40 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



extensive variations in the domain of the adult postcava of the 

 cat can be clearly interpreted genetically 25 on the basis of 

 this common groundplan, through abnormal persistence of one 

 or more of the embryonal pathways usually destined for complete 

 obliteration, thus producing farreaching modifications in the 

 structure and relations of the resulting atypical postrenal seg- 

 ment of the adult postcava. 



This periaortic axial venous lattice with connecting trans- 

 verse branches (Fig. A) contains four components on each side, 

 which develop in the following order : 



1. The postcardinal veins (1). 



2. The subcardinal veins (2}. 



3. As secondary derivatives of these two veins, the preaortic 

 cardinal collateral channels (3). 



4. As secondary dorsal derivatives of the postcardinal trunks, 

 the supracardinal veins (4). 



In the course of normal venous development along the line 

 typical for the great majority of placental mammals the right 

 supracardinal vein (4) obtains the preponderance and furnishes 

 the postrenal segment of the adult postcava, thus freeing the 

 ureter from its primitive retro-venous position. 



A part of the early capillary periaortic reticulum, out of which 

 this vessel develops, is secondarily replaced by extraintimal 

 lymphatic spaces, which through their confluence form the rela- 

 tively small retro-aortic lymph channel (4')> following in the adult 

 the dorso-lateral circumference of the postcava (4). (Figs. B and 

 C). 



The right and left postcardinal veins (1} are in part retained 

 as the terminals of the sex veins, in part replaced by the accom- 

 panying lymphatic trunks (!'}. (Figs. B and C). 



The left supracardinal vein (4), and both subcardinal veins 

 (2} , below the cross-anastomosis, as well as both cardinal collat- 

 eral veins (3), retrograde and are entirely replaced secondarily 

 by lymphatic channels. 



The lymphatic replacing the loft supracardinal vein (4') forms 

 normally in the adult the main retro-aortic lymphatic sinus. 

 (4 r , left, in figs. Band C). 



