356 



Veterinary Obstetrics 



the allantois : these, though separable, adhere somewhat in- 

 timately, the two together constituting the sac usually designated 

 as the amnion. Similarly, the external wall of the allantois 

 presses against the mesoblastic layer of the amniotic chorion or 

 false amnion, fuses with it and constitutes the allantois- chorion. 



Amniotic 

 Chorion 



Allantois 



■Allantois Chorion 



Coelom 



Vitelline Sac 



Sinus Termina,lis 



Circula 

 Ridge' 



Border Zone 



Central Portion of 

 Vitelline Area 



Fig. 74. Schematic illustration of fetal annexes of the embryo of 

 the horse, 28 days after fecvindation, as in Fig. 73. Perpendicu- 

 lar section through the embryo and its envelops. 



The embryo in black. — Ectoblast Vitelline layer. 



Parietal mesoblast. Visceral mesoblast. (Bonnet). 



In ruminants and swine, unlike the globular or spheroidal 

 blastoderm of the horse as shown in Figs. 73 and 74, it becomes 

 very greatly elongated, as shown in Figs. 76 and 77, longer even 

 than the uterine cavity, necessitating its folding upon itself. 

 This elongated • sac does not persist, but soon atrophies and 

 almost wholly vanishes long before birth. The formation of the 

 allantois follows more or less the plan of the blastoderm and its 

 yolk sac and in these animals becomes much elongated. In 

 swine the apices of the allantoic sac are destitute of placental 



