THE DEVELOPMENT OF THE VASCULAR SYSTEM 



259 



extensive than in the other forms. The dorsal arm of the loop, named the 

 supracardinal vein, extends from the iliac vein to the original anastomosis 

 between the subcardinals and cardinals. In the course of further develop- 

 ment the supracardinals approach each other and finally fuse, forming a 

 large single vessel which becomes the portion of vena cava caudal to the 

 renal veins. In this event the portions of both cardinals forming the ventral 

 arms of the venous loops atrophy and disappear. 



Xear 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. 



i)uct cf Cuvier.' 



Duct of Cuviet- 



Right umbilical - 



Right omphalomesenteric - 



Ductus venosus 



..Left umbilical 



Left omphalomesenteric 



FIG. 237. Diagrams illustrating two stages in the transformation of the omphalomesenteric 

 and umbilical veins in the liver. Hochstetter. 



With the atrophy of the mesonephroi, the subcardinal veins diminish in 

 size and finally disappear for the greater part. The part of the right sub- 

 cardinal cranial to the point of junction with the vena cava disappears- 

 entirely. The portion of the left subcardinal cranial to the anastomosis 

 between the two subcardinals becomes much reduced in size, but persists 

 as the left suprarenal vein. The left suprarenal vein is thus a branch of the 

 left renal vein, since the latter represents the anastomosis itself (Figs. 232, 

 2 33? 236). The right suprarenal vein probably does not represent a per- 

 sistent right subcardinal, but is a new vessel opening into the vena cava. 

 The portion of each subcardinal caudal to the anastomosis probably dis- 

 appears entirely, but this has not been definitely determined. 



The observations on the development of the azygos veins in the human 

 embryo are only fragmentary. In the rabbit the portions of the posterior 

 cardinal veins immediately cranial to the anastomosis between the sub- 



