152 THE FROG. 



teeth, the uppermost of a column of cells, the more deeply placed 

 members of which are indifferent epithelial cells, but which as 

 they approach the surface become first flattened, then cupped, and 

 finally hollow cones fitting into one another. As in the labial 

 teeth, the hardness is due to cornificatiou of the cells, invading 

 first the outer surface and ultimately the entire cell. As the 

 biting edge of the jaw gets worn away by use, it is constantly 

 renewed by the more deeply placed cells. 



The rest of the jaw consists of a dense mass of flattened and 

 cornified epithelial cells, which become firmly fused together, and 

 which, like the cells of the cutting edge, are renewed from the 

 indifferent epithelial cells of the deeper layers. Into this deeper 

 layer vascular papillas of the dennis project, increasing the 

 extent of the nutritive surface of the jaw. 



At the metamorphosis the horny jaws are cast off, and lost. 



4. The Pharynx. 



The characteristic feature of the pharynx, both in the tadpole 

 and in the adult frog, is its great width from side to side (cf. 

 Figs. 74 and 68) ; and this is acquired, as already described, at 

 a very early developmental stage. In horizontal section the 

 pharynx of the tadpole is somewhat lozenge-shaped (Fig. 74), 

 narrowing rather gradually in front to open into the buccal 

 cavity, and much more abruptly behind, where it passes back 

 into the oesophagus. 



The roof of the pharynx may be divided into two regions : 

 an anterior part, clothed by a flattened pavement epithelium, 

 and bearing taste bulbs and sensory papillae ; and a posterior 

 part, covered by a ciliated epithelium, and containing numerous 

 multicellular glands. 



The gills, which are the most important structures in con- 

 nection with the sides of the pharynx, will be described in the 

 next section (pp. 157 to 163). 



The tongue is formed on the floor of the pharynx, but does 

 not appear until shortly before the metamorphosis ; it then 

 grows rapidly and soon attains its adult shape and proportions 

 (Fig. 89, TN). 



5. The Thyroid Body. 



About the time of hatching of the tadpole, or a little earlier, 



