The Chorion 



357 



tufts, as shown in Fig. 77. In ruminants, Fig. 80, the allantoic 

 sac becomes much elongated, while its apex, along with the 

 amniotic chorion, finally undergoes necrosis, as shown on the 

 right of the figure, or the apex of the amniotic chorion alone be- 

 comes necrotic without the allantois having penetrated it, as 

 shown at the left. 



In case of single pregnancy, the allantoic sac of the ruminant 

 occupies both cornua. The arrangement of the allantois in 

 ruminants differs greatly from that of the horse. The allantoic 

 sac does not completely surround the fetus but, lying along its 

 ventral side, greatly elongated, only partially encloses the amnion. 



Amnion 



Epithelial Thickenings 

 ■ of I he Amnion 



Amniotic Chorion 



Hippomanei 



^^jjnner^ Layer 



Allantois 



Jiippomanes 



Fig. 75. Schematic longitudinal section of a horse embryo in its annexes, 

 at about 5 mos. gestation. Reduced to about ^. (Bonnet). 



This applies, however, only to the inner or hypoblastic layer of 

 the allantois, which constitutes the immediate wall of the sac. 

 The vascular, or mesoblastic, layer extends completely around 

 the fetus and its amnion but, over that portion of the amnion 

 where the allantoic sac does not extend, the amnion, vascular 

 layer of the allantois and amniotic chorion are unseparated by 



