172 Venous System of Didelphys Marsupialis (L) 
tem is now represented in birds and the rabbit, as in reptiles, (1) by an 
anterior and posterior pair of revehent veins which hold the same rela- 
tions to the unpaired postcava and cross anastomosis between the sub- 
cardinals, as the anterior and posterior revehent veins in reptiles; and 
(2) by a portion of the unpaired postcava (pars subcardinalis) which 
consists, approximately, of that portion of the right subcardinal vein 
which is included between the hepatico-subcardinal junction and the 
cross anastomosis. 
It is evident from Hochstetter’s figures and description of the veins 
in Lacerta that the right side of the venous ring which is formed around 
the origin of the omphalomesenteric artery, and which enters into the 
formation of the unpaired postcava is derived from the right subcardinal 
vein. Such being the case, we then have in Lacerta a portion of the un- 
paired posteava which corresponds in its relations to the subcardinal por- 
tion of the postcava described above for birds and the rabbit, since the 
right side of the venous ring in Lacerta is composed of that portion of 
the right subeardinal, which is included between the hepatico-subcardinal 
junction and the original cross anastomosis between the two subcardinals. 
Miller, 03, has shown that in chick embryos the subcardinal veins may 
occasionally, as in reptiles, form a venous ring around the origin of the 
omphalomesenteric artery and has kindly permitted the writer to pub- 
lish his reconstruction of the same (Text Fig. 8). Although Miller did 
not publish this figure in his paper, he described it as follows on page 
291: “ At about the stage from which Fig. 6 was taken (fifth day of in- 
cubation) the writer found a most interesting exception to the general 
plan of development of the subeardinal system in birds, which exception 
shows a striking combination of the conditions described by Hochstetter 
in reptiles and Echidna.. Anterior to the origin of the A. omphalomes- 
enterica and ventral to the aorta there is present a large anastomosis 
between the right and left subeardinals, just caudal to the point where 
the posteava joins the right subeardinal. Such a remarkable similarity 
to the conditions found in the earlier stages of reptilian development is 
certainly unusual.” The reference to Echidna mentioned in the above 
quotation does not refer to the formation of a venous ring about the 
artery, but to the secondary anastomosis between the subcardinals, as 
shown in the reconstruction. 
There can be no doubt as to the subeardinal character of the right side 
of the venous ring in Miller’s figure of the chick and, also, that it corre- 
sponds, in all essential details, to the right side of the ring in Lacerta. 
At the stages of development represented by Text Figs. 3, 4 and 6, it 
is, therefore, seen that the unpaired postcava, as thus far developed, con- 
