CH. LVIII.] DEVELOPMENT OF THE DIGESTIVE GLANDS. 853 



abdominal cavity by means of a lengthening mesentery formed 

 from the splanchnopleur 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. 670). 



Pancreas and Salivary Glands. The principal glands in con- 

 nection with the intestinal canal are the salivary glands, pancreas, 

 and the liver. In mammalia, each salivary gland first appears as 

 a simple canal with bud-like processes, lying in a mass of 

 mesoblast, and communicating with the cavity of the mouth. 



Fig. 672. Diagram of part of digestive tract of a chick (4th day). The black line 

 represents hvpoblast, the outer shading mesoblast. /.</, 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. (Ootte.) 



As the development of the gland advances, the canal becomes 

 more and more ramified (fig. 671). The pancreas is developed 

 exactly in the same way, but its cells are derived from the 

 hypoblast lining the intestine, while those of the salivary glands 

 are formed from the epiblast lining the mouth. In both cases 

 the blood-vessels and connective tissues are formed from the 

 mesoblast into which the glandular structure grows. 



The Liver. The liver is developed by the protrusion of a 

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

 branches (figs. 672, 673). The inner portion of the cones consists 

 of a number of solid cylindrical masses of cells, derived from 

 the hypoblast, which become gradually hollowed by the formation 

 of the hepatic ducts, and among which blood-vessels are rapidly 



