450 



more polygonal in form. This process is accompanied by an increase 

 in the thickness of the mucosa and a further flattening ot the mnermost 

 layer of cells, until in an embryo of 11 cm, the stratified columnar 

 epithelium is transformed into a simple form of squamous epithelium. 

 This differentiation continues until, at 21,5 cm, the innermost rows 



>3X'^oK 





~A 



IC. 



OL 



Fig. 7. Cross section of the Oesophagus of a pig 23 cm long taken a the level 

 of the bifurcation of the Trachea. Same preparation as Fig. 1. X 80 /^JJ^ 

 MP Basement membrane. SM Submucosa. J/i/ Muscularis muco ae^ ^^7 4 Pri- 

 and OL Outer layer of the Tunica muscularis. D Duct of oesophageal gland. A rn 

 mitive alveolus of the oesophageal gland. 



