CIRCULATORY ORGANS. 2gI 
portion of which now forms a new trunk, carrying blood from the 
posterior part of the body to the heart (figs. 294, 295). 
With the appearance of the postcava changes are introduced in the 
embryonic renal portal circulation ( p. 280) which may be summarized 
as follows: The subcardinals lose their connexion with the caudal vein 
and become connected with each other by transverse vessels (interrenal 
veins) while parts of the postcardinals adjacent to the nephridial 
organs separate from the parts in front, while they grow backward 
Fic. 295.—Development of postcaval system in birds (A, B, sparrow; C, D, chick), 
schematized after A. M. Miller. In A the postcardinals have extended nearly to the 
pelvic region and the subcardinals are appearing as isolated spaces. In B the 
subcardinal spaces are uniting and the capillary system connecting with the postcardinals 
is developing, while the postcava is arising. In C the postcava has united with the 
subcardinal of the right side. az, ischiadic artery; aie, external iliac artery; au, umbilical 
(hypogastric) artery; da, dorsal aorta; m, mesonephric veins; om, omphalomesenteric 
artery; ~, postcava and its anlagen; sc, subcardinal and its elements; vei, external iliac 
vein; vi, ischiadic vein. F 
and connect with the caudal vein (fig. 295). These posterior parts 
of the postcardinals now become the advehent veins of a second 
renal portal system, bringing blood from the tail and hind limbs to the 
excretory organs (mesonephroi). The subcardinals of the two sides 
usually fuse in the middle line, a process initiated by the appearance 
of the interrenal veins, and now act as a revehent vessel, carrying 
blood from the excretory organs to the postcava and the anterior 
