204 Venous System of Didelphys Marsupialis (L) 
The general principles, stated above, concerning the origin of the three 
main types of postcaval variations are, without doubt, fundamentally 
eorrect. The reconstructions of the pouch young clearly show that the 
formation of a particular type of postcava depends upon the manner as 
well as upon the extent to which certain veins that lie dorsal and ventral 
to the umbilical arteries (common iliacs in pouch young) are affected 
by atrophy. This general principle is well illustrated by the reconstruc- 
tion of the venous system of a 14 mm. pouch young (Text Fig. 15) in 
which a posteava of Type III, A, in which the principal union between 
the internal and external iliac vein les ventral to the common iliac 
arteries, is already established. In this particular case the internal iliac 
vein of the right side unites with the external iliac of the same side by 
means of two veins which lie dorsal and ventral, respectively, to the 
common iliac artery; while on the left side the union between these two 
veins takes place exclusively on the ventral aspect of the common iliac 
artery. Furthermore, it is evident, as the result of a complete atrophy 
of the ventral portion of the venous ring which encircles the right com- 
mon iliac artery, that a postcava of Type III, C could be established. 
In the latter case, the internal iliacs would then unite with the external 
iliacs by means of two vessels, subequal in caliber, which lie dorsal and 
ventral, respectively, to the common iliac arteries. The important ques- 
tion to be determined in this case, as well as in connection with other 
reconstructions of the pouch young, is the extent to which the vessels that 
le dorsal and ventral to the umbilical arteries in the 8 mm. embryos are 
involved in the formation of the vessels in the pouch young and adults 
which occupy corresponding positions with respect to the common iliac 
arteries. The only doubt that can exist as to their correspondence is the 
circumstance that the venous rings of the pouch young (Text Fig. 15) 
as well as those which occasionally persist in the adult (Fig. 17, right 
side, Plate IV, Part I) occupy a slightly different position with respect to 
the external and internal iliac veins than is the case in the 8 mm. embryos 
(Text Fig. 13). In the 8 mm. embryos (Text Fig. 13) the umbilical 
arteries, as well as the dorsal and ventral portions of the venous rings, 
lie somewhat craniad of the point of confluence of the internal and ex- 
ternal iliac veins; while in the pouch young (Text Fig. 15) and adult 
(Fig. 17, right side, Plate IV, Part I) these structures occupy a more 
caudal position, so that the dorsal portion of the ring is now formed by 
the section of the postcardinal which constitutes the internal iliac vein, 
and the ventral portion of the ring by a vein which joins, at its caudal 
end, the internal iliac vein. Although a difference exists regarding the 
position of the rings and arteries with respect to the internal and ex- 
