DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 307 



ary between the small and large intestine (Fig. 263). At this stage, therefore, 

 all the great divisions of the intestinal tract are distinguishable, viz. : the duodenum 

 with the ducts of the liver and pancreas; the mesenterial small intestine with the 

 yolk stalk; and the colon extending from the caecum to the caudal end. There 

 are, however, practically no differences between the regions, either in structure 

 or in size. 



In further development the duodenum comes to lie more nearly transversely 

 across the body, thus assuming its adult position. Its mesentery fuses with the 

 peritoneum of the dorsal body wall and the duodenum thus becomes a fixed 

 portion of the intestinal tract (p. 350; also Fig. 301). It enlarges a little more 



FIG. 2 64. Reconstruction of the liver and intestine of a human embryo of 17 mm. Mall. 



G.B., gall bladder; H. V., hepatic vein; U.V., umbilical vein; i -6, primary bends in the long 



intestinal loop; i represents the duodenum. 



rapidly than the rest of the small intestine and acquires a greater diameter. In 

 embryos of 12 to 13 mm. the lumen becomes obliterated by an overgrowth of the 

 mucous membrane caudal to the ducts of the liver and pancreas. In embryos 

 of about 15 mm., however, the lumen reappears. It seems difficult to find a 

 cause for this peculiar growth of the mucosa. 



Very shortly after the formation of the long loop in the intestine, six bends 

 become recognizable in the portion between the stomach and the apex of the 

 loop (Fig. 264). These bends later form distinct loops which are destined to 

 become definite parts of the small intestine. The first loop is the duodenum, 

 the development of which has already been considered, and which maintains 

 practically its original position. The other five loops continue to elongate and 

 form secondary loops, all of which push their way into the umbilical coelom 



