DEVELOPMENT OF THE THORACIC DUCT 129 



B. Detailed consideration of the individual stages in the develop- 

 ment of the azygos segment of the thoracic duct 



Figs. 194 and 195 show transverse sections of the middle thoracic 

 region in an 11 mm. cat embryo (series 213, slide xi, sections 

 9 and 10, X 225.) The right and left azygos veins (3, 6) lie ventral 

 to the sympathetic nerves (1) on each side of the aorta (7). 

 Between the latter and the right azygos vein are seen portions of 

 the ventro-medial azygos plexus (34). 



Figs. 196 and 197 show transverse sections of the same region of 

 this embryo further caudad at the level of a pair of dorsal inter- 

 segmental arteries (2) (series 213, slide xi, sections 27 and 29, X 

 225). The main azygos veins (3, 6} are reduced in comparison 

 with the previous sections, while, on the other hand, the radicles of 

 the ventro-medial azygos plexus (34) are increased in number 

 and size. This is especially so on the right side, in the space 

 between sympathetic nerve (1) and intersegmental artery (2) 

 dorsad, aorta (7) mesad, and main azygos trunk (3) laterad. In 

 the later stages (14 mm.) this is the site of the earliest and most 

 distinct extraintimal replacement of the venous plexus by the 

 lymphatic anlages of the right thoracic duct (cf. figs. 214 to 251). 



The full development of the ventro-medial azygos plexus is seen 

 in the 12 mm. stage. Figs. 198, 199 and 200 show three succes- 

 sive transverse sections of a 12 mm. embryo (series 217, slide x, 

 section 27, 28 and 29, X 225) at the [level of origin of a pair of 

 dorsal intersegmental arteries (2). Ventrad of these and of the 

 sympathetic strand (1), in the interval between the main azygos 

 trunks (3, 6} and the aorta (7) are numerous radicles of the plexus 

 (34) - The same has developed into a more extensive reticulum, 

 compared with the preceding stage, and now occupies the typical 

 ventro-medial position in relation to the parent azygos trunks. 

 Figs. 201, 202 and 203 show three sections of the same embryo 

 further cephalad (series 217, slide x, sections 18, 30 and 31). 

 They are of interest because they illustrate the condition of part 

 of the plexus just prior to its replacement by the extraintimal 

 anlages of the thoracic ducts. Some of the components of the 

 plexus, occupying the site of the future lymphatic channel, begin 



