DEVELOPMENT OF THE THORACIC DUCT 133 



nation of the antecedent venous kernel, and the establishment of 

 a lymphatic channel with clear and empty lumen. In the follow- 

 ing section, 5, (fig. 229) the lymphatic channel ends blindly in the 

 mesoderrn, indicating one of the intervals dividing the still separ- 

 ate and distinct links in the chain of lymphatic segments which are 

 forming in this stage along the line of the future continuous 

 thoracic ducts. (Cf. reconstruction of this embryo, shown in fig. 

 190, at level of first right intercostal artery). 



Figs. 230 to 235 inclusive show transverse sections from the 

 central region of the next slide of the same embryo (series 214, 

 slide xv, sections 10, 11, 14, 16, 19, 21, x 225). Sections 10 and 

 11 (figs. 230 and 231) repeat practically the conditions above 

 observed and recorded in sections 15, 16, 17, 19, 20 and 22 of 

 slide xiv. A large empty central endothelial bag (4) represents 

 the decadent ventro-medial azygos tributary around which the 

 extraintimal lymphatic anlage of the thoracic duct (5) is forming. 

 In sections 14, 16 and 19 (figs. 232, 233 and 234) both the central 

 venous kernel (4) and the replacing perivenous lymphatic space 

 (5) are diminishing, just as they are doing in figs. 220 to 224, 

 sections 26, 27, 28, 32 and 34 of the preceding slide xiv of this 

 embryo. In section 21 of slide xv (fig. 235) the perivenous 

 lymphatic space stops, and only the central venous kernel (4), 

 still connected with the ventro-medial azygos plexus of the right 

 side, persists. 



Finally figs. 236 to 240 inclusive give five successive sections of 

 the same embryo in the terminal part of slide xv (series 214, 

 slide xv, sections 27, 28, 29, 30 and 31, X 225), in which the stage 

 of the early formation of the extraintimal anlage of the azygos 

 portions of the thoracic duct is shown. The decadent ventro- 

 medial azygos tributary (4) is almost completely separated from 

 the azygos plexus, although the path of its original connection 

 with the same can still be traced by the arrangement of the peri- 

 venous mesodermal cells. The extraintimal lymphatic anlage 

 of the thoracic duct (<5) develops at first on the lateral aspect of the 

 atrophying component of the ventro-medial azygos plexus, and at 

 some distance ventral to the main azygos trunks and to the inter- 

 azygos anastomosis. This series is, therefore, of considerable 



