OLFACTORY ORGANS. 533 



front end of the brain. It is most probably the homologue of 

 the olfactory pits of the true Vertebrata. 



In the true Vertebrata the olfactory organ has usually the 

 form of a pair of pits, though in the Cyclostomata the organ is 

 unpaired. 



In all the Vertebrata with two olfactory pits these organs 

 are formed from a pair of thickened patches of the epiblast, on 

 the under side of the fore-brain, immediately in front of the 

 mouth (fig. 307, ol). Each thickened patch of epiblast soon 

 becomes involuted as a pit (fig. 308, N}, the lining cells of 

 which become the olfactory or Schneiderian epithelium. The 

 surface of this epithelium is usually much increased by various 

 foldings, which in the Elasmobranchii arise very early, and are 

 bilaterally symmetrical, diverging on each side like the barbs of 

 a feather from the median line. They subsequently become 

 very pronounced (fig. 309), serving greatly to increase the 

 surface of the olfactory epithelium. At a very early stage the 

 olfactory nerve attaches itself to the olfactory epithelium. 



In Petromyzon the olfactory organ arises as an unpaired thickening of 

 the epiblast, which in the just hatched larva forms a shallow pit, on the 

 ventral side of the head, immediately in front of the mouth. This pit 

 rapidly deepens, and soon extends itself backwards nearly as far as the 

 infundibulum (fig. 310, of). By the development of the upper lip the opening 

 of the olfactory pit is gradually carried to the dorsal surface of the head, and 

 becomes at the same time narrowed and ciliated (fig. 47, ol). The whole 

 organ forms an elongated sack, and in later stages becomes nearly divided 

 by a median fold into two halves. 



It is probable that the unpaired condition of the olfactory organ in the 

 Lamprey has arisen from the fusion of two pits into one ; there is however 

 no evidence of this in the early development ; but the division of the sack 

 into two halves by a median fold may be regarded as an indication of such a 

 paired character in the later stages. 



In Myxine the olfactory organ communicates with the mouth through 

 the palate, but the meaning of this communication, which does not appear 

 to be of the same nature as the communication between the olfactory pits 

 and the mouth by the posterior nares in the higher types, is not known. 



The opening of the olfactory pit does not retain its em- 

 bryonic characters. In Elasmobranchii and Chimaera it becomes 

 enclosed by a wall of integument, often deficient on the side of 

 the mouth, so that there is formed a groove leading from the 

 nasal pit towards the angle of the mouth. This groove is 



