DEVELOPMENT OF THE THORACIC DUCT 137 



fine series to the kindness of my colleague, Prof. C. F. W. McClure. 

 The embryo shows in the mediastinal and in the cephalic portion 

 of the azygos region the typical extraintimal formation of the 

 thoracic duct anlages, while in the more caudal part the ventro- 

 medial tributary venous plexus of the azygos trunks is still fully 

 developed and not yet invaded by the enveloping and replacing 

 lymphatics anlages. Intermediate between these two districts 

 are areas where the decadent ventro-medial azygos venous tribu- 

 taries can be clearly traced into the surrounding replacing extrain- 

 timal lymphatic plexus. The general disposition of the azygos 

 veins and of the thoracic duct anlages in this embryo is shown in the 

 ventral view of the reconstruction of slides (xxiv to xxxii given in 

 fig. 189. 



The perivenous lymphatic spaces are fully established in the 

 cephalic part of the azygos and in the preazygos portions, al- 

 though not yet joined into longer channel segments. In the caudal 

 area the plexus of the ventro-medial azygos tributaries predomi- 

 nates, while the accompanying and enveloping extraintimal lym- 

 phatic anlages are much reduced in size and number. Intermed- 

 iate between these two areas, and corresponding to slides xxvii 

 and xxxviii, lymphatic development is actively proceeding, especi- 

 ally along the right azygos tributary venous line 34 in figure 189, 

 by the formation of extraintimal mesenchymal spaces around the 

 venules of the azygos plexus. Sections in this area furnish con- 

 clusive early pictures of the decadent venous radicles, almost com- 

 pletely detached from the main channel and surrounded by peri- 

 venous lymphatic anlages. Figs. 246 to 251 show six successive 

 transverse sections (17 to 22 inclusive) of slide xxviii, in the 

 region indicated in the figure of the reconstruction (fig. 189) by 

 the lines A-B. 



In fig. 246 (section 17, X 225) the small central venous rem- 

 nant (4) surrounded by the extraintimal lymphatic anlage (5) 

 lies in the interval between aorta (7) mesad, right azygos vein (3) 

 laterad, oesophagus (8) ventrad, and elements of the interazygos 

 anastomosis dorsad. 



In section 18 (fig. 247) the same structures are seen. The cen- 

 tral vein (4) in the interior of the enveloping lymphatic anlage 



