TWINNING IN RUMINANTS 105 



anastomose, we should have the conditions that produce a sterile 

 freemartin in cattle. The possibility of their occurrence in sheep 

 is therefore given. 



The fertile freemartin in cattle may be due to causes similar 

 to those normal for sheep. Unfortunately, when the first two 

 cases of normal cattle freemartins that I have recorded came 

 under observation, I was not yet aware of the significance of the 

 membrane relations, and the circulation was not studied. But I 

 have in my notebook in each case that the connecting link of the 

 two halves of the chorion was narrow, and this is significant. 

 In the third case the two chorions were entirely unfused; this 

 constitutes an experimentum crucis. The male was 10.4 cm. 

 long; the female, 10.2 cm. The reproductive organs of both 

 were entirely normal. The occurrence of the fertile freemartin is 

 therefore satisfactorily explained. 



The sterile freemartin enables us to distinguish between the 

 effect of the zygotic sex-determining factor in mammals and the 

 hormone sex-differentiating factors. The female is sterilized 

 at the very beginning of sex-differentiation, or before any morpho- 

 logical evidences are apparent, and the male hormones circulate 

 in its blood for a long period thereafter. But in spite of this, the 

 reproductive system is, for the most part, of the female type, 

 though greatly reduced. The gonad is the part most affected; 

 so much so, that most authors have interpreted it as a testis; a 

 gubernaculum of the male type also develops, but no scrotal sacs. 

 The ducts are distinctly of the female type much reduced, and 

 the phallus and mammary glands are definitely female. The 

 genital somatic habitus inclines toward the male side. Male 

 hormones circulating in, the blood of an individual zygoticaUy 

 female have a definitely hmited influence even though the action 

 exists from the beginning of morphological sex-differentiation. 



The drawing of twin calves shown in Fig. 39 shows 

 very clearly the intra-uterine relations of bovine twins. 

 For permission to use this figure I am much indebted to 

 Professor LiUie, whose monograph on cattle twins is 

 now in course of preparation. The figure shows twins 



