THE EAK, AND ALIMENTARY CANAL. 281 



the labyrinth, and becoming specially developed at certain places, 

 to form the auditory epithelium. The auditory nerve, as noticed 

 above, very early conies in contact with the anterior and inner 

 wall of the auditory vesicle, fusing completely with this by the 

 fiftieth hour. This fused patch, by division and subsequent separa- 

 tion of the several portions, gives rise to all the special patches 

 of auditory epithelium present in the adult labyrinth. 



The accessory organs of hearing. The development of the 

 Eustachian tube, tympanic cavity, and tympanic membrane 

 will be described in the section dealing with the development of 

 the pharynx and gill-clefts (p. 283). The development of the 

 columella, or auditory ossicle, will be described with the skeleton 

 (p. 330). 



DEVELOPMENT OF THE ALIMENTARY CANAL. 



1 . General Account. 



The alimentary canal of the chick, like that of the frog, is 

 developed in three portions, of independent origin, and of very 

 unequal length. 



(i) The stomatodseum, or mouth invagination, is formed by a 

 pitting-in of the epiblast at the anterior end of the alimentary 

 tract ; from it the anterior part of the buccal cavity, and the 

 pituitary body are developed. 



(ii) The mesenteron gives rise to almost the entire length 

 of the alimentary canal, from the hinder part of the buccal cavity 

 to the cloaca ; and from it the lungs, liver, pancreas, and other 

 important structures arise as outgrowths. The mesenteron is 

 the tubular cavity formed within the embryo, as the result of the 

 process of folding or constriction by which the embryo becomes 

 pinched off from the yolk-sac (Figs. 112, 114, and 123). It is lined 

 by hypoblast along its whole length. Owing to the mode of its 

 formation, it communicates freely, through the yolk-stalk, with 

 the yolk-sac ; and, so long as the yolk-stalk remains tubular, the 

 mesenteron may be described as consisting of three lengths : the 

 fore-gut (Figs. 112, GF, and 114, TP, TO), which is the part included 

 in the head-fold, and has complete roof, sides, and floor : the mid- 

 gut (Figs. 114 and 123, YS), which opens into the yolk-stalk, and 

 which therefore has roof and sides, but no floor ; and the hind-gut 

 (Figs. 114 and 123, GH), which is the part included in the tail-fold, 

 and has, like the fore-gut, complete roof, sides, and floor. As the 



