170 SYSTEMIC LYMPHATIC VESSELS IN DOMESTIC CAT 



though the former is still the larger and better defined. It occu- 

 pies the original position between aorta and ventromesal surface 

 of the right azygos vein, ventral to the intercostal artery, dorsal 

 to the oesophagus. 



E. Definite lymphatic stage. From the 16 mm. embryo on. 

 Illustrations in body of text : figs. 259 to 262 ; figs. 265 to 270. 



Here all the thoracic duct anlages have united into the contin- 

 uous and connected plexiform channel of the thoracic ducts, 

 ventral to the azygos trunks, dorsal to the aorta. 



The early independent genetic history of the spaces,, which I 

 have above described as the first anlages of the thoracic duct 

 channels in the embryos of the cat, and the fact that in subsequent 

 stages they appear consistently and in every possible combination 

 as extraintimal or perivenous mesenchymal spaces, following and 

 surrounding the branches of the ventral azygos plexus, exclude to 

 my mind the possibility of regarding them as derived directly from 

 ventral azygos venous plexus, or from socalled 'venous outgrowths' 

 of the main azygos trunks, subsequently detached from the parent 

 trunks. 



In the foregoing genetic history of part of the thoracic duct 

 lymphatic plexus, by fusion of extraintimal and perivenous mesen- 

 chymal spaces, accompanying and surrounding certain of the ven- 

 tral venous tributaries of the right azygos trunk, the following 

 points should be noted: 



1. The portion of the entire adult thoracic duct complex here 

 considered develops in its earliest stages at some distance ventro- 

 mesad to the right azygos vein. It only subsequently, by extend- 

 ing proximad along the venule which it replaces, reaches the posi- 

 tion in close relation to the ventral surface of the azygos which it 

 occupies in the 16 mm. stage. 



2. In the 13.5 and 14 mm. embryos the radicles of the ventro- 

 medial tributary plexus are in the process of being surrounded by 

 the growing perivenous extraintimal lymphatic spaces which are 

 approaching along this line the ventral surface of the azygos, but 

 in the 14 mm. stage have not yet acquired this position. The 16 

 mm. embryo shows the lymphatic anlages in their definite relation 

 to the azygos trunk, and the primitive ventral venous tributary 



