THE MAMMALIAN VENA CAVA POSTERIOB 45 



marsupial, these give rise in the lumbar region to the vena 

 cava posterior. The cardinal collaterals in the embryo cat 

 (C.C., figs. 1, 4, and 5), as in marsupials, lie ventral and 

 ventrolateral to the aorta and extend throughout the lumbar 

 region, between the iliac and the renal veins (C.C. in figs. 4 

 and 5). While typical circumumbilical venous rings are pres- 

 ent in the human embryo, the cardinal collateral system of 

 veins in the lumbar region between the iliac and the renal 

 veins is not so prominently developed in the embryo of man as 

 in that of the cat (McClure and Butler, '25). Since cardinal 

 collateral channels are normally met with during the course 

 of ontogeny, they like other normal embryonic veins are po- 

 tential in the adult stage. As a matter of fact, we know of 

 four such instances, two in the cat and two in man. 



Figure 61 represents a case in which the embryonic car- 

 dinal collateral (C.C. in fig. 1), in conjunction with the right 

 and the left posterior cardinal veins (A and D in fig. 1), has 

 persisted in an adult cat. The vena cava posterior repre- 

 sented by this variant is one which we may regard as being 

 a combination of Type AD and of the so-called Marsupial 

 Type. If we compare figures 18 and 19, each of which is a 

 good example of Type AD, with figure 61, we observe that in 

 one case the iliac veins unite dorsal, and in the other, ventral 

 to the aorta, to form the vena cava posterior. Also, if we 

 compare the variant in figure 61 with the marsupial cava in 

 figure 60, we observe that, with exception of the relation 

 which the right ureter bears to the right caval vein, the con- 

 ditions are in each case identical, as regards the relations 

 between the aorta and the right caval vein. 



In the preceding pages we have seen that there are fifteen 

 ways in which the lumbar cava may be formed and carried 

 into the adult stage, due to the persistence of one or more of 

 the embryonic right and left supracardinal and the embryonic 

 right and left posterior cardinal veins. It is therefore evi- 

 dent we may rightly expect that the cardinal collateral vein 

 (C.C. in fig. 1) might persist in the adult cat in conjunction 

 with any one of these fifteen possibilities or Types of caval 



