relation of the caecal bud to the surrounding incipient convolutions of the small intestine, with the larger part of these coils situated ventrad of the caecum and only a single coil of larger curve placed dorsally; the caecal pouch, derived from the ascending limb of the umbilical loop, is situated between these two divisions, turning its concave border to the right and embracing the parent tube. At the time when the human caecum first appears as a distinct structure it forms a small conical pouch with blunt extremity whose shape is well illustrated by the caecum of some of the new-world monkeys, as _Mycetes fuscus_, the brown howler monkey (Figs. 449 and 450). The outgrowth develops rapidly in length and very soon assumes a distinct crescentic shape, gradually tapering toward the extremity, a type which is found reproduced in the caecum of _Ateles ater_, the black-handed spider monkey (Fig. 443). There is as yet no constriction or demarcation separating the distal segment (future appendix) from the proximal part (caecum proper) but the entire pouch gradually narrows funnel-like to its termination. [Illustration: FIGS. 488-496.--Series of schematic figures illustrating stages in the rotation of the intestinal canal.] II. CHANGES IN THE POSITION OF THE CAECUM AND APPENDIX DURING NORMAL DEVELOPMENT, DEPENDING UPON THE ROTATION OF THE INTESTINE AND THE SUBSEQUENT DESCENT OF THE CAECUM. The primary cause leading to the rotation of the intestinal canal and inaugurating the successive stages which produce the adult disposition of the tube is to be found in the rapid increase in length of the small intestine. Numerous convolutions of this tube succeed to the few primary coils noted in the first stages. This condition is illustrated in Fig. 498, taken from an embryo of 4.4 cm. cervico-coccygeal measure, and the arrangement of the intestine is indicated in schema, Fig. 491. The caecum is found nearly in the median line imbedded among the surrounding coils of the small intestine, which by their rapid increase have pushed the pouch cephalad nearly into contact with the caudal surface of the liver. [Illustration: FIG. 497.--Human embryo of 11 mm. cervico-coccygeal measure. Enlarged view of ventral and left aspect of intestinal canal. (Columbia University, Study Collection.)] [Illustration: FIG. 498.--Human embryo of 4.4 cm. cervico-coccygeal measure. Intestinal canal; Liver removed. (Columbia University, Study Collection.)]