THE FACTORS WHICH DETERMINE SEX 695 



development the vessels of the chorion of the sterile twin anastomose 

 with those of its fellow, which becomes a fertile hull, and that sexual 

 differentiation in the male precedes that of the female. Consequently 

 the male twin exerts an influence over the reproductive system of 

 its fellow at an early stage when the female organs are still un- 

 developed, and it does this through an internal secretion which is 

 present in the common circulating medium. The growth of the 

 generative organs is thereby partly inhibited in the female twin, 

 while the vestigial male structures are stimulated so as to undergo 

 some development. The Free-Martin, therefore, is an intersexual 

 individual whose development is due to the influence of a hormone 

 elaborated in the male twin, and carried thence to its fellow through 

 a common circulatory system. 



The fact that the embryos arise from different eggs and not from 

 one, as had been previously suggested, is shown by the presence of 

 two corpora lutea in the ovaries. Assuming Lillie's theory to be 

 correct there is no apparent reason, as Huxley has pointed out, why 

 the ovarian internal secretion of the mother should not penetrate 

 to the fo3tuses in any mammal and so produce intersexual individuals, 

 and one can only suppose that there is some special protective 

 mechanism to prevent this from occurring. 1 



Minoura, 2 by removing a portion of the shell from the eggs of 

 fowls during the second week of incubation and transplanting on 

 to the chorio-allantoic membrane a piece of gonad from another 

 individual, has succeeded in producing artificial " Free-Martins " 

 showing varying grades of intersexuality. The grafts were obtained 

 from both other chicks and older birds but the effects were similar. 

 Minoura found that in female-type embryos the right gonad, which 

 normally atrophies, might be got to persist as a result of an ovarian 

 graft. 



Hammond, 3 after describing a partially hermaphrodite pig, ex- 

 presses the view that the development of the accessory genital organs 

 in such cases is under the control of the gonad, which was for the 

 time being better developed or more potent. A similar explanation 

 has been suggested by Stone 4 to account for a comparable condition 

 in a goat. 



The experiments on sex-reversal by Steinach, Lipschutx, Pc-zard, 



1 For an account of tin- ' Structures and Homologies of the Free-Martin 

 Gonads," see Willier, .//";. </' A'.///. Z<H>l. t vol. xxxiii., 1921. 



'- Minoura, " A Study of Testis and Ovary Grafts on the Hen's Egg, etc.," 

 Jour, "f K.rp. Zool., vol. xxxiii., 1921. 



:t Hammond, "A Case of Hermaphroditism in the 1'itf," Jour, of Ant. ",/</ 

 /'///W., vol. xlvi., 1912. Of. Crew, \',-t. J<>,<r., vol. Ixxviii., I'liM. 



4 Stone, "A Typical Male Sex-ensemble in the Domestic Goat," <'/>ii"t 

 Mcd. Jo>n:, (November) 1920. Many other such cases are known. 



