DEVELOPMENT OF (ESOPHAGUS, STOMACH, AND INTESTINES. 1249 



Tongue. The tongue is formed in two portions, anterior and posterior, in the floor of 

 the pharynx. 



The anterior portion, forming the anterior two-thirds of the organ, is formed from the 

 tissues on each side of the tuberculum impar, which grow up and enclose that elevation, 

 and from the tuberculum itself. Evidence of this bilateral origin of the tongue is found 

 in those cases of bifid tongue which occur, though rarely (see p. 45). 



The hinder portion, forming the posterior third of the tongue, is formed from the 

 tissues covering the inner ends of the second pair of arches (see p. 45). 



These arches, as has been pointed out, meet and fuse in a common mass in front of 

 the furcula, in the floor of the sinus arcuatus. 



Between the ridge which they form and the tuberculum impar in front, a slight 

 median depression is found, from which the median thyreoid diverticulum is formed, and 

 which persists as the foramen caecum of the tongue. On each side of this depression a 

 groove runs obliquely laterally and upwards (the sulcus terminalis) immediately behind 

 the region of the vallate papillae, and marks the union of the anterior and posterior 

 portions of the tongue. 



The tongue mass formed by the union of these different parts increases in size, rises 

 upwards from the floor of the pharynx, and projects forwards. 



The tissue forming its interior becomes transformed into the muscular substance of 

 the tongue, and is derived largely from the first branchial region, and not from the 

 musculature of the visceral arches. 



The investing epithelium of the anterior two-thirds gives rise to the papillae and the 

 taste buds, while that covering the posterior portion remains smooth. The papillae appear 

 about the third month as elevations of the corium, covered with epithelium. 



The vallate papillae are formed by ingrowths of the epithelium in rings, around a 

 central core. The superficial layers of the epithelium desquamate and form the trench 

 surrounding the papilla. 



Submaxillary and Sublingual Glands. These glands are formed in the alveolo- 

 lingual groove in the floor of the primitive pharynx, immediately behind the first arch, 

 by outgrowths somewhat similar to those described in connexion with the parotid (q.v.). 



The submaxillary outgrowth occurs about the fifth week, and the sublingual, several 

 in number, on the outer side of it at the ninth week. 



Palatine Tonsil. The glosso-palatine arch arises in the position occupied earlier by 

 the second visceral arch, behind which, in the embryo, lies the pharyngeal portion of 

 the second visceral cleft. The dorsal extremity of that cleft enlarges, and forms a recess 

 termed the sinus tonsillaris. From the lower and greater part of the sinus tonsillaris the 

 palatine tonsil is developed ; the upper part of the sinus persists, however, as the supra- 

 tonsillar fossa. The palatine tonsil at first is a smooth depression of the mucous membrane. 

 About the fourth month of foetal life downgrowths of the epithelium take place, which are 

 afterwards converted into the tonsillar crypts. Subsequently lymph cells accumulate 

 around the downgrowths and form the lymph tissue, which constitutes the mass of the 

 organ. The plica triangularis is formed from a tubercle, which becomes flattened, and 

 forms a fold on the anterior and medial aspect of the inferior part of the palatine tonsil. 



DEVELOPMENT OF (ESOPHAGUS, STOMACH, AND INTESTINES. 



(Esophagus. The oesophagus is formed from the foregut. The lengthening of the 

 thoracic region of the trunk, which occurs with the growth and development of the 

 heart and lungs, causes this portion of the alimentary tube to become greatly lengthened. 

 The entodermal lining forms the epithelial layer, and the mesoderm the other coats of 

 the oesophagus. 



Stomach. As early as the fourth week, the foregut exhibits a fusiform enlargement 

 in the region of the developing heart, which is the first evidence of the differentiation of 

 the stomach : this enlargement takes the form first of an outgrowth on the dorsal 

 border to form the fundus. Soon, however, as the diaphragm is being formed, the 

 stomach passes into the abdomen, and its dorsal wall the future greater curvature 

 begins to grow more rapidly than the ventral wall. As a result the whole organ 

 becomes somewhat curved, and its inferior end is carried forwards from the posterior 

 abdominal wall, giving rise to the curvature of the duodenum. The excessive growth 

 of its posterior wall causes the stomach to turn over on to its right side, which now 

 becomes posterior or dorsal. In this rotation the upper or cardiac portion moves to 

 the left of the median plane, and the whole organ assumes an oblique direction across 



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