216 FREDERICK TILNEY 



veins designate not morphological but physiological units. The term 

 post cava only indicates a hydro-dynamic line. The variously named 

 cardinals are merely dilated portions of the reticulum along the major 

 hydro-dynamic lines, which, responding to the large volume of blood 

 they transmit, dominate the picture. 



Although the facts cited above account for the acquisition of 

 a pre-aortic post-renal segment of the cava in the adult Tragulus, 

 they do not furnish a complete explanation of the process by 

 which the ultimate relations of this vessel are attained. In the 

 20 mm. and 23 mm. embryos the post-renal segment of the cava 

 presents two portions, each of which is pre-aortic in position, 

 namely, the paired portion and the unpaired portion. The un- 

 paired portion in these stages, constitutes about one-third of 

 the entire post-renal segment of the cava (fig. 13, A and B). In 

 the adult, while the paired and unpaired elements still enter into 

 the formation of the post-renal segment, their proportions have 

 greatly changed. The unpaired portion instead of being one- 

 third as long as the paired portion is now six times longer. In 

 other words five-sixths of the post-renal segment of the cava is 

 represented in the adult by a single unpaired pre-aortic channel, 

 the remaining one-sixth being represented by the paired portion 

 (fig. 1). This marked change in proportion may be due to one 

 of three possibilities; 1, the fusion of the two cardinal collateral 

 veins across the median line in the cephalic two-thirds of their 

 course; 2, a caudal migration of the angle of confluence of the 

 cardinal collaterals; and 3, the longitudinal expansion of the 

 inter-renal segment alone. The position and relations of the 

 sex veins considerably lessen the difficulties in deciding which 

 of these processes is the active one. It has already been shown, 

 in the older embryos, that the renal veins mark the cephalic 

 limits of the inter-renal segment while its caudal limits are in- 

 dicated by the sex veins. The portion of the cava between these 

 limits, therefore, must be considered the inter-renal segment in 

 all stages. Upon this basis, the change in proportion of the 

 two elements of the post-renal cava, observed in passing from 

 the embryonic to the adult stages, may be explained by the longi- 

 tudinal expansion of the inter-renal segment. The ultimate wide 



