230 



CHORDATE ANATOMY 



cardiac glands occupy a relatively small area near the cardiac orifice 

 and resemble closely the glands of the esophagus. Each cardiac gland 

 consists of a group of parallel tubules opening into a single duct or pit. 

 The walls of the tubules are formed of cells which secrete zymogen or 

 pepsinogen granules, of parietal cells which secrete the chemical precursor 

 of hydrochloric acid, and of mucus-secreting cells. 



Most of the glands of the stomach are gastric each of which, Hke the 

 cardiac glands, consists of a duct or pit connected with a group of straight 



pORSAL PANCREAS 



^•VENTRAL PANCREAS 



INTESTINE 



-CAECUM PHARYNX- 



PERITDNEAL CAVITV 

 ^MESONEPHRIC 



PANCREAS 

 VENTRAL PANCREAS 

 URO SINUS 



UMBIUCAL CORO' 



Fig. 217. — 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, F. T. Lewis and 

 Arey.) 



or slightly curved tubules. The pits are relatively short and are Hned 

 with mucous gland cells like those which cover the inner surface of the 

 stomach, while the 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, pepsinogen becomes pepsin, which spUts 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. 



