66 Diseases of the Geriital Organs 



III. Sometimes, also, as shown in Plate II, the chorion be- 

 comes necrotic where it passes across the os uteri internum. 

 The common, small necrotic tips, shown in Figs. 35 and 36, 

 are designated as "normal" by embryologists but these pass 

 by imperceptible gradations into the degree of necrosis 

 shown in Plate III which is clearly and grossly pathologic. 

 The allantoic sac, fully distended with fluid, fills the uterine 

 cavity in such a manner that violent movements of the 

 mother cannot readily cause any material displacement be- 

 tween the uterine wall and the chorion, but both are moved 



Fig. 34. Same as Fig. 33. x 900. (Pomayer). 



as a unit, obviating largely any dragging upon the placenta 

 which might threaten to cause injury thereto. The fetus 

 floats within the amniotic fluid and the amniotic sac, in 

 turn, floats within the allantoic fluid. In solf^ edj. swine 

 and^ carnivora, the amniotic sac floats freely within the 

 allantoic fluid, except for the allantoic segment of the um- 

 bilic cord ; in ruminants, the dorsum of the amniotic sac is 

 intimately fused with the chorion, obliterating the allantoic 

 cavity in this area. The fetuses of solipeds, swine, and 

 carnivora may be expelled from the uterus through a rent 

 in the chorion, with the more or less intact amnion envel- 

 oping them, but in ruminants the adhesion of the amnion 



