220 FREDERICK TILNEY 



venous return. Lest it be argued that the perimesonephroic 

 plexus from which the above named vessels arise is a special 

 case in Tragulus, it may be stated that this plexus has already 

 been described by Brown ('11) in cat embryos, and has been 

 observed by the writer in several sauropsid forms (chick and 

 scleroporus embryos). The organization of the post-renal seg- 

 ment of the cava also appears to be an instance of the emergence 

 from a plexus of selected axial channels; in this case the channels 

 happen to be the cardinal collaterals. The plexus itself surrounds 

 the aorta; the cardinal collateral vessels mark its ventral limits; 

 the dorsal limits are formed by the post cardinals. A pre-aortic 

 post-renal segment of the cava is established in part by the 

 selection of the cardinal collateral veins as axial channels and 

 in part by a marked longitudinal expansion of the inter-renal 

 segment. These conditions observed in Tragulus definitely ally 

 its post-cava with the marsupial type. The similarity thus 

 established between the venous organization of this aberrant 

 ungulate and that of the marsupials has a clear phylogenetic 

 significance. It does not, however, shed much light on the more 

 fundamental problems involved in the development of the post- 

 cava. In fact, it merely serves to open the question as to what 

 hydro-dynamic and other mechanical factors must control the 

 selection of the ultimate venous drainage channels in the axial 

 line of the body. 



In conclusion, the writer desires to acknowledge his indebted- 

 ness and express his appreciation to Professor Huntington for 

 his direction and assistance in preparing this paper. 



