VEINS 315 



333), a portion of which now forms a new trunk, ca rrying blood frq5Q_ 

 theposterior part of t he body to the hear t;_ 



With the appearance of the postcava changes are introduced in 

 the embryonic renal portal circulation (p. 312) which may be sum- 

 marized 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, and 

 grow backward and connect with the caudal vein (fig. 333). These 

 posterior parts of the postcardinals now become the advehent veins 

 (fig. 332, C) of a second renal portal system, bringing blood from 

 th£_tajl and hind li mbs to th e e xcretor y orga,ns_(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 revehe nt vess el, c arrying blood fr om the excretory organs to the 

 postcava and the anterior portion of the postcardinals which have 

 joined the anterior ends of the subcardinals (fig. 332, C). The 

 changes in the postcardinals and the renal portal system of mammals 

 will be described below. 



In Ceratodus (dipnoi, fig. 334, A ) there are some differences from the above 

 account. Thus the anterior portion of the right postcardinal (not shown in the 

 figure) loses its connexion with the vessels behind and acts as a vertebral vein, 

 taking the blood from the intercostal veins of that side back to the heart. The 

 caudal and the subcardinals form a continuous trunk, the revehent vessels form- 

 ing side branches. The posterior portions of the postcardinals grow back into ' 

 the tail as paired vessels, forming no connexion with the caudal vein. In Prolop- 

 terus the vertebral vein is lacking, the subcardinals are not fused behind, while the 

 advehent veins are connected with the caudal. 



The development of lungs brings about the appearance of one or 

 more pairs jof pulmonary veins which carry the (arterial) bjoodjrorn^ 

 these organs to th e hejrt. These arise as an outgrowth from the 

 left atrial portion of the heart, dividing farther back to reach the two 

 lungs. At no time do the pulmonary veins connect with the sinus 

 venosus, but they always empty into the left atrium (figs. 322, 334). 



The Festal Circulation 



Some features of the foetal circulation of the amniotes have 

 already been alluded to, but the whole may be summarized here. In 

 the amniotes, with the development of a large yolk sac and of the 

 allantois, the vessels on the ventnd side of the body become corre- 



