VENOUS SYSTEM 



331 



A renal portal system occurs in. connection with the embryonic 

 kidney in all Sauropsida, and traces of it can also be recognised 

 in embryos of Echidna. In adult Reptiles, renal portal veins give 

 off branches into the permanent kidney (metanephros, p. 346) : in 



IT 



FIG. 270. DIAGRAM ILLUSTRATING THREE STAGES IN THE DEVELOPMENT OF 

 THE HEPATIC PORTAL SYSTEM. (See next page for c.) 



H, heart ; Sv, sinus venosus ; DC, DC, precavals ; Ci, postcaval ; L, liver ; Om, 

 Om l , Om", the three sections of the omphalo-mesenteric vein (the first still 

 shows its originally paired nature at ft : in stage B, the second section of this 

 vein, which passes through the liver, disappears, so that Om and Om 2 are 

 only connected by capillaries : in stage C, the first section (Om) has quite 

 disappeared, and the umbilical vein (Umb] has become developed); DA, 

 ductus venosus ; *, connection of the umbilical vein with the capillaries of 

 the liver; Vr, revehent veins; Vad, advehent veins; Mes., mesenteric 

 vein, which later gives rise to the hepatic portal ( V.port), receiving 

 blood from the alimentary canal (D) ; Az., azygos ; J7, iliac vein; JY, 

 kidney. 



Birds only a slight indication of such a renal portal system exists, 

 and in Mammals it is entirely wanting. 



As in Fishes, the first veins to appear in the embryo are the 

 omphalo-mesenteric veins (Fig. 270, A), bringing back the blood from 



