SUMMARY AND CONCLUSIONS 171 



along which the lymphatic spaces expanded to reach this position, 

 has disappeared. The successive stages of this process can be 

 followed in such detail and in such conclusive preparations that all 

 doubt as to the accuracy of the statements here made seems to me 

 to disappear. It is merely a question of suitable material, accur- 

 ately fixed, sectioned and stained, and sufficiently abundant to 

 pffer complete corroboration in a number of series of the same age. 

 There can then be no question as to the genetic principles involved. 



3. The portion of the thoracic duct, whose genetic history has 

 just been outlined, forms only a component of the entire lym- 

 phatic complex entering into the formation of the adult duct. Two 

 other equivalent components combine with it to produce the adult 

 conditions, viz., a corresponding segment on the left side, devel- 

 oped in the same way in association with the ventral tributary 

 plexus of the left azygos vein, and an intermediate plexiform lym- 

 phatic reticulum which arises as the result of fusion of extraintimal 

 lymphatic spaces formed along the ventral aspect of the supra- 

 aortic interazygos venous plexus. 



In addition to these three cardinal elements entering into the 

 organization of the adult thoracic ducts, these channels, when once 

 they have gained their position ventro-mesad to the azygos trunks, 

 grow partly by their own expansion, partly by addition of num- 

 erous new extraintimal lymphatic spaces which form along the 

 azygos veins, expecially on the left side. 



The eventual reduction of the left azygos vein, and of the cau- 

 dal part of the right trunk, by transferance of the blood current 

 to the vertebral chain, accounts for the topographical relations 

 of the replacing thoracic duct complex of the adult. 



