EMBRYOLOGY OF THE MARSUPIALIA. 7 



the lattei' fewer and shorter branches are given off. The 

 blood from these fine vessels apparently passes over into the 

 vascular area at the line of junction of the splanchnopleure 

 with the vascular omphalopleure. 



Semon (2) has also described the occurrence of similar fine 

 branches of the vitelline artery in the two forms examined 

 by him (^pyprymnus rufescens and Phascolarctus 

 cinereus), and such also occur in Macropus parma. 



The blood from the vascular area is returned by two 

 vitelline veins (figs. 1 and 2, vit. y.),each of which is formed 

 by the union of two main factors^ shortly after they enter 

 the yolk-sac splanchnopleure. As in Stage T), the two 

 vitelline veins run back in the splanchnopleure over the left 

 side of the head of the embryo (fig. 1), enclosing between 

 them a narrow triangular area altogether devoid of vessels. 

 The two veins gradually approximate as they pass back and 

 eventually run side by side. In this region the amnion is 

 found connected with the yolk splanchnopleure by a narrow 

 wedge-shaped band of mesoderm which separates the two 

 veins (PI. 1, fig. 5, mes.). This narrow connection is all 

 that is now left of the much larger pro-amniotic remnant 

 seen in Stage D. Not only is the area now less in extent, 

 but the mesoderm has penetrated completely across it. The 

 ectoderm of the amnion over the remnant is thickened (fig. 

 5j ect.), the cells being club-shaped in form, with the nucleus 

 situated in the outer projecting part of the cell. The ento- 

 derm of the yolk-sac splanchnopleure over the remnant is 

 likewise thickened, but irregularly (fig. 5, eni.). 



As the vitelline veins are traced back towards the umbilical 

 stalk they gradually become smaller, and at the same time 

 pass over from the yolk splanchnopleure towards the amnion, 

 in the now greatly thinned bridge of mesoderm of the pro- 

 amniotic remnant, which posteriorly becomes continuous with 

 the thin, attenuated yolk-stalk carrying the vitelline artery. 

 At the anterior margin of the umbilical stalk the two veins 

 unite into a single trunk. This runs up in a thin fold of 

 mesoderm continuous with that of the pro-amniotic remnant, 



