Fig. 140 indicates the results of adhesion of the duodenum, pancreas and mesoduodenum to the parietal peritoneum as it normally occurs in the human subject. It will be seen that the primary parietal peritoneum can be traced mesad over the ventral surface of the right kidney as far as the point _X_, and that from here on to the median line the peritoneum is _secondary_ parietal peritoneum, consisting of the visceral peritoneal investment of the ventral surface of the duodenum and of the original left leaf of the mesoduodenum, beneath which the ventral surface of the pancreas is seen. Pancreas and duodenum occupy in the adult secondarily a "retro-peritoneal" position, _i. e._, the peritoneum now covering the ventral surface of these viscera appears as a continuation of the parietal peritoneum, the transition between primary and secondary parietal peritoneum occurring along the line marked _X_ in Fig. 140. The opposed peritoneal surfaces indicated by the dotted lines have become adherent and converted into loose connective tissue in which the pancreas and duodenum lie imbedded. In the human embryo this process of adhesion begins in the eighth week, starting at the duodeno-jejunal flexure and ascending gradually toward the pylorus. At the end of the fourth month the union is complete. Proceeding caudad it will next be observed that the peritoneum of the mesentery occupies the narrow neck of the duodeno-colic isthmus, and that large vessels (the superior mesenteric) pass between its two layers at this point to supply the segments of the intestine forming the loop. In conformity with the greatly increased length of the intestine it will be found that the mesentery expands from the narrow pedicle at the neck in a fan-shaped manner in order to develop a sufficiently long margin for attachment to the intestine. The following points should be carefully borne in mind in studying the mesentery with the intestines in this position: 1. The mesentery presents two free surfaces, right and left. With the coils of the small intestine turned over to the right, the left leaf of the mesentery is turned toward the observer. 2. Inasmuch as the descending limb of the embryonic loop has developed the greater part of the small intestine, while a portion of the large intestine (caecum and colon up to the isthmus) is the result of differentiation within the ascending or returning limb of the loop, it will be at once apparent that the double peritoneal layer which extends between the duodeno-colic isthmus and the attached border of the gut is partly mesentery of the small intestine, partly mesocolon passing to the large intestine (caecum and proximal colon). This condition may be indicated schematically in Fig. 141. [Illustration: FIG. 141.--Schematic representation of mesentery of umbilical loop, common to small intestine and proximal portion of large intestine.]