260 



TEXT-BOOK OF EMBRYOLOGY. 



original anastomosis between the vena cava and the subcardinal and cardinal. 

 On the left side the anastomosis persists simply as the proximal part of the 

 renal vein (Fig. 236); on the right side the renal vein is a new structure which 

 develops after the kidney has attained practically its final position, and opens 

 into the vena cava secondarily. The inferior vena cava itself is a composite vessel 

 derived from four different anlagen. i. The part which extends from the 

 ductus venosus to the right subcardinal is of independent origin. 2. A short 

 portion is derived from a part of the right subcardinal. 3. Another short por- 

 tion is derived from the cross-anastomosis between the subcardinals and 



Aorta 



Post, cardinal vein 



Mesonephric duct "7T~~f"fl$ ''?''} -y 



Omphalomesenteric cartery S; 



Dorsal mesentery 

 .Coelom 



Left umbilical vein 



FIG. 235. From a transverse section of a 5 mm. human embryo, at the level of the 

 Omphalomesenteric (vitelline, superior mesenteric) artery. 



cardinals. 4. The caudal end is a derivative of the caudal part of the right 

 cardinal (compare Figs. 232, 233, 236). 



Before the caudal end of the left cardinal vein atrophies, an interesting and 

 important change occurs in the relations of the ureters and cardinals. Pri- 

 marily the cardinal veins develop to the ventral side of the ureters. But later 

 a collateral of each cardinal develops to the dorsal side of the ureter. These 

 join the cardinal cranial and caudal to the ureter. In other words, a venous 

 loop is formed around the ureter (Fig. 233). The ventral arm of the loop then 

 atrophies and disappears, leaving the dorsal arm as the direct part of the car- 

 dinal vein. On the right side, where the cardinal persists as a portion of the 



