ALIMENTARY TRACT AND ITS APPENDAGES 309 



In the young chick the thymus forms a voluminous tract of lobu- 

 lated aspect, extending the entire length of the neck; later it 

 atrophies and in old subjects one finds only traces of it. (Verdun.) 



Epithelial vestiges are formed from the ventral wall of the 

 intermediate portions of the third and fourth visceral pouches; 

 these come to lie together at the hinder end of the thymus in the 

 base of the neck. They are found in the adult near the lower 

 pole of the thyroid (Fig. 178). 



The postbranchial bodies have been called lateral rudiments 

 of the thyroid; in their differentiation, however, they do not form 

 thyroid tissue, but two main kinds of epithelial tissues similar 

 to the tissues of the thymus and epithelial vestiges respectively. 

 They are to be regarded, therefore, as a fifth pair of visceral 

 pouches, for which there are other reasons, as we have seen before. 

 The constituent elements, however, do not separate as in the case 

 of the third and fourth visceral pouches, but form a rather ill- 

 defined mass situated a short distance behind the thyroid (Fig. 

 178). 



The epithelial derivatives of the embryonic pharynx in the 

 chick are, therefore; 1. thyroid; 2. thymus (from III, IV); 

 3. epithelial vestiges (from III, IV); 4. postbranchial bodies, 

 including thymus V and epithelial vestiges V. The thyroid 

 develops in essentially the same manner in all vertebrates. In 

 the case of the thymus it may be said in general that more visceral 

 pouches are concerned in the lower than in the higher vertebrates. 



III. THE (ESOPHAGUS, STOMACH AND INTESTINE 



During the third and fourth days a very pronounced lateral 

 curvature of the alimentary canal develops, the convexity being 

 turned to the left and the concavity therefore to the right. The 

 part involved extends from the posterior portion of the oesopha- 

 gus to the end of the duodenum. As the duodenum is at first 

 very short, the stomach is the part principally affected at the 

 start. The depth of the mesogastrium (dorsal mesentery of 

 the stomach) is considerably increased by the displacement; in the 

 region of the greatest curvature it descends directly in the middle 

 line, then bends sharply to the left and is attached to the dorsal 

 wall of the stomach; the accessory mesentery arises at the bend. 

 (See Chap. XI.) The stomach does not rotate on its long axis so 

 as to carry the attachment of the mesogastrium to the extreme 



