288 GENETICS 



dimorphism are, however, secondary to gamete pro- 

 duction. 



In mammals and birds these so-called secondary 

 sexual characters are found to be largely dependent 

 for their proper development upon the normal presence 

 and activity of the gonads. For example, castration 

 of young male mammals results in individuals lacking 

 in many ways the attributes of normal males.*^Among 

 cattle and horses, which have undergone this opera- 

 tion, the fiery males become docile and lack the thick 

 neck common to their kind. They also put on fat more 

 readily. In man the voice fails to change, the beard 

 is weak, the epiphyses of the bones do not fuse and the 

 spirit is dulled. Females deprived of ovaries early in 

 life fail to develop normal mammary glands, while 

 certain of their skeletal characters are likewise much 

 altered. Extensive experiments have proved that in 

 birds and mammals secretions of the gonads, known 

 as hormones, are essential to normal development. The 

 castration of young male rats followed by ingraft- 

 ing of ovaries causes these individuals to become femin- 

 ized in character. 



Perhaps no better case of the influence of hormones 

 is known than that of the "free martin," adequately 

 explained by the observations of Lillie. He found in 

 cattle that when the chorionic coverings of twin em- 

 bryos of opposite sex fuse so that the blood vessels 

 anastomose, the more rapidly developing male embryo 

 sends out hormones into the circulation which inhibit 

 the normal development of the female embryo. The 

 much modified female embryo may then be born as a 



