Every Chromosome from a Chromosome. In the
establishment of the concept of genetic continuity, the
decade following 1873 was a crucial period. During
these years the details of mitotic cell division were
worked out, step by step, by a considerable number
of cytologists, among whom Eduard Strasburger,
working on plant materials, and Walther Flemming,
working on animal materials, were leaders. Many of
these researches were closely connected with the study
of the events of gametogenesis and fertilization. Here
we shall look simply at the discovery of the sequence
of events in the division of the cell and its nucleus,
a towering achievement of nineteenth-century biology,
fully as important as the Cell Theory itself. One may
sharply contrast this remarkable development of bio-
logical science with the advent of Mendelian genetics,
or of Darwin's Theory of the Origin of Species by
means of Natural Selection, for both of those achieve-
ments were largely the creation of single men, whereas
in the unfolding of mitosis many individuals contrib-
uted essential parts. In that respect the discovery of
mitotic cell division was an advance more like those
of genetics in the twentieth century, when the Chro-
mosome Theory of Heredity and the elucidation of the
roles of DNA and the nature of the genetic code have
required the labors of many persons, even though some
individuals may stand out as leaders or originators.
In the year 1900 the American cytologist E. B.
Wilson, whose own work on genetic continuity was
to be so fruitful, wrote in the first edition of The Cell
in Development and Inheritance (p. 46) the following
words: “It was not until 1873 that the way was opened
for a better understanding of the matter. In this year
the discoveries by Anton Schneider, quickly followed
by others in the same direction by Otto Bütschli,
Hermann Fol, Eduard Strasburger, Eduard van
Beneden, Flemming, and Hertwig, showed cell-division
to be a far more elaborate process than had been
supposed...”—supposed, that is, by Remak and
others, who thought that nuclear and cell division
represented simply a pinching in two of the nucleus
and the body of the cell.
First it became evident that cell division is regularly
associated with the formation in the cell of an achro-
matic (nonstainable) figure called the spindle. Fol saw
the initiation and growth of two asters in each dividing
cell of the sea urchin egg, and Otto Bütschli observed
that a spindle-shaped structure, also achromatic, is
formed between the asters and is eventually cut
through by a deepening constriction or furrow around
the cell in the plane of the equator of the spindle. By
1875 Strasburger had shown that in the typical plant
cell things happen somewhat differently. A spindle is
indeed produced, but there are no asters at its poles,
and no furrow constricts the dividing cell. Instead, a
cell plate is formed across the equator of the spindle,
and gradually extends beyond the spindle until it meets
the old cell walls on all four sides. The new, rigid cell
wall separating the daughter cells is then deposited in
layers on either side of the cell plate.
As for the nuclear elements themselves, Fol showed
that they can be brought back into view after the
nuclear membrane has dissolved, and then in 1873 A.
Schneider, and shortly thereafter I. Tschistiakoff,
stained and observed the bodies later to be named
chromosomes. These structures of the cell were espe-
cially well observed in the studies of Strasburger on
dividing plant cells and of Balbiani on those of a grass-
hopper. The stained structures, rodlike in the grass-
hopper but often angled or V-shaped in the plant
material, were found to cluster on the center of the
spindle. They then divided—Strasburger thought it to
be transversely—and the two parts thus formed moved
to opposite poles of the spindle. Oscar Hertwig showed
that these two groups of chromosomes reconstitute the
nuclei of the daughter cells; and Strasburger showed
that in his plant cells, well before the spindle is formed,
the chromosomes are to be seen within the nucleus
as long, twisted double threads, which later shorten
and thicken.
Walther Flemming confirmed that this is also char-
acteristic of animal cells, and in 1879 he added a most
significant observation: the division of each chromo-
some to make two is longitudinal, not transverse. The
succession of the stages of mitosis deduced from fixed
and stained material was shown to be correct by Flem-
ming and W. Schleicher by observing cell division in
living material. By 1880 and 1882, when Strasburger's
third edition of Zellbildung und Zelltheilung (“Cell
Structure and Cell Division”) and Flemming's Zellsub-
stanz, Kern und Zelltheilung (“Cell Substance, Nu-
cleus, and Cell Division”) respectively appeared, the
story of mitosis was almost complete. The final proof
that the longitudinal halves of each split chromosome
separate and move to opposite poles was provided for
animal cells by van Beneden in 1883 and for plant cells
by F. Heuser in 1884.
It is a striking fact that the two greatest contributors
to the unfolding of the nature of the mitotic process,
Strasburger and Flemming, were each responsible for
a serious misconception that plagued later students for
many years. Strasburger's error, the conception of the
transverse division of the chromosomes, offered a seri-
ous block to recognition that the elements of heredity
might be linearly arranged within the chromosomes;
for of course, if a chromosome really divided trans-
versely, either its two parts would be genetically
different, or else each chromosome could contain only
a single genetic element to be duplicated and appor-
tioned to the daughter cells. Strasburger recognized
his error, however, in a few years. Flemming, on the
other hand, clung to his erroneous view that all the
chromosomes are at first united into one long continu-
ous thread, a “spireme,” which later breaks up into
the separate chromosomes. This conception, which was
based simply on inadequate observations of the number
of free chromosome ends in the early prophase nuclei,
was less in conflict with any of the principles of genet-
ics, and had a much longer life. Even in the middle
of the twentieth century, textbooks and teachers could
still be found perpetuating this error, in spite of the
fact that a careful look at Flemming's own figures of
nuclei in early prophase shows quite clearly that more
than two chromosome ends are apparent in various
prophase nuclei!
Every chromosome from a chromosome—how
sharply this continuity contrasts with the mass division
of the cytoplasm, which may be very unequal in
amount. The significance of this understanding was
quickly apparent. Wilhelm Roux in 1883 suggested that
the longitudinal splitting of the chromosomes implies
the existence of a linear array of different hereditary
“qualities” along the length of each chromosome.
In 1884 Carl Nägeli, a botanist noted for his work
in plant physiology and plant hybridization, and re-
ferred to already, proposed what he called a “mecha-
nistic-physiological theory of descent.” In part he was
undertaking to criticize Darwin's theory of natural
selection, but in part he was also attempting to supply
a conceptual scheme for a physical system to account
for heredity. Strangely and unaccountably, just as he
ignored Mendel's discoveries, he ignored entirely all
the contemporary developments in knowledge of the
roles of the nuclei of the germ cells during fertilization,
as well as the indications of the genetic significance
of the chromosomes that were to be drawn from
mitotic cell division. Instead, reasoning that the sperm
and the egg, in spite of their differences in size, have
an equal share in the determination of the hereditary
characteristics of the offspring (see Maupertuis), Nägeli
concluded that the hereditary material is not the entire
substance of the egg but only some special part of it.
This restricted hereditary substance he called the “idio-
plasma.” He supposed it to be dispersed in a sort of
network through the entire substance of the cell,
through nucleus and cytoplasm alike. By division of
the fertilized egg into cells, the idioplasm would be-
come distributed to every new cell and give to each
its hereditary character. Evolution was thought to take
place through changes in the idioplasm, changes going
on continuously and impelled by some inherent force
toward inevitable change. For a man who so insistently
proclaimed that he was a mechanistic biologist, this
inconsistency was truly remarkable, but Nägeli did not
seem to notice that it was in the least illogical.
Perhaps a word should be permitted to characterize
a long, voluminous record of analogies between hered-
ity and memory, best exemplified by a lecture given
by the physiologist Ewald Hering in 1870. The dia-
lectic progresses from the idea that memory must have
an unconscious organic, or material, basis to the analo-
gous idea that a material basis must be involved in
the transmission from one generation of living orga-
nisms to the next of the “memory” that guides its
development. The weakness is quickly apparent in the
purely speculative mechanism, which like Nägeli's was
conceived in total disregard of the superb cellular
discoveries that at the very time were laying a sound
basis for understanding the real nature of genetic con-
tinuity. The reason is readily found. Hering clearly
hoped to provide an organic basis for his Lamarckian
conviction that acquired characteristics can become
inherited. Among others, Ernst Haeckel in 1876,
Samuel Butler in 1878, H. B. Orr in 1893, and finally
R. Semon in 1904 all elaborated magnificent specula-
tions about heredity in the same amazing oblivion of
the developing knowledge of cell division, chromosome
individuality and persistence, and the Chromosome
Theory of Heredity. Like Darwin, in an effort to ac-
count for supposed heritable effects of the environ-
ment, they assumed the existence of “plastidules” or
other living units that could be modified in various
body parts, and were then transmitted through the
reproductive cells to members of the next generation.
Yet unlike Darwin, not one of them made an effort
to check his theory by further experiments. Not one
of them, in fact, reasoned as clearly or tested his system
as carefully against the known facts as Maupertuis had
done, over a century before. On the contrary, it seems
to have escaped these nature philosophers that memory
must at best be a poor analogy for heredity, since
memory exists demonstrably only in animals, whereas
heredity is just as characteristic of plants. Herbert
Spencer, in his Principles of Biology (1864), was equally
speculative and equally fallow. In postulating biologi-
cal units determinative of development, he clearly
revealed less breadth of knowledge and biological
perspicacity than Charles Bonnet had exhibited a cen-
tury earlier.
It was August Weismann, once a student of Nägeli,
who undertook the task of properly relating Nägeli's
concept of the idioplasm to the recent developments
of cytology. In his first famous paper on the subject
of heredity, in 1883, Weismann defined the germplasm
as the unbroken lineage of cells connecting the fertil-
ized egg from which an individual springs with that
individual's own gametes, which through their union
form the fertilized eggs of the next generation. “We
have an obvious means by which the inheritance of
all transmitted peculiarities takes place,” he said, “in
the continuity of the substance of the germ cells, or
germplasm.” Weismann stressed two principles about
the germplasm. The first principle was the Continuity
of the Germplasm. According to this concept, the
substance of the body (the somatoplasm) is in each
generation produced as an offshoot of the germplasm,
or germ-line, so that whatever characteristics are in-
herited must be transmitted from the germplasm to
the somatic part of the body. “Changes in the latter,”
Weismann stated, “only arise when they have been
preceded by corresponding changes in the former.” He
deduced also that characteristics acquired by the so-
matic cells cannot be transmitted to the next genera-
tion unless there is some physical mechanism to transfer
material substances or particles from the somatic cells
to the germplasm. Weismann believed that any such
transfer of particles was highly improbable, and in
subsequent years he set himself to test the inheritance
of acquired characteristics by experiment. All of his
later work confirmed the noninheritance of whatever
characteristics were acquired by the somatic cells, and
from this experience he derived his second major prin-
ciple, the Isolation of the Germplasm. By this he meant
that effects of the environment which are inherited
must be exerted directly on the germplasm and cannot
be produced in somatic tissues and thence be trans-
ferred to the germplasm.
Weismann, like Hertwig and Strasburger, identified
Nägeli's idioplasm with the chromosomes, but Weis-
mann extended the conception to the postulate that
each chromosome is made up of hereditary elements
he called “ids,” which in turn are composed of heredi-
tary determinants for each inherited characteristic.
During somatic development, the ids were supposed
to release their determinants and so to be used up. Only
in the germ cells would the undiminished quota of ids
be retained. Moreover, in Weismann's view, every
chromosome was like every other. In spite of growing
evidence of the individuality of the chromosomes, as
well as their longitudinal division, already noted,
Weismann resisted all objections to his schema. Here,
if ever, we have a supreme example of a scientist who
commences with great insight and who hardens, in
devotion to some favored conceptual model, into dog-
matic resistance to all evidence that would force him
to change his views!
A theory far more like our modern views was put
forward by Hugo de Vries in 1889, under the name
of “Intracellular Pangenesis.” De Vries wished to re-
strict the hereditary elements, or pangenes as he called
them, to the nucleus and the chromosomes, and also
to limit their activities to the particular cell within
which they might lie. That was what he meant by
“intracellular.” De Vries' pangenes differ little from
the conceptual genes of the twentieth century. In his
view they constituted the chromosomes, but could
migrate into the cytoplasm and become active there,
thus controlling the development of the cell. A repre-
sentative group of them, however, would always re-
main behind within the nucleus, to be handed on by
mitotic division to both body cells and gametes. Can
one fail to be struck by the profound similarity between
these pangenes supposed to remain in the chromosomes
and the current concept of genes composed of DNA
(deoxyribonucleic acid) and restricted to the chromo-
somes, or on the other hand between the pangenes
supposed to migrate into the cytoplasm in order to
regulate development and to control the hereditary
characteristics and the current views of messenger
RNA (ribonucleic acid)? Since the pangenes were lim-
ited to the cell and corresponded one to one with
particular hereditary characteristics, and since they
were always represented in full measure in the nucleus,
the conceptual model developed by de Vries was con-
sonant with the principle of the isolation of the germ-
plasm and the noninheritance of acquired charac-
teristics. Unfortunately, the use of the term “pangenes”
made everyone recall the speculative theory which
Darwin evoked to allow for some supposed inheritance
of acquired characteristics. Consequently de Vries is
often thought, by persons who have never read his
massive volume on Intracellular Pangenesis, to have
held views quite the opposite of his real ones.
By the turn of the century, when the rediscovery
of Gregor Mendel's work really gave birth to modern
genetics, the cytological basis of genetic continuity had
been established. Omnis chromosoma e chromosoma:
every chromosome from a chromosome.
