398 JAS. P. HILL. 



fig. 38j Tat", ivj. Ot the vitelline vessels unfortunately I can 

 give only a very incomplete account. In opening up the uteri 

 in both Stages C and D the vascular area was partly destroyedj 

 so that I can only state the course of the main trunks. As in 

 ^pypryranus and Phascolarctus, according to Semon's account 

 (8), the yolk-sac is supplied by a vitelline artery and a vitelline 

 vein. 



The vitelline artery on leaving the yolk-stalk runs obliquely 

 backwards in the yolk splanchnopleure, and finally passes over 

 into the vascular oraphalopleurCj where it bifurcates into a right 

 and left trunk, which together constitute the circular sinus 

 terminalis (text fig., s. t,). From the sinus smaller branches 

 pass off into the vascular area. Whether the two trunks 

 actuall}' inosculate or are only connected by capillary anasto- 

 mosis I am unable to state. 



The vitelline vein is formed close to the base of the yolk- 

 stalk by the union of two main factors which arise in the 

 vascular omphalopleure by the union of tributaries coming 

 from the capillary network of the vascular area. These two 

 main vitelline trunks coming from opposite sides of the 

 vascular area pass over from the vascular omphalopleure to 

 the yolk splanchnopleure, and there run posteriorly over the 

 left side of the head of the embryo. They gradually approxi- 

 mate as they approach the yolk-stalk, near the base of which 

 they unite to form the single vitelline vein. 



The last point to which we need refer in this general 

 account concerns the existence of a persistent remnant of the 

 proamnion somewhat similar to the " proamnion-rest" de- 

 scribed by Semon (8) for Phascolarctus. As in that form, the 

 persistent connection between the amnion and the here non- 

 vascular yolk splanchnopleure forms a small pear-shaped area* 

 bounded laterally by the main factors of the vitelline vein, 

 and extending from their point of union up to about the level 

 of the eye on the left side of the embryo. A section through 

 this proamniotic area is represented in fig. 14. It will be 

 seen therefrom that both the ectoderm and entoderm over the 

 area are considerably modified. 



