Darwin and Modern Science (1909)
Edited by A.C. Seward
VI. THE MINUTE STRUCTURE OF CELLS IN RELATION TO HEREDITY.
By EDUARD STRASBURGER,
Professor of Botany in the University of Bonn.
ince 1875 an unexpected insight has been gained into the internal
structure of cells. Those who are familiar with the results of
investigations in this branch of Science are convinced that any modern
theory of heredity must rest on a basis of cytology and cannot be at
variance with cytological facts. Many histological discoveries, both such
as have been proved correct and others which may be accepted as probably
well founded, have acquired a fundamental importance from the point of view
of the problems of heredity.
My aim is to describe the present position of our knowledge of Cytology.
The account must be confined to essentials and cannot deal with far-
reaching and controversial questions. In cases where difference of opinion
exists, I adopt my own view for which I hold myself responsible. I hope to
succeed in making myself intelligible even without the aid of
illustrations: in order to convey to the uninitiated an adequate idea of
the phenomena connected with the life of a cell, a greater number of
figures would be required than could be included within the scope of this
article.
So long as the most eminent investigators (As for example the illustrious
Wilhelm Hofmeister in his "Lehre von der Pflanzenzelle" (1867).) believed
that the nucleus of a cell was destroyed in the course of each division and
that the nuclei of the daughter-cells were produced de novo, theories of
heredity were able to dispense with the nucleus. If they sought, as did
Charles Darwin, who showed a correct grasp of the problem in the
enunciation of his Pangenesis hypothesis, for histological connecting
links, their hypotheses, or at least the best of them, had reference to the
cell as a whole. It was known to Darwin that the cell multiplied by
division and was derived from a similar pre-existing cell. Towards 1870 it
was first demonstrated that cell-nuclei do not arise de novo, but are
invariably the result of division of pre-existing nuclei. Better methods
of investigation rendered possible a deeper insight into the phenomena
accompanying cell and nuclear divisions and at the same time disclosed the
existence of remarkable structures. The work of O. Butschli, O. Hertwig,
W. Flemming H. Fol and of the author of this article (For further reference
to literature, see my article on "Die Ontogenie der Zelle seit 1875", in
the "Progressus Rei Botanicae", Vol. I. page 1, Jena, 1907.), have
furnished conclusive evidence in favour of these facts. It was found that
when the reticular framework of a nucleus prepares to divide, it separates
into single segments. These then become thicker and denser, taking up with
avidity certain stains, which are used as aids to investigation, and
finally form longer or shorter, variously bent, rodlets of uniform
thickness. In these organs which, on account of their special property of
absorbing certain stains, were styled Chromosomes (By W. Waldeyer in
1888.), there may usually be recognised a separation into thicker and
thinner discs; the former are often termed Chromomeres. (Discovered by W.
Pfitzner in 1880.) In the course of division of the nucleus, the single
rows of chromomeres in the chromosomes are doubled and this produces a
band-like flattening and leads to the longitudinal splitting by which each
chromosome is divided into two exactly equal halves. The nuclear membrane
then disappears and fibrillar cell-plasma or cytoplasm invades the nuclear
area. In animal cells these fibrillae in the cytoplasm centre on definite
bodies (Their existence and their multiplication by fission were
demonstrated by E. van Beneden and Th. Boveri in 1887.), which it is
customary to speak of as Centrosomes. Radiating lines in the adjacent
cell-plasma suggest that these bodies constitute centres of force. The
cells of the higher plants do not possess such individualised centres; they
have probably disappeared in the course of phylogenetic development: in
spite of this, however, in the nuclear division-figures the fibrillae of
the cell-plasma are seen to radiate from two opposite poles. In both
animal and plant cells a fibrillar bipolar spindle is formed, the fibrillae
of which grasp the longitudinally divided chromosomes from two opposite
sides and arrange them on the equatorial plane of the spindle as the so-
called nuclear or equatorial plate. Each half-chromosome is connected with
one of the spindle poles only and is then drawn towards that pole. (These
important facts, suspected by W. Flemming in 1882, were demonstrated by E.
Heuser, L. Guignard, E. van Beneden, M. Nussbaum, and C. Rabl.)
The formation of the daughter-nuclei is then effected. The changes which
the daughter-chromosomes undergo in the process of producing the daughter-
nuclei repeat in the reverse order the changes which they went through in
the course of their progressive differentiation from the mother-nucleus.
The division of the cell-body is completed midway between the two daughter-
nuclei. In animal cells, which possess no chemically differentiated
membrane, separation is effected by simple constriction, while in the case
of plant cells provided with a definite wall, the process begins with the
formation of a cytoplasmic separating layer.
The phenomena observed in the course of the division of the nucleus show
beyond doubt that an exact halving of its substance is of the greatest
importance. (First shown by W. Roux in 1883.) Compared with the method of
division of the nucleus, that of the cytoplasm appears to be very simple.
This led to the conception that the cell-nucleus must be the chief if not
the sole carrier of hereditary characters in the organism. It is for this
reason that the detailed investigation of fertilisation phenomena
immediately followed researches into the nucleus. The fundamental
discovery of the union of two nuclei in the sexual act was then made (By O.
Hertwig in 1875.) and this afforded a new support for the correct
conception of the nuclear functions. The minute study of the behaviour of
the other constituents of sexual cells during fertilisation led to the
result, that the nucleus alone is concerned with handing on hereditary
characters (This was done by O. Hertwig and the author of this essay
simultaneously in 1884.) from one generation to another. Especially
important, from the point of view of this conclusion, is the study of
fertilisation in Angiosperms (Flowering plants); in these plants the male
sexual cells lose their cell-body in the pollen-tube and the nucleus only—the
sperm-nucleus—reaches the egg. The cytoplasm of the male sexual cell
is therefore not necessary to ensure a transference of hereditary
characters from parents to offspring. I lay stress on the case of the
Angiosperms because researches recently repeated with the help of the
latest methods failed to obtain different results. As regards the
descendants of angiospermous plants, the same laws of heredity hold good as
for other sexually differentiated organisms; we may, therefore, extend to
the latter what the Angiosperms so clearly teach us.
The next advance in the hitherto rapid progress in our knowledge of nuclear
division was delayed, because it was not at once recognised that there are
two absolutely different methods of nuclear division. All such nuclear
divisions were united under the head of indirect or mitotic divisions;
these were also spoken of as karyo-kineses, and were distinguished from the
direct or amitotic divisions which are characterised by a simple
constriction of the nuclear body. So long as the two kinds of indirect
nuclear division were not clearly distinguished, their correct
interpretation was impossible. This was accomplished after long and
laborious research, which has recently been carried out and with results
which should, perhaps, be regarded as provisional.
Soon after the new study of the nucleus began, investigators were struck by
the fact that the course of nuclear division in the mother-cells, or more
correctly in the grandmother-cells, of spores, pollen-grains, and embryo-
sacs of the more highly organised plants and in the spermatozoids and eggs
of the higher animals, exhibits similar phenomena, distinct from those
which occur in the somatic cells.
In the nuclei of all those cells which we may group together as gonotokonts
(At the suggestion of J.P. Lotsy in 1904.) (i.e. cells concerned in
reproduction) there are fewer chromosomes than in the adjacent body-cells
(somatic cells). It was noticed also that there is a peculiarity
characteristic of the gonotokonts, namely the occurrence of two nuclear
divisions rapidly succeeding one another. It was afterwards recognised
that in the first stage of nuclear division in the gonotokonts the
chromosomes unite in pairs: it is these chromosome-pairs, and not the two
longitudinal halves of single chromosomes, which form the nuclear plate in
the equatorial plane of the nuclear spindle. It has been proposed to call
these pairs gemini. (J.E.S. Moore and A.L. Embleton, "Proc. Roy. Soc."
London, Vol. LXXVII. page 555, 1906; V. Gregoire, 1907.) In the course of
this division the spindle-fibrillae attach themselves to the gemini, i.e.
to entire chromosomes and direct them to the points where the new daughter-
nuclei are formed, that is to those positions towards which the
longitudinal halves of the chromosomes travel in ordinary nuclear
divisions. It is clear that in this way the number of chromosomes which
the daughter-nuclei contain, as the result of the first stage in division
in the gonotokonts, will be reduced by one half, while in ordinary
divisions the number of chromosomes always remains the same. The first
stage in the division of the nucleus in the gonotokonts has therefore been
termed the reduction division. (In 1887 W. Flemming termed this the
heterotypic form of nuclear division.) This stage in division determines
the conditions for the second division which rapidly ensues. Each of the
paired chromosomes of the mother-nucleus has already, as in an ordinary
nuclear division, completed the longitudinal fission, but in this case it
is not succeeded by the immediate separation of the longitudinal halves and
their allotment to different nuclei. Each chromosome, therefore, takes its
two longitudinal halves into the same daughter-nucleus. Thus, in each
daughter-nucleus the longitudinal halves of the chromosomes are present
ready for the next stage in the division; they only require to be arranged
in the nuclear plate and then distributed among the granddaughter-nuclei.
This method of division, which takes place with chromosomes already split,
and which have only to provide for the distribution of their longitudinal
halves to the next nuclear generation, has been called homotypic nuclear
division. (The name was proposed by W. Flemming in 1887; the nature of
this type of division was, however, not explained until later.)
Reduction division and homotypic nuclear division are included together
under the term allotypic nuclear division and are distinguished from the
ordinary or typical nuclear division. The name Meiosis (By J. Bretland
Farmer and J.E.S. Moore in 1905.) has also been proposed for these two
allotypic nuclear divisions. The typical divisions are often spoken of as
somatic.
Observers who were actively engaged in this branch of recent histological
research soon noticed that the chromosomes of a given organism are
differentiated in definite numbers from the nuclear network in the course
of division. This is especially striking in the gonotokonts, but it
applies also to the somatic tissues. In the latter, one usually finds
twice as many chromosomes as in the gonotokonts. Thus the conclusion was
gradually reached that the doubling of chromosomes, which necessarily
accompanies fertilisation, is maintained in the product of fertilisation,
to be again reduced to one half in the gonotokonts at the stage of
reduction-division. This enabled us to form a conception as to the essence
of true alternation of generations, in which generations containing single
and double chromosomes alternate with one another.
The single-chromosome generation, which I will call the HAPLOID, must have
been the primitive generation in all organisms; it might also persist as
the only generation. Every sexual differentiation in organisms, which
occurred in the course of phylogenetic development, was followed by
fertilisation and therefore by the creation of a diploid or double-
chromosome product. So long as the germination of the product of
fertilisation, the zygote, began with a reducing process, a special DIPLOID
generation was not represented. This, however, appeared later as a product
of the further evolution of the zygote, and the reduction division was
correspondingly postponed. In animals, as in plants, the diploid
generation attained the higher development and gradually assumed the
dominant position. The haploid generation suffered a proportional
reduction, until it finally ceased to have an independent existence and
became restricted to the role of producing the sexual products within the
body of the diploid generation. Those who do not possess the necessary
special knowledge are unable to realise what remains of the first haploid
generation in a phanerogamic plant or in a vertebrate animal. In
Angiosperms this is actually represented only by the short developmental
stages which extend from the pollen mother-cells to the sperm-nucleus of
the pollen-tube, and from the embryo-sac mother-cell to the egg and the
endosperm tissue. The embryo-sac remains enclosed in the diploid ovule,
and within this from the fertilised egg is formed the embryo which
introduces the new diploid generation. On the full development of the
diploid embryo of the next generation, the diploid ovule of the preceding
diploid generation is separated from the latter as a ripe seed. The
uninitiated sees in the more highly organised plants only a succession of
diploid generations. Similarly all the higher animals appear to us as
independent organisms with diploid nuclei only. The haploid generation is
confined in them to the cells produced as the result of the reduction
division of the gonotokonts; the development of these is completed with the
homotypic stage of division which succeeds the reduction division and
produces the sexual products.
The constancy of the numbers in which the chromosomes separate themselves
from the nuclear network during division gave rise to the conception that,
in a certain degree, chromosomes possess individuality. Indeed the most
careful investigations (Particularly those of V. Gregoire and his pupils.)
have shown that the segments of the nuclear network, which separate from
one another and condense so as to produce chromosomes for a new division,
correspond to the segments produced from the chromosomes of the preceding
division. The behaviour of such nuclei as possess chromosomes of unequal
size affords confirmatory evidence of the permanence of individual
chromosomes in corresponding sections of an apparently uniform nuclear
network. Moreover at each stage in division chromosomes with the same
differences in size reappear. Other cases are known in which thicker
portions occur in the substance of the resting nucleus, and these agree in
number with the chromosomes. In this network, therefore, the individual
chromosomes must have retained their original position. But the
chromosomes cannot be regarded as the ultimate hereditary units in the
nuclei, as their number is too small. Moreover, related species not
infrequently show a difference in the number of their chromosomes, whereas
the number of hereditary units must approximately agree. We thus picture
to ourselves the carriers of hereditary characters as enclosed in the
chromosomes; the transmitted fixed number of chromosomes is for us only the
visible expression of the conception that the number of hereditary units
which the chromosomes carry must be also constant. The ultimate hereditary
units may, like the chromosomes themselves, retain a definite position in
the resting nucleus. Further, it may be assumed that during the separation
of the chromosomes from one another and during their assumption of the rod-
like form, the hereditary units become aggregated in the chromomeres and
that these are characterised by a constant order of succession. The
hereditary units then grow, divide into two and are uniformly distributed
by the fission of the chromosomes between their longitudinal halves.
As the contraction and rod-like separation of the chromosomes serve to
isnure the transmission of all hereditary units in the products of division
of a nucleus, so, on the other hand, the reticular distension of each
chromosome in the so-called resting nucleus may effect a separation of the
carriers of hereditary units from each other and facilitate the specific
activity of each of them.
In the stages preliminary to their division, the chromosomes become denser
and take up a substance which increases their staining capacity; this is
called chromatin. This substance collects in the chromomeres and may form
the nutritive material for the carriers of hereditary units which we now
believe to be enclosed in them. The chromatin cannot itself be the
hereditary substance, as it afterwards leaves the chromosomes, and the
amount of it is subject to considerable variation in the nucleus, according
to its stage of development. Conjointly with the materials which take part
in the formation of the nuclear spindle and other processes in the cell,
the chromatin accumulates in the resting nucleus to form the nucleoli.
Naturally connected with the conclusion that the nuclei are the carriers of
hereditary characters in the organism, is the question whether enucleate
organisms can also exist. Phylogenetic considerations give an affirmative
answer to this question. The differentiation into nucleus and cytoplasm
represents a division of labour in the protoplast. A study of organisms
which belong to the lowest class of the organic world teaches us how this
was accomplished. Instead of well-defined nuclei, scattered granules have
been described in the protoplasm of several of these organisms (Bacteria,
Cyanophyceae, Protozoa.), characterised by the same reactions as nuclear
material, provided also with a nuclear network, but without a limiting
membrane. (This is the result of the work of R. Hertwig and of the most
recently published investigations.) Thus the carriers of hereditary
characters may originally have been distributed in the common protoplasm,
afterwards coming together and eventually assuming a definite form as
special organs of the cell. It may be also assumed that in the protoplasm
and in the primitive types of nucleus, the carriers of the same hereditary
unit were represented in considerable quantity; they became gradually
differentiated to an extent commensurate with newly acquired characters.
It was also necessary that, in proportion as this happened, the mechanism
of nuclear division must be refined. At first processes resembling a
simple constriction would suffice to provide for the distribution of all
hereditary units to each of the products of division, but eventually in
both organic kingdoms nuclear division, which alone insured the qualitative
identity of the products of division, became a more marked feature in the
course of cell-multiplication.
Where direct nuclear division occurs by constriction in the higher
organisms, it does not result in the halving of hereditary units. So far
as my observations go, direct nuclear division occurs in the more highly
organised plants only in cells which have lost their specific functions.
Such cells are no longer capable of specific reproduction. An interesting
case in this connection is afforded by the internodal cells of the
Characeae, which possess only vegetative functions. These cells grow
vigorously and their cytoplasm increases, their growth being accompanied by
a correspondingly direct multiplication of the nuclei. They serve chiefly
to nourish the plant, but, unlike the other cells, they are incapable of
producing any offspring. This is a very instructive case, because it
clearly shows that the nuclei are not only carriers of hereditary
characters, but that they also play a definite part in the metabolism of
the protoplasts.
Attention was drawn to the fact that during the reducing division of nuclei
which contain chromosomes of unequal size, gemini are constantly produced
by the pairing of chromosomes of the same size. This led to the conclusion
that the pairing chromosomes are homologous, and that one comes from the
father, the other from the mother. (First stated by T.H. Montgomery in
1901 and by W.S. Sutton in 1902.) This evidently applies also to the
pairing of chromosomes in those reduction-divisions in which differences in
size do not enable us to distinguish the individual chromosomes. In this
case also each pair would be formed by two homologous chromosomes, the one
of paternal, the other of maternal origin. When the separation of these
chromosomes and their distribution to both daughter-nuclei occur a
chromosome of each kind is provided for each of these nuclei. It would
seem that the components of each pair might pass to either pole of the
nuclear spindle, so that the paternal and maternal chromosomes would be
distributed in varying proportion between the daughter-nuclei; and it is
not impossible that one daughter-nucleus might occasionally contain
paternal chromosomes only and its sister-nucleus exclusively maternal
chromosomes.
The fact that in nuclei containing chromosomes of various sizes, the
chromosomes which pair together in reduction-division are always of equal
size, constitutes a further and more important proof of their qualitative
difference. This is supported also by ingenious experiments which led to
an unequal distribution of chromosomes in the products of division of a
sea-urchin's egg, with the result that a difference was induced in their
further development. (Demonstrated by Th. Boveri in 1902.)
The recently discovered fact that in diploid nuclei the chromosomes are
arranged in pairs affords additional evidence in favour of the unequal
value of the chromosomes. This is still more striking in the case of
chromosomes of different sizes. It has been shown that in the first
division-figure in the nucleus of the fertilised egg the chromosomes of
corresponding size form pairs. They appear with this arrangement in all
subsequent nuclear divisions in the diploid generation. The longitudinal
fissions of the chromosomes provide for the unaltered preservation of this
condition. In the reduction nucleus of the gonotokonts the homologous
chromosomes being near together need not seek out one another; they are
ready to form gemini. The next stage is their separation to the haploid
daughter-nuclei, which have resulted from the reduction process.
Peculiar phenomena in the reduction nucleus accompany the formation of
gemini in both organic kingdoms. (This has been shown more particularly by
the work of L. Guignard, M. Mottier, J.B. Farmer, C.B. Wilson, V. Hacker
and more recently by V. Gregoire and his pupil C.A. Allen, by the
researches conducted in the Bonn Botanical Institute, and by A. and K.E.
Schreiner.) Probably for the purpose of entering into most intimate
relation, the pairs are stretched to long threads in which the chromomeres
come to lie opposite one another. (C.A. Allen, A. and K.E. Schreiner, and
Strasburger.) It seems probable that these are homologous chromomeres, and
that the pairs afterwards unite for a short time, so that an exchange of
hereditary units is rendered possible. (H. de Vries and Strasburger.)
This cannot be actually seen, but certain facts of heredity point to the
conclusion that this occurs. It follows from these phenomena that any
exchange which may be effected must be one of homologous carriers of
hereditary units only. These units continue to form exchangeable segments
after they have undergone unequal changes; they then constitute
allelotropic pairs. We may thus calculate what sum of possible
combinations the exchange of homologous hereditary units between the
pairing chromosomes provides for before the reduction division and the
subsequent distribution of paternal and maternal chromosomes in the haploid
daughter-nuclei. These nuclei then transmit their characters to the sexual
cells, the conjugation of which in fertilization again produces the most
varied combinations. (A. Weismann gave the impulse to these ideas in his
theory on "Amphimixis".) In this way all the cooperations which the
carriers of hereditary characters are capable of in a species are produced;
this must give it an appreciable advantage in the struggle for life.
The admirers of Charles Darwin must deeply regret that he did not live to
see the results achieved by the new Cytology. What service would they have
been to him in the presentation of his hypothesis of Pangenesis; what an
outlook into the future would they have given to his active mind!
The Darwinian hypothesis of Pangenesis rests on the conception that all
inheritable properties are represented in the cells by small invisible
particles or gemmules and that these gemmules increase by division.
Cytology began to develop on new lines some years after the publication in
1868 of Charles Darwin's "Provisional hypothesis of Pangenesis" ("Animals
and Plants under Domestication", London, 1868, Chapter XXVII.), and when he
died in 1882 it was still in its infancy. Darwin would have soon suggested
the substitution of the nuclei for his gemmules. At least the great
majority of present-day investigators in the domain of cytology have been
led to the conclusion that the nucleus is the carrier of hereditary
characters, and they also believe that hereditary characters are
represented in the nucleus as distinct units. Such would be Darwin's
gemmules, which in conformity with the name of his hypothesis may be called
pangens (So called by H. de Vries in 1889.): these pangens multiply by
division. All recently adopted views may be thus linked on to this part of
Darwin's hypothesis. It is otherwise with Darwin's conception to which
Pangenesis owes its name, namely the view that all cells continually give
off gemmules, which migrate to other places in the organism, where they
unite to form reproductive cells. When Darwin foresaw this possibility,
the continuity of the germinal substance was still unknown (Demonstrated by
Nussbaum in 1880, by Sachs in 1882, and by Weismann in 1885.), a fact which
excludes a transference of gemmules.
But even Charles Darwin's genius was confined within finite boundaries by
the state of science in his day.
It is not my province to deal with other theories of development which
followed from Darwin's Pangenesis, or to discuss their histological
probabilities. We can, however, affirm that Charles Darwin's idea that
invisible gemmules are the carriers of hereditary characters and that they
multiply by division has been removed from the position of a provisional
hypothesis to that of a well-founded theory. It is supported by histology,
and the results of experimental work in heredity, which are now assuming
extraordinary prominence, are in close agreement with it.
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