EMBRYOLOGY OF THE MARSUPIALIA. 5 



vessels branch out over the inner (coelomic) wall of the 

 vesicular part of the allantois. The two arteries — one running 

 forwards to supply the anterior portion of the placental area, 

 the other passing back to similarly supply its posterior 

 portion — eventually divide, each into two branches. These 

 pass outwards, dividing as they go, to the margin of the 

 allantoic vesicle, round which they and their branches pass 

 to reach its outer or placental surface, where they break up 

 into capillaries in the allanto-chorionic mesenchyme. 



The allantoic vein is formed by the union of two main 

 factors (fig. 2, all. v.) which accompany the corresponding 

 arteries. The main arterial branches are each accompanied 

 by a corresponding venous trunk, but the smaller arterial 

 branches may or may not be so accompanied. The vitelline 

 artery (fig. 2, vit. a.), emerging from under the distal 

 margin of the umbilical stalk, passes in the yolk-sac 

 splanchnopleure, at first almost directly backwards, then 

 curves mesially and slightly forwards to reach the margin of 

 the chorionic zone, where it passes over into the vascular 

 omphalopleure. There it at once divides into its two 

 branches, which, diverging in opposite directions, form the 

 sinus terminalis (figs. 1 and 2, s. t.), the two branches being 

 united by a delicate anastomosis. 



The vascular omphalopleure (figs. 1 and 2, vase, omph.) is 

 readily distinguishable macroscopically from the bilaminar 

 omphalopleure (figs. 1 and 2, Ml. omph.) lying outside the 

 sinus, by its more opaque appearance and yellowish colour. 



Owing to the fact that the vitelline artery at once divides 

 on reaching the vascular omphalopleure, the vascular area 

 has at this point no existence, but as the two branches of the 

 artery pass out, they at the same time diverge away from the 

 margin of the placental area, and the vascular ai*ea thus 

 gradually increases in width until a maximum of 5 mm. is 

 reached about opposite the point of bifurcation of the 

 vitelline artery. The vascular area has a maximum diameter 

 of 12"5 mm. It will be shown later that the extent of the 

 vascular area of the Perameles embryo of this stage is 



