DEVELOPMENT OF THE THORACIC DUCT 139 



the components of the ventro-medial azygos plexus (4), partially 

 separated from the main venous trunks, but not, as yet, sur- 

 rounded by extraintimal lymphatic anlages, as, further cephalad, 

 (slide xxviii) in the sections just described, (figs. 246 to 251). 



The 15 mm. and 15.5 mm. embryos show, in the majority of 

 instances, an advance over the preceding stage in two directions : 



I. The central venous core of the 14 mm. embryo is further 

 reduced or entirely eliminated, leaving the lymphatic anlage, 

 which formerly surrounded it, with a clear lumen. 



II. The individual scattered lymphatic areas have united to 

 form longer segments of the future lymphatic channel system. 

 This condition is well shown in the ventral view of the reconstruc- 

 tion of an average 15 mm. embryo (series 219) in fig. 191 (cf. 

 supra, p. 126). 



Figs. 252 to 258 inclusive show transverse sections of the lower 

 thoracic region in a 15.5 mm. cat embryo, (series 143, slide xviii 

 section 7, 11, 12, 13, 14; slide xvii, sections 31 and 32, X 225), 

 and fig. 192 gives the ventral view of the reconstruction of the 

 azygos region in this embryo, comprising slides xiv, xv, xvi, 

 xvii, and xviii. 



The sections (figs. 252 to 258) show the following changes in the 

 vascular structures, compared with the preceding stages 



I. The right (3) and left (6) azygos veins are concentrated, 

 more sharply defined and relatively smaller, although still equally 

 developed on both sides. 



II. The interazygos anastomosis is still well developed and 

 uniform (cf. also fig. 192). 



III. The entire azygos complex is much more closely applied 

 to the dorsal and dorso-lateral circumference of the aorta (7), 

 resulting in the practical obliteration of the interval between the 

 main veins and the artery seen in the preceding stages. 



IV. This topographical rearrangement has forced the ventro- 

 medial azygos tributaries, where they still persist (34 in figs. 252, 

 253, 257), and the thoracic duct anlage (36) to pass from the 

 ventro-medial to the ventral aspect of the main azygos trunks. 



V. Both azygos veins lie in the region ventro-lateral to the inter- 

 segmental arteries. The supra-aortic area between them is still 



