10 CHARLES F. W. McCLUEE AND GEORGE S. HUNTINGTON 



of the lumbar region at about the level at which the embryonic 

 renal veins (R.V.) are found. In this region, by extensive 

 anastomoses, the veins almost completely encircle the aorta, 

 forming a circumaortic venous ring, which we have designated 

 the renal collar. In order to reach the prerenal division of the 

 vena cava, blood in the lumbar supracardinals must pass, on 

 each side of the body, through the lateral portion of the 

 renal collar (Subc.Sprc.Anast.), and blood from the posterior 

 cardinals (Pc., A and Z>), which drain the gonads and the 

 mesonephroi, reaches the prerenal division of the cava 

 through the subcardino-posterior cardinal anastomosis (Subc. 

 Pc.Anast.) without passing through the lateral portion of the 

 renal collar. 



On the right side of the embryo (fig. 2), a complete peri- 

 ureteric venous ring is present, through which the ureter 

 (Ur.) passes. The ventral circumference of this ring is 

 formed by the right lumbar posterior cardinal (Pc.Dext., J.), 

 and its dorsal circumference by the right lumbar supracar- 

 dinal vein (Sprc.Dext., B). On emerging from the kidney, 

 the ureter at first passes dorsal, and farther caudad, ventral 

 to the posterior cardinal, lying between the aorta and this 

 vein. On the left side of the embryo (fig. 3), no periureteric 

 venous ring is present. Its absence is due to the loss of a 

 caudal connection which previously existed between the left 

 lumbar supracardinal (Sprc.Sin., C) and the left lumbar pos- 

 terior cardinal vein (Pc.8in.,D). The relations which the 

 ureter bears to the posterior cardinal and the supracardinal 

 veins remain the same, however, on the left as on the right 

 side. 



The posterior cardinal veins join the renal collar at its 

 ventrocaudal border and collect blood from the mesonephroi 

 and the sex glands (gonads). With the disappearance of the 

 mesonephroi, the posterior cardinal veins retain their connec- 

 tion with the gonads through the subcardinal veins and, 

 together with the subcardinals, form the internal spermatic 

 veins of the adult. 



