FIGURES 18 AND 19 



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

 258, slide XVIII, section 9), X 225. 



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

 (series 253, slide XXIV, section 9), X 225. 



1 Sympathetic nerve. 



2 Intersegmental arteries. 



3 Right azygos. 



4 Atrophying embryonal vein. 



5 Extraintimal or perivenous lymphatic space. 



6 Left azygos. 



7 Aorta. 



15 Interazygos venous plexus. 



36 Azygos segment of thoracic duct. 



