ACTION OF SEX HORMONES IN FOETAL LIFE 389 



^ Case 7 was received in my absence, and the entire uterus was placed in 

 formalin; preservation of its contents was bad, and condition of chorion must be 

 recorded as doubtful. 



- Case 13 uterus injured by butcher; chorion cut in two. 



^ Case 16 uterus injured bj' l)utcher; chorion cut in two. 



^ Case 22 uterus injured liy l)utcher; chorion cut in two. 



'•' Cases 42 and 44 are not included because they were selected heterosexual 

 pairs taken after birth. 



Til. THE TIME OF -FUSION OF THE TWIN CHORIONS AND THE DE- 

 VELOPMENT OF THE VASCULAR ANASTOMOSES 

 BETWEEN THE TWINS 



In order to form an estimate of the probable time of fusion of 

 the twin chorions it is necessary to present a few data concern- 

 mg the development of the usual single chorion. Figure 1 shows 

 the non-pregnant uterus of the cow partly dissected. It will be 

 noted that the horns of the uterus open by constricted apertures 

 into the small body. The blastodermic vesicle forms in the horn 

 of the uterus on the same side as the ovary from which the ovum 

 was derived, as I have observed in numerous cases. It grows out 

 into a long strand-like sac extending both distally and centrally. 

 The embryonic area forms near the. center in the sheep (Bonnet) 

 and presumably also in the cow. The grow^th of the strand-like 

 vesicle in length is extraordinarily rapid, and it soon enters the 

 body of the uterus centrally, and penetrates into the opposite 

 horn. By the time that the embryo is 10 mm. long the vesicle 

 has extended completely through the body of the uterus and far 

 into the other horn (two cases observed) ; the embryo is thus 

 excentrically placed in the very long vesicle. The allantois 

 forms later than the blastodermic vesicle; it grows from the 

 embryo both centrally and distally, and ultimately completely 

 fills the blastodermic vesicle and occludes its cavity. In the 

 case of an embryo of 19 mm. length the allantois had passed 

 from the horn of the uterus containing the embryo well into the 

 body of the uterus. In another case of an embryo of 21 mm. 

 length the allantois had extended through the entire horn of the 

 uterus opposite to that containing the embryo. 



I have one case of a twin pregnancy in the cow in which the 

 embryos were only 15 mm. long (no. 49). Unfortunately the 



THE JOUKNAL OF EXPERIMENTAL ZOOLOGY, VOL. 23, NO. 2 



