Charles F. W. McClure 185 
inclined to believe that some of the abnormalities which are met with in 
adult mammals, in which the internal iliac artery passes through a fora- 
men in the common iliac vein, as described by Treadwell, 96, McClure, 
oo, (1) and Weysse, 03, may be accounted for on the ground that they 
represent instances in which these embryonic circumarterial venous rings 
have persisted in the adult. 
Up to this point we have considered more or less in detail the general 
plan of the venous system as met with in the 8 mm. embryo of Didelphys 
and, since the latter represents the youngest stage of Didelphys possessed 
by the writer, its venous system may be taken as the starting point from 
which may be traced the subsequent transformations that lead up to the 
adult condition. From now on, therefore, and beginning with the 8 mm. 
embryo of Didelphys, we will trace in a connected manner through the 
different stages of embryos and pouch young possessed by the writer the 
transformations which the different portions of the venous system un- 
dergo before arriving at the adult stage. 
THr AzycGcos VEINS. 
In the adult of Didelphys there is, as a rule, but one azygos vein pres- 
ent and that is situated on the left side (Fig. 28, Plate I). At its cranial 
end it opens into the left precava about opposite the head of the third 
rib, while at its caudal end it invariably joins the postcava caudad of the 
renal veins and about opposite the second lumbar vertebra. Between its 
point of union with the postcava and about the middle of the tenth tho- 
racic vertebra, the left azygos vein hes dorsal to the segmental branches 
of the aorta; between the tenth thoracic vertebra and its connection with 
the precava, however, it lies ventral to these branches (see McClure, 03, 
pp- 381-2 and Fig. 28, Plate I, at the end of this paper). 
The right azygos vein, when present in the adult, opens into the pre- 
cava about opposite the head of the second rib. It is always a small and 
insignificant vessel, and its tributaries are confined to the first five inter- 
costal spaces of the right side. 
In the 8 mm. embryo of Didelphys, as stated above, each postcardinal 
receives a tributary slightly caudad of its junction with the duct of Cu- 
vier (Text Figs. 10 and 11). Each tributary, which can be traced caudad 
for only a short distance, lies lateral or dorsolateral to the aorta (Fig. 31, 
Plate Il) and ventral to the latter’s segmental branches. ‘These two 
tributaries, as stated above, which appear to be formed through a longi- 
tudinal anastomosis between the somatic branches of the postcardinals, 
together with the proximal ends of the two postcardinals, undoubtedly 
constitute the anlages of the right and left azygos veins. 
13 
