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. 



Allantois Chorion 



Sinus Terminal' 



Circula 

 Fidge' 



Central Portion of 

 Vitelline A rea 



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

 the horse, 2S days after fecundation, as in Fig. 73. Perpendicu- 

 lar section through the embryo and its envelops. 



The embryo in black. — Fctoblast Vitelline layer. 



Parietal mesobla.st. — — — Visceral mesoblast. (Bonnet). 



In ruminants and swine, unlike the globular or spheroidal 

 blastoderm of the hor.se 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 



