THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



227 



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 portion is derived from the cross-anastomosis 

 between the subcardinals and cardinals. 4. The caudal end is a derivative 

 of the caudal part of the right cardinal (compare Figs. 194, 195, 198.) 



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

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

 Primarily 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 



Inf. vena cava 



Suprarenal gland -f- 



Suprarenal vein (right) 



Renal vein (right) f" 

 \ ' 



Int. spermatic (right) 

 Ureter 



Inf. vena cava 

 (right post, cardinal) 



Common iliac (right) 



Int. vena cava 



Suprarenal gland 



r.- Suprarenal vein (lefU 



..]... Kidney 



*- Renal vein (left) 



Int. spermatic"(left) 

 (post, cardinal) 



% i Ureter 



Common iliac (left) 



Ext. iliac 



'* - Int. iliac 



.-.,...,, Common iliac (right) 



B 



FIG. 198. Diagrams representing final stages in the development of the inferior vena cava 

 (compare with Fig. 195). Slightly modified from Hochstetter. 



venous loop is formed around the ureter (Fig. 195). The ventral arm of the 

 loop then atrophies and disappears, leaving the dorsal arm as the direct part 

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

 portion of the vena cava, the latter vessel comes to lie ventral to the ureter 

 (Fig. 198, A). On the left side the cardinal atrophies, leaving only the por- 

 tion cranial to the loop as the proximal end of the internal spermatic (testicular 

 or ovarian ) vein (Fig. 198, B). Since on the left side the original anastomosis 

 between the subcardinals and cardinals persists as the renal vein, the left 

 internal spermatic is a branch of the renal. The right internal spermatic 

 vein probably represents a branch of the vena cava which is independent 

 of the cardinal. 



In the cat embryo the venous loop around the ureter is much more 



