The innermost primary germ layer that originates during gastrulation, which gives rise to the alimentary tract and other internal organs of the body.
See Blastopore, Blastula, Differentiation (embryology), Ectoderm, Embryo, Epithelium, Gastrulation, Germinal (or germ) layers, Invagination, Mesoderm, Organogenesis, Pancreas, Pharyngeal arches