided that the phenomenon observed was one of in-
complete division, and not one of fusion. In this case,
as in Aspidium falcatum, apogamy is offset not by apospory
but by an abnormal course of events in the sporangium.
In Aspidium falcatum the sporophyte arises as in the
examples mentioned in this paragraph, but because of

APOGAMY, APOSPORY, AND PARTHENOGENESIS 315

the presence of a cell and nuclear fusion it is classified under
le.]
(6) The gametophyte is diploid (see under Apospory):

(a) The sporophyte is developed from the diploid oosphere:
observed in some Pteridophyta, viz. certain ferns (Farmer
1907), Athyrium Filix-fcemina, var. clarissima, Scolopend-
rium vulgare, var. crispum-Drummondce, and Marsilia
(Strasburger 1907); also in some Phanerogams, viz.,
Composite (Taraxacum, Murbeck 1904; Antennaria alpina,
Juel 1898, 1900; sp. of Hieracium, Rosenberg 1906):
Rosacese (Eu-Alchemilla sp., Murbeck 1901, 1904, Stras-
burger 1905 [Fig. 125, C]): Ranunculacese (Thalictrum
purpurascens, Overton 1902). [Also in the lily, Atamosco
(Pace 1913), and Burmannia (Ernst 1909). Besides this
form of apogamy ("ooapogamy" or "generative apo-
gamy") Antennaria may also develop embryos from
diploid synergids ("vegetative apogamy") and from cells
of the nucellus ("sporophytic buddirg"). A similar
variety of embryo origins is found in certain other angio-
sperms. In many cases the chromosome number in
apogamous species is about twice as large as that of nearly
related forms reproducing sexually (Rosenberg 1909).]
(/3) The sporophyte is developed vegetatively from the gameto-
phyte: observed (Farmer [and Digby] 1907) in the fern
Athyrium Filix-foemina, var. clarissima.

In all cases enumerated under Eu-apogamy, apogamy is

associated with some form of apospory except Nephrodium

molle, full details of which have not yet been published.

[It is possible that a behavior like that in Aspidium

falcatum (le) or in Nephrodium hirtipes (2a/3) may occur

in Nephrodium molle.] Many other ferns are known to be

apogamous, but they are not included here because the

details of their nuclear structure have not been investigated.

Apospory. — The known modes of apospory may be arranged as

follows :

1. Pseudapospory: a spore is formed but without meiosis, so that it is diploid
— observed only in heterosporous plants, viz. certain species
of Marsilia (e.g. Marsilia Drummondii) where the megaspore has a
diploid nucleus (32 chromosomes) and the resulting prothallium and
female organs are also diploid (Strasburger 1907); and in various
Phanerogams, some Compositse (Taraxacum and Antennaria alpina,
Juel 1898, 1900, 1904), some Rosacese (Eu-Alchemilla, Strasburger
1905), and occasionally in Thalictrum purpurascens (Overton 1902),
where the megaspore ([and] embryo-sac) is diploid; in some species

316 INTRODUCTION TO CYTOLOGY

of Hieracium it has been found (Rosenberg 1906) that adventitious
diploid embryo-sacs are developed in the nucellus: these plants
are also apogamous. [In Marsilia Drummondii, which Shaw
(1897) and Nathansohn (1909) had shown to be apogamous, Stras-
burger (1907) found that, although normal reduction occurs in
some of the megasporocytes, giving spores with 16 chromosomes,
other megasporocytes undergo two divisions neither of which is
reductional: the first division is homceotypic in character and the
second is an additional vegetative mitosis without a homologue

FIG. 126.

A, gametophyte with antheridium (anth.) and rhizoids (r) arising aposporously from
tissue of sorus in Polystichum angulare var. pulcherrimum; sp, sporangia. X 70. (After
Bower.) B, gametophyte with archegonia arising from tip of pinnule in Polystichum.
X 10. (After Bower.)

in the normal cases. The resulting spores are therefore diploid,
and ooapogamy follows.]

2 Eu-apospory: no spore is formed — of this there are two varieties:
(a) With meiosis: this occurs in some Thallophyta which form no
spores; the sporophyte of the Fucacese bears no spores, con-
sequently meiosis takes place in the developing sexual organs.
The Conjugate Green Algae also have no spores, meiosis
taking place in the germinating zygospore which develops
directly into the sexual plant.

APOGAMY, APOSPORY, AND PARTHENOGENESIS 317

(b) Without meiosis: the gametophyte is developed upon the sporo-
phyte by budding; that is, spore-reproduction is replaced by a
vegetative process: for instance, in mosses it has been found
possible to induce the development of protonema, the first stage
of the gametophyte, from tissue cells of the sporogonium:
[In this way fil. and £m. Marchal (1909, 1912) were able to
produce in Mnium, Bryum, Phascum, and Amblystegium
diploid gametophytes; these in turn produced tetraploid
sporophytes which bore diploid spores. In one case (Ambly-
stegium) a tetraploid gametophyte was regenerated from
cells of the tetraploid sporophyte.] Similarly, in certain ferns
(varieties of Athyrium Filix-foemina, Scolopendrium vulgare,
Lastrcea pseudo-mas, Polystichum angulare, and in the species
Pteris aquilina and Asplenium dimorphum), the gametophyte
(prpthallium) is developed by budding of the leaf of the sporo-
phyte [commonly from the margin of the leaf or from the tissue
of the sorus (Fig. 126)], and in some of these cases it has been
ascertained that the gametophyte so developed has the same
number (2x) of chromosomes in its nuclei as the sporophyte
that bears it — that is, it is diploid.

Apospory has been found to be associated frequently with
apogamy [in the life cycle]; in fact, in the absence of meiosis,
this association would appear to be inevitable."

PARTHENOGENESIS IN ANIMALS1

The natural development of an egg without having been fertilized by
a male gamete is a phenomenon which is apparently of much more
frequent occurrence in animals than in plants. The best known examples
are found among the rotifers, crustaceans, and insects, parthenogenesis
being the regular mode of reproduction in some species. Other modes
also usually occur in such organisms under certain conditions or after a
certain number of generations. Parthenogenesis is reported in some
protozoa (Plasmodium vivax, Schaudinn 1902), where the macrogamete,
after certain nuclear changes, continues the life cycle without fusing
with a microgamete. Moreover, as has already been described in the
preceding chapter, parthenogenesis may be artificially induced in the
eggs of other animal groups, notably echinoderms, mollusks, and amphi-
bians, and around this fact centers much of the significant work of modern
experimental biology. In commenting upon parthenogenetic develop-
ment Minchin (1912, p. 137) points out that "... the gamete which has
this power is always the female ; but this limitation receives an explanation
from the extreme reduction of the body of the male gamete and its

1 The cytological results of researches on maturation and development in cases of
parthenogenesis have recently been summarized by Paula Hertwig (1920).

318 INTRODUCTION TO CYTOLOGY

feeble trophic powers, rendering it quite unfitted for independent repro-
duction, rather than from any inherent difference between the two
sexes in relation to reproductive activity."

Many normally parthenogenetic animal eggs are known to have the
diploid chromosome number as the result of a failure of reduction, a
condition paralleling that known as ooapogamy in plants. On the
contrary, there are some which, unlike any known vascular plant, are
haploid, reduction having taken place in the normal fashion. Partheno-
genesis is often associated with certain irregularities in the behavior
of the polar bodies, as will be noted in the following descriptions of some
well known examples. In the majority of recorded cases the partheno-
genetic egg produces but one polar body; in some, however, two are
formed as in all zygogenetic eggs (those developing after having been
fertilized).

It was long ago noticed by Blochmann (1888; see Wilson 1900, pp.
281-4) that in Aphis both zygogenetic and parthenogenetic eggs are
produced; the former produce the usual two polar bodies while the
latter have but one. It was also seen that the polar bodies are not
budded off as separate cells, but remain within the membrane of the
egg. Weismann (1886, 1887), working on rotifers, concluded that the
second polar body has something to do with parthenogenetic develop-
ment; and Boveri (1887d, 1890), who had seen the chromosomes of the
second polar body transform themselves into a nucleus in the egg of
A scam, made the suggestion that this second polar body might unite
with the egg nucleus and so initiate development. Brauer (1894) an-
nounced that this is precisely what occurs in Artemia, a phyllopod
crustacean. In this organism two types of parthenogenesis are found.
In some cases the nucleus of the second polar body, with 84 chromosomes,
actually does unite with the egg nucleus, likewise with 84, causing
"fertilization" and the resulting development of an individual with the
diploid number (168) of chromosomes. In other cases only one polar
body is produced, but reduction is accomplished in the division forming
it, and the resulting haploid egg develops parthenogenetically into an
individual with only 84 chromosomes.

In Phylloxera carycecaulis (Morgan 1906, 1908, 1909, 1910, 1915)
only one polar body appears, but here no reduction occurs: the diploid
egg develops parthenogenetically. In Nematus lacteus (Doncaster 1906)
two polar bodies are produced, but reduction fails and the diploid egg
proceeds to develop as in Phylloxera.

It has long been known that the eggs of the honey bee, Apis mellifica,
will develop either zygogenetically into females or parthenogenetically
into males. It has been shown in both cases that there are two polar
bodies (Blochmann) and that a normal reduction in the number of
chromosomes occurs (Nachtsheim 1912, 1913). The fertilized eggs

APOGAMY, APOSPORY, AND PARTHENOGENESIS 319

develop into workers or into queens with the diploid number (32) of
chromosomes; those not fertilized develop into drones with the haploid
number (16). (At the time of spermatogenesis in the drone no further
reduction in chromosome number occurs: the spermatozoa retain the
number present in the body cells (16).)

In the gall-fly, Neuroterus lenticularis, Doncaster (1910-1911) has
shown that there are two classes of parthenogenetic females. The egg of
the first class gives off no polar bodies, retains the diploid number (20)
of chromosomes, and develops parthenogenetically into a sexual female.
The egg of the second class gives off two polar bodies, retains the reduced
number (10) of chromosomes, and develops parthenogenetically into
a male. (The offspring of the sexual females and males constitute the
next generation of parthenogenetic females.)

There are thus several organisms in which both zygogenetic and
parthenogenetic eggs are produced. In some of them, such as the bee,
in which the same egg can develop in either way, the two classes of eggs
show no morphological differences. In other forms, such as a species of
Melanoxanthus (a plant louse) and Sida crystallina (crustacean), they
may differ considerably. The parthenogenetic egg, for example, may
contain much less yolk than the zygogenetic one : it is less highly differen-
tiated, and "still retains the capacity to initiate dedifferentiation and
reconstitution independently of union with a male gamete. In this
respect it resembles the less highly specialized cells of other tissues
rather than the gametes" (Child 1915, p. 408).

It has recently been shown that frogs which have been induced to
develop parthenogenetically from punctured eggs (Bataillon's method) are
of both sexes (Loeb 1921). The chromosome number in the females has
not been determined, but both Parmenter (1920) and Goldschmidt (1920)
report the diploid number in males so derived. The origin of this diploid
condition has not been satisfactorily explained. Parmenter suggests
that it may be due to the retention of one polar body, or to a premature
division of the chromosomes without cytokinesis just before the first
cleavage. This promises to be an interesting case in connection with
the mechanism of sex-determination.

Conclusion. — To review the various theories which have been advanced
to account for the origin of parthenogenesis, its relation to other forms of
reproduction, and its significance in the life history, is a task which lies be-
yond the scope of the present work: it has been our purpose only to indicate
some of the outstanding cytological facts in certain conspicuous instances
of the phenomenon. The cytological features have been accurately
ascertained in only a very few cases, and these show little agreement.
Furthermore, it is in artificially induced rather than in natural partheno-
genesis that the physiological conditions are best known. In view of
these facts it appears more than probable that many more cytological

320 INTRODUCTION TO CYTOLOGY

and physico-chemical data must be secured before any theory ad-
equately harmonizing all the observed phenomena of parthenogenesis can
be formulated.

Bibliography 13

Apogamy; Apospory; Parthenogenesis

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CHILD, C. M. 1915. Senescence and Rejuvenescence. Chicago.
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Jour. Micr. Sci. 51: 101-114. pi. 8.
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21

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CHAPTER XIV
THE ROLE OF THE CELL ORGANS IN HEREDITY

The chief interest of cytology at the present time probably lies in the
relation which it bears to the subject of heredity. From the time when
the problems of cell research first began to take definite shape, especially
since a connection between the activities of the cell and the phenomena
of inheritance was suggested, the efforts of most cytologists have con-
tributed directly or indirectly to the solution of two great and closely
interrelated problems of biology: the problem of ontogerietic develop-
ment and the problem of heredity. The aid which cytology has afforded
in these respects has been invaluable. Not only has it been able to
discover a large number of the significant facts of individual development,
or ontogeny, but it has also thrown a flood of light upon many obscure
matters in the field of heredity, and has so come to be an important factor
in the study of phylogeny and evolution.

The Law of Genetic Continuity. — "The most fundamental contribu-
tion of cell-research to the theory of heredity," says Wilson (1909), "is
the law of genetic continuity by cell-division. Cells arise only by the
division of preexisting cells ... In each generation the germinal stuff
runs through the same series of transformations; hence that reappearance
of the same traits in successive generations that we call heredity."

It is by the light of the above law that we are enabled to see some-
thing of the nature of the material continuity which exists between suc-
cessive stages of the ontogenetic development, and also between success-
ive generations. It is to be remembered, in the first place, that all the
cells of the adult multicellular organism are derived by repeated division
from the single cell (ordinarily a zygote or a spore) with which develop-
ment starts, so that the causes of events occurring at any particular stage
are to be sought largely in the reactions of cells at earlier stages; and, in
the second place, that the material link connecting two successive gen-
erations is a single organized cell, usually a gamete or a spore, which
means that the heritage of a long ancestry is in some way represented in
this single cell and its capabilities. "The conception that there is an
unbroken continuity of germinal substance between all living organisms,
and that the egg and the sperm are endowed with an inherited organiza-
tion of great complexity, has become the basis for all current theories of
heredity and development" (Locy, 1915, p. 224).

323

324 INTRODUCTION TO CYTOLOGY

Cytological studies have therefore centered mainly about the general
organization of the egg (chiefly that of animals) as related to the character
of the organism arising from it (the problem of development), and about
the roles played by the various cell organs of the gamete in the transmis-
sion of heritable characteristics from one generation to the next (the
problem of heredity). The character of the principal modern theory of
heredity to which these studies have led is due in no small measure to the
influence of a number of earlier hypotheses, such as those of Darwin and
deVries, and especially that of Weismann. These hypotheses will be
reviewed in Chapter XVIII, where their relation to the modern cytolog-
ical interpretation of heredity, set forth in this and the following three
chapters, will be discussed.

It is obvious that an account of the physical basis of heredity would
require for completeness not only a description of the structural changes
by which visible materials are transmitted and distributed during game-
togenesis, fertilization, and development; but also a review of many phy-
siological processes which accompany these changes, and through which
many characters are brought to expression. In these chapters attention
will be limited largely to the structural aspects of the problem.
Among the physiological changes those occurring at the time of fertiliza-
tion are best known, and have already been discussed in Chapter XII.

The R6le of the Nucleus.— It was Ernst Haeckel (1866) who first
advanced the hypothesis that "the nucleus of the cell is the principal
organ of inheritance." Cytological evidence in support of this view,
announced by Haeckel as a speculation, was brought forward by O.
Hertwig (1875 etc.), Strasburger (1878, 1884), van Beneden (1883 etc.),
and a number of other investigators, who described the behavior of the
nucleus in the various stages of the life cycle, particularly in somatic
cell-division, maturation, and fertilization. Two of these workers, O.
Hertwig and Strasburger, who had discovered the fusion of the gamete
nuclei at the time of fertilization in animals and plants respectively,
definitely announced the theory, now supported by a considerable body
of observational and experimental evidence, that the nucleus is the
"vehicle of heredity." They held that hereditary transmission is
through the nuclei of the gametes, and that the chromatin is the special
inheritance material, or "idioplasm," about which there had been so
much speculation. This view was at once widely adopted by biologists.

The efforts' of many cytologists were now directed toward the further
elucidation and verification of this nuclear hypothesis of heredity, and
many observations and experiments apparently demonstrated its essen-
tial correctness. It was noted that, so far as could be discerned, the
spermatozoon in many cases brings nothing but nuclear material into
the egg, so that hereditary transmission from the male parent must be
through the nucleus alone. A similar condition was later reported in

THE ROLE OF THE CELL ORGANS IN HEREDITY

325

plants, Guignard, Nawaschin (1910), and Welsford (1914) pointing out
that in Lilium only the male nucleus enters the egg, its accompanying
cytoplasm being rubbed off and left behind. (See p. 299.) Certain
ingenious experiments of Boveri (1889, 1895; also 1909 and 1918) led
to the same conclusion regarding the nucleus. Boveri induced the fer-
tilization of enucleated fragments of Sphcerechinus eggs (a phenomenon

FIG. 127.

A, egg of Sphcerechinus granularis undergoing artificially induced cleavage mitosis;
spermatozoon of Strongylocentrotus lividus has entered and taken the form of a chromosome
group. B, cytokinesis beginning; one blastomere will have a purely maternal nucleus,
and the other a hybrid nucleus. (Diagrammed after Hcrbst, 1909.)

FIG. 127 bis. — Diagram showing the irregular distribution of the chromosomes by a
quadripolar mitotic figure. (After Boveri.)

known as merogony) by spermatozoa of Echinus, and obtained larvae
which were purely paternal in character. From this it was argued that
it is the sperm nucleus alone, and not the egg cytoplasm, that transmits
the hereditary characters from one generation to the next in this case.
Other experiments of a similar nature, however, turned out differently,
as will presently be noted. Certain echinoderm hybrids, furthermore,
show paternal larval characters even when the egg nucleus has not been
removed.

326 INTRODUCTION TO CYTOLOGY

A strong piece of evidence supporting Boveri's conclusion was fur-
nished by Herbst (1909). By treating eggs of Sphcerechinus with valeri-
anic acid Herbst caused them to undergo cleavage artificially. While
the cleavage mitosis was in progress a spermatozoon of Strongylocentrotus
was allowed to enter the egg, where it at once gave rise to its group of
chromosomes (Fig. 127). These, however, arriving too late to join
regularly in the mitosis, were incorporated in neither of the daughter
nuclei of the first cleavage : they resumed the form of a nucleus, and this
was included in one of the blastomeres. This blastomere therefore
contained two nuclei, one maternal and one paternal, which combined
during subsequent stages, whereas the other blastomere had a maternal
nucleus only. Herbst regarded such nuclear behavior as responsible for
the frequently found larvae which are hybrid in character on one side
and purely maternal on the other. This experiment has been held to
show not only that it is the nucleus of the spermatozoon which brings in
the paternal characters, but also that it is the chromosomes alone that are
responsible. It is assumed, though perhaps without sufficient evidence,
that the other nuclear materials (karyolymph etc.) which may be present,
as well as any cytoplasmic elements, have opportunity to mix generally
with the egg cytoplasm, since the membrane of the male nucleus breaks
down and leaves the chromosomes lying free before the egg divides into
the two blastomeres. The paternal characters, however, appear only
where the chromosomes come to be located — that is, in the cells compos-
ing one-half of the organism. In his work on multipolar mitoses in di-
spermic eggs (Fig. 127 bis; see also p. 163) Boveri (1902, 1907) was able
to show further that abnormal chromosome distribution is associated
with abnormalities in development in a very definite way; and that if
isolated blastomeres resulting from such abnormal divisions be made to
develop 'independently, completely normal larvae result only where there
is statistical reason to believe that a full complement of the qualitatively
different chromosomes is present.

The nuclear theory had its opponents from the beginning. Verworn,
Waldeyer, Rauber, and other early investigators held that the cytoplasm
as well as the nucleus must be concerned in the hereditary process, since
the spermatozoon in many cases does bring cytoplasm into the egg, and
also because neither nucleus nor cytoplasm can function independently
of the other. This view received support in certain experiments which
seemed to discount the power of the nucleus in controlling heredity.
Loeb (1903) found that when a sea urchin egg was fertilized by a starfish
sperm the resulting larva possessed purely maternal characters, the
sperm nucleus exerting no visible hereditary effect. The same thing was
noted by Godlewski (1906) in crosses between sea urchins and crinoids.
Godlewski made the further significant observation that when enucleated
egg fragments of Parechinus (sea urchin) were fertilized by sperm of

THE ROLE OF THE CELL ORGANS IN HEREDITY 327

Antedon (crinoid) the larvae so produced, contrary to Boveri's results,
were maternal in character: they were like the mother, which had pre-
sumably contributed cytoplasm only, and not like the father, which had
furnished the nucleus. Fertilization by a spermatozoon had here pro-
duced a developmental stimulus but no amphimixis (the combining of
hereditary lines), so far as could be judged from the appearance of the
larva?. Even if the male cytoplasm were admitted to have no heredi-
tary role, it nevertheless seemed that the cytoplasm of the egg was clearly
so concerned.

In his work on sea urchin hybrids Baltzer (1910) was able to show
why it is tha.t some such larvae are maternal in character while others
have the characters of both parents. When an egg of Strongylocentrotus
fertilized by a spermatozoon of Sphcerechinus undergoes its first cleavage
division the paternal chromosomes behave irregularly ; they fail to become
incorporated in the daughter nuclei and are lost. Those individuals
which develop far enough show maternal skeletal characters. In the
reciprocal cross, on the contrary, all of the chromosomes behave normally
and the resulting larvae are truly hybrid in character. Thus in the first
cross, in which the paternal chromosomes are lost, the spermatozoon
furnishes only a developmental stimulus and has no appreciable effect
on the character of the new individual; whereas in the second cross, in
which the paternal chromosomes are included in the blastomere nuclei,
the spermatozoon not only furnishes the developmental stimulus but
also contributes paternal characters to the new individual. This is
particularly convincing evidence in favor of the view that the chromo-
somes are in some way responsible for the development of parental
characters in the offspring.

In a posthumous paper Boveri (1918) reported an additional observa-
tion which, he believed, goes far toward explaining the conflicting results
of different investigators. He found that egg fragments, and even whole
eggs, may often have chromatin in a form that easily escapes observation,
but which can exert its usual influence on development. In accordance
with his earlier observations, enucleate egg fragments of Sphoerechinus
fertilized by spermatozoa of Strongylocentrotus may develop into purely
paternal larvae. Most of them, however, are intermediate in charac-
ter, resembling the maternal parent also in certain features. Having
previously (1895, 1905) demonstrated that the size of the nuclei in
merogonic larvae is proportional to the number of chromosomes they
contain (see Chapter IV), Boveri was able to show that the nuclei of the
intermediate larvae are diploid rather than haploid, so that it is clear that
the supposedly enucleate fragments in such cases must have contained
chromosomes. It is probable, Boveri believed, that the maternal larvae
obtained by Godlewski may be accounted for in a similar fashion.

It was pointed out by Strasburger (1908), who had come to believe in

328 INTRODUCTION TO CYTOLOGY

the complete monopoly of the nucleus in the transmission of hereditary
characteristics, that the maternal character of Godlewski's larvae could
be explained on the assumption that the early developmental stages do
not require the expression of the hereditary capabilities of the nucleus,
but are dependent more directly upon mechanical causes. Boveri (1903,
1914), as a result of his hybridization experiments, strongly emphasized
the view that the spermatozoon has an influence upon all of the larval char-
acters; but he pointed out that the larval stages by themselves are not
sufficient grounds upon which to establish complete conclusions regarding
the respective roles of nucleus and cytoplasm, since the general course
of the early developmental stages in such organisms is immediately
dependent to a very large extent upon the general organization of the egg.

This brings us to a brief consideration of the "promorphology" of
the highly organized animal egg, and of the relation which exists between
this organization and the character of the organism developing from it.
Of the large amount of work done in this field only a hint can be given
here.

The Promorphology of the Ovum. — There arose very early two views
regarding the organization of the egg which recall the older theories of
preformation and epigenesis (Chapter I). According to one, most fully
expressed in W. His's Theory of Germinal Localization (1874; see Wilson
1900, p. 397), the embryo is prelocalized in the general cytoplasm of the
egg — not preformed in the old sense of Bonnet, but having its various
parts represented by substances with definite relative positions. This
view found support in those cases in which a single isolated blastomere of
the two-celled stage develops into a half-larva instead of a complete
smaller larva (Roux on the frog, 1888; Crampton on the marine gastropod,
Ilyanassa, 1896) ; and especially in Beroe, a ctenophore, which produces an
incomplete larva even if a portion of the unsegmented egg be removed
(Driesch and Morgan, 1895).

Opposed to the above view was that which held the egg to be isotropic
and without any predetermination of embryonic parts. Certain well
known experiments appeared to bear out this conclusion. It was found
in the frog (Pfluger 1884; Roux 1885), the sea urchin (Driesch 1892), an
annelid (Wilson 1892), and A scarfs (Boveri 1910) that very abnormal types
of cleavage can be artificially induced, but that normal larvae nevertheless
result. A number of cases were also described in which complete embryos
arose from single isolated blastomeres of the two-celled stage (Fundulus,
Morgan 1895; and other forms), or even from those of the sixteen-celled
stage (Clytia, Zoja 1895). Had there been any prelocalization of parts
in the egg it is difficult to see how normal or complete embryos could
have arisen in such abnormal ways as these.

It appears that the eggs of different animal species vary greatly
in the degree and fixity of their internal differentiation. In some cases

THE ROLE OF THE CELL ORGANS IN HEREDITY 329

the egg is virtually isotropic, and through several succeeding cell gene-
rations the blastomeres are equipotential, that is, equally capable of
developing into any part of the body or even into the whole of it. Thus
the embryonic parts, and hence many of the individual's characters, are
not definitely marked out until a comparatively late stage. On the
contrary, there are forms in which the axis of polarity and certain funda-
mental embryonic parts are roughly delimited in the egg cytoplasm in
such a way that an alteration in the relative positions of the egg materials
brings about a corresponding alteration in the character of the resulting
individual. As an illustration of such internal differentiation may be
taken the case of Styela, an ascidian, described by Conklin (1915). In
the egg of Styela there are four or five distinct kinds of plasma arranged
in a definite order and distributed in a regular manner as cleavage pro-
ceeds, each kind eventually giving rise to a certain portion of the embryo.

CTENOFHOflE TURBELLAKIAN ECHINOBERM A5CIBIAM

FIG. 128. — Eggs of various animals, showing the patterns assumed by the material s
which give rise to the various body regions. In the first three the egg has undergone
division, and the plasmas becoming ectoderm, mesoderm, and endoderm are represented
in clear white, cross-hatching, and parallel ruling respectively. In the fourth egg two
divisions have occurred, and several definitely arranged substances are distinguishable
(After Conklin, 1915.)

Substancss which are yellow, gray, slate-blue, and colorless give rise
respectively to muscle and mesoderm, nervous system and notocord,
endoderm, and ectoderm. " Thus within a few minutes after the fertiliza-
rion of the egg, and before or immediately after the first cleavage, the
anterior and posterior, dorsal and ventral, right and left poles are clearly
distinguishable, and the substances which will give rise to ectoderm,
endoderm, mesoderm, muscles, notocord and nervous system are plainly
visible in their characteristic positions" (Conklin 1915, p. 118). If
such eggs are placed in a centrifuge the various substances may be
made to assume an entirely abnormal stratified arrangement, which
in turn "may lead to a marked dislocation of organs; the animal may be
turned inside out, having the endoderm on the outside and its skin and
ectoderm on the inside, etc." (p. 321). Such a behavior emphasizes
the determinative character of the cytoplasmic pattern clearly present
in many eggs. It has further been noted that the eggs of various animal
phyla are characterized by distinct patterns in the arrangmeent of their
visibly different materials (Fig. 128). "The polarity, symmetry and

330 INTRODUCTION TO CYTOLOGY

pattern of a jellyfish, starfish, worm, mollusk, insect or vertebrate are
foreshadowed by the characteristic polarity, symmetry and pattern of the
cytoplasm of the egg either before or immediately after fertilization"
(Conklin, p. 172-5). That the arrangement of the embryonic parts is
not solely dependent upon such visible egg substances is shown by the
observations of Morgan (1909c, 1910e) and Boveri (19106) on centrifuged
eggs of Arbacia, Ascaris, the frog, and other forms. Here it is found
that the displacement of the various substances does not necessarily
cause a dislocation of the body parts of the embryo: hence the setting
apart of the embryonic regions must be dependent upon a polarity in the
egg which at least in many cases is not disturbed by the experimental
alteration in position of the visible egg substances. But in either case
differentiation appears to be related to a cytoplasmic organization.

From this it would appear that the characters which such an organism
inherits from the preceding generation do not belong to one category and
are not transmitted in the same way. There are first those general
characteristics of organization which are the direct outgrowth of a corre-
sponding organization in the egg cytoplasm. Secondly, there are the
Mendelian characters which appear later in the ontogeny and which there
is every reason to believe are represented in some way in the chromosomes
of the gamete nuclei (Chapter XV). Boveri thus distinguished two
periods after fertilization : an early one in which the course of develop-
ment is dependent on the organization of the egg cytoplasm, only general
metabolic functions of the chromosomes being active; and a later one in
which the specific hereditary powers of the chromosomes are brought to
expression, the right chromosomal combination then proving to be
necessary for normal development.

The question naturally arises as to how much the cytoplasmic organ-
ization may be due in turn to the activity of the nucleus during the
differentiation of the egg — as to whether the general characters which are
the direct outgrowth of this organization may or may not be ultimately
dependent, as are the clearly Mendelian characters, on nuclear factors.
Conklin (1915) comments upon this point as follows: " In this differentia-
tion and localization of the egg cytoplasm it is probable that certain
influences have come from the nucleus of the egg, and perhaps from the
egg chromosomes. There is no doubt that most of the differentiations of
the egg cytoplasm have arisen during the ovarian history of the egg, and
as a result of the interaction of nucleus and cytoplasm; but the fact
remains that at the time of fertilization the hereditary potencies of the
two germ cells are not equal, all the early stages of development, includ-
ing the polarity, symmetry, type of cleavage, and the pattern, or relative
positions and proportions of future organs, being foreshadowed in the
cytoplasm of the egg cell, while only the differentiations of later develop-
ment are influenced by the sperm. In short the egg cytoplasm fixes the

THE ROLE OF THE CELL ORGANS IN HEREDITY 331

general type of development and the sperm and egg nuclei supply only
the details" (p. 176).

Plastid Inheritance. — Certain cases of "plastid inheritance" have
been brought forward to show that the character of an organism may not
be entirely due to factors delivered to it by the gamete or spore nuclei.
It has been pointed out that two successive generations of cells repro-
ducing by division resemble each other for the obvious reason that the
organs of any given cell may actually become the corresponding organs of
the daughter cells. Thus in the case of a unicellular green alga the
daughter individuals are like the mother individual in being green because
the chloroplast of the mother cell is divided and passed on directly to
them. In those algae in which a swarm spore germinates to produce a
multicellular individual (Ulothrix etc.), or associates with others of its
kind to form a colony (Hydrodidyon, Pediastrum; Harper 1908, 1918a6),
the color of the successive colonies or multicellular individuals is a charac-
ter that i's transmitted directly by the repeated division of chloroplasts.
Thus, as Harper urges, the nucleus is not required here to account for the
resemblance between successive generations of cells or individuals, so far
as this character is concerned.

A similar interpretation has been placed by some geneticists upon the
inheritance of "chlorophyll characters" in the higher plants, the supposi-
tion being that plastids, multiplying only by division, are responsible
for the distribution, in the individual plant and through successive
generations, of those characters which manifest themselves in these
organs. Abnormalities in chlorophyll coloring are accordingly held to be
due to an abnormal condition or behavior of the chloroplasts.

Such a case is that of Mirabilis jalapa albomaculata, described by
Correns (1909). In plants of this race there are some branches with
normal green leaves, some with white leaves, and some with "checkered"
(green and white) leaves. Flowers are borne on branches of all three
types. In all cases crosses between unlikes result in seedlings with the
color of the maternal parent: inheritance is strictly maternal. For
instance, if a flower on a green branch is pollinated with pollen from a
flower on a white branch the offspring are all green. In the reciprocal
cross the offspring are all white, and soon die because of the lack of
chlorophyll. In neither case does the pollen affect the color of the
resulting individual. The explanation offered by Correns for the color-
less condition is that it is due to a cytoplasmic disease which destroys
the chloroplasts. It is therefore delivered directly to the next generation
in the egg cytoplasm, and is not transmitted by the male parent because
no male cytoplasm is brought into the egg at fertilization. If it had been
due to nuclear factors it would have been transmitted by both parents,
since the nuclear contributions of the two are equal. This condition is
analogous to that occasionally found in animals, in which bacteria may

332 INTRODUCTION TO CYTOLOGY

be carried from one generation to the next in the egg cytoplasm, causing
a direct inheritance of the disease. But such pathological cases are not
to be confused with, or thought to contradict, normal Mendelian heredity,
which, as will be seen in the following chapter, is closely bound up with
nuclear phenomena. They are rather to be regarded as examples of
repeated reinfection.

Results differing from those of Correns were obtained by Baur (1909)
in his researches on Pelargonium zonale albomarginata. This form, which
is characterized by white-margined leaves, often has pure green and pure
white branches, as in Mirabilis. Crosses either way between flowers on
these two kinds of branches result in every case in mosaic (green and
white) offspring: inheritance is here not purely maternal as in Mirabilis.
Although Baur admits for this case the possibility of a Mendelian inter-
pretation if a segregation of factors for greenness and whiteness in the
somatic cells be allowed, he thinks it more probable that inheritance in
this instance is not a matter of chromosomes and Mendelism at all, but
is rather due to a sorting out of green and colorless plastids, themselves
permanent cell organs, in the somatic cells. In order to account for
inheritance through both the male and the female, Baur assumes that
primordia of plastids are brought in through the male cytoplasm as well
as the egg cytoplasm, a conclusion directly contradictory to that of
Correns. Ikeno (1917), working on variegated races of Capsicum annum.
obtained results similar to those of Baur on Pelargonium, and concluded
that transmission of variegation is not through the nucleus, but through
plastids contributed by both parents.

Although the results and interpretations of Correns and Baur are at
present irreconcilable except on the basis of assumptions not warranted
by known facts, they agree in the conclusion that plastid inheritance is
not Mendelian, but is due rather to extra-nuclear factors. Baur reports
corroborative evidence in Antirrhinum (1918). Opposed to this con-
clusion is that of Lindstrom (1918), who has clearly shown in the case of
certain variegated races of maize that the inheritance of characters due
to unusual plastid behavior is strictly Mendelian. This means that the
distribution or degree of prominence of the plastids, although these may
be organs with their own individuality, depends upon the activity of
Mendelian factors in the chromosomes, which represent the only known
cell mechanism in which there is at present any hope of rinding an expla-
nation for the distribution of Mendelian characters (Chapter XV). In
Lindstrom's plants plastid inheritance appears to be as much a nuclear
matter as the inheritance of any other character manifested in the extra-
nuclear portion of the cell.

On the basis of the data at hand the tentative conclusion seems fully
justified that all cases of chlorophyll inheritance do not belong to one
category. Some of them are clearly to be accounted for on the same basis

THE ROLE OF THE CELL ORGANS IN HEREDITY 333

with other Mendelian characters, whereas others appear to require an
explanation of another kind. The results of work in progress at Cornell
University on variegated races of maize points in this direction. One
. of the most interesting problems in cytology and genetics at present is
that concerning the manner in which extra-nuclear bodies, such as plas-
tids and their primordia, may account for certain types of inheritance,
and the extent to which their behavior may be influenced by the
nucleus.

Aleurone Inheritance. — We may here refer to the attempt which has
been made to explain the inheritance of aleurone color in maize endosperm
on the basis of a somatic segregation of special cell organs in the form of
granular primordia, which multiply by fission and develop into aleurone
bodies of various types and colors. But since aleurone and other endo-
sperm characters are inherited in Mendelian fashion, as shown by East
and Hayes (1911, 1915), Collins (1911), and Emerson (1918), and since
there has been adduced in support of the supposed sorting out of primor-
dia no evidence approaching in cogency that upon which the chromosome
theory has been built up, geneticists generally are of the opinion that the
chromosomes with their well known mechanism of segregation offer the
best promise of an explanation of the inheritance of aleurone characters,
though all admit that other organs may play a part in bringing these
characters to expression. Furthermore, the case for the self-perpetuity
of the aleurone grain is much weakened by the fact of their artificial pro-
duction by Thompson (1912).

The theory that chondriosomes are concerned in heredity has been
discussed in Chapters VI and XII.

General Conclusions. — In conclusion the statement may again be
made that as genetical researches multiply it becomes increasingly clear
that the characters in which an individual resembles that from which it-
sprang are not in every case transmitted to it in the same manner. Those
characters which are inherited according to Mendelian rules, to anticipate
a conclusion based on evidence to be presented in the next chapter, in all
probability owe their repeated appearance in successive generations to
"factors" of some sort which are transmitted by the chromosomes of
the nucleus. This applies also to those characters which, while Men-
delian in distribution, depend for their expression upon the presence
of other cell organs (plastids) which may have an individuality of their
own.

All or nearly all of the hereditary contribution made by the male
parent must in most organisms be in the above form, since the male
gamete consists almost exclusively of nuclear material. The female
gamete, or egg, in addition to the clearly Mendelian characters repre-
sented by factors in its nucleus, may at least in the case of many animals

334 INTRODUCTION TO CYTOLOGY

contribute certain general characters, such as polarity, symmetry, and
general type of early development, which are the direct outgrowth of an
elaborate organization present in the egg cytoplasm. It is true that this
organization is the result of processes in which the nucleus cooperates
during the differentiation of the egg, and those who hold to the universal
applicability of the Mendelian interpretation would assume that the
type of organization must depend upon Mendelian factors carried in the
nucleus. However this may be, the fact remains that the two gametes
at the time of fertilization are not equal in hereditary potency, as
Conklin states. So far as the clearly Mendelian characters are con-
cerned, however, all evidence goes to show that they are precisely
equal.

The direct inheritance of metidentical characters, such as the above
mentioned green plastid color in Pediastrum, and the indirect inheritance
of colony characters in the same form, afford other examples of hereditary
transmission otherwise than through the nucleus. With respect to
colony characters, Harper has shown in a striking manner, both in Hydro-
dictyon and Pediastrum, that the characteristic form and type of organiza-
tion assumed by the colony are the results of interactions between the
form, polarities, adhesiveness, surface tension, etc. of the free-swimming
swarm spores which aggregate to build it up. The swarm spore has an
individual organization of a particular type, but its capabilities show it to
be devoid of any arrangement of its protoplasmic parts corresponding
either to its future position in the colony or to the arrangement of
the cells in the colony as a whole. The character of the colony
thus depends upon the interactions of its component units and is
in no way represented in any one of them. Consequently it is held by
Harper that no system of spatially arranged factors in a special germ
plasm is required to account for the regular reappearance of such cell and
colony characters in these organisms, and that such facts must be reck-
oned with in attempting to explain heredity and development in terms
of the cell.

By whatever means they are transmitted, it is evident that most
characters must be brought to expression through the activity of the cell
system as a whole, the process involving a long series of reactions in
which all or nearly all of the cell constituents play their parts. At the
present time little or nothing is known of the real nature of the " factor"
or of the manner in which it may influence the development of a character.
In general, then, we may say that the heritage bequeathed by an indi-
vidual to its offspring is in most organisms transmitted mainly through the
nucleus, since it is very largely upon this organ that the development or
non-development of particular characters in the organism depends; but
also that the development of the characters in the offspring, however these

THE ROLE OF THE CELL ORGANS IN HEREDITY 335

may be transmitted, involves all of the cell organs as well as a complicated
and orderly series of intercellular reactions and responses. These two
phases, the transmission of a heritage of factors and the develop-
ment of the organism's characters as the result of their influence,
must both be very much more fully known before either can be adequately
understood.

To some of the more cogent evidence upon which these general con-
clusions are based we shall now turn.

Bibliography at end of Chapter XVIII.

MENDELISM AND MUTATION

MENDELISM

The classic researches carried out by Mendel a half-century ago on
the hybridization of garden peas are now so well known that a detailed
description of them would be superfluous here. Moreover, since the
main principles of Mendelism are illustrated in the results of the simplest of
Mendel's experiments, a review of one or two of the latter will for our
purposes- be sufficient.1

A Typical Case of Mendelian Inheritance. — Mendel crossed plants of a
pure bred race of tall peas (6 to 7 feet in height) with plants of a pure
bred dwarf race (^ to 1% feet in height) (Fig. 129). All the plants
of the first hybrid generation (Fi) were tall like one of their parents.
When these tall hybrids were self-fertilized or bred to one another, it
was found that the second hybrid generation (Fz) comprised individuals
of the two grandparental types, tall and dwarf, in the relative numerical
proportion of 3:1. It was further found that the tall individuals of this
generation, though alike in visible characters, were unlike in genetic con-
stitution: one-third of them, if bred for another generation, produced
nothing but tall offspring, showing that they were "pure" for the
character of tallness; whereas the other two-thirds, if similarly bred,
produced again in the next generation both tall and dwarf plants in the
proportion of 3 : 1, showing that they were hybrids with respect to tallness
and dwarfness. The dwarf plants of the second hybrid generation (Fa)
produced nothing but dwarfs when interbred; they were "pure" for
dwarfness. From these facts it was evident that the plants of the F2
generation, although they formed only two visibly distinct classes, were
in reality of three kinds: pure tall individuals, tall hybrids, and pure
dwarfs, in the relative numerical proportions of 1 :2 :1.

The explanation offered by Mendel for these phenomena may be
briefly stated as follows (Fig. 129). The germ cells produced by the
pure tall plant carry something (now termed a factor, represented here

1 Detailed accounts of the many facts of Mendelism may be found in more special
works on the subject. See Morgan et al. 1915, Chapters 1 and 2; Bateson 1913;
Castle, Coulter et al. 1912; Castle 1916; Coulter and Coulter 1918; Babcock and
Clausen 1918, Chapter 5; Punnet 1919; Darbishire 1911; Morgan l&19a; Thomson
1913; East and Jones 1919.

336

MEN DELI SM AND MUTATION

337

by T) which makes the resulting plant tall. The germ cells of the dwarf
plant carry something (t) causirg the dwarf condition. In the first

PMENT5

T T

00

-© ©00

# m FZ

FIG. 129. — A typical Mendelian cross between tall and dwarf peas, showing dominance
of the tall over the dwarf condition in the first hybrid generation (Fi), and the 3: 1 ratio of
tall plants to dwarfs in the second hybrid generation (Ft). At the right is shown the
corresponding distribution of the Mendelian factors for tallness (T) and dwarf ness (t).

hybrid generation (F\) both factors are present, T coming from one parent

and t from the other, but T "dominates" and prevents the expression of
22

338

INTRODUCTION TO CYTOLOGY

the "recessive" t, so that the plants of this generation are all tall. When
the hybrid (Fi) produces germ cells the two factors for tallness and
dwarfness separate, half of the germ cells receiving T and the other half
receiving t. Each gamete therefore carries either one or the other of the
two factors in question, but never both: a given gamete is "pure"
either for T or for t. This segregation in the germ cells of factors pre-
viously associated in the individual without their having been altered by
this association is the central feature of the entire series of Mendelian
phenomena, and is often referred to as Mendel's first law. Since, now,
the gametes, both male and female, produced by the hybrid plants of the
Fi generation are of two kinds (half of them bearing T and half bearing ft

MIRABIUS JALAPA

FIG. 130. — Blending inheritance ("incomplete dominance") in Mirabilis jalapa, showing
1:2:1 ratio of three genotypes in Ft. (Adapted from Correns.)

four combinations are possible : a T sperm with a T egg, a T sperm with a
t egg, a t sperm with a T egg, and a t sperm with a t egg. These four
combinations result respectively in a tall plant (pure dominant, TT), two
tall hybrids (Tt and tT), and a dwarf plant (pure recessive, tt). It is
obvious that in the long run these three types will occur in the ratio of
1 :2:1.

Mendel's researches on peas included also a study of six other pairs
of heritable characters (now known as allelomorphic pairs), the two
members of each pair behaving toward each other in a manner similar to
that described above for tallness and dwarfness. He further observed
that the seven pairs are entirely independent of each other in inheritance
(Mendel 's second law; now modified; see p. 384). All these phenomena he
interpreted on the basis of the hypothesis that each character is in some
way represented by a factor in the cells, new combinations of factors

MEN DELI SM AND MUTATION 339

being formed at fertilization and the members of each allelomorphic pair
of factors separating when the germ cells are formed.

The Mendelian proportion of pure forms and hybrids is more easily
followed in cases of "incomplete dominance," the pure dominants here
being visibly distinguishable from the hybrids. Such a case is that of
Mirabilis jalapa, the four-o'clock (Fig. 130). If plants bearing pure red
flowers (var. rosea) are crossed with those bearing pure white flowers
(var. alba) the result is an F} generation of intermediate pink-flowered
plants. When these pink hybrids are bred among themselves the result-
ing FZ generation comprises plants of three visibly different types: pure
dominants with red flowers, hybrids with pink flowers, and pure recessives
with white flowers, in the numerical ratio of 1:2:1.

Terminology. — We may here introduce certain terms prominent in the
literature of genetics. The genotype is the entire assemblage of factors which an
organism actually possesses in its constitution, irrespective of how many of
these may be expressed in externally visible characters. The phenotype is the
aggregate of externally visible characters, irrespective of any other factors,
unexpressed in characters, which may be present in the organism. For illustra-
tion: in the case of the tall and dwarf peas there are in the second hybrid genera-
tion (F2) three genotypes (with respect now only to the single character pair
discussed): TT, Tt, and tt; represented respectively by pure tall plants, tall
hybrids, and dwarfs; but there are only two- phenotypes : tall and dwarf, because
of the fact that the complete dominance of tallness over dwarfness renders the
hybrids externally indistinguishable from the pure tall individuals. Thus one
phenotype (tall plants) here includes individuals with two genotypic constitu-
tions, and the two can be distinguished only by a study of their progeny. In
Mirabilis, however, there are in the F% generation not only three genotypes
represented, but also three phenotypes, since the incomplete dominance renders
the hybrids externally unlike either of the pure forms.

An individual is said to be homozygous for a given allelomorphic character
pair if it has received the same factor from the two parents — a pea, for example,
with the constitution TT or tt. If it has both members of the pair, such as Tt,
it is said to be heterozygous. It may be homozygous for some allelomorphic pairs
and heterozygous for others, or it may conceivably be either homozygous or
heterozygous for all of its characters. Thus an organism with the genotypic
constitution AABbcc is homozygous for the characters represented by A A and
cc, and heterozygous for those represented by Bb. It is thus a pure dominant
with respect to A and a, a pure recessive with respect to C and c, and a hybrid
with respect to B and b. The phenotypic appearance of the organism would be
determined by the dominant factors A and B and by the recessive c; a given
dominant factor dominates only its recessive allelomorph, and not the recessive
factors belonging to other pairs. It is a common practice to represent dominant
factors or characters by capital letters and their respective recessive allelomorphs
by the corresponding small letters.;

The Cytological Basis of Mendelism. — Having before us some of the
principal facts of Mendelism and Mendel's interpretation of them, we

340

INTRODUCTION TO CYTOLOGY

may now turn to the cytological basis of the Mendelian phenomena, and
inquire what visible mechanism there is in the cell which will in any way
help us toward an understanding of the striking behavior of the Mendelian
characters.

The behavior of the chromosomes at the critical stages of the life
cycle as described in the chapters on reduction and fertilization must
first be recalled. (See Fig. 131.) It has been shown that their history
is as follows. Each parent furnishes the offspring with a set of chromo-
somes, the two sets (represented in the diagram by A BCD and abed)
being associated in all the cells of the offspring. When gametes (or
spores followed later by gametes in the case of higher plants) are to be

FEHTILIZATIOH
Union of simplex groups

CLEAVAGE
Duplex groups
ABCD abed

SOMATIC DIVISIOMS
Duplex groups

REDUCTION
DIVISION

GERM CELLS
Simplex groups

FIG. 131. — Diagram showing the history of the chromosomes in the typical life cycle of
animals. (After Wilson, 1913.) See also Fig. 77.

formed by the new individual the chromosomes pair two by two (synap-
sis), the two homologous members of each pair coming from the two
parental sets. In the first maturation division (usually) the two members
of each pair separate and enter different daughter cells: this is reduction,
or the separation of entire chromosomes, presumably qualitatively differ-
ent, instead of qualitatively similar halves of chromosomes as in somatic
division. In the second maturation division all the chromosomes split
longitudinally (equationally), so that as the result of the two divisions
there are four gametes (or spores), two of them differing from the other
two in chromatin content. The somatic chromosomes are therefore
segregated into two unlike groups : each gamete (or spore) has a single set
of chromosomes, the set being composed of one member of each of the
pairs formed at synapsis. This set represents the contribution made to
the following generation.

MEN DELI SM AND MUTATION

341

It will be recognized at once that the above is precisely the sort of
distribution shown by the characters in Mendel's experiments: two groups

PARLNT5

FIG. 132. — Mendelian inheritance in black and albino guinea pigs.

FIG. 133. — Chromosome history in the cross represented in Fig. 132, showing the
parallelism between the distribution of a single homologous pair of chromosomes and that
of a single allelomorphic pair of Mendelian characters.

of (factors for) characters are brought together at fertilization and are
associated in the body of the offspring. When the germ cells are formed

342 INTRODUCTION TO CYTOLOGY

the (factors for the) two characters forming each allelomorphic pair
separate and pass to different gametes (or spores). Thus the chromo-
somes and the characters alike form a duplex group in the body cells and
a simplex group in the gametes (or spores) : the chromosomes, like the
characters, form new combinations at fertilization and are segregated
when the gametes (or spores) are formed. In the diagram the letters
ABCDabcd stand equally well either for chromosomes or for characters.
In view of these facts it appears extremely probable that chromosomes
and Mendelian characters have a definite causal relationship of some
kind: it is scarcely conceivable that the exact and striking parallelism
that they show can be without significance.

The precise nature of this correspondence between chromosome be-
havior and character distribution can be even more clearly shown by a
consideration of the history of a single homologous pair of chromosomes
in a typical Mendelian cross. If a pure white (albino) guinea pig be mated
to an individual of a pure black strain the offspring are all black ; black
is completely dominant over white (Fig. 132). If these black hybrids
are bred among themselves they produce in the Fz generation three black
animals to one white, or, more precisely, one pure black to two black
hybrids to one pure white. Let us now follow a single pair of chromo-
somes of each of the original animals through these two generations.

At the left in Fig. 133 are represented the two animals, pure black
and pure white, their chromosomes being drawn in solid black and outline
respectively. In the black animal the two chromosomes pair at synapsis
and separate to the two daughter cells at the first maturation mitosis,
and split longitudinally at the second, so that each of the gametes re-
ceives a single chromosome representing a longitudinal half of one of the
original pair. A similar process occurs in the white individual. Unions
between the gametes of the two animals now result in the F\ hybrids,
each of which has one chromosome from its black parent and one from
its white parent (not counting the chromosomes of other pairs) . When
these hybrids form gametes, as is seen at once in the diagram, the pa-
ternal and maternal members of the chromosome pair separate, with the
result that half the gametes receive one of them and half the other.
There are thus two kinds of spermatozoa and two kinds of eggs, one kind
carrying the paternal chromosome and the other carrying the maternal
one. Chance combinations now result in a generation (F2) of animals,
one-quarter of which have derived both chromosomes of the pair in
question from the black grandparent, one-half of which have derived
one chromosome of the pair from each grandparent, and one-quarter of
which have derived them both from the white grandparent. Moreover,
these animals are respectively pure black, hybrid black, and pure white,
in the proportion of 1:2:1. Thus it is seen that there is a direct paral-
lelism, not only between chromosome sets and character groups, but also

MEN DELI SM AND MUTATION

between the distribution of a given homologous pair of chromosomes and
thai of a single allelomorphic pair of Mendelian characters.

This is exactly the condition which would result if two material
units, each representing one of the characters of an allelomorphic pair,
were located in two homologous chromosomes that pair and separate at
reduction. The chromosomes afford precisely the type of mechanism
required to account for the distribution of characters if the latter are
associated with a definite material basis. It is this parallelism between
the behavior of the chromosomes in reduction and that of Mendelian
factors in segregation, first emphasized by Boveri and by Button, which
has led geneticists generally to the view that the characters are actually

FIG. 134. — Diagram showing the 16 genotypic constitutions which may be present in the
gametes of an organism with only 4 pairs of factors. (After Wilson, 1913.)

represented in the chromosomes by material factors, or genes, which in
some unknown manner control the development of the characters in
the body.

The earlier view that each character is thus represented by a single
material unit or determiner has now given way to the more fully devel-
oped Factorial Hypothesis, according to which, on the one hand, a
character may be due to the cooperative action of two or more factors
("duplicate" or "cumulative" factors); and, on the other hand, a single
factor may have "manifold effects," influencing the development of several
characters. The factors, or genes, are thought by some to constitute a
complex reaction system, interactions between genes having a marked
effect upon their activity in producing characters.1 "The factorial

1 A simple and brief explanation of the effects of cumulative factors is given by
Coulter and Coulter (1918).

344 INTRODUCTION TO CYTOLOGY

hypothesis does not assume that any one factor produces a particular
character directly and by itself, but only that a character in one organism
may differ from a character in another because the sets of factors in the
two organisms have one difference." "It can not ... be too strongly
insisted upon that the real unit in heredity is the factor, while the charac-
ter is the product of a number of genetic factors and of environmental
conditions" (Morgan et al, 1915, pp. 210, 212).

The abundant opportunity for the formation of new factor combina-
tions should be noted in this connection. An organism with four pairs of
chromosomes in its body cells, and only one pair of factors in each chromo-
some pair, could form, as the result of the independent distribution of the
four pairs of chromosomes, gametes with as many as 16 different geno-
typic constitutions (Fig. 134). Such a diversity being present in the
gametes of both sexes, this means that more than 200 different com-
binations are possible at fertilization. The 12 pairs of chromosomes in
man may in the same way form several million such combinations. Since
there is good reason to believe that each chromosome carries more than
one factor the number of variations actually produced by these means is
almost incalculable. This subject will be pursued further in the chapter
on Linkage (Chapter XVII), where the evidence for the presence of many
factors in a single chromosome will be presented and the consequences of
this condition pointed out.

MUTATION

Although opinion is divided over the question of the real nature of the
phenomenon of mutation, particularly in (Enothera Lamarckiana, one
school (deVries et al.) holding that it represents the actual origin of new
forms, and another (Bateson, Davis, Lotsy) regarding it as the result of
segregation in an organism of hybrid constitution, the observed facts in
either case are nevertheless very significant with respect to the chromo-
some theory of heredity. The mutations observed to arise from (Enothera
Lamarckiana fall into two general classes: first, those accompanied by
alterations of the normal chromosome number (seven pairs), and second,
those in which the number undergoes no change.1

Mutations Accompanied by Changes in Chromosome Number. — It
is to be noted first of all that the mutants belonging to this class do not
behave in a typically Mendelian fashion when bred to other forms, and
that this is correlated with the serious disturbance of the chromosome
mechanism. (Enothera mutants with many abnormal chromosome num-
bers have been observed; Gates, for example, found them with 15, 20,
21, 22, 23, 27, 28, 29, and 30 chromosomes.

1 Our knowledge of the cytology of the (Enotheras is due mainly to the researches
of Gates, Davis, Stomps, and Miss Lutz.

MEN 'DELI 'SM AND MUTATION

345

The 28-chromosome Mutants (gigas group}. — In the mutant forms of
this group, of which (Enothera gigas is a member, the somatic number of
chromosomes is 28 rather than 14; the plants are tetraploid (Fig. 135, B).
How this condition arises is not certainly known. Stomps (1912)
believed it to be the result of the union of two unreduced gametes, whereas
Gates (19096) suggested that "the doubling in the chromosome number
had probably occurred as the result of a suspended mitosis in the fertilized
egg or in an early division of the young embryo." Strasburger (19106)
also adopted the latter view.

FIG. 135. — Chromosomes in (Enothera mutants.

A, interkinesis in (E. Lamarckiana; 7 split chromosomes. B, same in (E. gigas; 14
split chromosomes. C, somatic cell of (E. semilata; 15 chromosomes. D, metaphase of
homoeotypic mitosis in (E. biennis lala, showing 8 chromosomes on one spindle and 7 on
the other. Spores and gametes with these numbers will result. E, the 21 chromosomes
in a mutant from (E. Lamarckiana. (A and B after Davis, 1911; C and D after Gates and
Thomas, 1914; E after Lutz, 1912.)

The mutants of the gigas group are characterized chiefly by an unusu-
ally large size, not only of the plant as a whole but also of its anatomical
constituents. In the tetraploid mutant (Enothera stenomeres, for instance,
Tupper and Bartlett (1916) found that the change from the diploid to
the tetraploid condition is concomitant with a 50 per cent increase in
the length of the vessel, a 150 per cent increase in the area of its cross
section, a 50 per cent increase in the length and diameter of the tracheids,
an increase in the three dimensions of the medullary ray cells, and a break-
ing up of the_tall multiple ray into a number of thin simple rays.

346 INTRODUCTION TO CYTOLOGY

A further significant observation on mutants of this lypo \vas that of
(In-gory (1914) on a tetraploid Prhnn-la. Ho showed by brooding experi-
ments that twice the normal number of Mendelian factors are present :
thus when the chromosome number is tetraploid the number of factors is
also tetraploid; each allelomorphic pair is represented twice.

It should be stated that not all cases of gigantism are accompanied
in this manner by an increase in the chromosome number. In Phragmites
communis, for example, Tischler (1918) finds abnormally large size asso-
ciated with an increase in the size of the chromosomes, but not in their
number. Stomps (1919) points out that among gigas mutants of (Eno-
thera, Narcissus, and Primula there are diploid as well as tetraploid
individuals, which must mean that the altered chromosome number is not
the sole cause of such mutation but is rather one of the characters of the
mutant.

The 15-chromosome Mutants (lata group). — The presence of an extra
chromosome in the cells of (Enothera lata and other members of this group
is due to the fact that the members of one pair of chromosomes in (Eno-
thera Lamarckiana fail to separate at the reduction division, both of them
going to one daughter cell. This phenomenon is known as non-disjunc-
tion. As a consequence there are gametes with eight and six chromosomes
rather than the normal seven; and a union of an 8-chromosome gamete
with a normal 7-chromosome gamete results in an individual with 15
chromosomes instead of the normal 14 (Fig. 135, C).

In her study of 15-chromosome mutants Miss Lutz (1917) found 11 or
12 types belonging to this group; only two of them were of the usual lata
type. This condition may be accounted for on the hypothesis that it is
sometimes one pair of chromosomes and sometimes another which fails
to separate at the time of reduction, so that the extra chromosome is not
in all cases the corresponding one of the complement. If the various
chromosomes of the complement differ in hereditary value, as there is
much reason to believe, it is evident that this would allow for a great
variety of mutants with the same aberrant chromosome number. In
(Enothera scintillans Hance (1918) has shown by careful measurements
that the extra chromosome can be distinguished from the regular 14.
Two classes of gametes are formed, some with seven chromosomes and
some with eight (Fig. 135, D) The union of two 7-chromosome gametes
gives (Enothera Lamarckiana, the form from which (E. scintillans sprang
as a mutant; whereas a union of a 7-chromosome gamete with an 8-
chromosome gamete gives (E. scintillans. Hance therefore points out
that the scintillans characters are plainly associated with the extra
chromosome. (E. scintillans was further observed to give rise to a type
resembling (E. oblonga. It is possible that this was due to the union of
two 8-chromosome gametes.

MEN DELI SM AND MUTATION 347

The 21-chromosome Mutants (semigigns group).— The 21-;chromo8ome

condition is brought about by the union of a normal 7-chromosomo gamete
with a 14-chromosomc gamete produced either by a mutant of the gigas
group or by a normal plant through failure of reduction. These mutants
are triploid (Fig. 135, E).

As might be expected in forms with aberrant chromosome numbers,
especially in those with an odd number like 21, certain irregularities
occur in the maturation mitoses, with the result that gametes, and hence
progeny, with abnormal chromosome numbers are produced. Accom-
panying this irregular behavior is an unsettled hereditary condition, the
plants showing various unusual character combinations and differing
markedly from generation to generation. In the course of such a series
of generations there is a gradual settling down to the normal number
through the loss of chromosomes in irregular mitoses. When the nor-
mal number (14) is finally reached the plants become much more stable
in their hereditary behavior: the hereditary mechanism again appears
to be in equilibrium. In one such series of mutant forms, which had
arisen in the first place from (Enolhera Lamarckiana, deVries and Stomps
found that after the normal chromosome number had thus been settled
upon the plants were not of the Lamarckiana type. Although the num-
ber was that characteristic of the original Lamarckiana individual from
which the series originated, the assortment was apparently a new one :
during the settling down process some chromosome pairs of the com-
plement had been lost completely while others had been duplicated.
The plants consequently had certain characters represented in duplicate,
while others present in the orignal ancestral plant were entirely lacking.
Upon the theory that the chromosomes of the complement differ in here-
ditary effect, the above facts are readily explained.

Conclusion. — All the evidence goes to show that in the above described
mutations the change in chromosome number and the change in the visible
characters of the organism occur simultaneously. This fact constitutes a
strong support to the theory that in the chromosomes there are factors
representing heritable characters, and indicates that mutations, whatever
may be their ultimate nature, are causally connected with alterations,
often visible, in the cell mechanism.

Bearing on the Origin of Species and Varieties. — The question nat-
urally follows as to what extent the origin of new species and varieties
may be bound up with fluctuations in chromosome number. Although
the experimental evidence, aside from that of the (Enothera mutants
which many do not regard as species at all, is as yet quite meager, the
following facts are nevertheless very suggestive in this connection.

In published lists of chromosome numbers it is strikingly evident
that the numbers shown by the species of a given genus or even of an
entire family very commonly form a series of multiples. Also, one or

348

INTRODUCTION TO CYTOLOGY

two species of such a group frequently have but one or two chromosome
pairs more or less than one of the numbers of the multiple series. From
this it is to be inferred, as suggested by McClung (1905, 1907), that there
is a relationship of some sort between the constitution of the chromosome
complement and the externally visible taxonomic characters. Certain
illustrative examples will now be cited.

Plants. — In the Leguminosae most of the species which have been
examined possess either 6, 12, or 24 pairs; Pisum has 7. Common
numbers in the Rosaceae are 8, 16, and 32; some species have 6.
In a new study of a large number of species of Rosa Tackholm (1920)
finds the fundamental haploid chromosome number in this genus to
be seven rather than eight. The various species of Chrysanthemum
have 9, 18, 27, 36, and 45 pairs. In both Triticum (Sakamura 1918) and
Avena (Kihara 1919) the pairs number 7, 14, and 28 (Fig. 136). From

a b c

FIG. 136. — The chromosomes of 3 species of Avena. a, A. strigosa; 14 chromosomes, b,
A. barbata; 28 chromosomes, c, A. sterilis; 42 chromosomes. (After Kihara, 1919.)

these and many other similar cases it is inferred that tetraploid species
have been derived from diploid species in much the same way that
(Enothera gigas with its 14 pairs has been observed to arise from (E.
Lamarckiana with seven; that triploid species have arisen either by
dispermy or by a union of diploid and haploid gametes as in the semigigas
group of (Enothera mutants; that hexaploid species have in turn arisen
from the triploid ones; and that those forms with numbers not belonging
to the regular multiple series have resulted from further irregularities in
chromosome behavior. Among such irregularities are the failure of the
two members of a homologous pair to separate at reduction (non-dis-
junction), and the segmentation of certain chromosomes at their points
of constriction (Sakamura 1920; Kuwada 1919). Changes in chromo-
some number are thus looked upon as an important factor in the origin
of new species.

The above conclusion is supported further by the observations of
Sakamura (1918) and Kihara (1919) on wheat. Sakamura finds that the
one-grained wheats (Triticum monococcum) have 14 chromosomes (dip-
loid), the emmer wheats (T. dicoccum, T. polonicum, T. durum, and T.
turgidum) 28 (tetraploid), and the spelt wheats (T. spelta, T. vulgar e,

MEN DELI KM AND MUTATION 349

and T. compactum) 42 (hexaploid). He concludes that the one-grained
wheats are the ancestral forms from which the emmer and spelt wheats
have arisen through changes in chromosome number. This is precisely
the conclusion which Schulz (1913) and Zade (1914, 1918) had reached
on other grounds. Kihara (1919) found further that by crossing emmer
and spelt wheats fertile hybrids with 35 chromosomes could be obtained,
and that these in future generations produced forms with varying chromo-
some numbers because of the irregular manner in which the chromo-
somes are distributed at the time of reduction. (See p. 253.) As a
general rule hybrids produced by crossing forms, with different chromo-
some numbers are sterile, but when they are fertile and their chromosome
number is odd there is usually an irregularity in genetic behavior for
several generations until the number again becomes settled. In some
cases the number thus settled upon is that of the original ancestor with
the lower number (the (Enothera mutants of deVries and Stomps cited
above), whereas in other cases (Triticum) it is that of the ancestor with
the higher number. The manner in which many such changes in number
occur is not yet known.

A study of the chromosomes in the genus Crepis has been made from
this point of view by Rosenberg (1918, 1920). He finds four species,
including C. irirens, with three pairs of chromosomes, eight species with
four, four species with five, one species with eight, one species with nine,
and three species with 21. In Crepis the chromosomes differ markedly
in size, and Rosenberg concludes that the species with three, four, and five
pairs have arisen through such irregular distribution of the smaller chro-
mosomes as has actually been observed in the maturation divisions,
together with recombinations occurring at fertilization. The segmenta-
tion of the larger chromosomes of the complement is not thought to
occur.

In an extensive investigation of the chromosomes of Zea Mays
Kuwada (1919) has found cytological confirmation of the conclusion of
Collins (1912) that this species, which for some years has played a
conspicuous role in genetical investigations, is in all probability a hybrid
between Euchlcena mexicana (teosinte) and some other unknown form
belonging to the nearly related Andropogonese. Owing to their in-
equality in size Kuwada is able to distinguish what he considers to be
the chromosomes of the two supposed ancestral derivations in the cells
of certain races of maize. Thus gemini with components of unequal
size are frequently observed in the microsporocytes.

Animals. — The most complete description of the chromosomes in a large
number of closely related animal species is that given by Metz (1914, 19166)
for the DrosophilidaB. In about 30 species Metz has identified no less than
12 main types of chromosome groups, all but one of them being found in
the genus Drosophila. In Fig. 137 are shown dia grammatically the 12

350 INTRODUCTION TO CYTOLOGY

principal types as they appear with their characteristic arrangements at
the time of cell-division. Type A is that found in Drosophila mclano-
gaster (formerly known as D. ampelophila) , upon which the greater part
of the genetic work in these flies has been done; it is also characteristic
of several other species. Here there are two pairs of large bent "euchro-
mosomes," one pair of sex chromosomes, and one pair of very small
"m-chromosomes." In some of the other species it is seen that the
position of one or both of the large pairs is occupied by two pairs but

H

*•••? *.'' M M

•

9 J °w " /^\? J d1

IT (ji i» XA? L

FIG. 137. — The 12 principal types of chromosome groups found in the cells of the Droso-

philidse. (After Melz, 1916.)

half as large, and there is much evidence to show that these have arisen
by a segmentation of the large chromosomes. Furthermore, the m-
chromosomes do not appear in some species, but it is not yet certain
whether they are actually lost through irregular mitoses or are fused
with some of the larger chromosomes of the group. Since irregular
mitoses resulting in abnormal distributions of chromosomes are actually
observed in Drosophila, and are known to be accompanied by changes
in the hereditary constitution, there can be little doubt that by this and
other means all the types of chromosome groups have been derived from

MENDELISM AND MUTATION 351

one or two original types, and that the specific differences exhibited by
the organisms are related to these differences in their chromosome
complements.

This conclusion is supported by the observations of McClung, Robert-
son, and others on the chromosomes of other insect families. In the
grasshoppers, for example, Robertson (1916) finds that the various
chromosomes of the complement form a regular graded series and
can be identified in all the species and genera studied. The nearer the
relationship the more nearly similar are the chromosome complements.
Robertson states that the degree of relationship is as clearly expressed
in the nucleus as in the externally visible characters, and that the
evidence indicates that descent by variation from a common ancestral
series of chromosomes is paralleled by degrees of variation in somatic
structures.

Mutations Accompanied by No Change in Chromosome Number. —
In most of the examples of this class it has been found that the mutant
behaves in a strictly Mendelian manner, usually being recessive to the
type from which it sprang. This observation falls into line with the fact
that the number and behavior of the chromosomes remain the same; the
operation of the Mendelian mechanism is not disturbed. Consequently
if the origin of mutations of this class is dependent on the chromosomes
it must be due to a change of some kind occurring within the chromosome
and affecting the character of its factors, or genes. That such factor
mutations do take place is the hypothesis upon which a large school of
geneticists is attempting to account for many of the observed phe-
nomena of inheritance. Such mutations may involve either a single
gene only ("point mutation") or a group of genes occupying a given
region of a chromosome ("regional mutation"). Furthermore, they
may apparently occur either in the germ cells or in the somatic cells,
but seem to be most frequent in the former at the time of maturation,
for the reason that only one or two gametes (or spores followed later by
gametes in most plants) among the large number produced reveal the
presence of the altered gene in their effect upon the offspring. The
mutation in the gene must here take place after the multiplication of
the germ cells has been nearly or quite completed; otherwise the effect
would be manifested by a larger number of gametes (or spores) . If, as
is true of the majority of cases, the mutation is such as to result in a
recessive character, this character does not manifest itself until it meets
a similarly mutated gene in the homozygous individual. Thus such
an alteration may remain latent for many generations, or may never
come to expression at all.

Factor mutations occurring in somatic (meristematic) cells result in
what are known as "vegetative mutations." These are of two principal
kinds: bud sports and chimeras. In the case of the bud sport, which is

352 INTRODUCTION TO CYTOLOGY

believed to be due to a factor mutation in the very young bud, the entire
product of the bud — branch, flower, or flower cluster — has a new geno-
typic constitution and exhibits an appearance often strikingly different
from that of the other branches or flowers of the plant. Such a factor
mutation occurring in a partially developed shoot or organ results in a
chimera, in which a distinct portion of the mature structure, commonly a
sharply defined sector, differs genotypically and in appearance from the
other portions. By some geneticists vegetative mutations are thought to
be due to a somatic segregation of allelomorphic factors in a heterozygous
individual and their consequent independent activity in different por-
tions of the body.

The nature of the change which may thus occur in the gene is unknown,
since the nature of the gene itself is entirely a matter of conjecture.
Although it has been suggested that the gene, because of its relative
stability, may be simply a molecule, it is more probably a more complex
colloidal aggregate, possibly enzymatic in nature, which is capable of
growth and division. As such it could not be expected to be absolutely
stable, as some geneticists have thought, but changes would in all likeli-
hood take place occasionally by addition, loss, or rearrangement of the
constituent atoms of the molecule. The probable rate of change of the
genes in Drosophila has been calculated by Muller and Altenburg (1919).
What the agencies are which cause such changes is also unknown. Some
evidence has been brought forward to show that genes may be modified
by external influences, but by many it is regarded as of very doubtful
value. The stimuli to which the genes respond by undergoing some
constitutional change are probably for the most part internal ones.
Although the number of ways in which a gene may change is limited by
its own organization, the possible changes are nevertheless numerous,
so that it is very probable that many variations which constitute initial
steps in evolution originate in this manner.1

Conclusion. — The following paragraphs are quoted from East and
Jones (1919, pp. 76 ff.):

"The relation between fact and theory in the Mendelian conception of
inheritance is this : Various kinds of animals and of plants were crossed and the
results recorded. With the repetition of experiments under comparatively
constant environments these results recurred with sufficient regularity to justify
the use of a notation in which theoretical factors or genes located in the germ cells
replaced the actual somatic characters found by experiment. Later, the ob-
served behavior of the chromosomes justified localizing these factors as more or
less definite physical entities residing in them. Now the data from the breeding
pen or the pedigree culture plot and the observations on the behavior of the
chromosomes during gametogenesis and fertilization are facts. The factors are
part of conceptual notation invented for simplifying the description of the breed-

1 See the discussion of these points by Conklin (1919-1S20).

MENDELISM AND MUTATION 353

ing facts in order to utilize them for the purposes of prediction, just as the chemi-
cal atom is a conception invented for the purpose of simplifying and making
useful observed chemical phenomena. As used mathematically, both the geneti-
cal factor and the chemical atom are concepts, but biological data lead us to
believe that the term factor represents a biological reality of whose nature we
are ignorant, just as a molecular formula represents a physical reality of a nature
yet but partly known.

" With this distinction in mind, one may treat the factor — or the atom — from
two points of view, either as a mathematical concept or a physical reality. As
a mathematical concept it is the unit of heredity, and a unit in any notation must
be stable. If one describes a hypothetical unit by which to describe phenomena
and this unit varies, there is really no basis for description. He is forced to
hypothecate a second fixed unit to aid in describing the first.

" The point at issue in this connection may be explained as follows : Characters
do vary from generation to generation, and the question to be decided is, how
much of this variation is due to the recombination of factors (considered now as
physical entities) and how much is due to change in the constitution of the
factors themselves . . .

"... We believe there should be no hesitation in identifying the hypotheti-
cal factor unit with the physical unit factor of the germ cells. Occasional changes
in the constitution of these factors, changes which may have great or small
effects on the characters of the organism, do occur; but their frequency is not
such as to make necessary any change in our theory of the factor as a permanent
entity. In this conception biology is on a par with chemistry, for the practical
usefulness of the conception of stability in the atom is not affected by the knowl-
edge that the atoms of at least one element, radium, are breaking down rapidly
enough to make measurement of the process possible."

Bibliography at end of Chapter XVIII.

23

CHAPTER XVI
SEX

In the present chapter attention will be devoted to the cytological
aspects of the inheritance and determination of sex.1 Much that is not
cytological in nature will enter into consideration, but it is far from
irrelevant: it has a direct bearing on the main cytological problem and
must be included in order that the latter may be placed in its proper
setting, and that the larger problem of sex may not be misrepresented by
being considered only from the cytological point of view.

From early times few essentially biological matters have been of more
interest to man than that of the determination of sex. Until recent
years this interest has been prompted largely by practical motives: the
ability to control sex in man and his domesticated animals is something
which has long been, desired. Of the many early ideas entertained on the
subject the majority were the outcome of defective generalization and
superstitious conjecture, and may be encountered in thinly veiled form at
the present day, but a review of them all would be out of place here. The
modern scientific interest in the problem of sex is far from being a purely
practical one. The great bulk of recent research has been done not
merely for the sake of the practical benefits which knowledge in this
field might confer, but mainly in the hope that it may lead to an under-
standing of the origin, nature, and biological significance of sex itself,
and to a solution of some of the problems of heredity. For this reason
studies have not been confined to man and his economically important
animals; any animal or plant, no matter how obscure, that will yield
evidence is exhaustively investigated, and there can be no doubt that
knowledge gained from such studies will, if sound, be directly applicable
to practical ends.

Experimental Evidence for Sex-determination. — During the closing
decades of the nineteenth century many researches were carried out in the
hope of identifying the controlling agency in sex-determination with one
or more of the environmental factors. The effects of light, temperature,
moisture, and nutrition were examined, and although a number of workers
believed their methods to be in a certain measure successful, the results
were on the whole inconclusive. Among all the ideas put forward the
most suggestive, in view of what has more recently been ascertained,

1 See Correns (1907), Correns and Goldschmidt (1913), Morgan (1913), Doncaster
(1914), and the works cited at the beginning of the preceding chapter.

354

SEX 355

was that of Geddes and Thomson (1889), namely, that the two sexes
differ primarily in the character of their metabolism, the female sex
being characterized by the preponderance of anabolic processes and the
male sex by those essentially katabolic in nature. This conception is
important not only in connection with the question of sex control, but
chiefly with respect to the more fundamental problem of the nature of
sex itself.

The long early period during which sex was looked upon as a character
more or less under the control of the environmental factors was succeeded
by one in which it came to be regarded as something automatically
regulated by some mechanism or condition within the cell, and as rela-
tively unalterable by external agencies. This conclusion, definitely
reached by Cue'not (1899) for animals and by Strasburger (1900) for
plants, received the support of a number of experimental researches on
animals and dioecious plants. Some of the latter will first be mentioned.

It was found by Blakeslee (1906) that in certain strains of a mold,
Phycomyces, there are produced in the germ sporangium two kinds of
asexual spores, which give rise to "plus" and "minus" mycelia respec-
tively. The "plus" (male?) mycelium later produces spores which
develop only into "plus" mycelia and so on indefinitely, while the
"minus" (female?) strain perpetuates only the "minus" condition: in
both cases the sex seems to be fixed by some mechanism functioning at
the time of spore formation.

In certain dioecious mosses (£l. and fim. Marchal 1906, 1907) two
kinds of spores are produced in equal numbers in the capsule. Those of
one kind develop into male gametophytes (bearing antheridia only) and
those of the other kind produce female gametophytes (with archegonia
only). In no way were the Marchals able to alter the sexes of these
plants. Furthermore, new gametophytes formed by regeneration from
the old ones were just as rigidly fixed as to sex. Protonemata regenerated
from the tissue of the sporophyte, however, gave rise to leafy branches
bearing both antheridia and archegonia. Both sex potentialities were
therefore present in the sporophytic tissue and in the diploid game-
tophytes regenerated from it, whereas the normal haploid gametophytes
produced from spores were either purely male or purely female.1 The
gametophytes of Marchantia (Noll; Blakeslee 1906) are similarly fixed
as to sex: if propogated repeatedly from gemmae the sex in any given line
remains the same in spite of alterations in the environmental conditions.
In Sphoerocarpos Douin (1909) and Strasburger (1909) were able to show
that two spores of a single tetrad produce male gametophytes while the
other two produce females.

The obvious conclusion to be drawn from the above cases is that in
such forms a separation of the sexes takes place during sporogenesis.

1 Diagrams of these experiments are given by Morgan (1919o, pp. 152-3).

356 INTRODUCTION TO CYTOLOGY

Both sexes are represented in the sporophyte (diploid) generation, as
shown particularly by the mosses, but the spores, though morphologically
similar, are of two distinct kinds: male-producing and female-producing.
Since it is precisely at sporogenesis that reduction occurs, the natural
inference is that a separation of qualitatively different sex-factors of some
kind occurs in the heterotypic division, the sexes of the future gameto-
phytes thus being automatically determined.

That a somewhat similar qualitative difference may exist in the
microspores of many angiosperms, resulting in the frequent dioecious
condition of the sporophyte, was concluded by Correns (1907) from his
researches on Bryonia hybrids. He was best able to interpret the
phenomena observed on the hypothesis that the eggs are all similar in
having the female sex "tendency;" that there are two kinds of micro-
spores and hence two kinds of male gametes, with male and female
"tendencies" respectively; and that in the sporophyte the male tendency
dominates the female. Darling (1909) was inclined toward a similar
conclusion for Acer Negundo.

Strasburger (1910) in a general discussion of the subject growing out
of his researches on Elodea, Mercurialis, and other plants, summarized
the situation in plants as follows. In monoecious mosses the separation
of the sexes occurs in the somatic divisions at the time the sex organs
are formed. The separation has been secondarily joined with reduction,
so that in the derived dioecious mosses it occurs at sporogenesis. In the
homosporous pteridophytes it takes place at some stage in the game-
tophyte before the formation of the sex cells, as in monoecious mosses,
though in some cases (Equisetum, Onoclea, and others) a marked physio-
logical dioecism is present. In heterosporous pteridophytes and all
seed plants the gametophytes are dioecious but the sporophytes may be
either monoecious (hermaphroditic) or dioecious. In monoecious forms the
sexes are separated at some stage prior to the development of megaspores
and microspores, whereas in dioecious forms it must take place at some
other point in the life cycle, since the two kinds of sporophytes (mega-
spore-bearing and microspore-bearing) are distinct from their initial
stages. There is some evidence to show that such dicecism is due to a
differentiation among the pollen grains and hence among the male
gametes: some grains have a strong male tendency which dominates the
female tendency of the egg, male progeny resulting; while other grains
have a weak male tendency dominated by the female tendency of the egg,
female offspring being produced. In brief, as sex separation became
joined with reduction in forms with monoecious gametophytes, the
dioecism of the gametophyte and heterospory (first physiological and then
morphological) followed; and this in turn led to the dioecism of the
sporophyte also. Finally, in such advanced forms there appears to be a
differentiation among the spores of one sex, the microspores, giving male
gametes of two types. The sex of the resulting offspring therefore

SEX 357

depends here upon the type of male gamete functioning, as is known
to be the case in so many animals. It has been suggested by Allen (1919)
that the separation of the sex-factors in dioecious seed plants may possibly
occur in the division which differentiates the two male nuclei in the
pollen tube, rather than at the divisions producing the microspores.
That this interpretation cannot be applied to Mendelian factors in general
is evidenced by the fact that in maize hybrids the embryo, with very
rare exceptions, has been found to be like the endosperm with respect
to factors introduced by the pollen parent. So far as these factors are
concerned, therefore, the two male nuclei must be qualitatively similar.

Strasburger's conclusion regarding moncecious mosses is confirmed
by the recent experiments of Collins (1919) on Funaria hygrometrica. In
this species the gametophytes arising from spores are bisexual (monce-
cious), but if gametophytes are produced by regeneration from the
antheridia or perigonial leaves of a single "male flower," they all bear
antheridia only. Collins thinks it possible that dio3cism may have
arisen as a result of vegetative multiplication following such a somatic
segregation in the tissue of the moncecious gametophyte.

In animals also there is much evidence, aside from that afforded by
the chromosomes to be discussed below, in favor of the view that sex is
internally controlled. The following illustrative cases may be cited.
The egg of the bee may develop either parthenogenetically or after
fertilization by a spermatozoon : in the former case a male (drone) results
and in the latter a female (queen or worker, depending on the nature of
the food). In Phylloxera (Morgan 1906, 1908, 1909, 1910) there are two
sizes of eggs produced by the females of the second parthenogenetic
generation: both may develop parthenogenetically after forming one
polar body, the larger ones into females and the smaller into males.
Fertilized eggs always develop into females. In Hydatina (Whitney
1914, 1916, 1917) the female-producing eggs form one polar body while
the male-producing eggs form two. Two kinds of eggs are also produced
in Dinophilus (Malsen 1906; Nachtsheim 1919), but in this form both
are regularly fertilized. In the nine-banded armadillo (Newman and
Patterson 1909, 1910) one fertilized egg commonly gives rise to four new
individuals, and the four are invariably all male or all female. Analogous
instances of polyembryony are also known in insects. Human twins,
if "identical" (produced by the same egg), are invariably of the same
sex; if "fraternal" (produced by different eggs) they may or may not be
of the same sex. It would therefore seem that sex in such cases as these
must be determined either in the egg before fertilization or at the moment
fertilization occurs.1

1 The determination of sex in the egg before fertilization, as in Phylloxera and
Dinophilus, is termed by Haecker "progamic" sex-differentiation; if determination
occurs at the moment of fertilization, as in the bee, it is "syngamic" sex-differentia-
tion ; and if it occurs after fertilization, as may possibly be the case in some forms, it
is "epigamic" sex-differentiation.

358 INTRODUCTION TO CYTOLOGY

Sex-Chromosomes. — The theory of the automatic determination of
sex and its relative, if not absolute, fixity has had one of its strongest
supports in the results of certain researches on the spermatogenesis of
animals. In 1891 Henking noticed in certain insects that half of the
spermatozoa contain an extra body, which he thought might be a
nucleolus. It was subsequently shown by Paulmier (1899), Montgomery
(1901), and de Sine'ty (1901) that this body is not a nucleolus but an
extra or "accessory" chromosome. Henking's misinterpretation had
apparently been due to the fact that the accessory chromosome often
does not transform into a portion of the reticulum along with the
other chromosomes ("autosomes"), but remains condensed and closely
resembles a nucleolus. Half of the spermatozoa in these animals there-
fore have one more chromosome than the others: hence the male is
said to be "heterogametic," or "digametic." It was at once suggested
by McClung (1902) that the accessory chromosome in some way
determines sex — that eggs fertilized by one kind of spermatozoon
develop into females, while those fertilized by the other kind become
males. This represents the first attempt to connect a given character
with a particular chromosome. An extensive series of researches was
now undertaken by Wilson, Miss Stevens, McClung, and a number of
other cytologists, who discovered among insects many striking instances
of the phenomenon. Accessory chromosomes (also referred to as sex-
chromosomes, heterochromosomes, idiochromosomes, x-chromosomes,
x-elements, and supernumerary chromosomes) of a number of different
types were found, not only among insects, where they are best displayed,
but also in certain echinoderms, nematodes, mollusks, and vertebrates,
including birds and man. A number of representative cases will now
be described.

Male Heterogametic. — In the threadworm, Ascaris (Boveri) (Fig. 138) !
there is in each body cell and primary spermatocyte of the male a single
heterochromosome, which seems to be attached to, or to constitute a
portion of, one of the four autosomes. At the time of reduction this
passes undivided to one daughter cell at the first division and divides at
the second, so that half of the sperms only receive it. In the female there
are two such heterochromosomes, every egg receiving one. If, now, an
egg is fertilized by a sperm without a heterochromosome the resulting
individual has only one (that from the egg) and develops into a male.

1 For the sake of brevity and clearness these diagrams are drawn as if only one
maturation mitosis occurred in spermatogenesis and oogenesis. It will be understood
that there are two divisions, resulting in four sperms instead of the two shown, and
in an egg and three polar bodies instead of the two eggs shown. The diagrams merely
indicate that two sorts of sperms and one kind of egg are produced, and how this
is brought about. In the cases of Lygceus and Prionidus the number of autosomes
shown (4) is not the actual number present. See the review of the subject of sex
chromosomes by Wilson (1911).

SEX

359

If, on the other hand, an egg is fertilized by a sperm with a hetero chromo-
some, the resulting individual receives two, one from each gamete, and
this individual develops into a female.

In Ancyracanthus (Mulsow 1912) (Fig. 138) the male has a single
heterochromosome which, since it has no homologue with which to pair,
passes to half the sperms, while in the female there are two such elements,
every egg receiving one. The two types of union result in individuals
of the two sexes, as in Ascaris. In Ancyracanthus Mulsow states that
the five and six chromosomes can actually be counted in the living
spermatozoa.

ANCYRACANTHUS

FIG.

138. — The behavior of the sex-chromosomes in Ascaris (Boveri), Ancyracanthus
(Mulsow, 1912), Lygasus (Wilson, 1905), and Prionidus (Payne, 1909).

In Lygoeus (Wilson 1905) (Figs. 138; 139, A) there are in the male
two heterochromosomes, one small and one large (an "XY" pair); in the
female there are two large ones ("XX"). Half the sperms receive the
X and half the Y, and every egg has an X. Fertilization by an X sperm
results in a female (XX), and by a Y sperm in a male (XY).

In Prionidus (Payne 19091) (Figs. 138; 139, B) the male has three
small heterochromosomes and also a much larger one. At the time of
reduction the three small ones behave as a unit and pair with the large
one: half of the sperms therefore carry the former and half the latter.
In the female there are six small heterochromosomes, and the eggs are all
alike in having three each. Fertilization now results in females with
six small elements and males with three small and one large.

1 In this paper Payne gives diagrams of several other types of heterochromosomes.

360

INTRODUCTION TO CYTOLOGY

In the fruit fly, Drosophila melanogaster (Stevens 1907; Morgan 1911;
Metz 1914) (Fig. 140), there are four pairs of chromosomes, including in
the male an XY pair and in the female an XX pair. Reduction in

«

FIG. 139. — Sex-chromosomes in various insects.

A, spermatocyte of Lygaeus, showing the .X-chromosome at left and y-chromosome
above, both split. X 2250. (After Wilson.) B, prophase in spermatocyte of Prionidus,
showing sex-chromosomes enclosed in plasmosome. X 2294. (After Payne.) C, pro-
phase in spermatocyte of Protenor. X 2250. (After Wilson.) D, metaphase of hetero-
typic mitosis in spermatocyte of Protenor. X 2250. (After Wilson.) E, anaphase of
heterotypic mitosis in spermatocyte of Musca domestica; h, heterochromosomes. X 1500.
(After Stevens.) F, the two daughter chromosome groups in the anaphase of the hetero-
typic mitosis in the oocyte of Phragmatobia fuliginosa, showing 28-29 distribution. X 4080.
(After Seiler.)

spermatogenesis gives sperms of two sorts: all contain four chromosomes,
but in half of them one of the four is the X, and in the other half it is
the Y. Since every egg contains an X, two kinds of union are possible
at fertilization : an X with a Y, giving a male fly, and an X with an X,

SEX

361

giving a female. In this case, a typical example of the XY form of sex
inheritance, extensive researches have shown that the y-chromosome
carries no factors for sex; the presence of one X-chromosome is associated
with maleness and that of two X-chromosomes with femaleness. Morgan
(1914, 19196) and Morgan and Bridges (1919) find that gynandromorph-
ism frequently appears in Drosophila females as the result of the
elimination of one sex-chromosome in abnormal mitosis.

FIG. 140. — The behavior of the sex-chromosomes in Drosophila (Stevens, Morgan, Metz),
Man (Wieman), Phragmatobia (Seiler), and the fowl (Guyer).

Malone (1918) reports that there are present in the spermatocyte of
the dog 10 pairs of autosomes and one large unpaired X-chromosome.
The X passes undivided to one pole in the first mitosis and divides in the
second, so that half of the spermatids, and hence spermatozoa, receive
an X while half do not. When measurements of these spermatozoa are
plotted a bimodal curve results, showing that the chromosome difference
is correlated with a size dimorphism. The same condition, except for
the number of autosome pairs, is reported for the spermatozoa of horses,
pigs, and cattle (Wodsedalek 1913, 1914, 1920).

In the case of man also the evidence at hand indicates a digametic
condition on the part of the male, but certain striking discrepancies in
the findings of various investigators have afforded a puzzle which up to
the present time has not been satisfactorily solved. Hemming (1898)
counted 24 chromosomes in the cells of the cornea, and Duesberg (1906)
found 12 in the spermatocytes. The same numbers were found by
Montgomery (1912). In 1910 Guyer reported that the spermatogonia
of the negro contain 22 chromosomes; these in the spermatocyte form 10
bivalents and two distinguishable accessories. At the first maturation
mitosis both of the latter go to one pole and at the second mitosis both
divide, so that half of the sperms have 10 chromosomes and half have 12;
the two accessories in the latter case are visible as "chromatin nucleoli"
in the resting stage. This difference in the gametes Guyer regarded as
probably associated with sex-determination. Gutherz (1912) failed to

362 INTRODUCTION TO CYTOLOGY

confirm Guyer's report of a dimorphism among the sperms, though he also
observed the accessories. Guyer in 1914 reasserted his conclusions of
1912, but added that he was finding a much larger chromosome number
in the cells of the white man.

Von Winiwarter (1912), also working upon the white man, found in
the spermatogonia and spermatocytes 47 chromosomes, including one
accessory. Since this accessory passes undivided to one pole in the first
mitosis and divides in the second, half of the sperms receive 23 and half
24 chromosomes. In the cells of the female there are 48. From these
data von Winiwarter logically concluded that the egg has 24 chromosomes,
and that it develops into a male when fertilized by a sperm with 23
chromosomes, and into a female when fertilized by one with 24. Such a
large number is found in the white man by Evans1 also, but he finds 48
chromosomes rather than 47 in the spermatogonia, indicating the presence
of an XY pair as in Drosophila.

A

FIG. 141. — Sex-chromosomes in man. A, primary spermatocyte, negro. B, same,
white. C, metaphase of heterotypic mitosis, negro. D, interkinesis, white. XY, the
sex-chromosomes; P, plasmosome. (After Wieman, 1917.)

Wieman (1917), using both negro and white material, finds 24 chromo-
somes in the spermatogonia, two of them being distinguishable as an
unequal XY pair which remains condensed while the autosomes form the
reticulum (Figs. 140, 141). In the spermatocyte 12 pairs are evident,
including the XY pair. At the first maturation mitosis the 11 autosome
pairs separate into univalents as usual, but the X and Y divide longitud-
inally; thus each daughter cell (secondary spermatocyte) receives 11 auto-
somes and an XY pair. At the second mitosis the 11 autosomes divide
longitudinally in the normal fashion and the X and Y separate. As a
result all of the sperms receive 12 chromosomes : in half of them one of the
12 is the X and in the other half it is the Y. Although for a time it ap-
peared that the white man had twice as many chromosomes as the negro,
a difference ordinarily great enough to mark them as distinct species,
Wieman shows clearly that in his material the two have the same num-
ber, and is inclined to regard von Winiwarter's material as in some way
abnormal. Sex inheritance in man is evidently of the XY type, as
Wieman's researches and genetic data indicate with considerable clearness;
but why some material should plainly show twice as many chromosomes

1 Unpublished work cited by Babcock and Clausen, p. 538.

SEX 363

as other material is a question which only future investigation can answer.
It is not improbable, however, that a segmentation of the chromosomes at
points of constriction may be mainly responsible for this condition.

In all of the cases reviewed above the male produces two sorts of
spermatozoa differing visibly in chromatin content: in the language of
Mendelism, the male is "heterozygous for sex." The female produces
but one kind of egg; she is "homozygous for sex." The sex of the off-
spring is clearly dependent on the kind of spermatozoon which functions,
and is therefore definitely correlated with the chromosome mechanism.

Female Heterogametic. — There are also on record a number of cases,
chiefly among moths and birds, in which the female produces two kinds
of eggs differing in chromosome content, while the male produces but one
kind of spermatozoon: the female is heterozygous for sex, and therefore
heterogametic, while the male is homozygous. Certain cases of this type
will now be reviewed.

In the moth, Phragmatobia, Seiler (1913) has described the following
condition. In the male the somatic number of chromosomes is 56,
including 54 autosomes and two Z-chromosomes1 (Figs. 139 F; 140).
Each sperm receives 28 (27 + Z). In the female the somatic number
is likewise 56, but includes a ZW pair instead of the ZZ pair of the
male. Half the eggs receive 27 + Z and the other half 27 + Ww (the
TT-chromosome breaks temporarily into two parts during maturation).
An egg with 28 (27 + Z) chromosomes fertilized by a sperm with
28 (27 + Z) develops into a male moth with 56 (54 + ZZ). An egg
with 29 (27 + Ww} chromosomes fertilized by a sperm with 28 (27 +
Z) develops into a female moth with 57 (54 + ZWw). The W and
w subsequently reunite to form a single W, both sexes then having
the same number, 56. Since some embryos show more than the normal
number of chromosomes Seiler thinks it probable that the Z-chromo-
some is compound and may under certain conditions subdivide into
smaller elements. The same investigator has recently (1919) reported
a digametic condition in the female in two other moths, Talceporia
tubulosa and Fumea casta. In the former the eggs have 29 and 30
chromosomes, and in the latter 30 and 31.

In the moth, Abraxas grossulariata (Doncaster 1914), sex inheritance
is apparently of the WZ type, though there is often an aberrant behavior
on the part of the chromosomes which has not been entirely explained.

In the common fowls Guyer (1909, 1916) has made observations which
he interprets as follows (Fig. 140). In the male there are 18 chromo-
somes : 16 autosomes and two accessories. Both of the latter go to one
pole in the first maturation mitosis, and in the second mitosis they sepa-

1 It is customary to refer to the sex-chromosomes in species with sexually hetero-
zygous females as W and Z, instead of Y and X as in the more common sexually
heterozygous males.

364 INTRODUCTION TO CYTOLOGY

rate. Half the spermatids, and therefore sperms, receive nine chromo-
somes (8 + 1 accessory), while the other spermatids failing to receive an
accessory apparently degenerate. In the female there are 17 chromo-
somes: 16 autosomes and one accessory. Since the accessory passes
undivided to one pole at the first maturation mitosis and divides at the
second, half of the eggs receive nine chromosomes (8 + 1 accessory) and
half receive eight. An egg with nine fertilized by a sperm with nine
develops into a male with 18 (16 + 2 accessories). An egg with eight
fertilized by a sperm with 9 develops into a female with 17 (16 + 1
accessory).

Cases of Parthenogenesis. — The cytological phenomena in those animals
reproducing in part by parthenogenesis (see p. 357) are of much interest
in this connection. In the honey bee the male, which develops from
an unfertilized egg, has the haploid number of chromosomes in his cells,
whereas the female, arising from a fertilized egg, is diploid. Similar in
some respects is the case of the gall-fly (Neuroterus), in which eggs that
have undergone reduction develop into haploid males, while other eggs
are formed without reduction and develop into diploid females. In the
male-producing eggs of Phylloxera there are two sex-chromosomes, two
others being lost in the polar body; in the female-producing egg all four
are present. Two kinds of sperms are produced, half of them with a
sex-chromosome and half of them without it. The latter kind degen-
erate, leaving only the former functional. All eggs, if fertilized, develop
into females. In Hydatina senta those eggs producing one polar body
and developing into females are diploid, whereas those giving off two
polar bodies and developing into males are haploid. In all of these
cases maleness accompanies the haploid, and femaleness the diploid
condition1.

Plants. — Up to the present time a visible chromosome difference
between the two sexes in plants has been established only in Sphcero-
carpos, the genus of dioacious liverworts in which Douin (1909) and Stras-
burger (1909) found two of the spores of a single tetrad to be male and the
other two female. In Sphcerocarpos Donnellii (Allen 1917, 1919) (Figs.
142, 143) there are in the cells of the female gametophyte seven autosomes
which differ somewhat in length, and one very large X-chromosome. In
the cells of the male gametophyte there are seven autosomes and a very
small F-chromosome. The sporophyte therefore has eight pairs: seven
autosome pairs and the XY pair. Although all the stages in the divisions
at sporogenesis have not been seen, the evidence is sufficient to show that
the X and Y separate in the heterotypic mitosis and divide longitudinally
in the homoeotypic. Two spores of a tetrad therefore receive an X-
chromosome in addition to the seven autosomes; these spores develop
into female gametophytes. The other two spores of the tetrad receive

1 Compare the case of the frogs developing by artificial parthenogenesis, page 319.

SEX

365

the Y in place of the X and develop into male gametophytes. The same
condition has been reported for Sphcerocarpos texanus (Miss Schacke
1919).

Although the situation in Sphcerocarpos suggests the XY type of sex
inheritance in Drosophila and other forms, it differs in several respects,

mini

Kale garaetophyte

IIIHH'

X

Fertillzad Sporophyte Sporocyte

egg

Maturation divislono

5PMAEROCARP05

Female gametophyte

FIG. 142. — The history of the chromosomes in the life cycle of Sphcerocarpos. (After
data of C. E. Allen, 1917, 1919.)

as Allen (1919) points out. In Sphcerocarpos the separation of the XY
pair results in the production of two kinds of spores which develop directly
into haploid organisms (gametophytes) of two sexes, whereas in Droso-
phila the corresponding separation results in two sorts of male gametes
which determine the sexes of the
diploid organisms developing from
the eggs they fertilize. Furthermore,
in animals with sex-chromosomes some
forms show the presence of XX to be
correlated with femaleness and X or

X Y with maleness (male heterOZygOUS FIG. 143.— Chromosome groups from

for sex), while in other forms XX maje and female gametophyte cells of

,

Sphcerocarpos Donnelhi.

Allen, 1919.)

(After C. E.

is correlated with maleness and A

or XY with femaleness (female

heterozygous for sex1). There is evidence to show that the F-chromo-

some carries no sex-factors in these cases,2 though its absence may result

in sterility (Bridges). (See Chapter XVII.) In Sphcerocarpos, on the

1 Sex-chromosomes referred to in this case as Z and W rather than X and Y.

2 See in this connection Castle 1921.

366 INTRODUCTION TO CYTOLOGY

other hand, the presence of X is correlated with femaleness, F with
maleness, and XY with the non-sexual condition of the sporophyte.
Here the F is apparently as important as the X in the transmission of
sex-factors. Allen suggests that secondary sexual differences of the
gametophytes, such as size, may be connected with the relative amounts
of chromatin in the nuclei: the female gametophyte, having the large
X-chromosome and therefore a distinctly greater mass of chromatin,
develops more rapidly and becomes much larger than the male gameto-
phyte with its small F-chromosome. The primary sexual differences
he regards as due to other factors.

Although Sphcerocarpos affords the only known example of hetero-
chromosomes in plants it is not improbable that other cases will be
discovered.

Conclusion. — In all of the organisms included in the foregoing review
the individuals of the two sexes differ visibly in their chromosome com-
plements. Moreover, in most of them the sexual differences are definitely
correlated with special distinguishable chromosomes, which are accord-
ingly known as sex-chromosomes. The distribution of these bodies at
the time of reduction results in the production of two kinds of male
gametes or two kinds of female gametes, and in at least one case two
kinds of spores. In all of these the chromosome differentiation in the
cells correponds to the sexual differentiation of the organisms into which
they develop. The conclusion appears unavoidable that the differentia-
tion of the sexes is here determined by a cell mechanism, and that the hetero-
chromosomes have a definite causal relationship with sex. How close this
relationship may be, and to what degree it is a fixed one, are as yet by no
means clear, but it is beyond question that the heterochromosomes are
not the sole determining cause of sex, as some workers have hastily
concluded. To this question we shall subsequently return.

Sex -chromosomes and Mendelism. — The heterochromosome phe-
nomena are intimately bound up with the whole matter of Mendelian
inheritance. According to the Mendelian interpretation the sexes are
due, like other heritable characters, to factors carried by the chromo-
somes— by the heterochromosomes where these are present. The ap-
proximate 1 : 1 ratio of the sexes in most organisms is accounted for in
the following manner. Referring to our typical Mendelian pair of charac-
ters in the pea, tall and dwarf, it is found that when a plant heterozygous
for tallness (Tt) is crossed with a pure recessive (tt) the resulting off-
spring are half tall (Tt) like one parent and half dwarf (tt) like the other,
a 1 : 1 ratio. If it is assumed in a similar manner that there is a pair of
factors for sex, one sex (the male, Correns; the female, Bateson) being
heterozygous and the other a homozygous recessive, a 1 : 1 ratio of the
sexes results.

Largely because of the observed behavior of sex-chromosomes the

SEX

Mendelian interpretation has been restated as follows. There is a single
factor for sex. In some organisms the presence of two of these factors
is correlated with femaleness and one with maleness (Fig. 144, A): the
male, having only one sex-factor (S), is heterozygous and produces
gametes of two kinds, with and without the factor; the female, having two
sex-factors, is homozygous and produces eggs of one kind, with one sex-
factor each. Two types of union are here possible, giving males and
females with one and two sex-factors respectively. This interpretation
is directly applicable to those cases in which the male has one sex-chromo-
some and the female two (Ancyr acanthus, Ascaris), and also to those
having an XY pair in the male and an XX pair in the female (Droso-
phila, Lygceus). Each sex-factor is thus thought to be located in an X-

6(i)0 CD
\ X /

\ / \
OQ0O

\ A _/

FIG. 144. — Mendelian interpretations of sex-inheritance. Explanation in text.

chromosome. In other organisms (moths and birds) these conditions
are reversed (Fig. 144, C), the presence of two sex-factors being correlated
with maleness and one with femaleness. The female is thus heterozygous
and produces eggs of two kinds, with and without the factor. In the
next generation the male receives two sex-factors (in the Z-chromosomes
of the egg and sperm) and the female one (in the Z-chromosome of the
sperm; the TF-chromosome of the egg carries no sex-factors). In view of
these two contrasted conditions as regards the quantitative relationship
between factors and sex, it is probable that the sex-factors carried by the
X-chromosomes are in some manner different from those in the Z-chromo-
somes. It has been suggested that in some cases (Fig. 144, B) the male
may have no sex-factors at all, the heterozygous female thus having one
more sex-factor than the male, as in the homozygous females of Fig.
144, A.

Experimental Alteration of the Sex Ratio. — Although most organisms
approximate closely the 1 : 1 sex ratio called for on the basis of the

368 INTRODUCTION TO CYTOLOGY

Mcndelian theory of sex, constant deviations from this ratio are fre-
quently found. Still more significant is the fact that in many cases the
ratio can be markedly altered by changing the environmental condi-
tions. Thus R. Hertwig (1906, 1912) and Kuschakewitsch (1910) found
that if the eggs of the frog are allowed to become over-ripe before
fertilization, in which case they take up an abnormal amount of water,
the resulting individuals show an unusually high percentage (even 100
per cent) of males. Conversely, Miss King (1907-1912) lowered the
water content of toad eggs, and with a mortality of only a little over 6
per cent obtained 80 per cent females.

In Dinophilus, as already noted, there are two kinds of eggs laid:
large ones developing into females and small ones developing into males.
Malsen (1906) found that by altering the temperature the relative pro-
portion of the two sexes could be changed, but this effect was brought
about through an influence on the laying of the eggs: both kinds were
produced as usual, but the laying of one kind was hindered.

The rotifer, Hydatina senta (Whitney 1914, 1916), if scantily fed on
Polytoma, continues to produce generations of parthenogenetic females,
but when copiously fed on Euglena females appear which lay male-pro-
ducing eggs, and sexual reproduction then occurs. According to Shull
and Ladoff (1916) the percentage of males is here correlated with the
supply of oxygen which counteracts certain agencies (accumulated
substances in the water) tending to decrease male production. Whitney
(1919), however, contends that oxygep is not a factor affecting sex in
Hydatina.

In interpreting such results as these considerable care should be
exercised in distinguishing an actual determination of the sex of an
individual from a number of other phenomena which, though they may
appear like sex-determination, are not to be regarded as such in the
strict sense. In many cases in which changed environmental factors
have been shown to have an influence on the sex ratio it is clear that the
results are not due to an actual reversal or determination of the sex in
any individual, but rather to the fact that the new conditions imposed
have caused a greater mortality among the eggs or embryos of one sex, so
that those of the other sex preponderate. Although the ratio of the sexes
may here be subject to an experimental control, the sex of no given
individual is actually determined or altered.

Indirect control of another type is seen in organisms whose sex is
dependent upon the form of reproduction (zygogenetic or partheno-
genetic). Environmental conditions may in such cases influence the form
of reproduction resorted to, and therefore the sex of the animals resulting;
but the sex of no individual, once started, is altered. Morgan points out
that the change of diet in Hydatina, instead of altering sex directly,
induces the formation of a new type of female which may either function

SEX 369

sexually or produce eggs which develop parthenogenetically into
males.

Another case in point is that of Dinophilus, in which abnormal temper-
ture conditions prevent the laying and development of eggs from which
the individuals of one sex normally arise. As Thomson (1913, p. 502)
remarks, " . . .if nutritive and other environmental influences are
operative, it is, in the main, by affecting the production and survival of
sexually-predestined germ cells."

A clear case of sex-determination in the strict sense would be one in
which a given parthenogenetic egg, fertilized egg, spore, or young individ-
ual, in a unisexual organism, could be made to develop at will into either
sex; or in which such an organism, already showing the essential characters
of one sex, could be made to develop into the other sex (sex-reversal).
Evidence that in certain cases such a determination or reversal can
actually be accomplished has had much to do with the development of
recent metabolic theories of sex.

Metabolic Theories of Sex — Animals. — According to the metabolic
theories of sex, the difference between the two sexes is primarily one of
metabolism, the difference being not necessarily in the kind of metabolic
processes, as Geddes and Thomson thought, but more probably in their
rate or level. One of the most prominent exponents of this type of
theory is Riddle (1912, etc.), who has continued the important researches
on pigeons begun by Whitman many years ago. At each laying these
birds produce a pair, or clutch, of eggs. Under normal conditions the
first egg laid develops into a male and the second into a female. By a
careful analysis of a large number of eggs Riddle has been able to show
that the yolk of the first egg is the smaller, and has the smaller amount of
storage material (fats and phosphorus-bearing compounds), the higher
water content, and the higher oxidizing capacity, or higher metabolism.
The second or female-producing egg therefore differs in having a larger
yolk, more storage material, less water, and a lower metabolism. The
chromosomes of the pigeon are not accurately known, but it is probable,
as indicated by the behavior of the sex-linked factors (see next chapter),
that the WZ type of sex-chromosome differentiation found in other birds
is present here. Whatever their cytological differences may be, the two
sexes are characterized in the egg stage by different levels of metabolism, the
male having the higher level and the female the lower; and this difference has
been shown (as indicated by the metabolism of the blood) to persist into
the adult stage.

The seasonal change in the amount of storage, and consequently in the
level of metabolism, is definitely correlated with the percentage of a
given sex and the degree of its manifestation. As the season advances
both eggs of the clutch store more energy-containing materials, and the
birds developing from the second (female-producing) egg, while often

24

370 INTRODUCTION TO CYTOLOGY

quite "masculine" in their secondary sex characters early in the season,
become increasingly "feminine " toward the end of the season. Moreover,
the total percentage of females increases. By influencing storage, water
content, and the general metabolic condition of these eggs Riddle has
been able to induce experimentally an actual reversal of their natural sex
tendencies. Hence he concludes that sex is a quantitative, modifiable,
and "fluid" character; the two sexes do not represent two qualitatively
distinct, mutually exclusive properties, but are rather two conditions of
one general property — two levels in a continuous series of metabolic
states passing gradually one into another. Accordingly, if the two levels
normally maintained can be sufficiently altered a series of intergrades
between the two sexes and an alteration of the sex itself should be possible,
and this Riddle has apparently accomplished.

On the basis of this metabolic theory of sex Riddle interprets the
results obtained by Miss King with toad eggs, by Hertwig with those of
the frog, by Whitney and Shull with rotifers, and by other investigators
to be mentioned below. The correlation of high and low water content
with maleness and femaleness respectively in toads and frogs, and that of
change of food and increased oxygen supply with maleness in rotifers, are
held to be in harmony with similar correlations which have been shown to
exist in pigeons.

.The theory that sex is a quantitative, reversible state closely asso-
ciated with metabolic conditions is strongly supported by the researches of
Goldschmidt (1916a6, 1917), Banta (1916), and Lillie (1917). In the
gypsy moth, Lymantria dispar, which is cytologically heterogametic,
Goldschmidt has been able to induce experimentally a large series of "sex
intergrades " between the male and female conditions. It appears that an
individual of either sex, after beginning its development, may be made to
develop the characters of the other sex partially or completely, the degree
of alteration depending upon the time at which the change sets in. Gold-
schmidt concludes that normal individuals of both sexes must have both
sex capabilities, the sex manifested depending upon the relative strength
with which they are caused to act by certain conditions. He thinks it prob-
able that the hereditary characters have their material basis in enzymes
or substances of a similar nature, those associated with femaleness and
maleness being termed " gynase " and " andrase " respectively. Although
for a time he interpreted the behavior of the sexes in Lymantria on the basis
of Mendelian factors, he is now inclined to view this form of explanation
as inadequate. The chromosome behavior in these moths is not well
known, but breeding experiments with an intersexual female functioning
as a male show the results which would be expected on the assumption
that she had the TFZ-chromosome constitution. This would mean that
the moth in question had had its sex reversed without any visible
change in its chromosome complement, but this intrepretation awaits
cytological confirmation.

SEX 371

In the phyllopod crustacean, Simocephalusvetulus, Banta (1916) finds
that under certain experimental conditions " . . . the same individual,
even the same sex gland, may develop eggs and sperms at the same time or
sperms at one time and eggs at another time." Banta looks upon sex
as not a fixed or definite state but rather "a purely relative thing"
dependent upon the general physiological state of the organism which in
turn is under the influence of environmental factors. He also reports
sex intergrades in Daphnia longispina (1918).

Lillie (1917a6c), as the result of a study of twins in cattle, concludes
that "sex-determination in mammals is zygotic, but it does not imply an
irreversible tendency to the indicated sex-differentiation. Intensification
of the male factors of the female zygote from the time of onset of sexual
differentiation by action of sex hormones may bring about very exten-
sive reversal of the indicated sex-differentiation" (1917c).

Intersexes have recently been described in Drosophila simulans by
Sturtevant (1920). All the intersexual flies are modified females and
show the same grade of intersexuality. Sturtevant believes the condition
to be due to the action of a mutant gene. If this interpretation is
applied generally to the phenomenon of intersexuality it is evident
that certain genes at least are modifiable by environmental conditions,
for the reason that such conditions so often evoke the intersexual state.

A very striking instance of the control exercised by environmental
factors over the sex of a given individual is found in Bonellia (Baltzer
1914). In this gephyrean worm the male is very small and degenerate,
and lives parasitically upon the long proboscis and in the nephridium
of the much larger female. If young individuals are kept in an aquarium
by themselves they develop into females, but if they are placed with
mature females they settle upon the probosces of the latter and develop
into males. By allowing them to remain on the probosces for varying
lengths of time Baltzer was able to obtain many intergrades between the
typical male and female conditions. It is plain that the proboscis
exercises an influence over the sex of the young animal, but whether this
is through a secretion or some other agency is not known.

A somewhat similar case is that of Crepidula plana, described by
Gould (1917). This mollusk is hermaphroditic but completely protan-
dric, i.e., the male and female phases are entirely separate in time, the
former developing first. The development of the male phase is dependent
upon the presence of a larger individual (not necessarily a female) of the
same species. If a male is removed from the neighborhood of a larger
individual the male organs degenerate, and after a period of sexual
inactivity the animal becomes a female. The nature of the stimulus
exerted by the larger individual has not been ascertained.

Plants. — Experiments of a corresponding nature on sex modification
in plants are on the whole less conclusive than those on animals, for the

372 INTRODUCTION TO CYTOLOGY

reason that many of them have been carried out with angiosperms, in
which hermaphroditism is so common and which are regarded by some
botanists as all potentially bisexual. As a case illustrating the latter
point may be cited the hemp plant, Cannabis sativa. This species is
normally unisexual, but if the flowers are removed it will produce flowers
of the other sex (Pritchard 1916).

Intersexes of many grades between the normal plants in dicecious
angiosperms are frequently found, as in Myrica Gale (Davey and Gibson
1917), Cannabis sativa, Salix amygdaloides, and Morus alba (Schaffner
1919a&), whereas the relative proportions of maleness and femaleness in
hermaphroditic forms are apparently very easily influenced by the
environment. In Plantago lanceolata (Bartlett 1911; Correns 1908;
Stout 1919) the stamens and pollen are developed in various degrees in
different flowers, or even in the same flower; in some cases they are only
rudimentary, leaving the flowers functionally female. Stout inclines
to the view that dicecism results from the suppression of femaleness or
maleness in organisms originally monoecious, and points out that the
many "degrees of maleness" in Plantago fill the gap completely in this
case. He accordingly concludes, in agreement with Riddle, Banta,
and Goldschmidt, that sex is a labile, reversible character; that maleness
and femaleness, both present in the somatic cells of all sporophytic
individuals, are relative and not absolute conditions; and that "sex-
determination, at least in hermaphrodites, is fundamentally a phe-
nomenon of somatic differentiation that is ultimately associated with
processes of growth, development, and interaction of tissues, and subject
to modification or even complete determination by them." The sex-
chromosome theory is held to be inadequate in the case of hermaphro-
dites: sex is an "epigenetic," not a "preformed" character.

Yampolsky (1919, 1920) thinks it probable that male and female
gametes with graded potencies are produced in Mercurialis annua, and
that the sexes cannot be due to a segregation of sex-factors at reduction.

The sex of the haploid phase (gametophyte) of the plant life cycle,
when this phase is unisexual (dicecious), behaves as a much more nearly
irreversible character, as shown by the experiments on mosses, Marchantia,
and Phycomyces cited early in the present chapter. How widely this holds
true in thallophytes and bryophytes is not known. Allen regards
the quantitative theory of sex as quite inapplicable to the case of
Sphcerocarpos. In the homosporous pteridophytes the gametophytes are
commonly monoecious, and the fact that here, as in many liverworts,
the male organs appear early and the female organs later suggests a
physiological basis of sex-determination. Some fern gametophytes, on
the contrary, are dicecious under all ordinary conditions. In Onoclea
(Wuist 1913), such a normally dicecious form, the female gametophytes
under certain experimental conditions produced antheridia in addition

SEX 373

to their archegonia, but the male gametophytes could not be made to
develop archegonia. In Osmunda regalis japonica and Asplenium nidus,
which are monoecious, Nagai (1915) found that the concentration of the
Knop's nutrient solution used has a controlling influence over the kind
of sex organ appearing, the number of antheridia in general decreasing
with the concentration.

These results, taken together with the fact that many gametophytes,
especially those of homosporous pteridophytes, are monoecious, show that
the capabilities of both sexes can be present in the haploid nucleus, as
Strasburger thought, although in many forms the visibly developed sexes
may be automatically determined by some cell mechanism.

General Discussion. — We have now reviewed some of the evidences
which have led to two general theories of sex and its determination. One
theory represents an attempt to account for the phenomena in question
on the basis of a morphological cell mechanism whereby Mendelian or
other factors are distributed in a definite and fixed manner, whereas on
the other theory it is held that they are the results of a physiological
differentiation manifesting itself chiefly in alterable levels of metabolism.
Although these two conceptions may appear to be mutually exclusive
if expressed in too uncompromising a form, both must contain elements
of truth. It is beyond question that the two manifestations of sexual
differentiation, the physiological and the morphological, are both of
importance and cannot be ultimately irreconcilable: our task is to
determine their relative significance and to discover the nature and
degree of their mutual interdependence. It seems clear that the digam-
etic condition when present in dioecious forms does regulate the ratio
of the sexes: under all ordinary circumstances the sex of the individual
is here dependent upon the kind of gamete which gives rise to it (or the
kind of spore in the case of certain gametophytes). But it is to be
emphasized that the dimorphism shown by such gametes or spores is not
entirely a morphological one, or even mainly so : in many cases no morpho-
logical difference can be detected although the two are clearly different in
physiological behavior, as shown by the spores of Phycomyces and
certain bryophytes. It is generally inferred that here a structural
difference, although invisible, is nevertheless present.

In this connection it may be recalled that the differences between the
male and female gametes, irrespective of any differentiation which may be
present among those of either kind, are both physiological and morpho-
logical. The primary characters of sex are those possessed by the
gametes themselves, and the principal distinction between the male and
female gametes seems to be a physiological one which is manifested in
their mutual attraction and fusion. Any visible morphological differ-
entiations that they may possess are to be regarded as secondary adapta-
tions to unlike functions, because of the fact that in many of the lower

374 INTRODUCTION TO CYTOLOGY

organisms there is no discernible structural difference between the
male and female gametes, and further because structural differences
may be annulled in certain instances (some gregarines). Any material
differences present in visibly similar gametes are more probably chemical
in nature, the structural difference being of a molecular order. Indeed,
there is probably no physiological difference without a structural differ-
ence of this sort. If the term structure be extended to include molecular
constitution the discussion over the relative priority of structural and
physiological differentiation becomes futile, for at this level the two are
aspects of one and the same change. It is only when we restrict the term
structure to the grosser, visible features that we can speak of physiological
differentiation as preceding alteration in structure . Ultimately structural
and functional changes are indistinguishable. Just as in the gametes
of the two sexes, so also in the unisexual individuals which the gametes
produce there may be striking differences of both morphological and
physiological natures; but if we use the term morphological only with
reference to visible features the primary distinction between the sexes
in organisms of all grades is apparently one of physiological state, this
distinction and its result (sexual reproduction) be.ing of the greatest
biological importance.

Taking into consideration all organisms, low and high, it seems
probable that any dimorphism among the gametes of one sex or the other
has in some way been developed in connection with the maintenance of
the above mentioned difference in physiological state in organisms of a
certain level of advancement. Different organisms show all degrees in
the differentiation among the gametes of one sex: some are marked by an
absence of any visible difference either in the gametes or in the her-
maphroditic individuals produced; in others the gametes (of one sex) are
visibly similar but result in male and female individuals in regular ratio ;
and finally there are those in which the gametes are of two kinds both
physiologically and morphologically, the two kinds controlling the pro-
duction of individuals of the two sexes. It is in organisms of the last type
especially that the question of sex-determination finds the adherents of
the chromosome-Mendelian theory and those of the metabolic theory
in disagreement. Is the sex of the individual inevitably dependent
upon the type of gamete functioning (usually the kind of sperm fer-
tilizing the egg), or is it possible to overcome the effect which the
chromosome mechanism may have by influencing sufficiently the
metabolism of the organism? If the sex of an individual is so changed,
does the chromosome mechanism undergo a corresponding alteration?

Those who have developed the chromosome-Mendelian theory have
perhaps too often held that the two sexual states, maleness and female-
ness, are in their ultimate analysis mutually exclusive — that they are two
fundamental and qualitatively different alternative characters depending

SEX 375

unalterably upon unit factors. The hereditary factors or genes are
usually regarded by geneticists as unmodifiable except by sudden muta-
tions. Failure to distinguish between the modification of genes and the
modification of the interaction of genes during ontogenesis has led many
to the view that the sex of the individual must be rigidly fixed at fertiliza-
tion in digametic and dioecious forms, or at reduction in the case of dice-
cious gametophytes like those of Sphcerocarpos. It is very difficult to
reconcile any inflexible theory of this nature with the great diversity of
situations known without resorting to hypotheses of somatic segregation
of factors, alterations in dominance, and other assumptions not well
supported by observational evidence. Such difficulties are encountered
in the common hermaphroditic condition of gametophytes, which are
haploid; in the possibility of causing the development of the second
sex in gametophytes normally unisexual (Onoclea) ; and especially in the
numerous cases of sex intergrades, in which it is possible not only to con-
trol the relative amounts of maleness and femaleness in hermaphroditic
forms, but also to produce all intermediate grades between male and
female individuals, and furthermore to reverse the sex in unisexual forms,
even in certain species with sex-chromosomes (moths and probably
pigeons).

If, in accordance with the ideas of many biologists, sex is held to
be a quantitative, "fluid" character associated with a continuous series
of physiological states which may pass into one another, the way is open
for the explanation on a common basis of all cases of hermaphroditism in
both haploid and diploid individuals, of sex intergrades, and of the experi-
mental modification of sex. At the same time the influence of the sex-
chromosomes or even of smaller factors within them may be allowed.
As Riddle (1917) states, organisms have had the problem of producing
germs of two metabolic levels, and in some cases this has led to the
establishment in the two sexes of two amounts of chromatin or even of two
different chromosome complements. The sex-chromosomes, or units
contained within them, act with others in the maintenance of two
diverse levels of metabolism in the gametes and in the offspring, and with
these levels are correlated the two conditions which we distinguish as
male and female. Even if the sexes in such cases do not differ in the
quality of their chromatin, they at least differ quantitatively in this
respect, in agreement with the theory that sex is a quantitative character.
The chromosome difference being only one factor in a complex system
producing the two sexual states, and no single element in this system
being the sole determiner of sex, it is not impossible that the effect of this
one factor should be annulled by sufficiently altering the other factors
and thus modifying the action of the factorial system as a whole. The
same is to be said of other characters also. What an organism inherits
is not simply this or that character or sex, but rather a tendency to

376 INTRODUCTION TO CYTOLOGY -

develop a definite group of characters, including a particular sex, under
a given set of environmental conditions.

In those organisms possessing heterochromosomes the sex of the
individual under all ordinary circumstances is dependent upon the kind
of sperm (or, in some cases, the kind of egg) functioning at fertilization,
and does not change thereafter. Furthermore, it apparently cannot be
changed by many methods commonly supposed to be efficacious in this
respect. So far the chromosome theory is valid; but it does not follow
from this that the sex which is characterized by a certain physiological
state and is correlated with a certain type of chromosome complement
and a variety of secondary sexual characters, is so firmly fixed that it
cannot be altered by any extraordinary means. The metabolic state,
even though its regulation may be accomplished in part through a visible
mechanism, is the resultant of a complex series of reactions which may be
interfered with at many points. In some cases this metabolic state has
been artificially altered to a degree sufficient to bring about an actual
reversal of the sex. It is admitted that other heritable characters de-
finitely associated with constant genes are greatly modified in the manner
and degree of their expression by environmental influences, and the
evidence now at hand indicates that no exception to this rule can be made
in the case of sex.

In criticizing the results and interpretations of Riddle, Morgan (1919a)
points out that the behavior of a certain sex-linked character worked out
by Strong (1912) indicates that the females which were "changed into
males" have the male chromosome complement, and that sex is as much
a matter of chromosomes here as elsewhere. He declares further that
there is as yet no known case in which the sex determined by a chromo-
some mechanism has been changed by other agencies in spite of the
chromosome arrangement. The evidence here points to the conclusion
that when an alteration of the sex is induced, this does not occur with-
out a corresponding alteration in the chromosome mechanism. In Droso-
phila, however, Sturtevant (1920) finds that the intersexes observed by
him are modified females with the usual two X-chromosomes. Here,
therefore, certain male characters at least are present in an organism with
the female chromosome complement.

The number of instances of change of sex in forms normally controlled
by a chromosome mechanism will probably increase as the nature of
sexual differentiation becomes more fully known and as experimental
technique improves; but it is also probable that in animals the sex of
which we are most desirous of controlling, practical difficulties may pre-
vent the attainment of satisfactory results. Slight differences in the
responses of the two kinds of male gametes (or female gametes) might
conceivably make possible a control over the kind functioning, but it
seems more probable that the sex of the individual will be found to be

SEX 377

more easily influenced, if at all, after fertilization. But it is quite un-
known whether or not an individual which had undergone a reversal of
sex would be as successful biologically as a normal one. Unusual fea-
tures may be expected in a sexually functional organism with the chromo-
some complement normally accompanying the other sex, if such an
organism should be found; but such interesting questions can only be
answered by the facts yielded by further research.

All questions of sex control are secondary to the main problem of the
ultimate nature of sex, a problem which reveals itself with increasing
clearness as primarily a physiological one. The question of the origin
and significance of sex is one which lies outside the scope of this work.

Bibliography at end of Chapter XVIII.

CHAPTER XVII
LINKAGE

In Chapter XV attention was directed to the remarkable parallelism
which exists between the distribution of the Mendelian characters and
that of the chromosomes. A vast number of breeding experiments with
both plants and animals have shown that new combinations of characters
are formed at the time of fertilization, when two parental sets of chromo-
somes are brought together, and that a segregation of characters occurs
at the time of reduction, when the chromosomes are sorted out into two
groups. Moreover, the distribution of a single allelomorphic pair of
Mendelian characters parallels precisely that of a single homologous pair
of chromosomes. These facts indicate clearly that chromosomes and
characters are in some manner causally related. This conclusion is
strongly supported by the cytological aspects of sex inheritance, maleness
and femaleness in a large number of reported cases being definitely corre-
lated with the activity of certain distinguishable chromosomes.

It has also been pointed out that the hypothesis upon which these
phenomena are generally interpreted is that the characters are repre-
sented in the chromosomes by material factors, or genes, which in some
way control the development of characters in the individual. Since, now,
an organism usually has many more Mendelian character pairs than it has
chromosome pairs, one pair of chromosomes must as a rule carry genes
for more than one pair of characters. Furthermore, the different pairs
of chromosomes are entirely independent of one another in distribution.
It would therefore follow that if two allelomorphic character pairs have
their genes located in different chromosome pairs, they will be quite
independent of each other in their inheritance through a series of genera-
tions; whereas, if their genes are located in the same pair of chromo-
somes, they will be inherited together. The latter condition — the
persistent association of characters belonging to different allelomorphic
pairs through a series of generations — actually exists and is known as
linkage,

The phenomenon of linkage was discovered in 1906 by Bateson and
Punnett in the sweet pea. They found flower color to be linked with the
shape of the pollen grain: purple flowers nearly always had long grains,
while red flowers had round grains. The possible relationship between
linkage and the chromosome hypothesis was pointed out by Lock in the
same year. Linkage relations have since been worked out in a consider-

378

LINKAGE

379

able number of plants and animals, and are especially well known in
the case of the fruit fly, Drosophila melanogaster, owing to the exhaustive
researches of Morgan and his coworkers.

A Typical Case of Linkage. — As a typical example may be taken the
following case of linkage in Drosophila (Fig. 145). A male fly with white
eyes and yellow body is mated to a female with red eyes and gray body.

LINKAGE IN DROSOPHILA

99%

1%

FIG. 145. — Linkage in Drosophila. Red eyes in black; gray bodies stippled; white eyes

and yellow bodies unshaded.

The flies of the FI generation all have red eyes and gray bodies, since red
is dominant over its allelomorph white, and gray is dominant over its
allelomorph yellow. If,rnow, the females of this generation are "back-
crossed" to males with both of the recessive characters,1 white and yellow,

1 This back-crossing to the pure recessive is the common method of testing the
genotypic constitution of hybrids.

380 INTRODUCTION TO CYTOLOGY

flies of four types appear in the next generation, as shown in the dia-
gram: white-yellow, white-gray, red-yellow, and red-gray. Since one
parent in this last cross is a double recessive, these results show that
the FI red-gray female must produce eggs of these four genotypic
constitutions.

All four types, however, are not produced in equal numbers. Those
flies with the same combinations as were shown by their grandparents,
white-yellow and red-gray, together comprise 99 per cent of the total
number; only 1 per cent show the other possible combinations, white-
gray and red-yellow. It thus appears that if the two characters, red
eyes and gray body, enter a hybrid together (i.e., are contributed by the
same parent) they come out together in the'next generation in nearly all
cases: they are definitely linked, and this is explained on the hypothesis
that their genes are located in the same chromosome. The same is
obviously true of their respective allelomorphs, white eyes and yellow
body: their genes are carried in the other chromosome of the homologous
pair. Were the two allelomorphic pairs of genes carried in different pairs
of chromosomes no such linkage would occur : the two characters red and
gray, and similarly the two characters white and yellow, would then be
present together in 50 per cent of the flies, the chance frequency, rather
than 99 per cent. How it is that the linkage is broken in some cases,
giving 1 per cent with exceptional combinations, is a question to which
we shall return later in the chapter.

Sex-linkage. — A very interesting special case of linkage is seen in
the phenomenon of sex-linkage, which may be illustrated by the following
example (Fig. 146) (Morgan 1910). If a red-eyed wild male is mated to a
white-eyed female (a member of a race descended from white-eyed
mutants) the FI individuals are white-eyed males and red-eyed females —
each eye color has been transferred from one sex to the other, a case of
"criss-cross" inheritance. If these FI flies are bred together, the F2
generation comprises four types: red-eyed males and females, and white-
eyed males and females. Turning now to the chromosomes, if the dis-
tribution of the sex-chromosomes is followed through these generations
this striking fact is revealed : red eye color appears in every fly, male or
female, which possesses the .XT-chromosome of the original red-eyed male ;
and white eyes appear in every male which receives one of the X-chromo-
somes of the original white-eyed female, and in every female receiving
two of them. This is taken by Morgan to mean that the original male's
X-chromosome carries the dominant factor for red eyes, while each of
the X-chromosomes of the original white-eyed female carries a recessive
factor for white eyes. In all the flies it is seen that the presence of one
X-chromosome is correlated with maleness, and that of two X-chromo-
somes with femaleness (compare Fig. 140); and that the two types of
eye color under consideration follow the distribution of these chromo-

LINKAGE

381

somes — they are sex-linked characters.1 So far as is known, the Y-
chromosome of the male carries 110 sex- or sex-linked factors. This
general interpretation is directly applicable to the reciprocal cross (white-
eyed male X red-eyed female), in which, however, the relative proportions
of red-eyed and white-eyed flies in F\ and F2 are different: in FI all flies
of both sexes have red eyes, while in Fz all the females and one-half of
the males have red eyes, white eyes appearing only in one-half of the
males. (See Morgan et al. 1915, pp. 16-20; Babcock and Clausen 1918,
pp. 74-77.)

FIG. 146. — Sex-linkage in Drosophila. Three successive generations at left; red eyes
shown in black. The history of the sex-chromosomes through these generations shown at
right; X -chromosome of original male shown in black. (Adapted from Morgan.)

In such cases as the above it is evident that characters other than
sex may be referred to certain chromosomes of the complement: it is
possible not only to tell which chromosomes have to do with sex, but
also to identify the ones concerned in the production of red and white
eye colors. A large number of such sex-linked characters have been
identified in Drosophila, and several have been found in other animals.
Human colorblindness is a character which is inherited in a manner
analogous to that of sex-linked characters in Drosophila, and its me-
chanism is apparently the same. The presence of this defect more
commonly in men than in women, and its appearance in so few individ-
uals in affected lines, are due to the fact that it is both a recessive and a

1 Sex-linked characters are not to be confused with sex-limited characters. The
latter are those found exclusively in one sex, and are now referred to as secondary
sexual characters.

382 INTRODUCTION TO CYTOLOGY

sex-linked character, precisely like white eyes in Drosophila. It occurs
in a woman only if both of her X-chromosomes bear factors for it,
which means that such a factor must have been received from each
parent; whereas one such factor is sufficient to produce colorblindness in
a man, because his F-chromosome carries no factors which might domi-
nate it. Furthermore, since the JT-chromosome of the male is always
derived from the mother, a man can inherit colorblindness from his
mother but not from his father. From these facts it follows that a
colorblind woman transmits the defect to all of her sons and to half of
her grandsons and granddaughters; whereas a colorblind man transmits
the defect to none of his children and only to one-half of his grandsons.1

The first sex-linked character known in plants was that of narrow
rosette leaves in the red campion, Lychnis dioica (Shull 1910, 1911), a
plant which appears to have the XY type of sex inheritance, but in which
no distinguishable sex-chromosomes have been identified.

Non-disjunction. — The chromosome interpretation of sex and of
sex-linkage has received an interesting confirmation in a phenomenon
discovered by Bridges (1913). In a certain strain of Drosophila the
white-eyed females were observed to give rise to a certain proportion of
"unexpected " forms. Most of their offspring (92 per cent) were red-eyed
females and white-eyed males, as expected in such an experiment, but
some of them (8 per cent) were white-eyed females and red-eyed males.
A long series of crosses showed that these results could be accounted for
if it were assumed that in the original white-eyed female both of the X-
chromosomes passed together to one pole in the reduction division instead
of separating as they should. This was termed non-disjunction (Fig. 147) .
As a result the eggs, instead of having the normal single X-chromosome,
would have either two or none, and the distribution of the sexes and the
sex-linked characters would be altered in the manner observed. In a
cytological examination of the flies in which these abnormal phenomena
appear Bridges showed that this non-disjunction of the X-chromosomes
does occur occasionally in the female (Fig. 148). The chromosome
theory thus received confirmation. "An abnormal distribution of the
sex-chromosomes goes hand in hand with an abnormal distribution of all
sex-linked factors" (Morgan). Additional genetic and cytological data
have since been furnished by Bridges (1916) and Safir (1920).

Linkage Groups. — An extensive series of studies on linkage relations
in various plants and animals has brought out the fact that the Mendel ian
characters of a given organism fall into a certain number of "linkage
groups," the members of each group being linked to one another in
various degrees but showing no linkage with the members of other groups.
It appears further, when the relationships of enough characters have
been worked out, that the number of linkage groups is the same as that

1 This case is fully explained by Babcock and Clausen (1918, p. 197).

LINKAGE

383

of the chromosome pairs. Drosophila melanogaster, in which linkage
relations have been most fully analysed, has four pairs of chromosomes
(Fig. 148) : two large " euchromosome " pairs, one pair of sex-chromosomes,

FIG. 147. — Non-disjunction and its results in Drosophila. The two large circles in
first row represent male and female flies producing sperms and eggs respectively. Non-
disjunction in the female gives 2 kinds of eggs, with XX and with no sex-chromosomes,
instead of the normal single kind with one X. At fertilization there are possible 4 combi-
nations rather than 2, as shown in the large circles of second row. Owing to the several
ways in which her 3 sex-chromosomes may be distributed at maturation, the female repre-
sented by the third circle produces 4 kinds of eggs. When mated to a normal male (below
horizontal line) with his 2 kinds of sperms, 8 combinations are possible (last row). Nos.
1, 4, and 5 are normal flies and give the usual types of progeny. Nos. 2, 6, and 7, owing
to the presence of 3 sex-chromosomes, give exceptional results when bred. Types No. 3
and No. 8 do not appear in the cultures, probably because they die very early. The
original male has red eyes and the original female white eyes. Red eyes (represented by
dots) appear in every fly bearing the .X"-chromosome of the original male, as in Fig. 146.
Compare Morgan 1919a, Figs. 93 and 94. (Diagram based on data of Bridges and Morgan.)

FIG. 148. — The chromosomes of Drosophila melanogaster as they appear during mitosis in a
female, a male, and a non-disjunctional female. (After Morgan.)

and one pair of very small "m-chromosomes." The Mendelian charac-
ters in Drosophila fall into four linkage groups, and it is noteworthy
that one of these groups contains only two known characters. Each

384 INTRODUCTION TO CYTOLOGY

chromosome pair therefore seems to be responsible for a certain group
of characters. It has been shown above that one of these groups,
the sex- and sex-linked characters, can be definitely assigned to the pair
of sex-chromosomes; and Morgan further believes that the factors
for the two characters of the small linkage group are located in the
m-chromosomes. The two remaining linkage groups, which contain
many characters each, are assigned to the large euchromosome pairs.
Each chromosome is accordingly regarded as a body containing the fac-
tors or genes for a considerable number of characters; and on the basis
of the evidence to be presented below it is concluded that these genes,
differing thus in their hereditary potencies, are arranged in the chromo-
some in a linear series as suggested by Roux many years ago.

In plants the best known cases of linkage are in Zea Mays, in which
Emerson and his students at Cornell have identified six linkage groups,
and Pisum, which has so far shown four linkage groups (White 1917).
Since maize has 10 pairs of chromosomes, four more groups may be
expected, while in Pisum, which has seven pairs, three more groups will
probably be established; in fact seven independently inherited characters
are known. It is an interesting fact that Mendel, in his famous researches
on Pisum, happened to select for study seven pairs of characters belonging
to the seven different groups, and so did not detect the phenomenon of
linkage.

From the foregoing considerations there arises an interesting and
very important question. If two homologous chromosomes, each carry-
ing factors for a certain group of characters (those of one group allelo-
morphic to those of the other group), separate into different gametes
(or spores) at the time of reduction, how does jt happen that occasionally
there appears an individual with some of the characters of each group ?
And if a single chromosome carries a series of factors for a certain group
of characters, how shall we account for the occasional individual with
some of these characters but not the rest? To state the problem in the
terms of linkage, if each group of linked characters is represented by a
series of genes in a given chromosome, how is the linkage broken in
a certain percentage of cases, with the resulting formation of new link-
age groups, as shown by the exceptional red-yellow and white-gray flies
in the experiment described at page 379? A solution to this problem has
been offered in the Chiasmatype Theory.

The Chiasmatype Theory. — In our discussion of chromosome con-
jugation it was pointed out (p. 257) that various opinions have been
entertained regarding the nature of the association between the members
of the synaptic pair. Some workers have held that the chromosomes
fuse completely and lose their identity, and that the two chromosomes
appearing on the first maturation spindle are not to be looked upon as
identical with those which entered into conjugation. On the contrary,

LINKAGE

385

there are those who deny any fusion at all between the members of the
pair, holding rather that their identity is not impaired in any way during

FIG. 149. — The behavior of the conjugating homologous chromosomes according to
the Chiasmatype Theory of Janssens. Single crossing over at left; double crossing over
at right. (Adapted from Babcock and Clausen.)

FIG. 150. — Crossing over between 2 of the 4 chromatids of the chromosome tetrad, giving
2 crossover and 2 non-crossover gametes. (Adapted from Janssens, 1909.)

FIG. 151. — Chromosomes of Batracoseps attenuatus, showing chiasmas. (After Janssens.)

their intimate association: the two chromosomes appearing on the first
maturation spindle are exactly the same as those which conjugated.

386 INTRODUCTION TO CYTOLOGY

Between these two extremes lie other views, the most suggestive of them
being that proposed by Janssens (1909).1

The theory of Janssens in its simplest possible form may be stated as
follows. The members of the conjugating pair twist about each other
and come into very intimate association at certain points. When they
again separate a break occurs at the point or points of closest contact,
but along a new plane, so that each of the two separating chromosomes
is made up of portions of both conjugating members (Fig. 149). This
process is known as chiasmatypy or crossing over. Such a behavior might
occur at various stages in the heterotypic prophase: most probably it
takes place at an early stage, when the conjugating chromosomes are in
the form of simple thin threads (Fig. 83). In other cases it may take
place at a later stage, when, in the case of animals, each of the chromo-
somes has split preparatory to the second mitosis, forming a tetrad of
chromatids. Here the crossing over may occur between only two of the
four chromatids (Fig. 150, Janssens's typical case; see also Fig. 151),
or between all four. If only two of the four chromatids are concerned,
only two of the resulting gametes (or spores) will be "crossover gametes"
(or spores), as in Fig. 150; whereas, if the crossing over takes place between
all four of the chromatids, or between the two yet unsplit threads in the
earlier prophase, all four of the gametes (or spores) will be "crossover
gametes" (or spores).

Application of the Chiasmatype Theory to the Problems of Linkage.—
It is the above interpretation of the nature of. chromosome conjugation
that lies at the basis of the work of Morgan and his students on Drosophila.
As already pointed out, these workers have found good evidence for the
conclusion that each chromosome is responsible for a certain group of
characters, the members of the group showing a strong tendency to remain
associated because their genes are borne by a common carrier. They
further believe that the evolution of new character groupings has been
brought about not only through the crossing of different hereditary
strains, but also through the evolution of chromosomes with new con-
stitutions by the process of crossing over. On the basis of the frequencies
in which the new types of grouping occur the relative positions (loci)
of the genes for the different characters have been plotted in the
chromosomes.

The above points are illustrated in Fig. 152, which summarizes what
is supposed to have occurred in a certain series of crosses between flies
with yellow body, white eyes, and miniature wings, and flies with gray
body, white eyes, and long wings. In the cells of the hybrid there is

1 Janssens has recently (1919a6) published an outline of his views of the maturation
phenomena in Orthoptera in which he again makes use of the chiasmatype interpreta-
tion. His results are discussed in some detail from the cytological and genetic points
of view by Wilson and Morgan (1920).

LINKAGE

387

a chromosome from one parent bearing the genes for the three linked
characters, Y, W, and M, and a chromosome from the other parent with
the' genes for the three linked characters, G, R, and L. The first three
characters are allelomorphic to the last three respectively, and the two
chromosomes are homologous.1 Upon breeding from the females of
these hybrids it is found that in the majority of cases (first column) the
F2 flies show the same genetic consitution as do the grandparents (with
respect to these particular characters). This is taken to mean that the
two homologous chromosomes forming the synaptic pair and separating
at reduction have maintained their substance intact — there has been

DROSOPHIUA

YEU.OW lelY N f C.KHY »ojr

WH.T. .... X { „» ~«

nimoTnt WIHM J ^ LON» WIN»I

1361 2O89.

FIG. 152. — Diagram illustrating the evolution of new linkage groups through crossing over.
Explanation in text. (Adapted from Morgan.)

no crossing over. In a certain number of cases (second column) new
groupings of the characters in question are observed in the F2 flies : some
have Y, W, and L, while others show G, R, and M . This is interpreted
on the assumption that a break has occurred along a new plane (dotted
line) at a point of contact, so that at reduction some of the gametes,
and hence the F% flies to which they give rise, receive a chromosome with
Y, W, and L, while others receive G, R, and M . In a smaller number
of cases (third column) the new combinations YRL and GWM are formed
in a similar manner, the break and reunion occurring at another point.
In a very small number of cases (fourth column) the combinations
YRM and GWL appear, which can be explained on the basis of a double

1 The hybrids have gray bodies, red eyes, and long wings, because G, R, and
L are dominant over Y, W, and M . (Ordinarily the genes have other designations.)

388

INTRODUCTION

TO CYTOLOGY

CHROJIOSOMB I

CHRCUOSOIB If

CKROMOSOIB III

0.0 \ /yellow, scute 0.0±

§:!%

J lethal 7
broad
prune

-2.0
0.0

telegraph
star

0.0

roughoid

1.5 —

— white, etc.

1.0

aristaless

3.0—

notch, facet

~~~~ abnormal

4.0

expanded

5.5

echinus

7.3 —

bifid

— -ruby

9.0

truncate

11.0

gull

13.7

crossveinlees

13.0
14.0

pink-wing
streak

16.7

club

20.0

cut

21.0

ringed

22.3

crea»-b

25.3
25.8

sepia
hairy

27.5

tan

28.0

flipper

29.0

dachs

32.0

divergent

33.0

Tennllion

33.0

ski

33.5
34.0

cream-Ill
dwarf old

si:?—

. tiny-bristles
— -miniature

35.5

squat

35.0

scarlet

37.5
J8.0

dusky
furrowed

38.3
38.5

tilt
dichaete

40.5

ascute

.

41.5

deformed

43 0

sable

43.0

maroon

44.4

garnet

44.0

nlnute-6

W.O
V5.0

curled
dwarf

46.5

black

»5.5

pink

46.7

Jaunty

48.5

apterous

53.5
54.5

small-wing
rudimentary

52.5

purple

*,

two-bristle
spineless

56.5

forked

55.0

bi thorax

57.0

bar

56.0

bithonxold

58.5

snail-eye

58.0

safranin

59.5

fused

59.0
60.0

glass
kidney

61.0

trefoil

60.5

giant

61.0

spread

65.0

cleft

65.0

Testigial

telescope

63.5
65.5

delta

hairless

67:*

dash

67.5

ebony

68.0

band

70.0
71.0

lobs
minute-5II

70.0

cm

72.0

white- ocelli

73.5

curved

,

75.0

dachsous

77.0

roof

V

*.V

V

^ • ^ v

•K-rfML.

minut«-2

^ . ^ /. . \/J

o»S«v; A

*

86.5

rough

"""•?£.

\ humptjr"
•n,-.f -~T

C..J&

beaded

95.0

purpleoid

B

tssss

97.5

arc

98.5

plexus

100.0

lethal-II«

101.0

brown

10J.O

blistered

104.5

roorula

105.0
105.5

speck
btlloon

eyeless 0.0±

FIG. 153. — The chromosome map of Drosophila melanogasier, showing the loci of the
genes as determined by Morgan and his associates. Corrected to November, 1920. Figure
kindly furnished by Professor Morgan.

LINKAGE 389

crossing over as shown in the diagram. Most of the F2 flies show the
same combinations as their grandparents, which indicates that no crossing
over has occurred in the majority of the gamete-forming cells. Since
there are many more F2 individuals with YWL or GRM (second column)
than with YRL or GWM (third column), it is believed that the distance
between the genes W and M (and between R and L) in the original chromo-
some must be greater than that between Y and W (and between G and R),
so that there is more chance for crossing over to occur in the lower part
than in the upper. Thus if two linked characters are often separated
(i.e., have their linkage broken) their genes are thought to lie relatively
far apart in the chromosome, whereas linked characters separated only
rarely are supposed to have their genes located very near each other.
Recombinations involving a double or even more complex crossing over
(fourth column) would be expected very rarely. In this way the genes
for the various characters have been assigned to their loci in the chromo-
somes on the basis of the frequencies in which the various new combi-
nations in F2 appear.

In Fig. 153 is shown the "map" of the chromosomes of Drosophila
as determined by Morgan and his associates, each factor being placed
a certain number of units of distance from the end. Many other known
genes are not shown in the diagram. As the unit of distance is taken
that space separating two factors whose linkage is broken (i.e., between
which crossing over occurs) in 1 per cent of the cases. Thus crossing
over occurs between "yellow" and " bifid" in 7.3 per cent of the observed
cases; the factors are therefore placed 7.3 units apart (first chromosome).
Furthermore, if some linkage relations are known, it is possible to calcu-
late certain other linkages in advance. For example, if it were known
that crossing over occurred between "sepia" and "pink" (third chromo-
some) in 20.2 per cent of the flies, and also that "sepia" and "kidney"
showed a 34.7 per cent crossing over, the prediction that "pink" and
"kidney" would show a 14.5 per cent crossing over (not including the
modifying effects of double crossing over, should this occur) would be
borne out by experimental results. Such an agreement of the results of
new crosses with predictions made on the basis of known linkages has
occurred over and over again in the experiments of Morgan and his
students. The chiasmatype hypothesis as thus elaborated obviously
fits the observed facts remarkably well.

Interference. — Another piece of evidence brought forward in support of
the hypothesis that the factors have a linear arrangement within the
chromosome is the phenomenon of interference, which has been elucidated
by Sturtevant (1913), Weinstein (1918), and particularly by Muller. If
the factors or genes are arranged in a series as supposed on the above
hypothesis, it would be expected that when crossing over occurs at a given
point in a pair of chromosomes, the regions immediately on either side of

390

INTRODUCTION TO CYTOLOGY

this point would be prevented from crossing over at the same time, for the
reason that the twisting of the chromosomes about each other is not close
enough to allow two crossovers so near each other. Thus in Fig. 154, if
crossing over occurred between the two factor pairs Dd and Ee, breaking
the linkages between D and E and between d and e, there would be at the
same time no such break between Cc and Dd or between Ee and Ff,
since for mechanical reasons a second crossing over could not take place
at either of these points simultaneously with that between Dd and Ee.
Crossing over at one point would thus interfere with crossing over which
might otherwise occur at nearby points. The amount of
this interference would progressively decrease at points
farther and farther from the first crossover point, until
at a certain distance (measured by the length of the loops
usually formed by the twisting chromosomes), as atLM,
it would vanish entirely; here crossing over would occur
with its normal frequency irrespective of any crossover
atDE.

Muller has found that the characters behave accord-
ing to these expectations. If two characters have their
linkage broken in a certain percentage of cases, this
percentage is noticeably lowered if breaks in linkage
occur between two other characters having normally a
fairly close linkage with the first two. In other words,
one linkage break interferes with other linkage breaks
within the same linkage group ; and the degree of this
interference varies from a high value in the case of a
closely linked series of characters to zero in the case of
characters very loosely linked. This, it is pointed out,
is just what should occur among characters represented
by a linear series of genes in chromosomes which un-
dergo crossing over, but which cannot twist about one
another with more than a certain degree of closeness.
The phenomenon of interference thus indicates another
point in which the chiasmatype hypothesis as developed by Morgan fits
the experimental facts.

A further point is of interest in this connection. In Drosophila it is
only in the females that crossing over takes place; it is in the eggs, and
not in the spermatozoa, that new factor combinations appear as the result
of this process. The absence of crossing over in the male may be asso-
ciated with the fact that the F-chromosome carries no known factors;1
the male is heterozygous for sex. In the fowl, in which the female
rather than the male is heterozygous for sex, it has been shown that
crossing over occurs in the male but not in the female. Crossing over for

^ee, however, Castle (1921).

FIG. 154.— Dia-
gram illustrating
interference. Ex-
planation in text.

LINKAGE 391

some reason is limited in these cases and some others to the sex which is
heterozygous for the sex-factors. On the contrary, in the grasshopper,
Apotettix, Nabours (1919) has shown that some crossing over occurs in
both sexes, and the same appears to be true in Primula (Gregory; Alten-
burg 1916), the rat (Castle and Wright 1916), and Zea (Emerson). In
Paratettix, in which no crossing over has been demonstrated, Miss
Harman (1920) reports that the homologous chromosomes do not
conjugate until the end of the prophase, and suggests that their indepen-
dence during the early stages may account for the absence of crossing
over.

General Discussion. — In the foregoing pages a brief account has been
given of the main points in the theory developed by those who have made
the most thoroughgoing attempt to relate the phenomena of heredity to a
visible cell mechanism. To follow out the details of its application does
not lie within the scope of this chapter : it is here intended only to furnish
a starting point for cytological studies in this field by indicating the
common ground upon which cytology and genetics meet. It is import-
ant, however, to differentiate between evidence which is genetical and
that which is cytological in nature; and further to remind ourselves to
what extent observed fact and hypothesis respectively have been woven
into the theory. Caution is particularly necessary in this latter regard,
since the general nature of many of our ideas of inheritance is traceable
in part to the speculative theories of Weismann. Weismann's theories
of heredity and development, which are summarized in the next chapter,
were primarily "corpuscular" or " particulate " theories: the phenomena
of heredity and development were referred to distinct material units
which in some way were able to bring about the development of the
heritable characters in the individual and their transmission from one
generation to the next. Bearing in mind the phenomena of inheritance
reviewed in the preceding chapters, especially the behavior of the Men-
delian characters, it is difficult to escape the conclusion that differential
factors of some sort, which in an unknown manner initiate the series of
reactions resulting in the several characters, are carried in the nucleus.
To determine the nature of these factors and to discover the real relation
existing between them and the developed characters are among our
greatest problems. That the factors or genes are discrete units is a
hypothesis which is not only plausible, but has also proved itself to be
most useful. If such factors exist, the chromosomes afford a means of
precisely the kind required to account for the observed distribution of
characters throughout a series of generations. Hence from Roux and
Weismann onward the factors have been lodged in the chromosomes.

But it is when these factors are directly sought with the aid of the
microscope that disappointment is met. The frequently observed
granules or chromomeres in the chromatin thread or chromosome are

394 INTRODUCTION TO CYTOLOGY

without recourse to the hypothesis of chiasmatypy, and that the observa-
tions and figures of Janssens do not prove the existence of that phe-
nomenon. Robertson, for example, holds that the "chiasma" figures are
more simply explained as the result of a tendency on the part of the four
chromatids to open out partly along the conjugation plane and partly
along the plane of splitting, without any actual breaking and recombina-
tion (Fig. 156). Wilson (Wilson and Morgan 1920), however, thinks it
"highly probable that the cytological mechanism of crossing-over must be
sought in some process of torsion and recombination in the earlier stages
of meiosis — perhaps during the synaptic phase or slightly later — and that
this process may leave no visible trace in the resulting spireme-threads."

During the time when the homologous chromosomes in the form of
slender threads are twisted about each other in the early prophase of the
heterotypic mitosis there is abundant opportunity for the required break-
ing and union to occur, and appearances often lend themselves well to
such an interpretation. If the side-by-side position is not assumed until
later in the prophase (Scheme B, Chapter XI) the time during which such
interchanges might occur is much shorter. But it is a matter of extreme
practical difficulty in either case to determine whether or not the plane of
separation is the same as the plane of union at a given crossing point.
The forces controlling the breaks and recombinations as well as the fre-
quencies with which they occur in the different chromosome pairs are
even more difficult to imagine. The difficulty of accounting for these
phenomena, however, does not weigh heavily as an argument against
their occurrence. Whether or not chiasmatypy actually takes place is a
question which must be settled primarily by direct evidence, and the need
for careful search for such evidence cannot be too strongly emphasized. In
the opinion of the cytologist the behavior of each chromosome as a whole
must be much more thoroughly known before cytological interpretations
of the phenomena of inheritance involving any smaller units which the
chromosome may contain can be regarded as more than hypotheses,
valuable as these hypotheses may be in the correlation of genetic data.

At this point it may be well to recall (see Chapter XI) that all cytolo-
gists do not agree that the synaptic mates maintain such an independ-
ence (except at crossover points) as is presupposed by the advocates of
the chiasmatype theory. A number of observers have reported an actual
fusion of the conjugating threads, the resulting pachyt&ne thread sub-
sequently splitting, probably along the line of this fusion. It may
accordingly be suggested, in line with the hypotheses discussed by Allen
(1905), that if the threads actually carry or consist of discrete units or
genes, and if these units do not themselves fuse during the process, such
a resplitting of the pachytene thread, if not wholly along the fusion plane
because of a twist of the thread or of the plane itself, would bring
about a redistribution of the genes as effectively as would the chiasmatype

LINKAGE 395

process as originally proposed.1 The splitting of chromatic threads is,
moreover, a process already known to occur in all other mitoses. But
interpretations involving the actual fusion of the conjugating threads
have been adversely criticized with much effect by Gre"goire (1910), and
only further research on the cell can lead us to an adequate evaluation of
the above outlined suggestion.

Other Theories of Linkage. — The chiasmatype hypothesis is not the
only one which has been advanced to account for the phenomenon of
linkage. Most prominent among other attempts to solve this problem is
that of the English geneticists, especially Bateson, Punnett, and Trow,
who have advanced what is known as the Reduplication Hypothesis. In-
stead of accounting for the new factor combinations manifested by a cer-
tain percentage of the gametes on the basis of an interchange of factors in
the chromosomes at the time of reduction, these investigators seek to
explain them by postulating a series of differential divisions in the earlier
cells of the germ cell lineage, whereby Mendelian factors, not carried by
chromosomes, are segregated in such a manner that the observed types of
gametes are produced. Furthermore, the various factors are supposed to
be segregated successively, and at such stages in the cell lineage that the
proliferation or reduplication of the cells with new combinations of factors
shall account for the ratios in which the new types appear. Although
the differentiation of the somatic and early germ cells is accompanied by
visible differences in the constitution of their cytoplasm (see p. 406),
there is at hand no cytological evidence for such a segregation of heredi-
tary units as is thought to occur by the proponents of the Reduplication
Hypothesis. So long as this is the case, discussion of the hypothesis,
together with the subhypotheses formulated to meet certain serious objec-
tions, hardly belongs to cytology. One fact, however, pointed out by
Morgan (1919a) as very significant in this connection, is found in the
results of some experiments by Plough (1917). Plough investigated the
effects of temperature on the frequency of linkage breaking (crossing over)
in Drosophila. Not only did he find that temperature does affect the
amount of crossing over, but the effect was clearly produced at the time
of maturation and not earlier. This evidence is directly opposed to the
view that the new factor combinations are formed during cell-divisions
some time prior to reduction.

Another effort to account for the results of crossing over without
resorting to the chiasmatype process is represented in a suggestion
(Goldschmidt 19176) to the effect that the two factors of an allelomorphic
pair are held to their places in the two homologous chromosomes by a
pair of variable forces, which allow them to exchange places in a certain

1 A form of this interpretation of synapsis has suggested itself also to Dr.
C. W. Metz and Dr. E. G. Anderson, who inform the author that such a hypothesis
will in all probability conform satisfactorily to the data of genetics.

394 INTRODUCTION TO CYTOLOGY

without recourse to the hypothesis of chiasmatypy, and that the observa-
tions and figures of Janssens do not prove the existence of that phe-
nomenon. Robertson, for example, holds that the "chiasma" figures are
more simply explained as the result of a tendency on the part of the four
chromatids to open out partly along the conjugation plane and partly
along the plane of splitting, without any actual breaking and recombina-
tion (Fig. 156). Wilson (Wilson and Morgan 1920), however, thinks it
"highly probable that the cytological mechanism of crossing-over must be
sought in some process of torsion and recombination in the earlier stages
of meiosis — perhaps during the synaptic phase or slightly later — and that
this process may leave no visible trace in the resulting spireme-threads."

During the time when the homologous chromosomes in the form of
slender threads are twisted about each other in the early prophase of the
heterotypic mitosis there is abundant opportunity for the required break-
ing and union to occur, and appearances often lend themselves well to
such an interpretation. If the side-by-side position is not assumed until
later in the prophase (Scheme B, Chapter XI) the time during which such
interchanges might occur is much shorter. But it is a matter of extreme
practical difficulty in either case to determine whether or not the plane of
separation is the same as the plane of union at a given crossing point.
The forces controlling the breaks and recombinations as well as the fre-
quencies with which they occur in the different chromosome pairs are
even more difficult to imagine. The difficulty of accounting for these
phenomena, however, does not weigh heavily as an argument against
their occurrence. Whether or not chiasmatypy actually takes place is a
question which must be settled primarily by direct evidence, and the need
for careful search for such evidence cannot be too strongly emphasized. In
the opinion of the cytologist the behavior of each chromosome as a whole
must be much more thoroughly known before cytological interpretations
of the phenomena of inheritance involving any smaller units which the
chromosome may contain can be regarded as more than hypotheses,
valuable as these hypotheses may be in the correlation of genetic data.

At this point it may be well to recall (see Chapter XI) that all cytolo-
gists do not agree that the synaptic mates maintain such an independ-
ence (except at crossover points) as is presupposed by the advocates of
the chiasmatype theory. A number of observers have reported an actual
fusion of the conjugating threads, the resulting pachyt£ne thread sub-
sequently splitting, probably along the line of this fusion. It may
accordingly be suggested, in line with the hypotheses discussed by Allen
(1905), that if the threads actually carry or consist of discrete units or
genes, and if these units do not themselves fuse during the process, such
a resplitting of the pachytene thread, if not wholly along the fusion plane
because of a twist of the thread or of the plane itself, would bring
about a redistribution of the genes as effectively as would the chiasmatype

LINKAGE 395

process as originally proposed.1 The splitting of chromatic threads is,
moreover, a process already known to occur in all other mitoses. But
interpretations involving the actual fusion of the conjugating threads
have been adversely criticized with much effect by Gre"goire (1910), and
only further research on the cell can lead us to an adequate evaluation of
the above outlined suggestion.

Other Theories of Linkage. — The chiasmatype hypothesis is not the
only one which has been advanced to account for the phenomenon of
linkage. Most prominent among other attempts to solve this problem is
that of the English geneticists, especially Bateson, Punnett, and Trow,
who have advanced what is known as the Reduplication Hypothesis. In-
stead of accounting for the new factor combinations manifested by a cer-
tain percentage of the gametes on the basis of an interchange of factors in
the chromosomes at the time of reduction, these investigators seek to
explain them by postulating a series of differential divisions in the earlier
cells of the germ cell lineage, whereby Mendelian factors, not carried by
chromosomes, are segregated in such a manner that the observed types of
gametes are produced. Furthermore, the various factors are supposed to
be segregated successively, and at such stages in the cell lineage that the
proliferation or reduplication of the cells with new combinations of factors
shall account for the ratios in which the new types appear. Although
the differentiation of the somatic and early germ cells is accompanied by
visible differences in the constitution of their cytoplasm (see p. 406),
there is at hand no cytological evidence for such a segregation of heredi-
tary units as is thought to occur by the proponents of the Reduplication
Hypothesis. So long as this is the case, discussion of the hypothesis,
together with the subhypotheses formulated to meet certain serious objec-
tions, hardly belongs to cytology. One fact, however, pointed out by
Morgan (1919a) as very significant in this connection, is found in the
results of some experiments by Plough (1917). Plough investigated the
effects of temperature on the frequency of linkage breaking (crossing over)
in Drosophila. Not only did he find that temperature does affect the
amount of crossing over, but the effect was clearly produced at the time
of maturation and not earlier. This evidence is directly opposed to the
view that the new factor combinations are formed during cell-divisions
some time prior to reduction.

Another effort to account for the results of crossing over without
resorting to the chiasmatype process is represented in a suggestion
(Goldschmidt 19176) to the effect that the two factors of an allelomorphic
pair are held to their places in the two homologous chromosomes by a
pair of variable forces, which allow them to exchange places in a certain

1 A form of this interpretation of synapsis has suggested itself also to Dr.
C. W. Metz and Dr. E. G. Anderson, who inform the author that such a hypothesis
will in all probability conform satisfactorily to the data of genetics.

396 INTRODUCTION TO CYTOLOGY

proportion of cases without involving a rupture of the chromosomes. These
forces, however, are purely conjectural. It is pointed out by Jennings
(1918), moreover, that the gametic ratios theoretically resulting from
such a process do not agree with the actual ratios observed in Drosophila.

Value of the Chromosome Theory of Heredity. — Whatever judgment
may ultimately be rendered on the chromosome theory of heredity as
outlined in these chapters, it must be agreed that the value of this theory
in the present state of our knowledge can hardly be overestimated.
Through its use a huge number of the observed facts of inheritance are
being reduced to order: the painstaking investigation of the interrelation-
ships of all the known heritable characters of even a single organism
such as Drosophila cannot fail to be a great service to biological science.
Its appeal to the cytologist, as Wilson states, is largely through the man-
ner in which it seeks to make use of known cell mechanisms rather than
entirely hypothetical processes. Those portions of the theory which
are as yet unsupported by the results of direct cytological observation,
though not contradicted thereby, at least have the virtue of affording a
useful and graphic representation of the mutual behavior of hereditary
characters. Notwithstanding the statement that " the graphic repre-
sentation of the location of the factors is a type of representation common
to every set of phenomena which can be expressed as percentages" (Trow
1916), these hypotheses are of great value, for by aiding in the correla-
tion of the facts of inheritance they serve to increase the number of
observed phenomena statable in terms of order; and the reduction of
experience to order and the statement of this order in simple formulae,
together with the search for new truth, constitute the principal tasks
of science. If this work of correlation has been well done the whole
body of facts can readily be placed under another theoretical interpre-
tation and described in a new set of terms should occasion require.

Although it may be that the chiasmatype hypothesis of linkage is in
certain points inadequate, mathematically (Trow 1916) or otherwise, it is
nevertheless true, as we have already seen, that it fits the case very well.
At the same time we may remind ourselves that the fact that a hypothesis
works well is no guarantee of its ultimate truth. But even if the chiasma-
type interpretation should have to be ever so greatly modified as new
facts accumulate, it is scarcely to be doubted that the chromosome theory
of heredity in some form will turn out to be in accord with the truth.
With respect to this general theory Wilson (1909) writes as follows:

"I stand with those who have followed Oscar Hertwig and Strasburger in
assigning a special significance to the nucleus in heredity, and who have recog-
nized in the chromatin a substance that may in a certain sense be regarded as the
idioplasm. This view is based upon no single or demonstrative proof. It
rests upon circumstantial and cumulative evidence, derived from many sources.
The irresistible appeal which it makes to the mind results from the manner in

LINKAGE 397

which it brings together under one point of view a multitude of facts that other-
wise remain disconnected and unintelligible. What arrests the attention when
the facts are broadly viewed is the unmistakable parallel between the course of
heredity and the history of the chromatin-substance in the whole cycle of its
transformation. In respect to some of the most important phenomena of
heredity it is only in the chromatin that such a parallel can be accurately traced.
It is this substance, in the form of chromosomes, that shows the association of
exactly equivalent maternal and paternal elements in the fertilization of the
egg. In it alone do we clearly see the equal distribution of these elements to
every part of the body of the offspring. In the perverted forms of development
that result from double fertilization of the egg and the like it is only in the
abnormal distribution of the chromatin-substance by multipolar division that we
see a physical counterpart of the derangement of development. Only in the
chromatin-substance, again, do we see in the course of the maturation of the
germ cells a redistribution of elements that shows a parallel to the astonishing
disjunction and redistribution of the factors of heredity that are displayed in the
Mendelian phenomenon."

With more particular reference to the chiasmatype hypothesis Wilson
(1913) says:

"This, admittedly, is a bold venture into a highly hypothetical region. Its
justification is the pragmatic one that it 'works.' The hypothesis gives us the
only intelligible explanation that has yet been offered for a series of undoubted
facts ; and it is certainly worthy of the most attentive further examination . . .
We have much to gain and nothing to lose by the use of explanatory hypotheses
that are naturally suggested by the facts and help us to formulate them for
analysis, so long as such hypotheses are not allowed to degenerate into dogmas
accepted as an act of faith, but are only used as instruments for further observa-
tion and experiment."

Bibliography at end of Chapter XVIII.

CHAPTER XVIII
WEISMANNISM AND OTHER THEORIES

The theory of heredity described in the foregoing chapters, though
resting on its own foundation of observational and experimental evidence,
shows in some of its features the influence of certain earlier speculative
hypotheses, particularly those set forth by Weismann.1 Some of the
conceptions embodied in these hypotheses are consequently involved in
cytological and genetical discussions of the present day, and for this
reason we shall here outline their main points, briefly indicating wherein
our modern theory has advanced beyond them.

Although conceptions of other types arose very early, many of the
hypotheses in question were based on the assumption that the phenomena
of heredity and development are the result of the activity of ultimate
living particles of ultramicroscopic size. Thus Herbert Spencer (1864)
built up a theory of considerable proportions about his 'physiological
units/ and these formed the prototype of the units postulated in many
later theories. Of these theories the most prominent were those of
Darwin, Nageli, de Vries, and Weismann.

Darwin's Hypothesis of Pangenesis. — In his Variations of Plants and
Animals under Domestication (1868) Darwin included a chapter on his
"Provisional Hypothesis of Pangenesis," which, though offered only as a
suggestion, excited great interest in the field of biology, especially after
the advances made in cytology a few years later. In several points it
closely resembled a theory propounded by Buffon more than a century
earlier (1749). Darwin clearly saw in the cytological aspects of heredity
one of the great biological problems of the future, but his only specula-
tions on the subject were embodied in the pangenesis hypothesis, which
may be stated as follows:

All the cells of the organism at all stages of development give off small
particles, or gemmules, which multiply by fission and circulate throughout
all parts of the body. These gemmules pass to the germ cells, carrying
with them the power to reproduce cells like those from which they came.
In this way units representing all the kinds of cells composing the organ-
ism are collected in the gametes (or spores or buds) and are thus passed
on to the next generation. During the embryogeny of the new individual
the gemmules are so distributed that at the proper times and places they

1 See Kellogg (1907), Delage and Goldsmith (1913), Thomson (1899, 1913), and
Conklin (1915).

398

WEISMANN1SM AND OTHER THEORIES 399

develop into cells like those from which they originally migrated, in
this manner building up a new individual like the parent. Some of the
gemmules do not function until a comparatively late stage in the onto-
geny, and others may remain latent through several generations: on
these two assumptions it is possible to account for the late appearance of
certain characters and for the fact that others may "skip" one or more
generations. It is further supposed that some gemmules remain latent
in the individuals of one sex: thus, for instance, the characters normally
present only in the male may be transmitted through the female.

In the many criticisms of this hypothesis the tendency has been to
judge and condemn it solely on the ground of the supposed transportation
of the gemmules from the body cells to the germ cells, for which no direct
evidence has ever been discovered. It should not be forgotten, however,
that Darwin suggested an explanation for the phenomena of heredity
on the basis of representative material units in the cells, a conception
which was of the greatest importance in that it constituted the starting
point for later fruitful investigations and theories. The migration of
the gemmules was postulated largely to account for the phenomena of
regeneration and the inheritance of acquired somatic modifications.
Since regeneration may be explained as well on other grounds, and since
the evidence for the inheritance of acquired somatic modifications is
for the most part of such extremely doubtful value, such a migration of
representative units, first denied by Galton (1875), has come to be re-
garded as unnecessary. A theory postulating representative particles
but no such migration was that of de Vries.

De Vries's Theory of Intracellular Pangenesis. — According to de
Vries (1889) the particles of hereditary substance, or pangens, do not
represent different kinds of cells as Darwin thought, but stand rather for
different elementary characters or qualities out of which the many visible
characters of the organism are built up. Furthermore, these living
elements or pangens do not pass from cell to cell, but merely circulate
between the nucleus, where a complete outfit of them is conserved, and
the other parts of the cell — hence the term " intracellular pangenesis."
In this way the characters brought into the new individual through the
nucleus are delivered to the cell as a whole. Contrary to the idea of
Darwin and especially to that of Weismann (see below), all the cells (or
nuclei) of the body contain pangens for all the hereditary characters:
they are not sorted out as development proceeds.

Nageli's Idioplasm Theory. — rA highly speculative theory of a some-
what different type was that formulated by Nageli (1884) five years
before that of de Vries. Protoplasm was thought by Nageli to be made
up of a vast number of fundamental living units; these he called micellce.
As a result of the ways in which these molecular complexes or micellae
may be arranged, there are in the cell two kinds of protoplasm : in nutri-

400 INTRODUCTION TO CYTOLOGY

live protoplasm, or trophoplasm, the micellae have no regular orienta-
tion, whereas in idioplasm they are oriented in a particular manner.
According to Nageli the phenomena of heredity are due to the constitu-
tion and transmission of this idioplasm; idioplasm is the physical basis of
inheritance. It is not confined to the nucleus, but forms an elaborately
constituted network extending throughout all the cells of the organism.
By arranging themselves in various groupings within this network the
micellae are able to bring about the development of the many specific
characters. The further details of this highly "fragile" hypothesis are
summarized in convenient form by Delage and Goldsmith (1913), who
point out that in spite of its many unsupported assumptions it did involve
two fertile ideas: first, that there are two kinds of protoplasm, one of
which carries the characters of the organism; and second, that there are a
limited number of elementary characters which combine in various ways
to produce the many visible characters.

Weismann's Theory. — The most highly developed and influential of
all such speculative theories was that of Weismann. On the basis of the
conception of pangens Weismann built up the highly involved system
of hypotheses set forth in his Das Keimplasma of 1892 and in more
elaborated form in his Evolution Theory of 1902. Certain modifications
were later made.

As Delage and Goldsmith have noted, Weismann incorporated in his
theory several of the stronger points of earlier theories, such as Darwin's
conception of representative particles, Nageli's elementary characters,
and de Vries's intracellular migration of particles. With Nageli he dis-
tinguished between nutritive morphoplasm and hereditary idioplasm or
germ-plasm, but unlike Nageli he identified the idioplasm with the
chromatin of the nucleus. His conception of the constitution of the
idioplasm was essentially as follows:

The ultimate unit in all living matter is the biophore, which is a
minute living particle capable of growth and reproduction — a vital unit.
The many kinds of biophores in a given cell represent the many characters
of that individual cell: they are not bearers of the characters as such
(though Weismann often spoke of them in this fashion), but are rather
factors upon whose presence the development of the characters depends.
The biophores are grouped to form vital units of a higher order, known as
determinants. The determinant, since it is composed of the many kinds
of biophores in the cell, has the power of determining the development of
a certain type of cell or group of cells. In general, therefore, there are as
many sorts of determinants in the organism as there are types of cells,
or "independently variable parts," to be developed. The determinants
are in turn grouped into ids. A single id contains all the kinds of deter-
minants, and so stands for the sum of all the characters of the organism:
it contains the "complete architecture" of the germ-plasm. The ids

WEISMANNISM AND OTHER THEORIES 401

in a given species differ only slightly among themselves, the differences
corresponding to the variations observed within the species : they are the
"ancestral germ-plasms" which have been contributed by past genera-
tions. The ids are identified with the visible chromatin granules in the
nuclear reticulum or in the chromatin thread during mitosis. In most
cases the ids are grouped to form idants, or chromosomes. In some forms
which have a large number of granular chromosomes it is possible that
each is composed of but one id. The id therefore, rather than the
chromosome, is the unit of primary importance. In case there are
several ids in a chromosome (idant) they are arranged in a linear series.
The idea that the chromosomes are all alike since they carry closely simi-
lar ids was later (1913) modified by Weismann, largely as the result of
the demonstration that very minute characters are segregated in
Mendelian fashion.

With the aid of this elaborate mechanism Weismann explained onto-
genetic development in the following manner. In the fertilized egg from
which the individual is to develop all the kinds of determinants are
present: thoes of the female parent are contained in the egg nucleus
and those of the male parent are brought in by the nucleus of the
spermatozoon. During the long series of cell-divisions beginning with
the fertilized egg and ending with the completion of the mature organism,
the many kinds of determinants are sorted out through a progressive
disintegration of the ids, and are distributed in a definite and orderly
manner to the different parts of the body. Many somatic mitoses are
therefore regarded not as equational (erbgleich) , but in reality qualitative
(erbungleich) . When a given determinant finally reaches the proper cell,
i.e., when that cell is finally formed, the determinant splits up into its
constituent biophores; and these, through their action upon the cell
elements, give to the cell its specific characters. The general character
of a cell is accordingly due to the type or types of determinant which
it receives. For Weismann, therefore, development (ontogenesis) was
definitely bound up with the evolution or unfolding of a complex struc-
ture contained in the fertilized egg. Although he did not hold that
the units in the egg have the same spatial relations as their corresponding
characters or structures in the adult, it has been said with some degree
of truth that he transferred preformationism to the nucleus.

Such being Weismann's conception of development, how did he account
for heredity? If the various kinds of body cells in an individual are
characterized by different types of determinants, how is it that the germ
cells, or gametes and fertilized egg through which this individual is
to give rise to the next generation, possess a complete outfit of deter-
minants? According to Darwin's hypothesis, outlined in the foregoing
pages, representative particles or gemmules are contributed by all the
body cells at all stages to the germ cells, by which they aro transmitted

26

402

INTRODUCTION TO CYTOLOGY

to the next generation. (See Fig. 157, A.) Such a contribution of
elements from the body cells to the germ cells was denied completely by
Weismann. He held rather that a certain portion of the complete germ-
plasm (idioplasm; chromatin) of the fertilized egg is carried along un-

A

B

C

FIG. 157. — Diagram illustrating the hypotheses of Darwin and Weismann. The large
circles represent successive generations of individuals, and the small circles their germ
cells. For the sake of simplicity inheritance is shown as uniparental rather than biparental.
A, Darwin's Hypothesis of Pangenesis. The branching solid lines ending in arrows repre-
sent the sorting out of the hereditary units (gemmules) during ontogenesis; the dotted
arrows show the migration of gemmules from the body cells to the germ cells, by which
they are carried into the next generation. B, C, Weismann's theory of the continuity of
the germ-plasm, with no contribution of hereditary units from the body cells to the germ
cells. In one case (B) the germ cells are set aside at the beginning of ontogenesis, and in
the other (C) much later. In both cases the "complete germ-plasm" is delivered to the
germ cells through a shorter or longer series of equational divisions (heavy lines).

changed and delivered intact to the germ cells. It had been shown
(Haeckel 1874; Rauber 1879; Jaeger 1878; Nussbaum 1880; Galton)
that in certain animals the primitive germ cells are set aside at once when
development begins, and Weismann pointed out that they are therefore
differentiated before any sorting out of the hereditary units can have

WEIXMANNISM AND OTHER THEORIES 403

taken place. Hence the germ cells are really produced by the germ cells
of the previous generation and not by the individual's own soma (body)
at all: they are present from the beginning of development with the full
hereditary outfit, and by a few equational divisions they give rise to the
gametes. This is represented in Fig. 157, B. In the more usual case of
those animals and plants in which the germ cells appear later in the onto-
geny Weismann held that, although a sorting out of the units occurs in the
majority of the cells during ontogenesis, those meristematic cells which
constitute the chain connecting the fertilized egg with the germ cells —
the germ track (Keimbahn) — maintain the undiminished germ-plasm
(Fig. 157, C). Thus in this case as in the other there is a continuity of
the germ-plasm, if not a continuity of the germ cells (unless meristematic
cells also be regarded as germ cells). Since the germ-plasm of any
generation is derived directly from that of the preceding one, it is continu-
ous through an unlimited number of generations; and the successive somas
(bodies) are, so to speak, side branches given off at intervals from the main
stream of the germ-plasm.

In elaborating the above views Weismann (1885, 1892) insisted
strongly upon the independence of the "potentially immortal" germ-
plasm and the transient and mortal soma1. He argued that since there
is no contribution of hereditary elements from the soma to the germ cells,
somatic changes being in no way impressed upon the germ cells from
which the next generation is to arise, there can be no inheritance of
acquired somatic modifications. In multicellular animals the only
inherited variations are those originating in the germ-plasm of the germ
cells or germ track as responses to internal (nutritive etc.) or external
environmental stimuli, or as the result of recombinations of hereditary
units at the time of fertilization (amphimixis). Weismann admitted
that the germ-plasm, though remarkably stable, might be altered directly
by the environment or even by modifications in the surrounding soma;
but he denied that in the latter case the alteration would be of such a
nature as would cause the reappearance of the same somatic modifica-
tion in the next generation. With Weismann, as with Mendel, the main
problem of heredity was not to discover how the characters of the organ-
ism get into the germ cells which it produces, but rather how the char-
acters of an organism are represented in the germ cell from which it is pro-
duced (Darbishire 1911, Chapter 12). He attempted to show how it is
that the stream of germ-plasm on the one hand maintains a stability suffi-
cient to account for the resemblance between the successive bodies spring-
ing from it at intervals, and on the other hand undergoes orderly changes
responsible for the evolutionary advance shown in a long series of
generations. In the words of Agar (Bower, Kerr, and Agar, 1919),
"According to Darwin, parents truly transmit their characteristics to

1 See discussion of senescence in Chapter VII.

404 INTRODUCTION TO CYTOLOGY

their offspring (by means of the gemmules). According to the modern
view [Mendel; Galton; Weismann], however, children resemble their
parents not, strictly speaking, because the latter have passed something
on to them, but because both have been produced from the same
germ-plasm" (p. 91). "The parent is rather the trustee of the germ-
plasm than the producer of the child" (Thomson 1913).

Weismann attempted further to account for the variations effective
in evolution on the basis of his theory of Germinal Selection. He sup-
posed that the determinants, while multiplying in the germ cells, are
subject to selection like all other organic units. Some determinants,
being better placed with respect to the nutritive conditions, are favored
thereby and grow stronger and more influential, while others undergo
changes in the opposite direction. The cells or parts of the organism
receiving the determinants which have had the advantage in the struggle
become better developed than those receiving the weaker determinants.
As this process continues from generation to generation the new variation
gradually increases until it becomes pronounced enough to be laid hold of
by natural selection. In this manner Weismann accounted for the
preservation of small variations not yet of selective value, and for
continued variation along definite lines (orthogenesis) in both plus and
minus directions. Thus for him selection was the cardinal principle
which ruled not only over organisms, but also over cells, ids, deter-
minants, and biophores. As he himself stated it, "This extension of the
principle of selection to all grades of vital units is the characteristic
feature of my theories."

Some Modern Aspects of Weismannism. — Although the distinction
between soma cells and germ cells is not now drawn so sharply as in the
days of Weismann, it is nevertheless of interest to note certain facts
adduced in support of his contention that the germ-plasm is continuous.

In Ascaris megalocephala (Boveri 1887c, 1889, 1891, 1892, 1904;
Zacharias 1913) it is observed that at the second cleavage mitosis the
chromsomes in one blastomere remain entire, while in the other blastomere
they become broken up into smaller pieces, some of which are lost in the
cytoplasm and are not included in the daughter nuclei (Fig. 158, A).
This process is called "chromatin diminution." At the third and fourth
cleavage mitoses a similar diminution occurs in all the blastomeres
but one; in this one the chromosomes remain entire. At the fifth division
it is seen that in the one undiminished cell no further diminution occurs
as it divides, and its descendants become the germ cells. The primary
germ cell is therefore set apart at the fourth mitosis; and, whereas the
other embryonic cells giving rise to somatic structures have undergone a
diminution, the entire chromatin outfit is delivered to the germ cells
through the undiminished cells of the germ track. A similar condition
is present in Miastor (Kahle 1908; Hegner 1912, 1914). In Ascaris

WEISMANNISM AND OTHER THEORIES

405

cam's (Walton 1918) the germ cells are similarly set aside at the seventh
cleavage mitosis.

In criticizing this supposed evidence for the independence and con-
tinuity of the germ-plasm Child (1915) points out that, since undi-
minished cells may give rise to other cells as well as germ cells in the early
divisions, the process observed may represent merely a segregation of
different organs rather than a separation of the germ-plasm from the
soma; and that the non-diminution of the chromatin in the germ track
may be the result of the differentiation of the germ cells rather than its
cause, the differentiation at this stage being primarily a physiological

B

FIG. 158.

A, chromatin diminution in Ascaris megalocephdLa. The second cleavage mitosis is in
progress: all the chromatin is retained in the upper blastomere, from which the germ
cells are to arise, whereas chromosome diminution occurs in the lower blastomere, which
is to give rise to the somatic cells. (After Boveri.) B, third nuclear division in the yet
unsegmented egg of Chironomus confinis showing the early setting aside of the primitive
germ cells at the lower end. (After Hasper.)

(metabolic) one. He refers to certain later researches of Boveri (1910),
which apparently show that "the occurrence or non-occurrence of chro-
matin diminution in a nucleus depends, not upon its qualitative con-
stitution, but upon its cytoplasmic environment." From this it is
concluded that "the 'germ path' is a feature of the cytoplasm, and the
cytoplasm is not, properly speaking, a part of the germ-plasm at all,
but represents the soma of the cell" (p. 327).

In support of this conclusion we may cite, as does Child, those cases
among insects (Hasper on Chironomus, 1911 ; Hegner on Miastor, 1912,
1914) and copepods (Haecker 1897, 1902; Amma 1911) in which the
substance of the future germ cells may be distinguished very early in
the embryogeny, even in the undivided egg, either as a visibly differen-

406 . INTRODUCTION TO CYTOLOGY

tiated region of the cytoplasm (Keimbahn-plasma) (Fig. 158, B), or
by the presence of certain cytoplasmic granules or inclusions (Keimbahn
determinants). These latter are ultimately delivered to the definitive
germ cells, the nuclei at the same time showing no differences in the germ
and soma cells. Although such cases seem to show that "the factors
determining what shall become germ cells and what somatic structures
apparently exist in the cytoplasm and not in the nuclei" (Child 1915,
p. 329), it is nevertheless very significant for the chromosome theory of
heredity that only in the germ cells, whatever the cause of their differ-
entiation from the other cells of the body, should the chromatin be
retained in the complete state in the cases of A scans and Miastor.
Whatever may be the relation of the chromatin to differentiation, and
whatever may be the degree of its independence of the soma-plasm, it
is noteworthy that here it is precisely in the germ cells and in the cells of
the germ track — the cells especially important in heredity — that the
chromatin shows an unbroken continuity from cell to cell and conse-
quently from generation to generation. Were the chromosome mechan-
ism disturbed in these cells as it is in the somatic cells, or should
"diminished" cells regenerate a completely normal organism, a serious
obstacle would be in the path of the chromosome interpretation of here-
dity as now formulated. The actual behavior of the chromatin in the
germ track of Ascaris argues for rather than against the chromosome
theory, at least as regards hereditary transmission.

A much used argument against Weismann's theory of development
(ontogenetic differentiation) is found in the phenomenon of regeneration.
It is well known that in certain animals and especially in plants a portion
of the body consisting solely of differentiated cells may under certain
conditions give rise to a complete individual with functional germ cells.
Weismann accounted for such regeneration on the basis of an additional
hypothesis which stated that during the sorting out of the hereditary
units in the process of cell differentiation certain "supplementary
determinants" are carried along unaltered, and that later, if occasion
arises, these cause the development of the differentiated cells into an
organism with all the usual characters. Since in certain cases (Begonia)
almost any cell of the body may undergo regeneration into a complete
plant, it is evident that all of the body cells must have a "complete"
germ-plasm. Hence the distinciton between a germ-plasm limited to
cells capable of producing an entire individual, and a soma-plasm present
only in somatic cells without such power, becomes of no value. Every
cell capable of regeneration — germ cell, meristem cell, or differentiated
somatic cell — contains the complete germ-plasm, which appears to be
simply the chromatin possessed by all the cells alike. Lack of power to
regenerate is not due tola lack of complete germ-plasm but to other
conditions associated with the degree of differentiation shown by the

WEISMANNISM AND OTHER THEORIES 407

cells. In thus using the terms germ-plasm and soma-plasm (somato-
plasm) synonymously with chromatin and cytoplasm respectively,
Weismann's conception of the chromatin as the substance especially
important in heredity remains, although his theory of the dependence
of ontogenetic differentiation upon a sorting out of qualitatively differ-
ent units of this substance during development is no longer held.

This use of the term germ-plasm is general among geneticists, who
are concerned with the problems of heredity, and may be distinguished
from that of certain students of the physiology of development, by whom
germ-plasm is regarded as "any protoplasm capable, under the proper
conditions, of undergoing regression, rejuvenescence, and reconstitution
into a new individual, organism, or part " (Child 1915, p. 462). From this
latter point of view the germ-plasm would be regarded as either the
complete protoplast capable as acting as so described, or, as Child is
inclined to believe, only an abstract idea — merely a term standing for
heredity.

Weismann's theory of the sorting out of hereditary units during onto-
genesis was abandoned not only because of irs inapplicability to the
results of certain experiments, but also because no support for it ^ould
be found in a direct study of the cell mechanism. Strasburger and other
investigators insisted strongly that so far as can be ascertained the
division of the chromatin at each somatic mitosis is exactly equational,
there being not the slightest indication of such a difference in the chro-
matin of the two daughter cells as might be expected were the divisions
qualitative (erbungleich) . To this Weismann had only to reply that
since the differentiation is a matter not of ids or of idants but of determi-
nants, the two nuclei would be visibly alike in spite of their qualitative
difference. Although certain cases have been described in which growth
is not equal in all of the chromosomes during the early stages of develop-
ment, and although the two daughter nuclei may become differentiated
through unequal nutrition after their formation, as Strasburger suggested,
most biologists have adopted the view that all of the somatic nuclei are
qualitatively alike in their chromatin content so far as its hereditary
powers are concerned. They have thus followed de Vries (1889) in
holding that factors for all of the hereditary characters are present in
all of the somatic cells, a conclusion strongly supported by the facts of
regeneration. The ontogenetic differentiation of the cells which mani-
fests itself largely in cytoplasmic changes, as well as the relative regen-
erative powers which these cells possess, are attributed for the most part
to physiological causes, the latter in large measure determining what
hereditary capabilities of the various cells shall come to expression.
The distinction between the view of Weismann and that of more recent
investigators is made clear in the two diagrams of Fig. 159, which have
been copied from Conklin (1919-1920).

408

INTRODUCTION TO CYTOLOGY

Notwithstanding the fact that many changes have been made in its
details, Weismann's theory of heredity proved to be of much greater value
than his theory of development. Morgan (Morgan et al. 1915, pp. 223-
227) points out that Weismann made three contributions to the study of
genetics, which may be stated in three propositions: (1) The germ-plasm

SOMATIC CELLS

B

SOMATIC CELLS

FIG. 159. — The behavior of the hereditary units in ontogenesis according to Weismann
(A) and the current interpretation (B). In A the determinants in the nucleus (1, 2, 3, 4)
are supposed to be distributed differentially to the various somatic cells. In B the genes
(1, 2, 3, 4) are distributed equally to every cell, but the cytoplasm is distributed differen-
tially. The same genes working upon different cytoplasms produce different results in
various somatic cells. (Diagrams and legend from Conklin, 1919-1920.)

contains independent elements which may be substituted one for another
without undergoing change; (2) a segregation of maternal and paternal
factors, pair by pair, occurs at one period in the history of the germ cells ;
(3) the behavior of the chromosomes is specifically applicable to the
problems of heredity. In these principles are found "the basis of our

WEISMANNISM AND OTHER THEORIES 409

present attempt to explain heredity in terms of the cell," for upon them
is founded the Factorial Hypothesis, now supported by a large mass of
experimental evidence.

In our conception of the nature of the heredity units or factors we
have departed widely from Weismann. For him each of the ids arranged
in a series in the chromosome represented the sum of the characters of a
complete organism; the smaller parts were represented by the smaller
units (determinants) composing the id, and these units in turn were made
up of biophores, which were ultimate and independent living particles.
According to our modern hypothesis each of the serially arranged factors
or genes exerts an influence on the development of one or more characters,
but does not stand for a complete organism as did the id, or for a part
of it as did the determinant. Moreover, it is generally regarded as a mass
of some complex chemical substance whose activities are due to its defi-
nite though imperfectly known physico-chemical properties, rather than
to forces exerted by hypothetical vital units.

In justice to Weismann it should be pointed out that the frequently
made criticism that his theory was a vitalistic one is warranted only to a
limited extent. Although his ultimate hereditary units, the biophores,
were regarded as actually living particles, Weismann stated that "they
are not composed in their turn of living particles, but only of molecules,
whose chemical constitution, combination, and arrangement are such as
to give rise to the phenomena of life." He was careful to point out that
in spite of the fact that it cannot be proved that no peculiar vitalistic
principle exists, we should hold fast to a purely physico-chemical basis
of life "until it is shown that it is not sufficient to explain the facts, thus
following the fundamental rule that natural science must not assume
unknown forces until the known ones are proved insufficient . . . We
can quite well believe that an organic substance of exactly proportioned
composition exists, in which the fundamental phenomena of all life —
combustion with simultaneous renewal — must take place under certain
conditions by virtue of its composition" (1902, lecture 36).

The manner in which hereditary factors are segregated at gameto-
genesis has been found to be different from that conjectured by Weismann.
As indicated in the chapter on reduction, he supposed it to occur through
a transverse division of the chromosome, whereas it is now known that
it is accomplished by the disjunction of pairs of entire chromosomes,
the separating members of each pair being qualitatively different. The
"reduction" predicted by Weismann was found to occur, but not in the
manner he supposed. As shown above, his idea of a further qualitative
segregation of units of a lower order in the somatic divisions has not
been substantiated. Notwithstanding the abandonment of his theory
of development and the changes made in his theory of heredity, Weis-
mann's influence on both cytology and genetics was enormous, largely

410 INTRODUCTION TO CYTOLOGY

because of his emphasis upon the need for careful studies of the cell
mechanism at the critical stages of the life history, and upon the idea
that this mechanism is in some way bound up with the phenomena of
heredity. "It has been Weismann's great service to place the keystone
between the work of the evolutionists and that of the cytologists, and
thus bring the cell-theory and the evolution-theory into organic con-
nection" (Wilson 1900, p. 13).

We may further point out, with Morgan (1915), that the factorial
hypothesis assumes only three things about the factors: they are constant,
they are usually in duplicate in each body cell and immature germ cell,
and they usually segregate in the maturing germ cells. The hypothesis,
and the Mendelian theory in general, therefore have to do only with
heredity : they do not attempt to explain the causes of development. They
seek rather to account for the initial resemblances or differences in here-
ditary potentiality which are observed to exist between the germ cells
from which successive generations arise. Between the materials com-
posing the initial factors and the fully expressed characters of the or-
ganism "lies the whole world of embryonic development," to which the
application of the theories under consideration has not yet been extended
in any systematic or satisfactory manner. Nevertheless many investi-
gators, though realizing the failure of Weismann's attempt to explain
development in terms of representative particles, are strongly inclined
to the view that since the Mendelian characters appearing toward ma-
turity behave as though associated with discrete units in the germ, the
course of ontogenetic development in its earlier stages must also be due in
large part to the activity of factors carried by the nucleus. Development
is thus held to be predetermined or controlled by an internal mechanism :
external agencies act only by affecting the operation of this mechanism.
The factors control the character and behavior of the cells, and upon
these in turn the organism, which is a cell aggregate, is alone dependent
for its characters and activities. In place of the early hypothesis on
which it was supposed that the development of characters is controlled
by the migration of determiners or pangens from the nucleus into the
cytoplasm at precisely the right times and places, we now have the theory
that the factors in the nucleus probably produce their effects by initiating
series of chemical reactions which involve all parts of the cell. As Mor-
gan (1920) "states, "Granting that differences may exist in the nuclei of
different species, different end products are expected. The evidence
that such differences may be related to specific substances in the nucleus
is no longer a speculation but rests on the analytical evidence from Men-
delian heredity. In what way and at what times the nuclear materials
take part in the determination of characters we do not know. The
essential point is that we are in no way committed to any interpretation.
Stated negatively we might add that there is nothing known at present
to preclude the possibility that the influence is a purely chemical process."

WEISMANNISM AND OTHER THEORIES 411

Non-factorial Theories. — The above theory of the dependence of t he
course of development upon the operation of an internal factorial mech-
anism is essentially an "elementalistic" conception: the attempt is
made to explain the organism in terms of its constituent parts, namely,
the cells and smaller elements contained by them. As noted in our
historical sketch, a number of botanists and zoologists many years ago
called attention to the fact that limits must be set to the conception of
the cell as the unit of structure and function ; and they have been followed
by a school, made up largely of experimental embryologists, which holds
that organization is not the result of cell formation, but rather precedes
and regulates the latter. From this " organismal " standpoint the or-
ganism as a whole, and not one or another of its elementary parts, is
regarded as the primary individual. This individual is something more
than the cell aggregate pictured by Schleiden and Schwann: it dominates
the activity of its constituent members from the beginning of the life
cycle onward, and behaves as a unit irrespective of the manner and degree
of its subdivision into special centers of action, the cells. The condition
present in ccenocytic plants is especially noteworthy in this connection,
as are also those cases among animals in which a derangement of the
early embryonic cells does not prevent the eventual attainment of the
normal form. As urged with much force by Ritter (1919), "the organism
in its totality is as essential to an explanation of its elements as its
elements are to an explanation of the organism."

The factorial theory may also be said to represent preformationism
in a very modern form. "We are sailing nearer the preformation coast
than at any time since the modern study of development began under
von Baer" (Conklin 1913). Directly opposed to corpuscular and fac-
torial theories of development are those which seek to explain the course
of ontogenesis not by an internal mechanism but rather as the result of
the influence of external agencies and the physiological responses shown
by protoplasm in the form of cells to such influence: development is
held to be truly epigenetic. The control exercised by environmental
factors during the organism's early developmental stages, and the effects
of various tropisms and tactisms between the component cells upon the
type of organization resulting, have been especially emphasized by O.
Hertwig, Hartog, Roux, Herbst, Driesch, and others. The most sug-
gestive recent work of this nature in plants is that of Harper (1908,
1918a6) on colonial algae. In Hydrodictyon and Pediastrum a number of
free-swimming cells come together and build up colonies of very definite
forms, and a series of experiments has shown that the position in the colony
of any given cell is in no way predetermined. As already pointed out
in Chapter XIV, Harper contends that the type of multicellular organiza-
tion thus built up in successive life cycles is to be explained as the result
of physico-chemical interactions between independent cells organized as

412 INTRODUCTION TO CYTOLOGY

swarm spores, and not as the product of the activity of a system of
spatially arranged factors in a special germ-plasm.

In this connection the name of Driesch (1907-8, 1914) has become
particularly prominent, not only because of his great experimental ingen-
uity, but also because of his decision that the facts of ontogenetic devel-
opment cannot be accounted for on the basis of any mechanical theory,
either now or in the future. As a result he takes the unscientific step of
assuming the existence of a non-mechanical, non-spatial, non-psychic,
non-energetic "entelechy," which presides over and controls develop-
ment. Such non-experiential agencies, manufactured for the purpose of
solving difficult problems, lead to experimental indeterminism and tend
only to obscure the points at issue : they may furnish convenient names
for great gaps in our knowledge, but they never give more than pseudo-
explanations. Nevertheless, in spite of his tendencies to mysticism, as
Harper (1919) remarks, Driesch has shown the impossibility of an exact
parallelism in spatial configuration between the germ-plasm and the
multicellular organism as a whole: there can be no strict preformation
in development. On the other hand, the work of the Mendelians shows
clearly that development cannot be completely epigenetic : nothing seems
clearer than that development is at least in part dependent upon the
orderly operation of an internal organization or mechanism. Wilson
(1909, pp. 106 ff.), in discussing the relation of the chromatin to heredity
and development, writes as follows:

"But do we really need to employ the pangen symbolism in the consideration
of this question? It seems a sufficient basis for our present attack on the problem
to assume that the control of the cell-activities is at bottom a chemical one and is
effected by soluble substances that may pass from nucleus to protoplasm and
from protoplasm to nucleus. Certainly it is to such a view that very many of the
chemical and physiological studies in this field are now unmistakably pointing.
The opinion is gaining ground that the control of development is fundamentally
analogous, perhaps closely similar, to the control of specific forms of physiological
action by soluble ferments or enzymes . . . We are thus led to something more
than a suspicion that the factors of determination, and therefore of heredity,
are at bottom of chemical nature . . . The conclusion thus becomes highly
probable that the characteristic differences of metabolism between different
species, including those involved in development, are traceable to initial chemical
differences in the germ cells. In so far as the chromatin theory expresses the
truth, the primary basis of these differences may be sought in the nuclear
substance."

A Chemical Theory of Heredity. — Among the theories based on the
conception of the idioplasm as a substance with a special chemical consti-
tution, rather than as a system of determinants, may be mentioned that
of Adami (1908, 1918). As indicated in Chapter III, Adami attributes
the phenomena of life to the activities of a protein-like "biophoric mole-

WEISMANNISM AND OTHER THEORIES 413

cule," which is made up of a chain or ring of amino-acid radicles to which
side-chains of various kinds may become attached. With regard to
individual development it is supposed that "in the ovum there is one
common idioplasm of simple type, to which, when distributed in the
various cells derived from that ovum, different side-chains become
attached, according to the relationships assumed by those cells, so that
the cells of different orders are controlled and formed around proto-
plasmic or idioplasmic molecules composed of those central rings plus
varying series of side-chains" (p. 145). With Driesch it is held that "the
structure of the cells in a multicellular organism is a function of their
position," since "the position of the cell determines the modification under-
gone by its idioplasm." Furthermore, "the greater the change impressed
upon the idioplasm of these cells, and the longer that idioplasm is sub-
jected to the conditions inducing this change, the more permanently will
the daughter cells exhibit the peculiar alteration in the idioplasm, with con-
sequent modified structure wherever they find themselves in the economy.
We have, in short, to recognize that two orders of forces determine the
structure of every cell in the body : (1) the previous influences acting upon
its idioplasm and causing it to be of a particular chemical constitution;
and (2) the position in which the cell finds itself, and the forces acting
momentarily and immediately upon its idioplasm. Or, briefly, these two
series of forces are inheritance and environment, and inheritance and
environment determine the constitution of the idioplasm and the struc-
ture of the cells" (p. 151).

"In terms of this theory, therefore, inheritance essentially depends
upon the chemical constitution of the idioplasm or the life-bearing or
biophoric protoplasm of the germ cells, not upon the number of the sepa-
rate ids or biophores or ancestral plasms or pangens contained in the idio-
plasm; and variation, whether slight and individual, or extensive and
leading to the production of new species, is ultimately the expression of
modification in the constitution of that idioplasm brought about by envi-
ronment. Whereas Weismann's theory lays stress upon relative fixity
in the constitution of the idioplasm, this theory admits freely the capacity
for change in structure of the same. So long as the surrounding condi-
tions are unaltered the idioplasm is unchanged ; alter these conditions and
the idioplasm is liable to variation in constitution" (pp. 152-3).

Adami cites certain calculations of the probable size of inorganic and
organic molecules to show that the existence of a system of determinants
or other representative particles of the Weismannian type is a physicial
impossibility. He also points out that since the idioplasm must increase
enormously in bulk by the addition of new material and become repeat-
edly subdivided as cells and individuals multiply, there can be no actual
continuity of the germ-plasm through countless generations: what is
eternal is rather a potential continuity of molecular arrangement and

414 INTRODUCTION TO CYTOLOGY

constitution, i.e., the physical and chemical properties of the germ-plasm
rather than the substance itself.

Conclusion. — In the foregoing pages we have touched upon some of
the most important biological problems toward the solution of which
cytology must make her further contributions. With regard to individual
development it must be determined on the one hand to what extent the
course of ontogenesis is dependent upon the operation of an internal cell
mechanism and how this mechanism brings about its results, and on the
other hand how far it is controlled by external environmental agencies:
a way must be found between the "Scylla of preformation and the
Charybdis of epigenesis" (Conklin 1913). Furthermore, the manner
and the causes of the progressive modification of the hereditary mechan-
ism must be better known in order that evolutionary advance
may be accounted for. With respect to both development and heredity
the roles of the two individualities, the cell and the organism as a whole,
must be more fully ascertained and correlated.

It is obvious that no adequate solution of any of these problems can
be reached until the physico-chemical constitution of protoplasm,
especially that of the idioplasm or inheritance material, is more
clearly disclosed. Only further research can show whether we shall
continue to regard the idioplasm or chromatin as a heterogeneous
system of discrete molecules or molecular complexes (factors or genes)
with a definite spatial arrangement, as is supposed on our current
Mendelian theories, or shall come to look upon it as a single enormously
complex chemical substance in which varying side-chains or other portions
of the molecule are responsible for the variety of results observed. It is
at any rate a striking fact that "in the Mendelian phenomenon we see a
synthesis, splitting apart, and recombination of determinative factors
that is singularly like that of chemical elements or radicles" (Wilson 1909,
p. 108); and nothing appears more clearly evident than the truth of
Wilson's assertion that ".. . .in the union of cytology and biochemistry
lies our greatest hope of future advance."

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416 INTRODUCTION TO CYTOLOGY

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HEREDITY: SEX 417

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27

418 INTRODUCTION TO CYTOLOGY

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HEREDITY; SEX 419

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420 INTRODUCTION TO CYTOLOGY

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HEREDITY; SEX 421

1916. Chromosome studies in the Diptera. III. Additional types of chromosome

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422 INTRODUCTION TO CYTOLOGY

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424 INTRODUCTION TO CYTOLOGY

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426 INTRODUCTION TO CYTOLOGY

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INDEX

Bold-face numbers indicate pages bearing illustrations

Abies (fir), 295
Abraxas (moth), 363
Accessory body, 88, 89

chromosome, 358 ff.
Acer (maple), 159, 176, 236, 239, 356
Achromatic figure, 66, 145

in animals, 177 ff.

in higher plants, 175 ff.

operation of, 182 ff.

origin of, 180
Achromatin, 64
Acids, organic, 135
Acquired characters, inheritance of, 399,

403

Acrididse, 243
Acrosome, 273, 276
Actinophrys (protozoan), 282
Actinosphcerium (protozoan), 211
Activation of egg, 284 ff.
Acton, 202, 205
Adami, 50, 412 ff.
Adiantum (fern), 91, 122
Adoxa (angiosperm), 236
Adsorption, 37
Mthalium (slime mold), 39
Agar, ix, 162, 403
Agathis (conifer), 295
Agave (century plant), 134, 239
Age, 136, 137, 138
Akinetic division, 143
Albugo (fungus), 289, 290
Albumen crystals, 135
Albumin, 40, 41, 117
Alchemilla (angiosperm), 313, 315
Aleurone, 134, 135, 333
Alexieff, 118

Alicularia (liverwort), 111
Allelomorphic pairs, 338 ff., 378 ff.
Allen, C. E., cell plate, 190

chromomeres, 155, 394

plastids, 113

reduction, 222, 236

sex-determination, 357, 364-365

spermatogenesis, 88, 89

Allen, R. F., 91, 313, 314
Allium (onion)

achromatic figure, 182

eanaliculse, 48

cell wall, 191

chondriosomes, 116

nucleolus, 65

reduction, 234, 236, 239

somatic chromosomes, 149, 150
Allolobophora (annelid worm), 244, 245
Altenburg, 391

Alternation of generations, 311
Altman, 33, 40, 48, 115
Alveolar theory of protoplasm, 33
Alveolation of chromosomes, 148, 149,

242

Alytes (toad), 237, 256
Amblystegium (moss), 317
Ambystoma (salamander), 160
Amici, 6, 13
Amides, 135
Amitosis, 210 ff.

and heredity, 212-215

criticism of evidence, 213-215
Amma, 405

Amoeba (protozoan), 42, 43, 44, 207, 222
Amphiaster, 145, 177, 178, 286
Amphibia, 239
Amphimixis, 403
Amphioxus (lancelet), 237
Amphitene, 231, 233
Amphvuma (amphibian), 150
Amygdalus (angiosperm), 134
Amylodextrin, 107
Amyloplast, 104, 106, 122
Amylose, 107
Anabaenin, 204
Analysis of gametes, 286
Anaphase

heterotypic, 234, 236, 238, 239, 242

somatic, 144, 145, 147
Anasa (bug), 244, 245, 249
Anastomoses, 147, 148, 149, 150
Ancel, 256

Ancyracanthus (nematode worm), 359,
367

427

428

INDEX

Anderson, 395

Androcyte, 88, 89

Androgone, 88, 89

Aneura (liverwort), 82, 87, 239

Angiosperms

fertilization, 298, 299

sporogenesis, 225
Antedon (echinoderm), 327
Antheridium, 87, 222, 289, 290
Anthoceros (.liverwort)

apical cell, 27

chondriosomes, 122

fertilization, 293

plastid, 103, 104, 105, 113, 114

pyrenoid, 104, 108, 109
Anthocyanin pigments, 122, 135
Anti-fertilizin, 287
Antirrhinum (snap dragon), 332
Aphis (aphid), 318
Apical body, 89
Apical cell, 27
Apical growth, 27
Apis (bee), 318
Apogamy, 311 ff., 312
Apospory, 311, 315 ff., 316
Apotettix (grasshopper), 391
Apposition, 192

Arbacia (sea urchin), 78, 160, 330
Arber, 212

Arcella (protozoan), 62, 281
Archicarp, 223, 290, 312, 313
Archoplasm, 115, 181
Arctostaphylos (angiosperm), 134
Ariscema (angiosperm), 104, 105, 248
Aristotle, 1
Armadillo, 357
Arnold, 239

Artemia (crustacean), 318
Artichoke, 134
Artificial cytasters, 78, 280
Artificial parthenogenesis, 284 ff.
Ascaris (nematode worm)

centrifuged egg, 330

centrosome, 77

chondriosomes, 118

chromatin diminution, 404, 405

chromosomes (somatic), 150

cleavage, 404, 406

fertilization, 276, 277, 280, 281

hybrids, 164

individuality of chromosome, 157,
164

polar body, 318

Ascaris (nematode worm) reduction, 219,
239, 248, 256, 276

sex-chromosomes, 358, 359, 367

spermatozoon, 274
Ascidian egg, 329

Ascobolus (fungus), 80, 81, 223, 290, 291
Ascogonium, 312
Ascomycetes, centrosomes, 80

fertilization, 290 ff.

mitosis, 179

reduction, 223

Ascophanus (fungus), 291, 312
Ascospore wall, 80, 291
Asexual reproduction, 137
Askenasy, 193
Asparagus, 121
Aspergillus (fungus), 290, 312
Aspidium (fern), 91, 313, 314, 373
Assimilation, 106
Aster, 26, 76, 177, 178, 183, 189, 279,

280, 286

Asterella (liverwort), 87
Astrosphere, 76, 177, 178
Atamosco (lily), 315
Athyrium (fern), 315, 317
Atkinson, 223, 248, 290
Atoms and factors, 353
Atrichum (moss), 88
Attraction sphere, 26, 76, 177
Auerbach, 16
Autogamy, 282
Autosome, 358
Autumnal colors, 136
Avena (oats), 348
Axial filament, 273, 275

gradient, 139
Axon, 29, 30

B

Bacillus Butschlii, 67
Bacteria, 3, 66, 108
Bacterium gammari, 67
Bailey, 63, 177, 192
Balbiani, 61, 155
Ballowit/, 274
Baltzer, 160, 327, 371
Bancroft, 35, 44
Banta, 370, 371
Barber, 38
Barley, 122
Barry, 15
Bartlett, 372

INDEX

429

de Bary, 11, 12, 32, 108, 312
Basal granules or corpuscles, 96, 97
Basichromatin, 64, 66, 70, 158
Basidiomycetes, centrosomes, 81

fertilization, 292

reduction. 224
Basidiospores, 224
Basidium, 224
Bataillon, 285, 319
Bateson, 344, 366, 395
Bateson and Punnett, 378
Batracoseps (salamander), 237, 385
Baur, 332
Beauverie, 135
Bechamp, 48
Bechhold, 37
Beckwith, 118, 123
Bee, 318, 319, 357
Beer, 110, 196, 239
Begonia (angiosperm), 137, 256
Beijerinck, 49
Belajeff, 86, 90, 95, 288
Benda, 115, 119
van Beneden, achromatic figure, 181, 183

attraction sphere, 76

centrosome, 77, 180

early work on mitosis, 143, 144

fertilization, 279

individuality of chromosome, 157

polarity, 138

reduction, 219
Bensaude, 292
Bensley, 47
Berberis (barberry), 47
Berghs, nucleus and mitosis, 61, 66, 158,
176, 209

reduction, 231, 236, 257
Bernard, 69
Berrihardi, 51
Beroe (ctenophore), 328
Berridge and Sanday, 295
Bertholletia (Brazil nut), 134
Biochemistry, 23, 414
Biococci, 69
Biogen, 42, 50

Biophore (Adami), 50, 412 ff.
Biophore (Weismann), 49, 226, 400 ff.
Bioplasts, 33, 48

Birds, sex-determination, 363, 367, 369
Bivalent chromosomes, 161, 227, 233,

239

Black, 292
Blackman, M. W., 255

Blackman, V. H., 224, 291, 292, 295, 312

Blackman and Fraser, 290, 312

Blackman and Welsford, 291, 298, 313

Blakeslee, 290, 355

Blasia (liverwort), 87, 92, 95

Blastomere, 188, 325 ff.

Blending inheritance, 338

Blepharoplast, 83 ff.

relation to centrosome, 94-97

Blochmann, 318

Blood cells, 29, 60

Boletus (mushroom), 81

Bolleter, 87

Bone, 30

Bonellia (gephyrean worm), 371

Bonnet, C., 3

Bonnet, J., 223

Bonnevie somatic chromosomes, 150

152, 155, 159
reduction, 250, 251, 257

Boquet stage, 247, 248

Bordered pit, 191, 192

Borelli, 2

Borzi, 46

Botrychium (fern), 236

Boudiera (fungus), 290

Bourquin, 106, 109

Boveri, amitosis, 213

archoplasm, 115, 181
centrosome, 77, 78, 180
chromosome number, 163, 164
experiments on echinoderms etc.,

163, 164. 325-330
fertilization, 279, 286
germ cells of Ascaris, 404, 405
hereidtary substance, 210
individuality of chromosome, 157
nuclear size, 63
polar body in Ascaris, 318
reduction, 228, 248, 251
somatic chromosomes, 149

Bower, 316

Brachymeiosis, 223

Brachystola (grasshopper), 158, 160,
252, 253, 255

Brauer, 77, 155, 227, 248, 318

Braun, 11

Bridges, 382

Brooks, F. T., 291

Brooks, W. K., 49

Brown, H. B., 291

Brown R., 6, 7

Brown W. H., 291. 299

430

INDEX

Brownian movement, 7
Bryonia (angiosperm), 356
Bryophyllum (angiosperm), 137
Bryophytes, blepharoplast, 86

centrosome, 82

fertilization, 292, 293

reduction, 224

sex-determination, 355, 356, 364
Bryum (moss), 317
Buchtien, 89
Bud sport, 351
Buffon, 48, 398
Bufo (toad), 241
Burmannia (angiosperm), 315
Buscalioni, 211
Biitschli, achromatic figure, 181

bacteria, 66

blue-green algae, 202

centrosome, 79

ectoplasm, 42

furrowing, 188

polar bodies, 15

protoplasm, 33, 49, 183

Cceoma (rust fungus), 312

Calcium salts, 135, 194

Caldwell, 93

Calkins, 183, 213, 248

Calopogon (orchid), 300

Calycanthus (angiosperm), 159, 236,

248, 256

Calypogeia (liverwort), 110
Cambarus (crayfish), 237
Cambium, 27, 63, 177
Camerarius, 13
Campanula (angiosperm), 159
Campbell, 89, 195
Canaliculse, 48
Cancer (crab), 237
Canis (dog), 237, 361
Cannabis (hemp), 256, 372
Capsicum (angiosperm), 332
Carbohydrates, 41
Cardiff, 236, 257
Carex (sedge), 159
Carleton, 65
Carnoy, 33, 248

and Lebrun, 280
Carothers, 160, 255
Carotin, 104, 121
Carpogonium, 289

Carrel, 138

Carruthers, 136, 223, 313
Carter, 60, 105, 209
Cartilage, 29, 30
Carus, 15
Castle, 390

and Wright, 391
Castrada (flatworm), 274
Catalyzer in egg, 285
Cattle, sex-determination, 361, 371
Cavers, 119, 122
Cell, description, 24

discovery, 2

evolution of, 69

a system, 26

Cell-division, Chaps. VIII-X
Cell-formation, early views, 5
Cell plate, 176, 190

sap, 135

Theory, 7, 9, 12

wall, 26, 28, 190 ff.
Cellulose, 135, 194
Central spindle, 177, 178
Centrifuged eggs, 329, 330
Centriole, 77, 83
Centrodesmose, 83, 209
Centronema, 84
Centrosome, 26, 76 ff.

individuality, 77

in fertilization, 279

in mitosis, 145, 146, 177, 178, 180

in plants, 79 ff.
Centrosphere, 26, 76
Cephalotaxus (conifer), 295
Ceratiomyxa (slime mold), 222
Ceratozamia (cycad), 220, 294
Cerebratulus (nemertean worm), 275
Chcenia (protozoan), 61, 68
Chcelopterus (annelid worm), 275
Chamberlain, 48, 82, 93, 94, 294, 295,

298

Chambers, observations on living cell,
38, 43, 62, 77, 118, 119, 120,
123, 183, 189, 214, 287
Chara (stonewort) and other Charales,
86, 90, 95, 108, 116, 210, 211,
222, 288

Chemical theory of development, 412
of heredity, 412

theories of life, 50

Chiasmatype hypothesis, 257, 384 ff.
Chick cells in tissue cultures, 146
Child, amitosis, 213

INDEX

431

Child, colloids, 34

germ-plasm, 407

germ-track, 405

metabolic gradient, 139

metabolism in maturation, 260

metaplasm, 133

nucleoplasmic ratio, 63

parthenogenesis, 319

physiology of fertilization, 284

polarity, 139

senescence, 136, 137

vital activity, 49, 50, 51
Chiloscyphus (liverwort), 248, 249
Chimera, 351

Chironomus (midge), 61, 406
Chitin, 194

Chlamydomonadacese, 112
Chloralized cells, 136, 164, 248, 249,

259, 260

Chlorogonium (green alga), 85
Chlorophyll, 104, 105, 124

inheritance, 331
Chlorophyllogen, 105
Chloroplast, 104, 105, 113

in heredity, 331-334
Chmielewskij, 109
Chodat, 202
Cholesterin,. 41
Chondriokont, 116
Chondriomite, 116

Chondriosome, 26, 48, 97, 111, 115 ff.,
134

division, 118

in fertilization, 120, 280

microtechnical methods, 117

other functions, 121

and oxidation, 123

relation to plastid, 121 ff., 124

rank as cell organ, 124

r61e in inheritance, 119, 122, 124,

279

Chorthippus (grasshopper), 168, 237
Christman, 291, 292, 312
Chromatid, 230, 246, 247
Chromatin, 25, 40, 64

contributions of parents, 279, 340

diminution, 404, 405

extrusion, 136

qualitatively varied, 226

Weismann's conception of, 400
Chromatoider Nebenkorper, 86, 89
Chromatophore, 104, 136, 202
Chromidia, 67, 68, 117, 118, 133, 208

Chromidial fragmentation, 208
Chromidiogamy, 281, 282
Chromioles, 155
Chromomere vesicle, 158, 162
Chromomeres, 154 ff., 155, 238, 243,

258, 391, 392
Chromosomal vesicle, 158
Chromosomes, aberrrant behavior, 302

alveolation, 148, 149

in artificial parthenogenesis, 286

bivalent, 227, 233

in cleavage division, 278, 295

conjugation, 227, 233, 251 ff.

in Cyanophycese, 203, 204

early work on, 143

in endosperm, 301

equational division, 156, 226

in fertilization, 277-279, 295 ff.,
340

in heredity, 326, 330, 333, 340 ff.,
396

individuality, 157-168

in life cycle, 221, 340

linear organization, 162, 226

in man, 361, 362

map, 388, 389

in mitosis, 144 ff.

multiple chromosomes, 164, 165

and mutation, 344 ff.

number, 162 ff., 347 ff.

pairing before maturation, 255

pairs, 160, 161, 227, 251

in parthenogenesis, 318, 319, 364

qualitative division, 227

reduction of, 219-261

sex, 358 ff.

size and shape, 160, 252 ff.

and species, 347 ff.

specificity of, 162, 250

splitting of, 145, 152, 153, 236, 239

tetrads, 230, 233, 243 ff.

transverse division, 227
Chroococcus (blue-green alga), 68, 205,

206

Chrysanthemum (angiosperm), 348
Chun, 211

Cicada (insect), 120, 121
Cienkowski, 12, 32
Cilia, 30, 45, 96
Ciliated cells, 96, 212
Cirri, 30, 45

Cladophora (green alga), 60, 85, 105, 112,
209

432

INDEX

Claussen, 81, 224, 290, 291
Cleavage centrosomes in, 279, 280

chromosomes in, 278, 295 ff.

division, 278, 329

furrowing, 146, 186 ff.
Cleland, 109, 223
Closing membrane, 191, 192
Closterium (desmid), 186, 222
Clowes, 35, 36, 44
Clytia (coelenterate), 328
Coagulation, 38
Cobcea (angiosperm), 177
Coctidium (protozoan), 207, 208
Coenocentrum, 289
Co3nocytes, 60, 411
Cohn, 11
Coker, 295

Coleochcete (green alga), 113, 222, 236
Collema (lichen), 291
Collins, E. J., 357
Collins, G. N., 333, 349
Colloids, 34

Colonial algae, 140, 331, 334, 411
Colorblindness, 381
Components (of colloids), 35
Compositae, 236, 315
Coniferales, fertilization, 294 ff.
Conjugate, 288, 316
Conjugate division of nuclei, 292
Conjugation in Paramoecium, 283, 284
Conklin, achromatic figure, 183

amitosis, 213-214

centrosomes, 78

fertilization, 279, 280

individuality of chromosome, 159

nucleoplasmic ratio, 63

performationism, 411, 414

promorphology of ovum, 329, 330

role of chro matin, 70

Weismannism, 408
Connecting fibers, 145, 176
Connective tissue, 29, 30
Conocephalus (liverwort), 86
Constrictions in chromosomes, 160, 161,

162, 166, 248, 249, 255, 260
Continuity of chromosomes : see chromo-
somes, individuality

of germ-plasm, 402, 403, 413
Contractile vacuole, 30, 44, 46, 84
Contractility in achromatic figure, 183
Copepods, 245, 249, 405
Coprinus (mushroom), 292
Corallina (red alga), 80, 223

Cork, 2, 194

Corpuscles de rebut, 283, 284

Corpuscular theories, 391, 398 ff.

Correlation, 139

Correns, 331, 332, 338, 356, 366, 372

Corsinia (liverwort), 82, 293

Corti, 6

Corydalis (angiosperm), 211

Coulter. 288

and Land, 295
Cowdry, E. V. 116-118
Cowdry, N. H., 116, 117, 123
Crampton, 328

Crepidula (mollusk), amitotic appear-
ances, 213

fertilization, 279, 280
individuality of chromosome, 159
nuclear size, 63
sex-determination, 371
Crepis (angiosperm), 159-162
reduction, 239, 252, 254
species and chromosomes, 349
Crinoid, 326, 327
Criss-cross inheritance, 380
Crossing over, 385, 386 ff.
Cruciferae, 159
Cryptobranchus (amphibian), 159, 160,

278, 279

Cryptomeria (conifer), 294
Crystalloid, 35, 65
Crystals, 134, 135
Ctenolabrus (fish), 160
Ctenophore, 328, 329
Cuenot, 355

Culex (mosquito), 165, 256
Cumingia (mollusk), 286
Cumulative factors, 343
Cunninghamia (conifer), 295
Cuscuta (angiosperm), 47
Cuticle, 194
Cuticularization, 194
Cutin, 194
Cutinization, 194
Cutleria (brown alga), 222, 241
Cutting, 312

Cyanophyceae, cell-division, 202 ff.
nucleus, 68
plastids, 108

Cyanophycin granules, 203, 206
Cyanoplasts, 203
Cycas (cycad), 93, 294
Cyclops (copepod), 159, 160, 237, 245,
279

INDEX

433

Cystolith, 134, 135

Cytoblast, 8

Cytoblastema, 8, 143

Cytokinesis, 186 ff.

by cell plate, 176, 190
by furrowing, 186

Cytolysis, 285, 286

Cytomorphosis, 117

Cytoplasm, 25, 40

in fertilization, 280, 294, 299
in heredity, 326, 328, 330
pattern in ovum, 329
reducing action, 123

Czapek, 44

1)

Dahlgren, 225
Dahlia (angiosperm), 256
Dale, 290, 312

Dangeard, 84, 85, 86, 122, 202, 290, 291
Daphnia (copepod), 371
Darling, 159, 356
Darwin, 16, 49, 398, 402
Datura (angiosperm), 134
Davey and Gibson, 372
Davis, B. M., blepharoplast, 85
centrospheres, 80, 82
cytokinesis, 187
fertilization, 289, 290
CEnothera mutants, 344, 345
plastid, 113, 114
reduction, 241
Davis, H. S., 239
Death, 136, 138

Debaisieux, 237

Dedifferentiation, 137

Degeneration, 211 ff., 259

Dehorne, 150, 159

Delage, 63

and Goldsmith, 400

Delia Valle, 163

Dellinger, 45

Dendrite, 30

Denke, 195

Derbesia (green alga), 85, 86

Dermatosome, 193

Dermocarpa (blue-green alga), 204

Derschau, 66, 123, 136

Des Cilleuls, 212

Desmid, 105, 186, 222

Determinant, 226, 400 ff.

Deton, 237

28

Deutoplasm, 134

Development (Ontogenesis), 323, 324,
335, 410

chemical control of, 412

non-factorial theory of, 411

Weismann's theory of, 401, 407
Developmental mechanics, 411
Devise", 182
Dextrin, 107
Diakinesis, 231, 232, 233
Dicrocoelium (flatworm), 245
Dictyota (brown alga), cell wall, 186

centrosome, 79, 80, 82

reduction, 223
Didiploid nuclei, 260
Didymium (slime mold), 187
Dietel, 224
Differentiation of cells, 26, 137

products, 133 ff.
Digametic condition, 358 ff.
Digby, extruded chromatin, 136

nucleolus, 65

prochromosomes, 159

reduction, 239, 240, 248

somatic chromosomes, 149, 152
Dinophilus (annelid worm), 357, 368
Dioncea (angiosperm), 47
Dioon (cycad), 62, 93, 94, 294
Diplotene, 231, 233
Diptera, 256
Dispermy, 163, 326
Dissosttira (grasshopper), 119
Dixon, 211
Dobell, 67
Dodge, 223
Dog, 361

Dominance, 337, 339
Doncaster, ix, 319, 363
Double fertilization, 14, 298, 299, 302
Double heterotypic spireme, 241-243
Double reduction, 223, 224, 291
Douin, 355, 364
Downing, 275

Draparnaldia (green alga), 104, 108
Driesch, 328, 412

and Morgan, 328

Drosera (angiosperm) 159, 236, 251, 252
Drosophila (fruit fly), chromosome com-
plement, 360, 383
map, 388, 389
crossing over, 387 ff.
intersexes, 371, 376
linkage, 379

434

INDEX

Drosophila (fruit fly), non-disjunction,
382, 383

sex-chromosomes, 360 ff.

sex-linkage, 380
Drosophilidse, 349, 350
Druery, 311
Druner, 181, 183
Druse, 134, 135
Dublin, 255
Dubreuil, 121
Duesberg, 115-120, 361
Dujardin, 11
Dupler, 295

Duplex chromosome group, 340
Duplicate factors, 343
Dytiscus (beetle), 237

E

Eames, 295

and Hayes, 135, 333
East and Jones, 352

Echinoderm hybrids, 160, 163, 325-327
Echinus (sea urchin), 160, 325
Ectocarpus (brown alga), 222, 314
Ectoderm, 329
Ectoplasm, 42, 44, 45
Ectoplast, 25, 43, 44, 85
Egg, segmentation in animal, 188, 189

fragments, 287
Eisen, 66, 155
Elceis (palm), 134
Elaioplast, 109, 110
Elasmobranchs, 239
Elastin, 194

Electrical charge of nucleus and
cytoplasm, 62, 184

phenomena in egg, 287

theories of mitosis, 184
Elementalism, 12, 411
Elodea (angiosperm), 37, 122, 356
Emberger, 122
Embryo sac, 299
Embryogeny (see Development)

historical, 13, 15
Embryonal mitosis, 278, 295 ff.
Embryonic cells, 137
Emerson, 135, 302, 333, 384, 391
Emulsion, 34-36, 44
Emulsoid, 35
Enchenopa (bug), 237
Enchylema, 33, 34
End piece, 275
Endoderm, 329

Endoplasm, 42, 44

Endosperm, development of, 302

mosaic effects, 302

nuclear behavior in, 300-301
Endospore, 195, 196
Energy change in cell, 26, 52, 70
Entelechy, 412
Enteroxenos (mollusk), 237
Entz, 83, 84
Enucleated cells, 69, 70
Environment and development, 411, 413
Enzyme, 39

Ephedra (gymnosperm), 295
Epigamic sex-determination, 357
Epigenesis theory, 4
Epithelial cells, 30, 83, 96
Equational division of chromatin, 156,

226, 228, 401

Equisetum (pteridophyte), chromosome
tetrad, 248

dicecism, 356

spermatogenesis, 90-92, 95

spore coat, 196

stem, 135
Ergatule, 49
Erhard, 96
von Erlangen, 189
Ernst, 300, 301, 315
Erysiphe (fungus), 80, 81, 290
Escoyez, 79, 87, 210
Etiolation, 105
Euchlcena (angiosperm), 349
Eudorina (green alga), 112
Euglena (flagellate, eyespot, 112

flagellum, 45

mitosis, 209

paramylum, 108
Euschistus (bug), 237
Exine, 196
Exospore, 195, 196
Extruded chromatin, 136
Eye color in Drosophila, 380
Eyespot, 46, 84, lllff., 112

Factor (see Gene), 333, 334, 336, 353,

409, 410

Factorial hypothesis, 343, 410
Farlow, 311
Farmer, 220, 248

and Digby, 136, 239, 312-315

and Moore, 237, 239

INDEX

435

Farmer and Reeves, 82

and Shove, 239

and Williams, 80, 222
Farr, 18&-188
Fasciola (flatworm), 247
Fasten, 237, 257
Fats, 41, 134

and chondriosomes, 121, 122
Faull, 80, 81, 179, 223, 291
Faure-Fremiet, 115, 117, 118
Federley, 252

Fegatella (liverwort), 86, 87
Ferguson, 295
Fertilization, animals, 273 ff.

centrosome in, 279

cone, 277

historical, 13

membrane, 277, 284

parental chromosome groups in,
159, 254, 276, 278, 279, 295,
300, 340

physiology of, 284 ff.

plants, 287 ff.

relation to maturation of ovum, 275
Fertilzin, 287
Fiber theory, 6

Fibrillar theory of protoplasm, 33
Fick, 163, 249, 250
Ficus (rubber plant), 134, 135
Filaria (round worm), 120
Fischer, 34, 66, 108, 202, 204
Fish hybrids, 159, 160
Fitting, 194, 195
Fitzpatrick, 224, 291
Fixation, 38, 117

Flagellates, blepharoplast, 83, 111
Flagellum, 45, 84, 96, 112

of spermatozoon, 273-275
Flemming, amitosis, 211, 213

cell-division, 143

centrosome, 77

chondriosomes, 115

chro matin, 64

chromosomes in man, 361

nuclei by division only, 10

protoplasm, 33, 34

reduction, 220, 227

spindle, 180
Florideae, 108, 116, 223
Floridean starch, 108, 109
Florin, 82, 248, 249
Flower colors, 135
Focke, 302

Faeniculum (angiosperm), 134
Fol, aster and astrosphere, 76

cell-division, 143

centrosomes in fertilization, 280

chromomeres, 155

fertilization, 15

streaming of protoplasm, 183
Font ana, 6
Food materials, 133
Foot and Strobell, 213, 244, 245
Forenbacher, 118, 122
Forficula (earwig), 239
Fossombronia (liverwort), 248
Fowl, 361, 363
Franze, 112
Fraser, brachymeiosis, 223

centrosome, 81

fusion in ascomycetes, 290, 312, 313

reduction, 238, 239, 240

somatic chromosomes, 149
Fraser and Brooks, 81, 223

and Snell, 149

and Welsford, 81, 223
Free, 44
Fremy, 193
Fries, 224
Friesendahl, 236
Fritillaria (angiosperm), amitosis, 211

constricted chromosomes, 162

fertilization, 298, 300, 301
Frog, artificial parthenogenesis, 319

blastomeres, 328

centrifuged eggs, 330

cleavage, 188

organization of egg, 330

sex-determination, 368, 370
Fromman, 33
Fuchsia (angiosperm), 239
Fucus, centrosome, 80, 82

eyespot, 111

fertilization, 14

gametes, 288

parthenogenesis, 314

plasma membrane, 43

reduction, 222, 223, 241

spindle, 179
Fujii, 92

Fuligo (slime mold), 39, 41, 187
Fumea (moth), 363
Funaria (moss), 106, 357
Function and structure, 374
Fundulus (fish), 158, 159, 160, 328
Funkia (angiosperm), 236, 256

436

INDEX

Furrowing (cytokinesis), 186 jf.
Fusion, centrosomes, 280, 291

chromosomes, 257, 394

gametes, 276 ff.

nuclei, 15, 219, 277 ff., 299, 301

sporocytes, 313, 314

three nuclei in endosperm, 299, 301

O

Gaertner, 13

Gaillardia (angiosperm), 110
Gall-fly, 319, 364
Gallactinia (fungus), 80
Gallardo, 184
Galton, 399, 402, 404
Galtonia (angiosperm), extrusion of
chromatin, 136

nucleolus, 65

reduction, 236, 239

somatic pairing, 256

Gametes, 137, 273 ff., 288, 293 ff., 333 ff.
Gametophytes, sex-determination in,

355, 356, 364, 372
Garber, 292
Gardiner, 46, 47

and Hill, 46
Gardner, 108, 202, 204
Gargeanne, 110, 111
Gastrodia (orchid), 314
Gatenby, 120
Gates, 63, 149, 241, 252, 344, 345

and Thomas, 345
Gaudissart, 121

Geddes and Thomson, 355, 369
Geerts, 241, 252
Gegenbaur, 15
Gel, 35

Gelatin, 37, 194
Gelation, 38
Gemini, 233
Gemmule, 49, 226, 398
Generative apogamy, 315
Generatule, 49
Genes, 343

cytological evidence for, 391 ff.

in development, 410

linked, 380, 386

modification of, 351-353

nature of, 352, 391, 409
Genetic continuity, 15, 323
Genotype, 339
Geotriton (salamander), 118

Gerassimow, 61, 69

Germ cells and soma cells, 402 ff.

Germ-plasm, 226, 334

and soma-plasm, 407

Weismann's conception of, 400 ff.
Germ-track, 403 ff.
Germinal localization theory, 328

selection, 404

vesicle, 62, 221, 275
Giard, 15
Gigantism, 346
Giglio-Tos, 250
Gille, 160

Ginkgo (gymnosperm), 92, 122, 236, 294
Gladiolus (angiosperm), 177
Glaser, 40, 213
Glceocapsa (blue-green alga), 204, 205,

206

Globular theory, 6
Globulin, 40
Glucose, 106
Glucoside, 135
Glycogen, 108
Gnetales, 294
Gnomonia (fungus), 291
Godlewski, 119, 213

echinoderm hybrids, 326-328
Goldschmidt, chromidia, 133

parthenogenesis, 319

reduction, 244, 245, 256

sex-determination, 370
Gonium (green alga), 112
Gonotokont, 225
Goodsir, 10
Goroschankin, 46
Gottsche, 110
Gould, 371

Grafted tissue in apogamy, 312-314
Graham, 82, 293
Granata, 250

Granular theory of protoplasm, 33, 49
Grasshopper, 156, 158, 162, 233, 237,

243, 258, 351, 391, 392
Gregoire, chromomeres, 258

nuclear reticulum, 64, 150, 152, 155

reduction, 227, 231, 234, 236, 245,

248, 257, 259
Gregoire and Berghs, 82

and Wygaerts, 64, 149, 152, 155
Gregory, 239, 346, 391
Grew, 2

Griffithsia (red alga), 223
Griggs, 239, 248

INDEX

437

Gromia (protozoan), 46

Ground substance, 33

Growth period in oocyte, 158, 221, 233,

234, 274, 276
Gruber, 61, 69, 70
Gryllotalpa (mole cricket), 118, 119
Guignard, blepharoplast, 90, 94

eyespot, 111

fertilization, 14, 298, 300, 325

reduction, 220
Guilliermond, bacteria, 67

centrosome, 80

chondriosomes, 115-122

reduction, 224

yeasts, 211, 292
Guinea pigs, 274, 341, 342
Gurwitsch, 213
Gutherz, 361
Gutta percha, 135
Guyer, 285, 361, 363
Gymnogramme (fern), 90
Gyrodactylus (flatworm), 160

H

Haberlandt, 46, 61, 103, 104, 105, 193
Haeckel, 16, 48, 324, 402
Haecker, fertilization, 279

hereditary substance, 210

individuality of chromosome, 157,
159

Keimbahn-plasma, 405

nucleolus, 66

pseudoamitosis, 211, 213

reduction, 227, 244, 245

sex-determination, 357
Hsematochrom, 112
Haller, 6
Hamm, 14
Hammarsten, 40
Hance, 165, 346
Hannig, 196

Hanstein, 12, 24, 33, 46, 133
Hardy, 62
Hargitt, 213
Harman, 160, 213, 391
Harper, centrosomes in ascus, 80, 81

chondriosomes, 123

cleavage furrow, 187

development in colonial algae, 334,
411

evolution of cell structure, 207

fertilization in ascomycetes, 290

Harper, inheritance in colonial algae, 331,
334

mitosis in myxomycetes, 208

plastids, 103, 105, 107, 115

polarity, 138, 140

protoplasm, 38, 49, 51

reduction in ascomycetes, 223
Hartmann and Noller, 84
Hartog, 104, 185
Harvey, E. E., 162
Harvey, E. N., 284
Harvey, Wm., 4
Hasper, 405
Hatschek, 36, 49, 138
Hautschicht, 42
Heartwood, 194
Hegler, 108, 202, 203
Hegner, 62, 404, 405
Heidenhain, 64, 70, 138, 144, 183
Heilbrunn, 189, 286, 287
Helix (snail), 96, 256
Helleborus (angiosperm), 159
Helvetia (fungus), 136, 223, 291, 313
Hemiptera, 239, 255
Hemp, 372

Henking, 227, 244, 251, 256, 358
Henle, 10

Henneguy, 70, 95, 96
Herbst, 213, 325, 326
Heredity, cell organs in, 323 ff.

chemical theory of, 412

in colonial algae, 331, 334

early theories of, 398 ff.

Mendelian, 336 ff.

non-factorial theories of, 411

Weismann's theory of, 401
Herla, 157, 164
Herlant, 285, 286
Hermann, 144, 180, 183
Herrick, 274
Hersilia (copepod), 237
Hertwig, O., basal granules, 96

cell-division, 143

fertilization, 15, 273, 276

nucleus in heredity, 16, 324

spindle, 180
Hertwig, R., amitosys, 211, 213

chromidia, 133

division in protozoa, 207

heterotypic prophase an abortive
mitosis, 259

nucleoplasmic ratio, 63, 70

sex-determination, 368, 370

438

INDEX

Hesperotettix (grasshopper), 164, 165
Heterochromosomes, 358 ff.
Heterodera (fungus), 260
Heterogametic females, 363

males. 358 ff.
Heterotypic mitosis, 220, 231 ff.

compared with somatic mitosis, 228

and degeneration, 259-260
Heterozygous state, 339
Heuser, 144
Hexads, 249
Hick, 46

Hieradum (angiosperm), 315, 316
Hill, 46
Hilum, 106
Hirase, 92; 294
His, 328
Hober, 51

Hofmeister, 10, 13, 14
Holt, 165

Homarus (lobster), 274
Homceotypic mitosis, 220, 231 ff.
Homologous chromosome pairing, 161,

252-254, 340, 342
Homozygous state, 339
Homunculus, 3
Hooke, 2, 23
Hoppe-Seyler, 40
Hormones, 139
Horse, 361
Hoven, 121
Humaria (fungus), 81, 223, 290, 291,

312, 313
Httppe, 67
Hutchinson, 295 ff.
Huxley, 12, 13, 168, 273
Hyaloplasm, 33, 42, 183

sphere, 76
Hybrid echinoderms, 160, 163

fishes, 159, 160

Hydatina (rotifer), 357, 364, 368
Hydnobolites (fungus), 81, 224
Hydra (coelenterate), 276
Hydrocharis (angiosperm), 256
Hydrodictyon (green alga), 85, 105, 107,
109, 187

development of, 334, 411

inheritance in, 331, 334
Hydrogel, 37
Hydrophilus (beetle), 239
Hydrosol, 37

Hygrophorus (fungus), 224
Hymenomycetes, 224, 292

Hypertonic solutions, 285
Hypothesis, role of, 396-397

Id, 226, 400 ff.
Idant, 226, 401
Idiochromatin, 208, 209
Idiochromosome, 358
Idioplasm, 258, 324, 400

chemical conception of, 412

theory of Nageli, 399
Idiosome, 77
Ikeda, 248

Ikeno, 86, 89, 93, 94, 294, 332
Ilyanassa (mollusk), 328
Immortality of certain cells, 138, 403
Independence of chromosome pairs, 255
Individuality, aleurone grain, 135

alga colony, 140

centrosome, 77

chromosome, 157 ff., 168

organism, 168

plastid, 113
Infusoria, 60

Inheritance of acquired somatic changes,
399, 403

colony characters, 331, 334

Mendelian characters, 336 ff.

plastid characters, 331 ff.

sex, 354 ff.

Insects, 160, 239, 359 ff.
Inter-alveolar substance, 34
Interference, 389, 390
Inter-filar substance, 33
Interkinesis, 232, 236
Interphase, 145, 150, 151, 236
Intersexes, 370 ff.
Intracellular pangenesis, 399
Intranuclear spindle, 179
Intra-vitam stains, 117, 287
Intussusception, 193
Inulin, 134, 135
Inversion of phases, 36, 44
Ipomoea (angiosperm), 196
Iris (flag), 121, 122, 177
Ishikawa, 256
Isoetes (pteridophyte), 114
Isonandra (angiosperm), 135
Isotropy of egg, 328

Jaeger, 402

Jahn, 59, 208, 222

INDEX

439

Janssen, 1

Janssens, 237, 257, 385, 386, 393

and Willems, 237, 256
Jennings, 63, 396
Johnson, 111
Johow, 210
Jonsson, 46
Jordan, 213
Jorgensen, 178, 213
Juel, 224, 315

Juglans (walnut, butternut), 134
Juniperus (juniper), 294, 295

K

Kabsch, 193
Kahle, 404
Karsten, 79, 209, 222
Karyochondria, 118
Karyogamy, 282
Karyokinesis, 143 (see Mitosis)
Karyolymph, 25, 64, 149, 182, 184
Karyoplasm, 25
Karyoplast, 84
Karyosome, 25, 65, 151, 209
Kassowitz, 133
Keene, 110, 290
Keimbahn, 403

-determinants, 406

-plasma, 406
Kemp, 259
Keratin, 194
Kernplasma relation, 62
Keuten, 209
Kienitz-Gerloff, 46, 47
Kieser, 5
Kihara, 252, 348
Kildahl, 295
Kinetonucleus, 84
Kinetosomes, 88
King, 241, 368
Kingery, 116, 259
Kingsbury, 117, 123, 124

and Hirsch, 259
Kinoplasm, 34, 43, 181
Kinoplasmic caps, 175
Kite, 38, 43, 62, 64
Klebs, 111, 112, 193
Klein, 33, 181, 183
Kniep, 224
Knoche, 213
Koelreuter, 13

Kofoid, 208, 209

Kohl, 46, 108, 202, 203

von Kolliker, 10, 15, 16, 34

Koltzoff, 274

Konopacki, 213

Korff, 95, 96

Kornhauser, 237, 257

Korotneff, 118

Korschelt, 61, 244

Kossel, 40, 42

Kostanecki and Wierzyski, 276

Kowalevsky, 144

Kuczynski, 84

Kuhla, 46, 47

Kiihne, 12

Kurssanow, 113, 222

Kuschakewitsch, 368

Kusano, 314

Kiister, 104, 110

Kuwada, 65, 256, 348, 349

Kylin, 222, 314

Laboulbenia (fungus), 179, 224, 291
Lachnea (fungus), 80, 81, 223, 290, 291,

312

Lagerburg, 236
Laibach, 159
Land, 294
Lang, 213

Larix Oarch), 122, 182, 191, 192, 295
Lastrcea (fern), 312, 313, 314, 317
Laticiferous vessel, 59
Lauterborn, 79

La Valette St. George, 14, 115, 120
Lawson, fertilization, 294, 295

nuclear membrane, 64

reduction, 239

spindle, 177, 184
Lecithin, 41
van Leeuwenhoek, 3
van Leeuwen-Reijnvaan, 224
Leguminosse, 348
Lenhossek, 96
Leotia (fungus), 291
Lepeschkin, 44, 49
Lepidosiren (fish), 162
Leptonema, 231-233
Leptotene, 231
Lerat, 237, 245

Leucoplast, 104, 106, 109, 113, 121, 122
Levine, 81, 224

440

INDEX

Lewis, C. E., 87

Lewis, I. F., 223

Lewis, I. M., 239

Lewis, M. and W., 116, 118, 123, 146

Lewitski, 115, 118, 121

Leydig, 33

Libocedrus (conifer), 294

Lignin, 194

Lilium (lily), amitosis, 211

chondriosomes, 116

extruded chromatin, 136

fertilization, 298-301, 325

megaspore nuclei, 225

protoplasmic connections, 47

reduction, 236, 238, 239
Lillie, F. R., fertilization, 273 ff., 286

regeneration, 69

sex-determination, 370, 371
Lillie, R. S., 62, 70, 184, 185, 285
Limosphere, 88, 89
Lindstrom, 332
Linin, 25, 64, 180, 181
Link, 5
Linkage, 378 ff.

calculation of, 389

groups, 382 ff.
Lipoid, 41, 43, 44
Liposomes, 123
de Litardiere, 236
Liverworts, blepharoplast, 86

centrosome, 82

fertilization, 293

sex-chromosomes, 364, 365, 372
Lloyd, 44
Lock, 378
Locy, 323

Loeb, J., 284, 319, 326
Loeb, L., 138
Loeb and Bancroft, 285

and Wasteneys, 70, 287
Lopezia (angiosperm), 248
Lotsy, 225, 344
Lowschin, 117, 118
Lundegardh, 123, 144, 149, 155, 236
Luther, 274
Lutman, 186
Lutz, 345, 346

Lychnis (angiosperm), 256, 382
Lycopodium (pteridophyte), 91
Lygceus (bug), 359, 360, 367
Lymantria ^moth), 370
Lyon, 194, 195
Lysin, 285

M

Macallum, 66

Macdougal, 41

Macfarlane, 47

Macormick, 290

Macronucleus, 30, 60

Macrosomes, 38

Maggi, 48, 115

Magnolia (angiosperm), 188

Maier, 96

Maire, 80, 224

Maize, chlorophyll inheritance, 332

crossing over, 391

hybrid nature? 349

linkage groups, 384
Makinoa (liverwort), 87
Male cells and nuclei, 294, 298, 299

cytoplasm in fertilization, 294
Malone, 237, 361
Malpighi, 2
Malsen, 357, 368
Malva (angiosperm), 194
Man, centrosome, 77

colorblindness, 381

reduction, 237

sex-chromosomes, 361, 362
Mangin, 194

Manifold effects of factor, 343
Mantle fibers, 145, 176, 177
Marchal, 317, 355

Marchantia (liverwort), blepharoplast,
centrosome, and spermatogene-
sis, 82, 86 ff.

cell-formation, 5

chondriosomes, 122

sex-determination, 355
Marcus, 248, 259

Marshal, 155, 158, 210, 234, 237, 257
Mark, 15

Marsilia (water fern), apogamy, 315
316

blepharoplast and centrosome, 91,
92, 95

nucleus, 61, 66, 210

spindle, 176

spore coat, 194
Martin, 33

Martins Mano, 155, 236
Masdevallia (Orchid), 47
Massee, 46
Mast, 112
Mastigella (flagellate), 83

INDEX

441

Mastigina (flagellate), 83

Mathews, 42, 70

Maturation divisions, 220 ff.

Maximow, 213

Mayer, Rathery, and Schaeffer, 123

McAllister, 109, 236

McAvoy, 239

McClendon, 188, 285

McClung, chromosomes in insects, 351

chromosome individuality, 160,
163-168

reduction, 245, 252, 393

sex-chromosome, 358
McCubbin, 291
McLean, 212
Mead, 78
Mechanism of cytokinesis, 188

of karyokinesis, 182
Mechanistic hypothesis, 52
Meek, 185
Meganucleus, 30, 60
Megaspore, Lilium, 225

Marsilia, 194

Physostegia, 225

Selaginella, 195
Meiosis, 220

Melandrium (angiosperm), 256
M elanoxanthus (plant louse), 319
Melin, 248
Membranellse, 45
Mencl, 67
Mendelism, 336-344

and sex, 366
Menidia (fish), 160

Mercurialis (angiosperm), 256, 356, 372
Merismopedia (green alga), 205, 206
Meristem, 27, 63, 137
Mermiria (grasshopper), 165
Merogony, 63, 325
Merriman, 149, 209, 210
Mesoderm, 329
Mesospore, 195
Metabolic gradient, 139

rate or level, 137, 139, 260

theories of sex, 369 ff. .
Metabolism, 69, 70, 260
Metachromatin, 205, 206
Metachrome, 122
Metaphase, 144, 145, 147, 148
Metaplasm, 26, 133 ff.

and senescence, 136
Metasyndese, 228
Metcalf, 208

Metz, 256, 349, 350, 360, 361, 395
Meves, amitosis, 213

centrosome in sperm, 95

chondriosomes, 115-121
in fertilization, 120, 280

reduction, 227, 250

spermatozoon, 274

spindle, 178
Meyen, 6

Meyer, A., 46, 106, 107, 113, 123
Meyer, K., 82, 293
Miastor (fly), 404-406
Micellae, 107, 193, 399
Microcycas (cycad), 93
Microdissection, 38, 62
Micromeric theories of protoplasm, 48
Micronucleus, 30, 60
Microscope, 1
Microsome, 33, 122, 190
Microsphcera (fungus), 81
Microsporocyte, cytokinesis, 187

reduction, 225, 235, 238
Microzyme, 48
Mid-body, 178, 179, 188
Middle lamella, 190, 191, 192, 194

piece, 273, 275, 281
Miescher, 40, 41
Migula, 67

Mimosa (.angiosperm), 47
Minchin, 44, 45, 60, 68, 83, 207, 208, 222,

281, 282, 317
Minot, 63

Mirabilis (angiosperm), plastid inheri-
tance, 331

Mendelian behavior, 338, 339

spore coat, 196
Mirande, 116
Mirbel, 5

Mitochondria, 115 ff.
Mitokinetism, 185
Mitome and paramitome, 33
Mitoplast, 122
Mitosis, duration of phases, 146

heterotypic, 220, 231 ff.

mechanism of, 182 ff.

somatic, 143 ff.

compared with heterotypic, 228
summarized, 156

in Cyanophycese, 202 ff.

in other thallophytes, 179

in protozoa,. 207 ff.
Mixochromosome, 257
Miyake, 87, 92, 236, 290, 295

442

INDEX

Mnium (moss), 87, 88, 317

Moenkhaus, 160

von Mohl, cell-formation, 6, 9, 10

cell wall, 190, 192

plastid, 12, 103
Moira (echinoderm), 160
Moldenhawer, 5
Molisch, 193
Monascus (fungus), 291
Monaster, 286
Monkey, centrosomes, 77
Monotropa (angiosperm), 70
Monteverde and Lubimenko, 105
Montgomery, 65, 163, 228, 237, 239,

251, 256, 257, 281, 358, 361
Moore, A. C., 248
Moore, B., 43

Moore, J. E. S., 95, 227, 255
Moore and Embleton, 239
Moreau, 118, 122, 187
Morgan, achromatic figure, 183

blastomeres, 328

centrifuged eggs, 330

conjugation in Paramcecium, 283

crossing over, 379 ff.

cytasters, 78, 286

factorial hypothesis, 344, 410

linkage, 379 ff.

sex-determination, 357, 360, 361,
368, 376

on Weismann, 408
Moroff, 213
Morphoplasm, 400
Morris, 160

Morus (mulberry), 256, 372
Mosaic endosperm, 302

leaves, 331 ff.
Mosses, regeneration, 317

sex-determination, 355 ff.

spermatogenesis, 88 ff.
Moths, sex-determination, 363, 367
Motile cells, 83
Mottier, blepharoplast, 86, 87

cell plate, 186

centrosomes, 79, 80

chondriosomes, 119^.

chromomeres, 155, 238

fertilization, 298, 300

reduction, 238, 239
Mucor (mold fungus), 62
Muller, 389, 390

and Altenburg, 352
Muller, 149, 152, 155, 156, 253, 256

Mulsow, 359

Multiple chromosomes, 164, 165

complex, 165
Multipolar mitosis, 163, 325, 326

stage of mitosis, 176, 177
Murbeck, 313, 315
Murrill, 295
Mus (mouse), 237
Musa (banana), 159
Musca (fly), 360
Muscle cells, 28, 29, 120-121
Mutation, 344-352

and chromosome number, 344, 351

of gene, 351-353

rate of, 352

vegetative, 351
Myoneme, 46
Myrica (angiosperm), 372
Myricaria (angiosperm), 236
Myristica (angiosperm), 134
Myxine (fish), 237
Myxomycetes, chondriosomes, 116

cytokinesis, 187

mitosis, 208, 209

nucleus, 59

protoplasm, 32

reduction, 222

N

Nabours, 391

Nachtsheim, 318, 357

Nagai, 373

von Nageli, cell-formation by division, 9

cell wall, 190, 192

idioplasm theory, 399

plastid and starch, 12

starch grain, 107
Nagler, 282

Najas (angiosperm), 160, 161, 253
Nakahara, 66, 211, 239
Nakanishi, 67

Narcissus (angiosperm), 346
Narcotics, 259, 260
Nathansohn, 211, 213, 316
Nawaschin, M., 160, 162
Nawaschin, S., 14, 162, 298, 300, 301,

325

Nebenkern, 91, 120
Nemalion (red alga), controsomes, 80

pyrenoid, 108, 109

reduction, 223
Nematus (saw-fly), 318

INDEX

443

Nemec, 149, 152, 213, 259
Neotiella (fungus), 81, 224
Nephrodium (fern), apogamy, 313-315

fertilization, 293

reduction, 232, 236

spermatogenesis, 91

spindle, 176

Nereis (annelid worm), 274-277, 280
Nerve cells, 29, 30
Neuroterus (gall-fly), 319, 364
Newman and Patterson, 357
Newport, 15
Nichols, 255
Nichols, G. E., 295
Nicotiana (tobacco), 187
Nidularia (fungus), 224
Nienburg, 291
Nissl substance, 30
Noll, 193, 355

Non-disjunction, 346, 348, 382, 383
Non-factorial theories of heredity and

development, 411
Noren, 294, 295
Nothnagel, achromatic figure, 182

fertilization, 298-301

reduction, 239, 240
Nowikoff, 213
Nuclear membrane, 63, 149, 154

migration, 312
Nucleic acid, 40
Nuclein, 40, 64
Nucleolini, 65
Nucleolo-centrosome, 209
Nucleolus, 25, 65, 150, 154, 181

and chromosomes, 209, 210
Nucleoplasm, 25
Nucleoplasmic ratio, 62
Nucleo-proteins, 40
Nucleus, division, 143 ff.

discovery, 7

distributed nucleus, 59, 68

early views on division, 10, 143

evolution of, 206

in fertilization, 277

functions, 69

in heredity, 16, 324 ff.

kinetic nucleus, 84

nuclear membrane, 63> 149, 154

in protista, 66

relation to metabolism, 69

sphere of influence, 62

structure, 63, 160

trophic nucleus, 84

Nucleus, vesicular, 69
Numerical reduction, 229, 249
Nussbaum, 402
Nymphcea (water lily), 115, 116

O

Octads, 249
(Edamatin, 64

(Edogonium (green alga), blepharoplast,
85

fertilization, 14, 288

reduction, 222
(Elkers, 222
(Enothera, cytology of mutants, 344 ff.

fragmentation of chromosomes, 165

synapsis in hybrids, 252
Ogata, 65
Oil, 110, 111, 122, 134, 194

body, 110, 134

droplet, division of, 189

vacuole, 48, 111, 134
Olive, 59, 108, 202, 204, 208, 222
Oliver, 47

Oniscus (crustacean), 255
Onoclea (fern), 288, 356, 372, 375
Ontogenesis, 323, 324, 398, 401, 407, 411
Ooapogamy, 315

Oocytes or ovocytes, 220, 233, 274, 275
Oogenesis, 221, 274, 276
Oogonia or ovogonia, 221
Oogonium (of plants), 179, 222, 223, 288
Oomycetes, 289
Open spireme, 238

Ophryotrocha (annelid worm), 237, 244
Orchis (orchid), 104
Organic acids, 135

Organism as a whole, 12, 52, 140, 411
Or man, 118
Orthogenesis, 404, 410
Orthoptera, 386, 393
Oryza (rice), 256

Oscillatoria (blue-green alga), 202, 204
Osmosis in mitosis, 184
Osmunda (fern), apical cell, 27

reduction, 234, 236, 239, 240

sex-determination, 373
Osterhout, 70, 248
Otidea (fungus), 81, 223
Ovarian egg, 221, 274
Overton, E., 43, 46, 111, 220
Overton, J. B., parthenogenesis, 314, 315

prochromosomes, 159

444

INDEX

Overton, reduction, 223, 236, 248~, 249

somatic pairing, 256, 257
Ovum, 221, 274, 275, 276

activation of, 284 ff.

organization of, 328 ff.
Oxidation, and chondriosomes, 123, 124

in egg, 285, 287

and nucleus, 70

and sex-determination, 368
Oxychromatin, 64, 66, 70

Pace, 300, 315

Pachynema, 231, 232, 233

Pachytene, 231, 233, 258

Padina (brown alga), 223

Palladin, 135

Pallavicinia (liverwort), 122, 220, 248

Pangenesis hypothesis, 398, 399

Pangenosomes, 258

Pangens, 258, 399

Parallelism of chromosomes and

characters, 342-343
Paramitome, 33
Paramoecium, 30, 60, 207, 283
Paramylum, 108, 112
Parasynapsis, 228, 231, 233, 234, 235,

243, 248, 250, 251, 257
Parasyndese, 228
Paratettix (grasshopper), 391
Parechinus (sea urchin), 326
Parental chromosome sets, 254, 279, 340
Paris (angiosperm), 301
Parmenter, 160, 163, 319
Parthenogenesis, 314, 317, 364

artificial, 284 ff.
Passiflora (angiosperm), 181
Patterson, 213
Paulmier, 95, 244, 245
Pay en, 11, 193
Payne, 119, 359, 360
Pea, Mendelism, 336, 337

linkage, 378, 384
Pearl, 138
Pearson, 52
Pectates, 194
Pectose, 194
Pediastrum (green alga), 140, 331, 334,

411

Pedicellina (flatworm), 255
Pelargonium (angiosperm), 104, 332
Pellia (liverwort), 82, 87, 88
Pellicle, 45

Pensa, 117, 122
Pentose and pentosan, 41
Peperomia (angiosperm), 299
Peptone, 40

Peranema (flagellate), 83
Percnosome, 89
Perforatorium, 273, 276, 277
Perikaryoplasm, 177
Perinium, 195
Peripatus (arthropod), 281
Periplaneta (cockroach), 239
Periplasm, 289
Periplast, 44
Perispore, 195, 196
Perivitelline space, 277
Perla (stone-fly), 239
Permeability, 37, 43, 287
Peronospora (fungus), 290
Peziza (fungus), 80, 81, 223, 291
Pfeffer, aleurone grain, 134

amitosis, 211

cell wall, 193

ectoplast, 42

oil body, 110

protoplasmic connections, 47
Pfitzner, 33, 155
Pfluger, 328
Phseophyceae, 108, 222
Phajus (orchid), 106
Phascum (moss), 317
Phases in colloids, 35

of mitosis, 145
Phenotype, 339
Phillips, 202
Phosphatid, 117
Phospholipin, 117
Phosphoric acid, 40
Photosynthesis, 105, 107, 108
Phragmatobia (moth), 360, 361, 363
Phragmidium (fungus), 291, 312
Phragmites (angiosperm), 346
Phragmoplast, 176
Phrynotettix (grasshopper), 156, 158,

162, 233, 237, 243, 258, 392
Phycocyanin, 104
Phycoerythrin, 104
Phycomyces (fungus), 110, 186, 187, 355,

373

Phycomycetes, 289
Phyllactinia (fungus), 80, 81, 290, 291
Phyllodadus (conifer), 295
Phylloglossum (pteridophyte), 91
Phyllopneuste (bird), 274

INDEX

445

Phylloxera (aphid), 318, 357, 364
Physa (snail), 276
Physiological units, 48, 398
Physiology of fertilization, 284 ff.

of sex, 369 ff.

Physostegia (angiosperm), 176, 225
Phytelephas (angiosperm), 46
Picard, 147, 230
Pictet, 51

Pieris (butterfly), 61, 211
Pig, chromosomes, 165, 361
Pigcera (butterfly), 252
Pigeons, sex-determination, 369, 376
Pigments, 104, 105, 122, 135, 136
Pilobolus (fungus), 187
Pinguicula (angiosperm), 158
Pinney, 159

Pinnotheres (crustacean), 274
Pinus (pine), cell plate, 176

chondriosomes, 121, 122

fertilization, 294, 295

protoplasmic connections, 46

reduction, 239
Pisdola (leech), 178
Pisum (pea), 61, 121, 122, 249, 256, 348

linkage in, 378, 384
Pits, 28, 191, 192
Planaria (flatworm), 139, 237
Plantago (angiosperm), 372
Plasma membrane, 25, 42
Plasmahaut, 42

Plasmatic microsomes, 206, 206
Plasmodermal blepharoplast, 95, 97
Plasmodesmen, 46
Plasmodium (protozoan), 317
Plasmogamy, 292
Plasmosome, 25, 65
Plastid, 26, 103 ff.

individuality of, 113

inheritance, 331-334

primordia, 115, 122, 124, 332,

333

Plastidome, 122
Plastidule, 48
Plastochondria, 118, 121
Plough, 395

Plumbagella (angiosperm), 225
Podocarpus (conifer), 213
Podophyllum (angiosperm), 159, 239
Point mutation, 351
Polar bodies, 15, 221, 275, 276

in parthenogenesis, 318, 319, 364
Polar nuclei, 299, 300, 301

Polarity, 138 ff., 329, 334

Polianthes (angiosperm), 110

Polioplasm, 42

Politis, 110

Pollen grain wall, 196

tube, 294, 298, 299
Polygonella (angiosperm), 25
Polypodium (fern), 236
Polysiphonia (red alga), centrospheres,
79, 80

fertilization, 289

reduction, 223, 236
Polystichum (fern), 316, 317
Poly stigma (fungus), 291
Polytoma (green alga), 83, 84
Polytomella (green alga), 83
Polytrichum (moss), 88, 89, 224
Popoff, 120, 259
Porella (liverwort), 87
Postreduction, 245, 248
Potato starch, 107
Pratt and Long, 237
Preformation theory, 3
Preissia (liverwort), 82, 293
Prenant, 184, 185
Prereduction, 245, 248
Prevost and Dumas, 14, 15
Primsertypus, 245

Primula (primrose), 239, 248, 346, 391
Pringsheim, 14, 311
Prionidus (bug), 359, 360
Pritchard, 372
Prochromosomes, 158, 159
Progamic sex-determination, 357
Promitosis, 208

Promorphology of ovum, 328 ff.
Pronucleus, 275, 276, 277
Prophase, somatic, 144, 145, 150-153

heterotypic, 231 ff.
Protandry, 371
Protein, 39, 40, 51, 134

crystals, 135
Protenor (bug), 360
Prothallial nuclei, 294
Protokaryon, 68
Protoplasm, chemical nature, 39 jf., 50

colloidal nature, 34 ff., 51

doctrine, 11

early observations, 6, 32 ff.

physical properties, 32

senescence, 136

structural theories, 33

substratum of life, 32, 48 ff. -_.

446

INDEX

Protoplasm, a system, 32, 49, 52

varieties of, 41
Protoplasmic connections, 46
Protoplasmid, 41
Protoplast, 12, 24
Protoxylem, 192
Protozoa, ectoplast, 44

mitosis, 207 ff.

nucleus 68

reduction 222
Pseudapogamy, 312
Pseudapospory, 315
Pseudoamitosis, 211
Pseudogamy, 292
Pseudomitosis, 204
Pseudopodium, 45
Psilotum (pteridophyte), 239
Pteridophytes, fertilization, 292, 293

sex-determination, 356

spermatogenesis, 89 ff.
Pteris (fern), 248, 312, 317
Puccinia (rust fungus), 312
Punnett, 395
Purkinje, 11
Pustularia (fungus), 116
Pyrenoid, 104, 108
Pyronema (fungus), 81, 224, 290, 291
Pyrrochoris (bug), 244
Pythium (fungus), 290

Q

Quadrille of centers, 280
Quadripolar division, 325
Quadrivalent chromosomes, 223
Qualitative division of chromosome, 227,

229, 297, 401
Quantitative theory of sex, 370 ff.

R

Rabbit, 212, 237

Rabl, 157, 181, 279

Raciborski, 110

Ramlow, 291

Random assortment of chromosome

pairs, 255
Ranunculacese, 315
Raphides, 134, 135
Rat, 391

vom Rath, 211, 227, 244, 245
Rauber, 326, 402
Rawitz, 77

Reboulia (liverwort), 293
Recessive, 338, 339, 351
Recombination of factors, 343, 344
Reducing action of cytoplasm, 123
Reduction of chromosomes, in animals,
220 ff.

with chromosome tetrads, 243 ff.

defined, 229

discovery, 219, 220

modes of, 230 ff.

numerical, 229

in plant groups, 222-225

in somatic cells? 259

Weismann's theory of, 226

without qualitative change, 249
Reduplication hypothesis, 395
Reed, 65, 70

Refractive body, 118, 274
Regaud, 115, 117
Regeneration, 69, 137, 406
Regional mutation, 351
Reichert, 42
Reinke, 64

and Rodewald, 39, 41
Rejuvenescence, 137
Remak, 10, 43
Reserve starch, 106, 107
Resin, 194

Respiration, 123, 124
Resting stage of nucleus, 144, 150, 151,

157

Reticular theory of protoplasm, 33, 49
Reticulum of nucleus, 25, 64, 144, 150,

157

Rhabdites (nematode worm), 189
Rheum (angiosperm), 134
Rhizina (fungus), 291
Rhizonema, 83, 84
Rhizoplast, 83
Rhizopus (mold fungus), 116, 186, 187,

290

Rhumbler, 183
Ribes (gooseberry), 236
Riccardia (liverwort), 82, 236
Riccia (liverwort), 87, 293
Richardia (angiosperm), 159, 248, 249
Richards, 158, 159, 213
Ricinus (castor bean), 134
Riddle, 369, 375
Ritter, 411
Rivett, 111
Robertson, A., 295
Robertson, T. B., 189

INDEX

447

Robertson, W. R. B., 160, 165, 237, 245,
249, 257, 351, 393, 394

Rosa (rose), 348

Rosacese, 315, 348

Rosen, 65

Rosenberg, 160, 161
apogamy, 315, 316
chromosomes in Crepis, 349
reduction, 236, 251, 252

Rosenvinge, 46

Rotifers, 318, 364, 368, 370

Roux, 154, 156, 328, 384, 392

Roux-Weismann theory, 225, 391

Rubber, 135

Riickert, 159, 227, 244, 245, 279

Rudolph, 123

Rudolphi, 5

Ruhland, 43

Russow, 46

Rusts, 60, 224, 292

Ruzicka, 67

Rytz, 187

S

Sabaschnikoff, 248
Sachs, 12, 49, 62
Safir, 382
Sagitta, 256
Saguchi, 96, 212

Sakamura, chromosomes and species,
348

deranged mitosis, 212, 214, 259

extruded chromatin, 136

false tetrads, 248, 249, 259

reduction, 236, 254

somatic mitosis, 160, 162, 164, 165
Salamandra (salamander), achromatic
figure, 178, 184

attraction sphere, 77

chromosomes, 150

reduction, 237
Salix (willow), 372
Salter, 107
Salts, 135
Sands, 81
Sap in cell, 135
Sapehin, 88, 114, 123
Saprolegnia (fungus), 187, 290
Sarcode, 11
Sargant, 211
Sax, 298-300
Schacht, 13

Schacke, 365
Schaeffer, 42
Schaffner, 87, 239, 372
Schaudinn, 67, 207, 282, 317
Schaxel, 116
Scheben, 274
Schellenberg, 247
Scherrer, 105, 113, 118, 123
Schikorra, 291
Schiller, 213
Schilling, 112

Schimper, 12, 107, 108, 113
Schleicher, 143
Schleiden, 7, 8, 13
Schleip, 237, 257
Schmitz, 14, 108
Schneider, A., 33, 143
Schneider, EL, 236, 257
Schottlander, 82, 86, 90
Schreiner, 231, 235, 237, 257, 258
Schuberg, 96
Schultze, M., 11
Schultze, W. H., 70
Schulz, 349
Schurhoff, 66, 213
Schwann, 8, 9, 15
Schwarz, 64
Schweigger-Seidel, 14
Scolopendra (centipede), 255
Scolopendrium (fern), 315, 317
Scopolia (angiosperm), 61
Scorpion, 119
Scott, 202

Sea urchin, artificial parthenogenesis,
284, 287

blastomeres, 328

hybrids, 325-327

Second contraction, 232, 233, 238, 239
Secondary thickening of cell wall, 191
Secretions, 28, 30, 66
Secretory cells, 30

Segmentation, of chromosomes in
fertilization, 296

of egg, 188

of spireme, 145, 239
Segregation, of chromosomes, 229, 340

Mendelian, 338, 343, 408, 409

somatic, 352, 395
Seifriz, 38, 39, 43
Seller, 360, 361, 363
Selachians, 237

Selaginella, plastids, 103, 104, 105, 114,
122

448

INDEX

Selaginella, spore coat, 194, 195
Senescence, 63, 136 iff.
Sequoia (conifer), 294, 295
Sex, chromosomes, 358 ff., 380 ff.

determination of, 354 ff., 373 ff.

intergrades, 370 ff.

linkage, 380 ff.

Mendelian interpretation of, 366

practical control of, 376

quantitative theory of, 370

ratio, alteration of, 367 ff.

reversal, 370 ff.
Sex-limited characters, 381
Sex-linked characters, 381
Sexual reproduction and rejuvenescence,

137

Shaffer, 118, 120, 121
Sharp, 64, 87, 90, 92, 147 ff., 160, 176,

225, 242, 254
Shaw, 91, 316

Shull, A. F., and Ladoff, 368
Shull, G. H., 382
Sida (crustacean), 319
Sieve tube, 28
Silica, 135, 194
Silk gland, 66

Simocephalus (crustacean), 371
Simplex chromosome group, 340
de Sinety, 358
Siphoneae, 59
Slime globules, 203
de Smet, 150, 159, 160
Smilacina (angiosperm), 236, 239
Smith, B. G., 159, 160, 278, 279
Smith, G. M., 108
Smith, H. L., 79
Sokolow, 119
Sol, 35
Somatic mitosis, 143 ff.

comparison with heterotypic, 228

pairing of chromosomes, 256

segregation, 352, 395
Sordaria (fungus), 81
Sorodiscus (slime mold), 208, 209
Spallanzani, 14
Species, origin of, 347
Specificity of chromosomes, 162, 250
Spek, 189

Spencer, 48, 49, 398
Spermatia, 312
Spermatid, 87, 220
Spermatocyte, 220, 235
Spermatogenesis, in animals, 95, 96, 220

Spermatogenesis, chondriosomes in, 119,
120

in man, 361, 362

in plants, 86 ff.
Spermatogonia, 220
Spermatozoids, 86 jf., 288 ff., 293, 294
Spermatozoon, discovery, 14

in fertilization, 277 ff.

in heredity, 324, 328

structure, 273, 275

types, 274

Sphacelaria (brown alga), 79, 186
Sphcerechinus (sea urchin), 326-327
Sphcerocarpos (liverwort), 355, 364, 365,

372, 375

Sphceroplea (green alga), 222
Sphcerotheca (fungus), 290
Sphaerraphides, 135
Sphagnum (moss), 248
Spherome, 122

Spinacia (angiosperm), 248, 256
Spinax (fish), 237
Spindle, 145, 147, 154, 175
Spiral stage of chromosome, 150, 152, 155

wall thickening, 28, 192
Spireme, 145, 150, 163, 154, 232, 238

in fertilization, 300, 301

in heterotypic prophase, 241-243
Spirogyra (green alga), amitosis, 211

conjugation, 288

cytokinesis, 186

enucleated cells, 69

mitosis, 209

nuclear fusion, 14, 273

plastid, 104

pyrenoid, 104, 108

reduction, 222

Spirostomum (protozoan), 61, 62
Spitzer, 70

Splitting of chromosomes, in first em-
bryonal division, 295, 300; in matu-
ration, 236, 239, 242, 258; in somatic

mitosis, 163, 242, 392
Spoehr, 41
Spore membrane, 196, 196

tetrads, 223-225, 355

walls, 194
Sporocyte, 225, 232

Sporodinia (mold fungus), 110, 187, 290
Sporogenesis, in various plant groups,
222 ff.

behavior of plastids in, 114

without reduction, 315, 316

1NJ)EX

449

Sporogenesis, and sex-determination,

355-356

Sporophytic budding, 312, 315
Sprengel, C. K, 13
Sprengel, K., 5
Stains, 117

Stangeria (cycad), 294, 298
Staphylea (angiosperm), 239
Starch, 106, 107, 112, 134
Starfish, 285, 287
Steil, 288, 313, 314
Stein, 61

Stemonitis (slime mold), 85
Stentor (protozoan), 60, 62, 69
Stevens, F. L., 289, 290
Stevens, N. M., 256, 358, 360, 361
Stomps, 155, 248, 256, 345-347
Storage, 66, 106, 133
Stout, 159, 372
Strasburger, amitosis, 213

apogamy and apospory, 311, 315,
316

blepharoplast, 85

cell-division, 143

cell wall, 190, 193

centrosphere, 76, 79, 80

chromomeres, 155, 156, 258

chromosome number, 163, 164

chromosomes (somatic), 149, 158

cytokinesis, 186, 190

ectoplast 43

eyespot, 111, 112

fertilization, 300

mitosis in myxomycetes, 208

nuclear fusion, 14, 273

nuclei by division only, 10

nucleolus, 66

nucleoplasmic ratio, 62

nucleus in heredity, 16, 324, 327
of Marsilia, 210

protoplasm, 34

protoplasmic connections, 47

reduction, 220, 222, 236

sex-determination, 355, 356, 364

somatic pairing, 256

spindle, 180, 181

spore wall, 194

starch, 106

tetraploidy, 345
Stratiotes (angiosperm), 212
Streaming in cell-division, 183, 189
Strepsinema, 231, 233, 238
Strepsitene, 231, 233
29

Stroma, 105

starch, 109
Strong, 376
Strongylocentrotus (sea urchin), 287, 326,

327

Structure and function, 374
Studnicka, 96
Sturgeon, 274
Sturtevant, 371, 376, 389
Styela (ascidian), 329
Stypocaulon (brown al ga), 79
Suberin, 194
Sugars, 106, 135, 136
Supernumerary chromosome, 358 ff.
Surface tension in cytokinesis, 188
Surirella (diatom), 79
Suspensoid, 35
Sutton, 158, 160, 228, 245, 251, 252, 253,

255

Swarczewsky, 281
Swarm spores and colony, 140, 331, 334,

411

Swingle, D. B., 186, 187
Swingle, W. T., 79, 186
Sykes, 256

Symmetry, 139, 329, 334
Synapsis,'227, 233, 251 ff.

and degeneration, 259

nature of union, 256, 384, 394
Synaptene, 231, 233
Synaptic mates, relationship, 251-254
Synchytrium (fungus), 62, 187
Syndiploid nuclei, 259, 260
Synergids, 299

Syngamic sex-determination, 357
Syngamy, 222
Synizesis, 231, 233, 237, 238, 255

in somatic cells? 259
Synkaryon, 278, 282, 292
Synochocystis (blue-green alga), 205

Tackholm, 248, 348

Tahara, 256

Tamus (angiosperm), 47

Tangl, 46, 47

Tannin, 194

Tapetal cells, 59, 210

plasmodium, 194, 196
Taraxacum (dandelion), 315
Tassement polaire, 149
Taxodium (conifer), 295

450

INDEX

Taxus (conifer), 122, 295

Taylor, M., 256

Taylor, W. R., 176

Teleosts, 160, 237

Teliospore or teleutospore, 292

Telophase, somatic, 144, 145, 148

Telophasic splitting? 149, 240, 242

Telosynapsis, 228, 239, 244, 245, 248

Tennent, 160

Teosinte, 349

Terletzki, 46

Terni, 118

Tertiary thickening of wall, 191

Tetrads, of cells, 220 ff.

of chromosomes, 230, 234, 239,

243 ff.; in plants, 248, 249
Tetraploidy, 317, 345
Tetraspora (green alga), 109
Tetraspores, 223
Tettigidae, 243
Thalictrum (angiosperm), 159, 236, 248,

315

Thelygonium (angiosperm), 236
Theophrastus, 1
Thorn, 91
Thompson, 52, 333
Thomson, 369, 404
Thuja (conifer, arbor vitse), 239, 294
Thuret, 14

Thysanozoon (flat worm), 237
Timberlake, 85, 105, 109, 187, 190
Tischler, chondriosomes, 118

gigantism, 346

prochromosomes, 159

reduction, 236

spore coat, 196
Tissue cultures, 138, 146
Toad eggs, 368, 370
Tolypothrix (blue-green alga), 203
Tomopteris (annelid worm), 235
Tonoplast, 25, 47, 111
Torreya (conifer), 295
Torus, 191, 192
Townsend, 69

Toxopneustes (sea urchin), 160
Tracheid, 28, 191, 192, 345
Trachelomonas (protozoan), 112
Tradescantia (angiosperm), protoplasm,
6, 25, 33, 37

reduction, 236, 239

somatic mitosis, 147, 148, 150, 162,

242
Tragopogon (angiosperm), 239

Transverse division of chromosome, 227

Tretjakoff, 248

Treub, 190

Treviranus, 5, 6

Trichites, 107

Trichocyst, 30, 45

Trichogyne, 289

Tricyrtis (angiosperm), 248

Trillium (angiosperm), 149, 155, 300,

301

Trimerotropis (grasshopper), 255
Triple fusion in angiosperms, 299, 301
Triploidy, 347

Triticum (wheat), chromosomes and
species, 348

fertilization, 299

synapsis in hybrids, 252, 253
Triton (salamander), 274
Trollius (angiosperm), 236
Trondle, 109, 222
Trophochromatin, 208, 209
Trophoplasm, 34, 181, 400
Trow, 290, 395, 396
Trypanosoma (flagellate), 45, 84
Tschernoyarow, 160, 161, 253
Tschirch, 107, 134
Tsuga (conifer, spruce), 295
Tulasne, 13

Tulipa (angiosperm), 298
Tumors, 163
Tunicata, 237
Tupper and Bartlett, 345
Turbellarian egg, 329
Twins, 357
Twiss, 118
Tyloses, 193
Tyndall effect, 35

U

Ulothrix (green alga), 104, 222, 288, 331

Undulating membrane, 45, 274

linger, 9

Unio (mollusk), 280

Unna, 34

Uredinese, 60, 224, 292

Uromyces (rust fungus), 312

Vacuole, 25, 47, 60, 111, 135

contractile, 30, 44, 46, 84
Vacuome, 122

INDEX

451

Vandendries, 224

Vanessa (butterfly), 61

Van Hook, 82

Vanilla (angiosperm), 109

Varieties, origin of, 347

Vaucheria (green alga), 60, 85

Vegetative apogamy, 314, 315

mutation, 351
Vejdowsky, 67, 250, 257
Velten, 33

Vermiform nuclei, 298, 299
Verworn, 42, 50, 326
Vesperugo (bat), 274
Vida (horse bean), chromosome comple-
ment, 160, 161

deranged mitosis, 211, 212

reduction, 235, 236, 238, 254

somatic mitosis, 147, 153, 242
Vines, 31 Iff.
Virchow, 10, 15

Viscosity of protoplasm, 39, 62, 189
Viscum (mistletoe), 47
Vital units, 48, 398 ff.
Vitalism, 49, 52, 409, 412
Vitelline membrane, 286
Voinov, 118, 119
Volutin, 41

Volvocaceae (green algae), 112
von Voss, 257
de Vries, intracellular pangenesis, 399

(Enothera mutants, 344, 347, 349

pangens, 49, 258, 399

tonoplast, 47

W

W-chromosome, 363
Wager, 13, 108, 112, 290
Wakker, 109, 110
Waldeyer, 144, 326
Walker, 224

Wall of cell, 26, 28, 190 ff.
Walton, 248, 279, 405
Warburg, 285, 287
Wasielewski, 211, 212, 213
Wassilief, 118
Watase, 181

Water in protoplasm, 37
Webber, 83, 92, 93, 288, 294, 302
Weinstein, 389
Weinzieher, 236
Weismann, 16, 49, 225
parthenogenesis, 318

Weismann, reduction, 226

theories of heredity and develop-
ment, 391, 400 ff.
Wells, 40

Welsford, 290, 298, 299, 312, 325
Weniger, 298-301
Wenrich, 156, 158, 162, 233, 237, 243,

257, 258, 392, 393
West and Lechmere, 136
Wheeler, 279
Wheldale, 135
Wherry, 70
White, 384
Whitman, 369
Whitney, 357, 368, 370
Wieman, 361, 362
Wiesner, 48, 109, 193
Wildman, 118, 120, 121, 281
Wille, 46

Williams, C. L., 181
Williams, J. L., 80, 82, 223
Wilson, E. B., abnormal cleavage, 328

achromatic figure, 181-185

chemical nature of gene, 412, 414

chondriosomes, 119

chromatin and cytoplasm, 69

chromosomes and heredity, 396-397

conjugation in Paramoecium, 284

crossing over, 393, 394

genetic continuity, 323

hyaloplasm sphere, 76

individuality of chromosome, 157

protoplasm, 34, 49

sex-chromosomes, 358 ff.

somatic chromosomes, 152

spermatozoon, 275
Wilson, H. V., 138
Wilson, M., 88, 89
Wilson and Morgan, 386, 393
Winge, 208, 209
von Winiwarter, centrosomes, 77

chondriosomes, 123

chromosomes in man, 362

reduction, 228, 231, 237
von Winiwarter and Sainmont, 257
Winkler, 62, 311
van Wisselingh, 210
Wodsedalek, 361
Wohler, 50
Wolfe, 80, 223
Wolff, 4, 5

Wollenweber, 111, 112
Woodburn, 293

452

INDEX

Woolery, 239
Wortman, 56
Woyciki, 295
Wuist, 372

X-chromosome, 359 ff., 380 ff.
Xanthophyll, 104, 105
Xenia, 302

Xylaria (fungus), 291
Xyris (grass), 236

Y-chromosome, 359 ff., 380 ff., 390
Yamanouchi, achromatic figure, 176, 179

apogamy, 314

blepharoplast, 91

centrosomes, 79, 80, 82

eyespot, 111

fertilization, 289, 293

pyrenoid, 109

reduction, 222, 223, 232, 236
Yampolsky, 372
Yeasts, 206, 210, 292
Yolk, 134
Young, 213
Yucca (angiosperm), 256

Z-chromosome, 363
Zacharias, 65, 193, 202, 404

Zade, 349

Zamia (cycad), 93, 288, 294
Zanardinia (brown alga), 111, 222
Zea (maize), 65, 122

aleurone inheritance, 333

chlorophyll inheritance, 332

chromosomes, 349

crossing over, 391

linkage groups, 383
Zentralkorner, 203
Zettnow, 67
Zigzag stage of chromosomes, 151, 152,

153

Zimmermann, 110, 193
Zoja, 164

Zodyonus (worm), 237, 244, 245
Zoospores, 85, 86, 112, 137, 222
Zukal, 202
Zweiger, 239
Zygnema (green alga), fertilization, 113

mitosis, 210

plastid, 103, 106, 108, 109

pyrenoid, 109

reduction, 222
Zygogenetic eggs, 318, 319
Zygomycetes, 290
Zygonema, 231, 233
Zygospore, 113, 222
Zygote, 222, 223
Zygotene, 231, 233
Zymogen, 66