SEXUAL REPRODUCTION 219 



mistakable, and is of considerable import- 

 ance in its connection with the sexual act. 

 The chief character which urges itself on 

 our notice consists in the relatively large 

 size of the egg or female gamete, and the 

 small size of the other, the male or sperm. 



The egg not only becomes large, but it 

 loses the power of independent motility. 

 It consists of a bulky mass of cytoplasm, in 

 which nutritive matter is often present, and 

 it also contains a large and somewhat watery- 

 looking nucleus. 



The sperm, on the other hand, is small and 

 compact. It is nearly always actively motile, 

 though this character is almost or entirely 

 abandoned in certain groups, such as the 

 highest flowering plants in which this has 

 evidently occurred as the result of correlation 

 with other secondary changes connected with 

 pollination, which render motility useless 

 or even disadvantageous. In another im- 

 portant respect the sperm also differs from 

 the egg, inasmuch as it tends to become 

 composed almost entirely of the cell nucleus, 

 the cytoplasm being merely represented by 

 the cilia and a thin skin which sheaths the 

 nucleus as a whole. 



One of the results secured by fertilisation 

 has already been pointed out, namely, 

 the vigorous development so characteristic 

 of the sexually produced organism. But 

 there is another and perhaps hardly less 

 important consequence, namely, that the 



