DEVELOPMENT OF THE THORACIC DUCT 131 



surrounded by the extraintimal anlage (5) of the intermediate 

 segment of the azygos portion of the thoracic duct. The extrain- 

 timal anlage of the right segment of the duct (not labelled in 

 figure) is seen in this and some of the succeeding sections ventral to 

 the right azygos vein, surrounding an atrophying radicle of the 

 ventro-medial azygos plexus. The two succeeding sections (16 

 and 17 of slide xi, figs. 207 and 208) show the same extraintimal 

 lymphatic anlage and the enclosed venous radicle in its relation to 

 the right intercostal artery (2 in fig. 208), and it can be followed 

 through the remaining figures (209 to 213) always in the same 

 position, in the interval between the interazygos plexus, the aorta, 

 and the right azygos, and ventral to the right intercostal arteries. 

 In stages slightly less developed than the present embryo the lym- 

 phatic anlages of the central thoracic duct division are usually 

 interrupted at the level of the intersegmental arteries, and present 

 in many cases a regular segmental character. Later, as in the 

 present instance, these segmental anlages become confluent 

 across the ventral aspect of the intercostal arteries. The figure of 

 the reconstruction (fig. 188) still shows the earlier segmental 

 arrangement of the duct anlages in the cephalic part, in relation 

 to the first three right intercostal arteries. 



In the succeeding stage, 14 mm., the isolated and interrupted 

 lymphatic anlages of the 13.5 mm. stage have usually coalesced 

 to form much longer continuous channel segments. 



It is necessary to examine a large number of successive sections 

 in many embryos in order to become convinced of this marked 

 advance in lymphatic development. Thus, for example, the lym- 

 phatic anlage of the azygos segment of the thoracic duct just 

 described in certain sections of the preceding 13.5 mm. embryo 

 (series 76, figs. 206 to 213), is found in the identical position and 

 relation to surrounding structures, and much more fully developed 

 in the 14 mm. embryo shown in figs. 214 to 240 inclusive, and in 

 reconstruction in fig. 190. 



As a matter of fact every phase in the early extraintimal devel- 

 opment of the thoracic duct anlage can be perfectly illustrated by 

 selections from the 72 sections composing slides xiv and xv of 

 this series. 



