44 CHARLES F. W. MCCLURE AND GEORGE S. HUNTINGTON 



a form at the renal level as that shown in figure 59. In all 

 three cases, however, the portion of the venous ring which lies 

 dorsal to the aorta has resulted from the persistence of an 

 intersupracardinal anastomosis (Int.Sprc.Anast., fig. 1), 

 between the supracardinal veins. 



The three cases above mentioned represent the only in- 

 stances on record in which a venous ring, encircling the aorta 

 at or near the renal level, has been carried into the adult. 



18. Persistence of subaortic plexus (embryonic cardinal 

 collateral veins) 



Marsupial Type of vena cava posterior. (See C.C. in fig. 1 and in figs. 4 and 5, 

 for cardinal collateral veins). Figure 61, cat (Darrach Series); 62, cat 

 (Princeton Morphological Museum, no. 603), and 63, man (Wicke, '27) 



In most placental mammals the iliac veins unite to form the 

 vena cava posterior dorsal to the aorta and to the iliac 

 arteries ; and, between the iliac and the renal veins, the cava 

 lies free of the ureter and to the right of the aorta. This is 

 the type of cava (Type B) which is ordinarily met with in 

 placental mammals. On the other hand, in most of the Aus- 

 tralian marsupials, the iliac veins unite to form the vena cava 

 posterior ventral to the aorta and to the iliac arteries; and, 

 between the iliac and the renal veins, the cava lies free of the 

 ureters and ventral to the aorta. A comparison of figure 26 

 of the cat with figure 60 of an Australian marsupial (Petro- 

 gale) clearly illustrates the difference between the typical 

 vena cava posterior of a placental mammal (Type B) and 

 that of a marsupial. 



In both cat (Huntington and McClure, '20) and man (Mc- 

 Clure and Butler, '25) the umbilical arteries of the embryo 

 are at one time encircled by venous rings, the ventral circum- 

 ference of which is formed by a vein, designated cardinal col- 

 lateral vein (C.C. in fig. 1). The term cardinal collateral 

 was first applied by McClure ('06), in marsupials, to certain 

 embryonic veins, distinct from the posterior cardinals. In 

 the lumbar region these veins lie ventrolateral and ventral to 

 the aorta, and ventral to the umbilical arteries. In the adult 



