128 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



The latter (47) is seen in the cephalic part of the reconstruction 

 along the lateral border of the right azygos vein. The formei 

 shows a caudal enlarged and fairly symmetrical segment, caudad 

 of line A B, in which the right and left thoracic ducts, connected 

 by several broad transverse anastomoses, follow the ventro-medial 

 aspect of the right and left azygos veins. 



The latter in this region have secondarily assumed the supra- 

 cardinal position, by developing, as the main channels, out of 

 the preceding interazygos plexus. They hence lie dorsal to the 

 lower intersegmental arteries. A remnant of the former (post- 

 cardinal) azygos trunks, ventral to the lower three intercostal 

 arteries, is seen on each side between the thoracic ducts and the 

 sympathetic nerves (1), as a slender longitudinal venous channel, 

 connected at numerous points with the main azygos trunks. 

 Above the level of the line A B, the entire and now united and 

 continuous lymphatic channel swings cephalo-sinistrad and ascends 

 on the ventral aspect of the interazygos anastomosis and of the 

 reduced left azygos vein. It represents the further development 

 of the left and intermediate lines of azygos lymphatic organization 

 of the earlier stages. Each of these lines is represented now by a 

 continuous marginal lymphatic channel, lateral to or perforated 

 by the intersegmental arteries, and interlacing in a multiple 

 plexiform anastomosis with its fellow of the opposite side. 



The mesenteric lymphatics of the dorsal mesogastrium (51} are 

 beginning to establish their connections with this now completed 

 main axial lymphatic trunk. Further development of the same 

 is in the direction of more definite condensation of the lymphatic 

 plexus and relative reduction of the lumen, as seen in the adult 

 preparations previously figured and described (cf. figs. 95 to 99), 

 p. 79. 



