THE EVOLUTION OF SEX 23 



earlier accounts, the germ-cells were described as appear- 

 ing late in embryonic development, it has been shown 

 that the germ-cells can be detected at a very early stage 

 in the walls of the digestive tract (Fig. 9). Thence they 

 migrate to their definitive position, and give rise to 

 the cells from which the eggs or the sperm arise. 



The germ-cells are in fact often the earliest cells to 

 specialize in the sense that they are set aside from the 

 other cells that produce the soma or body of the in- 

 dividual. 



THE APPEARANCE OF THE ACCESSORY ORGANS OF 



REPRODUCTION 



As animals became larger the problem of setting free 

 the germ-cells was a matter of great importance. Sys- 

 tems of outlets arose — the organism became piped, as it 

 were. In the lower animals the germ-cells are brought 

 to the surface and set free directly, and fertilization is a 

 question of the chance meeting of sperm and egg ; for 

 there is practically no evidence to show that the sperm 

 is attracted to the egg and much evidence that it is 

 not. Later, the copulatory organs were evolved in all 

 the higher groups of animals by means of which the 

 sperm of the male is transferred directly to the female. 

 This makes more certain the fertilization of the egg. 



In the moUusks, in the insects and crustaceans, and 

 in the vertebrates the organs of copulation serve to 

 hold the individuals together during the act of mating, 

 and at the same time serve to transfer the semen of the 

 male to the oviduct, or to special receptacles of the 

 female. Highly elaborated systems of organs and 

 special instincts, no less elaborate, serve to make the 



