As previously stated the normal type of the human embryonal caecum is represented by the pouch of some of the new-world monkeys, as _Ateles_ (Fig. 443) or of certain lemurs, of which _Nycticebus_ (Fig. 420) furnishes an excellent example. The caecum is distinctly crescentic, turning its concave margin, after completed intestinal rotation, upwards and to the left, toward the lower margin of the ileum. The distal diminished segment of the pouch in _Ateles_ has already assumed the character of a caecal appendage in _Nycticebus_ and becomes by further reduction the typical appendix in man and the anthropoid apes, while the proximal portion develops into the capacious sacculated caecum proper. Consequently the initial curve of the caecum tends to carry the root of the appendix upward and to the left toward the ileo-colic junction. This curve of the pouch, combined with the mechanical effects produced by the adhesion of the colon during the caecal descent, accounts for the frequency with which the caecum in the later months of foetal life and at birth is found curved backward, upward and to the left, placing the root of the appendix under cover of the terminal ileal convolutions (Fig. 548). We have seen that this disposition of the structures accounts for the preponderance of that type of adult caecum which results from the further and unequal development and dilatation of the segment of the pouch situated to the right of the origin of the appendix. [Illustration: FIG. 548.--Human foetus at term. Ileo-colic junction and caecum. Early colic and caecal adhesion with retroverted appendix. (Columbia University Museum, Study Collection.)] Bearing in mind the three elements just considered, viz., the effect of adhesion during the caecal descent, the inherent shape of the pouch and the unequal alterations in caliber in the development of the adult type, we can at once take up the resulting variations in the peritoneal relations of the adult caecum and appendix which have an important influence on the progress of pathological processes in this region. It should be remembered that in the following schematic figures the colon, caecum and appendix are represented in the profile view in a straight line, without indicating the characteristic turn of the crescentic caecal pouch upwards and to the left. Fig. 549 shows the arrangement in unimpeded caecal descent without adhesion of colon and mesocolon to the parietal peritoneum. This disposition of the structures, if carried into adult life, would produce the permanently free ascending colon and mesocolon which we encountered exceptionally in the human subject (cf. p. 82) and normally in certain of the cynomorphous monkeys (p. 83). The ascending colon and mesocolon can, under these conditions, be turned mesad, lifting them away from the primary parietal peritoneum investing the ventral surface of the right kidney. Caecum and appendix have, of course, a complete serous investment. [Illustration: FIG. 549.]