differentiation of the first multicellular organisms by division of
labour[133]. If we desire to investigate the relation between
germ-cells and somatic cells, we must not only consider the highly
developed and strongly differentiated multicellular organisms, but we
must also turn our attention to those simpler forms in which phyletic
transitions are represented. In addition to solitary unicellular
organisms, we know of others living in colonies of which the
constituent units or cells (each of them equivalent to a unicellular
organism) are morphologically and physiologically identical. Each unit
feeds, moves, and under certain circumstances is capable of reproducing
itself, and of thus forming a new colony by repeated division. The
genus _Pandorina_ (Fig. I), belonging to the natural order
_Volvocineae_, represents such ‘homoplastid’ (Götte) organisms. It
forms a spherical colony composed of ciliated cells, all of which are
exactly alike: they are embedded in a colourless gelatinous mass. Each
cell contains chlorophyll, and possesses a red eye-spot, and a
pulsating vacuole. These colonies are propagated by the sexual and
asexual (Fig. II) methods alternately, although in the former case the
conjugating swarm-cells cannot be distinguished with certainty as male
or female. In both kinds of reproduction, each cell in the colony acts
as a reproductive cell; in fact, it behaves exactly like a unicellular
organism.
[Illustration: I. _Pandorina morum_ (after Pringsheim), a swarming
colony. II. A colony divided into sixteen daughter colonies: all the
cells alike. III. A young individual of _Volvox minor_ (after Stein),
still enclosed in the wall of the cell from which it has been
parthenogenetically produced. The constituent cells are divided into
somatic (_sz_), germ-cells (_kz_).]
It is very interesting to find in another genus belonging to the same
natural order, that the transition from the homoplastid to the
heteroplastid condition, and the separation into somatic and
reproductive cells, have taken place. In _Volvox_ (Fig. III) the
spherical colony consists of two kinds of cells, viz. of very numerous
small ciliated cells, and of a much smaller number of large germ-cells
without cilia. The latter alone possess the power of producing a new
colony, and this takes place by the asexual and sexual methods
alternately: in the latter a typical fertilization of large egg-cells
by small spermatozoa occurs. The sexual differentiation of the
germ-cells is not material to the question we are now considering; the
important point is to ascertain whether here, at the very origin of
heteroplastid organisms, the germ-cells, sexually differentiated or
not, arise from the somatic cells _at the end of ontogeny_, or whether
the substance of the parent germ-cell, during embryonic development, is
_from the first_ separated into somatic and germ-cells. The former
interpretation would support Nägeli’s view, the latter would support my
own. But Kirchner[134] distinctly states that the germ-cells of
_Volvox_ are differentiated during embryonic development, that is,
before the escape of the young heteroplastid organism from the
egg-capsule. We cannot therefore imagine that the phyletic development
of the first heteroplastid organism took place in a manner different
from that which I have previously advocated on theoretical grounds,
before this striking instance occurred to me. The germ-plasm
(nucleoplasm) of some homoplastid organism (similar to _Pandorina_)
must have become modified in molecular structure during the course of
phylogeny, so that the colony of cells produced by its division was no
longer made up of identical units, but of two different kinds. After
this separation, the germ-cells alone retained the power of
reproduction possessed by all the parent cells, while the rest only
retained the power of producing similar cells by division. Thus
_Volvox_ seems to afford distinct evidence that in the phyletic origin
of the heteroplastid groups, somatic cells were not, as Nägeli
supposes, intercalated between the mother germ-cell and the daughter
germ-cells in each ontogeny, but that the somatic cells arose directly
from the former, with which they were previously identical, as they are
even now in the case of _Pandorina_. Thus the continuity of the
germ-plasm is established at least for the beginning of the phyletic
series of development.
The fact, already often mentioned, that in most higher organisms the
separation of germ-cells takes place later, and often very late, at the
end of the whole ontogeny, proves that the time at which this
separation of the two kinds of cells took place, must have been
gradually changed. In this respect the well-established instances of
early separation are of great value, because they serve to connect the
extreme cases. It is quite impossible to maintain that the germ-cells
of Hydroids or of the higher plants, exist from the time of embryonic
development, as indifferent cells, which cannot be distinguished from
others, and which are only differentiated at a later period. Such a
view is contradicted by the simplest mathematical consideration; for it
is obvious that none of the relatively few cells of the embryo can be
excluded from the enormous increase by division, which must take place
in order to produce the large number of daughter individuals which form
a colony of polypes. It is therefore clear that all the cells of the
embryo must for a long time act as somatic cells, and none of them can
be reserved as germ-cells and nothing else: this conclusion is moreover
confirmed by direct observation. The sexual bud of a _Coryne_ arises at
a part of the Polype which does not in any way differ from surrounding
areas, the body wall being uniformly made up of two single layers of
cells, the one forming the ectoderm and the other the endoderm. Rapid
growth then takes place at a single spot, and some of the young cells
thus produced are transformed into germ-cells, which did not previously
exist as separate cells.
Strictly speaking I have therefore fallen into an inaccuracy in
maintaining (in former works) that the germ-cells are themselves
immortal; they only contain the undying part of the organism—the
germ-plasm; and although this substance is, as far as we know,
invariably surrounded by a cell-body, it does not always control the
latter, and thus confer upon it the character of a germ-cell. But this
admission does not materially change our view of the whole subject. We
may still contrast the germ-cells, as the undying part of the Metazoan
body, with the perishable somatic cells. If the nature and the
character of a cell is determined by the substance of the nucleus and
not by the cell-body, then the immortality of the germ-cells is
preserved, although only the nuclear substance passes uninterruptedly
from one generation to another.
G. Jäger[135] was the first to state that the body in the higher
organisms is made up of two kinds of cells, viz., ontogenetic and
phyletic cells, and that the latter, the reproductive cells, are not a
product of the former (the body-cells), but that they arise directly
from the parent germ-cell. He assumed that the formation of germ-cells
takes place at the earliest stage of embryonic life, and he thus
believed the connexion between the germ-plasm of the parent and of the
offspring had received a satisfactory explanation. As I have previously
mentioned in the introduction, Nussbaum also brought forward this
hypothesis at a later period, and also based it upon a continuity of
the germ-cells. He assumed that the fertilized egg is divided into the
cells of the individual and into the cells which effect the
preservation of the species, and he supported this view by referring to
the few known cases of early separation of the sexual cells. He even
maintained this hypothesis when I had proved in my investigations on
Hydromedusae that the sexual cells are not always separated from the
somatic cells during embryonic development, but often at a far later
period. Not only is the hypothesis of a direct connexion between the
germ-cells of the offspring and parent broken down by the facts known
in the Hydroids, and in the Phanerogams[136] which resemble them in
this respect, but even the instances of early separated germ-cells
quoted by Jäger and Nussbaum do not as a matter of fact support their
hypothesis. Among existing organisms it is extremely rare for the
germ-cells to arise directly from the parent egg-cell (as in Diptera).
If, however, the germ-cells are separated only a few cell-generations
later, the postulated continuity breaks down; for an embryonic cell, of
which the offspring are partly germ-cells and partly somatic cells,
cannot itself possess the nature of a germ-cell, and its idioplasm
cannot be identical with that of the parent germ-cell. In order to
prove this, it is only necessary to refer to the arguments as to the
ontogenetic stages of the idioplasm. In the above-mentioned instances,
the continuity from the germ-substance of the parent to that of the
offspring can only be explained by the supposition that the somatic
nucleoplasm still contains some unchanged germ-plasm. I believe that
the fundamental idea of Jäger and Nussbaum is quite correct: it is the
same idea which has led me to the hypothesis of the continuity of the
germ-plasm, viz., the conviction that heredity can only be understood
by means of such an hypothesis. But both these writers have worked out
the idea in the form of an hypothesis which does not correspond with
the facts. That this is the case is also shown by the following words
of Nussbaum—‘the cell-material of the individual (somatic cells) can
never produce a single sexual cell.’ Such production undoubtedly takes
place, not only in Hydroids and Phanerogams, but in many other
instances. The germ-cells cannot indeed be produced by any indifferent
cell of embryonic character, but by certain cells, and under
circumstances which allow us to positively conclude that they have been
predestined for this purpose from the beginning. In other words, the
cells in question contain germ-plasm, and this alone enables them to
become germ-cells.
As a result of my investigations on Hydroids[137], I concluded that the
germ-plasm is present in a very finely divided and therefore invisible
state in certain somatic cells, from the very beginning of embryonic
development, and that it is then transmitted through innumerable
cell-generations, to those remote individuals of the colony in which
sexual products are formed. This conclusion is based upon the fact that
germ-cells only occur in certain localized areas (‘Keimstätten’) in
which neither germ-cells nor primitive germ-cells (the cells which are
transformed into germ-cells at a later period) were previously present.
The primitive germ-cells are also only formed in localized areas,
arising from somatic cells of the ectoderm. The place at which
germ-cells arise is the same in all individuals of the same species;
but differs in different species. It can be shown that such differences
correspond to different phyletic stages of a process of displacement,
which tends to remove the localized area from its original position
(the manubrium of the Medusa) in a centripetal direction. For the
purposes of the present enquiry it is unnecessary to discuss the
reasons for this change of position. The phyletic displacements of the
localized areas are brought about during ontogeny by an actual
migration of primitive germ-cells from the place where they arose to
the position at which they undergo differentiation into germ-cells. But
we cannot believe that primitive germ-cells would migrate if the
germ-cells could be formed from any of the other young cells of
indifferent character which are so numerous in Hydroids. Even when the
localized area undergoes very slight displacement, e.g. when it is
removed from the exterior to the interior of the mesogloea[138], the
change is always effected by active migration of primitive germ-cells
through the substance of the mesogloea. Although the localized area has
been largely displaced in the course of phylogeny, the changes in
position have always taken place by very gradual stages, and never
suddenly, and all these stages are repeated in the ontogeny of all
existing species, by the migration of the primitive germ-cells from the
ancestral area to the place where the germ-cells now arise.
Hartlaub[139] has recently added a further instance (that of _Obelia_)
to the numerous minute descriptions of these phyletic displacements of
the localized area, and ontogenetic migrations of the primitive
germ-cells, which are given in my work already referred to. The
instance of _Obelia_ is of especial interest as the direction of
displacement is here reversed, taking place centrifugally instead of in
a centripetal direction.
But if displacements of the localized areas can only take place by the
frequently roundabout method of the migration of primitive germ-cells,
we are obliged to conclude that such is the only manner in which the
change can be effected, and that other cells are unable to play the
role of the primitive germ-cells. And if other cells are unable to take
this part, it must be because nucleoplasm of a certain character has to
be present in order to form germ-cells, or according to the terms of my
theory, the presence of germ-plasm is indispensable for this purpose. I
do not see how we can escape the conclusion that there is continuity of
the germ-plasm; for if it were supposed that somatic idioplasm
undergoes transformation into germ-plasm, such an assumption would not
explain why the displacement occurs by small stages, and with extreme
and constant care for the preservation of a connexion with cells of the
ancestral area. This fact can only be explained by the hypothesis that
cell-generations other than those which end in the production of the
cells of the ancestral area, are totally incapable of transformation
into germ-cells.
Strasburger has objected that the transmission of germ-plasm along
certain lines, viz. through a certain succession of somatic cells, is
impossible, because the idioplasm is situated in the nucleus and not in
the cell-body, and because a nucleus can only divide into two exactly
equal halves by the indirect method of division, which takes place, as
we must believe, in these cases. ‘It might indeed be supposed,’ says
Strasburger, ‘that during nuclear division certain molecular groups
remain unchanged in the nuclear substance which is in other respects
transformed, and that these groups are uniformly distributed through
the whole organism; but we cannot imagine that their transmission could
only be effected along certain lines.’
I do not think that Strasburger’s objections can be maintained. I base
this opinion on my previous criticism upon the assumed equality of the
two daughter-nuclei formed by indirect division. I do not see any
reason why the two halves must always possess the same structure,
although they may be of equal size and weight. I am surprised that
Strasburger should admit the possibility that the germ-plasm, which, as
I think, is mixed with the idioplasm of the somatic cells, may remain
unchanged in its passage through the body; for if this writer be
correct in maintaining that the changes of nuclear substance in
ontogeny are effected by the nutritive influence of the cell-body
(cytoplasm), it follows that the whole nuclear substance of a cell must
be changed at every division, and that no unchanged part can remain. We
can only imagine that one part of a nucleus may undergo change while
the other part remains unchanged, if we hold that the necessary
transformations of nuclear substance are effected, by purely internal
causes, viz. that they follow from the constitution of the nucleoplasm.
But that one part may remain unchanged, and that such persistence does,
as a matter of fact, occur is shown by the cases above described, in
which the germ-cells separate very early from the developing egg-cell.
Thus in the egg of Diptera, the two nuclei which are first separated by
division from the segmentation nucleus, form the sexual cells, and this
proves that they receive the germ-plasm of the segmentation nucleus
unchanged. But during or before the separation of these two nuclei, the
remaining part of the segmentation nucleus must have become changed in
nature, or else it would continue to form ‘pole-cells’ at a later
period instead of forming somatic cells. Although in many cases the
cell-bodies of such early embryonic cells fail to exhibit any visible
differences, the idioplasm of their nuclei must undoubtedly differ, or
else they could not develope in different directions. It seems to me
not only possible, but in every way probable, that the bodies of such
early embryonic cells are equal in reality as well as in appearance;
for, although the idioplasm of the nucleus determines the character of
the cell-body, and although every differentiation of the latter depends
upon a certain structure of its nucleoplasm, it does not necessarily
follow that the converse proposition is true, viz. that each change in
the structure of the nucleoplasm must effect a change in the cell-body.
Just as rain is impossible without clouds, but every cloud does not
necessarily produce rain, so growth is impossible without chemical
change, but chemical processes of every kind and degree need not
produce growth. In the same manner every kind of change in the
molecular structure of the nucleoplasm need not exercise a transforming
influence on the cytoplasm, and we can easily imagine that a long
series of changes in the nucleoplasm may appear only in the kind and
energy of the nuclear divisions which take place, the cell-substance
remaining unchanged, as far as its molecular and chemical structure is
concerned. This suggestion is in accordance with the fact that during
the first period of embryonic development in animals, the cell-bodies
do not exhibit any visible differences, or only such as are very
slight; although exceptional instances occur, especially among the
lower animals. But even these latter (e.g. the difference in appearance
of the cells of the ectoderm and endoderm in sponges and Coelenterata)
perhaps depend more largely upon a different admixture of nutritive
substances than upon any marked difference in the cytoplasm itself. It
is obvious that, in the construction of the embryo, the amount of
cell-material must be first of all increased, and that it is only at a
later period that the material must be differentiated so as to possess
various qualities, according to the principle of division of labour.
Facts of this kind are also opposed to Strasburger’s view, that the
cause of changes in the nucleoplasm does not lie within this substance
itself but within the cell-body.
I believe I have shown that theoretically hardly any objections can be
raised against the view that the nuclear substance of somatic cells may
contain unchanged germ-plasm, or that this germ-plasm may be
transmitted along certain lines. It is true that we might imagine _a
priori_ that all somatic nuclei contain a small amount of unchanged
germ-plasm. In Hydroids such an assumption cannot be made, because only
certain cells in a certain succession possess the power of developing
into germ-cells; but it might well be imagined that in some organisms
it would be a great advantage if every part possessed the power of
growing up into the whole organism and of producing sexual cells under
appropriate circumstances. Such cases might exist if it were possible
for all somatic nuclei to contain a minute fraction of unchanged
germ-plasm. For this reason, Strasburger’s other objection against my
theory also fails to hold; viz. that certain plants can be propagated
by pieces of rhizomes, roots, or even by means of leaves, and that
plants produced in this manner may finally give rise to flowers, fruit
and seeds, from which new plants arise. ‘It is easy to grow new plants
from the leaves of _Begonia_ which have been cut off and merely laid
upon moist sand, and yet in the normal course of ontogeny the molecules
of germ-plasm would not have been compelled to pass through the leaf;
and they ought therefore to be absent from its tissue. Since it is
possible to raise from the leaf a plant which produces flower and
fruit, it is perfectly certain that special cells containing the germ
substance cannot exist in the plant.’ But I think that this fact only
proves, that in _Begonia_ and similar plants, all the cells of the
leaves or perhaps only certain cells contain a small amount of
germ-plasm, and that consequently these plants are specially adapted
for propagation by leaves. How is it then that all plants cannot be
reproduced in this way? No one has ever grown a tree from the leaf of
the lime or oak, or a flowering plant from the leaf of the tulip or
convolvulus. It is insufficient to reply that, in the last-mentioned
cases, the leaves are more strongly specialized, and have thus become
unable to produce germ-substance; for the leaf-cells in these different
plants have hardly undergone histological differentiation in different
degrees. If, notwithstanding, the one can produce a flowering plant,
while the others have not this power, it is of course clear that
reasons other than the degree of histological differentiation must
exist; and, according to my opinion, such a reason is to be found in
the admixture of a minute quantity of unchanged germ-plasm with some of
their nuclei.
In Sachs’ excellent lectures on the physiology of plants, we read on
page 723[140]—‘In the true mosses almost any cell of the roots, leaves
and shoot-axes, and even of the immature sporogonium, may grow out
under favourable conditions, become rooted, form new shoots, and give
rise to an independent living plant.’ Since such plants produce
germ-cells at a later period, we have here a case which requires the
assumption that all or nearly all cells must contain germ-plasm.
The theory of the continuity of the germ-plasm seems to me to be still
less disproved or even rendered improbable by the facts of the
alternation of generations. If the germ-plasm may pass on from the egg
into certain somatic cells of an individual, and if it can be further
transmitted along certain lines, there is no difficulty in supposing
that it may be transmitted through a second, third, or through any
number of individuals produced from the former by budding. In fact, in
the Hydroids, on which my theory of the continuity of the germ-plasm
has been chiefly based, alternation of generations is the most
important means of propagation.
II. The Significance of the Polar Bodies.
We have already seen that the specific nature of a cell depends upon
the molecular structure of its nucleus; and it follows from this
conclusion that my theory is further, and as I believe strongly,
supported, by the phenomenon of the expulsion of polar bodies, which
has remained inexplicable for so long a time.