Germ layers

Germ layers

The primitive cell layers, or first tissues, which appear early in the development of animals and from which the embryo body and its auxiliary membranes, when present, are constructed. These are more or less distinct anatomically, but do not necessarily have sharp boundaries of demarcation. Germ layers are almost universal among animal embryos and appear to establish discontinuities of architectural importance without complete loss of continuity. Three kinds of germ layers are recognizable: (1) the ectoderm or outer skin, (2) the endoderm or inner skin, and (3) the mesoderm or middle skin. The layers have been named in accordance with their positions in the spherical type of gastrula such as that of the sea urchin or amphibian. The terms epiblast, mesoblast, and hypoblast are sometimes used as synonyms for ectoderm, mesoderm, and endoderm, respectively. The three primary germ layers are present as a basic structural plan in all Metazoa with the exception of the coelenterates and the Porifera, in which a distinct mesodermal layer is absent. See Gastrulation