PART II 



THE DEVELOPMENT OF THE PREAZYGOS AND AZY- 

 GOS SEGMENTS OF THE THORACIC DUCTS 



In certain developmental stages in the cat, the embryonic 

 anlages of the thoracic ducts appear as potentially symmetrical 

 bilateral lymphatic channels. 



Subsequently, the cephalic portion of the left duct, having 

 secondarily assumed the function of conveying the lymph from 

 the trunk and the posterior extremities, forms, cephalad to the 

 aortic arch level, the main continuation of the thoracic duct 

 proper, emptying, through the intervention of the left jugular 

 lymph sac, into the general venous system at the left common 

 jugular confluence, or at the jugulo-subclavian junction, or at 

 both of these points, according to the type of adult lymphatico- 

 venous connection established in individual cases (26) > (2?; > (34) > (35;> 



The corresponding cephalic preazygos portion of the right lym- 

 phatic duct then remains as a more or less rudimentary structure, 

 draining the anterior mediastinal region and emptying into the 

 right jugular lymph sac. 



Caudad of the level of the aortic arch, the adult thoracic duct 

 is a composite of parts of three embryonic lymph channels pre- 

 senting a great range of individual variation. Two of these 

 anlages develop as bilateral extraintimal lymphatic trunks along 

 the ventral aspect of the right and left azygos veins. The third 

 intermediate channel, which is the first to distinctly differentiate 

 in the embryo and is the main element in the typical organiza- 

 tion of the adult duct, develops in the caudal portion from ex- 

 traintimal lymphatic anlages, which replace part of the ventro- 

 medial tributary plexus of the right azygos vein. In ascending 



35 C. F. W. McClure and C. F. Silvester: "A comparative study of the Lym- 

 phatic-Venous communications in Adult Mammals," Anat. Rec., Vol. Ill, pp. 

 634-551. 



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