THE INTESTINAL CANAL. 



683 



embryo, just below the protovertebrse, is lined internally by 

 hypoblast, and covered externally by mesoblast. The cephalic 

 or anterior extremity of the canal is formed by uncleft mesoblast ; 

 the rest of the canal is formed by the splanchnopleural layer of 

 the mesoblast. 



A dilatation of a part of the fore gut gives origin to the primi- 

 tive stomach ; this is quite straight at first, lying below the ver- 

 tebral column, with which it is connected by mesoblast. After 

 a time the stomach becomes turned to the right side, so that the 



FIG. 273. 



Position of the various parts of the alimentary canal at different stages. A. Embryo 

 of five weeks; B. Of eight weeks ; C. Of ten weeks. (Allen Thomson.) I. Pharynx. . 

 Stomach, t. Small intestine. '. Large intestine, g. Genital duct. u. Bladder, d. 

 Cloaca, c. Caecum, vi. Ductus vitello-intestinalis. si. Urogeuital sinus, v. Yelk sack. 



left surface of the organ comes to lie anteriorly and the right sur- 

 face posteriorly, the mesoblast connecting it with the vertebral 

 column, being developed into the peritoneal processes of the organ. 



The lower part of the fore gut is of much smaller calibre than 

 the dilated portion forming the stomach ; it becomes the duode- 

 num, in connection with which arise two important viscera, the 

 liver and the pancreas. 



The mid gut and hind gut form the small and large intestines, 

 these being at first one straight tube, of which the small intestine 



