Charles F. W. McClure 209 
of a 15 mm. pouch young in which a posicava of Type III, B, has al- 
ready been established ; that is, the type in which the principal union be- 
tween the internal and external iliac veins lies dorsal to the common 
iliac arteries. In this case, however, the connection between the internal 
and external iliac veins which lies ventral to the arteries does not consist 
of a single median vessel as in Text Fig. 17, but has the appearance of 
being formed as the result of a fusion between two medianly situated 
vessels, one of which, the right, follows a somewhat curved course. 
These median ventral vessels in Text Fig. 18 undoubtedly have the same 
origin as that of the single median ventral vessel in Text Fig. 17, al- 
though it is a difficult matter to determine in either case whether the 
ventral portions of one or of both of the embryonic circumarterial venous 
rings are involved in their make-up. The anastomosis between the ven- 
trally situated veins and the right internal iliac in Text Fig. 18 is most 
interesting, however, since it appears to explain the presence in the adult 
variations of a similarly situated vessel which lies ventral to the arteries 
and extends between the internal iliac vein and the external iliac of the 
opposite side. See Figs. 15, 16 and 18 on Plate IV, Part I, for examples 
of this type of variation. ‘There is one marked difference to be noted in 
comparing the venous systems of these two 15 mm. pouch young as rep- 
resented by Text Figs. 17 and 18. In Fig. 17 the ventral portions of 
the lateral rings anastomose with the external iliac veins, while in Fig. 
18 this anastomosis is wanting. Whether this latter condition indicates 
that the ventral portions of the lateral rings are undergoing atrophy or 
that they are merely in the process of formation, it is impossible to 
state, although I believe the former view to be correct, on account 
of the large size and prominence of the vessels which unite the iliacs 
dorsal to the arteries. These dorsal vessels in Text Fig. 18, as well as 
the similarly situated vessels in all of the other pouch young, are de- 
rived from the postcardinal veins. 
Text Fig. 16 represents still another reconstruction of the venous 
system of a 15 mm. pouch young in which a postcava of Type II is al- 
ready established. In this case the connections between the internal and 
external iliac veins lie exclusively dorsal to the common iliac arteries ; 
there being no indication of any vessels which unite these veins ventral 
to the arteries, except the ventral portions of the lateral rings which, as 
in Text Fig. 18, are either in the process of formation, or, as 1s more 
likely, are undergoing atrophy. 
Finally, Text Fig. 14 represents the reconstruction of a 14 mm. pouch 
young in which a postcava of Type IT is also already established. In 
this case there is no anastomosis formed ventral to the common iliac 
