1. The middle section of the digestive tract in a vertebrate embryo from which the ileum, jejunum, and portions of the duodenum and colon develop.
2. The middle section of the embryonic intestine, which develops into the stomach, small intestine, and most of the large intestine.
3. The middle portion of the digestive tract of certain invertebrates; such as, arthropods, lined with an enzyme-secreting tissue and serving as the main site of digestion and absorption.