DEVELOPMENT LYMPHATIC -SYSTEM IN AMNIOTES 309 



stream (fig. 19, C, 4)- After this evacuation of their early blood 

 contents the axial lymphatic channels are retained as the per- 

 manent avian thoracic ducts (fig. 19, D, 5). The bird, therefore, 

 in comparison with the reptile, presents identical morphogenetic 

 characters, only differing in the degree of their development. 

 The reptile unfolds the simple periaortic lymphatic channels by 

 confluence of innumerable independently developed mesenchymal 

 spaces. The bird follows the same genetic plan, but the inter- 

 cellular mesenchymal spaces at once come into close relation with 

 the periaortic haemopoetic mesenchyme which is not developed 

 in the reptile. Hence in the early phase the avian thoracic duct 

 becomes a haemophoric lymphatic channel. 



In the mammal (fig. 20), as shown by a number of recent 

 investigations, the anlages of the thoracic ducts develop as inde- 

 pendent intercellular mesenchjinal spaces in close association with 

 the temporary ventro-medial tributary plexus of the azygos 

 veins (fig. 20, A andj5, 4), Subsequently these venous radicles 

 are surrounded by the growing lymphatic spaces (fig. 20, C, 6), 

 become detached from the azygos veins, atrophy, and are finally 

 replaced topographically by the thoracic ducts (fig. 20, D, 7). 



This mode of lymphatic development has been described by 

 McClure and myself as the 'extra-intimal type,' because the 

 lumen of the lymphatic anlage is always outside of the intimal 

 lining of the degenerating vein which the resulting lymphatic 

 channel is destined to replace. 



The mammalian embryo, compared with the embryos of the 

 two remaining amniote classes, offers specialized conditions in 

 the axial region of the trunk by reason of the presence of the 

 azygos or supracardinal system of veins. Differentiation of 

 mesenchyme in situ into developing blood cells occurs in this 

 region as it does in avian embryos (fig. 20, A, 3). The products 

 of this haemopoesis are, however, conveyed in the mammal 

 directly into the azygos veins through the temporary ventro- 

 medial tributary plexus (fig. 20, B, 5) . This plexus atrophies after 

 perfomiing this haemophoric function, and is topographically 

 replaced by the thoracic ducts (fig. 20, C, 6; fig. 20, D, 7). In the 

 bird, on the other hand, the absence of axial veins in this area 



