420 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



venous trunks laid down as in the Lung-fish or Elasmobranoh. 

 There is within the group considerable variability but the variations 

 are as a rule derivable from a primitive type like that of Lepidosiren. 

 Thus in Salmo the subintestinal vein bifurcates in front into the 

 two paired vitelline veins while in numerous other Teleosts (Usox, 

 Belone, Syngnathus, Hippocampus, Gobius) it passes forwards into 

 a median unpaired vitelline vein. Each of these conditions is 

 obviously derivable from that illustrated by Lepidosiren, by the 

 disappearance, on the one hand, of the median, and, on the other, of 

 the paired vitelline veins. 



Amphibia. — In Salamandra (Choronshitzky, 1900) two lateral 

 vitelline veins are described, the right comparatively small in size. 

 Behind the liver rudiment they lie close together near the mid- 

 ventral line and passing forwards they diverge, passing one on each 

 side of the liver rudiment to unite in front of it and form the hind 

 end of the heart. The two veins undergo fusion behind the liver to 

 form the subintestinal vein and in front of the point of fusion the 

 right vein disappears so that, as in the Lung-fish, all the blood 

 passes to the heart round the left side of the liver. The mesenteric 

 vein develops as a branch of the right vitelline vein close to its front 

 end and after the disappearance of the greater part of the right 

 vitelline vein the mesenteric is seen replacing it as the right limb of 

 a horseshoe-shaped arrangement of veins which embraces the diver 

 rudiment from in front. The portion of the vitelline veins in front 

 of the mesenteric breaks up into the hepatic network. The vitelline 

 vein shrinks to an inconspicuous vestige while the mesenteric 

 becomes relatively large and forms the hepatic portal of the adult. 



The posterior cardinal veins (Hochstetter, 1888) run alongside 

 the archinephric ducts, which they more or less surround, to the 

 region of the pronephros where each dilates to form a large 

 pronephric sinus. In the opisthonephric region the vein forms two 

 main channels an inner and an outer (Fig. 194, B, op), the former 

 eventually undergoing fusion with its fellow to form an inter-renal 

 vein which becomes later the renal portion of the posterior vena 

 cava. The outer channel, as in the Lung-fish, becomes continuous 

 with the caudal vein to form the renal portal, while at the front end 

 of the opisthonephros it loses its connexion with the part of the vein 

 lying farther forwards. A venous connexion is established between 

 the front end of the inter-renal vein and the tip of the liver, and the 

 channel which so arises, commencing behind in the inter-renal vein, 

 traversing the substance of the liver and ending in the hepatic vein, 

 forms the posterior vena cava (Fig. 194, C, p.v.c). In later stages 

 the liver tissue disappears over the greater part of the posterior 

 vena cava so that it passes naked through the splanchnocoele. 



The anterior cardinal vein with its intercalated section of lateral 

 cephalic persists as the internal jugular vein of the adult. The 

 Duct of Cuvier also persists in the adult and is now termed the 

 anterior vena cava. 



