INTRODUCTION 



599 



apposition of the oral evagination of the foregut and the stomodaeal invagina- 

 tion of the epidermal tube, constitutes the oral or pharyngeal membrane. Ecto- 

 derm and entoderm thus enter into the composition of the pharyngeal mem- 

 brane. This membrane normally atrophies. 



2) Head Gut or Seessel's Pocket. This structure represents the extreme an- 

 terior end of the foregut which projects forward toward the anterior end of 

 the notochord and brain. It extends cephalad beyond the region of contact of 

 the stomodaeum with the oral evagination of the foregut. During its earlier 

 period, the head gut is intimately associated with the anterior end of the 



PHARYNGEAL REGION OF 

 FOREGUT, 



DULT CONDITION '-'■ 



Fig. 279. Morphogenesis of the digestive structures in the dog fish, Sqiialus acanthias. 

 See also Figs. 29 IC and 296A. 



