THE DETERMINATION OF SEX 289 



free martin in which the ovaries tend to form tubules 

 quite like those of a testis. 



In birds the activities of the gonads likewise control 

 to a large extent the development of the secondary 

 sexual characters, as has well been shown by Goodale 

 and by Morgan in castration and transplantation ex- 

 periments on ducks and fowls. Most striking is the 

 case of female birds which, when castrated while still 

 young, develop male plumage and posture. 



It has been clearly demonstrated that the genes for 

 secondary sexual characters lie in the autosomes and 

 thus both male and female have determiners for the 

 secondary sexual characters of both sexes. For ex- 

 ample, normally in cases where the male is heteroga- 

 metic, the presence of a single ^-chromosome in all of 

 its cells, together with the endocrine secretion of its 

 gonads, causes the male genes for secondary sexual 

 characters to develop and those of the female to be 

 suppressed. By castration and transplantation the 

 normal condition may be upset and the female secon- 

 dary sex genes brought into action. 



The whole problem of sex-hormones is very compli- 

 cated since it has been shown that the secretion from 

 the gonads is merely one link in the chain of endocrine 

 factors which tend to set into action the genes for 

 determining secondary sexual characters. In the de- 

 velopment of sex in the vertebrates the genes for the 

 production of these sex-hormones are second in impor- 

 tance only to those genes which determine whether ova 

 or sperm shall be formed in an individual. 



