12 CHARLES F. W. McCLURE AND GEORGE S. HUNTINGTOIST 



renal vein. In this manner we can account for the difference 

 which exists on opposite sides of the body, in the junction 

 of the internal spermatic vein with the cava. 



V. THE COMPOSITE ONTOGENETIC VENOUS PLAN OF THE CAT 



We may now pass to a consideration of the composite onto- 

 genetic plan of the veins (fig. 1). In this diagram are shown 

 two important sets of vessels not represented in the recon- 

 struction of the veins in the 16-mm. cat. These are the sub- 

 cardinal and the cardinal collateral veins. In the 7- to 8-mm. 

 cat embryo the subcardinals form a bilaterally symmetrical 

 system of veins, which anastomose with the posterior car- 

 dinal veins and, ventral to the aorta, with each other. They 

 receive blood from the mesonephroi and the sex glands. The 

 cardinal collateral veins, which are not shown in their entirety 

 in the diagram, reach the height of their development at 

 approximately the 29-mm. stage, and for this reason were 

 not evident in the 16-mm. cat. It is also to be noted that, 

 except for its cranial end, the thoracic division of the pos- 

 terior cardinal veins has not been shown in the diagram 

 (fig.l). . 



If we compare figure 1 with figures 2 and 3, we shall see 

 that in none of its fundamental aspects does the composite 

 plan of the embryonic veins exaggerate the condition actually 

 observed. 



If we examine the composite ontogenetic venous plan of the 

 cat (fig. 1), we observe that three pairs of longitudinal bi- 

 laterally symmetrical venous channels are represented in 

 the lumbar region and that they consist of the posterior car- 

 dinals ( A and D), the supracardinals (B and (7), and the sub- 

 cardinals (Subc.} into which the sex veins (Vv. spermaticae, 

 8.V.) open. It is to be observed that the supracardinals 

 anastomose with the posterior cardinals (Pc.Sprc.Anast.} 

 and, dorsal to the aorta, with each other (Int.Sprc.Anast.} ; 

 and that the subcardinals anastomose with the posterior car- 

 dinals (Subc.Pc.Anast.) and, ventral to the aorta, with each 

 other (Int.Subc.Anast.). In addition to the three pairs of 



