to the end of the ovarian development of the egg is inconsistent with
the preservation of the other half of the dividing nucleus, I should be
inclined to reply that a ‘reducing division’ of the mature egg,
resulting in the production of two eggs, was probably the phyletic
precursor of the present condition. I imagine that the division of the
mature egg-cell—although it is now so extremely unequal—was equal in
very remote times; but that for reasons of utility, connected with the
specialization of the eggs of animals, it gradually became more and
more unequal. It is now hardly possible to give in detail the various
reasons of utility which have brought about this condition, but it may
be assumed that the enormous size attained by many animal egg-cells has
been especially potent in producing the change.
A careful consideration of this last point seems to me to be demanded
by a comparison of the egg-cells with the male germ-cells. Just as the
female germ-cells of animals are distinguished by the attainment of a
large size, the male germ-cells are generally remarkable for their
minute proportions. In most cases it would be physiologically
impossible for a large egg-cell, rich in yolk, to attain double its
specific size in order to undergo division into two equal halves and
yet to remain of the characteristic size. Even without the additional
difficulties imposed by the necessity for such a division, all
means—such as cells used as food, or the passage of food from
follicular cells into the ovum, etc.—are employed in order to bring the
egg-cell to the greatest attainable size. Furthermore, the ‘reducing
division’ of the nucleus cannot take place before the egg has attained
its full size, because the ovogenetic nucleoplasm still controls the
egg-cell, and must be removed before the germ-plasm can regulate its
development. By arguments such as these I should attempt to render the
whole subject intelligible.
But the case is entirely different with the sperm-cells, which are
generally minute: here it is quite conceivable that a ‘reducing
division’ of the nuclei may take place by an equal division of the
sperm-cells, occurring towards the end of the period of their
formation; that is to say, in such a way that both products of division
remain sperm-cells, and neither of them perishes like the polar bodies.
But the other possibility also demands consideration, viz. that the
reducing division may occur at an earlier stage in the development of
sperm-cells. At all events, the arguments adduced above, which proved
that the consequence would be a want of variability in the egg-cells,
would not apply to an equal extent in the case of the male germ-cells.
Among the egg-cells it may be very important that each one should have
its special individual character, produced by a somewhat different
composition of its germ-plasm, inasmuch as a considerable proportion of
the eggs frequently developes, although this is never the case with all
of them. But the production of sperm-cells is in most animals so
enormous that only a very small percentage can be used for
fertilization. If, therefore, e. g. ten or a hundred spermatozoa
contained germ-plasm with exactly the same composition, so that, as far
as the paternal influence is concerned, ten or a hundred identical
individuals would result if they were all used in fertilization, such
an arrangement would be practically harmless, for only one spermatozoon
out of an immense number would be employed for this purpose. From this
point of view we might expect that the ‘reducing division’ of the
sperm-nucleus would not take place at the end of the development of the
sperm-cell, but at some earlier period. There is no necessary reason
for the assumption that this division must take place at the end of
development, and without some cause natural selection cannot operate.
It is, of course, conceivable that the causes of other events may also
involve the occurrence of this division at the end of development; but
we do not at present know of any such causes. I should not consider the
influence of the specific histogenetic nucleoplasm, i.e. the
spermatogenetic nucleoplasm, to be such a cause, because the
quantitative proportions are very different from those which obtain in
the formation of egg-cells, and because it is not inconceivable that
the small quantity of true germ-plasm which must be present in the
nuclei of the sperm-cells at every stage in their formation might enter
upon a ‘reducing division’ with the spermatogenetic nucleoplasm, even
when the latter preponderated.
As soon as we can recognize with certainty the forms of nuclear
division which are ‘reducing divisions,’ the question will be settled
as far as spermatogenesis is concerned. It has been already established
that various forms of nuclear division occur at different periods of
spermatogenesis. I make this assertion, not only from my own
observations, but also from observations which have been made and
insisted upon by others. Thus, van Beneden and Julin[266] stated in
1884 that direct and karyokinetic nuclear divisions alternate with each
other in the spermatogenesis of _Ascaris megalocephala_. Again,
Carnoy[267] distinctly states that the different cell-generations in
the same testis may not uncommonly exhibit considerable differences as
regards karyokinesis. ‘This may go so far that direct and indirect
division may proceed simultaneously.’ Platner[268], in his excellent
paper on karyokinesis in Lepidoptera, also points out that the
karyokinesis of the spermatocytes is essentially different from that of
the spermatogonia. According to his description, the latter form may be
very well interpreted as a ‘reducing division,’ for no equatorial plate
is formed, and the chromatin rods (or granules, as they are better
called in this case) remain from the first on both sides of the
equatorial plane, and finally unite at the opposite poles to form the
two daughter-nuclei. Furthermore, if Carnoy has correctly observed, the
form of karyokinesis which I have previously interpreted as a ‘reducing
division’ occurs in the sperm-mother-cells—a karyokinesis in which the
chromatin rods either do not divide longitudinally, or else divide in
this way after they have left the equatorial plate and are proceeding
towards the poles. Carnoy does not himself attach any special
importance to these observations, for he only considers them as proofs
that the longitudinal splitting of the loops may occur at various
periods in different species—either at the equator, or on the way
towards the poles, or even at the poles themselves. We cannot conclude
from the author’s statements whether this form of nuclear division only
occurs in a single cell-generation during spermatogenesis, as it must
do if it really represents a ‘reducing division.’ Until this point is
settled, we cannot decide with certainty whether the described form of
karyokinesis is to be considered as the ‘reducing division’ for which
we are seeking. Fresh investigations, undertaken from these points of
view, are necessary in order to settle the question. It would be
useless to seek further support for the theory by going into further
details, and by critically examining the numerous observations upon
spermatogenesis which have now been recorded.
I will only mention that among the various nuclei and other bodies in
different animals which have been considered by different observers as
the polar bodies of the sperm-cells, or the cells which form the
latter—in my opinion the paranucleus (‘Nebenkern’) of the ‘spermatides’
described by La Valette St. George[269] has the highest claim to be
considered as the homologue of a polar body. But I am inclined to
identify it with the first rather than the second polar body of the
egg-cells, and to regard it as the histogenetic part of the nucleoplasm
which has been expelled or rendered powerless by internal
transformations. There are two reasons which lead me to this
conclusion: first, as I have tried to show above, it is probable that
the ancestral germ-plasms are not removed by expulsion, but by means of
equal cell-division; secondly, my theory asserts that the histogenetic
nucleoplasm cannot be rendered powerless until the close of
histological differentiation.
The whole question of the details of the transformations undergone by
the nucleus of the male germ-cells is not ready for the expression of a
mature opinion. From the very numerous and mostly minute and careful
observations which have been hitherto recorded, we cannot conclude with
any degree of certainty when and how the ‘reducing division’ of the
nucleus takes place, nor can we decide upon the processes which signify
the purification of the germ-plasm from the merely histogenetic part of
the nucleoplasm. But perhaps it has not been without value as regards
future investigation that I have tried to apply to the male germ-cells
the views gained from our more certain knowledge of the corresponding
structures in the female, and thus to indicate the problems which now
chiefly demand solution.
IV. THE FOREGOING CONSIDERATIONS APPLIED TO PLANTS.
It remains to briefly consider the case of plants. Obviously, the
‘reducing division’ of the germ-nuclei, if it takes place at all,
cannot be restricted to the germ-cells of animals. There must be a
corresponding process in plants, for sexual reproduction is essentially
the same in both kingdoms; and if fertilization must be preceded by the
expulsion of half the number of ancestral germ-plasms from the eggs of
animals, the same necessity must hold in the case of plants.
But whether the process always takes place in the form of polar bodies,
and not perhaps principally, or at any rate frequently, in the form of
equal cell-division, is another question. It is true that polar bodies
occur in numerous plants, as we chiefly know from Strasburger’s
researches[270]. Strasburger shows that cells are separated by division
from the germ-cells, and perish. But it seems to me doubtful whether we
must always regard their formation as the removal of half the number of
ancestral germ-plasms rather than the histogenetic nucleoplasm of the
germ-cell. It appears to me that histogenetic nucleoplasm must be
present in the highly differentiated vegetable germ-cells, especially
in the male cells, and also that it must be removed during the
maturation of the cell, if my idea of the histogenetic nucleoplasm be
accepted. It is very possible, as I have already mentioned, that there
may be quite indifferent germ-cells, viz. cells which are entirely
without specific histological structure, and in such cases histogenetic
nucleoplasm would be absent; and during the maturation of such
germ-cells no polar body would be formed for its removal. This view
accords with the fact that polar bodies are absent in many plants.
Furthermore, I am far from maintaining that in the cases where polar
bodies occur, they must have the above-mentioned significance. I only
wish to point out that the reduction assumed to be necessary for the
nucleus of the vegetable germ-cells is not necessarily to be sought for
at the close of their maturation, but perhaps even more frequently in
an equal division of the germ-cells during some period of their
development.
It also seems to me to be not impossible that a number of these
vegetative ‘polar bodies’ may have an entirely different significance,
viz. to perform some special function accessory to fertilization, as in
the so-called ‘ventral canal-cells’ of the higher cryptogams and
conifers. As we know that even the two polar bodies of the animal egg
are not identical—although externally they are extremely similar, and
although they arise in a precisely similar manner—I am even more
inclined than before to consider that the very various ‘polar bodies’
of plants possess very different meanings.
But I do not feel justified in criticizing in detail the results of
botanical investigation. I must leave the decision of such questions to
botanists, and I only desire to state distinctly that a ‘reducing
division’ of the nuclei of germ-cells must occur in plants as well as
in animals.
V. CONCLUSIONS WITH REGARD TO HEREDITY.
The ideas developed in the preceding paragraphs lead to remarkable
conclusions with regard to the theory of heredity,—conclusions which do
not harmonize with the ideas on this subject which have been hitherto
received. For if every egg expels half the number of its ancestral
germ-plasms during maturation, the germ-cells of the same mother cannot
contain the same hereditary tendencies, unless of course we make the
supposition that corresponding ancestral germ-plasms are retained by
all eggs—a supposition which cannot be sustained. For when we consider
how numerous are the ancestral germ-plasms which must be contained in
each nucleus, and further how improbable it is that they are arranged
in precisely the same manner in all germ-cells, and finally how
incredible it is that the nuclear thread should always be divided in
exactly the same place to form corresponding loops or rods,—we are
driven to the conclusion that it is quite impossible for the ‘reducing
division’ of the nucleus to take place in an identical manner in all
the germ-cells of a single ovary, so that the same ancestral
germ-plasms would always be removed in the polar bodies. But if one
group of ancestral germ-plasms is expelled from one egg, and a
different group from another egg, it follows that no two eggs can be
exactly alike as regards their contained hereditary tendencies: they
must all differ. In many cases the differences will only be slight,
that is, when the eggs contain very similar combinations of ancestral
germ-plasms. Under other circumstances the differences will be very
great, viz. when the combinations of ancestral germ-plasms retained in
the egg are very different. I might here mention various other
considerations; but this would lead me too far from my subject, into
new theories of heredity. I hope to be able at some later period to
develope further the theoretical ideas which are merely indicated in
the present essay. I only wish to show that the consequences which
follow from my theory upon the second division of the egg-nucleus, and
the formation of the second polar body, are by no means opposed to the
facts of heredity, and even explain them better than has hitherto been
possible.
The fact that the children of the same parents are never entirely
identical could hitherto only be rendered intelligible by the vague
suggestion that the hereditary tendencies of the grandfather
predominate in one, and those of the grandmother in another, while the
tendencies of the great-grandfather predominate in a third, and so on.
Any further explanation as to why this should happen was entirely
wanting. Others even looked for an explanation to the different
influences of nutrition, to which it is perfectly true that the egg is
subjected in the ovary during its later development, according to its
position and immediate surroundings. I had myself referred to these
influences as a partial explanation[271], before I recognized clearly
how extremely feeble and powerless are the influences of nourishment,
as compared with hereditary tendencies. According to my theory, the
differences between the children of the same parents become
intelligible in a simple manner from the fact that each maternal
germ-cell (I shall speak of the paternal germ-cells later on) contains
a peculiar combination of ancestral germ-plasms, and thus also a
peculiar combination of hereditary tendencies. These latter by their
co-operation also produce a different result in each case, viz. the
offspring, which are characterized by more or less pronounced
individual peculiarities.
But the theory which explains individual differences by referring to
the inequality of germ-cells, may be proved with a high degree of
probability by an appeal to facts of an opposite kind, viz. by showing
that identity between offspring only occurs when they have arisen from
the same egg-cell. It is well known that occasionally some of the
children of the same parents appear to be almost exactly alike, but
such children are without exception twins, and there is every reason to
believe that they have been derived from the _same_ egg. In other
words, the two children are exactly alike because they have arisen from
the same egg-cell, which could of course only contain a single
combination of ancestral germ-plasms, and therefore of hereditary
tendencies[272]. The factors which by their co-operation controlled the
construction of the organism were the same, and consequently the
results were also the same. Twins derived from a single egg are
identical: this is a statement which, although not mathematically
proved, may be looked upon as nearly certain. But there are also twins
which do not possess this high degree of similarity, and these are even
far commoner than the others. The explanation is to be found in the
fact that the latter were derived from two egg-cells which were
fertilized at the same time. In most cases, indeed, each twin is
enclosed in its own embryonic membranes, while much less frequently
both twins are enclosed in the same membranes. In one point only the
proof is incomplete; for it has not yet been shown that identical twins
are always derived from a single egg, since such an origin, together
with a high degree of similarity, could only be established as
occurring together in a small proportion of the cases. We therefore see
that under conditions of nutriment which are as identical as possible,
_two_ egg-cells develope into unlike twins, _one_ into identical twins;
although we cannot yet affirm that the latter result invariably
follows. It is conceivable that the stimulus for the production of two
eggs from one may be afforded by the entrance of two spermatozoa, but
these latter, as was shown above, could hardly contain identical
hereditary tendencies, and thus two identical twins would not arise. It
appears indeed that some cases have been observed in which differences
have been exhibited by twins which were enclosed in the same embryonic
membranes; but nevertheless I believe that two spermatozoa are not
necessary to cause the formation of twins by a single egg. We know, it
is true, from the investigations of Fol[273], that multiple
impregnation produces the simultaneous beginning of several embryos in
the eggs of star-fishes. But several embryos and young animals are not
developed in this way, for embryonic development soon ceases, and the
egg dies.
The recent observations of Born[274] upon the eggs of the frog also
make it very probable that a double development is produced by the
entrance of two spermatozoa into the egg, but here also only
monstrosities, and not twins, were produced. On the other hand, it has
been shown that in birds twins may be produced from the same egg, and
there is no reason for the belief that their production is due to
multiple impregnation. But if it may be assumed that human twins, when
identical, have been derived from a single egg, it seems to me to be
extremely probable that fertilization was also effected by a single
sperm-cell. We cannot understand how such a high degree of similarity
could have been produced if two sperm-cells had been made use of, for
we are compelled to assume that two such cells would very rarely
contain identical germ-plasms.
It is most probable that the egg-nucleus coalesces with the nucleus of
a single spermatozoon, but the resulting segmentation-nucleus divides
together with the cell-body itself, without the occurrence of those
ontogenetic changes in the germ-plasm which normally take place. The
nucleoplasm of the two daughter-cells still remains in the condition of
germ-plasm, and its ontogenetic transformation begins afterwards—a
transformation which must of course proceed in the same way in both
cells, and must lead to the production of identical offspring. This is
at least a possible explanation which we may retain until it has been
either confirmed or disproved by fresh observations,—an explanation
which is moreover supported by the well-known process of budding in the
eggs of lower animals.
VI. RECAPITULATION.
To bring together shortly the results of this essay:—the fundamental
fact upon which everything else is founded is the fact that _two_ polar
bodies are expelled, as a preparation for embryonic development, from
all animal eggs which require fertilization, while only _one_ such body
is expelled from all parthenogenetic eggs.
This fact in the first place refutes every purely morphological
explanation of the process. If it were physiologically valueless, such
a phyletic reminiscence of the two successive divisions of the
egg-nucleus must have been also retained by the parthenogenetic egg.
In my opinion the expulsion of the first polar body implies the removal
of ovogenetic nucleoplasm when it has become superfluous after the
maturation of the egg has been completed. The expulsion of the second
polar body can only mean the removal of part of the germ-plasm itself,
a removal by which the number of ancestral germ-plasms is reduced to
one half. This reduction must also take place in the male germ-cells,
although we are not able to associate it confidently with any of the
histological processes of spermatogenesis which have been hitherto
observed.
Parthenogenesis takes place when the whole of the ancestral
germ-plasms, inherited from the parents, are retained in the nucleus of
the egg-cell. Development by fertilization makes it necessary that half
the number of these ancestral germ-plasms must be first expelled from
the egg, the original quantity being again restored by the addition of
the sperm-nucleus to the remaining half.
In both cases the beginning of embryogenesis depends upon the presence
of a certain, and in both cases equal, quantity of germ-plasm. This
certain quantity is produced by the addition of the sperm-nucleus to
the egg requiring fertilization, and the beginning of embryogenesis
immediately follows fertilization. The parthenogenetic egg contains
within itself the necessary quantity of germ-plasm, and the latter
enters upon active development as soon as the single polar body has
removed the ovogenetic nucleoplasm. The question which I have raised on
a previous occasion—‘When is the parthenogenetic egg capable of
development?’—now admits of the precise answer—‘Immediately after the
expulsion of the polar body.’
From the preceding facts and considerations the important conclusion
results that the germ-cells of any individual do not contain the same
hereditary tendencies, but are all different, in that no two of them
contain exactly the same combinations of hereditary tendencies. On this
fact the well-known differences between the children of the same
parents depend.
But the deeper meaning of this arrangement must doubtless be sought for
in the individual variability which is thus continuously kept up and is
always being forced into new combinations. Thus sexual reproduction is
to be explained as an arrangement which ensures an ever-varying supply
of individual differences.
------------------------------------------------------------------------
Footnotes for Essay VI.
Footnote 234:
See Berichten der Naturforschenden Gesellschaft zu Freiburg i. B.,
Band III. (1887) Heft I, ‘Ueber die Bildung der Richtungskörper bei
thierischen Eiern,’ by August Weismann and C. Ischikawa.
Footnote 235:
Vol. I. p. 60.
Footnote 236:
The most recent example of this kind is afforded by the excellent
work of O. Schultze, ‘Ueber die Reifung und Befruchtung des
Amphibieneies,’ Zeitschr. f. wiss. Zool., Bd. XLV. 1887. Schultze has
proved that two polar bodies are expelled from the egg of the Axolotl
and of the frog, although all previous observers, including O.
Hertwig, had been unable to find them. Thus the latter authority
states as the result of an investigation specially directed towards
this point, that the nucleus is transformed in a peculiar manner
(‘Befruchtung des thierischen Eies,’ III. p. 81).
Footnote 237:
O. Hertwig, ‘Beiträge zur Kenntniss der Bildung, Befruchtung, und
Theilung des thierischen Eies,’ Morpholog. Jahrbuch, I, II, and III.
1875-77.
Footnote 238:
H. Fol, ‘Recherches sur la fécondation et le commencement de
l’hénogénie chez divers animaux.’ Genève, Bâle, Lyon, 1879.
Footnote 239:
Bütschli, ‘Entwicklungsgeschichtliche Beiträge,’ Zeitschr. f. wiss.
Zool. Bd. XXIX. p. 237. 1877.
Footnote 240: