8H 



HANDBOOK OF PHYSIOLOGY. 



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 vitel- 

 line or omphalo-mesenteric duct. 



From the fore-gut are formed the pharynx, O3sophagus, 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 maxil- 

 lary and mandibular processes, which becomes deeper and deeper till it 

 reaches the blind end of the fore-gut, and at length communicates freely 

 with the pharynx by the absorption of the partition between the two. 



At the other end of the alimentary canal the anus is formed in a pre- 

 cisely similar way by an involution from the free surface, which at length 



D 



Fig. 508. Outlines of the form and position of the alimentary canal in successive stages of 

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

 eight weeks ; D, at ten weeks ; Z, the primitive lungs connected with the pharynx ; s, the stomach ; 

 d, duodenum; t, the small intestine; i', the large; c, the caecum and vermiform appendage; r, the 

 rectum ; cZ, in A, the cloaca ; a, in B, the anus distinct from s i, the sinus uro-genitalis ; v, the 

 yolk-sac ; v i, the vitello-intestinal duct ; w, the urinary bladder and urachus leading to the al- 

 lantois; gr, genital ducts. (Allen Thomson.) 



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 alimen- 

 tary 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 more and closed in till its originally wide communication with the 

 yolk-sac becomes narrowed down to a small duct (vitelline). This duct 

 usually completely disappears in the adult, but occasionally the proximal 

 portion remains as a diverticulum from the intestine. Sometimes a 

 fibrous cord attaching some part of the intestine to the umbilicus, re- 

 mains to represent the vitelline duct. Such a cord has been known to 

 cause in after-life strangulation of the bowel and death. 



