Charles F. W. McClure 205 
ternal iliac veins it is seen, on comparing the figures, that the relations 
of the dorsal and ventral portions of the venous rings to the arteries 
which they encircle are the same in all cases in that the ventral portion 
of the venous ring lies nearer the median line than the dorsal portion. 
In view of this last-mentioned relation to the arteries, as well as the cir- 
cumstance that the posteardinal vein (int. iliac) still forms the dorsal 
portion of the venous rings, I am convinced that the two cases cited 
above for the pouch young (Text Fig. 15) and the adult (Fig. 17, Plate 
IV, Part I), respectively, represent instances in which the ventral portion 
of the circumarterial venous rings of the 8 mm. embryo has been re- 
tained, and that the change in the position of the rings has been brought 
about secondarily as the result of a growth of the embryo. 
It has already been stated that the Jeft internal iliac vein of the 14 mm. 
pouch young in question (Text Fig. 15) joins the external iliac of the 
same side, exclusively on the ventral aspect of the left common iliac 
artery. It is plain from what we have learned from the reconstructions 
of the 8 mm. embryos that this ventral connection cannot have been 
formed by the posteardinal vein since the latter les dorsal to the um- 
bilical arteries of the embryo. This ventral union, therefore, can be ac- 
counted for only on the grounds that it has either been formed through 
the persistence of the same class of vessels as those that le ventral to the 
umbilical arteries in the embryo ; or, as will be described later on, through 
the persistence of a vein which has been secondarily developed in connec- 
tion with the V. pudendovesicalis. Whatever the case may be, the large 
size of the ventral anastomosis between the internal and external iliac 
veins of the left side is undoubtedly correlated with the complete atrophy 
of the vessel which, in the embryo, formed a union between these two 
veins dorsal to the artery. 
Text Fig. 17, which is a reconstruction of the venous system of a 
15 mm. pouch young, presents a somewhat different arrangement ol the 
veins which unite to form the posteava, from that just described for the 
14 mm. pouch young (Text Fig. 15). In this case (Text Fig. 17) the 
internal iliac veins unite with the external iliacs to form the posteava by 
means of five vessels, three of which lie ventral and two dorsal to the 
common iliac arteries. It is further seen that these dorsal and ventral 
connections between the iliac veins form, on each side, two complete cir- 
cumarterial venous rings, both of which encircle the common iliac artery. 
The ventral portion of the more medially situated rings is formed by a 
common vessel which is situated in the mid-ventral line and which is 
continuous caudad with the caudal veins; while the dorsal portion of 
the medial rings is formed by the postcardinal veins which also form 
