The foregut and hindgut end blindly at the oral and cloacal membranes, circumscribed median areas where the endoderm and ectoderm are in direct contact, with no intervening mesoderm (see Figure 3-2).

These mem­branes form the floors of surface depressions known as the Stomodeum and proctodeum. The depressions are deepened by the relatively rapid growth of the sur­rounding tissue; when the membranes break down the depressions become confluent with the gut, extending it at each end by a short passage lined with ectoderm.

The foregut differentiates to form the pharynx, esophagus, stomach, and first part of the duodenum together with the structures formed by outgrowth from these parts. The midgut forms the remainder of the small intestine, the cecum, and the larger part of the colon. The hindgut forms the distal part of the colon, the rectum, and, after partitioning, part of the urogenital tract.