THE SENSE ORGANS 



40,3 



which connects the anterior part of the nasal passage with the mouth 

 cavity. (Fig. 354) 



Ontogenesis. In elasmobranchs, each olfactory organ develops from 

 a placode-like thickening at the anterior termination of the series of lateral- 

 line organs. Subsequently each placode, by invagination, is converted 

 into a pit, which Hes in close apposition to the telencephalic vesicle. In 

 man a groove similar to the nasobuccal groove of elasmobranchs connects 

 each olfactory pit with the corner of the mouth. The nasal passages, 

 however, are not formed by the fusion of the edges of this groove, as in 



NASO-OPTIC 

 GROOVE 



NASO-BUCCAL 

 GROOVE 



MANDIBULAR, 

 PROCESS 



DOUBLE HARE-LIP 



Fig. 355. — The development of the nasal passages in A, chick and B, man. In 

 phylogenesis the nasal 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 nasal 

 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. (.4 redrawn 

 after B. Patten, and B after Corning.) 



some amphibians 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 nasal passages into the mouth correspond 

 with those of amphibians and reptiles. The secondary and definitive 

 choanae arise posterior 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 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 human ontogenesis. 

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



