The disposition of the structures illustrated by this example probably depends upon delayed adhesion of the colic embryonal tube to the dorsal parietal peritoneum. The caecum and appendix appear to have descended freely until the final position in the right iliac fossa has been nearly attained, adhesion and fixation of the colon taking place just before the descent is completed, and thus producing the backward turn of the caecal end of the tube. Further development of the caecum to form the adult caput coli in these cases leads to the unequal and exaggerated expansion of the ventral and lateral walls of the pouch, as compared with the fixed and adherent dorsal wall. The former are distended and pushed downwards, producing a relative recession of the root of the appendix upward and to the left, until it comes into relation with, or even under cover of, the ileo-colic junction and of the terminal ileal coil entering the colon at this point. The resulting characteristic adult position of the appendix in these cases is as follows: The termination of the caecum proper, and the root of the appendix are under cover of the terminal ileum and frequently adherent to the parietal peritoneum of the iliac fossa (Fig. 555). The distal portion of the appendix remains free, either hanging down and in over the brim of the pelvis (Fig. 542), or turned upwards and to the left and coiled in several turns (Figs. 504, 555 and 556). [Illustration: FIG. 555.--Human foetus at term. Ileo-colic junction and caecum; dorsal view. The area of peritoneal adhesion is seen to involve the dorsal aspect of the caecum as far as the root of the appendix. (Columbia University Museum, No. 1549.)] [Illustration: FIG. 556.--Human infant. Ileo-colic junction and caecum, with extensive adhesion to parietal peritoneum. (Columbia University Museum, No. 301.)] Finally the _erect vertical retro-caecal_ position of the appendix presents several important variations in the disposition of the peritoneal investment. In Fig. 503, taken from an embryo of 7.6 cm. vertex-coccygeal length, the early complete recession of the retro-caecal ileal convolutions has probably permitted an early apposition and adhesion of the beginning of the colon to the dorsal prerenal parietal peritoneum. The subsequent descent into the iliac fossa produces a bend in the ventral wall of the colic tube, with a marked convexity directed downwards and forwards, the apex of the bend situated at or near the level of the ileo-colic junction, while the dorsal colic wall is held by the adhesion to the parietal peritoneum, thus giving a backward inclination to the entire caecum and appendix. During the subsequent descent of the caecum proper this bend in the colon is gradually diminished and the tube becomes straightened but the apex of the caecum remains turned back and the appendix is placed in a more or less vertical erect position behind caecum and ascending colon.