146 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



tions, and ends blindly at either end in a gradually diminishing 

 pointed extremity. In a reconstruction of this embryo it would 

 correspond absolutely in extent and relations to the long lym- 

 phatic channel above referred to on the right side of series 214 

 (fig. 190). A comparison of the reconstruction (fig. 190) and of the 

 corresponding sections (figs. 214 to 240) of this latter embryo, 

 clearly reveal the significance of this, at first sight, somewhat 

 anomalous and puzzling structure. It will be noted that the recon- 

 struction of series 214 (fig. 190) shows, at the level of the line C, 

 the caudal end of a large ventro-medial tributary trunk (5^) of 

 the right azygos vein (S), which, in proceeding cephalad, becomes 

 imbedded in the centre of the long extraintimal lymphatic chan- 

 nel. The continuation of this venous tributary line, detached 

 from the functional veins, furnishes the central venous core (4) 

 which is surrounded by the extraintimal lymphatic space (5) in 

 all the sections of series 214 shown in figs. 214 to 240 inclusive. 

 If this ventro-medial tributary trunk {3J{. in fig. 190) had organized 

 in its entire length, by confluence of the radicles of the preceding 

 plexus, and had then become detached from the functional right 

 azygos vein, it would, before extraintimal development of the 

 surrounding lymphatic spaces began, present exactly the condi- 

 tions seen in series 211. In comparing, therefore, these two 14 

 mm. embryos, we find in series 214 the venous line, around which 

 this important segment of the thoracic duct develops and which it 

 subsequently replaces, present in the condition typical for the 

 average embryo of this period, as a detached and nearly empty 

 venous bag, surrounded by the extraintimal lymphatic channel. 

 On the other hand, series 211 shows the same tributary trunk, also 

 detached from the main vein, but still filled with blood, while the 

 enveloping extraintimal lymphatic anlages have not yet made 

 their appearance. Comparison with the reconstruction and with 

 the sections of series 214 prove its identity with the latter's venous 

 core (4) in the interior of the lymphatic channel (5). 



A 15 mm. embryo of the Princeton embryological collection 

 (series 53), which I had the opportunity of studying through the 

 courtesy of Professor McClure, shows conditions identical with 

 those just described in series 211. It is interesting and conclusive 



