290 The Development of the Postcaval Vein in Birds 
largement of the subcardinal veins the stem of the postcava attains cor- 
responding proportions. At the end of the 5th day of incubation the 
subcardinal system has reached its height of development, and at this 
stage is seen the immense importance of the subcardinal veins in the 
development of the postcava. They form the entwre efferent system, 
and moreover a great quantity of blood passes through the renal portal 
system, for the anterior ends of the postcardinals by this time have be- 
come much diminished in size and are unable to carry a very large pro- 
portion of the blood to the duct of Cuvier. 
The significance of these subcardinal veins in birds is even better 
appreciated by a glance at the conditions found in adult reptiles, where 
the efferent veins of the renal portal system persist in part as paired 
vessels which connect anteriorly with the unpaired portion of the post- 
cava (Hochstetter, 93). The efferent vessels in birds are undoubtedly 
ihe homologues of the paired portion of the postcava in reptiles. In 
mammals also Hochstetter (93, 96) has described a pair of vessels run- 
ning on the median side of the mesonephros, which unite with the stern 
of the postcava; McClure has observed the same conditions in Didelphys 
and Lewis (02) has described these vessels in the rabbit under the 
name of the subcardinal veins. The subcardinals in birds are certainly 
homologous with the vessels in mammals described by the above inves- 
tigators, even though a true renal portal system is not established in 
the higher mammals. Hochstetter has already mentioned the homology 
of the subcardinal veins, running through the three classes of animals 
mentioned above, but it has not been brought out with sufficient em- 
phasis. 
Fig. 6 (chick of 5 days’ incubation) shows the large above-mentioned 
anastomosis between the two subcardinals caudal to the A. omphalo- 
mesenterica. Note the great relative size of the subcardinal veins and 
the branches (a) from the mesonephroi emptying into them; there are 
also many smaller branches which have been omitted in the reconstruc- 
tion. The anterior ends of the postcardinals are seen to be considerably 
diminished in size. The stem of the postcava has become a relatively 
large vessel, for it now carries a great quantity of blood to the heart. 
The subcardinal veins have been indicated by crosses throughout the 
series of illustrations; and a comparison of Figs. 2, 4, 5 and 6 will 
enable one to trace clearly the progressive development of the subcar- 
dinal system up to its height at the end of the 5th day of incubation 
in the chick, and furthermore will enable one to appreciate the vast 
importance of the subeardinal system in the earlier stages of develop- 
ment. 
