DEVELOPMENT OF THE DIGESTIVE TUBE 621 



velops into the lungs or into structures which function as air bladders and 

 lungs. (See Chap. 14.) 



c. Morphogenesis and Histogenesis of the Esophagus and the Stomach 

 Region of the Metenteron 



The esophageal and stomach areas of the gut develop from that segment of 

 the foregut which extends from the pharyngeal area caudally to the area of 

 the developing gut tube from which the liver and pancreatic diverticula arise. 

 In Amphioxus and certain of the lower vertebrates, a true stomach is not 

 differentiated within this portion of the foregut. This condition is found in 

 the cyclostome, Petromyzon, in the lungfish, Protopterus, and various other 

 forms (fig. 291 A, B). In these species, this segment of the gut merely serves 

 to transport food caudally to the intestine, and the histogenesis of its walls 

 resembles that of the esophagus. On the other hand, a true stomach is de- 

 veloped in all other vertebrate species. The functions of the stomach are to 

 store food, to break it up into smaller pieces, and to digest it partially. As such, 

 the stomach comprises that segment of the digestive tract which lies between 

 the esophagus and intestine. It is well supplied with muscular tissue, is capable 

 of great distention, and possesses glands for enzyme secretion. 



In development, therefore, the foregut area between the primitive pharynx 

 and the developing liver becomes divided into two general regions in most 

 vertebrates: 



( 1 ) a more or less constricted, esophageal region, and 



(2) a posteriorly expanded, stomach segment (figs. 279-282). 



The latter tends to expand and to assume a general, V-shaped form, the 

 portion nearest the esophagus comprising the cardiac region, and the part 

 nearest the intestine forming the pyloric end. 



Many variations in esophageal-stomach relationships are elaborated in dif- 

 ferent vertebrate species. In the formation of the stomach of the pig or human, 

 for example, a generalized, typical, vertebrate condition may be assumed to 

 exist. In these forms, the stomach area of the primitive gut gradually enlarges 

 and assumes a broad, V-shaped form, with its distal or pyloric end rotated 

 toward the right (fig. 292A-C). Eventually, the entodermal lining tissue shows 

 four structural conditions: 



(a) There is an esophageal area near the esophagus, where the character 

 of the epithelial lining resembles that of the esophagus. 



(b) A cardiac region occurs, where the epithelium is simple, columnar in 

 form, and contains certain glands. 



(c) There is a fundic region, capable of being greatly expanded. The in- 

 ternal lining of the fundic area produces numerous, simple, slightly 

 branched, tubular glands, wherein pepsin is secreted by the chief cells 

 and hydrochloric acid by the parietal cells (fig. 293). 



