THE MAMMALIAN VENA CAVA POSTERIOR 9 



vena cava posterior which have been observed by us and by 

 others, especially in adult man 4 and the cat, and to show how 

 they may be interpreted as modifications of a normal onto- 

 genetic venous plan. Such a composite ontogenetic plan of 

 the veins is shown in figure 1. Here are represented the main 

 axial venous channels and their anastomoses, which, in the 

 cat, make their appearance at one time or another during 

 the course of ontogeny. Under normal conditions certain of 

 these veins function only temporarily and then disappear, 

 while others are carried into the adult stage. Atypical condi- 

 tions of the veins are brought about by the persistence of 

 some one or more of the embryonic channels which normally 

 are not carried into the adult stage. 



Before considering the composite ontogenetic plan of the 

 veins as depicted in figure 1, we will examine a typical recon- 

 struction of the veins of the cat, the basis upon which in 

 general the composite diagram (fig. 1) is built. Such a recon- 

 struction is represented in figures 2 and 3, which show the 

 main axial venous channels in the lumbar region on the right 

 (fig. 2) and the left (fig. 3) sides of a 16-mm. cat embryo. 

 These axial vessels consist of the right and the left supra- 

 cardinals (Sprc., B and C) and the right and the left posterior 

 cardinal veins (Pc., A and D}. At an earlier stage of develop- 

 ment both supracardinal and posterior cardinal veins ex- 

 tended as continuous vessels throughout the lumbar and the 

 thoracic regions, so that blood from the caudal region of the 

 body could pass directly to the heart through either the supra- 

 cardinal or the posterior cardinal veins. The separation of 

 these vessels into a lumbar and a thoracic division has been 

 brought about by the formation of the unpaired prerenal divi- 

 sion of the vena cava (P. Hep., P. Sub., and I*tJ9*bc*A*Qtt.), 

 through which, in the 16-mm. embryo, all blood collected by 

 the lumbar axial veins is now returned to the heart. The 

 prerenal division of the vena cava is joined by tin- axial veins 



* The publication by McClure and Butler, in 1925, on the development of the 

 vena cava inferior in man, has made it possible for the writer to confirm and to 

 enlarge upon our previous observations. 



