of the activities of different parts of the idioplasm: certain groups
of micellae in the idioplasm represent the germs (‘Anlagen’) of certain
structures in the organism: when any such germ reacts under stimulation
it produces the corresponding structure. It seems to me that this
hypothesis bears some resemblance to Darwin’s theory of pangenesis. I
think that Nägeli’s preformed germs of structures (‘Anlagen’) and his
groups of such germs are highly elaborated equivalents of the gemmules
of pangenesis, which, according to Darwin, manifest activity when their
turn comes, or, according to Nägeli, when they react under stimulation.
When a group of such germs, by their active growth or by their
‘irritation,’ have caused a similar active growth or a similar
irritation in the next group, the former may come to rest, or may
remain in a state of activity together with its successor, for a longer
or shorter period. Its activity may even last for an unlimited time, as
is the case in the formation of leafy shoots in many plants.
Here, again, we recognize the fact that Nägeli’s whole hypothesis is
intimately connected with the supposition that the entire mass of
idioplasm is continuous throughout the organism. Sometimes one part of
the idioplasm and sometimes another part is irritated, and then
produces the corresponding organ. But if, on the other hand, the
idioplasm does not represent a directly continuous mass, but is
composed of thousands of single nucleoplasms which only act together
through the medium of their cell-bodies, then we must substitute the
conception of ‘ontogenetic stages of development of the idioplasm’ for
the conception of germs of structure (‘Anlagen’). The different
varieties of nucleoplasm which arise during ontogeny represent, as it
were, the germs of Nägeli (‘Anlagen’), because, by means of their
molecular structure, they create a specific constitution in the
cell-bodies over which they have control, and also because they
determine the succession of future nuclei and cells.
It is in this sense, and no other, that I can speak of the presence of
preformed germs (‘Anlagen’) in the idioplasm. We may suppose that the
idioplasm of the first segmentation nucleus is but slightly different
from that of the second ontogenetic stage, viz. that of the two
following segmentation nuclei. Perhaps only a few groups of micellae
have been displaced or somewhat differently arranged. But nevertheless
such groups of micellae were not the germs (‘Anlagen’) of a second
stage which pre-existed in the first stage, for the two are
distinguished by the possession of a different molecular structure.
This structure in the second stage, under normal conditions of
development, again brings about the change by which the different
molecular structure of the third stage is produced, and so on.
It may be argued that von Baer’s well-known and fundamental law of
development is opposed to the hypothesis that the idioplasm of
successive ontogenetic stages must gradually assume a simpler molecular
structure. The organization of the species has, on the whole, increased
immensely in complexity during the course of phylogeny: and if the
phyletic stages are repeated in the ontogeny, it seems to follow that
the structure of the idioplasm must become more complex in the course
of ontogeny instead of becoming simpler. But the complexity of the
whole organism is not represented in the molecular structure of the
idioplasm of any single nucleus, but by that of all the nuclei present
at any one time. It is true that the germ-cell, or rather the idioplasm
of the germ-nucleus, must gain greater complexity as the organism which
arises from it becomes more complex; but the individual nucleoplasms of
each ontogenetic stage may become simpler, while the whole mass of
idioplasms in the organism (which, taken together, represent the stage
in question) does not by any means lose in complexity.
If we must therefore assume that the molecular structure of the
nucleoplasm becomes simpler in the course of ontogeny, as the number of
structures which it potentially contains become smaller, it follows
that the nucleoplasm in the cells of fully differentiated tissues—such
as muscle, nerve, sense-organs, or glands—must possess relatively the
most simple molecular structure; for it cannot originate any fresh
modification of nucleoplasm, but can only continue to produce cells of
the same structure, although it does not always retain this power.
We are thus brought back to the fundamental question as to how the
germ-cells arise in the organism. Is it possible that the nucleoplasm
of the germ-cell, with its immensely complex molecular structure,
potentially containing all the specific peculiarities of an individual,
can arise from the nucleoplasm of any of the body-cells,—a substance
which, as we have just seen, has lost the power of originating any new
kind of cell, because of the continual simplification of its structure
during development? It seems to me that it would be impossible for the
simple nucleoplasm of the somatic cells to thus suddenly acquire the
power of originating the most complex nucleoplasm from which alone the
entire organism can be built up: I cannot see any evidence for the
existence of a force which could effect such a transformation.
This difficulty has already been appreciated by other writers.
Nussbaum’s[123] theoretical views, which I have already mentioned, also
depend upon the hypothesis that cells which have once become
differentiated for the performance of special functions cannot be
re-transformed into sexual cells: he also concludes that the latter are
separated from all other cells at a very early period of embryonic
development, before any histological differentiation has taken place.
Valaoritis[124] has also recognised that the transformation of
histologically differentiated cells into sexual cells is impossible. He
was led to believe that the sexual cells of Vertebrata arise from the
white blood corpuscles, for he looked upon these latter as
differentiated to the smallest extent possible. Neither of these views
can be maintained. The former, because the sexual cells of all plants
and most animals are not, as a matter of fact, separated from the
somatic cells at the beginning of ontogeny; the latter, because it is
contradicted by the fact that the sexual cells of vertebrates do not
arise from blood corpuscles, but from the germinal epithelium. But even
if this fact had not been ascertained we should be compelled to reject
Valaoritis’ hypothesis on theoretical grounds, for it is an error to
assume that white blood corpuscles are undifferentiated, and that their
nucleoplasm is similar to the germ-plasm. There is no nucleoplasm like
that of the germ-cell in any of the somatic cells, and no one of these
latter can be said to be undifferentiated. All somatic cells possess a
certain degree of differentiation, which may be rigidly limited to one
single direction, or may take place in one of many directions. All
these cells are widely different from the egg-cell from which they
originated: they are all separated from it by many generations of
cells, and this fact implies that their idioplasms possess a widely
different structure from the idioplasm, or germ-plasm, of the egg-cell.
Even the nuclei of the two first segmentation spheres cannot possess
the same idioplasm as that of the first segmentation nucleus, and it
is, of course, far less possible for such an idioplasm to be present in
the nucleus of any of the later cells of the embryo. The structure of
the idioplasm must necessarily become more and more different from that
of the first segmentation nucleus, as the development of the embryo
proceeds. The idioplasm of the first segmentation nucleus, and of this
nucleus alone, is germ-plasm, and possesses a structure such that an
entire organism can be produced from it. Many writers appear to
consider it a matter of course that any embryonic cell can reproduce
the entire organism, if placed under suitable conditions. But, when we
carefully look into the subject, we see that such powers are not even
possessed by those cells of the embryo which are nearest to the
egg-cell—viz. the first two segmentation spheres. We have only to
remember the numerous cases in which one of them forms the ectoderm of
the animal while the other produces the endoderm, in order to admit the
validity of this objection.
But if the first segmentation spheres are not able to develope into a
complete organism, how can this be the case with one of the later
embryonic cells, or one of the cells of the fully developed animal
body? It is true that we speak of certain cells as being ‘of embryonic
character,’ and only recently Kölliker[125] has given a list of such
cells, among which he includes osteoblasts, cartilage cells, lymph
corpuscles, and connective tissue corpuscles: but even if these cells
really deserve such a designation, no explanation of the formation of
germ-cells is afforded, for the idioplasm of the latter must be widely
different from that of the former.
It is an error to suppose that we gain any further insight into the
formation of germ-cells by referring to these cells of so-called
‘embryonic character,’ which are contained in the body of the mature
organism. It is of course well known that many cells are characterized
by very sharply defined histological differentiation, while others are
but slightly differentiated; but it is as difficult to imagine that
germ-cells can arise from the latter as from the former. Both classes
of cells contain idioplasm with a structure different from that which
is contained in the germ-cell, and we have no right to assume that any
of them can form germ-cells until it is proved that somatic idioplasm
is capable of undergoing re-transformation into germ-idioplasm.
The same argument applies to the cells of the embryo itself, and it
therefore follows that those instances of early separation of sexual
from somatic cells, upon which I have often insisted as indicating the
continuity of the germ-plasm, do not now appear to be of such
conclusive importance as at the time when we were not sure about the
localization of the idioplasm in the nuclei. In the great majority of
cases the germ-cells are not separated at the beginning of embryonic
development, but only in some one of the later stages. A single
exception is found in the pole-cells (‘Polzellen’) of Diptera, as was
shown many years ago by Robin[126] and myself[127]. These are the first
cells formed in the egg, and according to the later observations of
Metschnikoff[128] and Balbiani[129], they become the sexual glands of
the embryo. Here therefore the germ-plasm maintains a true unbroken
continuity. The nucleus of the egg-cell directly gives rise to the
nuclei of the pole-cells, and there is every reason to believe that the
latter receive unchanged a portion of the idioplasm of the former, and
with it the tendencies of heredity. But in all other cases the
germ-cells arise by division from some of the later embryonic cells,
and as these belong to a more advanced ontogenetic stage in the
development of the idioplasm, we can only conclude that continuity is
maintained, by assuming (as I do) that a small part of the germ-plasm
persists unchanged during the division of the segmentation nucleus and
remains mixed with the idioplasm of a certain series of cells, and that
the formation of true germ-cells is brought about at a certain point in
the series by the appearance of cells in which the germ-plasm becomes
predominant. But if we accept this hypothesis it does not make any
difference, theoretically, whether the germ-plasm becomes predominant
in the third, tenth, hundredth, or millionth generation of cells. It
therefore follows that cases of early separation of the germ-cells
afford no proof of a direct persistence of the parent germ-cells in
those of the offspring; for a cell the offspring of which become partly
somatic and partly germ-cells cannot itself have the characters of a
germ-cell; but it may nevertheless contain germ-idioplasm, and may thus
transfer the substance which forms the basis of heredity from the germ
of the parent to that of the offspring.
If we are unwilling to accept this hypothesis, nothing remains but to
credit the idioplasm of each successive ontogenetic stage with a
capability of re-transformation into the first stage. Strasburger
accepts this view; and he believes that the idioplasm of the nuclei
changes during the course of ontogeny, but returns to the condition
of the first stage of the germ, at its close. But the rule of
probability is against such a suggestion. Suppose, for instance, that
the idioplasm of the germ-cell is characterized by ten different
qualities, each of which may be arranged relatively to the others in
two different ways, then the probability in favour of any given
combination would be represented by the fraction (1/2)^{10} = 1/1024:
that is to say, the re-transformation of somatic idioplasm into
germ-plasm will occur once in 1024 times, and it is therefore
impossible for such re-transformation to become the rule. It is also
obvious that the complex structure of the germ-plasm which
potentially contains, with the likeness of a faithful portrait, the
whole individuality of the parent, cannot be represented by only ten
characters, but that there must be an immensely greater number; it is
also obvious that the possibilities of the arrangement of single
characters must be assumed to be much larger than two; so that we get
the formula (1/_p_), where _p_ represents the possibilities, and _n_
the characters. Thus if _n_ and _p_ are but slightly larger than we
assumed above, the probabilities become so slight as to altogether
exclude the hypothesis of a re-transformation of somatic idioplasm
into germ-plasm.
It may be objected that such re-transformation is much more probable in
the case of those germ-cells which separate early from the somatic
cells. Nothing can in fact be urged against the possibility that the
idioplasm of (e. g.) the third generation of cells may pass back into
the condition of the idioplasm of the germ-cell; although of course the
mere possibility does not prove the fact. But there are not many cases
in which the sexual cells are separated so early as the third
generation: and it is very rare for them to separate at any time during
the true segmentation of the egg. In _Daphnidae_ (_Moina_) separation
occurs in the fifth stage of segmentation[130], and although this is
unusually early it does not happen until the idioplasm has changed its
molecular structure six times. In _Sagitta_[131] the separation does
not take place until the archenteron is being formed, and this is after
several hundred embryonic cells have been produced, and thus after the
germ-plasm has changed its molecular structure ten or more times. But
in most cases, separation takes place at a much later stage; thus in
Hydroids it does not happen until after hundreds or thousands of
cell-generations have been passed through; and the same fact holds in
the higher plants, where the production of germ-cells frequently occurs
at the end of ontogeny. In such cases the probability of a
re-transformation of somatic idioplasm into germ-plasm becomes
infinitely small.
It is true that these considerations only refer to a rapid and sudden
re-transformation of the idioplasm. If it could be proved that
development is not merely in appearance but in reality a cyclical
process, then nothing could be urged against the occurrence of
re-transformation. It has been recently maintained by Minot[132] that
all development is cyclical, but this is obviously incorrect, for
Nägeli has already shown that direct non-cyclical courses of
development exist, or at all events courses in which the earliest
condition is not repeated at the close of development. The phyletic
development of the whole organic world clearly illustrates a
development of the latter kind; for although we may assume that organic
development is not nearly concluded, it is nevertheless safe to predict
that it will never revert to its original starting-point, by backward
development over the same course as that which it has already
traversed. No one can believe that existing Phanerogams will ever, in
the future history of the world, retrace all the stages of phyletic
development in precise inverse order, and thus return to the form of
unicellular Algae or Monera; or that existing placental mammals will
develope into Marsupialia, Monotremata, mammal-like reptiles, and the
lower vertebrate forms, into worms and finally into Monera. But how can
a course of development, which seems to be impossible in phylogeny,
occur as the regular method of ontogeny? And quite apart from the
question of possibility, we have to ask for proofs of the actual
occurrence of cyclical development. Such a proof would be afforded if
it could be shown that the nucleoplasm of those somatic cells which
(e.g. in Hydroids) are transformed into germ-cells passes backwards
through many stages of development into the nucleoplasm of the
germ-cell. It is true that we can only recognise differences in the
structure of the idioplasm by its effects upon the cell-body, but no
effects are produced which indicate that such backward development
takes place. Since the course of onward development is compelled to
pass through the numerous stages which are implied in segmentation and
the subsequent building-up of the embryo, etc., it is quite impossible
to assume that backward development would take place suddenly. It would
be at least necessary to suppose that the cells of embryonic character,
which are said to be transformed into primitive germ-cells, must pass
back through at any rate the main phases of their ontogeny. A sudden
transformation of the nucleoplasm of a somatic cell into that of a
germ-cell would be almost as incredible as the transformation of a
mammal into an amoeba; and yet we are compelled to admit that the
transformation must be sudden, for no trace of such retrogressive
stages of development can be seen. If the appearance of the whole cell
gives us any knowledge as to the structure of its nuclear idioplasm, we
may be sure that the development of a primitive germ-cell proceeds
without a break, from the moment of its first recognizable formation,
to the ultimate production of distinct male or female sexual cells.
I am well aware that Strasburger has stated that, in the ultimate
maturation of the sexual cells, the substance of the nuclei returns to
a condition similar to that which existed at the beginning of
ontogenetic development; still such a statement is no proof, but only
an assumption made to support a theory. I am also aware that Nussbaum
and others believe that, in the formation of spermatozoa in higher
animals, a backward development sets in at a certain stage; but even if
this interpretation be correct, such backward development would only
lead as far as the primitive germ-cell, and would afford no explanation
of the further transformation of the idioplasm of this cell into
germ-plasm. But this latter transformation is just the point which most
needs proof upon any theory except the one which assumes that the
primitive germ-cell still contains unchanged germ-plasm. Every attempt
to render probable such a re-transformation of somatic nucleoplasm into
germ-plasm breaks down before the facts known of the Hydroids, in which
only certain cells in the body, out of the numerous so-called embryonic
cells, are capable of becoming primitive germ-cells, while the rest do
not possess this power.
I must therefore consider as erroneous the hypothesis which assumes
that the somatic nucleoplasm may be transformed into germ-plasm. Such a
view may be called ‘the hypothesis of the cyclical development of the
germ-plasm.’
Nägeli has tried to support such an hypothesis on phyletic grounds. He
believes that phyletic development follows from an extremely slow but
steady change in the idioplasm, in the direction of greater complexity,
and that such changes only become visible periodically. He believes
that the passage from one phyletic stage to another is chiefly due to
the fact that ‘in any ontogeny, the very last structural change upon
which the separation of germs depends, takes place in a higher stage,
one or more cell-generations later’ than it occurred in a lower stage.
‘The last structural change itself remains the same, while the series
of structural changes immediately preceding it is increased.’ I believe
that Nägeli, being a botanist, has been too greatly influenced by the
phenomena of plant-life. It is certainly true that in plants, and
especially in the higher forms, the germ-cells only make their
appearance, as it were, at the end of ontogeny; but facts such as these
do not hold in the animal kingdom: at any rate they are not true in the
great majority of cases. In animals, as I have already mentioned
several times, the germ-cells are separated from the somatic cells
during embryonic development, sometimes even at its very commencement;
and it is obvious that this latter is the original, phyletically
oldest, mode of formation. The facts at our disposal indicate that the
germ-cells only appear, for the first time, after embryological
development, in those cases where the formation of asexually produced
colonies takes place, either with or without alternation of
generations; or in cases where alternation of generations occurs
without the formation of such colonies. In a colony of polypes, the
germ-cells are produced by the later generations, and not by the
founder of the colony which was developed from an egg. This is also
true of the colonies of Siphonophora, and the germ-cells appear to
arise very late in certain instances of protracted metamorphosis
(Echinodermata), but on the other hand, they arise during the embryonic
development of other forms (Insecta) which also undergo metamorphosis.
It is obvious that the phyletic development of colonies or stocks must
have succeeded that of single individuals, and that the formation of
germ-cells in the latter must therefore represent the original method.
Thus the germ-cells originally arose at the beginning of ontogeny and
not at its close, when the somatic cells are formed.
This statement is especially supported by the history of certain lower
plants, or at any rate chlorophyll-containing organisms, and I think
that these forms supply an admirable illustration of my theory as to
the phyletic origin of germ-cells, as explained in my earlier papers
upon the same subject.