DEVELOPMENT OF THE LIVER. 



607 



The stomach originally has the same direction as the rest of 

 the canal ; its cardiac extremity being superior, its pylorus in- 

 ferior. The changes of position which the alimentary canal 

 undergoes may be readily gathered from the accompanying 

 figures. 



The principal glands in connection with the intestinal canal 

 are the salivary, pancreas, and the liver. In Mammalia, each 

 salivary gland first appears as a simple canal with bud-like 

 processes (Fig. 240), 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 rainitied, increasing at the expense of the blastema 



FIG. 241. 



FIG. 240. 



FIG. 240. First appearaace of the parotid gland in the embryo of a sheep. 

 FIG. 241. Lobules of the parotid, with the salivary ducts, in the embryo of the 

 sheep, at a more advanced stage. 



in which it is still inclosed. The branches or salivary ducts 

 constitute an independent system of closed tubes (Fig. 241). 

 The pancreas is developed exactly as the salivary glands. 



The liver in the embryo of the bird is developed by the 

 protrusion, as it were, of a part of the walls of the intestinal 

 canal, in the form of two conical hollow branches, which em- 

 brace the common venous stem (Fig. 242). The outer part of 

 these cones involves the omphalo-meseuteric vein, which breaks 

 up in its interior into a plexus of capillaries, ending in venous 



