ANATOMY OF A 17.8 MM. HUMAN EMBRYO 89 



which a short piece has been added in plate 4. It persists as 

 the terminal pari of the left renal vein. 



Simultaneously with the transformation of the subcardinal 

 veins that part of the right subcardinal, situated ventral to the 

 developing suprarenal gland has tapped the hepatic sinusoids as 

 described by Lewis ('02). In this manner there has developed 

 that segment of the vena cava inferior, intervening between the 

 subcardinal and common hepatic (V.hepxo7n.) veins. 



The entire vena cava inferior, as shown in plate 4, is composed, 

 therefore, of four parts. These are the supra-ureteral channel; 

 a segment of the original right subcardinal {V .scard.d.) interme- 

 diate between the renal anastomosis and the termination of the 

 anlage of the right suprarenal vein (V.s.-r.d.); a large channel 

 passing through the plica venae cavae formed by the tapping of 

 the hepatic sinusoids by the right subcardinal, and the teniiinal 

 part, the vena hepatica communis {V. hep.com.), which empties 

 into the right atrium through the sinus venosus (S.v.). 



The course of the vena azygos (V.az.) along the lateral side of 

 the aorta has been interrupted in the region of the suprarenal 

 gland, so that it now receives the first ten thoracic intersegmental 

 veins (the first three indirectly, for they unite to form a common 

 trvmk, the vena intercostalis suprema dextra). 



The vena azygos, a little cephalad of the termination of the 

 vena intercostalis suprema, also receives a small ventral trib- 

 utary, the cephalic remnant of an earlier mesonephric sinu- 

 soid, figured in pig embryos, by Lewis ('03), Davis ('10), and 

 Thyng ('11). This vessel has been designated by Davis ('10) 

 the ventro-lateral vein of the mesonephros. 



At the level of the 10th thoracic intersegmental vein the vena 

 azygos communicates with the hemiazygos by a transverse anas- 

 tomosis, passing dorsal to the aorta. Slightly cephalad of this 

 anastomosis tributaries from the suprarenal gland (referred to 

 above) join the azygos. 



Although the left posterior cardinal system has not been recon- 

 structed, the following observations are here recorded for the sake 

 of completeness. 



