852 



DEVELOPMENT. 



[CH. LVIII. 



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 precisely similar way by an involution from the free surface, 

 which at length 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 alimentary 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 and more 



Fig. 671. Lobules of the parotid, with the salivary ducts, in the embryo of the sheep, 

 at a somewhat advanced stage. 



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, remains to represent the vitelline duct. Such a 

 cord has been known to cause in after-life, strangulation of the 

 bowel and death. 



The alimentary canal lies in the form of a straight tube close 

 beneath the vertebral column, but it gradually becomes divided 

 into its special parts, stomach, small intestine, and large intestine 

 (fig. 670), and at the same time comes to be suspended in the 



