CH. LIX.] THE ALIMENTARY CANAL 867 



external auditory passage, are the remains of the first or hyo- 

 mandibular cleft. The membrana tympani divides the cavity of this 

 cleft into the tympanum, and the external meatus. The mucous 

 membrane of the pharynx, which is prolonged in the form of a diver- 

 ticulum through the Eustachian tube into the tympanum, and the 

 external cutaneous system come into relation with each other at this 

 point; the two structures are separated only by the membrane of 

 the tympanum. 



The pinna or external ear is developed from a process of integu- 

 ment in the neighbourhood of the first and second visceral arches, 

 and probably corresponds to the gill-cover (operculum) in fishes. 



The Nose. The nose originates, like the eye and ear, in a depres- 

 sion of the superficial epiblast at each side of the fronto-nasal process 

 (primary olfactory pit), which is at first in front of the cavity of the 

 primitive mouth, and gradually extends backwards, into its roof (p. 

 846). 



The olfactory bulbs of the brain lie in close relation with the 

 roofs of the olfactory pits, and the olfactory nerves are out- 

 growths from special bipolar cells in the epithelium of the pit (see 

 p. 736). 



Development of the Alimentary Canal. 



The alimentary canal in the earliest stages of its development 

 consists of three parts the fore- and hind-gut ending blindly at each 

 end of the body, and a middle segment which communicates freely 

 on its ventral surface with the cavity of the yolk-sac through the 

 vitelline or omphalo-mesenteric duct. 



From the fore-gut are formed the lower and back part of the 

 mouth, the pharynx, oesophagus, stomach, and first and second parts 

 of the duodenum; from the hind-gut, the lower end of the colon, the 

 rectum, and the bladder. The upper and front part of the mouth, 

 and the nasal chambers are developed from the stomadseal space 

 (p. 847). 



At the other end of the alimentary canal the anus is formed by 

 an involution from the free surface, which at length opens into the 

 hind-gut. When the depression from the free surface does not reach 

 the intestine, the condition known as imperforate anus results. A 

 similar condition may exist at the other end of the alimentary canal 

 from the failure of the involution which forms the mouth, to meet 

 the fore-gut. 



The middle portion of the digestive canal becomes more and more 

 closed in, till its wide communication with the yolk-sac becomes 

 narrowed down to a small duct (vitelline). This duct usually com- 

 pletely disappears in the adult, but occasionally the proximal portion 



