CHAPTER XIV. 



DEVELOPMENT OF THE ALIMENTARY CANAL AND 

 APPENDAGES. 



THE alimentary canal is formed, as already described (page 621), from 

 the internal blastodermic layer, surrounded by a lamina derived 

 from the mesoderm, which curves downward and inward, and is thus 

 converted into a cylindrical tube, closed at both extremities, and em- 

 braced on each side by the abdominal walls. As the abdominal walls 

 do not unite with each other on the median line until after the forma- 

 tion of the intestine, the abdomen of the embryo is at first widely open 

 in front, presenting a long oval excavation, within which the intestinal 

 tube is situated. 



Stomach and Intestine. The formation of the stomach takes place 

 in the following manner : The alimentary canal, originally straight, 

 soon presents two lateral curvatures at the upper part of the abdomen ; 

 the first to the left, the second to the right. The first of these curva- 

 tures becomes expanded into a wide sac, projecting laterally into the 

 left hypochondrium, and forming the great pouch of the stomach. 

 The second curvature, directed to the right, marks the boundary 

 between the stomach and the duodenum ; and the tube at that point, 

 becoming constricted and furnished with an unusually thick layer of 



muscular fibres, is converted 

 into the pylorus. Immediately 

 below the pylorus, the duode- 

 num again turns to the left ; and 

 similar curvatures, increasing 

 in number and complexity, form 

 the convolutions of the small 

 intestine. The large intestine 

 assumes a spiral direction ; as- 

 cending on the right side, then 

 crossing to the left as the trans- 

 verse colon, and again descend- 

 ing on the left side, to terminate, 

 through the sigmoid flexure, in 

 the rectum. 



The curvatures of the intes- 

 tinal canal take place in an an- 

 tero-posterior, as well as in a 

 lateral direction (Fig. 224). The abdominal walls are here imperfectly 

 closed, leaving a wide opening at a, 6, where the integument of the 



672 



FIG. 224. 



FORMATION OF THE ALIMENTARY CANAL. a, 6. Com- 

 mencement of amnion. c, c. Intestine, d. Pha- 

 rynx, e. Urinary bladder. /. Allantois or chorion. 

 g. Umbilical vesicle. 



