82 SEX 



Let us consider the case of fowls. For 

 three or four weeks after hatching, chickens 

 do not show any external marks of sex. In 

 size and colour of comb, in plumage and 

 limbs, pullets and cockerels are alike, and it 

 is not till towards the thirtieth day that the 

 external differentiation begins to be apparent. 

 By the forty-fifth day the comb is more 

 pronounced and more vividly coloured in the 

 male ; the wattles begin to develop ; the young 

 cocks crow towards the second month ; dif- 

 ferences in the plumage begin to differentiate 

 the two sexes more and more sharply. 



So far we are on familiar ground, and it 

 is also well-known that the removal of the 

 testes hinders the development of the secon- 

 dary sex-characters. But a recent investi- 

 gation by J. des Cilleuls has made matters 

 more precise. He has shown that the ap- 

 pearance of the secondary sex-characters in 

 the young cock coincides with the appearance 

 of interstitial cells in the testes ; that the 

 interstitial cells and the distinctively cock- 

 characters increase concurrently; and that 

 the cock-characters continue to be accen- 

 tuated till after the sixtieth day, while the 

 essential part of the testes the seminal 

 tubules remain embryonic. It would appear 

 therefore, that the internal secretion of the 

 interstitial cells serves as a stimulus for the 

 development of the secondary sex-characters. 



A great number of very interesting experi- 

 ments have been made in reference to the 

 r61e of the internal secretions in connection 

 with sex-characters, and while there seems 

 to be a tendency to hurry to very definite 



