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.
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.