CH. LVIII.] DEVELOPMENT OF THE ALIMENTARY CANAL. 851 



separated from the cavity of the mouth, and gradually extends 

 backwards and downwards till it opens into the mouth. 



The outer angles of the fronto-nasal process, \initing with the 

 maxillary process on each side, convert what was at first a groove 

 into a closed canal. The olfactory nerve which meets this is, like 

 the optic nerve, primarily a hollow process of the brain. 



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 



Fig. 670. Outlines of the form and position of the alimentary canal in successive stages 

 of its development. A, alimentary canal, &c., in an embryo of four weeks ; B, at six 

 weeka ; C, at eight weeks ; D, at ten weeks ; I, the primitive lungtt connected with the 

 pharynx ; s, the stomach ; d, duodenum ; t, the small intestine ; i", the large ; c, the 

 ceecum and vermiform appendage ; r, the rectum ; d, in A, the cloaca ; a, in B, the 

 anus distinct from ti, the sinus uro-genitalis ; v, the yolk-sac ; vi, the vitello-intentinal 

 duct ; u, the urinary bladder and urachus leading to the allantois ; </, genital ducts. 

 (Allen Thomson.) 



at each end of the body, and a middle segment which com- 

 municates 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 pharynx, oesophagus, and 

 stomach ; from the hind-gut, the lower end of the colon and the 

 rectum. The mouth is developed by an involution of the epiblast 

 between the maxillary and mandibular processes, which becomes 

 deeper and deeper till it reaches the blind end of the fore-gut, and 



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