VIII. ] THE MOUTH. 249 



or first visceral arch. 2. second visceral arch. x. first vis- 

 ceral cleft between the first and second visceral arches. 



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 and converted into canals. 



from the nasal pit above, into the cavity of the mouth 

 below, and places the two in direct communication. 

 This canal, whose lining consists of epiblast, is the 

 rudiment of the nasal labyrinth. 



By the seventh day (Fig. 82), not only is the union 

 of the superior maxillary and fronto-nasal processes 

 completed, and the upper boundary of the mouth thus 

 definitely constituted, but these parts begin to grow 

 rapidly forward, thus deepening the mouth and giving 

 rise to the appearance of a nose or beak (Fig. 82), 

 which, though yet blunt, is still distinct. The whole of 

 the lower boundary of the buccal cavity is formed by 

 the inferior maxillary processes. 



As we have before mentioned (p. 240), cartilage suc- 

 ceeded by bone is developed in the fronto-nasal process ; 

 the pterygo-palatine osseous bar (membranous ossifica- 

 tion) in the superior maxillary process; Meckel's cartilage 

 the main part of which atrophies, the proximal end only 

 ossifying as the articulare, and the quadrate succeeded 

 by bone in the inferior maxillary process; the other 

 bones which form the boundaries of the mouth in the 

 adult are developed later after all external trace of these 

 parts as separate processes has "disappeared. 



At first the mouth is a simple cavity into which the 

 nasal canals open directly. When however the various 



