Charles F. W. McClure 191 
ous vessel, and the blood from this section of the lumbar region is re- 
turned, as in the adult, directly to the posteava, by means of lumbar 
veins. These lumbar veins are undoubtedly formed through the per- 
sistence of the anastomoses that were formed at an earlier stage between 
the continuous lumbar azygos vein and the postcava. As these anas- 
tomoses may occur on either side or on both sides of the aorta, we find 
in them an explanation of the variations which were described in Part 
I of this paper in connection with the lumbar veins of the adult (p. 387), 
as well as those concerning the manner in which the caudal end of the 
azygos may join the posteava (p. 382).° 
The left azygos vein of the 17 mm. pouch young occupies exactly the 
same relative position with respect to the segmental branches of the aorta 
as in the adult (see Plate I, Fig. 28), in which along its cranial half it 
lies ventral and along its caudal half it lies dorsal to the segmental ar- 
teries of the left side. The transition from the ventral to the dorsal po- 
sition takes place at the caudal end of the thoracic cavity (about oppo- 
site the 10th thoracic vertebra in the adult) at a level which probably 
marks the embryonic point of union of the two originally separate com- 
ponents of the left azygos channel. It is, therefore, evident that the 
original positions occupied in the pouch young by the azygos veins of the 
thoracic and lumbar regions are retained in the adult. 
A tight azygos vein which opens into the right precava is also present 
in the 17 mm. pouch young. It is a small and insignificant vessel which 
lies ventral to the segmental branches of the aorta and which is confined 
to the anterior portion of the thoracic cavity. 
It is an interesting fact that the right azygos vein is apparently a con- 
‘In four of the five adult specimens of Petrogale penicillata recently 
examined by the writer two essentially independent subdivisions of the azygos 
system were met with similar to those described above for the pouch young 
of Didelphys. The thoracic region was drained chiefly by a right thoracic 
azygos vein, while the lumbar region was drained, for the most part, by a 
single continuous vein which extended forward, dorsal to the aorta, and which 
opened into the postcava near the opening into the latter of the renal veins. 
The lumbar azygos vein, by means of slight connections, anastomosed with 
the postcava at intervals along its course and also received the lumbar veins. 
*The lumbar veins of the adult may open into the postcava either in pairs 
or, on either side of the aorta, by means of a common trunk. The caudal end 
of the azygos vein of the adult may join the postcava either to the left, which 
is the usual method, or to the right of the aorta; or it may bifurcate into 
two branches on the ventral circumference of the aorta which join the post- 
cava on the right and left side of the aorta, respectively. In this case the 
aorta is encircled by a venous ring formed by the azygos and the postcava. 
