THE VASCULAR SYSTEM 327 



into the posterior cardinals near their fusion with the an- 

 terior ones to form the ductus Cuvieri. The subdavian vein 

 from the anterior fin enters either the lateral vein or the 

 posterior cardinal near the entrance of the latter. In the 

 former case, which may be considered the more primitive, we 

 have the suggestion of the early relation of the lateral 

 vein to the fin-fold, for this condition suggests strongly a 

 primitive one in which the lateral vein received a branch from 

 each metameric element of the fin-fold. When the definite 

 limbs were established by the hypertrophy of an anterior and 

 posterior region and the loss of the intermediate portion, the 

 veins corresponding to the regions retained became large and 

 important, while the rest were somewhat reduced. To ac- 

 count for the retention of a single vein for each appendage, 

 rather than one from each somite represented, one may sup- 

 pose either the retention of one and the loss of the others, or 

 the fusion of several. Since, in the pelvic fin of the skate, 

 there are, in addition to the principal iliac vein, one or two 

 small vessels which open independently into the lateral vein, 

 the former alternative is the more probable. 



The second system begins by a median caudal vein, which 

 starts at the tip of the tail and runs within the haemal arches, 

 upon the ventral side of the aorta. When near the cloaca 

 this vein divides into two lateral branches, which run along 

 the lateral margins of the long and narrow kidneys, and give 

 off to these organs numerous lateral branches, the vena renales 

 advehentes. These break up into a capillary system within the 

 substance of the kidneys and form the renal portal system. 

 'From this capillary net-work the blood is collected along the 

 medial margin of the kidneys by numerous vena renales 

 revehentes, the union of which into a common trunk forms 

 the origin of each posterior cardinal. 



The third, or hepatic portal system, is exactly as given in the 

 embryological sketch. It collects the blood from the intestines 

 and stomach into a common trunk, the portal vein, which enters 

 the liver upon its dorsal side and becomes resolved into capil- 

 laries, as in the former case. From this organ the blood is re- 



