216 Venous System of Didelphys Marsupialis (1) 
right and left veins much nearer to the middle line, and a further inter- 
esting change has taken place in that the right vein has become much 
reduced and no longer opens into the liver. It is now merely a small 
vein which brings some blood from the anterior abdominal wall into the 
umbilical sinus. The left vein which now carries all of the allantoic 
blood to the liver, runs only a little to the left of the middle, though the 
recti muscles are still widely apart.” 
“A little later all trace of the right vein disappears, and the left vein, 
though comparatively small, follows a course very similar to that of the 
umbilical vein in the higher mammals. At birth, of course, the circula- 
tion through the umbilical vein ceases. It will be observed that this 
doubling of the umbilical vein is very dissimilar to the condition found 
in the higher mammal, and very similar to that found in the early lacer- 
tilian embryo.” 
The Abdominal Veins——The abdominal veins are most prominently 
developed in the 8 mm. embryo of Didelphys and consist of two small 
vessels which he in the ventral body wall caudad of the umbilicus (Figs. 
40 and 41, Plate III). These two veins resemble in all their relations 
the posterior division of the abdominal veins in the embryos of reptiles, 
since thev receive tributaries from the body walls, open cranially into the 
umbilical veins and connect caudally with external iliac veins (Text 
Fig. 24). Veins occupying the same relative positions as the abdominal 
veins were also met with in the 11.5-12 mm. embryos of Didelphys, al- 
though they were much less prominent here than in the 8 mm. embryos. 
The abdominal veins are undoubtedly transitory in character and con- 
fined to the embryo during its uterine existence since no traces of them 
were met with in any of the pouch young examined. | 
The Omphalomesentertc Veins.—In the 8 mm. embryo of Didelphys 
the omphalomesenteric veins (Figs. 34, 35, 36, Plate II, and Figs. 37 
and 38, Plate III) are represented by a single large vein whose earlier 
transformations I have been unable to follow. There can be little doubt, 
however, that a venous ring is formed about the intestine at a previous 
stage, as in the rabbit, since such a ring is actually present in the 6 mm. 
embryo of Dasyurus (Fig. 29, Plate II). In the 8 mm. embryo the large 
omphalomesenteric vein enters at the umbilicus on the left side of the 
intestine and then curves dorsad until it lies dorsal to the same. In this 
position it enters the liver through which it passes, without at any time 
anastomosing directly with the umbilical veins, and opens into the post- 
cava slightly caudad of the opening of the umbilical veins (Text Fig. 10 
and Fig. 33, Plate II). In the 11.5-12 mm. embryos of Didelphys the 
omphalomesenteric vein, except for minor changes due to the elongation 
