400 FRANK R. LILLIE 



horn alone; the embryo in the right horn deriving nourishment 

 from the right part of the conception, that in the left from the 

 left portion of the same." He made similar observations on the 

 sheep, goat, and "other bisulcated animals" and notes that "in 

 the dog, rabbit, hog, and other animals that produce a consider- 

 able number of young at a litter, the thing is otherwise. In 

 these each foetus has two humors, these being severally surrounded 

 with their proper membranes." So far as I know there are no 

 other published observations on the foetal membranes of twins in 

 ungulates from Harvey's time to the present with the exception 

 of Bonnet's single case already referred to. Harvey's observa- 

 tions show that fusion of chorions is wide spread in twin preg- 

 nancies in ungulates; but he states definitely that in the deer the 

 umbilical vessels of each foetus are distributed to its own side 

 only, in which it resembles the sheep. A more careful examina- 

 tion of the female of two-sexed twin pairs in these animals would 

 be of interest in order to determine the possible sporadic occur- 

 rence of sterility. 



The theory requires that if the same condition of common 

 circulation of the foetal blood were to occur in other mammals 

 as in twins of cattle the sterile free-martin condition should occur 

 there also. Now in multiparous mammals such conditions cer- 

 tainly do not occur commonly; for, if they did, the very numerous 

 researches on their embryology would have brought them to light. 

 In the pig one can find occasional, but rare, fusions of adjacent 

 chorions, but I have never found any vascular connection. A 

 number of mammalian groups could be at once excluded from 

 consideration because the conditions of placentation are such as 

 to prohibit chorionic fusion; in mammalian groups such as 

 primates and many rodents in which the ovum becomes embedded 

 in the uterine mucosa, there is of course an insuperable bar to 

 early chorionic fusion. And in those mammalian groups in 

 which the placenta is a highly localized organ as in the remain- 

 der of the rodents, the insectivores, carnivores, and edentates 

 the circulation in the chorion outside of the placental area is so 

 restricted that, even if chorionic fusion did occur, it is difficult to 

 believe that the circulation of separate foetuses would intermingle 

 to any great extent. 



