134 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



interest, as exhibiting, at different levels in the same embryo, the 

 following phases of early extraintimal replacement of ventro- 

 medial azygos tributaries by the thoracic duct anlages : 



1. Detachment of venous elements of the plexus, as empty 

 endothelial tubes, surrounded by extraintimal lymphatic anlages : 

 Slide xiv, sections 15, 16, 17, 19, 20, 22, (figs. 214 to 219 inclusive). 

 Slide xv, sections 10 and 11, (figs. 230 and 231). 



2. Gradual recession of degenerating central venule, and 

 approach of same and of surrounding extraintimal lymphatic 

 space to ventral aspect of main azygos system: Slide xiv, sec- 

 tions, 26, 27, 28, 32, and 34, (figs. 220 to 224 inclusive). Slide 

 xv, sections 14, 16, and 19 (figs. 232, 233 and 234) 



3. Elimination of central atrophying venous kernel with 

 persistence of clear lumen of lymphatic channel segment: Slide 

 xiv, sections 1, 2, 3, and 4, (figs. 225, 226, 227 and 228). 



4. Blind ending of lymphatic channel anlage and develop- 

 ment of interval between the still disjointed links of the future 

 continuous lymphatic chain of the thoracic duct: Slide xiv, sec- 

 tion 5, (fig. 229) and reconstruction (fig. 190). 



5. Blind ending of extraintimal perivenous lymphatic anlage 

 with persistence of ventro-medial azygos tributary venule (4) : 

 Slide xv, section 21 (fig. 235). 



6. Early stage of development of azygos segment of thoracic 

 duct, in which the same appears as an extraintimal lymphatic 

 space (5} closely applied to the lateral circumference of a compo- 

 nent of the ventro-medial azygos venous plexus (4), still in com- 

 munication with the axial venous trunks, but destined subse- 

 quently to be separated from them and to undergo entire extrain- 

 timal replacement by the surrounding lymphatic space: Slide xv, 

 sections 27, 28, 29, 30 and 31 (figs. 236 to 240 inclusive). 



Another feature of the early development of the azygos segment 

 of the thoracic ducts, strikingly illustrated by the embryo under 

 discussion, is the segmental type of the originally isolated and 

 independent perivenous extraintimal lymphatic development. 

 Thus in the selected sections of the two slides xiv and xv just 

 described the following succession of conditions is to be noted. 

 Sections 1 and 2 (figs. 225 and 226) lead up, at the caudal end 



