182 



THE FIFTH DAY. 



[CHAP. 



stitutecl, but these parts begin to grow rapidly forward, thus 

 deepening the month, and giving rise to the appearance of a 

 nose or beak (Fig. 58), which, though yet blunt, is still 

 distinct. The whole of the lower boundary of the buccal 

 cavity is formed by the inferior maxillary processes. 



Fig. 58. 



r\ 





Z^x 



Head of a Chick of the Seventh Day from below. (Copied from Huxley's 



Elements of Comparative A natomy.) 



I a. cerebral vesicles, a. eye. g. nasal pits. ft. fronto-nasal process. I. superior 



maxillary process. I. first visceral arch. 2. second visceral arch. x. first 



visceral cleft. 



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 process (first pair of visceral arches) below. 



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

 along the whole length of the latter, with the exception of a small space in 

 front, where a narrow angular opening is left between the two. 



As we have before mentioned, the ethmovomerine car- 

 tilage is developed in the fronto-nasal process, the pterygo- 

 palatine bar in the superior maxillary process, Meckel's 

 cartilage and the quadrate in the inferior maxillary process ; 

 the other bones which form the boundaries of the mouth in 

 the adult are developed later after all externa] trace of these 

 parts as separate processes has disappeared. For an account 

 of their formation, however, we must refer the reader to the 

 chapter upon the development of the skull. 



At first the mouth is a simple cavity into which the 

 nasal canals open directly. When however the various 



