186 Venous System of Didelphys Marsupialis (L) 
It is a curious fact that the azygos veins are more advanced in develop- 
ment in the 11.5-12 mm. embryos than in the youngest of the pouch young 
studied by the writer (10.5 mm.). This circumstance clearly proves 
that the opossums are not born, in all cases, at a corresponding period of 
development, but that some are born at a more advanced stage than others. 
In order, therefore, to give a connected account of the development of the 
azygos system it will be necessary to describe the conditions as met with 
in the 11.5-12 mm. embryos after those of the youngest pouch young 
have been considered. 
In the 10.5 mm. pouch young two azygos veins are present in the tho- 
racic region which, as in the 8 mm. embryo, open dorsally into the ducts 
of Cuvier, the opening of the right vein being somewhat craniad of that of 
the left. These two veins, as stated above, are formed from the cranial 
ends of the two postcardinals as well as from veins which have united 
with the latter and which have probably been formed through a longi- 
tudinal anastomosis between the somatic branches of the postcardinals. 
The right azygos can be traced caudad from its connection with the 
duct of Cuvier for about 89 sections where it appears to termi- 
nate as a small capillary vessel which hes on the ventral surface of the 
vertebral column. The left azygos is, however, of much greater extent 
and can be traced caudad as a continuous vessel for about 156 sections 
where it then appears to terminate in the region slightly caudad of the 
point where the omphalomesenteric vein enters the liver. Each azygos 
vein, along its entire extent, hes dorsolateral to the aorta and ventral to 
the segmental arteries and, at intervals along its course, receives tribu- 
taries from the body walls contiguous to the vertebral column (Fig. 43, 
Plate III). Somewhat caudad of the apparent termination of the left 
azygos vein (37 sections) small capillary vessels are met with which lie 
in the tissue dorsal and dorsolateral to the aorta and dorsal to the seg- 
mental arteries, which become more prominent near the origin of the 
omphalomesenteric artery and especially so, further caudad, in the neigh- 
borhood of the permanent kidneys (Fig. 46, Plate IV). These vessels 
can be traced without difficulty caudad of the anastomosis between the 
pars subcardinalis and the postcardinal veins where they form frequent 
anastomoses with vessels which le in the tissue ventral to the aorta 
(Figs. 47 and 48, Plate IV). These latter or ventral vessels with which 
they anastomose (Figs. 47 and 48, Plate IV) are, in my estimation, rep- 
resentatives of the cardinal collateral veins which have been described 
above in connection with the 8 mm. embryos. They can be traced caudad 
almost as far as the origin of the umbilical arteries, but whether they 
join the posteardinal veins at their caudal ends I am unable to determine 
definitely. 
