Twin Pregnancy 403 



or the one is born alive, and there is observed a great variation 

 in the degree of development of the two, which sometimes leads 

 to the belief that they represent impregnations at widely sep- 

 separated times. 



When death of one fetus occurs early, it may undergo mummi- 

 fication and remain in the uterus as an inert body. In our 

 museum, there are, in the uterus of a cow, twin mummified 

 fetuses which had perished at about the 7th month of gestation. 



In the cow and ewe, each of the twin fetuses very largely 

 occupies one uterine cornu and usually presents with the 

 head toward the os uteri, but it is not rare to find them 

 presenting alternately, that is, one anteriorly and the other 

 posteriorly. 



It is interesting to note also that occasionally, in twin preg- 

 nancies, one twin is more or less enclosed within the body of the 

 other. This presumably results from fission of an ovum, by 

 which two embryos result from a single egg and, being unequal 

 in size and remaining intimately connected, the one grows 

 around and includes the other, until finally its inclusion is more 

 or less complete. 



