296 



HAND-BOOK OF PHYSIOLOGY. 



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

 length opens into the hind-gut. When the depression frt)m the free sur- 

 face 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 



FIG. 455. First appearance of the parotid gland in the embryo of a sheep. 



mouth, to meet the fore-gut. The middle portion of the digestive canal 

 becomes more and more closed in till its originally wide communication 

 with the yelk-sac becomes narrowed down to a small duct (vitelline). 

 This duct usually completely disappears in the adult, but occasionally the 



advanced 5 sta~e j0bUleS f the parotid ' with the salivai 7 ducts, in the embryo of the sheep at a more 



proximal portion remains as a diverticulum from the intestine. Some- 

 times 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 th^ bowel and death. 



