THE DIGESTIVE SYSTEM 



311 



tubular glands are relatively elongated and their walls are formed of 

 granular chief cells and of peripheral parietal cells. The chief cells 

 secrete two kinds of zymogen granules — pepsinogen and prochymosin. 

 When mixed with hydrochloric acid secreted by the parietal cells, pep- 

 sinogen becomes pepsin, which splits the molecules of albumen into 

 peptones, and the prochymosin becomes chymosin or rennin, which 

 changes casein into paracasein. It is also asserted that the gastric glands 

 secrete lipase, a fat-splitting enzyme. 



^.eOCFr STALK 



STOMOOEUM''' \\ 



YOLK STALK> 



'STOMACH 



pORSAL PANCflEAS 



" k -vtr/TRAL PANCREAS 



-SMALL INTESTINE 

 , -CAECUM PHARYNX' 



?ITDNEAL CAVITY 

 ^MESONEPHRIC 



ESOPHACUS 

 TRACHEA- 



DORSAL PANCREAS 

 VENTRAL PANCREAS 

 RO SINUS 



rCLK STALK' 



ANTOIS 

 PR0CT00€Um\ 



UMSIUCAL CORD' 



Fig. 261. — Stages A-F in the ontogenesis of the alimentary canal and associated 

 structures in the human embryo. A, Early embryo; B, Three- weeks embryo; C, Three 

 to four weeks embryo; D, Four weeks embryo; E, Five weeks embryo; F, Seven weeks 

 embryo. Notable among the changes represented are the great elongation of the canal, 

 the outgrowth of numerous appendages, and in the cloacal region the separation of the 

 organs of excretion and digestion. (Redrawn after Thompson, Ingalls, P. T. Lewis and 

 Arey.) 



The pyloric glands are limited to the pyloric portion of the stomach. 

 Their chief secretion is mucus, but the presence of some chief and parietal 

 cells suggests that they may also secrete some gastric juice. They differ 

 from gastric glands also in having relatively long pits and short, branched 

 and twisted tubules. Thus they resemble duodenal glands. 



Development of the Stomach. During ontogenesis, beginning with 

 the fifth week, the stomach arises as a local enlargement of the fore-gut. 

 Its lining, therefore, together with the glands derived from it, is endo- 

 dermal. The external peritoneal membrane is mesodermal; the remainder 



