Finally, in a certain number of cases, an intermediate condition between the types presented by Figs. 564 and 567 is encountered. In Fig. 568 the general arrangement of the parts corresponds pretty accurately to that seen in Fig. 566, but the transition from a completely free intermediate non-vascular fold to one which has begun to fuse with the dorsal vascular fold is evident. The caecum is bent upward and to the left, the caput coli being formed by the right sacculation. The appendix, 7.8 cm. long, takes a wide {~WREATH PRODUCT~}-shaped curve. The convexity of the proximal curve corresponds to the point where the proximal appendicular artery (2) passes to the tube. The non-vascular intermediate fold (3), measuring 2.2 cm., fuses with the dorsal vascular fold at this point. The three preparations illustrate serially the share which the peritoneal folds take in the formation of the posterior ileo-caecal fossa. In Fig. 566 the failure of the intermediate fold to meet and fuse with the dorsal vascular fold has left the caudal boundary of the fossa (between 2 and 3) incomplete, the ventral and dorsal walls being formed by the folds in question. Fig. 568, in which fusion between the non-vascular and the dorsal vascular folds has commenced, shows the shallow form of the complete fossa under these conditions, while in Fig. 567, with extensive union of the folds, the fossa has correspondingly increased in depth. [Illustration: FIG. 569.--Human adult. Ileo-colic junction and caecum. (Columbia University Museum, No. 1610.)] [Illustration: FIG. 570.--Human infant, four days old. Ileo-colic junction and caecum. (Columbia University Museum, No. 879.)] [Illustration: FIG. 571.--Human infant. Ileo-colic junction and caecum. (Columbia University, Study Collection.)] A similar series is shown in Figs. 569, 570 and 571. In Fig. 569, taken from an adult subject, the intermediate non-vascular fold is entirely free, the dorsal branch of the ileo-caecal artery passes to caecum and appendix in an area of adhesion between parietal peritoneum and the intestine which includes the dorsal vascular fold. There is consequently no caudal boundary to the ileo-caecal fossa. Figs. 570 and 571 are both taken from infantile preparations. In Fig. 570 the dorsal vascular and the intermediate folds nearly meet at the root of the appendix. They serve to outline the fossa, which appears completed in Fig. 571 by the actual meeting and fusion of the folds. _The Ileo-caecal Folds in the Anthropoid Apes.--(1) Chimpanzee, Troglodytes niger._ The structures in a juvenile specimen of this animal are shown in Figs. 460 and 461.