64:4 DEVELOPMENT OF THE ALIMENTARY CANAL. 



CHAPTER XV. 



DEVELOPMENT OF THE ALIMENTARY CANAL 

 AND ITS APPENDAGES. 



WE have already seen ; in a preceding chapter, that the intestinal 

 canal is formed by the internal layer of the blastodermic membrane, 

 which curves forward on each side, and is thus converted into a 

 nearly straight cylindrical tube, terminating at each extremity in 

 a rounded cul-de-sac, and inclosed by the external layer of the 

 blastodermic membrane. The abdominal walls, however, do not 

 unite with each other upon the median line until long after the 

 formation of the intestinal canal ; so that, during a certain period, 

 the abdomen of the embryo is widely open in front, presenting a 

 long oval excavation, in which the nearly straight, intestinal tube 

 is to be seen, running from its anterior to its posterior extremity. 



The formation of* the stomach takes place in the following man- 

 ner : 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 curvatures 

 becomes expanded into a wide sac, projecting laterally from the 

 median line into the left hypochondrium, 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 a circular 

 layer of muscular fibres, is converted into the pylorus. Immedi- 

 ately below* the pylorus, the duodenum again turns to the left ; and 

 these curvatures, increasing in number and complexity, form the 

 convolutions of the small intestine. The large intestine forms a 

 spiral curvature ; ascending on the right side, then crossing over 

 to the left as the transverse colon, and again descending on the left 

 side, to terminate by the sigmoid flexure in the rectum. 



The curvatures of the intestinal canal take place, however, in an 

 antero-posterior, as well as in a lateral direction, and may be best 

 studied in a profile view, as in Fig. 241. The abdominal walls are 



