THE MAMMALIAN VENA CAVA POSTERIOR 35 



11. Persistence of right and left supracardinal, and left 



posterior cardinal veins. Periureteric venous ring 



present on left side, through which ureter passes 



Type BCD (B, C, and D in fig. 1) 



So far as we are aware, no example of Type BCD has thus 

 far been observed either in the adult cat or in man. An inter- 

 esting example of Type BCD, however, in which the right and 

 the left supracardinal and the left posterior cardinal veins 

 are involved, has been found to occur as a transitory onto- 

 genetic stage in a 17-mm. embryo of the cat. Here, in figures 

 44 and 45, of the right and the left sides, respectively, of a 

 reconstruction of the veins, both supracardinals (Sprc.Dext. 

 and Sin. ) are still functional as embryonic caval veins and 

 the left supracardinal (Sprc.Sin., fig. 45), with the left pos- 

 terior cardinal vein (Pc.Sin., fig. 45), forms a complete peri- 

 ureteric venous ring through which the ureter passes. It is 

 evident that, potentially, the condition presented in this 

 embryo may be carried into the adult stage. The chances 

 are, however, that if development had proceeded to the adult 

 stage, only the right supracardinal (Sprc.Dext., fig. 44) and 

 the left posterior cardinal veins (Pc.Sin., fig. 45) would have 

 been retained to establish a cava of Type BD, as shown in 

 figures 46 and 47. The large size of the right supracardinal 

 (Sprc.Dext., fig. 44) and the left posterior cardinal veins (Pc. 

 Sin., fig. 45) leads one to infer that blood would still be di- 

 rected from the iliac veins into these two vessels, at the 

 expense of the smaller left supracardinal (Sprc.Sin., fig. 45) 

 and the left side of the renal collar (Subc.Sprc.Anast., fig. 45). 

 Although Type BCD has not thus far been observed in the 

 adult cat, it is of importance to recognize the fact that it is 

 found in the embryo and is therefore potential in the adult. 



12. Persistence of right supracardinal and left posterior 

 cardinal veins 



Type BD (B and D in fig. 1). Figures 46 and 47, cat (Darrach Series) 



Type BD represents a case of double vena cava posterior. 

 It differs from Type BC (fig. 33) in that the left cava has 



