Charles F. W. McClure 221 
arteries are invariably present in the embryos of birds, as thus far exam- 
ined (Hochstetter and Miller), although in the adult only the dorsal 
portion of the embryonic ring persists normally as a functional vessel. 
In the embryos of Echidna (Hochstetter) the internal iliac veins 
(posteardinals) at first le ventral to the iliac arteries, but subsequently 
anastomose with the external iliacs by means of two vessels, one on each 
side, which he dorsal to these arteries so. that the postcava is at one time 
formed through a union between its iliae tributaries which takes place 
both dorsal and ventral to the iliac arteries. The adult condition is 
reached as the result of the complete atrophy of the anastomosis between 
the iliac veins which lies ventral to the iliac arteries (see Text Figs. X 
and XI, Part I). In this, as in the preceding cases, however, a fixity of 
type normally prevails in the adult regardless of the conditions which 
prevail in the embryo. 
Among the Australian marsupials it is not as yet known whether cir- 
cumarterial venous rings encircle the umbilical arteries as in the em- 
bryos of Didelphys. There is no question, however, as to the constancy 
with which, in most of these animals, certain ventrally situated vessels 
aid in forming the stem of the postcava in the neighborhood of the iliac 
arteries. 
Cireumarterial venous rings have recently been figured by Lewis (02, 
Plate 2, Figs. 7 and 8) as encircling the origin of the umbilical arteries 
of a 14.5 mm. rabbit embryo. In this case the ventral portions of the 
rings normally atrophy, while only the dorsal or posteardinal element of 
the rings persists in the adult as the functional channel. The persistence 
of the ventral as well as the dorsal portion of such a venous ring may 
possibly account for those interesting abnormalities in the adult mammal 
in which the common iliac artery passes through a foramen in the com- 
mon iliae vein. 
In establishing the adult conditions in Didelphys this embryonic 
eround-plan, as stated above, is not modified in any one definite direc- 
tion, but in such a manner that the resulting condition may be repre- 
sented by any one of the possible combinations which such a ground- 
plan is capable of producing. These possible combinations constitute 
the three types of postcaval variations which have been described in the 
preceding pages under Types I, II and III. The production of these va- 
riations is in every sense a normal procedure, and it is an interesting fact 
that the variations described under Type I (Fig. 1, Plate I, Part I) re- 
semble the usual condition of the adult postcava in the Australian mar- 
supials, while those under Types III, B (Fig. 18, Plate IV) and Type IT 
(Figs. 6 and 7, Plate II, Part I) are identical, respectively, with the em- 
