184 Venous System of Didelphys ,Marsupialis (L) 
encircles the umbilical artery ventrally possesses a caliber either 
subequal with or greater than that which encircles it dorsally, as in Text 
Fig. 10 and Fig. 42, Plate III; the other, in which the portion of the 
venous ring which encircles the umbilical artery ventrally possesses a 
smaller caliber than that which encircles it dorsally as in Text Fig. 13 
and Fig. 41, Plate III. The latter type of ring is by far the more com- 
mon of the two since, with one exception, it was characteristic of all the 
S mm. embryos examined. 
As regards the veins which enter into the formation of these venous 
rings there is also considerable variation and, in some cases, it is quite 
impossible to determine definitely how these rings are formed. In all 
of the rings the portion which encircles the umbilical artery dorsally is 
formed by the posteardinal vein. The portion of the ring which encircles 
the umbilical artery ventrally, however, may be formed exclusively by 
the cardinal collateral vein as in Text Fig. 13 (left side), or by a vein 
which appears to be formed as the result of a fusion between the cardinal 
collateral and posterior revehent (subcardinal) veins. In addition to 
the above, in one embryo (Text Fig. 10) the circumarterial venous rings 
appear to be formed exclusively by the postcardinal veins, although it is 
impossible to determine in this case to what extent the cardinal collateral 
veins may have also entered into their formation. 
The variable character of these circumarterial venous rings fore- 
shadows the unusual variations recently described by the writer in Part 
I of this paper as regularly occurring in the adult, and the relationship 
which exists between the two will be considered in connection with an- 
other topic. 
The presence of circumarterial venous rings about the origin of the 
umbilical artery is not so uncommon as is generally supposed to be the 
case. The writer has recently observed these rings in the embryos of a 
lizard (Sceloporus undulatus). Hochstetter, 88, and Miller, 03, have ob- 
served them in the embryos of the chick and the English sparrow (Passer 
domesticus), respectively, and Lewis, (02, Figs. 7 and 8, Plate 2) has 
recently figured them as occurring in a rabbit embryo of 14.5 mm. in 
length. In Sceloporus the portion of the ring which encircles the um- 
bilical artery dorsally disappears before the adult condition is reached, 
while in birds, as well as in the rabbit, it is the ventral portion of the 
ring that atrophies. In Sceloporus and birds the portion of the ring 
which is not formed from the posteardinal vein appears to be formed 
through a longitudinal anastomosis of the somatic branches of the post- 
cardinals, while in the rabbit it appears from Lewis’ figures as if it might 
be formed from the subeardinal vein. Whatever the case may be, I am 
