DEVELOPMENT OF THE ITIOKACIC 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 {2/f) into its two main 

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

 proach (i4)> 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 {2If) laterally. Its pointed extremity is directed further 

 ventro-mesad toward the interval between the sympathetic nerve 

 {!) 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 artery (^4) 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 



