424 EMBBYOLOGY OF THE LOWEB VEETEBEATES oh. 



of the anus, where the ventral wall of the primitive alimentary 

 canal has become intensely modified in connexion with the storage 

 of yolk, the subintestinal vein has disappeared from the course of 

 development. 



As regards the dorsal venous system (Fig. 196), the two posterior 

 cardinal veins converge posteriorly and become continuous with the 

 caudal vein. The portions in the region of the opisthonephros 

 become resolved into their external and internal components connected 

 by numerous sinus - like spaces and channels amongst the kidney 

 tubules (Fig. 196, A). With the development of a capillary network 



a.c.u 



a.czr. 



v.c. 



Fig. 196.- 



-Diagram illustrating the development of the dorsal venous system in Lacerta 

 according to Hochstetter, as seen from the ventral side. 



a.c.v, anterior cardinal vein ; c, caudal vein ; d.C, duct of Cuvier ( = ant. vena cava) ; 

 p.c.v, posterior cardinal vein ; p.v.c, posterior vena cava ; s, subclavian vein. 



iliac vein ; 



in the substance of the opisthonephros the larger blood spaces 

 become divided into an afferent set connected with the external com- 

 ponent and an efferent set connected with the internal one. The 

 external channel remains continuous with the caudal vein and forms 

 the renal portal vein. The two internal components fuse together 

 in their anterior portion (Fig. 196, B) and become continuous with 

 the intrahepatic portion of the vena cava. Posteriorly they remain 

 separate and lose their continuity with the caudal vein (Fig. 196, B, C). 

 The blood from the kidneys being now able to pass to the heart by 

 the direct route through the posterior vena cava, the portions of 

 posterior cardinal lying in front of the kidneys are no longer required 

 and soon disappear (Fig. 196, C). 



