DEVELOPMENT OF THE THORACIC DUCT 127 



quently connected by a close plexiform anastomosis and together 

 form the azygos segment of the thoracic duct. (cf. fig. 193). 



Fig. 192 shows the ventral view of the central azygos region in 

 a 15.5 mm. embryo, series 143. 



The same plan of para-azygos lymphatic development, with 

 two lines caudad and three cephalad of the level C, is presented as 

 in the preceding series. The right and left lines are composed of 

 relatively long links of lymphatic channel segments. The inter- 

 mediate line above C is, in the part shown in the reconstruction, 

 not quite as far advanced as in the preceding series 219, or in the 

 14 mm. embryo (series 214, fig. 190). The latter, in fact, presents 

 an unusually advanced condition of this channel, and illustrates 

 the fact that in comparing different embryos of about the same 

 age lymphatic organization may, in certain regions, be either 

 ahead or behind the average level usually attained at the 

 period in question. This is especially true in the earlier lym- 

 phatic stages, in embryos between 13 and 15 mm. crown-rump 

 measure. 



Embryo 143 (fig. 192) shows, for example, in comparison with 

 series 214 (fig. 190) less advanced development of the intermediate 

 thoracic duct line above the level C. Correspondingly, the ven- 

 tro-medial venous ridge (84) of the right azygos trunk, upon and 

 around which these lymphatic spaces develop and which the re- 

 sulting lymphatic channel eventually replaces, is still distinct!}' 

 evident in the former (fig. 192), whereas in the latter (fig. 190) 

 all but its caudal portion is hidden by the enveloping lymphatic 

 trunk. 



Fig. 192 also shows, in the area of slide xvii, parts of the ventral 

 azygos plexus surrounding the lateral and ventral circumference 

 of the abdominal aorta, and the extraintimal lymphatic spaces 

 forming along them and draining into the right thoracic duct 

 channel. 



Finally the 16 mm. embryo (fig. 193, series 96) usually marks 

 the developmental stage in which the hitherto isolated and dis- 

 connected lymphatic segments have united with each other into 

 the continuous channel of the azygos portion of the thoracic and 

 right lymphatic ducts. 



