40 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



extensive variations in the domain of the adult postcava of the 

 cat can be clearly interpreted genetically^^ 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 bi-anches (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 

 supi-acardinal vein (4) obtains the preponderance and furnishes 

 the postrenal segment of the adult postcava, thus freeing the 

 ureter from its i)rimitive retro-venous position. 



A part of the earl}- 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 

 D. 



The right and I(>ft postcardinal veins (1) are in part retained 

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

 panying lymj)hatic trunks {!'). (Figs. B and C). 



TIh- left supracardinal vein (4), and both subcardinal veins 

 {2) . below tiie cros.s-anastomosis, as well as both cardinal collat- 

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

 by !\-mpli;itic chamiels. 



'J'he lymphatic replacing the left supracardinal vein {4') forms 

 iiornially in the adult the main retro-aortic Ivmphatic sinus. 

 U', left, in figs. HjuidC). 



