DEVELOPMENT OF THE THORACIC DUCT 141 



In comparison with the series just considered figs. 259 and 

 260 show transverse sections of the lower thoracic region of series 

 258, slide xviii, sections 1 and 9, X 225. This embryo is rated 

 as 17 mm. in crown-rump length, but the lymphatic develop- 

 ment of the azygos region is relatively retarded and only slightly 

 in advance of the conditions presented by the 15.5 mm, series 

 143. Direct comparison of figs. 252 to 258 with figs. 259 and 

 260 will show this correspondence, both at the lev^l of the inter- 

 segmental arteries (2 in fig. 260), and in the intervals between them 

 (fig. 259). In the older embryo the right and left thoracic duct an- 

 lages (36) are somewhat larger and form longer channel segments, 

 but otherwise the two series belong practically to the same develop- 

 mental stage, in spite of their difference in total length measure. 

 The correspondence, for example, between figs. 254 (series 143) 

 and 259 (series 258) is remarkable. 



On the other hand the majority of 16 mm., 16.5 mm. and 17 mm. 

 embryos offer decided advances in the lymphatic development of 

 this region beyond that normally found in the 15 mm. and 15.5 

 mm. stages. 



Two successive sections of a typical 16 mm. embryo (series 96) 

 in the azygos area are shown in figs. 261 and 262, and fig. 193 

 gives the ventral view of the reconstruction of the same embryo, 

 hi which the level of figs. 261 and 262 is indicated by the line A-B. 



Comparison with the immediately preceding series (143 and 

 258) will show that the following fundamental changes have oc- 

 curred in the azygos venous and lymphatic systems : 



I. The two azygos veins (3 and 6} have become reduced in size. 

 At the level, at which the sections of figs. 261 and 262 are taken, 

 both veins now lie dorsal to the intercostal arteries (2), whereas in 

 the earlier stages (e.g. figs. 260, 258, 257) the azygos trunks are 

 ventral to the arteries, and the interval between the latter, along the 

 dorsal aspect of the aorta, is occupied by the supracardinal inter- 

 azygos plexus. 



Reference to the reconstruction of series 96 (fig. 193) 

 shows that this topographical readjustment has occurred in the 

 caudal part of the azygos area, below the level A-B, by the longi- 

 tudinal hydrostatic definition of the main supracardinal azygos 



