284 



THE EVOLUTION THEORY 



do not dilior iioiii tlie rest either in origin or in appearance, only they 

 do not lieconie mature eggs, but at a definite time cease to make 

 progress, and tlien slowly break up, so that their substance may be 

 absorbed as food by the true ova. Thus there is a much greater and 

 at (lie sauu' time more rapid growth than could be attained through 

 nourishment from the blood alone. In the Daphnids the ovaries 

 consist of groups of four cells each, only one of which becomes an 

 ovum (Fig. 72, El), while the other three (i, 2, and 4) form nutritive 

 cells which break up. This is so in all summer eggs; but in the 

 winter eggs a much larger number of nutritive cells may take part 

 in equipping a single ovum, and in the genus Moina over forty 

 do so. But here the difference in size between the two kinds of 

 eggs is very marked, the winter eggs being twice the diameter of the 



summer eo-o-s. 



In many insects also, e. g. in beetles and bees, similar nutritive 



Wo 





FL 



Fig. 72. Sida crystallina, a Daphnid : a fragment of the ovary showing one of the 

 groups of four cells, of which i, 2, and 4 are nutritive cells, and only 3 becomes an 

 ovum. Magnified 300 times. 



cells occur, but there is in these forms a different arrangement which 

 serves at the same time for the formation of the shell, and the 

 supplying to the ovum of the necessary yolk-stuffs — the ovum is 

 surrounded with a dense layer of epithelial cells, a so-called ' follicle.' 

 In mammals and birds also these ' follicle cells ' certainly play an 

 important part in the nutrition of the ovum, though it is not yet 

 quite clear how they act — whether they produce within themselves 

 grains of yolk and other nutritive substances and convey these to the 

 ovum by means of fine radiating processes, or whether they themselves 

 ultimately migrate into the ovum and there break uj?. In any case 

 it is worthy of note that all these follicular cells in insects and 

 vertebrates have the same origin as the egg-cells, that is, they are 

 modified germ-cells. The case is therefore essentially the same as in 

 the nutritive cells of the Daphnids : nature sacrifices the greater 

 number of the germ-cells in order to be able to provide more 

 abundantly for the minority. She thus succeeds in raising the egg^ 



