618 



THE DIGESTIVE SYSTEM 



, ESOPHAGUS 



Fig. 292. Developing stomach regions of the digestive tract. (A-C) Three stages in 

 the development of the pig's stomach. Arrows indicate formation of omental bursa which 

 forms from the pocket-like enlargement of the dorsal mesogastrium and proceeds to the 

 left forming the omental bursa as the pyloric end of the stomach rotates toward the 

 right. The ventral aspect of the stomach is indicated by crosses. (D) Diagram of the 

 ruminant stomach. The abomasum corresponds to the glandular stomach of the pig or 

 human; the other areas represent esophageal modifications. (Redrawn from Kingsley, 

 1912. Comparative Anatomy of the Vertebrates, Blakiston, Phila.) 



developing tongue. The place of origin is near the anterior limits of the tongue. 

 Two of these epithelial outpushings occur on either side (fig. 290B). The sub- 

 maxillary-gland and sublingual-gland ducts open at the side of the frenulum of 

 the tongue (fig. 290C). The parotid glands arise as epithelial evaginations, at 

 the angle of the mouth, from the groove which separates the forming jaw and 

 the lip (fig. 290B, C). 



The various oral glands, such as the palatine, labial, tongue, and cheek 

 glands of mammals and lower vertebrates, the poison glands of snakes, etc., 

 arise as epithelial buds which grow out from the developing oral cavity in a 

 manner similar to those of the parotid, submaxillary, and sublingual glands of 

 mammals. The original epithelial outgrowths may branch and rebranch many 

 times to produce large, compound, alveolar glands, as in the parotid, sub- 

 maxillary, and sublingual glands of mammals and the poison glands of snakes. 



b. Development of the Pharyngeal Area 



1) Pharyngeal Pouches and Grooves. The pharynx is that region of the 

 early digestive tube which lies between the oral cavity and the esophagus. In 

 adult vertebrate species, the pharyngeal area is much modified and diflferen- 

 tially developed. However, in the early embryo, it tends to assume a generalized 

 sameness throughout the vertebrate series. 



The early formation of the pharynx results from a series of outpocketings 

 of the entoderm of the foregut, associated with a corresponding series of epi- 

 dermal inpushings; the latter tend to meet the entodermal outgrowths. As a 

 result of these two sets of movements, the one outward and the other inward, 

 the lateral plate mesoderm becomes isolated into dorso-ventral columns, the 

 branchial or visceral arches, between the series of outpocketings and inpush- 



