DEVELOPMENT. 815 



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. 508), and at 

 the same time comes to be suspended in the abdominal cavity by means 

 of a lengthening mesentery formed from the splanchnopleure which at- 

 taches it to the vertebral column. The stomach originally has the same 

 direction as the rest of the canal ; its cardiac extremity being superior, 

 its pylorus inferior. The changes of position which the alimentary canal 

 undergoes may be readily gathered from the accompanying figures (fig. 

 508). 



Pancreas and Salivary Glands. The principal glands in connec- 

 tion with the intestinal canal are the salivary, pancreas, and the liver. 

 In mammalia, each salivary gland first appears as a simple canal with bud- 



Fig. 509. Lobules of the parotid, with the salivary ducts, in the emhryo of the sheep, at a more 



advanced stage. 



like processes (fig. 509), lying in a gelatinous nidus or blastema, and 

 communicating with the cavity of the mouth. As the development of 

 the gland advances, the canal becomes more and more ramified, increas- 

 ing at the expense of the blastema in which it is still inclosed. The 

 branches or salivary ducts constitute an independent system of closed 

 tubes (fig. 509). The pancreas is developed exactly as the salivary 

 glands, but is developed from the hypoblast lining the intestine, while 

 the salivary glands are formed from the epiblast lining the mouth. 



The Liver. The liver is developed by the protrusion, as it were, 

 of a part of the walls of the fore-gut, in the form of two conical hollow 

 branches, which embrace the common venous stem (figs. 510, 511). The 



