176 NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER X. 

 THE OLFACTORY APPARATUS. 



The olfactory apparatus is treated here because of the physiolog- 

 ical relationship between it and the gustatory apparatus. Whether 

 a morphological relationship exists or not has yet to be determined. 



The peripheral organ of smell consists of a sensory epithelium 

 situated at the extreme anterior end of the head. It arises in 

 all vertebrates as a pair of thickenings of the ectoderm one at 

 either side of the lower or anterior border of the neuropore. The 

 thickenings are therefore formed from ectoderm which lies close 

 at the border of the neural plate before it rolls up into the neural 

 tube. While the brain and optic vesicles are developing and the 

 secondary divisions of the brain are taking form, the two epithelial 

 thickenings become depressed and sink into the angle between 

 the cephalic surface of the optic vesicles and the brain, and even- 

 tually form deep pits or sacs each with an external opening. At the 

 same time that these nasal pits are forming there appears below 

 and between them a single invagination which has been described 

 as the hypophysis (p. 66). The hypophysis is the remnant of 

 the ancient mouth and in some forms still has a connection with 

 the archenteron for a short time during development. It appears 

 that the olfactory organ was originally situated in the roof of the 

 mouth or in front of it. In this position the organ must have been 

 useful both in finding and selecting food. When the hypophysis 

 ceased to serve as mouth and a new mouth was formed from a 

 pair of gill slits the olfactory organ was farther removed from the 

 mouth but still in front of it. Finally, in air-breathing vertebrates 

 the olfactory sac comes to have a connection with the pharynx 

 and to serve as a passage for air in respiration. In this position 

 the organ is favorably placed for detecting odors and so for finding 

 food. 



In the wall of the nasal sac a part of the cells develop into sense 



