THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



261 



vena cava, the latter vessel comes to lie ventral to the ureter (Fig. 236, A) . On the 

 left side the cardinal atrophies, leaving only the portion cranial to the loop as 

 the proximal end of the internal spermatic (testicular or ovarian) vein (Fig. 

 236, B).» Since on the left side the original anastomosis between the subcar- 

 dinals 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. 



Inf. vena cava 



Suprarenal gland A- 



Suprarenal vein (right) 

 ■° Renal vein (right) 



Int. spermatic (right) 

 Ureter 



Inf. vena cava 

 (right post, cardinal) 



Common iliac (right) 



Inf. vena cava 



r\ Suprarenal gland 



-•"-". Suprarenal vein (left) 



Kidney 



Renal vein (left) 



Int. spermatic "(left) 

 (post, cardinal) 



'^ Ureter 



Common iliac (left) 



Ext. iliac 

 Int. iliac 

 ■* Common iliac (right) 



Fig. 236. — Diagrams representing final stages in the development of the inferior vena cava 

 (compare with Fig. 233). Slightly modified from Hochstetter. 



Very recent investigations (Huntington and McClure) on cat embryos have shown that 

 the venous loop around the ureter is much more extensive than in some other Mammals. 

 In fact the dorsal arm of the loop, to which the name supracardinal vein has been given, 

 extends from the iliac vein to the original anastomosis between the subcardinals and car- 

 dinals. In the further course of development the supracardinals approach each other 

 and finally fuse in the medial line, forming a large single vessel which becomes that portion 

 of the vena cava caudal to the renal veins. In this event both cardinals, which form the 

 ventral arms of the venous loops, atrophy. 



Near the caudal end of each cardinal vein a branch arises which receives the 

 blood from the corresponding lower extremity. Then a transverse anastomosis 

 appears between the two cardinals at this point (Fig. 236, A). Since the 

 portion of the left cardinal caudal to the renal vein atrophies, the anastomosis 

 itself constitutes the left common iliac vein (Fig. 236, B). The right common 

 iliac is, of course, the original branch of the right cardinal. As the iliacs 

 enlarge they form the two great branches of the vena cava. 



