nearly related species of Daphnids, as a glance at Fig. 66 will show;
and, on the other hand, how similar they may be in two species which
belong to different families, like _Bythotrephes longimanus_ (_b_),
and _Daphnella hyalina_ (_c_). The last fact may be explained as an
adaptation to similar conditions of fertilization. Both species have
effective copulatory organs, and their large delicate sperm-cells must
immediately adhere when they come into contact with the shell-less
ovum, and penetrate into it by means of amœboid processes. Conversely,
the difference between sperm-cells of allied species like _Sida
crystallina_ (_a_), _Moina rectirostris_ (_e_) and _M. paradoxa_ (_d_)
is related to different adaptations to nearly the same conditions of
fertilization. In _Sida_ (Fig. 66 _a_) the large flat sperm-cells, with
their fringed ends and their large soft surface, adhere easily to the
ova, and the same end is attained in _Moina rectirostris_ by means of
stiff radiating processes, while in the nearly related species, _Moina
paradoxa_, the male cell (_d_) resembles an Australian boomerang and
presses in like a wedge between the ova and the wall of the brood-sac.
[Illustration: FIG. 67. Spermatozoa of various animals, after
Ballowitz, Kölliker, and vom Rath. 1, man. 2, bat (_Vesperugo_). 3,
pig. 4, rat. 5, bullfinch. 6, newt. 7, skate (_Raja_). 8, beetle. 9,
mole-cricket (_Gryllotalpa_). 10, freshwater snail (_Paludina_). 11,
sea-urchin. Much magnified.]
In Fig. 67 a small selection of animal male cells is figured, all of
which take the form of sperm-threads or spermatozoa, and yet they
differ very much from one another in detail. It would undoubtedly
be of great interest to follow out these minute adaptations of the
sperm-cells to the conditions of fertilization, and to demonstrate
that their size, and especially their form, in the different species
of animals are adjusted to the special constitution of the ovum, its
envelope, and its micropyles, and to the ease or difficulty with which
it can be reached; but much information must be forthcoming before we
can even suggest, for instance, why the sperm-cell of the salamander
is so enormously long, large, and pointed at the head, while that of
Man (Fig. 67, 1) is comparatively short, with broad, flat head and a
recently discovered minute apex; or why those of Man and many fishes
(such as _Cobitis_) should be so much alike, and so on. From many
sides, however, we are led to conclude that even down to the minutest
details nothing is in vain, and that everything depends on adaptation.
In general, even the peculiarities of form already indicate this; thus
the spirally coiled structure of the head, which is especially well
developed in the spermatozoa of birds (Fig. 67, 5), in those of the
skate (7), and of the freshwater snail (_Paludina_) (10), works like a
corkscrew, and makes it possible for the spermatozoon to pierce through
the resistant envelope of the ovum. Similarly, the sharply pointed head
of the insect spermatozoon (Fig. 67, 8 & 9) seems adapted for slipping
through the minute pre-formed micropyle in the hard egg-shell.
Of the detailed and complicated structure of spermatozoa we have only
recently been made aware through the increasing perfection of the
microscope and of technical methods of investigation. Fig. 68 shows
one after a diagrammatic figure by Wilson. We see the apical point
(_sp_) for boring into the ovum, the nucleus (_n_) surrounded by a thin
layer of protoplasm, which together form the head, then the middle
portion (_m_) which contains the 'centrosome' (_c_), and the 'tail' or
'flagellum' which effects the movement of the whole and which itself
possesses a complex structure with an 'axial filament' (_ax_) and an
enveloping layer, the latter often drawn out into a spirally twisted,
undulating membrane of the most extreme delicacy, as is most clearly
seen in the newt (Fig. 67, 6).
[Illustration: FIG. 68. Diagram of a spermatozoon, after Wilson. _sp_,
apical point. _n_, nucleus. _c_, centrospere. _m_, middle piece. _ax_,
axial filament. _e_, terminal filament.]
Not in the Daphnids alone, but in other groups of Crustaceans as
well, sperm-cells of quite peculiar form occur, as, for instance, in
the crayfish and its marine relatives, the crabs and the long-tailed
Decapods. In these cases the spermatozoa bear long and stiff thorn-like
processes, which, as in the sperm-cells of _Moina_, make them adhesive,
and, according to Brandes, render it possible for them to cling among
the bristles on the abdomen of the female until one of the many eggs
leaving the oviduct comes within reach. For among these Crustacea
there is no true copulation, but the masses of sperm-cells are packed
together into sperm-packets or 'spermatophores,' and are affixed by the
male near the opening of the oviduct, where they burst and pour forth
their contents between the appendages of the female.
All these remarkable and widely divergent structures and arrangements
depend not upon chance or on the fantastic expression of a 'formative
power,' as an earlier generation was wont to phrase it; they are
undoubtedly without exception adaptations to the most intimate
conditions of fertilization in each individual case. I lay particular
stress upon a recognition of this, because it permits us to infer with
certainty that even the variations of the single cell, if they are
sufficiently important for the species, may be controlled by natural
selection. It is obvious that the adaptations of the sex-cells must
depend not on histonal selection, but only upon personal selection,
since it is indifferent for the individual sperm-cells whether
fertilization is accomplished successfully or not, while it is by
no means indifferent for the species. The organism dies without
descendants if its sperm-cells do not fertilize, and the carrying on
of the species must be left to those of its fellows which produced
sperm-cells which fertilize with more certainty; thus it is not
the sperm-cells themselves, but the individual organisms which are
selected, and that in relation to the quality of the sex-cells they
produce.
In contrast with the great diversity of form exhibited by the
spermatozoa, the differentiation of the ovum appears very uniform,
at least in regard to form and activity. The main form is spherical,
but it is subject to many variations in the way of elongation or
flattening. In the lower forms of life, as, for instance, among the
sponges, and also in the polyps and Medusæ the egg-cells possess, until
they are mature, the locomotor capacity of unicellular organisms;
they creep about after the manner of amœbæ, and indeed, as I showed
years ago, this movement from place to place in many polyps is exactly
regulated; thus at a definite time they may leave the place where they
originated and may, for instance, creep from the outer layer of cells
(ectoderm) of the animal into the inner layer (endoderm) by boring
through the so-called 'supporting lamella,' then they may creep further
in the endoderm, and finally return to quite definite and often remote
spots in the ectoderm (_Eudendrium_, Fig. 95). In another hydroid polyp
(_Corydendrium parasiticum_) the mature egg-cells leave their former
position within the endoderm and creep entirely outside of the animal
which produced them, establishing themselves in a definite spot on its
external surface, where they await the fertilizing zoosperms. Many ova
can accomplish slight amœboid movements, but in most animals these
do not suffice for movement from place to place, and the ova remain
quietly in the spot where they were developed, or are passively pushed
to another. Cases such as that of the polyp I have cited, in which the
ovum actually comes to meet the male element, are quite exceptional,
for in general the ovum is the passive and the spermatozoon the active
or exploring element in fertilization. The female cell is entrusted
with procuring and storing the material necessary to the building up of
the embryo; and its peculiarities depend chiefly on this.
[Illustration: FIG. 69. Ovum of the Sea-urchin, _Toxopneustes lividus_,
after Wilson. _zk_, cell-body. _k_, nucleus or so-called 'germinal
vesicle,' _n_, nucleolus or so-called 'germinal spot.' Below there is
a spermatozoon of the same animal, with the same magnification (750
times).]
It is true that in plants this stored material is seldom considerable,
and that is because the ovum so frequently remains even after
fertilization within the living tissues of the plant, and is thence
supplied, often very abundantly, with food-stuffs; and, moreover,
because the young plant that springs from the fertilized ovum maybe
very small and simple, and yet capable of immediately procuring its
own nourishment. But there are exceptions to this; thus the ova of the
brown sea-wracks, or Fucaceæ, for instance, are quite twenty times
as large as the ordinary cells of the algæ (Fig. 64), and contain a
quantity of food-stuff within themselves. In this case the ova are
liberated into the water even before fertilization, and the nutrition
of the embryo from the mother-plant is excluded.
In these Algæ we meet, for the first time, with a special organ in
which the ova arise. In animals this is much more generally the case,
and from sponges upwards there are always quite definite parts and
tissues of the body which are alone able to develop eggs, and these
are usually well-defined organs of special structure, the ovaries.
Similarly, in male animals the spermatozoa arise in special places, and
usually in special organs, the spermaries or testes.
Animal ova often consist of more than the simple cell-body, the
protoplasm and its nucleus; they almost always contain in the cell-body
a so-called 'Deutoplasm,' as Van Beneden has fittingly named the
yolk-substance. This consists of fats, carbohydrates, or albuminoids,
which often lie in the cell-body in the form of spherules, flakes, or
grains--a nutritive material that is often surrounded and enclosed
by a small quantity of living matter or formative protoplasm. Apart
from these stores of yolk it would be impossible for a young animal
to develop from the ovum of a snake or a bird, for such highly
differentiated animals could not be formed from an egg of microscopic
dimensions if this remained without some supply of food from outside of
itself during the period of development. There is obviously need for a
considerable amount of building material, so that all the organs and
parts, which are composed of thousands and millions of cells, may be
developed.
Thus the size of the animal-ovum depends essentially on the quantity of
yolk that has to be supplied to the egg, and this depends in the main
on whether the egg is still drawing nourishment from the mother during
the development of the young animal. Therefore, as a general rule,
eggs which are laid, and are surrounded and protected by a shell, are
usually much larger than the eggs of animals which go through their
development within the body of the mother. The best known illustration
of this proposition is offered by mammals and birds, animals of equally
high organization and comparable in bodily size. While the eggs of
birds may be as much as 15 centimetres in length, and may weigh 1½
kilogrammes, those of most mammals remain microscopically minute, and
scarcely exceed a length of 0.3 millimetres. The same principle is
often illustrated within one and the same small group of animals, and
even in the same species. Here, again, the Daphnids or water-fleas may
serve as an example.
Among these there are two kinds of eggs, summer and winter eggs, of
which the former go through their development into a young animal
within a brood-cavity on the back of the female, while the others are
liberated into the water, and are surrounded by a hard shell. The
summer eggs receive more or less nourishment from the mother by the
extravasation of the nutritive constituents of the blood into the
brood-cavity, and they thus require a smaller provision of yolk than
the winter eggs, which are thrown entirely upon their own resources.
Accordingly we find that in all Daphnids the summer eggs are at least
a little smaller and have less yolk than the winter eggs, as in the
genus _Daphnella_ (Fig. 70, _A_ and _B_), while in some species, e.g.
of _Bythotrephes_, this difference increases so much that the summer
eggs are almost without yolk, and therefore very minute (Fig. 71, _B_).
The reason of this lies in the fact that in this case the brood-sac
is filled with a nutritive fluid rich in albuminoid substances, so
that the embryo during its development is continually supplied with
concentrated nourishment. This is not the case with the winter eggs,
because these are liberated into the water, and we therefore find that
they are of enormous size and quite filled with yolk (Fig. 71, _A_).
[Illustration: FIG. 70. _Daphnella._ _A_, summer egg. _B_, winter egg.
_Oe_, 'oil-globules' of the summer egg.]
[Illustration: FIG. 71. _Bythotrephes longimanus._ _A_, the brood-sac
(_Br_) of the female containing two winter-ova (_Wei_), on which
five large sperm-cells (_sp_) are lying. _R_, dorsal surface of the
animal. _Dr_, glandular layer which secretes the shell-substance. _BK_,
copulatory canal. _B_, the brood-sac (_Br_) containing two summer-ova
(_Sei_). Both figures under the same magnification (100).]
In this instance, as in all the simpler eggs, the yolk constituents
are secretions of the cell-body of the ovum; but nature employs many
devices, if I may so speak, to bring up the mass of the egg, and
especially of the yolk, to the highest attainable point. Thus in many
orders of Crustaceans, for instance in the water-fleas just mentioned,
there are special egg-nourishing cells, that is, young ovum-cells
which do not differ from the rest either in origin or in appearance,
only they do not become mature eggs, but at a definite time cease to
make progress, and then slowly break up, so that their substance may
be absorbed as food by the true ova. Thus there is a much greater and
at the same time more rapid growth than could be attained through
nourishment from the blood alone. In the Daphnids the ovaries consist
of groups of four cells each, only one of which becomes an ovum (Fig.
72, _Ei_), while the other three (1, 2, and 4) form nutritive cells
which break up. This is so in all summer eggs; but in the winter eggs
a much larger number of nutritive cells may take part in equipping a
single ovum, and in the genus _Moina_ over forty do so. But here the
difference in size between the two kinds of eggs is very marked, the
winter eggs being twice the diameter of the summer eggs.
[Illustration: FIG. 72. _Sida crystallina_, a Daphnid: a fragment of
the ovary showing one of the groups of four cells, of which 1, 2, and 4
are nutritive cells, and only 3 becomes an ovum. Magnified 300 times.]
In many insects also, e.g. in beetles and bees, similar nutritive cells
occur, but there is in these forms a different arrangement which serves
at the same time for the formation of the shell, and the supplying to
the ovum of the necessary yolk-stuffs--the ovum is surrounded with a
dense layer of epithelial cells, a so-called 'follicle.' In mammals and
birds also these 'follicle cells' certainly play an important part in
the nutrition of the ovum, though it is not yet quite clear how they
act--whether they produce within themselves grains of yolk and other
nutritive substances and convey these to the ovum by means of fine
radiating processes, or whether they themselves ultimately migrate into
the ovum and there break up. In any case it is worthy of note that
all these follicular cells in insects and vertebrates have the same
origin as the egg-cells, that is, they are modified germ-cells. The
case is therefore essentially the same as in the nutritive cells of the
Daphnids; nature sacrifices the greater number of the germ-cells in
order to be able to provide more abundantly for the minority. She thus
succeeds in raising the egg beyond itself, so to speak, and provides
the means for a growth which could obviously not be attained by means
of the ordinary nourishment supplied by the blood.
[Illustration: FIG. 73. Diagrammatic longitudinal section of a hen's
egg before incubation, after Allen Thomson. _Bl_, germinal disk. _GD_,
yellow yolk. _WD_, white yolk. _DM_, vitelline membrane. _EW_, albumen.
_Ch_, chalaza. _S_, shell membrane. _KS_, shell. _LR_, air chamber.]
We now understand why the eggs of many animals should be of such
enormous size and often of such complex structure. The eggs of birds
are especially remarkable in this respect, and it has till recently
been disputed whether they are really morphologically equivalent to
a single cell. But this is undoubtedly the case, and though only the
small thin germinal disk (Fig. 73, _Bl_) with its nucleus is the active
part of this cell--the cell-body proper--yet all the rest--the enormous
sphere of yolk with its regular layers of yellow (_GD_) and white
(_WD_) yolk, the concentric layers of fluid albumen (_EW_) round about
this, the chalazæ (_Ch_), and finally, the delicate shell membrane
(_S_) and the limy shell (_KS_)--belong to this cell, and have arisen
in connexion with it (Fig. 73).
LECTURE XV
THE PROCESS OF FERTILIZATION
Cell-division and nuclear division--The chromatin as the material
basis of inheritance--The rôle of the centrosphere in the mechanism
of division--The Chromosomes--Fertilization of the egg of the
sea-urchin according to Hertwig--Of the egg of Ascaris according
to Van Beneden--The directive divisions, or the extrusion of the
polar bodies--Halving of the number of chromosomes--The same in
the sperm-cell--Reducing division in parthenogenetic eggs--In
the bee--Exceptional and artificial parthenogenesis--Rôle of the
centrosphere in fertilization and in parthenogenesis.
NOW that we have made ourselves acquainted with the two kinds of
germ-cells on the union of which 'sexual reproduction' depends, we may
proceed to a more detailed discussion of the process of fertilization
itself. But it is indispensable that we should take account of the
processes of nuclear and cell-division, as these have been gradually
recognized and understood in the course of the last decade. It may
appear strange that the processes of division should throw light on the
apparently opposite processes of cell-union, but it is the case, and
no understanding of the latter is possible without a knowledge of the
former.
From the time of the discovery of the cell until well on in the sixties
the process of cell-division was looked on as a perfectly simple
process, as a mere constriction in the middle of the cell. It was
observed that a cell in the act of dividing (Fig. 59, _A_) stretched
itself out, that its nucleus also became longer, became thinner in the
middle, assumed a dumb-bell form, and was then gradually constricted,
giving rise to two nuclei (_B_), whereupon the body of the cell also
constricted and the two daughter-cells were formed (_C_). In certain
worn-out or highly differentiated cells a cell-division of this kind
really seems to occur--the so-called 'direct' division--but in young
cells, and indeed in all vigorous cells, the process, which looks
simple, is, in reality, exceedingly complex. Not only is the structure
of the nucleus incomparably more complex than was recognized a quarter
of a century ago, but nature has placed within the cell a special and
marvellously intricate apparatus, by means of which the component parts
of the nucleus are divided between the two daughter-nuclei.
For a long time all that was distinguished in the cell-nucleus was
the nuclear membrane and a fluid content in which one or more nuclear
bodies or nucleoli float. But this does not by any means exhaust
what can now be recognized in the structure of the nucleus, and the
most important constituents are not even among these, for recent
researches, especially those of Häcker, have shown that the nucleolus
or the nucleoli, to which there was formerly an inclination to attach
a very high importance, must be regarded as only transient formations
and not living elements--in fact, as mere collections of organic
substance--'bye-products of the metabolism,' which at a definite time,
that is just before the division of the nucleus, disappear from the
nuclear space and are used up. We now know that in the resting cell,
that is, in the cell which is not in the act of dividing (Fig. 74,
_A_), a very fine network of pale threads, often very difficult to
make visible, fills the whole nuclear cavity, like a spider's web or
the finest soap bubbles, and that in this so-called nuclear framework
there are embedded granules of rounded or angular form (_A_, _chr_)
which consist of a substance which stains deeply with such pigments as
carmine, hæmatoxylin, all aniline dyes, &c., and which has therefore
received the name of chromatin. Often, indeed generally, these granules
are exceedingly small, but sometimes they are bigger, and in that case
they are less numerous and more easily made visible; in all cases,
however, they are in a certain sense the most important part of the
nucleus, for we must assume that it is their influence which determines
the nature of the cell, which, so to speak, impresses it with the
specific stamp, and makes the young cell a muscle-cell or a nerve-cell,
which even gives the germ-cell the power of producing, by continued
multiplication through division, a whole multicellular organism of a
particular structure and definite differentiation, in short, a new
individual of the particular species to which the parents belong. We
call the substance of which these chromatin granules consist by the
name first introduced into science by Nägeli, though only to designate
a postulated substance which had not at that time been observed,
but which he imagined to be contained within the cell-body--by the
name _Idioplasm_, that is to say, a living substance determining the
individual nature (εἶδος = form). I am anticipating here, and I reserve
a more detailed explanation until I can gradually bring together all
the facts which justify the conception I have just indicated of the
'chromatin grains' as an 'idioplasm,' or, as we may also call it, a
'hereditary substance.'
That this chromatin must be something quite special we see from the
processes of cell and nuclear division, which I shall now briefly
describe.
[Illustration: FIG. 74. Diagram of nuclear division, adapted from E.
B. Wilson. _A_, resting cell with cell-substance (_zk_), centrosphere
(_csph_) which contains two centrosomes, nucleolus (_kk_); and
chromosomes (_chr_), the last distributed in the nuclear reticulum.
_B_, the chromatin united in a coiled thread; the centrosphere divided
into two and giving off rays which unite the halves. _C_, the nuclear
spindle (_ksp_) formed, the rays more strongly developed, the nuclear
membrane (_km_) in process of dissolution, the chromatin thread
divided into eight similar pieces (_chrs_), the rays are attaching
themselves to the chromosomes. _D_, perfected nuclear spindle with
the two centrospheres at the poles (_csph_) and the eight chromosomes
(_chrs_) in the equator of the spindle, all now longitudinally split.
_E_, daughter-chromosomes diverging from one another, but still united
by filaments, the centrosomes (_cs_) are already doubled for the next
division. _F_, daughter-chromosomes, quite separated from one another,
are already beginning to give off processes; the cell-substance is
beginning to be constricted. _G_, end of the process of division:
two daughter-cells (_tz_) with similar nuclear reticulum (_tk_) and
centrospheres (_csph_), as in _A_.]
When a cell is on the eve of dividing we observe first that the
chromatin grains, which have till then been scattered throughout the
network of the nucleus, approach each other and arrange themselves
into a long thin thread which, irregularly intertwined, forms a loose
skein, the so-called coil-stage (Fig. 74, _B_). The thread then begins
to thicken, and somewhat later it can be seen to have broken up into
a number of pieces of equal length, as if it had been cut into equal
pieces with scissors (_C_).