The ileo-caecal junction takes place from right to left in a downward direction. Nearly the entire mass of the small intestine is situated below and to the left of caecum and colon, but a terminal ileal coil still occupies, although evidently in the process of withdrawal, the retro-caecal position, separating the caecum from direct contact with the dorsal parietal peritoneum. The withdrawal of this terminal coil of the small intestine is accompanied, or immediately followed, by a further turn of the colon cephalad and to the right, which brings it into contact with the caudal surface of the liver and completes the rotation, producing a change in the relative positions of the terminal ileal coils and the caecum. In the stages illustrated in Figs. 498 and 499 and shown schematically in Figs. 490 and 491, the terminal coils of the ileum pass from right to left behind the caecum to enter the colon, and the concavity of the caecal pouch is directed upwards and to the right. After the final rotation has occurred (schema, Fig. 492) the ileum enters the large intestine from the left and from below, and the concave border of the caecum is directed caudad and to the left. This change in relative position has been accomplished by a revolution of the colon and caecum through an arc of 180 deg. around its own long axis carrying the caecum above and behind the small intestine and bringing it into contact with the dorsal prerenal parietal peritoneum. At the same time the terminal coils of the ileum turn downwards and to the left. If this final step in the rotation of the large intestine fails to occur, with otherwise normal development of the parts, the ileum will persist in entering the large intestine from right to left after the caecum has obtained its final lodgment in the right iliac fossa. We have had occasion to refer previously to the significance of these instances of partially arrested development (cf. p. 61, Figs. 123, 127 and 128). [Illustration: FIG. 499.--Human embryo of 6.7 cm. vertex-coccygeal measure. Liver removed. (Columbia University, Study Collection.)] [Illustration: FIG. 500.--Human embryo of 4.9 cm. vertex-coccygeal measure. Ventral view of abdominal cavity, with liver partially removed. (Columbia University, Study Collection.)] [Illustration: FIG. 501.--The same embryo represented in Fig. 500. The colic coil further depressed and turned to the left; seen from the right side.]