FIGURES 18 AXD 19 



IS Transverse section of middle thoracic region in a 17 mm. cat embryo (series 

 258. slide XVIIL section 9), X 22o. 



19 Transverse section through mid-thoracic region of a 19 mm. cat embryo 

 (series 2.")3, slide XXIV, section \)), X 22.'i. 



1 Sympathetic nerve. 



2 Intersegmental arteries. 



3 Right azygos. 



4 .Atrophj-ing embryonal vein. 



5 E.\traintimal or perivenous Ij^mphatic space. 



6 Left azygos. 



7 Aorta. 



1.') Interazygos venous plexus. 



3(3 Azygos segment of thoracic duct. 



