3 o 4 POPULAR SCIENCE MONTHLY. 



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 V'olvocaceae. 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, pro- 

 tecting them in that way for long periods and of course such eggs must 

 naturally be quiescent. With respect to sperms the evolutionary tend- 

 encies 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 specializa- 

 tion 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 

 bigher levels of sexual differentiation important factors of heredity. 

 But these have not been very clearly separated, except that the egg 



