THE DIFFERENTIATION OF THE SEXES 3 



Moreover, grafting experiments have shown that the removal 

 of the gonad from its normal site in the body to an alien 

 situation does not affect the development of the associated 

 organs. Experiments of this nature soon suggested that the 

 gonads exert their control by means of some internal secretion, 

 and later work has substantiated this view. The extraction of 

 the active principles from the gonads has, however, lagged 

 behind work on the other internal secretory organs. This is 

 particularly true of the testis, from which no substance appears 

 to have been prepared capable of replacing its endocrine action 

 in the castrated male. As regards the mammalian ovary, at 

 least one definitely active substance has been prepared, and it 

 is reasonable to suppose that both ovary and testis control 

 the development of their accessory organs by means of 

 internal secretions. In the female mammal the study of the 

 endocrine activity of the gonad is comphcated by the cyclic 

 changes which take place in the reproductive organs during 

 the breeding season, and which have no analogue in the male. 



