Two of the more common positions of the appendix, viz., backwards, upwards and inwards behind the ileo-colic junction, and directly backward, erected vertically behind caecum and colon, can therefore in part be referred to the mechanical conditions obtaining normally during the descent of the caecum. Of course the shape of the caecal pouch and the later development of the adult type of caecum will modify this influence in individual cases. We have seen that this early adhesion and the resulting effects on the position of caecum and appendix depend on the direct apposition of the colic tube and mesocolon to the dorsal parietal peritoneum. Any condition which will prevent or delay this apposition will likewise perpetuate the original embryonal condition of the tube, completely invested by peritoneum and with a free mesocolon. Such an element is found in the persistence of the dorsal set of ileal convolutions in the original retro-caecal position beyond the usual period, as indicated in the schematic Fig. 492, _IV, a_. If the turn downward and to the left of these coils is for any reason delayed beyond the usual time the caecal extremity of the colon will descend from the subhepatic to the iliac position without coming directly into contact with the dorsal parietal peritoneum, and therefore without the usual peritoneal adhesion and obliteration of the apposed serous surfaces. The caecum under these conditions descends without making the backward bend, and the origin of the appendix is found at the lowest point of the pendent funnel-shaped pouch, causing it finally to hang downward or downward and inward over the pelvic brim. The resulting form of the caecum and the position of the appendix is the one above described as type _Ia_, _Ib_ and _Ic_ (Fig. 509). Fig. 510 from a foetus at term, and Figs. 515 and 516 representing infantile caeca, illustrate this form of the pouch, while the parts are shown in situ in Fig. 543 taken from a preparation of a five-month foetus. [Illustration: FIG. 546.--Human embryo, 6.5 cm. cervico-coccygeal measure. Abdominal cavity, with liver removed, seen from the right side. (Columbia University, Study Collection.)]