For this reason the comparative study of the corresponding parts in other vertebrates, especially in certain mammalia, is of the utmost value, if we seek to explain and understand the derivation, significance and typical arrangement of these folds. We have seen that the caecum as found in the large majority of mammalian forms is equivalent to the caecum and appendix of the human subject and anthropoid apes; that in other words the vermiform appendix represents the distal segment of a caecal pouch, originally uniform in caliber, which has remained undeveloped, while the proximal portion has progressed evenly with the general development of the alimentary canal to form the caecum proper. We have seen that this tendency to retain the distal portion of the pouch in a rudimentary condition, _i. e._, the production of an appendage to the caecum proper, is encountered in several of the lower forms, as certain Marsupials, Carnivores, Ungulates and Lemurs. The morphology of the ileo-caecal folds is hence best understood by considering these structures as they appear in connection with the various caecal types presented by the lower mammalia. Their arrangement and significance can here be readily made out. On the other hand, in studying these structures in the human appendix we are following lines which are already becoming indistinct on account of the rudimentary character of the organ, which we must regard as undergoing an exceedingly slow process of reduction, with a view to its ultimate elimination from the body. We have seen that the structural uncertainty impressed on caecum and appendix by this evolutionary influence finds its expression in the wide range of variation in size and arrangement which these parts present. Necessarily, of course, this tendency to variation is shared, and even exhibited to a more marked degree, by what we can term the accessory structures connected with caecum and appendix, viz., the mesenteric vascular and non-vascular serous folds passing to them from the ileum. We can most profitably begin our consideration of these folds in a form in which they are preserved in their entire and original development, and then successively trace the changes leading up to the normal disposition in the human subject. Such a type is presented by the caecum of _Ateles ater_, the black-handed spider monkey (Figs. 444 and 445). The caecum of this animal presents a uniform crescentic curve, with the concavity directed upward and to the left, and the gradual diminution in the caliber of the pouch, from the ileo-colic junction to the apex, denotes the tendency to retain the distal segment in a rudimentary condition, foreshadowing the eventual formation of a vermiform appendix. In the ventral view, with the terminal ileum lifted up, the following arrangement of folds passing between ileum and caecum is noted (Figs. 444 and 445).