312 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



As the mesonephroi (see Excretory Organs) reach the hinder end of 

 the coelom, the caudal vein loses its primitive connexion with the 

 subintestinal vein and becomes connected with a pair of vessels, the 

 subcardinal veins, which develop between the two mesonephroi and 

 ventral to the nephridial arteries (fig. 331, J5). The blood from the 

 tail now goes through the subcardinals and from them into the 

 excretory organs, passing through a system of capillaries, to be 



Fig. 332.; — Relations and modifications of the post- and subcardinal, abdominal and 

 postcaval veins in different stages of the amphibia. In A the veins (il) from the hind 

 limb return directly to the heart by the lateral abdominal veins {la), while the blood from 

 the tail (c) passes by way of the subcardinals (sc) through the mesonephroi to the post- 

 cardinals (pc). In B the lateral abdominals have united in front to form the anterior 

 abdominal vein (aa); the iliacs have sent a branch to the postcardinals, which have 

 grown back to join the caudals, while the subcardinals have lost their connexion with 

 the caudal and have acquired one with the postcava (/>), a backward growth from the 

 sinus venosus. In C the postcardinals have been interrupted, the posterior half of each 

 now forming an advehent vtin{av) while the subcardinals, as in B; form the revehent 

 veins (r). 



gathered again in the postcardinals and by them to be returned to 

 the heart. Here, then, there is another portal system (p. 309), the 

 first renal-portal system, which may be modified later, as will be 

 described below. 



With the development of the limbs corresponding veins arise 

 (fig. 332), a subclavian vein for each for ejimb, a common jUac for 

 jthe hind le g, t hese bringing the blood from the appendage to the 

 trunk. In the young each subclavian empties into the postcardinal 



