Wikipedia
Enterocoely (adjective forms: enterocoelic and enterocoelous) is a process by which some animal embryos develop. In enterocoely, a mesoderm (middle layer) is formed in a developing embryo, in which the coelom forms from pouches "pinched" off of the digestive tract (also known as the embryonic gut, or archenteron). This type of coelom formation occurs in deuterostome animals, which for this reason are also known as enterocoelomates.
Enterocoelous development begins once the embryo reaches the gastrula phase of development. At this point, there are two layers of cells: the ectoderm (outermost) and the endoderm (innermost) layers. The mesoderm begins to form as two "pockets" of tissue (one above the endoderm, and one below) are formed via folding of the endoderm. These "pockets" begin to grow larger, and as they do so, they extend towards each other. When the two "pockets" of cells meet, the mesoderm is formed – a complete layer of tissue right in between the endoderm and ectoderm layers. This then leads to the formation of a coelom.
Enterocoelous development is the stage of embryological development of deuterostomes in which the coelom forms. The stage starts with the gastrula; as the archenteron forms, pockets of migrating cells also form, creating another layer between the endoderm and ectoderm, the mesoderm. These pockets gradually expand to form the coelom.
In protostomes, the body cavity is formed by schizocoely.