160 SYSTEMIC LYMPHATIC VESSELS IN DOMESTIC CAT 



acic duct approach of the jugular lymph sac, in certain typical 14 

 mm. embryos, extends as a curved blunt conical process meso- 

 caudad for some distance between the main vein and the thyro- 

 cervical artery, towards the interval between the former and the 

 sympathetic nerve, where it ends blindly. Mesad of the thoracic 

 duct approach the internal and common jugular veins receive a 

 number of dorso-medial tributary radicles of the above mentioned 

 prevertebral venous plexus. Now, it will depend altogether upon 

 the degree of development of any particular embryo, and the selec- 

 tion of the sections, as to whether both the thoracic duct approach 

 and the dorso-medial tributary terminal appear side by side in the 

 same field, or whether one or the other, or both are absent at the 

 particular level examined. Thus in a 14 mm. embryo (series 122, 

 slide ix, section 28, fig. 58), the dorso-medial tributary (16) of the 

 main vein (left internal jugular, 25) together with the thoracic 

 duct approach of the jugular lymph sac (12), are both present in 

 the typical relation to the thyro-cervical artery (24) and the sym- 

 pathetic nerve (1). Traced back in series 122 to section 21 of 

 slide ix (fig. 57) the thoracic duct approach (12) is seen to arise 

 from the dorso-mesal aspect of the caudal end of that portion of 

 the jugular lymph sac, which, as the terminal of the ventral division 

 of the entire structure, forms the jugular approach (13). 



But since in section 28 of slide ix of series 122, this portion of the 

 future thoracic duct (lymph sac segment) is present in the classical 

 position between thyro-cervical artery and sympathetic nerve, 

 dorsal to the common jugular vein, together with the well developed 

 dorso-medial venous tributary (16), it must become evident that 

 the latter cannot be regarded as a 'venous outgrowth' represent- 

 ing the anlage of the thoracic duct, since it is manifestly impossible 

 to include the 'anlage' of a structure and the actual structure it- 

 self side by side in a single section. 



As a matter of fact, a selection of sections in this neighborhood 

 in series 122 (14 mm.) will furnish all possible combinations of the 

 venous dorso-medial tributary of the internal and common jugular 

 vein and of the thoracic duct approach of the jugular sac. 



The two structures, as can be clearly seen, both occupy in sec- 

 tions at the proper level the topographical position along the dor- 



