PLACENTA IN PERAMBLES 151 



reach the right side. Here it passes into the outer leaf, i. e. 

 from the yolk-sac splanchnopleiire to the vascular omphalo- 

 pleure. In the course of its passage along the body it gives 

 rise to the numerous fine and extremely characteristic vessels 

 which extend in tlieir peculiarly parallel manner into the 

 yolk-sac splanchnopleure which they supply. These branches 

 are extremely long and pass for some little distance over into 

 the vascular omphalopleure where they alternate with corre- 

 sponding venous factors to the vitelline vein. 



Immediately on entering the vascular omphalopleure the 

 vitelline artery divides to form the sinus terminalis (s.t.). 

 Each portion of the sinus passes forward and at first has the 

 usual course. Instead, however, of passing directly forward 

 in the usual way, each branch, at aljout the level of the fore- 

 limb, takes a sudden turn ventrally so that the anterior portion 

 of each lateral branch forms another curve with a ventral 

 convexity. The artery on each side unites with its fellow in 

 front of the head by anastomosis. 



The sinus gives off to the area vasculosa a wonderfully rich 

 plexus of ])ranches quite as complex or even better developed 

 than in any marsupial so far described. 



The peculiar arrangement of the sinus terminalis results 

 in the division of the vascular area on each side into two regions, 

 anterior and posterior, the former being somewhat smaller in 

 area than the latter. Each of these areas is drained by a 

 separate factor of the vitelline vein on each side. The posterior 

 factor from the posterior area is the larger, and after receiving 

 numerous fine capillaries and branches travels along the 

 dorsolateral aspect of the vascular omphalopleure, parallel to 

 and some little distance from its dorsal edge till the vein 

 reaches the height of the neck flexure of the embryo, where it 

 passes over into the yolk splanchnopleure. Here it receives the 

 anterior factor which drains the anterior area. The branches 

 and capillaries which go to make up the latter are extremely 

 rich. Each lateral vitelline vein {vit.v.) formed of the factors 

 just mentioned passes down and unites with its fellow just 

 before entering the body. Between them in this region is the 



