VEINS AND LYMPHATICS IN TRAGULUS 215 



upon the other channels. The venous plexus between the pre- 

 aortic and post-aortic veins becomes so extensive as to completely 

 invest the aorta except for a small interval immediately in front, 

 on either side of the median line. As it approaches the levels 

 marking the derivation of the hypogastric arteries the plexus 

 becomes more voluminous and finally communicates with the 

 iliac trunks. 



From the conditions of these embryonic stages it is obvious 

 that the venous return from the internal and external iliac veins 

 depends upon a large peri-aortic plexus rather than upon discrete 

 venous channels. The increasing prominence of certain axial 

 channels in this plexus clearly indicates the process by which 

 the plexus itself is to be replaced by veins in the line of the lon- 

 gitudinal growth of the body. The course of these axial channels 

 is parallel to the long body-axis; two are placed ventro-lateral 

 to the aorta, the cardinal collateral veins; two are situated dorso- 

 lateral to the aorta, the post-cardinals. At the indeterminate 

 stages represented by the 20 mm. and the 23 mm. embryos the 

 internal and external iliac veins may select one of several possi- 

 bilities for the continuance of their drainage lines toward the 

 heart. They may choose either or both of the post-cardinal 

 veins to the exclusion of the cardinal collaterals, so that the 

 venous blood would reach the inter-renal segment of the cava 

 by the plexiform channels of the inter-renal post-cardinal plexus. 

 They may select both cardinal collaterals to the exclusion of the 

 post-cardinals and so establish a communication with the inter- 

 renal segment of the cava. The adult conditions, showing that 

 the post-renal segment of the cava is pre-aortic in position, 

 clearly demonstrate that the selection has fallen upon the cardi- 

 nal collateral veins and that the post-cardinals play no part in 

 the formation of this portion of the cava. 



These possibilities of selection which must be reckoned with 

 in discussing the post-renal cava in Tragulus emphasize again 

 the statement of Schulte ('09) that 



homonymous venous channels are not necessarily morphological equiva- 

 lents but are rather homodynomous, agreeing in function because they 

 drain similar areas: — and it thus appears that the anatomical name of 



