166 SYSTEMIC LYMPHATIC VESSELS IN DOMESTIC CAT 



The only question at issue is the determination of the genetic 

 processes through which the embryonic azygos venous plexus is 

 replaced by the lymphatic network of the thoracic ducts in the 

 adult. 



Some observers have derived this portion of both ducts directly 

 from the ventral aspect of the azygos veins by comparing tran- 

 sections of 14 mm. and 16 mm. embryos, and finding in the former 

 ventral tributaries of the main channels, whose place in the latter 

 is occupied by the thoracic ducts. They hence conclude that 

 the ventral azygos plexus, forming 'the veno-lymphatic anlages' 

 of the thoracic ducts, separates at numerous points from the parent 

 vein, and that the resulting detached portions of the originally 

 continuous venous plexus, secondarily unite to form the channel 

 of the thoracic ducts. The latter structures are therefore, in their 

 estimation, direct derivatives of the azygos veins and their tribu- 

 taries. 



The preceding pages give my reasons for regarding this replace- 

 ment as having occurred in course of the development of the thor- 

 acic ducts by confluence of numerous isolated mesenchymal spaces, 

 formed along and around the azygos veins and their ventral trib- 

 utaries, but genetically independent of the latter and lined by an 

 endothelium not derived from the pre-existing haemal endothe- 

 lium. 



In the region under discussion these mesenchymal clefts develop, 

 owing to local conditions, very largely as extraintimal or peri- 

 venous spaces accompanying and surrounding the branches of the 

 retrograding ventromedial azygos plexus. The resulting thoracic 

 ducts thus come to occupy the place formerly filled by the ventral 

 azygos venous network, and later are led into direct ventral 

 contact with the main azygos veins, which they in their further 

 growth to a large extent secondarily replace. 



In the early (11-12 mm. stage) the azygos veins receive on their 

 ventral and ventro-medial aspect the terminals of a venous plexus 

 originating in the retro- and peri-aortic mesenchyme. While the 

 individual embryos offer many striking pictures at various levels, 

 I believe that my purpose of demonstrating the genesis of the 

 thoracic ducts in this area will best be accomplished by taking 



