Charles F. W. McClure yer dre 
into that of the anterior and posterior revehent veins. This is clearly 
shown to be the case by the reconstructions of the venous system of the 
6 mm. Dasyurus (Text Fig. 9) and 8 mm. Didelphys embryos (Text 
Fig. 10) in which the ground-plan is fundamentally the same as that de- 
scribed by Lewis, 02 (Plate I, Figs. 3 and 4), for a rabbit embryo of 
thirteen davs, where the right and left subcardinals have anatomosed in 
the median line, caudad of the origin of the omphalomesenteric artery, 
and the right subcardinal has been “ tapped” by the hepatic circulation. 
The point at which the right subeardinal vein makes connection with 
the hepatic circulation is designated by the writer in the following pages 
as the hepatico-subcardinal junction. 
On account of the bilateral symmetry of its subecardinal veins the 
6 mm. embryo of Dasyurus undoubtedly represents a stage of develop- 
ment which is relatively earlier than that of the 8 mm. embryo of Didel- 
phys, and, for purposes of comparison with the latter, a reconstruction 
of its venous system has been added to the text (Text Fig. 9). 
THE VENOUS SYSTEM OF THE 8 MM. EMBRYOS OF DIDELPHYS. 
The Postcardinal Veins.—In the majority of the 8 mm. embryos of 
Didelphys (Text Fig. 10) the postcardinal veins can be traced as contin- 
nous vessels between the ducts of Cuvier, into which they open dorsally, 
and the caudal end of the body where they are formed, on each side, 
through the union of the internal and external iliac veins. Caudal to the 
origin of the omphalomesenteric artery each postcardinal vein joins the 
root of the postcava by means of a single large anastomosis. The post- 
cardinals which le caudad of this anastomosis with the postcava are ves- 
sels of large size and constitute its principal, though not direct, caudal 
continuation ; the latter being formed by the right posterior revehent vein 
(subcardinal). The relation of the postcardinal veins to the umbilical 
arteries is most complex and will be treated more fully in connection with 
another topic. It may be mentioned here, however, that the umbilical 
artery of each side, instead of lying ventral to the postcardinal vein as in 
most mammalian embryos or dorsal to the same as in Echidna (Hoch- 
stetter) and the 6 mm. embryo of Dasyurus (Text Fig. 9), is encircled 
by a circumarterial venous ring. 
Craniad of the anastomosis with the postcava the postcardinals are 
much reduced in size and slightly caudad of their union with the ducts 
of Cuvier each postcardinal receives a tributary which can be traced 
caudad for only a short distance as a continuous vessel. ‘These two trib- 
utaries (Fig. 31, Plate II and Text Fig. 10) which le lateral or dorso- 
