892 THE DEVELOPING ENDOCRINE GLANDS 



It should be mentioned in this connection that in the marmoset, Oedipo- 

 midas geoffroyi, similar anastomoses between the placental blood vessels of 

 heterosexual twins fail to produce the freemartin condition, both twins being 

 normal. Species differences in the response to hormones or other sex-modifying 

 substances therefore occur (Wislocki, '32). 



The studies made in an endeavor to ascertain the influences which sex hor- 

 mones play in the development of the reproductive system and in sexual dif- 

 ferentiation have produced the following general results. 



Developing ovaries and testes and the reproductive ducts of birds, frogs, 

 and urodeles may show various degrees of sex reversal when the developing 

 young are exposed to hormones or other humeral substances of the opposite 

 sex. There is some evidence to the effect that sex reversal by sex hormones 

 is accomplished more readily and completely from the homogametic sex to 

 the heterogametic sex, suggesting, possibly, that the sex field of the hetero- 

 gametic sex is the stronger and more resistant. The reproductive ducts are 

 more responsive to change than are the gonads (Burns, '38, '39a; Domm, '39; 

 Mintz, Foote, and Witschi, '45; Puckett, '40; Willier, '39; and Witschi, '39). 



In mammals, the gonads (ovary and testis) appear iquite immune to the 

 presence of sex hormones, whereas the reproductive ducts respond partially 

 to the sex hormone of the opposite sex. The caudal parts of the genital pas- 

 sages are more sensitive to change than are the more anterior portions (Burns, 

 '39b, '42; Greene, Burrili, and Ivy, '42; and Moore, '41, '50). 



Castration experiments before and shortly after birth in mammals produce 

 the following effects: 



( 1 ) Removal of the testis results in retardation and suppression of the 

 male duct system, while it allows the female duct system to develop. 



(2) Removal of the ovary does not affect the female duct system until the 

 time of puberty. 



(See LaVelle, '51, and Moore, '50, for extensive references and discussion.) 



The general conclusions to be drawn from the above experiments, relative 

 to the differentiation of the reproductive ducts, are as follows: 



(1) The reproductive ducts are responsive to sex hormones after they 

 are formed in the embryo. 



(2) The male duct system normally responds to humeral substances, elabo- 

 rated by the developing testis soon after it is formed. 



(3) The female duct system probably is not dependent upon hormonal se- 

 cretion for its development until about the time of sexual maturity. 



(4) The developing ovary, unlike the developing testis, probably under 

 normal conditions does not elaborate sex hormones in large amounts 

 until about the time of sexual maturity. 



