492 LIFE : OUTLINES OF GENERAL BIOLOGY 



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 in the second 

 month; difterences 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 secondary 

 sex-characters. But an investigation by J. des Cilleuls (191 2) has 

 made matters more precise. He has shown that the appearance 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 pari passu; 

 and that the cock-characters continue to be accentuated till after 

 the sixtieth day, while the essential part of the testes — the seminal 

 tubules — remains 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. 



As Steinach and Kammerer have put it, the changes that occur in 

 the body when an animal becomes sexually mature are conditioned 

 by the internal secretion of the gonads, probably of the interstitial 

 tissue alone. The brain is influenced profoundly, it is "eroticised"; 

 it becomes susceptible to the attractions of the other sex. The 

 cerebral ganglia acquire a tendency to lower the tonus of certain 

 inhibiting centres in the spinal cord, and at the same time the ex- 

 citability of certain sympathetic ganglia is increased, so that they 

 react to peripheral stimulation. 



"The eroticising of the central nervous system also brings about 

 far-reaching changes in metabolism, such as increased blood-supply 

 to the genital and extra-genital sex-characters, which react to this 

 with vigorous, often annually renewed growth, and at the time of 

 puberty reach their full development". In these words Kammerer 

 expresses a conclusion based on experiment and of the highest 

 importance. The gonads by their hormones influence the whole 

 body and in particular the sex-characters, but the influence comes 

 under the regulation of the central nervous system. Yet it cannot 

 be concluded that the secretion of the gonads causes the sex-char- 

 acters, though it is a condition of their development in the individual. 

 The gonads are the nurses of sex-characters, but not their producers. 

 The characters arc in the hereditary treasure-box, even though they 

 are never exhibited. The internal secretion is a condition of their 

 normal development ; that is all that can be said. 



According to Tandler the criterion of a sex-character is that it 

 reacts to gonadial secretions in a particularly definite way. The 

 gonadial secretions punctuate the growth of the long bones and thus 

 affect the proportions of the body; they also influence the nervous 

 system and the grncral metabolism; thev work in harmony with 



