DEVELOPMENT OF THE ALIMENTARY CANAL. 

 Fig. 603. 



861 



Fig. 603. OUTLINES OP THE FORM AND POSITION OP THE ALIMENTARY CANAL IN 

 SUCCESSIVE STAGES OP ITS DEVELOPMENT. 



A, alimentary canal, &c., in an embryo of four weeks; B, at six weeks ; C, at eight 

 weeks ; D, at ten weeks ; I, the primitive lungs connected with the pharynx ; *, the 

 stomach ; d, duodenum ; i, the small intestine ; ', the large ; c, the caecum and vermi- 

 form appendage; r, the rectum; c I, in A, the cloaca ; a, in B, the anus distinct from 

 si, the sinus uro-geni tails ; v, the yolk sac; v i, the vitello-intestinal duct; u, the 

 urinary bladder and urachus leading to the allantois ; g, the genital ducts. 



The dilated portion of the tube which forms the stomach turns over on its right 

 side, so that the border, which is connected to the vertebral column by the membra- 

 nous fold (or true mesogastrium) comes to be turned to the left, the position of the 

 tube being still vertical, like the stomach of some animals. By degrees it becomes 

 more dilated, chiefly on what is now the left border but subsequently the great cur- 

 vature, and assumes first an oblique and finally a transverse position, carrying with 

 it the mesogastrium, from which the great omentum is afterwards produced. A 

 slight indication of the pylorus is seen at the third month. Upon the surface of the 

 part of the canal which immediately succeeds the stomach, and which forms the 

 duodenum, the rudiments of the liver, pancreas, and spleen are simultaneously depo- 

 sited : in connection with the two former, protrusions of the mucous membrane grow 

 into their blastemic mass and form the commencement of their principal ducts. 



The place of distinction between the small and the large intestine, which is soon 

 indicated by the protrusion of the caecum, is at a point just below the apex or middle 

 of the simple loop already mentioned. As the small intestine grows, the part 

 below the duodenum forms a coil which at first lies in the commencing umbilical 

 cord, but retires again into the abdomen about the tenth week ; afterwards it con- 

 tinues to elongate, and its convolutions become more and more numerous. 



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

 there is at first no caecum. 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 continues 

 to be developed with the rest of the intestine. The caecum now appears as a protru- 

 sion a little below the apex of the bend in the primitive intestinal tube, and, together 

 with the commencing colon, and the coil of small intestine, is at first lodged in the 

 wide part of the umbilical cord which is next the body of the embryo. The ileo- 

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

 intestine and caecum have retired from the umbilicus into the abdomen, the colon 



