194 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



ends of the pronephros, from which they return the blood to the 

 heart. With the development of the mesonephros they extend 

 farther back until they reach the posterior limits of the abdomi- 

 nal cavity. Each gives off intersegmental veins to bring back 

 a portion of the blood sent into the abdominal walls by the 

 intercostal and lumbar arteries, while in the early stages the 



IV 



VI 



Vlt 



VIII 



FlG. 205. Development of the venous system of selachians, after Rabl and 

 Hochstetter. C, caudal; CS, cardinal sinus; y, jugular; LO, left omphalomes- 

 araic; P, portal; PC, postcardinal ; RO, right omphalomesaraic ; S, sinus venosus ; 

 Sf, subintestinal ; V, vitelline (in V and VII, cloacal loop). Compare VII and 

 F///with Fig. 206. 



veins from both extremities (subclavians and hypogastrics) 

 empty into the same vessels. 



These post cardinals gradually develop a rich vascular plexus 

 in the mesonephros, receiving the blood brought from the tail by 

 the caudal vein, which runs forward between the two Wolffian 

 bodies. When this system is established the connection between 



