determined by the structure of the nucleoplasm, every kind of cell
which is histologically differentiated must have a specific
nucleoplasm. But the egg-cell of most animals, at any rate during the
period of growth, is by no means an indifferent cell of the most
primitive type. At such a period its cell-body has to perform quite
peculiar and specific functions; it has to secrete nutritive substances
of a certain chemical nature and physical constitution, and to store up
this food-material in such a manner that it may be at the disposal of
the embryo during its development. In most cases the egg-cell also
forms membranes which are often characteristic of particular species of
animals. The growing egg-cell is therefore histologically
differentiated: and in this respect resembles a somatic cell. It may
perhaps be compared to a gland-cell, which does not expel its
secretion, but deposits it within its own substance[141]. To perform
such specific functions it requires a specific cell-body, and the
latter depends upon a specific nucleus. It therefore follows that the
growing egg-cell must possess nucleoplasm of specific molecular
structure, which directs the above-mentioned secretory functions of the
cell. The nucleoplasm of histologically differentiated cells may be
called histogenetic nucleoplasm, and the growing egg-cell must contain
such a substance, and even a certain specific modification of it. This
nucleoplasm cannot possibly be the same as that which, at a later
period, causes embryonic development. Such development can only be
produced by true germ-plasm of immensely complex constitution, such as
I have previously attempted to describe. It therefore follows that the
nucleus of the egg-cell contains two kinds of nucleoplasm:—germ-plasm
and a peculiar modification of histogenetic nucleoplasm, which may be
called _ovogenetic nucleoplasm_. This substance must greatly
preponderate in the young egg-cell, for, as we have already seen, it
controls the growth of the latter. The germ-plasm, on the other hand,
can only be present in minute quantity at first, but it must undergo
considerable increase during the growth of the cell. But in order that
the germ-plasm may control the cell-body, or, in other words, in order
that embryonic development may begin, the still preponderating
ovogenetic nucleoplasm must be removed from the cell. This removal
takes place in the same manner as that in which differing nuclear
substances are separated during the ontogeny of the embryo: viz. by
nuclear division, leading to cell-division. The expulsion of the polar
bodies is nothing more than the removal of ovogenetic nucleoplasm from
the egg-cell. That the ovogenetic nucleoplasm continues to greatly
preponderate in the nucleus up to the very last, may be concluded from
the fact that two successive divisions of the latter and the expulsion
of two polar bodies appear to be the rule. If in this way a small part
of the cell-body is expelled from the egg, the extrusion must in all
probability be considered as an inevitable loss, without which the
removal of the ovogenetic nucleoplasm cannot be effected.
This is my theory of the significance of polar bodies, and I do not
intend to contrast it, _in extenso_, with the theories propounded by
others; for such theories are well known and differ essentially from my
own. All writers agree in supposing that something which would be an
obstacle to embryonic development is removed from the egg; but opinions
differ as to the nature of this substance and the precise reasons for
its removal[142]. Some observers (e. g. Minot[143], van Beneden, and
Balfour) regard the nucleus as hermaphrodite, and assume that in the
polar bodies the male element is expelled in order to render the egg
capable of fertilization. Others speak of a rejuvenescence of the
nucleus, others again believe that the quantity of nuclear substance
must be reduced in order to become equal to that of the generally
minute sperm-nucleus, and that the proportions for nuclear conjugation
are in this way adjusted.
The first view seems to me to be disproved by the fact that male as
well as female qualities are transmitted by the egg-cell, while the
sperm-cell also transmits female qualities. The germ-plasm of the
nucleus of the egg cannot therefore be considered as female, and that
of the sperm-nucleus cannot be considered as male: both are sexually
indifferent. The last view has been recently formulated by Strasburger,
who holds that the quantity of the idioplasm contained in the
germ-nucleus must be reduced by one half, and that a whole nucleus is
again produced by conjugation with the sperm-nucleus. Although I
believe that the fundamental idea underlying this hypothesis is
perfectly correct, viz. that the influence of each nucleus is as
largely dependent upon its quantity as upon its quality, I must raise
the objection that the decrease in quantity is not the explanation of
the expulsion of polar bodies. The quantity of idioplasm contained in
the germ-nucleus is, as a matter of fact, not reduced by one-half but
by three-fourths, for two divisions take place one after the other.
Thus by conjugation with the sperm-nucleus, which we may assume to be
of the same size as the germ-nucleus, a nucleus is produced which can
only contain half as much idioplasm as was present in the original
germ-nucleus, before division. Strasburger’s view leaves unexplained
the question why the size of the germ-nucleus, before the expulsion of
polar bodies, was thus twice as large; and even if we neglect the
theory of ovogenetic nucleoplasm and assume that this larger nucleus
was entirely made up of germ-plasm, it must be asked why the egg did
not commence segmentation earlier. The theory which explains the
sperm-cell as the vitalizing principle which starts embryonic
development, like the spark which kindles the gunpowder, would indeed
answer this question in a very simple manner. But Strasburger does not
accept this theory, and holds, as I do, a very different view, which
will be explained later on.
If, on the other hand, we assume that the germ-nucleus contains two
different kinds of nucleoplasm, the question is answered quite
satisfactorily. In treating of parthenogenesis, further on, I shall
mention a fact which seems to me to furnish a real proof of the
validity of this explanation; and, if we accept this fact for the
present, it will be clear that the simple explanation now offered of
phenomena which are otherwise so difficult to understand, would also
largely support the theory of the continuity of the germ-plasm. Such an
explanation would, above all, very clearly demonstrate the co-existence
of two nucleoplasms with different qualities in one and the same
nucleus. My theory must stand or fall with this explanation, for if the
latter were disproved, the continuity of the germ-plasm could not be
assumed in any instance, not even in the simplest cases, where, as in
Diptera, the germ-cells are the first-formed products of embryonic
development. For even in these insects the egg possesses a specific
histological character which must depend upon a specifically
differentiated nucleus. If then two kinds of nucleoplasm are not
present, we must assume that in such cases the germ-plasm of the newly
formed germ-cells, which has passed on unchanged from the segmentation
nucleus, is at once transformed entirely into ovogenetic nucleoplasm,
and must be re-transformed into germ-plasm at a later period when the
egg is fully mature. We could not in any way understand why such a
re-transformation requires the expulsion of part of the nuclear
substance.
At all events, my explanation is simpler than all others, in that it
only assumes a single transformation of part of the germ-plasm, and not
the later re-transformation of ovogenetic nucleoplasm into germ-plasm,
which is so improbable. The ovogenetic nucleoplasm must possess
entirely different qualities from the germ-plasm; and, above all, it
does not readily lead to division, and thus we can better understand
the fact, in itself so remarkable, that egg-cells do not increase in
number by division, when they have assumed their specific structure,
and are controlled by the ovogenetic nucleoplasm. The tendency to
nuclear division, and consequently to cell-division, is not produced
until changes have to a certain extent taken place in the mutual
relation between the two kinds of nucleoplasm contained in the
germ-nucleus. This change is coincident with the attainment of maximum
size by the body of the egg-cell. Strasburger, supported by his
observations on _Spirogyra_, concludes that the stimulus towards
cell-division emanates from the cell-body; but the so-called centres of
attraction at the poles of the nuclear spindle obviously arise under
the influence of the nucleus itself, even if we admit that they are
entirely made up of cytoplasm. But this point has not been decided
upon, and we may presume that the so-called ‘Polkörperchen’ of the
spindle (Fol) are derived from the nucleus, although they are placed
outside the nuclear membrane[144]. Many points connected with this
subject are still in a state of uncertainty, and we must abstain from
general conclusions until it has been possible to demonstrate clearly
the precise nature of certain phenomena attending indirect nuclear
division, which still remain obscure in spite of the efforts of so many
excellent observers. We cannot even form a decided opinion as to
whether the chromatin or the achromatin of the nuclear thread is the
real idioplasm. But although these points are not yet thoroughly
understood, we are justified in maintaining that the cell enters upon
division under the influence of certain conditions of the nucleus,
although the latter are invisible until cell-division has already
commenced.
I now pass on to examine my hypothesis as to the significance of the
formation of polar bodies, in the light of those ascertained facts
which bear upon it.
If the expulsion of the polar bodies means the removal of the
ovogenetic nucleoplasm after the histological differentiation of the
egg-cell is complete, we must expect to find polar bodies in all
species except those in which the egg-cell has remained in a primitive
undifferentiated condition, if indeed such species exist. Wherever the
egg-cell assumes the character of a specialized cell, e.g. in the
attainment of a particular size or constitution, in the admixture of
food-yolk, or the formation of membranes, it must also contain
ovogenetic nucleoplasm, which must ultimately be removed if the
germ-plasm is to gain control over the egg-cell. It does not signify at
all, in this respect, whether the egg is or is not destined for
fertilization.
If we examine the Metazoa in regard to this question, we find that
polar bodies have not yet been discovered in sponges[145], but this
negative evidence is no proof that they are really absent. In all
probability, no one has ever seriously endeavoured to find them, and
there are perhaps difficulties in the way of the proofs of their
existence, because the egg-cell lies free for a long time and even
moves actively in the tissue of the mesogloea. We might expect that the
formation of polar bodies takes place here, as in all other instances,
when the egg becomes mature, that is, at a time when the eggs are
already closely enveloped in the sponge tissue. At all events the eggs
of sponges, as far as they are known, attain a specific nature, in the
possession of a peculiar cell-body, frequently containing food-yolk,
and of the nucleus which is characteristic of all animal eggs during
the process of growth. Hence we cannot doubt the presence of a specific
ovogenetic nucleoplasm, and must therefore also believe that it is
ultimately removed in the polar bodies.
In other Coelenterata, in worms, echinoderms, and in molluscs polar
bodies have been described, as well as in certain Crustacea, viz. in
_Balanus_ by Hoek and in _Cetochilus septentrionale_ by Grobben. The
latter instance appears to be quite trustworthy, but there is some
doubt as to the former and also as regards _Moina_ (a Daphnid), in
which Grobben found a body, which he considered to be a polar body, on
the upper pole of an egg which was just entering upon segmentation. In
insects polar bodies have not been described up to the present
time[146], and only in a few cases in Vertebrata, as in _Petromyzon_ by
Kupffer and Benecke.
It must be left to the future to decide whether the expulsion of polar
bodies occurs in those large groups of animals in which they have not
been hitherto discovered. The fact, however, that they have not been so
discovered cannot be urged as an objection to my theory, for we do not
know _a priori_ whether the removal of the ovogenetic nucleoplasm has
not been effected in the course of phylogeny in some other and less
conspicuous manner. The cell-body of the polar globules is so minute in
many eggs that it was a long time before the cellular nature of these
structures was recognized[147]; and it is possible that their minute
size may point to the fact that a phyletic process of reduction has
taken place, to the end that the egg may be deprived of as little
material as possible. It is at all events proved that in all Metazoan
groups the nucleus undergoes changes during the maturation of the egg,
which are entirely similar to those which lead to the formation of
polar bodies in those eggs which possess them. In the former instances
it is possible that nature has taken a shortened route to gain the same
end.
It would be an important objection if it could be shown that no process
corresponding to the expulsion of polar bodies takes place in the male
germ-cells, for it is obvious that here also we should, according to my
theory, expect such a process to occur. The great majority of
sperm-cells differ so widely in character from the ordinary indifferent
(i. e. undifferentiated) cells, that they are evidently histologically
differentiated in a very high degree, and hence the sperm-cells, like
the yolk-forming germ-cells, must possess a specific nuclear substance.
The majority of sperm-cells therefore resemble the somatic cells in
that they have a specific histological structure, but their
characteristic form has nothing to do with their fertilizing power,
viz. with their power of being the bearers of germ-plasm. Important as
this structure is, in order to render it possible that the egg-cell may
be approached and penetrated, it has nothing to do with the property of
the sperm-cell to transmit the qualities of the species and of the
individual to the following generation. The nuclear substance which
causes such a cell to assume the appearance of a thread, or a stellate
form (in Crustacea), or a boomerang form (present in certain Daphnids),
or a conical bullet shape (Nematodes), cannot possibly be the same
nuclear substance as that which, after conjugation with the egg-cell,
contains in its molecular structure the tendency to build up a new
Metazoon of the same kind as that by which it was produced. We must,
therefore, conclude that the sperm-cell also contains two kinds of
nucleoplasm, namely, germ-plasm and spermogenetic nucleoplasm.
It is true that we cannot say _a priori_ whether the influence
exercised on the sperm-cell by the spermogenetic nucleoplasm might not
be eliminated by some means other than its removal from the cell. It is
conceivable, for instance, that this substance may be expelled from the
nucleus, but may remain in the cell-body, where it is in some way
rendered powerless. We do not yet really know anything of the essential
conditions of nuclear division, and it is quite impossible to bring
forward any facts in support of my previous suggestion. The germ-plasm
is supposed to be present in the nucleus of the growing egg-cell in
smaller quantity than the ovogenetic nucleoplasm, and the germ-plasm
gradually increases in quantity: thus when the egg has attained its
maximum size, the opposition between the two different kinds of
nucleoplasm becomes so marked, in consequence of the alteration in
their quantitative relations, that their separation, viz. nuclear
division, results. But although we are not able to distinguish, by any
visible characteristics, the different kinds of nucleoplasm which may
be united in one nuclear thread, the assumption that the influence of
each kind bears a direct proportion to its quantity is the most obvious
and natural one. The tendency of the germ-plasm contained in the
nucleus cannot make itself felt so long as an excess of ovogenetic
nucleoplasm is also present. We may imagine that the effects of the two
different kinds of nucleoplasm are combined to produce a resultant
effect; but when the two influences exerted upon the cell are nearly
opposed, only the stronger can make itself felt, and in such a case the
latter must exceed the former in quantity, because part of it is as it
were neutralized by the other nucleoplasm working in an opposite
direction. This metaphorical representation may give us a clue to
explain the fact that the ovogenetic nucleoplasm comes to exceed the
germ-plasm in quantity. For obviously these two kinds of nucleoplasm
exert opposite tendencies in at least one respect. The germ-plasm tends
to effect the division of the cell into the two first segmentation
spheres; the ovogenetic nucleoplasm, on the other hand, possesses a
tendency towards the growth of the cell-body without division. Hence
the germ-plasm cannot make itself felt, and is not able to expel the
ovogenetic nucleoplasm until it has reached such a relative size as
enables it to successfully oppose the latter.
Applying these ideas to the sperm-cells we must see whether the
expulsion of part of the nuclear substance, viz. of the spermogenetic
nucleoplasm, corresponding to the ovogenetic nucleoplasm, takes place
in them also.
As far as we can judge from thoroughly substantiated observations such
phenomena are indeed found in many cases, although they appear to be
different from those occurring in the egg-cell, and cannot receive
quite so certain an interpretation.
The attempt to prove that a process similar to the expulsion of polar
bodies takes place in the formation of sperm-cells has already been
made by those observers who regard such expulsion as the removal of the
male element from the egg, thus leading to sexual differentiation; for
such a theory also requires the removal of part of the nuclear
substance from the maturing sperm-cell. Thus, according to E. van
Beneden and Ch. Julin, the cells which, in _Ascaris_, produce the
spermatogonia (mother-cells of the sperm-cells), expel certain elements
from their nuclear plate, a phenomenon which has not been hitherto
observed in any other animal, and even in this instance has only been
inferred and not directly observed. Moreover the sperm-cells have not
attained their specific form (conical bullet-shaped) at the time when
this expulsion takes place from the spermatogonia, and we should expect
that the spermogenetic nucleoplasm would not be removed until it has
completed its work, viz. not until the specific shape of the sperm-cell
has been attained. We might rather suppose that phenomena explicable in
this way are to be witnessed in those sperm-blastophores (mother-cells
of sperm-cells) which, as has been known for a long time, are not
employed in the formation of the nuclei of sperm-cells, but for the
greater part remain at the base of the latter and perish after their
maturation and separation. In this case an influence might be exerted
by these nuclei upon the specific form of the sperm-cells, for the
former arise and develope in the form of bundles of spermatozoa in the
interior of the mother-cell.
It has been already shown in many groups of animals that parts of the
sperm-mother-cells[148] perish, without developing into sperm-cells, as
in Selachians, in the frog, in many worms and snails, and also in
mammals (Blomfield). But the attention of observers has been directed
to that part of the cell-body which is not used in the formation of
sperm-cells, rather than to the nucleus; and the proof that part of the
nucleus also perishes is still wanting in many of these cases. Fresh
investigation must decide whether the nucleus of the sperm-mother-cell
perishes as a general rule, and whether part of the nucleus is rendered
powerless in some other way, where such mother-cells do not exist.
Perhaps the paranucleus (Nebenkern) of the sperm-cell, first described
by La Valette St. George, and afterwards found in many animals of very
different groups, is the analogue of the polar body. It is true that
this so-called paranucleus is now considered as a condensed part of the
cell-body, but we must remember that it has been hitherto a question
whether the head of the spermatozoon is formed from the nucleus of the
cell or from the paranucleus; and that the observers who held the
former view were in consequence obliged to regard the paranucleus as a
product of the cell-body. But according to the most recent
investigations of Fol[149], Roule[150], Balbiani[151], and Will[152],
upon the formation of the follicular epithelium in the ovary of
different groups, it is not improbable that parts of the nucleus may
become detached without passing through the process of karyokinesis.
Thus it is very possible that the paranucleus may be a product of the
main nucleus and not a condensed part of the cell-body. This view is
supported by its behaviour with staining reagents, while the other
view, that it arises from the cell-substance, is not based upon direct
observation. Consequently future investigation must decide whether the
paranucleus is to be considered as the spermogenetic nucleoplasm
expelled from the nucleus. But even if this question is answered in the
affirmative, we should still have to explain why this nuclear
substance, remaining in the cell-body, does not continue to exercise
any control over the latter.
Strasburger has recently enumerated a large number of cases from
different groups of plants, in which the maturation of both male and
female germ-cells is accompanied by phenomena similar to the expulsion
of polar bodies. In this respect the phenomena occurring in the
pollen-grains of Phanerogams bear an astonishing resemblance to the
maturation of the animal egg. For instance, in the larch, the
sperm-mother-cell divides three times in succession, the products of
division being very unequal on each occasion; and exactly as in the
case of polar bodies, the three small so-called vegetative cells shrink
rapidly after separation, and have no further physiological value.
According to Strasburger, the so-called ‘ventral canal-cell,’ which, in
mosses, ferns, and Conifers, separates from the female germ-cell,
reminds us, in every way, of the polar bodies of animal eggs.
Furthermore, the spermatozoids in the mosses and vascular cryptogams
throw off a small vesicle before performing their functions[153]. On
the other hand the equivalents of ‘polar bodies’ (the ‘ventral
canal-cells’) are said to be absent in the Cycads, although these are
so nearly allied to Conifers. Furthermore, ‘no phenomenon occurs in the
oospheres (ova) of Angiosperms which can be compared to the formation
of polar bodies.’ Strasburger therefore concludes that the separation
of certain parts from the germ-cells is not in all cases necessary for
maturation, and that such phenomena are not fundamental, like those of
fertilization, which must always take place along the same
morphological lines. He further concludes that the former phenomena are
only necessary in the case of the germ-cells of certain organisms, in
order to bring the nuclei destined for the sexual act into the
physiological condition necessary for its due performance.
I am unwilling to abandon the idea that the expulsion of the
histogenetic parts of the nuclear substance, during the maturation of
germ-cells, is also a general phenomenon in plants; for the process
appears to be fundamental, while the argument that it has not been
proved to occur universally is only of doubtful value. The embryo-sac
of Angiosperms is such a complex structure that it seems to me to be
possible (as it does to Strasburger) that ‘processes which precede the
formation of the egg-cell have borne relation to the sexual
differentiation of the nucleus of the egg.’ Besides, it is possible
that the vegetable egg-cell may, in certain cases, possess so simple a
structure and so small a degree of histological specialization, that it
would not be necessary for it to contain any specific histogenetic
nucleoplasm: thus it would consist entirely of germ-plasm from the
first. In such cases, of course, its maturation would not be
accompanied by the expulsion of somatic nucleoplasm.
I have hitherto abstained from discussing the question as to whether
the process of the formation of polar bodies may require an
interpretation which is entirely different from that which I have given
it, whether it may receive a purely morphological interpretation.