Page:Popular Science Monthly Volume 62.djvu/310

304 quently receive two to four times more protoplasma and food material than the smaller male gametes. This is an important distinction and fundamental to all sexual evolution. The differentiation of one of the gametes as the cell specialized to hold the greater part of the food supply marks the first step in the series of changes that follow.

Eudorina and Volvox are heterogamous. The eggs are large, the sperms highly specialized (see Fig. 2, c and d) and in all respects the differentiation of sex seems to be carried about as far as in any group of algae. The eggs are formed singly in the mother cell (oogonium), which means that all the protoplasm and all the food available is reserved for that gamete and this is a great advance over the lower types of the Volvocaceae. The sperms are produced abundantly in each antheridium, 64 in Eudorina and sometimes more than 200 in Volvox. Naturally one would expect the sperms to be minute and they are proportionally very many times smaller in relation to these eggs than the male gametes of Chlamydomonas are to the female.

The condition of heterogamy and its advance over isogamy is the result of several well-defined factors at work wherever sex is present. It is advantageous to specialize one gamete for the purpose of holding as much nourishment as possible, thereby providing well for the vegetative possibilities of the next generation. The most effective way to accomplish this end is to reduce the number of female gametes produced in each oogonium, and the best results will obtain when all the protoplasm from such a mother cell goes to a single female gamete. The absence of motility characteristic of eggs is an accompaniment of the increased food supply. It is a very natural condition and advantageous to the species. In the first place large cells can not move through the water as easily as small cells, and again most of the higher algae have the habit of retaining the eggs in oogonia, protecting them in that way for long periods and of course such eggs must naturally be quiescent. With respect to sperms the evolutionary tendencies are very easily understood. Relieve them of the responsibility of contributing much food material to the egg and it is obviously a great advantage to the organism that sperms be produced as numerously as possible, consistent with economy of energy. This demands the reduction in size of the sperm and also results in that high specialization of form so characteristic of motile male elements.

It is evident that all the factors work for the good of the species and are so vital in their bearings as to fall well within the sphere of natural selection. There are doubtless physiological principles taking part in the developmental processes of gamete formation, and at the higher levels of sexual differentiation important factors of heredity. But these have not been very clearly separated, except that the egg