26 CHARLES P. W. MCCLURE AND GEORGE S. HUNTINGTON 



left embryonic posterior cardinal veins. Their posterior car- 

 dinal origin is plainly manifested by the typical relation 

 which the ureters bear to the veins (fig. 2). In Type AD 

 there enters into the formation of the caval veins neither 

 the lateral portions of the renal collar (Subc.Sprc.Anast. 

 Dext. and Sin., fig. 1) nor the supracardinal veins (B and C, 

 fig. 1). However, in addition to the posterior cardinals (A 

 and D, fig. 1), the right and the left subcardino-posterior car- 

 dinal anastomoses (Subc.Pc.Anast., fig. 1) have also been 

 retained. In figure 18 the right and the left caval veins are 

 about subequal in size, while in figure 19 the vein of the 

 right side is functionally the more important. There is con- 

 siderable variation in the iliac region, in the connections 

 between the caval veins of opposite sides. In some cases, as 

 in figure 20, no connection whatever is present between these 

 veins ; in figure 21 only a slight one exists, through a right and 

 a left fork of the caudal vein. Cases of incomplete fusion, in 

 which arteries pass dorsal through foramina in the caval 

 veins, recall early conditions in the embryo (figs. 3, 4, and 5), 

 and are shown in figures 22 and 23 ; cases in which the fusion 

 is complete in the iliac region between the veins of opposite 

 sides are shown in figures 18, 19, 22, 23, and 24. 



When either one or both of the caval veins have a posterior 

 cardinal origin, it is important to note that the internal 

 spermatic or sex veins, either single (fig. 19) or multiple 

 (fig. 18), join the cava (posterior cardinal) somewhat caudal 

 to the renal level. With the loss of the distal segment of the 

 left posterior cardinal (figs. 18 to 24, inclusive), it is evident 

 that the left spermatic vein would then, as under normal con- 

 ditions, open into the left renal vein, and that a considerable 

 portion of the left spermatic vein would be formed by the left 

 posterior cardinal. This clearly demonstrates that the left 

 spermatic vein of the adult is derived in part from the sub- 

 cardinal and in part from the left posterior cardinal vein. 



Type AD occurs with relative frequency in the adult cat. 

 Darrach found it in twenty cases (3.3 per cent) of his series 

 of 605 adults. In the cat it was first described bv Hochstetter 



