424 EMBRYOLOGY OF THK LoWKII VERTEBRATES CR 



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 



ac.v: 



C 



l'i'.. 196. Diagram illustrating the development of the dorsal venous system in l.nc<'iin 

 according to Hochstetter, as seen from the ventral M<1<-. 



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

 p.c.v, posterior cardinal vein ; /,.<-.<, posterior vena cava ; (, siibrlavian vein. 



in the substance of the opisthonephros the larger blood spares 

 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 {interior portion (Fig. 11)0, B) and become continuous \\ith 

 t.he intrahepatie port KM, ol' tin- vena cava. 1'ostrriorly they remain 



separate and loae their continuity with ttfe eaudal vein (Fig. I!'*;. B,0) 



Tin- iilood j'rniu the kidneys bring now able t< pass to the heart by 

 the direct route through the posterior vena ea\a. the portions of 

 posterior (ordinal lying in front of tin- kidneys are no longer n-juire(l 

 and soon disappear (Fig. 196, C). 



