number of animals in the most diverse groups; it is everywhere the same
in its essential features; there is always only one sperm-cell which
normally enters into conjugation with the ovum-nucleus, and in every
case the sperm-cell, however minute it may be to begin with, forms a
nucleus nearly or exactly as large as the nucleus of the ovum, and in
all cases it contains the same number of chromosomes as the nucleus of
the ovum. Of special interest, however, is the fact that this number
is always half the number of the chromosomes exhibited by the somatic
cells of the particular animal in question, and that the reduction of
the number of chromosomes to half the normal is effected in both male
and female germ-cells by the last divisions of these cells, which take
place before they have attained to a state of maturity. In the ovum the
reduction occurs in the directive divisions, to which we must therefore
turn our attention once more, with special reference to the number of
chromosomes.
We saw that, in the full-grown ovarian egg, the germinal vesicle rises
to the surface and there becomes transformed into the first polar
spindle. Now this shows, in its equatorial plane, double the number of
chromosomes normal to the species. This duplication comes about, not
directly before the nuclear division, but much earlier in the young
mother-egg-cell; it is only the change in the time of the splitting
of the chromosomes that is unusual. The first maturation division
takes place nevertheless in accordance with the usual plan of nuclear
division; it is, as I have called it, an 'equation division,' that is,
both daughter-nuclei again receive the same number of chromosomes as
the young mother-egg-cell had to start with, namely, the normal number
of the species. Thus, if the young mother-egg-cell had four chromosomes
(Fig. 76, _A_), this number would double to eight at an early stage
(_B_), but the first maturing division would give each daughter-nucleus
four (_C_ and _D_). In the second maturation division the case is
different, for here no splitting and duplicating of the number of
chromosomes takes place, but the existing number, by being distributed
between the two daughter-nuclei, is reduced to half in each (_E_ and
_F_). For this reason I have called it a 'reducing division.' In our
example, therefore, the ovum, as well as the second polar body, would
contain only two chromosomes (Fig. 76, _F_).
[Illustration: FIG. 76. Diagram of the maturation divisions of the
ovum. _A_, primitive germ-cell. _B_, mother-egg-cell, which has grown
and has doubled the number of its chromosomes. _C_, first maturation
division. _D_, immediately thereafter; _Rk1_, the first directive cell
or polar body. _E_, the second maturation spindle has been formed; the
first polar body has divided into two (2 and 3); the four chromosomes
remaining in the ovum lie in the second directive spindle. _F_,
immediately after the second maturation division; 1, the mature ovum;
2, 3, and 4, the three polar cells, each of these four cells containing
two chromosomes.]
I cannot enter into the details of the process here, for we are
dealing with essentials and not with isolated and, so to speak,
chance details, but I must emphasize the fact that the same process
of reduction of the number of chromosomes takes place in this or an
analogous manner in all animal ova, and can be demonstrated also
in most of the chief groups of plants. Whether it be, as many have
maintained, that the reduction is not always first effected by the
'maturation divisions,' but in some cases takes place earlier in the
primitive egg-cell[12], so much is certain, that the nuclei which come
together for 'fertilization' only contain half the normal number of
chromosomes, and this is true not only of the ovum but also of the
sperm-nucleus.
[12] See the discussion of this point in chapter xxii.
Arguing from general considerations, but especially from the theory
which regards the chromosomes as the bearers of the hereditary
substance, I had come to the conclusion, before there was any full
knowledge of the phenomena of the maturation of the ovum, that a
reduction of the chromosomes by half _must_ take place, and had
postulated a similar 'reducing division' for the sperm-cell, and
further, for plants as well as animals--indeed, for all sexually
reproducing forms of life. The two divisions in the sperm-cell
corresponding to the polar divisions of the ovum with their reduction
of chromosomes were demonstrated by Oscar Hertwig in the case of the
thread-worm of the horse (_Ascaris megalocephala_)--a form which
has proved so very important in relation to the whole theory of
fertilization. It is true that in this case the course of the phenomena
of reduction is less convincing than in some other forms which have
been investigated more recently, as, for instance, the mole-cricket and
the bugs. In these instances, at any rate, a 'reducing division' in
spermatogenesis, quite corresponding to that of the egg-cell, has been
demonstrated, and this demonstration is of particular value owing to
the fact that the development of the sperm-cell, as we shall presently
see, throws an entirely new light on that of the ovum, and especially
on the phyletic significance of the polar bodies.
We began our consideration of the processes of reduction with the
full-grown egg-cell, but now let us go back to the earliest rudiments
of the ovary of the embryo, and we find that it consists of a single
primitive egg-cell, from which, by division, all the other egg-cells
arise. In the same way the first rudiment in the testis or spermary
is formed by a primitive sperm-cell, which does not differ visibly
from the primitive egg-cell. Both now multiply by division for a
considerable time, and in the ovary this is followed by a period of
growth, during which multiplication ceases, and each cell increases
considerably in size and lays in a store of yolk. Each cell thus
ultimately reaches the condition with which we started previously, that
of the full-grown _mother-egg-cell_.
Although the primitive sperm-cells do not exhibit such pronounced
growth as the ova, they have likewise their period of growth, during
which multiplication by division ceases, and the cells increase only in
size (Fig. 77, _A_). When they have attained their maximum of growth
the number of chromosomes is seen to have been doubled by longitudinal
splitting (as in the diagram, Fig. 77, _B_, from four to eight).
From this _mother-sperm-cell_ there now arise by two divisions in
rapid succession (_C_-_F_) four sperm-cells, and the same reduction
of the number of chromosomes to half is effected as in the polar
divisions of the egg-cell. In the first division, four chromosomes
go to each daughter-cell (_D_), in the second, two (_F_). The only
essential difference between the corresponding processes in the
egg-cell and the sperm-cell lies in the fact that the divisions of the
so-called 'spermatocyte' or mother-sperm-cell are equal, so that four
granddaughter-cells of equal size arise, while in the mother-egg-cell
or 'ovocyte' the divisions are very unequal. In the former the result
of the divisions is _four_ cells capable of fertilizing, in the latter
_one_ cell capable of being fertilized and three minute 'polar cells'
which are incapable of conjugating with a sperm-cell and giving rise to
a new individual.
[Illustration: FIG. 77. Diagram of the maturation-divisions of the
sperm-cell, adapted from O. Hertwig. _A_, primitive sperm-cell. _B_,
mother-sperm-cell. _C_, first maturation division. _D_ 1 and 2, the two
daughter-cells. _E_, the second maturation division, by which the four
cells of _F_ arise, each with half the number of chromosomes.]
There can thus be no doubt that the polar cells, as Mark and Bütschli
long ago suggested, are abortive ova, that is, that, at a remote period
in the evolution of animal life, each of these four descendants of a
mother-egg-cell became a germ-cell capable of development. It is not
difficult to infer that the unequal division, which now leads to an
insufficient size in three of these descendants, has gone on _pari
passu_ with the continually increasing size of the mature ovum, and had
its reason in the fact that it was important above all things to store
in the ovum as much protoplasm and yolk as possible. We have already
seen that even the dissolution of a number of the sister-cells of the
ovum is sometimes demanded, so that the ovum may be surrounded by
nutritive follicular cells. In short, the greatest possible quantity of
nourishment is conveyed to the ovum in every conceivable way, and it
is thus stimulated to a growth which no single cell could attain to if
it were dependent on the ordinary nutrition supplied by the blood. And
we can understand that nature--to speak metaphorically--did not wish
to destroy her own work by finally distributing among four ova all the
nourishment she had succeeded in heaping up in all sorts of ways within
the mother-egg-cell.
But it may be asked, Why have all these unnecessary divisions been
maintained up till the present day? Why have they not long ago been
given up, since they can and do only lead to the production of three
abortive ova, which are foredoomed to perish? Are they mere vestiges,
processes which are in themselves meaningless, but have, so to speak,
been maintained by the principle of inertia? This principle is
certainly operative in some sense and to some extent even in living
nature; a process which has been regularly repeated through a long
series of generations does not at once cease to be performed when it is
no longer of use to the organism concerned. The eyes of animals which
have migrated to lightless depths do not disappear all at once and
leave no trace; they degenerate very gradually and only in the course
of many generations; and it would thus be quite possible to defend the
position that these polar or 'maturation divisions' of the ovum are
purely _phyletic reminiscences_ without actual significance.
But I cannot agree with this opinion. If it were actually so we should
expect that the formation of the polar bodies would not still take
place in all cases in almost the same manner, for all rudimentary parts
and processes vary greatly; we should expect that in many animal groups
the polar divisions would not occur, or perhaps that only half the
number would occur. But this is not so; in all multicellular organisms,
from the lowest to the highest, two reducing divisions take place,
and always in almost the same manner, with the exception of a single
category of ova, of which I shall presently have to speak. We shall see
later that even in unicellular organisms analogous processes may be
observed.
But it is also intelligible that this twice repeated division of
the mother-egg-cell is necessary if the reduction in the number of
chromosomes to half is only possible in this way, since _this reduction
is indispensable_. If each of the two conjugating germ-cells contained
the full normal number of chromosomes, the segmentation-nucleus
would contain a double number, and if that went on, the number of
chromosomes would increase in arithmetical proportion from generation
to generation, and would soon become enormous. Even though we were not
otherwise certain that these chromosomes are units of a permanent
nature, which only apparently break up in the nuclear reticulum,
but in reality persist, the fact of reduction would point in this
direction. For if they were not permanent structures and distinct from
one another, and if their number depended solely on the quantity of
chromatin which the nucleus contains, the reduction in number might be
secured if the chromosomes in the growing egg and sperm-cells increased
in size more slowly than the cell-body and the other parts of the cell.
But from the fact that the reduction takes place not in this simple
way, but, in sperm-cells and in ova which require to be fertilized,
only through cell-division and a specific mode of nuclear division, we
may conclude that it cannot happen otherwise, that chromosomes are not
mere aggregates of organic substance, but organs whose number can only
be reduced by the extrusion of some of them from the cell.
It is true that there are ova in which the process of reduction does
not follow the course we have described, but the exceptions only
serve to confirm our view of the reducing significance of the polar
divisions, and of their persistence because of the necessity for
reduction.
As far back as the middle of the nineteenth century it was known
that in various animals the eggs develop without fertilization. This
reproduction by 'parthenogenesis' was first established with certainty
by the German bee-keeper Dzierzon in 1845, and then scientifically
corroborated by Rudolph Leuckart and C. Th. von Siebold. Although
parthenogenesis was at first observed only in a few groups of the
animal kingdom, in bees and some nocturnal Lepidoptera (Psychidæ
and Tineidæ), it has become more and more apparent in the course of
years that this 'virgin reproduction' is by no means a rare form
of reproduction, and that it occurs regularly and normally in many
cases, especially in the very diverse groups of the great series of
Arthropoda. Thus among insects it is found in certain saw-flies,
gall-flies, ichneumon-flies, in the honey bee, and in common wasps, and
it is particularly widespread among plant-lice (Aphides) such as the
vine-aphis (_Phylloxera_), whose prodigious multiplication in a short
time depends partly on the fact that all the generations, with the
exception of one, consist only of females with a parthenogenetic mode
of reproduction.
Among the lower Crustaceans also parthenogenesis plays a large rôle,
and in many species it even occurs as the sole mode of reproduction,
but more often--as is also the case among insects--it occurs
alternately with bi-sexual reproduction. For parthenogenesis must not
be regarded as asexual reproduction, but rather as _unisexual_, that
is, as arising from sexually differentiated individuals (females),
and from germ-cells (true ova), but brought about by the agency of
individuals of only one sex, the female. These parthenogenetic eggs
emancipate themselves, so to speak, from the law that was previously
regarded as without exception, that all ova require fertilization to
enable them to develop. That this law admits of many exceptions is
now universally admitted; thus in the small family of water-fleas
(Daphnids) there are even two kinds of eggs, the summer and winter eggs
we have already mentioned, which are produced by the same female, and
yet the former kind develop without fertilization, while the latter
require to be fertilized before they can develop.
It was obviously important to learn the state of affairs in regard to
reducing divisions in parthenogenetic ova, to find out whether here
also, three, or, in some circumstances, two polar bodies were formed,
and whether the second polar division reduced the number of chromosomes
to half. If the theory previously advanced as to the importance of the
chromatin, and especially of the reducing effect of the second maturing
division be correct, we should expect the second division to be wanting
in parthenogenetic eggs, since otherwise the number of chromosomes
would be reduced to half in each generation, and would thus gradually
disappear or sink to one.
Having directed my attention to this problem, I succeeded in
establishing for a Daphnid, _Polyphemus_, that the second polar
division does not occur, and that only one polar body is formed.
Blochmann found the same in the parthenogenetic eggs of plant-lice or
Aphides, in which, moreover, the eggs requiring fertilization exhibit,
like the winter eggs of Daphnids, two polar divisions. It was thus
established that at least those eggs of Aphides and Daphnids which
are wholly parthenogenetic retain the full number of chromosomes
of their species, as is represented in the diagram, Fig. 78. When
parthenogenesis set in the polar divisions were limited to one, and
that this could happen justifies us in concluding _a posteriori_
that it could have happened also in the case of ova which required
fertilization if that had been necessary or even merely indifferent.
The polar divisions are thus not mere 'vestigial' processes; they have
an immediate significance, and it lies in the reduction of the number
of chromosomes.
But I must make a reservation here; it is not universally true of
parthenogenetic eggs that maturation takes place without the second
polar division. The first exception was observed in the salt-water
crustacean, _Artemia salina_. In this case only one polar body is
actually extruded and the number of chromosomes remains normal, as I
was able to demonstrate with the small number of ova at my disposal;
but according to the investigations of Brauer on more abundant material
it appears that, while the second polar division is suppressed in the
majority of the ova, and the external extrusion of a second polar body
never occurs, the second polar division does nevertheless sometimes
take place. The two daughter-nuclei arising from this division unite
again immediately afterwards to form a single nucleus, and this now
functions as a segmentation nucleus. Of course it again contains the
full number of chromosomes, namely, twice 84=168.
In _Artemia_, therefore, the adaptation of the ova to parthenogenetic
development is not yet fully established, and the complete abandonment
of the second polar division seems to be phyletically striven for,
since, although the division still takes place, its effect is
neutralized immediately afterwards.
[Illustration: FIG. 78. Diagram of the maturation of a parthenogenetic
ovum. The number of chromosomes normal to the species has been assumed
to be four. _Uei_, a primitive germ-cell. _M Eiz_, a mother-egg-cell,
with twice the normal number of chromosomes. _Eiz_, mature ovum after
the separation of the first and only polar body. _Rk_^1.]
Among bees the state of affairs is again exceptional. Here the female,
the so-called queen bee, possesses a capacious sperm-sac, in which
the spermatozoa received in copulation remain living for years,
and the fertilization of an ovum is effected in the usual way from
this sac while the egg from the ovary is passing down the oviduct.
The queen bee has the power of releasing some spermatozoa from the
receptacle, or of not doing so, and thus of fertilizing the egg, or
of not fertilizing it. Since the notable observations of Dzierzon and
the investigations of von Siebold and Leuckart which followed them, it
has been assumed that only those eggs were fertilized which were laid
in the cells destined for rearing females (workers or queens), while
those which were to give rise to 'drones' or males remained normally
unfertilized. Only in the last decade of the past century did the
bee-keepers begin to cast doubt on this so-called 'Dzierzon theory';
various violent and obstinate attacks were made upon it, and these
were supported by new and apparently convincing experiments. Dickel, a
teacher in Darmstadt, has been particularly strenuous in attempting to
overthrow the old theory, by emphasizing the fact that von Siebold's
old investigations on bee eggs afforded no convincing proof. Von
Siebold made his investigations on eggs freshly taken from the hive,
and was never able to find spermatozoa in 'drone eggs' (that is, eggs
laid in drone cells and therefore destined to develop into drones),
while he was often able to demonstrate the presence of from one to
four spermatozoa in 'worker eggs.' But he only examined 'drone eggs'
which were already twelve hours old, and in these, as we now know, he
would not have found spermatozoa in any case, even if they had been
fertilized, because in ova at that stage the development of the embryo
has already fully begun, and nothing remains of the spermatozoa. In
the bee, according to Buttel-Reepen, the fertilizing spermatozoon is
transformed in twenty minutes after it has penetrated into the egg into
a minute 'sperm-nucleus' which is almost invisible even in sections,
and certainly nothing whatever could be seen of it by the old method of
squeezing the fresh ovum.
It had therefore to be admitted that Dzierzon's theory rested on an
insecure foundation, and I accordingly set two of my students at that
time, Dr. Paulcke and Dr. Petrunkewitsch, to examine the eggs of the
bee anew with regard to the point in question, using the greatly
improved methods at their disposal. These investigations have been
carried out in the Freiburg Zoological Institute during the last three
years, and have resulted in establishing the absolute correctness of
Dzierzon's theory: the 'drone eggs' do remain unfertilized, while the
eggs from which females are to develop are fertilized without exception.
In this case, therefore, we have, in the same animal, eggs which
can be fertilized and eggs which, without fertilization, develop
parthenogenetically, and it is therefore of the greatest possible
interest to know the state of matters in them in regard to the
directive divisions and the reduction of the chromosomes.
Dr. Petrunkewitsch's investigations have shown that in both cases,
that is, whether a spermatozoon penetrates into the ovum or does
not, a twice-repeated division of the nuclear material in the ovum
takes place. Moreover, the two daughter-nuclei which result from the
second division do not, as Brauer showed was sometimes the case in
_Artemia_, unite again afterwards; they remain separate, and the
number of chromosomes--there are sixteen of them--is thereby reduced
to half in the segmentation nucleus. But this is not all, for before
embryonic development has begun the normal number can be again seen in
the segmentation nucleus; the chromosomes must therefore have _doubled
their number by division within the nucleus_.
It is probable that something similar takes place in the cases of
exceptional parthenogenesis which have long been known, but this point
has not yet been sufficiently investigated. Nevertheless I cannot pass
them over, as they are instructive from another point of view.
[Illustration: FIG. 79. The two maturation divisions of the 'drone
eggs' (unfertilized eggs) of the Bee, after Petrunkewitsch. _Rsp 1_,
the first directive spindle. _k 1_ and _k 2_, the two daughter-nuclei
of the same. _Rsp 2_, the second directive spindle. _k 3_ and _k 4_,
the two daughter-nuclei. In the next stage _k 2_ and _k 3_ unite to
form the primitive sex-nucleus. Highly magnified.]
In some silk-moths (Bombycidæ) and hawk-moths (Sphingidæ), especially
in the silk-moth proper (_Bombyx mori_), in _Liparis dispar_, and
in quite a number of other Lepidoptera, it sometimes happens that,
out of a large number of unfertilized eggs, a few will develop and
produce caterpillars. This is interesting enough, but it gains
increased importance through the investigations of the Russian
naturalist, Tichomiroff, who succeeded in considerably increasing
the number of unfertilized eggs that developed by gently rubbing
them with a paint-brush, or by dipping them for a little in dilute
sulphuric acid. It is thus possible to make eggs, which would not
ordinarily develop without being fertilized, capable of parthenogenetic
development by means of mechanical or chemical stimulus. This sounds
almost incredible, but it is beyond a doubt, and it is still further
corroborated by the fact that Prof. Jacques Loeb has succeeded in
inciting the eggs of a sea-urchin to parthenogenetic development by
means of a chemical stimulus. When he added to the sea-water in which
the eggs were laid a certain quantity of chloride of magnesium the
ova developed, and not only went through the process of segmentation,
but even reached the stage of the quaint easel-like Pluteus larva.
Quite recently Hans Winkler has made the interesting observation
that from sea-urchin sperms which have been killed by heat it is
possible to extract in aqueous solution a substance capable of exciting
unfertilized sea-urchin eggs to development, although they only go as
far as to the sixteen-cell stage.