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_.