188 Venous System of Didelphys Marsupialis (L) 
dorsal to this section of the postcava the lumbar azygos veins send tribu- 
taries into the tissue ventral to the aorta. These tributaries, if they do 
not already anastomose by means of capillaries with the postcava are 
at least preparing to do so, since direct anastomoses between these two 
veins are of constant occurrence in this region in more advanced stages. 
Slightly caudad of the junction of the two postcardinals with the post- 
cava the lumbar azygos system anastomoses with the left postcardinal 
vein( Fig. 50, Plate IV) and further caudad becomes continuous with 
small vessels (cardinal collaterals, Fig. 51, Plate IV) which lie, one on 
each side, ventrolateral to the aorta between the aorta and the ureter, 
and which frequently anastomose with each other ventral to the aorta 
(Fig. 52, Plate IV). As in the case of the 10.5 mm. pouch young, I 
have been unable to establish a connection between the caudal ends of 
these vessels and the postcardinal veins. 
In the 11.5-12 mm. embryos the permanent kidneys have not migrated 
as far forward as in the 11.5 mm. pouch young. The postcardinal veins 
present the same arrangement as in the 10.5 and 11.5 mm. pouch young 
and still form the principal route by means of which the blood reaches 
the root of the postcava from the mesonephroi, the hind limbs and pelvie 
region. The azygos system, however, appears to be more highly developed 
in these embryos than in either the 10.5 or 11.5 mm. pouch young, a 
circumstance which has already been mentioned. 
The right and left azygos veins in the thoracic region open, as in the 
preceding cases, into the ducts of Cuvier. Instead, however, of termi- 
nating blindly at their caudal ends, as in the 10.5 mm. pouch young, they, 
or at least the vein of the left side, are now directly continuous with the 
lumbar azygos veins so that a continuous chain of veins can be traced 
from the ducts of Cuvier to the hinder end of the body. The junction 
of the left anterior revehent vein with the postcava still forms a promi- 
nent landmark at this stage of development, and the section of the un- 
paired postcava which lies caudad of this junction has become somewhat 
elongated and anastomoses freely with the lumbar azygos veins (Fig. 54, 
Plate V). Also, shghtly caudad of the root of the postcava, the lumbar 
azygos veins, by means of ventral prolongations, anastomose with the 
posteardinals (Fig. 55, Plate V), and then become directly continuous 
with the cardinal collateral veins which lie ventrolateral and ventral 
(Fig. 56, Plate V) to the aorta between the two postcardinals. 
The azygos veins of the thoracic region still occupy a somewhat dif- 
ferent position from that occupied by the azygos veins of the lumbar re- 
gion. In the thoracic region they lie ventral to the segmental branches 
of the aorta, while in the lumbar region they lie, for the most part, dorsal 
to these branches. 
