[Illustration: FIG. 143.--Stage of umbilical loop. Differentiation of common dorsal mesentery of earlier stage into dorsal mesogastrium, mesoduodenum, primitive mesentery of umbilical loop, and descending mesocolon.] [Illustration: FIG. 144.--Final stage. With complete differentiation of large and small intestine, the primitive mesentery of the umbilical loop contains not only the mesentery of the future jejuno-ileum, but also the mesocola and the ascending and transverse colon, developed from the ascending or afferent limb of the umbilical loop.] Moreover, it will be appreciated that the entire extensive coil of intestines extending between the two boundaries of the duodeno-colic isthmus (_a_, _b_) is suspended from the back part of the abdomen by a narrow pedicle and that consequently rotation will readily occur around the axis drawn through the neck of the isthmus. Now proceed to illustrate on the cat the result of the rotation as it occurs normally during the development of the primate intestinal tract. Take the caecum and commencement of the colon and draw the same over to the right across the duodeno-colic isthmus and the duodenum. Twist or rotate the entire mass of small intestines around the isthmic pedicle, so that the original left leaf of the mesentery will look to the right and _vice versa_ (Fig. 145). The conditions thus established will be found to correspond to the schemata shown in Figs. 114 and 115. The main features of the intestinal tract in the rearranged position will be as follows: 1. The two points, _a_ and _b_, of the duodeno-colic isthmus (Fig. 145) are still close together, but reversed in position, _b_ is in front and to the right, _a_ behind and to the left, whereas before the rotation _b_ was situated below and to the left, _a_ above and to the right (Fig. 135). [Illustration: FIG. 145.--Abdominal viscera of cat, with intestines rotated to correspond to the stage in the development of the human canal in which the caecum has reached the subhepatic position, but before the establishment of the ascending colon. (From a fresh dissection.)] 2. The direction of the ileo-colic entrance is reversed, the ileum now entering the large intestine from below and the left upwards and to the right, instead of from right to left. 3. The descending duodenum is now situated dorsad to the colon. 4. The original left leaf of the mesentery has become the right, and _vice versa_. 5. The superior mesenteric artery crosses over the transverse portion of the duodenum, and with the exception of the inferior pancreatico-duodenal artery the original right-sided branches now arise from the left side of the vessel and _vice versa_. It is now time to compare the conditions established in the cat by the manipulations just detailed with the arrangement of the adult human intestinal tract and peritoneum below the level of the transverse colon and mesocolon.