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 239 III. Variations of Adult Caecum and Appendix 244 _A._ Shape of Caecum and Origin of Appendix. Types and Variations of Adult Caecum and Appendix 245 _B._ Position and Peritoneal Relations of Appendix 250 _C._ Ileo-Caecal Folds and Fossae 260 INTRODUCTION. In considering the anatomy of the human abdominal cavity and peritoneum in the following pages the explanation of the adult conditions encountered is based upon the development of the parts, and the successive human embryonal stages are illustrated by the examination of the lower vertebrates presenting permanent adult structural conditions which appear as merely temporary embryonal stages in the development of the higher mammalian alimentary tract. For the sake of clearness and brevity all discussion of the _theories of peritoneal development_ has been designedly omitted. The assumption of peritoneal _adhesion_, and consequent obliteration of serous areas, offers many advantages in considering the adult human abdominal cavity, especially from the standpoint of comparative anatomy. The same has consequently been adopted without reference to divergent views and theories. In studying the descriptive text and the diagrams the student should remember that the volume offers in no sense a complete or detailed account of the development of the abdominal cavity and its contents. The purpose is not to present the embryology of this portion of the vertebrate body, but to _utilize_ certain embryological facts in order to _explain_ the complicated adult conditions encountered. To avoid confusion, and to bring the salient points into strong relief, the majority of the diagrams illustrating human embryonal stages are purely schematic. Moreover, in order to avoid confusing and unnecessary details it is often desirable to disregard developmental chronology entirely. Many of the diagrams combine several successive developmental stages, showing different degrees of development in different portions of the same drawing. Again it is frequently necessary, for the sake of brevity and clearness, to actually depart from known embryological conditions. If, for example, the stomach and liver are treated as if they were from their inception abdominal organs, the student of systematic embryology will recall the fact that this position is only _obtained after_ their primitive differentiation by growth and migration.