84 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



cal conditions for any given stage. They afford absolutely no 

 proof of the histogenetic origin of vascular structure. The mere 

 fact that a lymphatic plexus occupies topographically the place 

 of an antecedent venous plexus is absolutely no proof of the 

 histogenetic identity of the channel system involved. However, 

 to return to the matter in hand, while the region under discussion 

 serves no better than any other part of the embryo for this pur- 

 pose, it fully and completely illustrates the relative condition of 

 embryonic veins and the replacing extraintimal lymphatics in the 

 genesis of the adult systemic lymphatic channels. 



The ontogenetic history of the preazygos segment of the thoracic 

 duct must be considered in reference to three regions: 



A. The development of a broncho-mediastinal or ventral 

 mediastinal lymphatic channel, draining the ventral and lateral 

 mediastinal areas, cephalad of the pericardium, and including 

 the lymphatic return from the thymic, tracheal, bronchial, and 

 oesophageal regions. 



B. The development of the proximal portion of the thoracic 

 duct proper, between the termination of the thoracic duct ap- 

 proach of the jugular lymph sac, and the beginning of the azygos 

 segment of the thoracic ducts, caudal to the level of the aortic 

 arch. 



C. The junction of these two independently formed lymphatic 

 trunks with each other and with the thoracic duct approach of 

 the jugular lymph sac. 



Adult Conditions 



Before taking up the embryological consideration of the develop- 

 ment of these channels and the establishment of their subsequent 

 connections, it seems advisable, on account of the complicated 

 topographical relations, to establish these clearly by reference 

 to a small selected series of adult animals with successful injection 

 of the lymphatic structures concerned. This becomes all the 

 more necessary because the adult variations observed in the 

 arrangement of the lymphatics of this region have an important 



