536 EXTERNAL AND INTERNAL NARES. 



part of its outer side. The inner and outer ridges, together 

 with the superior maxillary process, enclose a deep groove, con- 

 necting the original opening of the nasal pit with the mouth. 

 The process just described is illustrated by fig. 311 A, and it 

 may be seen that the ridge on the inner side of the groove 

 forms the edge of the fronto-nasal process (k). 



On the sixth day (Born, 394) the sides of this groove unite 

 together in the middle, and convert it into a canal open at both 

 ends the ventral openings of the canals of the two sides being 

 placed just within the border of the mouth, and forming the 

 posterior nares ; while the external openings form the anterior 

 nares. The upper part of the canal, together with the original 



FIG. 311. HEAD OF A CHICK FROM BELOW ON THE SIXTH AND SEVENTH DAYS 

 OF INCUBATION. (From Huxley.) 



/". cerebral vesicles ; a. eye, in which the remains of the choroid slit can still be 

 seen in A ; g. nasal pits ; k. fronto-nasal process ; /. superior maxillary process ; 

 i. inferior maxillary process or first visceral arch; 2. second visceral arch; x. first 

 visceral cleft. 



In A the cavity of the mouth is seen enclosed by the fronto-nasal process, the 

 superior maxillary processes and the first pair of visceral arches. At the back of it is 

 seen the opening leading into the throat. The nasal grooves leading from the nasal 

 pits to the mouth are already closed over. 



In B the external opening of the mouth has become much constricted, but it is 

 still enclosed by the fronto-nasal process and superior maxillary processes above, and 

 by the inferior maxillary processes (first pair of visceral arches) below. 



The superior maxillary processes have united with the fronto nasal process, along 

 nearly the whole length of the latter. 



nasal pit, is alone lined by olfactory epithelium ; the remaining 

 epithelium of the nasal cavity being indifferent epiblastic epi- 



