326 OUTLINES OF CHORDATE DEVELOPMENT 



The olfactory pit has already begun to deepen before the 

 division of its external aperture. The true olfactory epithe- 

 lium, which is distinguished from the adjacent non-sensory 

 epithelium by the fact that the former remains only one cell 

 thick, is limited to the deeper part of the pit, so that the olfac- 

 tory and respiratory portions of the olfactory chamber are 

 sharply distinguished even in these early stages. During the 

 fourth to eighth days, the internal nares are carried farther 

 back by the development of the palate, and the three pairs of 

 turbinates make their appearance, growing in from the outer or 

 lateral wall of the chamber. The lower turbinate extends into 

 the respiratory portion, the middle and upper turbinates into 

 the olfactory portion; later, however, the epithelium covering 

 the middle turbinate loses its olfactory character and becomes 

 like that of the respiratory part. 



The true olfactory epithelium contains neuroblasts as well as 

 ordinary epithelial cells. Superficially the former send out to, 

 or above, the surface of the epithelium, short processes which 

 are sensory or receptive in character. These same cells send 

 out also long processes (axons) which grow into the wall of 

 the adjacent telencephalon and form the true olfactory nerves. 

 The sensory epithelium of the olfactory organ is therefore a 

 neuro-epithelium. The I cranial nerve called the olfactory, is 

 composed of these fibers. 



III. THE ALIMENTARY TRACT AND ITS APPENDAGES 



We have already described the formation of the main divisions 

 of the embryonic gut; these are the fore-gut, with which we 

 described the pharynx and oral plate, the hind-gut with the 

 postanal gut, and the mid-gut connecting with the splanchnic 

 stalk. We shall now review the early development of the 

 various regions of the tract and of the appendages or derivatives 

 of each portion. 



1. Organs of the Fore-gut 



We left the fore-gut, at the thirtieth hour, as the short but 

 wide cavity of the head-fold, extending from the oral plate 



