CHAPTER XLI. 



THE DIGESTIVE TRACT 



After the splitting of the mesoderm and notochord from the ento- 

 derm, the wall of the digestive tract is but one cell in thickness, except 

 in the region of the mid-gut, where the yolk-mass is very large. 



The stomodaeum, already discussed, is a shallow depression just 

 below the olfactory and fore-brain region. This, by the time of hatch- 

 ing, while still very shallow, has its floor come to lie in contact with the 

 wall of the fore-gut. The region in which these fuse is called the oral 

 plate (Figs. 301, I, and 329, Pharyngeal membrane). It is at this point 

 that the mouth forms a few days after hatching. The oral sucker (Fig. 

 337) forms just below the stomodaeal invagination. 



The margins of the mouth form mandibular ridges which become 

 drawn out as an upper and lower lip, the lower being the larger and 

 freely movable. (Figs. 316, 317, 318.) 



Strands of cells from the deep layer of the epidermis push toward 

 the surfaces and as each cell arrives at the surface it becomes cornified 

 into a so-called "tooth." The upper lip has three rows of these 

 "teeth," while the lower has four rows. All of these are lost when the 

 tadpole assumes its adult shape. In the adult form, true teeth and jaws 

 form. 



From the fore-gut the following structures are derived. (Figs. 328, 

 329, 337) : 



The Pharyngeal Cavity, the large expansion in the anterior portion 

 of the fore-gut. 



The Oesophagus, that portion of the fore-gut narrowed immediately 

 dorsal to the yolk. 



The Visceral Pouches are seen as vertical solid foldings extending 

 to the surface ectoderm. Six pairs of these (compare this number with 

 those found in the chick) develop, increasing in size and importance pos- 

 teriorly. 



The Visceral Arches, not to be confused with the pouches. The 

 arches are the vertical rods of mesoderm lying between the pouches. 



The hyomandibular pouch is the name given the first pouch. 



The Mandibular arch is the first arch, lying in front of the hyoman- 

 dibular pouch, or in other words, between the hyomandibular pouch and 

 the mouth. 



The I to V Branchial, or gill pouches, are the remaining ones run- 

 ning posteriorly from the hyomandibular. The hyomandibular is the 

 first in point of position, but bears a separate name. The numbers thus 

 begin with the true second pouch. 



