TWINNING IN RUMINANTS 107 



about ten inches long taken out of the chorionic sac 

 through the openings shown in dark shading. The 

 double chorionic vesicle is shown somewhat collapsed 

 through loss of its fluid contents. The narrow part 

 in the middle is the point where two separate eggs have 

 fused at a much earher period. The flower-like pads 

 scattered over the membranous wall are the placental 

 areas, the so-called cotyledons, by means of which the 

 fetuses obtain nourishment from the uterine tissues. 

 The chorionic blood vessels and those of the fetuses are 

 shown clearly, the arteries in outline and the veins 

 black. It will be seen that the arteries of the two 

 fetuses are in direct communication across the placental 

 bridge. The veins of both fetuses in some cases enter 

 the same cotyledon. There is every opportunity for 

 the admixture of blood between the twin individuals. 

 It need hardly be pointed out that the individual on 

 the left is a male and that on the right a sterile female 

 or freemartin. 



LilHe's work has revealed the true nature of the 

 freemartin; it is a sterile female whose gonads remain 

 in the juvenile stage so that they resemble testes, and 

 which has certain secondary sexual characters of the male 

 due to the presence for a considerable period of male 

 hormones in the blood borrowed from its male co-twin. 

 The animal is a hermaphrodite only in a very limited 

 sense. The work leaves no question as to the dizygotic 

 origin, not only of opposite-sexed, but also of same- 

 sexed bovine twins. 



Whether real monozygotic twinning ever occurs 

 among the ungulates is highly questionable. Several 

 considerations lead to this conclusion. Polyembryony, 



