As a part of the general caecal pouch which persists in an early developmental stage and which we can regard as destined for further reduction and ultimate elimination in the course of evolution, the appendix shares with other vestigial structures a wide range of variation. Consequently the study of the development of this portion of the alimentary tract enables us to gain a clearer view of the primary arrangement of the structures and to trace the causes which are active in determining the adult conditions most frequently encountered. [Illustration: FIG. 486.--Alimentary canal and appendages of human embryo of 12.5 mm. x 12. (Kollmann, after His.)] [Illustration: FIG. 487.--_A_, schematic representation of alimentary canal, with umbilical loop and mesenteric attachments in human embryo of about six weeks. _B_ and _C_, stages in the intestinal rotation.] At the time when the umbilical loop of the intestine has formed and has begun to protrude into the cavity of the umbilical cord (fifth to sixth week), the first indication of the future caecum appears as a circumscribed thickening of the returning or ascending limb of the intestinal loop a short distance from the apex (Figs. 486-488). This rudiment indicates the derivation of the future definite intestinal segments from the elements of the loop. The descending limb, apex (site of embryonic vitelline duct, Meckel's diverticulum of adult) and a short succeeding portion of the ascending limb furnish the ileum and jejunum. The rest of the ascending limb develops into caecum and appendix, ascending and transverse colon. The increase in the length of the intestine is not uniform. The formation of convolutions begins in the seventh week in the apex and subsequently in the descending limb. By the eighth week a considerable number of jejuno-ileal coils have resulted from the growth in length of these parts of the original umbilical loop, while the growth of the segment which furnishes the colon is at this time still inconsiderable (Fig. 489). In the meanwhile the thickening of the tube which forms the first rudiment of the caecum has developed into a small sac-like enlargement of the gut, budding from the left and dorsal aspect of the ascending limb, crescentic in shape, turning its concavity toward the parent tube. In the majority of instances examined the small outgrowth is packed closely between the incipient ileal convolutions, lying under cover of the more prominent bulging coils of the umbilical protrusion, between them and a single coil of larger arc situated dorsally and belonging to the jejunal or proximal portion of the small intestine (Fig. 490). Fig. 497, taken from an embryo of 11 mm. cervico-coccygeal length, represents this stage in the development of the umbilical loop. The arrangement of the caecum which we can assume as the typical condition at this stage and which determines in part the subsequent final arrangement of the structures, is illustrated by this