140 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



occupied by the interazygos (supra-cardinal) anastomosis (cf. 

 infra, figs. 261 and 262). 



VI. The sections taken from the caudal portion of the thoracic 

 region (slides xvii and xviii) show the right and left thoracic 

 duct anlages (36) following the ventral aspect of the respective 

 azygos veins (cf. fig. 192). They are not yet united into a continu- 

 ous channel, but their lumen is clear, without trace of the venous 

 kernel around which they developed. 



Fig. 252 (slide xviii, section 7) shows remnants of the ventro- 

 medial azygos plexus (34) emptying on both sides into the azy- 

 gos trunks. Lateral to these venous radicles lies the thoracic 

 duct anlage of each side (36). 



In section 11 of the same slide (fig. 253) the right thoracic 

 duct anlage has enlarged and presents a beautifully clear picture 

 of the relation of this structure to the rightazygos vein at this stage. 



On the left side the conditions are as in the preceding figure, 

 both persistent ventral venous tributary (84) and left thoracic duct 

 anlage (36) lying side by side. 



In the two succeeding sections, 12 and 13, (figs. 254 and 255) the 

 right thoracic duct continues well developed and presents a 

 pointed dorso-mesal prolongation indicative of its situation in the 

 earlier stages, on the mesal aspect of the vein, between it and the 

 aorta, and ventral to the interazygos anastomosis, (cf. series 

 34 and 214). The left duct anlage continues as a small channel. 



In section 18, (fig. 256) both right and left lymphatic anlages 

 cease as distinct channels, marking one of the intervals between 

 the still disconnected segments of the future continuous duct. 



Figs. 257 and 258 (sections 31 and 32 of slide xvii) show on 

 alternating sides small thoracic duct anlages (36), and in the root 

 of the dorsal mesogastrium sections of the well-developed anterior 

 mesenteric lymphatics (51). Some of the later stages (series 253, 

 figs. 265 to 270) will show the full development of these mesen- 

 teric channels and their confluence with the thoracic duct. 

 This area, is, however, responsible for the development of what 

 I have above defined as the post-azygos segment of the thoracic 

 duct and will hence be considered in detail in the second volume 

 of this series. 



