THE LATER DEVELOPMENT OF THE FROG 159 



the mouth, enlarging the stomodseal cavity; the lower lip is the 

 larger and is freely movable (Fig. 58, B). The lips form the 

 important feeding organs of the tadpole, and as such they 

 become furnished with various horny structures, described 

 as teeth and jaws (Fig. 58, B), but not at all to be compared 

 with the true teeth and jaws which develop later. These horny 

 structures are purely larval organs and are entirely lost at the 

 time of metamorphosis. The "teeth" develop from strands or 

 piles of cells of the deeper layer of the epidermis. Each cell, 

 as it nears the surface, undergoing cornification, becomes conical 

 in form, and pushes through the skin, soon to be replaced by the 

 next cell (" tooth") underlying it. The upper lip bears three 

 medially interrupted rows of these " teeth," the lower lip four 

 complete rows. Toward the base of each lip, near the mouth, 

 a closely set row of teeth, together with intermediate horny 

 cells, form a continuous ridge; these ridges, of which that of the 

 lower lip is the larger, form the "jaws" or beak of the tadpole. 

 During metamorphosis the horny teeth and jaws are lost, the 

 lips are retracted, and as the mouth rapidly enlarges the true 

 teeth and definitive jaws are formed. 



1. The Derivatives of the Fore-gut 



The organs derived from the fore-gut region that we shall 

 describe, are the pharynx, oesophagus, stomach, liver and pan- 

 creas; in connection with the pharynx we shall describe the 

 development of the visceral pouches and arches, the internal 

 and external gills, the thymus, the ultimobranchial bodies, 

 epithelioid bodies, the thyroid body, tongue, ancj. the lungs. 



In front of the yolk-mass the fore-gut is widely expanded 

 transversely as the pharyngeal cavity (Fig. 37) ; antero-ventrally 

 its wall is fused with the ectoderm as the oral plate, while 

 postero-ventrally the liver region is indicated as a small cavity 

 extending backward beneath the yolk. Dorsal to the yolk the 

 fore-gut is narrowed as the rudiment of the oesophagus. Along 

 the sides of the pharynx a series of vertically elongated ridges, 

 or solid outfoldings, appear and extend to the surface ectoderm 



