bs 
220 Venous System of Didelphys Marsupialis (LL) 
iliac veins of the adult under three types. Type I, in which the internal 
iliac veins unite with the external iliacs ventral to the arteries to form 
the posteava (marsupial type); Type II, in which the internal iliac 
veins unite with the external iliacs dorsal to the arteries to form the post- 
cava; Type III, in which the internal iliac veins unite with the external 
iliacs, both dorsal and ventral to the arteries, to form the postcava. 
From an embryological standpoint it is not a difficult matter to deter- 
mine that the establishment of a particular type of postcava in the adult 
Didelphys depends upon the manner, as well as upon the extent to which 
certain well defined vessels in the embryos and pouch young, which le 
dorsal and ventral to the umbilical or common iliac arteries, are affected 
by atrophy during the subsequent stages of development. These dorsally 
and ventrally situated veins are usually met with in the embrvos and 
pouch young in the form of circumarterial venous rings which encircle 
the origin of the arteries in question. In establishing the three types of 
posteaval veins this embryonic ground-plan undergoes the following 
modifications : 
Type I is established as the result of the complete atrophy of the ves- 
sels which he dorsal, and Type II as the result of the complete atrophy 
of the vessels which he ventral to umbilical or common ilhac arteries. 
Type III, which is a combination of Types I and II, is established as 
the result of the persistence of vessels which lie both dorsal and ventral 
to the arteries in question. 
So far as known to the writer, no adult vertebrate has hitherto been 
described which presents such a series of normally occurring variations 
of the venous system as those described by the writer for Didelphys. 
Also, as far as I am aware, a fixity of type normally characterizes the 
main stem of the venous system of all other adult vertebrates so that va- 
riations, when they occur, are exceptions rather than the rule. 
It is an intereseting fact that an essentially similar ground-plan of the 
venous system as that described above for Didelphys, in which a cireum- 
arterial venous ring encircles the origin of the umbilical arteries, is also 
characteristic of the embryos of a number of other vertebrates. These 
vertebrates, however, differ from Didelphys in that the modifications 
which the embryonic ground-plan undergoes take place in a definite 
direction, so that a characteristic or fixed type of venous system normally 
results in the adult. For example: A circumarterial venous ring occa- 
sionally encircles the origin of the umbilical arteries in reptilian em- 
bryos (Sceloporus), although only the ventral or postcardinal portion of 
the ring normally persists in the adult. 
Circumarterial venous rings which encircle the origin of the umbilical 
