SUMMARY AND CONCLUSIONS 165 



II. AZYGOS SEGMENT OF THORACIC DUCT 



The genesis of the azygos segment of the thoracic ducts is in 

 my estimation the region in which the validity of divergent views 

 as to systemic lymphatic development can best be tested. 



Certain mechanical and genetic conditions combine to make 

 the investigation of the development of the thoracic ducts in this 

 region perhaps difficult in one sense, while in another the results 

 of thorough investigation are most conclusive and convincing. 



Part of the difficulty referred to lies in the relatively small 

 amount of the retro- and peri-aortic space available for the develop- 

 ment both of the azygos veins and of the subsequent lymphatic 

 ducts replacing the same to a large extent. Again, the process 

 of systemic lymphatic development in this region appears to pro- 

 ceed with great rapidity. The first anlages of the thoracic ducts 

 in a given stage are succeeded in stages only slightly older by well 

 marked longer segments of the future lymphatic channel. On the 

 other hand, the observer has the advantage of dealing with a cir- 

 cumscribed region and with axial venous trunks, which, both in 

 their main line and in their tributaries, are remarkably constant, 

 regular and easily distinguished. 



As above stated, in the earlier stages a ventral tributary plexus 

 of the azygos veins drains the periaortic region. In later stages 

 this ventral venous plexus in large part disappears, a bilateral 

 lymphatic duct occupying its former place. The two conditions 

 cover each other absolutely. 



No one, possessing a knowledge of the embryonic venous devel- 

 opment of the cat, can examine a number of injections of the thor- 

 acic ducts and azygos veins of the adult animal without becoming 

 convinced that in course of development the thoracic duct complex, 

 caudal to the aortic arch-level, replaces a large part of the embry- 

 onic azygos venous system, and comes to occupy absolutely its topo- 

 graphical position and relation to surrounding structures. The 

 relation of the dorsal intersegmental aortic branches to the inter- 

 azygos venous anastomosis of the embryo, compared with the 

 intercostal arteries of the adult perforating through the meshes 

 of the plexiform lymphatic channel of the thoracic ducts, will alone 

 absolutely demonstrate this mutual topographical correspondence. 



