192 Venous System of Didelphys Marsupials (L) 
stant character up to a late period of development, while in the adult its 
presence is an exception. Its constancy during the developmental period 
may account, however, for the fairly large percentage (30 per cent) of 
cases observed by the writer in which the vein was present in the adult, 
but in which it was extremely variable in character and confined to the 
first few intercostal spaces (McClure, 03, p. 383). 
A connection between the caudal end of the azygos vein and the post- 
cava is apparently not of constant occurrence among all adult marsupials. 
In Phalangista, Beddard, 95, found such a connection in only one of 
several individuals examined, and in this case, on account of its large 
size, it practically took the place of the posteava. 
The writer’s interpretation of the azygos and cardinal collateral veins 
seems the one best fitted to the conditions met with in the pouch young 
at hand, although it is possible that it might be shghtly modified if 
some of the material studied were in a better state of preservation. 
The cardinal collateral veins of Didelphys may possibly be regarded 
by some as corresponding to the derivative of the postcardinal 
veins which is formed in the embryos of some of the higher mammals on 
the dorsomedial side of the ureters (rabbit) and permanent kidneys 
(cat). I cannot accept this view, however, for the reasons that the 
cardinal collaterals occupy an entirely different position with respect to 
the aorta (Fig. 56, Plate V), and also appear to have a different mode 
of origin than the derivative of the postcardinal veins in question. 
THE COMPLETION OF THE POSTCAVA. 
Up to and including the stages of development represented by the 
11.5-12 mm. embryos and the 11.5 mm. pouch young the unpaired post- 
cava as met with in the adult is as yet incomplete, since the portion which 
forms the caudal continuation of the pars subcardinalis has not been 
fully established. In the 8 and 11.55-12 mm. embryos, as well as in the 
10.5 and 11.5 mm. pouch young, the postecava consists of an unpaired 
vessel which extends between the sinus venosus and a point in the lumbar 
region where it anastomoses with the two postcardinal veins which still 
form its principal caudal continuation. This unpaired portion of the 
postcava consists embryologically of two independent divisions: One 
formed between the sinus venosus and the hepatico-subeardinal junction, 
in a manner yet to be determined; the other between the hepatico-sub- 
cardinal junction and the junction between the posteava and the post- 
cardinals which is formed in part by the hepatic sinusoids, but chiefly by 
the right subeardinal vein. The unpaired postcava, as thus formed, re- 
ceives most of the blood collected by the tributaries of the external and 
