VEINS AND LYMPHATICS IN TRAGULUS 217 



separation between the renal and sex veins seems to indicate 

 that this was the process by which the post-renal cava has been 

 changed from an embryonic channel in greater part paired to 

 an adult vessel in greater part unpaired. The probability of 

 this explanation is further sustained by the fact that the incre- 

 ment of growth from the early stages has favored the inter-renal 

 segment (see page 213). The relations between the length of 

 the post-renal segment of the aorta and that of the inter-renal 

 segment of the cava also show changes which are significant in 

 this connection. The measurements of the aorta were taken 

 from the point of derivation of the right renal vein to the iliac 

 bifurcation; those of the inter-renal segment from the entrance 

 of the right renal vein to the point of confluence of the two cardi- 

 nal collateral veins. 



In the 20 mm. embryo the length of the inter-renal segment 

 of the cava was 0.33 that of the post-renal segment of the aorta. 

 In the 23 mm. embryo this value has increased to 0.40 and in 

 the adult to 0.87. Thus there has been a relative increase in 

 the rate of growth in the inter-renal segment of the cava as com- 

 pared with the post-renal segment of the aorta. 



The change in the relations of the sex veins and arteries is 

 further evidence of this relative increase in the inter-renal seg- 

 ment of the cava. Both of the older embryos show the sex veins 

 and arteries in close relation to each other (fig. 13, A and B). 

 The veins enter the inter-renal segment practically in common 

 with the entrance of the cardinal collateral veins. The sex arter- 

 ies arise separately from the aorta at a level only slightly caudal 

 to that of the veins. The adult specimen in the Columbia col- 

 lection shows the sex arteries arising from a short common trunk 

 given off from the aorta at a point 1.5 cm. above the iliac bifur- 

 cation, while the sex veins enter the common iliac veins (paired 

 portion of the post-cava) 4 mm. below the point of entrance of 

 these latter channels into the unpaired portion of the post-cava 

 (fig. 1). Thus the sex arteries,' which in the embryo arise from 

 the aorta caudad of the sex veins, in the adult arise from a level 

 distinctly cephalad of these veins. This marked change in rela- 

 tions appears to have its explanation in the relatively more rapid 



