[Illustration: FIG. 562.--Human adult. Ileum with Meckel's diverticulum, 131.5 cm. from ileo-colic junction; a distinct vascular fold is prolonged from the ileal mesentery to the margin of the diverticulum. (Columbia University Museum, No. 1849.)] In this connection it may be noted that the production of the caecal vascular folds and their relation to the mesentery is also very perfectly illustrated in some forms of Meckel's diverticulum. Thus in the preparation shown in Fig. 562, a broad triangular serous fold passes from the ileal mesentery to the margin of the diverticulum, carrying the blood vessels which supply the pouch. If the section of the intestine to the left of the figure is regarded as representing the terminal ileum, that to the right the colon, and the diverticulum the caecal pouch, the formation of the fold and its relation to the mesentery, blood vessels and intestine will correspond closely to the ileo-caecal vascular folds. Fig. 350 shows the ileo-colic junction and caecum of _Halmaturus derbyanus_, the rock kangaroo. The caecum here extends backwards along the free border of the ileum to which it is closely bound by the common investing visceral peritoneum for the greater part of its extent. In another marsupial form, a small species of opossum from Trinidad (Fig. 349), the caecum has separated itself more completely from the adjacent small intestine--thus drawing out the peritoneum into a narrow connecting fold. Finally, in the Virginia opossum (Fig. 348), the ileum has attained the usual position at right angles to caecum and colon. The former pouch is separated from the small intestine by a considerable interval and the angle between the two is filled out by a well-developed triangular serous fold, connecting the free margin of the terminal ileum and the adjacent left border of the caecum. This is the "intermediate non-vascular" ileo-caecal fold.