218 Venous System of Didelphys Marsupialis (L) 
sits of that portion of the postcava which les caudal to the renal veins. 
The following table shows the approximate relations which exist be- 
tween the veins in the embryo and the above-mentioned sub-divisions of 
the postcava in the adult: 
ADULT RABBIT EMBRYO DIDELPHYS EMBRYO 
BrehnepatiG irene ceet se V. hepatica communis... a 
FIeNaAtiC: tse eee ce Hepatic sinusoids ..... Hepatic sinusoids and a small 
portion of the right sub- 
cardinal vein. 
IRM Gooagoocoscuocaqamedn, Soloeerobimel 552 Right subcardinal. 
PoOstrenaillare sf aanide Right postcardinal ....Cross anastomosis, (formed 
by both subcardinals), and 
right and left cardinal col- 
lateral veins. 
1. After the permanent kidneys have completed their migration in 
the embryos of the rabbit and cat, renal veins are developed which open 
into the postcava, approximately, at the level at which the latter joins 
the two postcardinal veins. 
In Didelphys, on the other hand, after the permanent kidneys have 
completed their forward migration, renal veins are developed which open 
into the postcava some distance craniad of the latter’s junction with the 
two postcardinals; a circumstance which now explains why the post- 
cava of the adult opossum was never found by the writer to be bifurcated 
as far forward as the level at which the renal veins opened into the post- 
cava. 
The position of the renal veins in Didelphys, with respect to the june- 
tion of the postcardinal veins and the postcava, does not appear to indi- 
cate that the permanent kidneys in Didelphys have undergone, relatively, 
a more extensive migration than in the rabbit and eat, since the renal 
veins, as in the embryos of the rabbit and cat, open into the postcava only 
slightly caudad of the origin of the omphalomesenteric artery, and con- 
tiguous to the junction of the postcava and the left anterior revehent 
vein. The opening of the latter vein into the posteava, however, lies 
relatively, much further craniad of the junction of the postcava and post- 
cardinal veins in the pouch young than is the case in the 8 mm. embryos 
of Didelphys and the embryos of the rabbit and cat. This circum- 
stance I can only account for on the basis that in Didelphys the junction 
of the posteava and the postecardinal veins remains, more or less, as a 
fixed point, in front of which the vessels elongate more rapidly than 
those that lie behind. This growth in length, so far as the postcava 
“ After Lewis (02, page 242). 
