CHAPTER X 
THE ALIMENTARY TRACT AND ITS APPENDAGES 
Tue 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 cesophagus, 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 (ef. Fig. 175). In the floor the extent of 
the ectodermal component is much less. If the tongue is entirely 
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