GENERATION AND DEVELOPMENT. 



297 



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. 454), and at the 

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

 lengthening mesentery formed from the splanchnopleure which attaches 

 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. 454). 



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-like processes (Fig. 455) 3 lying in a gelatinous nidus or blastema, 



FIG. 457. 



FIG. 458. 



FIG. 457. Diagram of part of digestive tract of a chick (fourth day). The black line represents 

 hypoblast, the outer shading mesoblast: I g, lung diverticulum, with expanded end forming primary 

 lung- vesicle; S , stomach; J, two hepatic diverticula, with their terminations united by solid rows of 

 hypoblast cells; , diverticulum of the pancreas with the vesicular diverticula coming from it. 

 (Gotte.) 



FIG. 458. Rudiments of the liver on the intestine of a chick at the fifth day of incubation. 1, 

 heart; 2, intestine; 3, diverticulum of the intestine in which the liver (4) is developed; 5, part of the 

 mucous layer of the germinal membrane. (Muller.) 



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

 of the gland advances, the canal becomes more and more ramified, in- 

 creasing at the expense of the blastema in which it is still enclosed. The 

 branches or salivary ducts constitute an independent system of closed 

 tubes (Fig. 456). 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. 



Liver. The liver 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 embrace the common venous stem (Figs. 457, 458). The 

 outer part of these cones involves the omphalo-mesenteric vein, which 

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



