168 Venous System of Didelphys Marsupialis (1) 
presence of a renal portal system as is the case in selachians, (amphib- 
ians), reptiles and the embryos of birds. Their presence, therefore, in 
the embryos of mammals in which a renal portal system is usually want- 
ing” is most suggestive, and indicative of a “ ground-type” of venous 
system of which the subcardinal veins form a constituent element. It is 
evident, therefore, from what we at present know of the subcardinal 
veins that they can no longer be regarded as transitory structures of 
little importance, since they form an essential and important element of 
the embryonic venous system in a number of vertebrates, and are retained 
in the adult, to a greater or lesser degree, in accordance with the presence 
or absence there of a renal portal system. 
Since the subeardinal veins play such an important role in the devel- 
opment of the mammalian postcava (rabbit) it may be well, in order to 
better appreciate the conditions in the marsupials, to first give a com- 
parative sketch of the transformations which these veins undergo in rep- 
tiles, birds and the rabbit. , 
The figures recently published by Miller and Lewis show more clearly 
than has hitherto been observed the striking parallelism that exists, up 
to a certain period, between the development of the subcardinal system 
in reptiles, birds and the rabbit. In the latter stages, however, this parall- 
elism ceases to exist, owing to the divergence from the common ground- 
plan which occurs in birds and the rabbit in connection with a partial 
degeneration of the subcardinal veins. For convenience of description, 
therefore, the transformations which the subeardinal veins undergo will 
be considered as they occur under the following periods: I. Period of 
parallelism—(a) before posteava is formed, (b) after postcava is formed ; 
II. Period of divergence. j 
I. PrRIopD oF PARALLELISM. 
(a) Before Postcava is Formed.—According to Hochstetter, 92, 
upon whose investigations the following account of the development of 
the reptilian venous system is based (Lacerta), the veins which subse- 
quently become the Vy. revehentes anteriores and posteriores of the 
mesonephroi at first convey blood to these organs. These veins are 
branches of the caudal vein in Lacerta, but in Tropidonotus arise inde- 
pendently of this vein at the caudal end of the mesonephroi. They con- 
sist at first of two bilaterally symmetrical vessels (Text Fig. 1) which 
lie on the ventromedial side of the mesonephroi (see Hochstetter, 92, 
>See reference to Perameles under Résumé and General Considerations 
(page 223). 
