142 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



trunks through confluence of elements of the preceding interazy- 

 gos plexus. The original post-cardinal azygos veins, which in 

 series 258 and 143 are seen to form the main channels ventral to 

 the intercostal arteries, are, in the caudal region of series 96, 

 represented by two slender longitudinal trunks, on the lateral 

 aspect of the thoracic ducts between them and the sympathetic . 

 nerves (1). The main azygos veins have developed as distinct 

 circumscribed axial channels out of the antecedent supracardinal 

 interazygos plexus, dorsal to the intercostal arterial branches (2) . 

 Correlated with this new azygos development is the reduction of 

 the earlier postcardinal channels, ventral to the intersegmental 

 arteries, and their topographical replacement in large part by the 

 right and left thoracic ducts (36) (not labelled in fig. 193). 

 Cephalad of the level A-B the reconstruction shows the earlier 

 relation of the azygos trunks, ventro-lateral to the intercostal 

 arteries. In ascending the left azygos vein gradually diminishes, 

 while conversely the vessel of the right side increases in caliber. 



II. The individual separate lymphatic anlages of the azygos 

 segment of the thoracic duct of the preceding stages have united 

 into a continuous and uninterrupted lymphatic channel. 



Caudad of the level A-B (fig. 193) the right and left thoracic 

 ducts are fairly symmetrical bilateral channels, connected with 

 each other by several broad and by a larger number of slender 

 transverse anastomoses. They occupy the position of the earlier 

 postcardinal azygos trunks, before these shifted to the supra- 

 cardinal position, ventro-lateral to the intercostal arteries, along 

 the medial border of the reduced longitudinal tributary channel 

 representing on each side the remnant of the earlier post-cardinal 

 azygos, and draining dorsad into the newly established supra- 

 cardinal azygos line of each side. 



Figs. 261 and 262 give excellent views of the topographical 

 relations of the vascular structures, arterial, venous and lym- 

 phatic. Both sections show the right and left thoracic ducts (36), 

 ventral to the intercostal arteries (2), the lymphatic cross anas- 

 tomosis between the two along the dorsal circumference of the 

 aorta, and the main supracardinal azygos veins (3, 6) . Cephalad 

 of the level A-B (fig. 193) this plexiform anastomosis between the 



