THE STOMATODvEUM. 287 



becomes more clearly defined, as a slightly depressed, transversely 

 elongated patch, bordered by a thickened rim, which is formed 

 partly by the ventral ends of the anterior visceral arches, and 

 partly by the under surface of the head itself. 



By further thickening of this rim, the depression which it 

 surrounds becomes deepened ; and the pit formed in this way, 

 rather by building up of its walls than by lowering of its floor, 

 becomes the stomatodasum. 



Towards the end of the third day the floor of the stomatodaaal 

 )it thins away and becomes perforated, placing the pharynx for 



-NS 



Fig. 125. — The head of an Embryo Chick at the end of the fifth day of incuba- 

 tion ; seen from below. Compare Fig. 115 for a view of an embryo of the 

 same age from the side, x 8. 



BR', 'first branchial arch. BS, cerebral hemisphere. CH, note-chord. DS. 

 month. FP, fronto-nasal process. HM, hyoniandibular cleft. HY, hyoid arch. 

 MN, mandibular arch. MX, maxillary arch. NS, spinal cord, seen in section where 

 the neck has been cut across. OC, eye. OH, choroidal fissure. OK, olfactory pit. 

 OL, lens. 



the first time in direct communication with the exterior, and 

 forming the permanent mouth opening (Fig. 114, o). 



The Face. After the definite formation of the mouth opening, 

 the borders of the stomatodseal pit continue to develop, and 

 gradually give rise to the beak and the anterior part of the face of 

 the bird. At the end of the fifth day the mouth opening (Fig. 

 125, ds) is oblong in shape. Its anterior border is formed by 

 the fronto-nasal process, FP, a broad plate, notched in the median 

 line, and forming, at this stage, the under surface of the head. 

 The posterior border of the mouth opening is formed by the 

 ventral ends of the mandibular arches, mn, which meet each 



