212 



FREDERICK TILNEY 



enters the inter-renal segment immediately below the cephalic 

 angle (fig. 13, 57). 



Two vessels of equal size enter the inter-renal segment, one 

 at either caudal angle. They are practically parallel to each 

 other, are placed ventro-lateral to the aorta and separated from 

 each other by a mean distance of 60/z. Caudally they arise 

 from the junction of the internal and external iliac veins. The 

 proximity of these two parallel channels to the median line, 

 their position ventral to the aorta and mesial to the ureters 

 makes it probable that they are derivatives of the ventro-mesial 

 element of the peri-mesonephroic plexus and hence should be 

 considered the cardinal collateral channels (fig. 13, 21). 



Immediately above the entrance of each cardinal collateral 

 vessel, a vein from the mesonephros empties into the inter-renal 

 segment. This mesonephroic vein subsequently becomes the sex 

 vein and will hereafter be referred to by that term (fig. 13, Jfi). 



Caudad of its sub-cardinal portion, the axial drainage line is 

 therefore made up as follows, beginning from the confluence of 

 the internal and external iliac veins. 



1. The two cardinal collateral veins, constituting the so-called 

 paired portion of the post-cava (fig. 13, 21). 



2. The inter-renal segment, situated directly ventral to the 

 aorta and between the kidneys. It receives the cardinal col- 

 lateral and sex veins at its caudal angles while the renal veins 

 empty into it by its cephalic angles. At its right cephalic angle 

 it passes over into the sub-cardinal portion of the post-cava 

 (fig. 13, 59). 



The relative dimensions of the two portions change consider- 

 ably in the further development of the post-cava; the following 

 tabulation gives the length of the two segments in the older 

 embryos as compared with the adult conditions. 



Length of inter-renal segment 



Length of paired portion (cardinal col- 

 lateral) 



Distance between renal and sex veins 



20-MM. EMBRYO 23-MM. EMBRYO 



mm. 



0.71 



0.14 

 G.26 



mm. 



0.75 



1.25 

 0.30 



mm. 



65 



49 

 54 



