74 GENETICS 



the female never produces its characteristic secretion, and 

 so the male is not influenced. 



Such secretions circulating in the blood, and influencing 

 development, are known as hormones. 



Can such male and female hormones be extracted from 

 the blood, and used experimentally to produce male or fe- 

 male characteristics? This has been done to a certain extent. 

 The hormones have not yet been obtained in a pure condi- 

 tion. But enough has been done to show that male and fe- 

 male hormones do exist, that they circulate in the body, and 

 that they affect the sex characteristics. 



(The account given above relates to mammals. In birds 

 also sex hormones have been discovered, playing a role 

 similar to that of the sex hormones in mammals, though 

 with some striking differences. In insects on the other hand 

 hormones do not appear to play a part in producing sex dif- 

 ferences.) 



The Question Answered: The Means by Which the 

 Chromosomes Affect Sex 



The facts just summarized as to sex development in mam- 

 mals provide a general answer to the question as to the 

 method of operation of the chromosomes in determining 

 sex. The answer is that the chromosomes determine sex by 

 altering the chemical processes that occur in development. 

 The cells bearing XX produce a secretion that differs in its 

 physiological effects from that produced by the cells bearing 

 XO. These two diverse secretions bring about the different 

 developmental processes that result in the production of the 

 two sexes. 



As before seen, the difference in the secretions or hor- 

 mones results in some way from the fact that the XX cells 

 have two centers for the production of certain materials, 

 while the XO cells have only one center for the production 



