174 Venous System of Didelphys Marsupialis (L) 
hinder end of the body and the postcava. By this means the blood may 
flow directly to the heart without passing through the mesonephric cir- 
culation, as is the case in reptiles (Text Fig. 3) and the embryos of 
birds (Text Fig. 4 and 8) in which a renal portal system is present (see 
Miller, 03, Fig. 6). 
Il. Perrtop oF DIVERGENCE. 
The final changes which take place in connection with the development 
of the venous system, subsequent to those represented by Text Figs. 3, 
4 and 6, and which lead up to the adult condition are in reptiles, birds 
and the rabbit somewhat divergent. 
In reptiles the fundamental plan of the venous system as represented 
by Text Fig. 3, is, with slight modifications, retained in the adult. The 
posterior revehent veins which remain, for the most part, separate in 
Lacerta and snakes and fuse to form a single vein in turtles (Stromsten, 
05) function as the reyehent veins of the permanent kidneys. The ad- 
vehent veins, on the other hand, are formed from the caudal divisions of 
the postcardinals which, after giving up their connections with the ducts 
of Cuvier, return blood from the hinder end of the body to the permanent 
kidneys. 
In order to attain the adult condition in birds (Text Fig. 5), in which 
a renal portal system is absent,’ the Vv. renales magne (the revehent 
veins of the permanent kidneys) grow caudad from the caudal end of 
the postcava (pars subcardinalis) and, at the level of the external iliac 
veins, anastomose with the postcardinals. A continuous channel 
is thereby established, on each side, between the hinder end of the body 
and the posteava through which the blood may flow without previously 
passing through the mesonephric circulation (see Miller, 03, Fig. 7). 
The postcardinals which he craniad of their anastomosis with the 
great renal veins atrophy, while,those which le caudad of the same fuse 
at their caudal ends and persist in the adult as the so-called internal 
iliac veins (Miller). Unless the unpaired portion of the postcava to- 
gether with the internal ihac veins (postcardinals) may be regarded as 
representing a type of bifurcated or double posteava, it is evident that 
the postcardinal veins or any part of the same do not, in birds, as in the 
rabbit, enter into the formation of the adult postcava, since the latter 
° Parker and Haswell, 97 (page 375), figure and describe the presence of a 
partial renal portal system in adult birds (pigeon). This system in the adult, 
however, differs fundamentally from that in the embryo in that the revehent 
veins (Vv. renales magnae) are independent formations and are not formed 
from the subcardinal veins. 
