The Development of the Frog 45 



will form the anterior nares. The lining of 

 the nasal pits will become connected with the 

 brain by the olfactory nerves, and will form 

 the olfactory epithelium of the nose. From 

 the inner side of the nasal pits a diverticulum 

 grows down to open into the mouth cavity as 

 the posterior nares. 



Development of the Alimentary Tract 



The origin of the primitive digestive tract 

 or archenteron has already been given : it now 

 remains to describe the further changes that 

 take place in the digestive tract itself, and to 

 describe the development of the various struc- 

 tures that are derived from it. 



The digestive tract may, for convenience, 

 be divided into three regions: (i) the mesen- 

 teron (whose formation has been described), 

 which is lined with entoblast and from which 

 are developed the liver, pancreas, lungs, gill 

 clefts, etc. ; (2) the stomodcetim, which forms 

 the mouth ; (3) the proctodceum or cloacal re- 

 gion. The first region is lined throughout 

 with entoblast, while the latter two are both 

 lined with ectoblast. The reason for this dif- 

 ference will be seen when the development of 

 the mouth and anus is described. 



