there was lacking, apart from technical methods, an appreciation of the
morphological value of the ovum and the spermatozoon. It was necessary
to recognize both ovum and spermatozoon as _cells_ before their union
in fertilization could be regarded as the fusion of two cells, as a
copulation or conjugation of two minute elementary organisms. But this
knowledge only gained ground very gradually, and even in the sixties
opinions on the subject were very much divided. Moreover, there was an
entire absence of knowledge in regard to 'sexual' reproduction among
the lower plants, the Algæ, Fungi, Mosses, and Ferns, as well as of
any detailed acquaintance with the processes of fertilization among
flowering plants. All this had to be elucidated by the labours of many
distinguished observers before it was possible to say so much even as
this, that the process of fertilization depends in general on the union
of two cells.
I need not discuss the whole of this long process of scientific
development, and have only touched upon it because I wished to
emphasize that the conception of the process of fertilization was
for a long time quite erroneous, and has only attained to clearness
in recent times. Pairing as it is seen in the higher animals was for
long regarded as the essential part of the process, and a mysterious
life-awakening influence was assumed in regard to it; and even when it
was understood that not the copulation, but the union of two living
units which was always brought about thereby--the union of the male
and the female germ-cells--was the essence of 'fertilization,' this
was still regarded as a life-awakening process, and the way to a true
understanding of the facts was thus once more blocked.
The simplest form of sexual reproduction in many-celled animals
is found, among others, in the Volvocineæ, those green, spherical,
freshwater cell-colonies which we have already studied in relation to
reproduction by asexual germ-cells. Among them it is the rule that,
after a long series of generations producing only 'asexual' germ-cells,
colonies occur in which each germ-cell is no longer able to develop a
new colony alone, but can do so only after it has united with another
germ-cell.
Now, as we have seen, there are Volvocineæ in which the differentiation
of cells into those of the body (soma) and those concerned with
reproduction has not been established, and all the cells are therefore
alike. In these, as for instance in the genus _Pandorina_ (Fig. 62,
p. 257), when sexual reproduction is to occur the whole colony breaks
up into sixteen cells; these burst forth from the gelatinous matrix
in which they have been hitherto enclosed, swim about in the water
with the help of their two flagella, meet other similar free-swimming
cells and conjugate with these. The two swimming cells come close to
each other, draw in their flagella, sink to the ground in consequence,
and fuse completely both as to the cell-body and the nucleus. They
assume a spherical form, lose the eye-spot, become surrounded with a
tough cell-skin or cyst, and so remain for a longer or shorter time as
so-called 'zygotes' or lasting spores. Then they develop by repeated
cell-division into one of the sixteen-celled _Pandorina_ colonies with
which we are already familiar; this bursts forth from the capsule and
swims freely about in the water again.
Here, therefore, the so-called sexual reproduction depends on the
fusion of two cells similar in appearance, and when this phenomenon
was first known it was regarded as something quite different from the
corresponding reproduction in other multicellular organisms. But we
now know that quite nearly related Volvocineæ belonging to the genus
_Volvox_ and to other genera, which exhibit a differentiation into
body-cells and reproductive cells, may reproduce sexually by means of
two _different_ kinds of germ-cells; and we have also learned through
Goebel and others that even genera like _Pandorina_, which consist of
only one kind of cells, may yet produce male and female reproductive
cells differing essentially in form from one another. In _Eudorina_,
for instance, a gelatinous sphere containing sixteen or thirty-two
individual cells, asexual reproduction occurs in exactly the same way
as in _Pandorina_, that is, each of these cells divides four or five
times in rapid succession, and thus forms a new colony, which then
bursts forth; but when the time for sexual reproduction comes the
colonies behave differently, for some become female and some male.
In the former the cells remain as they were before, but in the male
colonies the sixteen or thirty-two cells undergo a peculiar process
of division, which ends in each becoming a mass (16-32) of so-called
'zoosperms,' that is, minute, narrow, longitudinally elongated cells
with two flagella (Fig. 63 at _D_ shows those of _Volvox_). In
_Eudorina_ they differ from the female germ-cells or ova externally
in form and size, as well as by being much more actively motile, and
they contain green and subsequently yellow colouring matter, and a
red eye-spot. We here find, for the first time among multicellular
organisms, the differentiation of male and female germ-cells; and we
learn from this that the essence of fertilization does not lie in this
differentiation, since it may be absent, but that this distinction
of female and male cells is only of secondary moment. From the fact
that the egg-cells are larger and less active, the 'sperm-cells'
or zoosperms smaller and livelier, we can already anticipate what
will be more definitely established as our knowledge of the facts
increases--that a differentiation according to the principle of
division of labour has taken place even in the germ-cells, and that the
first effect of this is to render the meeting of the cells destined for
conjugation easier and more certain. The much smaller and more slender
zoosperms swim about in the water in clusters until they come in
contact with a female colony; then they separate from each other, bore
their way into the soft jelly of the female colony, and 'fertilize' the
egg-cell, that is to say, each male cell fuses with a female cell and
forms a 'lasting spore,' exactly as in _Pandorina_.
[Illustration: FIG. 63. _Volvox aureus_, after Klein and Schenck.
_A_, besides the small flagellate somatic cells of the colony there
are five large egg-cells (_t_) which are capable of parthenogenetic
development, three recently fertilized egg-cells (_o_) and a number of
male germ-cells (_a_) in process of multiplication. From each of these,
by continued division, a bundle of spermatozoa arises. _B_, a bundle
of thirty-two sperm-cells in process of development, seen from above.
_C_, the same seen from the side. Magnified 687 times. _D_ individual
spermatozoa, magnified 824 times.]
In _Volvox_ the state of matters is similar to that in _Eudorina_;
here again, in addition to the 'asexual' reproduction through the
'Parthenogonidia' which are like egg-cells in appearance (Fig. 63,
_A_, _t_), there are also male and female germ-cells which are usually
produced alternately with the former, but sometimes at the same time,
as in Fig. 63. The egg-cells are large and have no flagella, the
sperm-cells lie together in clusters, and after they are mature (_D_)
they swim freely in the water and then bore into another colony, where
each unites with an egg-cell. The difference between the two kinds of
cells consists therefore in the much greater number, the much smaller
size, and the greater activity of the male cells, and in the smaller
number but much larger size of the female cells--a differentiation
in accordance with the principle of division of labour, depending on
the fact that the two kinds of cells must reach each other, and yet
must contain a certain mass of living protoplasm. While the small size
but large number of male cells, combined with their motility, gives
them an advantage in seeking out and boring into the female cells,
the large size of the latter, on the other hand, makes up for the
loss in mass which the fertilized egg would otherwise suffer from the
diminution in size of the male cell. This difference in size may be
greatly accentuated; thus in one of the brown sea-wracks, for instance,
the spermatozoa are only 5 micro-millimetres in length, while the ova
are spherical and have a diameter of 80-100 micro-millimetres, thus
containing a mass 30-60,000 times greater (Möbius). Fig. 64 shows an
ovum of this species surrounded by spermatozoa.
In the course of the evolution of species this contrast between female
and male germ-cells became more and more marked, not always in the same
direction, however, but in one or another according to the conditions
of fertilization. It would be erroneous to suppose that, with the
higher differentiation of the organism as a whole, the differentiation
of the germ-cells became increasingly complex. On the contrary we find
even among Algæ, as the case of _Fucus_ shows, a marked difference
between the sex-cells, which rather decreases than increases among many
of the higher plants. It is not on the more or less complex structure
of the organism itself that the nature and degree of the dimorphism
of the germ-cells depends, but on the special conditions which are
involved in each case, both in the union of the two kinds of sex-cells
and in the subsequent development of the product of this union, the
'fertilized ovum.'
[Illustration: FIG. 64. _Fucus platycarpus_, brown sea-wrack. _Ei_,
ovum, surrounded by swarming sperm-cells (_sp_). After Schenck.]
Thus it comes about that the male or 'sperm-cells' of the lower plants,
of the lower animals, and, again, of the highest animals are similar
in structure. In all these organisms the male germ-cells exhibit the
minuteness, the form, and the activity of the so-called 'zoosperms'
or 'spermatozoa,' that is to say, they are thread-like, very minute
corpuscles, which move rapidly forwards in water or other fluid with
undulatory movements, and penetrate into the ovum with similar boring
movements when they have been fortunate enough to reach their goal. At
the anterior end they possess a more or less conspicuous thickening,
the so-called 'head' in which the nucleus lies, and this is followed
by the 'tail,' a thread-like structure consisting of cytoplasm which
effects undulatory movements comparable to those of the flagella
of Infusorians and Volvocineæ. The whole spermatozoon is thus a
specialized 'flagellate cell.'
When these 'zoosperms' were recognized as the 'fertilizing elements' in
higher animals, and when 'sperm-threads' had been found, not only in
all mammals and birds, reptiles, amphibians, and fishes, but even in
many 'invertebrates,' the conclusion was suggested that the function
of fertilization might be discharged by this lively motile substance;
for until the eighth decade of the nineteenth century fertilization was
still regarded by many as an 'awakening of life' in the egg. Since life
depends on movement, in truth on infinitely fine molecular movements,
of which the movement of the whole germ-cell from place to place is
only a visible outcome, fertilization was pictured, by a not very
luminous process of reasoning, as the awakening of life in the ovum--in
itself incapable of further life--through the transference to it of
movement through the agency of the zoosperm. Some investigators even
went the length of regarding the ovum as 'dead organic material.'
I mention this at this point, though I do not propose in the meantime
to inquire further into the significance of the conjugation of the
sex-cells. But the view just referred to is so completely refuted even
by the external form of the male germ-cells in many groups of plants
and animals, that I cannot discuss these differences in form without at
the same time indicating the conclusions which they directly suggest.
The great majority of plants and animals exhibit the zoosperm form of
male germ-cells, and this must obviously be interpreted in the light of
the fact that the ova to be fertilized are not generally to be found in
direct proximity to the sperms shed by the male organism, but are at
some distance from them. Among Medusæ and Polyps both male and female
germ-cells are liberated into the water, simultaneously it may be,
but separated from each other by distances of some feet or yards. The
spermatozoa then swim about seeking the ova, which are also floating
freely in the sea, guided by a power of attraction on the part of the
latter--an attraction of whose nature we know nothing, though in the
case of certain fern-ova it has been traced to the secretion of malic
acid (Pfeffer).
The same conditions obtain among Sponges. Here, again, the persons or
stocks are either male or female; the latter produce large delicate
ova, which lie in the interior of the sponge and there await the
fertilizing sperms; the former give off the ripe sperms into the water
in such abundance that thousands and millions of zoosperms burst
forth simultaneously in all directions; these seek about for a female
sponge, penetrate into its canal system, and so ultimately reach the
ova. Of course only a very few of them reach their goal; the greater
number are lost in the water and become the prey of Infusorians,
Radiolarians, or other lowly animals. The fact that enormous numbers
thus miss their true aim shows us why these zoosperms must be produced
in such quantities; it is simply an adaptation to the extraordinarily
high ratio of elimination in these cells, just as the number of young
annually produced by an animal, or of seeds by a plant, is regulated
by natural selection according to the ratio of elimination of the
particular species. The more numerous the descendants which succumb
each time to unfavourable circumstances, to enemies, or to lack of
food, the more prolific must the species be. The same holds true of
the regulation of the number of male germ-cells to be produced by an
individual; there must be so many developed that, in spite of the
unavoidable enormous loss, on an average the number of mature ova
necessary to the maintenance of the species always receive spermatozoa.
Also associated with the prodigal production of zoosperms is their
minuteness, for the more zoosperms that can be developed out of a
given mass of organic substance the smaller they are. Each species
is restricted within definite limits of production by its size and
the volume of its body, and there is thus an advantage in having the
zoosperms of the smallest possible size whenever the chance of the
individual sperm successfully reaching an ovum is very small. In all
such cases nature has abstained from burdening the male germ-cell with
an appreciable contribution of material to the result of conjugation,
that is, to the foundation of the new organism; the passive ovum
contains in itself alone almost all that is necessary to the building
up of the embryo. Fertilization of the ovum by the liberation of the
sperm-cells into the water occurs not only in animals of low degree,
such as Sponges, Medusæ, Star-fishes, Sea-urchins and their relatives,
but also in much higher animals, such as many Fishes and Amphibians,
and in these the male cells have the form of motile threads. But the
spermatozoon-form of male cell does not occur only in animals and
plants which live in the water, or in those which, like mosses and many
vascular plants, are at least occasionally covered by a thin layer of
rain or dew, in which the zoosperms can swim to the ova, it occurs also
in a very large number of animals in which the sperms pass directly
into the body of the female, in those, therefore, in which copulation
takes place.
But even where copulation occurs we find that in most cases, as, for
instance, in Vertebrates, Molluscs, and Insects, the zoosperm-form
is retained. The reason for this is obviously twofold: in the first
place, in many cases the sperms do not directly reach the ovum as a
consequence of copulation, but may have to go a long way within the
body of the female, as in mammals; or even when the way is short and
certain, the ovum may be encased in a firm covering or shell difficult
to penetrate, and the thread-like zoosperm has to face the task of
boring its way through this shell, or slipping in through a minute
opening, the so-called micropyle. In either case it would be difficult
to imagine a form of sperm-cell better adapted to the fulfilment of
this task than that of a thread with a thin, pointed head-portion and
a long motile tail, which enables the zoosperm to twist itself like a
screw through a narrow opening in the egg-envelope, whether the opening
was previously present or not.
We can thus understand why, among insects for instance, the male cells
should always occur in the form of zoosperms, although in this case
they reach a special receptacle in the female reproductive organs,
the 'receptaculum seminis,' and are stored up in this. When a mature
ovum gliding downwards through the oviduct comes to the place where
this receptacle opens into it, the liberation of a few sperm-cells
suffices to fertilize it with certainty, provided that they possess the
thread-like form, which allows them to slip in through the very minute
opening in the egg-envelope. It might be inferred from the certainty
with which the ovum must in this case be found by the spermatozoon
that only a small number of the latter would require to be produced,
and yet even here the number is very large, though not so enormous
as in the sea-urchins and other marine animals, which simply allow
the sperm-cells to escape into the water. The large number in insects
is due to the fact that many of the sperms may miss the micropyle;
and also that in many insects a very large number of eggs have to be
fertilized in succession. In the course of a life lasting three or
four years the queen bee lays many thousand of eggs, most of which are
fertilized, and that from a seminal receptacle which has been filled
only once.
There are, however, other sperm-cells of thread-like form which are
not produced in such enormous multitudes, but in a much more moderate
number, perhaps a few hundreds in the testicle. This is so in the
little Crustaceans, known as Ostracods, all the freshwater species of
which possess zoosperms only moderately numerous and of quite unusual
size.
The comparatively small number is explained by the certainty with which
each of them reaches the ovum, and the large size may be accounted
for in part by the small number which suffices, and which, therefore,
admits of the male cell also carrying a considerable portion of the
material for the building up of the embryo. Probably, however, the
thickness and firmness of the covering of the ovum has something to
do with it, for it has no opening for the entrance of the male cell,
and it is fully hardened by the time fertilization takes place.
Perhaps nowhere can we see more clearly how every little detail of the
structure of the organism is dominated by the principle of adaptation
than in the arrangements for fertilization, and notably in those which
obtain in the Ostracods. I pass by the complicated apparatus for
copulation, since we do not yet understand it fully in all particulars.
According to my own investigations and those of my former students,
Dr. Stuhlmann and Dr. Schwarz, the essential point seems to be that
the colossally large zoosperms, which show no activity within the
body of the male, leave it one at a time, so to speak, in single
file. In copulation they are pressed out singly, one after the other,
through a very fine tube, and then they enter, still singly, through
the reproductive aperture of the female into an equally fine passage
with spiral windings, through which they ultimately reach a roomy
pear-shaped receptacle, the 'receptaculum seminis' of the female. There
they lie in a long band composed of several hundreds, and only now
attain their full maturity by throwing off an outer cuticle--moulting,
so to speak. It is only when they get into a fluid medium that they
show the power of undulatory movement, feeble at first, but gradually
more energetic and more violent. And these movements enable them to
penetrate like gimlets into the calcareous egg-shell. In the normal
course it happens that when a mature ovum is deposited from the opening
of the oviduct, one of the giant zoosperms at the same time, or shortly
afterwards, leaves the 'receptaculum seminis' of the female by way of
the spiral passage, and reaches the exterior just behind the ovum. The
actual process of penetration has not been observed as yet, but the
zoosperm has been seen at a slightly later stage spirally coiled inside
the ovum.
[Illustration: FIG. 65. Copulation in a Daphnid (Lyncæid). Emptying of
the sperm (_sp_) into the brood-chamber of the female (♀). _abd_ ♂, the
abdomen of the male. Magnified 100 times.]
In these Ostracods the sperms are often visible with the naked eye, and
in some species they are twice the length of the animal; they are thus
emphatically giant cells, which can develop a very considerable boring
power.
In respect to the various adaptations of the sperm-cells to the
conditions of fertilization there is hardly any group more interesting
than the water-fleas or Daphnids.
It is amazing how greatly the size of the sperms varies among the
Daphnids, and how it stands in inverse proportion to their number,
and how both are obviously regulated in relation to the difficulties
which stand in the way of each sperm-cell before it can reach the ovum.
In some species the sperm-cells are very large, in others extremely
small. In the genera _Daphnia_, _Lynceus_, and others, copulation
occurs as shown in Fig. 65; the sperm-cells (_sp_) are liberated by
the male into the capacious brood-cavity of the female, which at the
moment is closed to some extent by the abdomen of the male, in reality
closed only partially at the posterior end and at the sides. It seems
inevitable that a large proportion of the male elements should stream
out again and be lost because of the violent movements of both animals.
Accordingly, we find that the sperm-cells are only about the hundredth
part of a millimetre in length and of round or rod-like form, and are
voided in multitudes into the brood-cavity. Fig. 66, _f_, _g_, and
_h_, show such cells in different species, as they occur in the testes
to the number of many thousands. But in all the species in which the
brood-cavity is _closed_, and in which therefore there is not such a
serious loss of sperm-cells, the elements are much larger, and at the
same time less numerous. They are largest and least numerous in species
of genera like _Daphnella_, _Polyphemus_, and _Bythotrephes_, in which
the males have a copulatory organ, so that the possibility of loss
of the male cells is excluded. Thus the round, delicate, and viscid
sperm-cells of _Bythotrephes_ (Fig. 66, _b_) are more than a tenth of a
millimetre in length, but they are developed in proportionately smaller
numbers, so that more than twenty are never found in the testis, and
often only six or eight, while in copulation only from three to five
are ejected. But as there are only two eggs to be fertilized at a time,
and as the male cells are expressed into the brood-cavity directly upon
the eggs, so that they immediately adhere to them, this small number is
amply sufficient.
[Illustration: FIG. 66. Spermatozoa of various Daphnids. _a, Sida._
_b, Bythotrephes._ _c, Daphnella._ _d, Moina paradoxa._ _e, Moina
rectirostris._ _f, Eurycercus lamellatus._ _g, Alonella pygmæa._ _h,
Peracantha truncata._ All magnified 300 times.]