302 THE DEVELOPMENT OF THE CHICK 



a pharyngeal striictiire (in the embryological sense) the limit 

 of the ectoderm would lie near the angle between the tongue 

 and the floor of the mouth. In the side walls the boundary must 

 be near the lines uniting the dorsal and ventral points as thus 

 determined. 



To/7^//e. 



\ '- 



ff^pa/. 





{ i\ K 



T 



-f/sTi/d. 



Oor/?.//r 



Fig. 175. — Floor and roof of the mouth of the hen. The jaw muscles were 



cut through on one side, the lower jaw disarticulated and the entire floor 



drawn back. 



Corn. H., Cornu of the hyoid. Fis. pal., Palatine fissure. Fis. Tub., 

 Tubal fissure. Mu., cut surface of jaw muscles. 



We have already considered the formation of the boundaries 

 of the mouth (Chap. VI and Chap. VII), and of the palate (Chap. 

 IX, page 299). These data need not be repeated, so we have 

 left to consider only the development of the beak, egg-tooth, 

 tongue, and oral glands. 



Beak and Egg-tooth. The beak is a horny structure formed 

 by cornification of the epidermal cells around the margins of 



