478 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



-c 



that the set of cells which give rise to one are quite 

 separate from those which give rise to the other sexual 

 cells, but this is not always the case, as the herma- 

 phrodite gland of the snail and the generative cells of 

 the just-mentioned Ascaris are sufficient to bear 

 witness. 



The broad differences between a male and a 

 female element may be easily apprehended ; starting 

 from cells which are essentially similar in character, 

 those which are to become the 

 male bodies subdivide, and 

 each cell gives rise to a large 

 number of smaller bodies, 

 which typically, though by no 

 means always, consist of a 

 rounded head (which repre- 

 sents part of the nucleus of 

 the original cell), and a more 

 or less long, actively moving 

 tail (Fig. 201). The female 

 cell (Fig. 202), on the other 

 hand, increases rather than 

 diminishes in size, and often 



acquires considerable bulk from the large number of 

 yolk cells that are aggregated around it ; it frequently 

 also becomes invested in a membrane, the outermost 

 portion of which may, as in the familiar example of 

 the egg of the bird, form the basis for a shell, which 

 may be calcareous or chitinous. 



The generative cells are, in their simplest condition, 

 nothing more than modified elements of the epithelial 

 layer which lines the body cavity, and it is only with 

 increasing differentiation of structure that they 

 become aggregated into definite masses holding a 

 certain topographical relation to the other parts of 

 the organism. The influence of the male on the 

 female element will be described shortly (page 482). 



Fig. 202. Eipe Ovtun of Cat. 



a, Zona pellucida ; b, germinal 

 vesicle ; c, protoplasm. 

 (After Klein.) 



