THE MAMMALIAN VKNA CAVA POSTERIOR 23 



Another interesting feature of this embryo is the atrophied 

 character of the left side of the renal collar (Subc.Sprc. 

 Anast., fig. 13), into which the left embryonic renal vein opens 

 (R.V.). Ordinarily, the left side of the renal collar, with the 

 left embryonic renal vein, breaks away from the suprarar- 

 dinal (Sprc.), and the left renal vein of the adult cat is 

 thereby formed. The renal vein of the adult cat, formed in 

 this manner by the left embryonic renal vein (J?.F.), the left 

 side of the renal collar (Subc.Sprc. Anast.), and the left sub- 

 cardino-posterior cardinal anastomosis (Subc. PC. Anast., figs. 

 1 and 13), passes ventral to the aorta and receives the left 

 sex vein (posterior cardinal and sex-vein tributaries) before 

 joining the vena cava. However, if the left side of the renal 

 collar with the embryonic renal vein (R.V.) should give up 

 its connection with the subcardino-posterior cardinal anasto- 

 mosis (Subc.Pc.Anast.) and retain its connection with the 

 supracardinal vein (Sprc.), the left renal vein would then 

 pass dorsal to the aorta before joining the caval vein. This 

 latter condition is one not uncommonly found as a variant in 

 the adult cat and in man, and is the normal condition found in 

 the adult bat (Hochstetter, '93; Grosser, '01, and Butler, '27). 



6. Persistence of right posterior cardinal and left 

 supracardinal veins 



Type AC (A and C in fig. 1). Figures 14, 15, and 16, cat (Darrach Series) 



Of the two para-aortic axial veins shown in figures 14, 15, 

 and 16, the vein of the right side is of posterior cardinal 

 origin. This is evident from the relation which the right 

 ureter bears to the vein. As in Type A (fig. 6), the right 

 ureter on emerging from the kidney at first passes dorsal, 

 and, farther caudad, ventral to the right cava. 



The left para-aortic axial vein manifests considerable 

 variation in the three examples shown. In one case (fig. 14) 

 it extends only between the left iliac and the left iliolumbar 

 veins, at which point it passes dorsal to the aorta to join the 

 right caval vein (posterior cardinal). In another case (fig. 

 15) it forms a direct continuation of the left iliac vein and 



