778 DEVELOPMENT OF THE ALIMENTARY CANAL. 



The large intestine is at first less in calibre than the small. In the 

 early embryo there is at first no csecum. This part of the bowel 

 gradually grows out from the rest, and in the first instance forms a tube 

 of uniform calibre, without any appearance of the vermiform appendix: 

 subsequently the lower part of the tube ceases to grow in the same 

 proportion, and becomes the appendix, whilst the upper portion con- 

 tinues to be developed with the rest of the intestine. The caecum now 

 appears as a protrusion a little below the apex of the bend in the primi- 

 tive intestinal tube, and, together w'ith the commencing colon, and the 

 coil of small intestine, is at first lodged in the wide part of the um- 

 bilical cord W'hich is next the body of the embryo. The ileo-cascal 

 valve appears at the commencement of the third month. When the 

 coils of intestine and cjecum have retired from the umbilicus into 

 the abdomen, the colon is at first entirely to the left of the con- 

 volutions of the small intestines, but subsequently the first part of 

 the large intestine, together with the meso-colon, crosses over the 

 upper part of the small intestine, at the junction of the duodenum and 

 jejunum. The ca3cum and transverse colon are then found just below 

 the liver ; finally, the ca3cum descends to the right iliac fossa, and at 

 the fourth or fifth month the parts are in the same position as in the 

 adult. At first, villous processes or folds of various lengths are formed 

 throughout the whole canal. After a time these disappear in the 

 stomach and large intestine, but remain persistent in the intermediate 

 portions of the tube. According to Meckel, the villous processes are 

 formed from larger folds, which become sen-ated at the edge, and 

 divided into separate villi. 



The formation of the hinder part of the gut is complicated with the 

 development of the allantois, which arises as a projection or out- 

 growth of the hypoblast and mesoblast from the lower wall of its 

 terminal portion. This part rapidly buds out in the pleuro-peritoneal 

 space, having from a very early period a rich network of the branches 

 of the umbilical vessels in its outer layer. The anal or cloacal portion 

 remains behind the allantoid pedicle, and by the fifth day in the chick 

 the whole of the tissues which close the terminal fold thin rapidly away, 

 and become perforated so as to form the primitive anal, or rather the 

 cloacal opening. The separation of the permanent anus fi'om the uro- 

 genital orifice is the result of a later process of development. 



The mode of development of the alimentary canal explains, in some 

 measure, the complicated folds of the peritoneum. The stomacii 

 being originally more nearly in the line of the alimentary canal 

 and mesial in position, the small omentum aad gastro-phrenic ligament 

 are originally parts of a mesial fold with the free edge directed for- 

 wards, which afterwards forms the anterior boundary of the foramen 

 of Winslow. Thus the anterior wall of the sac of the omentum, as far 

 as the gi'eat curvature of the stomach, may be considered as formed by 

 the right side of a mesial fold, while the peritoneum in front of the 

 stomach belongs to the left side of the same, and a sac of the omentum 

 is a natural consequence of the version and disproportionate growth of 

 the tube between the duodenum and the cardiac orifice of the stomach. 

 It is obvious that the view of the omental sac, according to which its 

 posterior layers are held to return to the duodenum and posterior wall 

 of the body before proceeding to form the transverse meso-colon (p. 484) 

 is more consistent with the phenomena of development now described. 



