COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 205 



Fig. 55. — Diagrams to show the development of the veins in elasmobranchs. A, earliest stage, 

 showing the two vitelline veins a passing through the liver q into the sinus venosus p. B, the sub- 

 intestinal vein b has appeared; it connects with one of the vitellines, makes a loop t around the anus, 

 and continues into the tail as the caudal vein j. C, the anterior and posterior cardinal veins, c and /, 

 have appeared and connect with the sinus by way of the common cardinal vein d; the vitelline veins 

 are breaking up in the' liver at r and caudal to the liver are connected by ring-shaped anastomoses at s. 

 D, the vitelline veins have broken up into a network of capillaries in the liver at v; their proximal por- 

 tions remain as the hepatic veins e; their distal portions have formed the hepatic portal vein g, which 

 is continuous with the subintestinal b. E, the posterior cardinals / have extended posteriorly and at u 

 have joined the loop t formed by the caudal vein j around the anus; the subintestinal has severed its 

 connection with the caudal; the lateral vein i and its tributary, the subclavian vein h, have appeared. 



F, a new vein, the subcardinal vein k, has appeared between the kidneys and connects with the posterior 

 parts of the posterior cardinals / by means of cross-vessels 11 and also connects with the caudal vein j. 



G, the posterior cardinal veins have joined the subcardinals k at w, their intermediate portions m dis- 

 appearing; the posterior parts of the posterior cardinals persist as the renal portal veins x, which flow 

 into a network of capillaries in the kidneys; the lateral abdominal veins have grown posteriorly and 

 developed iliac tributaries I from the pelvic fins, a, vitelline vein; b, subintestinal vein; c, anterior 

 cardinal vein; d, duct of Cuvier or common cardinal vein; e, hepatic vein; /, posterior cardinal vein; 

 g, hepatic portal vein; h, subclavian vein; i, lateral abdominal vein; j, caudal vein; k, subcardinal 

 vein; /, iliac vein; m, obliterated portion of the posterior cardinals; n, communications between sub- 

 cardinals and renal portals; 0, capillary network in kidneys; p, sinus venosus; q, liver; r, branching 

 of vitelline veins in the liver; s, rings between the two vitellines; t, loop around the anus; it, union 

 of posterior cardinals with the caudal vein; v, capillary network between hepatic portal and hepatic 

 veins; w, union of posterior cardinals with subcardinals; x, renal portal veins. (Slightly modified 

 after Hochstetter in Hertwig's Handbuch der vergleichenden mid experhnentellen Entwickelimgslehre 

 der Wirbeltiere.) 



