EMBRYOLOGY OF THE MARSUPIALIA. 19 



occupies ratlier less than half the surface of the entire 

 embryonic formation. Its surface is also irregularly ridged. 

 The vitelline artery, leaving the yolk-stalk on the left side 

 of the embryo, courses in the yolk splanchnopleure backwards 

 over its dorsal surface to reach the junction of the latter 

 membrane with the vascular omphalopleure (Plate 2, fig. 9, 

 vit. a.). Here it passes over into the latter, does not at once 

 divide as in Perameles, but runs on for a distance of about 

 4 mm. before dividing into its two branches, which form the 

 sinus terminalis {s.t.). The sinus is completed by a narrow 

 anastomosis, as in Perameles. It measures in diameter 14 

 mm. by 12 mm. The vascular area has a greatest width of 

 8 mm. opposite the point of bifurcation of the vitelline artery, 

 and a least width of 4 mm. at that point. It is thus evident 

 from these measurements that in mere surface extent the 

 vascular area of this Macropod embryo very considerably 

 exceeds that of the Perameles embryo described in the pre- 

 ceding pages. And not only is the area greater in extent, 

 but its capillary system is also much more richly developed 

 than in that form, in correlation with its high functional 

 importance in the nutrition of the embryo. From the sinus 

 there pass off into the vascular area numerous branches, i 

 which divide up into much finer branches to form a rich 

 capillary system, from which the factors of the vitelline veins 

 take their origin. The numerous fine long wavy branches of 

 the vitelline artery which run in the yolk splanchnopleure 

 ai'e especially well developed (Plate 2, fig. 9, a. v.). Above, 

 these branches come off directly from the vitelline artery, 

 while below they arise from an independent branch of 

 the artery which runs parallel with the same. The blood 

 circulating in these vessels must, as in Perameles, pass over 

 into the vascular area at the junction of the yolk splanchno- 

 pleure with the vascular omphalopleure. After the vitelline 

 artery has passed over into the vascular omphalopleure, it 

 still continues to give off small branches from both sides 

 almost up to its point of bifurcation. 



I In the figure these branches have been represented too short. 



