DEVELOPMENT OF THE THORACIC DUCT 125 



plexus, between and in front of the series of right interseg- 

 mental arteries. As above stated, the earliest and best defined 

 anlages of the azygos segment of the thoracic ducts are usually 

 found in this region in the 14 mm. stage. In this particular 

 embryo (series 34) the process of replacement of the radicles 

 of the earlier ventro-medial azygos plexus by the extraintimal 

 lymphatic spaces has just begun, and the sections show the rela- 

 tion between the atrophying venous core and the enveloping 

 perivenous lymphatic anlage clearly. 



Figs. 244 to 251 show sections in the area marked A-B in the 

 reconstruction and afford definite pictures of this relationship. 



Another early character well exhibited by this embryo is the 

 full development of the ventro-medial azygos tributary plexus in 

 the caudal portion of the right side. A distinct plexiform venous 

 ridge (34) courses along the ventro-medial border of the right azy- 

 gos vein, with which it communicates at frequent intervals. 

 In the series under discussion it is just beginning to undergo regres- 

 sion and replacement by perivenous lymphatic anlages at a few 

 points. In the subsequent stages it becomes detached from the 

 main right azygos channel and forms the framework upon and 

 around which the large caudal element of the thoracic duct 

 forms on the right side. A similar plexiform ridge follows the left 

 azygos, but is usually less well pronounced. These lines of para- 

 azygos venous reticulum, preceding the actual development of 

 the extraintimal thoracic ducts, are the ones which McClure 

 followed and described in the paper on thoracic duct develop- 

 ment above quoted (15}. 



Fig. 190, showing the reconstruction of another 14 mm. embryo 

 (series 214), offers an interesting comparison with the two pre- 

 ceding preparations. The embryo is characterized by advanced 

 development of the segment of the thoracic duct which diverges 

 at the level C in fig. 188 from the right azygos line to turn up and 

 to the left in front of the interazygos network. The path of this 

 segment of the future thoracic duct is outlined in the two preced- 

 ing preparations, series 76 and 34, by a line of detached and still 

 isolated lymphatic spaces. In series 214 these spaces have, to a 

 large extent, united into a longer continuous segment of the lym- 



