78 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



sion of these as yet isolated lymphatic anlages the still lacking 

 intermediate bondpiece will be produced, which, by union of its 

 cephalic extremity with the thoracic duct approach of the jugular 

 lymph sac and junction of its caudal end with the cephalh ternrnnl 

 of the preazygos segment of the thoracic duct (35), will produce 

 the continuous and uninterrupted lymph channel of the succeed- 

 ing stage (se:ies 143, 15.5 mm. embryo, fig. 171). A study of the 

 reconstruction of series 218 makes it evident that the opportunity 

 still exists of developing this final junction either on the dorso- 

 lateral or ventro-medial side of the sympathetic cord, for the 

 lymphatic anlages exist in both situations. The end of the 

 thoracic duct approach of this embryo rides with a well marked 

 lateral and medial prolongation in the fork formed by the en trance 

 of the left superior intercostal vein (16-4-6} into the common 

 jugular. The lateral process, carrying the leader 12 in the figure, 

 is in position to join the line of separate lymphatic anlages extend- 

 ing caudad on the lateral side of the sympathetic, and a prominent 

 cephalic process of the already established segment of the preazy- 

 gos lymph channel extends cephalo-laterad across the dorsal 

 surface of the nerve, as if to meet the lymphatic line just described. 

 If development had proceeded along this line in this embryo, the 

 condition seen i.n series 245 might have resulted, with the modifi- 

 cation of swinging the thoracic duct approach to the lateral 

 instead of to the medial side of the left superior intercostal vein, 

 but carrying the thoracic duct along the dorso-lateral aspect 

 of the sympathetic nerve. In this case the large separate lym- 

 phatic anlage labelled 35' in fig. 170 would take an important 

 share in building up the resulting lymph channel. On the other 

 hand, the definite connection with the preazygos segment of the 

 thoracic duct might have been established through the ventro- 

 medial terminal prolongation of the thoracic duct approach. A 

 piece of the sympathetic nerve has been removed in the recon- 

 struction in order to show the position of this structure, in the 

 interval between sympathetic nerve (1) and vagus (22). The 

 long slender prolongation extending forward between vagus (22) 

 and common carotid artery (23) is the path of junction for the 

 ventral or broncho-mediastinal lymphatic trunk. I am under 



