DEVELOPMENT. 



705 



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. 489). 



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. 490), 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, in- 

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

 branches or salivary ducts constitute an independent system of closed 



FIG. 492. 



FIG. 493. 



FIG. 492. Diagram of part of digestive tract of a chick (4th day). The black line represents 

 hypoblast, the outer shading, mesoblast; Za,lung diverticulum. with expanded end forming primary 

 lung- vesicle; St, stomach; I, two hepatic diverticula, with their terminations united by solid rows 

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

 (Gotte.) 



FIG. 493. Rudiments of the liver on the intestine of a chick at the fifth day of incubation, a, 

 heart; 6, intestine; c. diverticulum of the intestine in which the liver (d) is developed; e, part of the 

 mucous layer of the germinal membrane. (Miiller.) 



tubes (Fig. 491). 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 fore-gut, in the form of two conical hollow 

 branches which embrace the common venous stem (Figs. 492, 493). 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 

 trunks for the conveyance of the blood to the heart. The inner portion 

 of the cones consists of a number of solid cylindrical masses of cells, de- 

 rived probably from the hypoblast, which become gradually hollowed by 

 45 



