452 



HOMER WHEELON 



twin and a common chorion stand in some kind of etiological relationship. 

 Furthermore, the fusion of the chorions occurs at or about the time of 



Fig. 12. .Normal male 26 cm. long; from twin one-sex pair. X 4/3. Single 

 chorion, constricted between the two fetuses. This figure shows the entire urogenital 

 system; the gubernacula have entered the scrotal sacs. The testes are drawn into the 

 vaginal sacs. The disposition of the teats for the normal male should be noted. 1, rec- 

 tum; 2, vasa deferentia; 3, epididymis; 4, testis; 5, urogenital sinus; 6, gubernaculum 

 withdrawn from scrotal sac; 7, cut end of umbilical cord; 8, prepuce; 9, wall of penial 

 sheath; 10, penis; 11, vesiculae seminales; 12, teats; 13, allantois; 14, gubernaculum in 

 scrotal sac; 15, retractor muscle of penis. (Lillie, 1917, Fig. 12, p. 434.) 



the beginning of sex differentiation. Hence, it may be concluded that 

 the sterile freeKmartin is zygotically a female modified by the sex hormone 

 of the male twin which circulates in the vessels of both individuals during 



