The tubular structure in embryos which differentiates into the alimentary canal. The method by which the primitive gut arises depends chiefly on the yolk content of the egg.
Eggs with small or moderate amounts of yolk usually develop into spherical blastulae which invaginate at the vegetative pole to form double-walled gastrulae. The invaginated sac extends in length to become the primitive gut.
Animals such as fish, reptiles, and birds, having more yolk than can be cleaved, form flattened gastrulae consisting of three-layered blastoderms surmounting the yolk. Mammals also belong in this group, although the yolk has been lost secondarily in all except the monotremes. The head is formed by a folding of the blastoderm upon itself. The entodermal layer within the head fold becomes the pharynx. This foregut is extended by an anterior growth of the whole head and by the union of lateral entodermal folds at its posterior boundary. In most forms, the hindgut arises by a similar folding in the opposite direction, the tail fold, at the posterior end of the blastoderm. See Cleavage (embryology), Gastrulation, Ovum