DEVELOPMENT OF THE THORACIC DUCT 77 



x and xi of this embryo, carried caudad to the level of the verte- 

 bral vessels (41, 4)- Comparison with fig. 90 will show the 

 correspondence of the two preparations in all essential points and 

 will accentuate the above described difference in the course of 

 the thoracic duct approach (12) and the preazygos segment of 

 the thoracic duct (35) in relation to the vagus-sympathetic strand. 



It is readily seen, in comparing the reconstructions of the two 

 older stages just described (figs. 90 and 91) with that of the 

 earlier embryo shown in fig. 89, that the latter, in the construction 

 of the terminal of its thoracic duct approach, combines poten- 

 tially the ability of developing into either one of the two more 

 advanced conditions just described. The process which the 

 thoracic duct approach turns nearly horizontally mesad into the 

 interval between vagus and sympathetic is present in all three 

 reconstructions. In many embryos, as will appear presently in 

 describing the critical stages in detail, the thoracic duct approach 

 appears to make its connection with the independently developed 

 anlages of the preazygos portion of the thoracic duct in this situa- 

 tion, and continued development will lead to the condition seen in 

 series 142 (fig. 91). If, on the other hand, the caudal extremity 

 of the thoracic duct approach of the 13.5 mm. embryo (fig. 89) 

 joins the independently developed lymphatic anlages at the root 

 of the left superior intercostal vein and along the common jugular 

 lateral to the sympathetic line, the resulting relation will be as seen 

 in series 245 (fig. 90). 



These observations are further supported by comparison of the 

 preparations just described with the corresponding portions of 

 the two reconstructions shown in figs. 170 and 171. 



Fig. 170 (series 218) shows the stage in a 15 mm. embryo just 

 prior to the junction of the thoracic duct approach with the inde- 

 pendently formed preazygos segment of the thoracic duct (35). 

 The latter is in its greater part already a distinct and considerable 

 segment of the future continuous duct channel. Between its 

 blind cephalic extremity and the blind caudal end of the thoracic 

 duct approach (12) are a number of scattered and still separate 

 lymphatic anlages along the common jugular and innominate 

 veins on each side of the sympathetic line. By continued exten- 



