Embryology any of the three primary layers of cells (ectoderm, endoderm, and mesoderm) from which the various organs and parts of the organism develop by further differentiation
See germ layer in American Heritage Dictionary 4
noun
Any of three cellular layers, the ectoderm, endoderm, or mesoderm, into which most animal embryos differentiate and from which the organs and tissues of the body develop through further differentiation.