606 



GENERATION AND DEVELOPMENT. 



semicircular canals are produced as diverticula of the vestibule, 

 which terminate by again communicating with the same cavity. 

 The Eustachian tube, the cavity of the tympanum, and the 

 external auditory passage, are remains of the first branchial 

 cleft. The membrana tympani divides the cavity of this cleft 

 into an internal space, the tympanum, and the external meatus. 

 The mucous membrane of the mouth, which is prolonged in 

 the form of a diverticulum through the Eustachian tube into 

 the tympanum, and the external cutaneous system, come into 

 relation with each other at this point ; the two membranes 

 being separated only by the proper membrane of the tym- 

 panum. 



Development of the Alimentary Canal. 



The alimentary canal, the early stage of whose development 

 has been already referred to (p. 585), is at first a uniform 

 straight tube, which gradually becomes divided into its special 

 parts, stomach, small intestine, and large intestine (Fig. 239). 



Outlines of the form and position of the alimentary canal in successive stages of 

 its development (from Quain). A, alimentary canal, &c., in an embryo of four 

 weeks ; B, at six weeks ; C, at eight weeks ; D, at ten weeks ; I, the primitive lungs 

 connected with the pharynx ; s, the stomach ; d, duodenum ; i, the small intestine ; 

 i', the large ; c, the caecum and vermiform appendage ; r, the rectum ; cl, in A, the 

 cloaca ; a, in B, the anus, distinct from si, the sinus uro-genitalis ; v, the yelk-sac ; 

 vi, the vitello-intestinal duct ; u, the urinary bladder and urachus leading to the 

 allantois; g, genital ducts. 



