168 SYSTEMIC LYMPHATIC VESSELS IN DOMESTIC CAT 



these in the characteristic manner above detailed, and continue 

 to develop as extraintimal or perivenous lymphatic spaces grad- 

 ually replacing the shrinking ventral azygos plexus. 



2. In the succeeding stage (13+ mm.) the aorta sinks more 

 deeply into the interazygos interval and is spanned on its dorsal 

 aspect by the horse shoe of the abundant supra-aortic cross anas- 

 tomosis between the azygos trunks of the two sides. The periven- 

 ous extraintimal lymphatic spaces surrounding and accompany- 

 ing the ventral azygos tributaries have increased in size, and a few 

 spaces of the same character begin to make their appearance along 

 the ventro-lateral aspect of the main azygos veins. 



3. In the 13.5 mm. embryo the extraintimal anlages replacing 

 the ventro-medial azygos tributaries have increased in number, 

 united with each other to form a series of longer continuous seg- 

 ments, and have, following the lead of the venules which they are 

 replacing, approached nearer to the ventral aspect of the main 

 azygos veins, and appear more closely interlocked with the ventro- 

 medial tributaries of the latter. 



C. IJf. mm. stage. Illustrations in body of text : figs. 214 to 240. 



The most instructive and conclusive pictures are obtained in 

 embryos of approximately this crown-rump measure. 



The sections are clearer and most unmistakable in their meaning 

 and the individual spaces have become confluent for longer 

 stretches, representing longer segments of the thoracic duct chan- 

 nel which is subsequently to be formed by their continued elon- 

 gation and fusion with each other. This becomes clearly apparent 

 in contrasting the ventral view of the reconstruction of series 76 

 (13.5 mm.) with that of series 214 (14 mm.) shown in figs. 188 and 

 190. 



The diffuse and scattered elements of the parazygos lymphatic 

 plexus anlages in the 13.5 mm. stage have, in the 14 mm. embryo, 

 become condensed and united into longer segments representing 

 the future line of right axial prevertebral lymphatic drainage. It 

 is noteworthy, as an important fact, that notwithstanding the 

 clear development of unmistakable longer segments of the future 

 right thoracic duct, these anlages are still placed at some distance 

 ventro-mesal to the right azygos vein and ventral to the inter- 

 segmental arteries. 



