THE SENSE ORGANS 



577 



pit with the corner of the mouth. The narial passages, however, are not 

 formed by the fusion of the edges of this groove, as in some amphibia and 

 fishes, but by the backward extension of the olfactory pits, which acquire 

 a secondary connexion with the mouth. Hare-lip in man results from 

 the imperfect obliteration of the nasobuccal groove. The primary 

 openings of the narial passages into the mouth correspond with those of 

 amphibia and reptiles. The secondary and definitive choanae rise poste- 

 rior to the primary pair, when paired palatine processes unite in the middle 

 of the roof of the mouth, and thus separate the naso-pharyngeal cavity 



NASO-OPTIC. 

 GROOVE 



NASO-BUCCAL 

 GROOVE 



MANDIBULAR 

 PROCESS 



DOUBLE HARE-LIP 



Fig. 476. — The development of the narial passages in A, Chick and B, Man. In 

 phylogenesis the narial passages are believed to have arisen through the approximation 

 and closure of the edges of the nasobuccal grooves. Such grooves appear in ontogenesis. 

 Failure of such grooves to close over is the best explanation of hare-lip and perforate 

 palate shown in B. In normal growth in the human embryo, however, the narial 

 passages are not formed by the closing over of grooves but by the backward growth of 

 an ectodermal cord which grows from the nasal pit to the mouth cavity. (A redrawn 

 after B. Patten, and B after Corning.) 



from the mouth cavity. Palatine processes appear first in a two-months 

 human embryo, and the formation of the palate is completed at five 

 months. 



The inferior or maxillary conchae arise early in ontogenesis. Five 

 more pairs are formed by outgrowth from the ethmoid. Of these, three 

 later disappear, leaving the maxillary and two ethmoid characteristic of 

 the adult. 



TASTE ORGANS 



The second of the chemical senses is taste which, as we have seen, has 

 a common origin with smell. All animals respond in one way or another 

 to substances dissolved in water. Specialized taste organs in the form 



