302 THE DEVELOPMENT OF THE CHICK 
a pharyngeal structure (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. 
Tongue 
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 
