CIRCULATORY ORGANS. 



291 



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 



FIG. 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, ai, ischiadic artery; aie, external iliac artery; au, umbilical 

 (hypogastric) artery; da, dorsal aorta; m, mesonephric veins; om, omphalomesenteric 

 artery; p, postcava and its anlagen; sc, subcardinal and its elements; vei, external iliac 

 vein; vi, ischiadic vein. 



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 



