germ layers (primary germ layers) The layers of cells in an animal embryo at the gastrula stage, from which are derived the various organs of the animal's body. There are two or three germ layers: an outer layer (see ectoderm), an inner layer (see endoderm), and in most animal groups a middle layer (see mesoderm). See also development.

germ layers In an embryo at the gastrula stage, the layers of cells that will develop to form the organs of the body. See GASTRULATION.