A. M. Miller 297 
umbilical artery, which seems to be a sort of anastomosis between the 
proximal ends of the neighboring vertebral veins (Fig. 10, x, sparrow, cor- 
responding to a chick at the end of 
the 5th day). This dorsal channel Ue 
increases rapidly in size and by the 
following day is as large as the 
ventral channel. The latter chan- 
nel decreases and, as a general rule, 
disappears about the 7th day, thus 
leaving the posteardinal vein.on the 
dorsal side of the umbilical artery. 
(Compare Figs. 4, 5, 6, 10 and 7.) 
c - Fra. 10. Reconstruction from a sparrow 
These observations were also embryo, corresponding to a chick of het 
. Showing venous toop around the umbilica 
made by Hochstetter (93) in the artery. Diet side. X15. (Explanation of 
= oC oC : lettering on page 284.) 
chick. In his article on the reptiles Cee 
(93, p. 495) he explains the formation of the loop around the umbilical 
artery in birds on a mechanical basis by saying that the large umbilical 
artery (in birds) exerts a pressure on the dorsal side of the neighboring 
portion of the postcardinal vein, which bends the vein downward. The 
blood then seeks a more direct course which results in the formation of 
a collateral channel dorsal to the artery, while the ventral portion of 
the loop then disappears. 
There is a possibility that Hochstetter’s explanation would suffice in 
some instances in birds where the loop is very short; but the writer has 
found cases where the loop is very much extended antero-posteriorly ; in 
one case, indeed, it extended almost from the level of the external iliac 
veins to the anastomosis, described above, caudal to the posterior lobe 
of the kidney. Furthermore, McClure (00, 02) has described varia- 
tions in Didelphys where the veins may be dorsal to the artery, or 
ventral to the artery, or both dorsal.and ventral to the artery; and cer- 
tainly no mechanical principle is adequate to account for such extreme 
variations, where a similar loop formation is present in the embryo. It 
seems to the writer that the explanation of the formation of the loop 
around the umbilical artery is to be sought in some underlying physio- 
logical principle. 
RESUME. 
Up to the end of the 5th day of incubation the subecardinal system 
increases in size and importance; from the 5th day on it decreases up 
to the adult stage, where it persists only as a small portion of the stem 
of the postcava and the genital and suprarenal veins. 
