THE MAMMALIAN VENA CAVA POSTERIOR 47 



The ventral position occupied by the left common iliac vein 

 clearly indicates that it has been derived from the ventral 

 portion of the embryonic circumumbilical venous rings (C.C. 

 in fig. 1). Furthermore, the presence of the anastomosis 

 between the right and the left common iliac veins (V.cm.cd.) 

 indicates the persistence of a complete embryonic circumum- 

 bilical venous ring on the right side, as is shown in the com- 

 posite diagram of the veins (C.C., fig. 1). 



It is of interest here to note that during the course of this 

 investigation our prediction that a cava of the Marsupial 

 Type would be found in both cat and man has been completely 

 fulfilled. 



The table that follows shows the relative frequency with 

 which the different types of vena cava posterior have been 

 observed to occur in the adult cat and in man. The data for 

 the cat include those cases observed by Darrach in his series 

 of 605 adults, as well as those recorded in the literature by 

 other investigators. The data for man include cases which 

 have been recorded in the literature available to us. While 

 the table is in no sense to be regarded as complete, it never- 

 theless brings out many interesting points regarding the rela- 

 tive distribution of the different types of venae cavae as 

 thus far determined bv us in the cat and in man. 



