THE EAR, AND THE DIGESTIVE SYSTEM. 399 



which is perhaps the most generally accepted view, or both 

 from the hyoidean bar, or one from each of these bars. There 

 are, at present, no recorded observations which determine the 

 matter satisfactorily in the case of the rabbit. 



The malleus can be recognised on the fifteenth day (Fig. 159, 

 ma) ; it is, from its first appearance, imbedded in the substance 

 of the tympanic membrane, eb, and is for some time continuous 

 with the posterior end of the mandibular bar, or Meckel's 

 cartilage. 



THE DEVELOPMENT OF THE DIGESTIVE SYSTEM. 

 A. The Alimentary Canal. 

 1 . General Account. 



The general history of the development of the alimentary 

 canal of the rabbit is closely similar to that of the chick. The 

 greater part of the length of the canal is formed from the 

 mesenteron, which, as in the chick, is a tubular cavity included 

 within the embryo by the process of constriction, through which 

 the embryo becomes separated from the yolk-sac (cf. I'igs. 146 

 and 147). Owing to this mode of formation of the mesenteron, 

 it necessarily communicates with the cavity of the yolk-sac in 

 the early stages, and so long as the yolk-stalk remains tubular. 

 The mesenteron may, therefore, as in the chick, be divided into 

 three lengths : fore-gut, mid-gut, and hind-gut ; the fore-gut 

 (Figs. 146 and 147, Gf) being the anterior portion, in which 

 roof, sides, and floor are all alike present ; the hind-gut, GH, 

 being the similar portion at the hinder end of the body ; and 

 the mid-gut, GT, being the median portion, which opens through 

 the yolk-stalk into the cavity of the yolk-sac, and which con- 

 sequently has no floor. Fore-gut and hind-gut increase in 

 length, at the expense of the mid-gut, as the embryo becomes 

 more and more sharply constricted from the yolk-sac ; and 

 ultimately, when the yolk-stalk becomes solid, about the thir- 

 teenth day, the mid-gut as a separate division of the alimentary 

 canal ceases to exist (cf. Figs. 146, 147, and 150). 



The mouth and anal openings are formed, as in other Ver- 

 tebrates, by stomatodeeal and proctodseal invaginations of the 

 epiblast at the anterior and posterior ends of the embryo 



