CHAPTER X 



THE ALIMENTARY TRACT AND ITS APPENDAGES 



The origin of the alimentary canal and of its various main 

 divisions and appendages has been considered in preceding chap- 

 ters. The subsequent history will now be taken up in the fol- 

 lowing order: 



1. The mouth and oral cavity. 



2. The pharynx and its derivatives. 



3. The oesophagus, stomach and intestine. 



4. The liver and pancreas. 



5. The respiratory tract. 



The history of the yolk-sac and allantois was considered with the 

 embryonic membranes (Chap. VII); the detailed history of the 

 mesenteries will be taken up in connection with the body cavities 



(Chap. XI). 



I. Mouth and Oral Cavity 



The oral cavity may be defined embryologically as that part 

 of the alimentary canal formed on the outer side of the oral plate. 

 Anatomically, however, such a definition is unsatisfactory both 

 because it is impossible to determine the exact location of the 

 oral plate in late stages, and also' because of the difference in 

 extent of the ectodermal component in roof and floor of the 

 mouth; the definitive mouth cavity includes part of the floor of 

 the embryonic pharynx. It is, however, of interest to determine 

 as nearly as possible the limits of the ectodermal component 

 of the oral cavity. In the roof this is not difficult because the 

 hypophysis, which arises just in front of the oral plate, retains 

 its connection with the mouth cavity until definitive landmarks 

 are formed. The median sagittal section of an eight-day chick 

 (Fig. 148) shows that this point is situated almost immediately 

 opposite to the glottis, that is, between the palatine and tubal 

 fissures in the roof (cf. Fig. 175). In the floor the extent of 

 the ectodermal component is much less. If the tongue is entirely 



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