22 CHARLES F. W. McCLTJEE AND GEORGE S. HUNTINGTON 



ment of both cat and man, it is a potential condition present 

 in the embryo, and, as a variant, might be carried into the 

 adult stage. 



5. Persistence of right posterior cardinal, right supracardinal, 

 and left posterior cardinal veins. Right periureteric 

 venous ring present, through which ureter passes 



Type ABD (A, B, and D in fig. 1) 



So far as we have determined, no example of Type ABD 

 has thus far been observed, either in the adult cat or in any 

 other mammal. As in the case of Type ABCD, Type ABD 

 occurs as a transitory stage in the development of both cat 

 and man and is therefore potential as a variant in the adult 

 stage. 



In this connection it is of interest to study a reconstruction 

 of the veins in the lumbar region of a 21-mm. cat embryo. 

 This reconstruction is instructive for the reason that it illus- 

 trates clearly the method by which venous transformations 

 in the embryo lead to the formation of definite types of cavae. 



.On the right side of this embryo (fig. 12), there is present 

 a periureteric venous ring, through which the ureter passes. 

 The relation which the ureter bears to the ring clearly indi- 

 cates that the veins which enclose the ring are the right pos- 

 terior cardinal (Pc.,A) and the right supracardinal veins 

 (Sprc.,B}. On the left side of the embryo (fig. 13), the left 

 posterior cardinal (Pc.,D) has given up its connection with 

 the supracardinal (Sprc.), though only recently, as is indi- 

 cated, near the ureter (Ur.), by the narrow break in the left 

 posterior cardinal vein (Pc.). We have here an instance in 

 which Type ABD has recently been transformed into Type 

 AB. Also, judging from the difference in size that exists 

 between the right supracardinal (Sprc., fig. 12) and the right 

 posterior cardinal veins (Pc.), it seems probable, if develop- 

 ment had proceeded further, that the right supracardinal 

 (Sprc.) would have continued to atrophy and that the right 

 posterior cardinal (Pc.) would alone have been retained in 

 the adult to form a cava of Type A. 



