Charles F. W. MecClure.- 189 
The cardinal collateral veins in the 11.5-12, as in the 8 mm. embryos, 
constitute an extremely complicated system of vessels which lie in the 
tissue ventral and ventrolateral to the aorta between the postcardinal 
veins (Fig. 56, Plate V). Here they form frequent anastomoses with 
the postcardinals and with veins which occupy the position of the pos- 
terior reyehent veins (subcardinals). They, also, at their caudal ends, 
join the posteardinals and appear, as in the 8 mm. embryos, to form 
the ventral portions of circumarterial venous rings which encircle the 
origins of the umbilical arteries. 
In the 14 and 15 mm. pouch young of Didelphys the azygos veins 
show a marked advance in their development over that met with in the 
preceding stages; an advance which is undoubtedly correlated with the 
degeneration of the mesonephroi and the mesonephric divisions of the 
postcardinal veins. 
A right and a left azygos vein are present in the thoracic region. The 
vein of the right side is smal! in ealiber and, on being traced caudad, 
appears to terminate in the thoracic region. The vein of the left 
side possesses a large caliber at its cranial end, but gradually dimin- 
ishes in size from before backward where, as a rule, it becomes 
directly continuous with a single azygos vein of the lumbar region. AlI- 
though a direct anastomosis is established between the azygos veins of 
the lumbar and thoracic regions the character of the connection is such 
that the two systems, even at this stage of development, are, as in the 
preceding stages, practically independent of each other. 
The right thoracic azygos along its entire extent hes ventral to the 
segmental branches of the aorta. The left thoracic azygos, for a portion 
of its course, occupies the same relative position, but, at the caudal end 
of the thoracic cavity, where it becomes continuous with the lumbar 
azygos vein, it les dorsal instead of ventral to these arteries. 
The lumbar azygos system is represented by a single vein which les 
dorsal to the segmental branches of the aorta. It increases in size from 
before backward and in places frequently possesses a caliber as large as 
that of the postcava (Fig. 57, Plate V). The renal veins have both been 
formed; the vein of the left side taps the left anterior revehent vein 
near its point of junction with the postcava, as will be described further 
on. Caudad of the renal veins and between the latter and the junction 
of the posteava with the two posteardinals, the lumbar azygos vein anas- 
tomoses in a number of places directly with the postcava, so that the 
blood collected by its tributaries is now returned to the heart by the 
posteava. The anastomoses which are formed between these two veins 
are extremely variable in their character, since they may be formed on 
