Infections of the Gravid Uterus 475 



without notable injury to the fetus. This is analogous to 

 the observations upon swine embryos, in which extensive 

 necrosis commonly occurs at both ends of the embryonic 

 sacs without invading the essential placental structures. In 

 this manner the peril to the embryo is limited chiefly to the 

 passage of bacteria through the necrotic area, while the 

 placenta continues to function physiologically. Rarely, the 

 placentitis of pregnancy is of such virulence that all cotyle- 

 dons become necrotic simultaneously or in rapid succession. 

 The fetus then perishes, the uterine walls are paretic, and, 

 if the mother survives, the embryo macerates or the uterus 

 undergoes abscessation. 



(e) Adventitious placental structures regularly follow 

 the necrosis of the cotyledons, whether it occurs during or 

 after pregnancy. Adventitious placentae are not seen in 

 heifers, but are common in cows, where they serve as proof 

 of the prior necrotic destruction of cotyledons. In the 

 uterus of the pregnant heifer, placental contact is regularly 

 limited to the distinctive cotyledonal or placental areas. 

 With each succeeding pregnancy, there is an increased 

 probability of necrosis of some of the cotyledons about the 

 os uteri internum. In the area denuded of cotyledons, ad- 

 ventitious placental structures develop, presenting macro- 

 scopically the appearance of the diffuse placenta of the mare. 

 When cotyledons are destroyed, there is no visible effort at 

 compensational hypertrophy of the remaining cotyledons, 

 but the compensation is effected through adventitious ac- 

 tivity of the intercotyledonal endometrium. 



The adventitious placental structures tend to invade the 

 endometrium about the os uteri internum and to form over 

 the os a virtually continuous placental contact, constituting 

 in effect a placental seal, separating the cervical canal from 

 the utero-chorionic space. When parturition begins, the 

 adventitious placental structures must give way before the 

 cervical canal can dilate and the parturient rent occur in 

 the chorion. The adventitious placental structures ordi- 

 narily radiate from the os uteri internum for only four to 

 ten inches, though occasionally they involve the entire uter- 



