262 



TEXT-BOOK OF EMBRYOLOGY. 



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

 and finally disappear for the greater part. The part of the right subcardinal 

 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 sub- 

 cardinals 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, 233, 236). The right suprarenal 

 vein probably does not represent a persistent right subcardinal, but is a new 

 vessel opening into the vena cava. The portion of each subcardinal caudal 

 to the anastomosis probably disappears 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 car- 



Duct of Cuvier I- 



Right umbilical 



Right omphalomesenteric 



-Duct of Cuvier 



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. 



dinal veins immediately cranial to the anastomosis between the subcardinals 

 and cardinals, that is, just cranial to the renal veins, disappear. The more 

 cranial portion of the right cardinal persists as the azygos vein which receives 

 the intercostal (segmental) branches and opens into the superior vena cava. 

 An oblique anastomosis is formed, dorsal to the aorta, between the two cardinals 

 (Fig. 233). This anastomosis and the portion of the left cardinal caudal to it 

 together form the hemiazygos vein. The portion of the left cardinal cranial 

 to the anastomosis loses its connection with the duct of Cuvier (or coronary 

 sinus) and becomes the accessory hemiazygos vein (Fig. 234). The ascending 

 lumbar veins, which join the azygos and hemiazygos, probably do not represent 

 persistent parts of the caudal ends of the cardinals, but are formed by longi- 

 tudinal anastomoses between the original segmental lumbar veins. 



The changes which occur in the region of the liver are of much importance 

 and result in conditions which bear no resemblance to the primary ones. As 



