170 Venous System of Didelphys Marsupialis (1) 
Text Fig. 7 and Figs. 1 and 2, Plate I), respectively, to attain a high 
degree of development and to consist, as in reptiles, of two bilaterally 
symmetrical vessels which hold the same relation to the mesonephroi and 
posteardinal veins as the subcardinals do in reptiles. 
(b) After Postcava is Formed.—In connection with the develop- 
ment of the postcava in reptiles the grouad-plan of the venous system, 
as represented by Text Fig. 1, undergoes considerable modification. The 
proximal or hepatic portion of the unpaired postcava in Lacerta grows 
caudad from the V. hepatica revehens dextra and, at a point slightly 
craniad of the origin of the omphalomesenteric artery, anastomoses with 
both subeardinal veins at a point which, for convenience of description, 
may be designated as the hepatico-subcardinal junction. The subcardinal 
veins also anastomose with each other caudad of this artery so that a 
complete venous ring, ventral to the aorta, is formed about the origin 
of the omphalomesenteric artery (Text Fig. 2). This condition is only 
temporary, however, since the anastomosis craniad of the omphalomes- 
enteric artery between the subcardinal of the left side and the hepatic 
portion of the postcava is not long retained, with the result that the right 
side of the venous ring (a portion of the right subcardinal) enters into 
the formation of a portion of the unpaired postcava (pars subcardinalis, 
Text Fig. 3). Correlated with the above changes the caudal vein (La- 
certa) gives up its connections with the subcardinals (Text Fig. 3) and 
joins the postcardinals so that the latter, after giving up their connec- 
tions with the ducts of Cuvier, function as the advehent veins of the 
mesonephroi. ‘The subcardinal veins, on the other hand, through their 
connection with the unpaired portion of the postcava, function as the 
anterior and posterior revehent veins of the mesonephroi. ‘The posterior 
and left anterior revehent veins open into the cross anastomosis between 
the subeardinals behind the omphalomesenteric artery; while the right 
anterior revehent vein opens into the unpaired portion of the postcava, 
somewhat craniad of the anastomosis at the hepatico-subcardinal june- 
tion (Text Fig. 3). 
A ground-plan of the venous system similar to that last described for 
reptiles (Text Fig 3) is also met with in the embryos of birds (chick, 
five days incubation) and the rabbit (thirteen days) as described and 
figured by Miller (03, Fig. 6) and Lewis (02, Figs. 3 and 4, Plate 1 and 
Figs. 5 and 6, Plate 2), respectively. In the case of both the birds 
(Text Fig. 4) and the rabbit (Text Fig. 6) the subcardinal veins have 
anastomosed with each other caudad of the origin of the omphalomesen- 
teric artery and the right subcardinal has been “ tapped” by the hepatic 
circulation at the hepatico-subcardinal junction. The subcardinal sys- 
