I04 THE BIOLOGY OF TWINS 



monochorial, but in spite of this nearly all are dizygotic [italics 

 mine] ; for in aU cases in which the ovaries were present with the 

 uterus a corpus luteum was present in each ovary [italics mine]; 

 in normal single pregnancies in cattle there is never more than 

 one corpus luteum present. There was one homosexual case 

 (males) in which only one ovary was present with the uterus 

 when received, and it contained no corpus luteum. This was 

 probably monozygotic. 



In cattle a twin pregnancy is almost always the result of the 

 fertilization of an ovum from each ovary; the development 

 begins separately in each horn of the uterus. The rapidly elongat- 

 ing ova meet and fuse in the small body of the uterus at the 

 same time between the lo mm. and 20 mm. stage. The blood 

 vessels from each side then anastomose in the connecting part 

 of the chorion; a particularly wide anastomosis develops, so that 

 either fetus can be injected from the other. The arterial circula- 

 tion of each overlaps the venous territory of the other, so that 

 constant interchange of blood takes place [Fig. 40]. If both are 

 males or both are females, no harm results from this; but if one 

 is male and the other female, the reproductive system of the female 

 is largely suppressed, and certain male organs even develop in the 

 female. This is unquestionably to be interpreted as a case of 

 hormone action. It is not yet determined whether the invariable 

 result of sterilization of the female at the expense of the male is 

 due to more precocious development of the male hormones, or 

 to a certain natural dominance of male over female hormones. 



The results are analogous to Steinach's feminization of male 

 rats and masculinization of female by heterosexual transplanta- 

 tion of gonads into castrated infantile specimens. But they are 

 more extensive in many respects because of the incomparably 

 earlier onset of the hormone action. In the case of the freemartin, 

 nature has performed an experiment of surpassing interest. 



Bateson states that sterile freemartins are found also in 

 sheep, but rarely. In the four twin pregnancies of sheep that I 

 have so far had the opportunity to examine, a monochorial 

 condition was found, though the fetuses were dizygotic; but the 

 circulation of each fetus was closed. This appears to be the 

 normal condition in sheep; but if the two circulations should 



