212 



THE ELASMOBRANCH FISHES 



sinuses. Where the two vessels fuse together the walls between the two sides 

 are more or less broken down and the remaining walls are supported by 

 trabeculae. These hepatic sinuses may empty by relatively small apertures 

 near the middle line into the sinus venosus (Scyllium). In certain types the 

 hepatic veins enter the outer tips of the duet of Cuvier (Torpedo, Raja) . 



DEVELOPMENT OF HEPATIC POETAL SYSTEM 



In the embryo, the vitelline veins from the yolk sac (v. v., fig. 195) are among 

 the first vessels to appear. These are followed by the subintestinal vein (s.i.), 

 previously mentioned, which unites with the vitelline to form an omphalo- 



^ 



i 



C 



D 



Fig. 195. Diagram of development of hepatic portal system in Elasmobranchs, ventral view. 

 (From Eabl, modified.) 



a.c.v., anterior cardinal vein; cd.v., caudal vein; 7;.^;., hepatic vein; Iv., liver capillaries; 

 om., omphalomesenteric vein; p.c, postcardinal ; p.i.v., posterior intestinal vein; r.p., renal 

 portal vein; s.i., subintestinal (intraintestinal) vein; s.v., sinus venosus; v.v., vitelline vein. 



mesenteric. The right vein remains rudimentary, but the left omphalomes- 

 enteric becomes an important vessel (om..). When the developing liver comes 

 in contact with the omphalomesenteric the latter vessel sends branches into 

 the tissue of the liver and divides into two parts, each of which forms a series 

 of capillaries in the liver (Iv. fig. 195c) . After the absorption of yolk, and the 

 consequent loss of the vitelline veins, the main stem of the hepatic portal sys- 

 tem is along the subintestinal line. This vein in the adult is carried in with the 

 develo])ing valve into the valvular intestine, as the intraintestinal, to drain the 

 free margin of the valve. There is next formed the posterior intestinal or 

 mesenteric vein (p.i.v., fig. 195b-c). Blood now empties by the united sub- 

 intestinal and posterior intestinal into the hepatic portal and this empties into 

 the liver. From the liver the blood is collected and passes to the sinus venosus 



