DEVELOPMENT OF THE THORACIC DUCT 143 



ducts increases in extent, and the entire lymphatic channel in- 

 clines, in ascending, to the left. This, as previously explained 

 (cf. supra, p. 129), is due to the greater development of the inter- 

 mediate lymphatic line, along the ventral aspect of the interazygos 

 anastomosis, and its union with the left thoracic duct to form the 

 plexiform channel of the adult. The corresponding lymphatic 

 of the right side, on the other hand, becomes reduced above the 

 level A-Bj and only its cephalic portion persists as the thoracic 

 segment of the right lymphatic duct (47 in fig. 193). 



The extraintimal lymphatic anlages ventral to the thoracic 

 ducts, which are eventually to establish the connection between 

 them and the mesenteric lymphatics, are in the process of develop- 

 ment in series 96. The venous core (4) and the replacing peri- 

 venous lymphatic (5) are shown on the left side in figs. 26 land 

 262, and equivalent segments of this channel are seen in the recon- 

 struction (51 in fig. 193). In other portions of the ventral and 

 ventro-lateral periaortic mesoderm numerous independent inter- 

 cellular spaces begonging to the same system are developing. 



While the thoracic ducts are in the majority of 16 mm. embryos 

 complete continuous and definite channels, as in the present aver- 

 age example, the acquisition of this final condition is not infre- 

 quently delayed considerably beyond this period. I have a number 

 of 16 and 16.5 mm. embryos in which the thoracic duct develop- 

 ment has not advanced beyond the stage typical of the average 

 14 mm. or 15 mm. embryo. This is another example of the con- 

 siderable range of chronological variation of lymphatic develop- 

 ment in individual embryos encountered in examining a large 

 number of series of about the same length measure. 



The 16 mm. embryo again furnishes conclusive evidence as to 

 the secondary character of the mutual relation between embryonic 

 venous and lymphatic vessels, and shows, in typical individuals, 

 the final stage of the process through which temporary venous 

 channels are replaced by the permanent components of the lym- 

 phatic system. -This replacement, as demonstrated by the suc- 

 cessive series of sections from the azygos region just considered, is 

 a gradual process. The lymphatic channel has its inception in 

 numerous small extraintimal mesenchymal spaces which form 



