2 INTERNAL SECRETIONS OF THE OVARY 



fertilized ovum, evacuation of the foetus, and subsequent 

 suckling of the young. 



The secondary sexual characters in mammals are extra- 

 ordinarily diverse. The characters usually consist in the 

 appearance or accentuation of some attribute which serves to 

 attract the opposite sex, or combat others of the same sex. 

 Since the male is usually the active partner in mammalian 

 reproduction, this sex has the more definite secondary 

 sexual characters. Thus the possession of fighting weapons and 

 a stronger skeleton are typical of the male mammal. Sex 

 differences in the voice and in the amount and distribution of 

 hair are also found. 



The differences in external appearance which result from the 

 possession of secondary sexual characters make it possible in 

 many animals to distinguish male from female without reference 

 to their genitalia. The degree to which these characters are 

 present, however, is subject to much specific variation, and 

 from the point of view of the experimental physiologist, their 

 distribution is disappointing. The common laboratory rodents, 

 for instance, have practically no secondary sexual characters, 

 and long experience is required to distinguish male from female 

 in such animals as mice, rats, guinea-pigs, and rabbits without 

 examination of the external reproductive organs. 



The mechanism of sexual differentiation. The factors which 

 cause a fertilized ovum to develop a testis or an ovary, 

 namely, to become male or female, are at present inadequately 

 known, but there can be little doubt that the development of the 

 indifferent embryo into one sex or the other is normally 

 dependent upon its chromosome constitution. This aspect of 

 the problem has been dealt with in full by many writers (8i, 

 147, 149, 168, 248). 



The nature of the subsequent sexual differentiation is better 

 understood, and it is clear that once the gonads have 

 developed, differentiation proceeds as the result of stimuli 

 from these organs. Gonadectomy experiments have shown 

 decisively that the development of the accessory organs and 

 secondary sexual characters is entirely dependent upon the 

 presence of the gonad, as are the skeletal and other structural 

 details typical of the two sexes. 



