DEVELOPMENT OF THE THORACIC DUCT 67 



Finally an intermediate condition is seen in another 14 mm. 

 embryo, series 122, of which selected sections are appended, from 

 slides ix and x (figs. 57 to 62). Section 21 of slide ix (fig. 57) 

 is taken at the level where the jugular lymph sac has divided 

 for the passage of the thyro-cervical artery (24) into its two main 

 terminal components, viz., the dorsally placed subclavian ap- 

 proach (14), and the ventrally situated jugular approach (13). 

 The latter appears in the interval between the internal jugular 

 vein (25) and the jugulo-cephalic trunk (29). These veins are 

 approaching each other to form the common jugular confluence 

 (fig. 39, '26), at which point the jugular approach usually makes 

 the secondary permanent lymphatico-venous tap. The thoracic 

 duct approach (12) arises from the dorsal aspect of the jugular 

 approach by a relatively narrow pedicle and then hooks, as an. 

 elongated and expanded curved pouch, ventro-mesad over the 

 dorsal aspect of the internal jugular vein towards the neuro- 

 venous recess. 



In section 28 of the same slide (fig. 58) the internal jugular 

 and jugulo-cephalic veins have met in the common jugular con- 

 fluence (26) which includes the jugular approach. Consequently 

 the latter structure only appears in its ventral prolongation which 

 is seen occupying the ventral angle of the common jugular junc- 

 tion (not labelled in fig. 58). 



The subclavian approach (14) has moved relatively nearer to 

 the dorsal surface of the main vein channel. 



The blind terminal end of the thoracic duct approach (12) is 

 seen dorsal to the common jugular vein, between a large dorso- 

 medial tributary of the latter (16) mesally, and the thyro-cervical 

 artery (24) laterally. Its pointed extremity is directed further 

 ventro-mesad toward the interval between the sympathetic nerve 

 (1) and the medial surface of the common jugular vein (26). 



In section 7 of slide x (fig. 59) the terminal of the thoracic 

 duct approach (12) has relatively receded and lies in the narrow 

 interval between thyro-cervical artefy (24) and common jugular 

 vein (26). 



In fig. 60 (section 9 of slide x) a large dorso-medial venous 

 tributary (16, left superior intercostal vein) empties into the 



