286 The Development of the Postcaval Vein in Birds 
In his article on the development of the postcaval vein in mammals 
Lewis (02) has attempted to explain the presence of the subcardinals 
on the ground that the course of the blood through the postcardinals is 
impeded by the mesonephric tubules and the recurrent bend of the duct 
of Cuvier, and hence these vessels are the result of an attempt of the 
postcardinals to disentangle themselves from the mesonephros. He has 
apparently overlooked the fact that the subcardinal veins have their an- 
cestors, so to speak, in similar structures in birds and reptiles, which 
have been described by Hochstetter and others under the name of the 
efferent veins of the*primitive kidney, and consequently have not been 
called into existence in mammals by any physiological reason. Further- 
more, Lewis states that the subcardinals are tributaries of the postcar- 
dinals, formed from certain branches of the latter veins which pass 
ventrad between the mesonephros and the aorta. However this may be 
in mammals, it is certainly not the case in the chick or sparrow. As has 
been stated earlier in this article, the subcardinals arise as unconnected 
vessels or islands, which are without a doubt independent structures ; 
and the connections with the postcardinals are formed later and sec- 
ondarily. 
It is a noteworthy fact that in birds, at least, the subcardinal veins 
appear and attain a considerable size before the postcava begins to de- 
velop. This is plainly seen in Figs. 4 and 5, where the postcava (V. ce. i.) 
is in its incipient stage, and in Fig. 4 is composed of merely a few islands 
situated in the anlage of the liver and the caval mesentery. 
The writer has carefully reconstructed in wax the veins in the liver 
region and has found almost a perfect agreement with Hochstetter’s de- 
scription and illustrations. However, to clear up some slight doubt in re- 
gard to the exact point where the postcava joins the ductus venosus, it has 
been thought best to insert a figure and brief description of the conditions 
found in the sparrow at a very early stage of development (Fig. 5, spar- 
row corresponding to a chick of 90 hours). As shown in this figure the 
right and left omphalomesenteric veins fuse at a very early stage to form 
the ductus venosus (D. V.). The sinus venosus (S. V.), which is virtu- 
ally a continuation of the ductus venosus, is formed by a union of the lat- 
ter vessel and the two ducts of Cuvier (D. C. d and s), and empties into 
the right auricle of the heart. The right and left umbilical veins 
(V.u.d ands) join the right and left ducts of Cuvier respectively some 
distance from the opening of the latter vessels into the sinus venosus. 
The posteaval vein (V. ec. 1.) at this stage has not joined the ductus 
venosus. But its point of connection, as shown by a closely succeeding 
stage, is found to be on the dorsal side of the ductus venosus just caudal 
