DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 113 



C. The junction of the two independently formed lymphatic chan- 

 nels above described, viz., the preazygos segment of the thoracic duct 

 and the ventral mediastinal trunk, with each other and with the tho- 

 racic duct approach of the jugular lymph sac. 



In the 15 and 15.5 mm. embryo of the cat the thoracic duct 

 approach of the jugular lymph sac has extended mesad between 

 the common jugular and innominate veins and the thyro-cervical 

 or subclavian arteries to the interval between the main vein and 

 the sympathetic nerve. In this region the process makes second- 

 ary connections with the independently developed ventral medias- 

 tinal lymphatic trunk and with the equally independently formed 

 preaortic segment of the thoracic duct. 



The order of this union, as previously explained, appears to 

 have an important influence on the subsequent topographical 

 position of this portion of the adult thoracic duct. If the jugular 

 sac approach first meets and unites with the ventral mediastinal 

 lymphatic channel, then the entire duct appears to pass ventrad to 

 the sympathetic strand, between it and the adjacent dorso-medial 

 surface of the main vein, to turn subsequently dorsad towards the 

 prevertebral region on the mesal aspect of the nerve (reconstruc- 

 tion, fig. 171, series 143, slides x and xi, figs. 172 to 183). 



If, on the other hand, the thoracic duct approach first joins 

 the preazygos segment of the thoracic duct, developed by the 

 confluence of the dorsal perivenous lymphatic anlages accom- 

 panying and surrounding the prevertebral venous plexus cephalad 

 of the aortic arch level, then the resulting thoracic duct passes 

 dorso-laterad to the sympathetic nerve, after having leceived 

 the ventral mediastinal trunk through the interval between this 

 nerve and the main vein (text figs., p. (72). 



Fig. 170 shows in a dorsal view the reconstruction of the left 

 vascular corhplex of the lower cervical and upper thoracic regions 

 in a 15 mm. cat embryo (series 218, slides VIII, sections 25-44; 

 ix, sections 1-48; x, sections 1-48; xi, sections 1-48; xii, 

 sections 1-48.) 



The reconstruction should be followed in the direction caudo- 

 cephalad. The caudal part of the reconstruction includes 



