DEVELOPMENT OF THE ORGANS OF SPECIAL SENSE 



A The Olfactory Organs- 



The Xosiv. 



The olfactory organs appear early in enil)ryonic life as thick- 

 ened patches of epiblast upon the antero-inferior part of the 

 head, which patches soon sink inwards to constitute the olfactory 

 pits, into the bottom of which the nerves of smell grow out from 

 the olfactory bulbs of the brain. 



Fig. 33. The under surface of the head of a human embryo, let- 

 tered by Professor His, Hn, and estimated as about twenty-nine 

 days old. X 7>^- From Marshall after His. 



BS, Cerebral hemisphere. D.S, Stomatodeum. FO, Processus 

 globularis, or lateral portion of fronto-nasal process. HM, hyo- 

 mandibular cleft. MN, Mandibular .irch. MX, Maxillary arch. 

 OC, Eye. OK, Olfactory pit. 



The olfactory pits are at first incomplete, their lower borders 

 being deeply notched, and communicate with the stomatodeum, 

 somewhat as an extension of that cavity. The olfactory pits 

 and the stomatodeum are connected by a narrow isthmus, 

 bordered inwardly by Wi^ processus globiilares of the fronto-nasal 

 process and externally by the maxillary processes of the maxillary 

 arch. Soon the maxillary processes approach and fuse with the 

 processus globulares anteriorly, to complete the circumference 

 of the olfactory pits and, by .separating them from the mouth, to 

 constitute the nostrils. For a short distance anteriorly, as 

 far as to the incisive foramen of the adult, the processus globulares 

 send projections inwards to constitute the anterior portion of 

 the palate, while, behind the foramen, the maxillary processes 

 send out .shelf-like projections, which meet on the median line to 

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