CHAPTBE XV. 





DEVELOPMENT OF THE ALIMENTAEY CANAL 

 AND ITS APPENDAGES. 



THE intestinal canal is formed, as already described (page 726), from 

 the internal blastodermic layer which curves downward and inward on 

 each side, and is thus converted into a cylindrical tube, terminating at 

 each extremity in a cul-de-sac, and inclosed by the external blastodermic 

 layer. The abdominal walls do not unite with each other upon the 

 median line until 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 intestinal tube is 

 situated, running from its anterior to its posterior extremity. 



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 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 an unusually thick 

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

 mediately below the pylorus, the duodenum turns to the left ; and these 

 curvatures, increasing in num- 



ber and complexity, form the con- 

 volutions of the small intestine. 

 The large intestine assumes 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 intes- 

 tinal canal, which take place in 

 an antero-posterior, as well as in 

 a lateral direction, may be best 

 studied in a profile view, as in 

 Fig. 287. The abdominal walls 

 are here still imperfectly closed, 



Fig. 287. 



Formation of the ALIMENTARY CANAL. 

 a, b. Commencement of amnion. c, c. Intestine. 

 d. Pharynx, e. Urinary bidder. / Allantois 

 or chorion. g. Umbilical \esicle. 



(775) 



