288 THE CHICK. 



other in the median plane at the chin; and the sides of the 

 opening are bounded by the maxillary arches, MX, which grow 

 forwards from the mandibular arches to meet the outer angles 

 of the fronto-nasal process. 



The olfactory pits, OK, lie just beyond the anterior and outer 

 angles of the mouth : the inner border of each pit is formed 

 by the side of the fronto-nasal process, or inner nasal process ; 



OJ 



OC 



^HM 

 CH 



NS 



FIG. 126. The head of an Embryo Chick at the end of the seventh day of 

 incubation ; seen from below, x 8. 



BS, cerebral hemisphere. CD, third eyelid, or nictitating membrane. CH, 

 notochord, seen in section where the neck has been cut across. DS, mouth. HM, 

 external auditory meatus. MW, mandibular arch. MX, maxillary arch. US, spinal 

 cord, seen in section. OC, eyeball. O J, epithelial knob on tip of beak. OK, external 

 nostril. OL, lens. 



the outer border is formed by a strip of the side of the head 

 between the olfactory pit and the eye, which is spoken of as the 

 outer nasal process. 



Between the outer nasal process and the maxillary arch there 

 is a slight depression, the lacrymal groove, which runs from the 

 under surface of the eye to the outer border of the olfactory pit. 

 Between the inner nasal process, or wing of the fronto-nasal 

 process, and the anterior end of the maxillary arch there is a 

 more conspicuous depression, the nasal groove, which becomes 



