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.'
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.