202 Venous System of Didelphys Marsupialis (L) 
and ventral to the common iliac arteries. On purely theoretical grounds, 
therefore, it is not difficult to explain the origin of all the adult varia- 
tions on the basis that the formation of a particular type of postcava, 
depends upon the manner, as well as upon the extent, to which the circum- 
arterial venous rings of the 8 mm. embryo might be affected by atrophy 
during the subsequent stages of development. On this basis the forma- 
tion of Type III, in which the postcava is formed through a union of 
the external and internal iliac veins which takes place both dorsal and 
ventral to the common iliac arteries, might be explained on the grounds 
that portions of the venous rings which le dorsal, as well as those which 
lie ventral to the umbilical arteries in the embryo, have been retained in 
the adult. See Text Figs. 19 and 20. 
Type III, as represented by these figures, in which both embryonic 
venous rings retain their integrity and individuality might then be re- 
garded as a ground-type” arrangement of the venous system, of which 
all of the postcaval variations which fall under Type III (Plates III, 
IV and V, Part I), are modifications, as well as are those variations 
which fall under Types I and II (Plates I and II, Part I). Thus Type 
III, A (see Plate III, Part I), in which the principal union between the 
internal and external iliac veins lies ventral to the common iliac arteries, 
might be formed as the result of the partial atrophy of the dorsal, and 
Type III, B (Plate IV, Part I), in which the principal union between 
the iliac veins lies dorsal to the arteries, might be formed as the result 
of the partial atrophy of the ventral portions of the circumarterial venous 
rings. Type III, C, in which there is practically no difference in the 
ealiber of the vessels which lie dorsal and ventral to the common iliac 
arteries( see Plate V, Part I), might represent a case in which atrophy 
had affected the dorsal and ventral portions of the circumarterial venous 
rings in a like manner. 
Type I (see Plate I, Part I), in which the internal iliac veins unite 
with the external iliacs ventral to the arteries to form the postcava, and 
Type II (see Plate II, Part I), in which the reverse is the case, might be 
formed as the result of the complete atrophy of the dorsal (Type I) and 
ventral (Type II) portions, respectively, of the circumarterial venous 
rings in the manner illustrated by Text Figs. 21 and 22. 
18 Among the 101 adult opossums examined by the writer one was met with 
in which the postcava was formed as in text Fig. 20, in which the common 
iliac vein unites with the external iliac of each side by means of two vessels 
which lie dorsal and ventral, respectively, to the common iliac artery. See 
Figs. 13 and 14 on Plate III, Part I, which are dorsal and ventral views, 
respectively, of the same preparation. 
