Germ layer

A germ layer, primary germinal layer, or germinal layer is a primary layer of cells that forms during animal embryonic development. Germ layers form during gastrulation, when the early embryo is formed of two or three layers of cells. Sponges do not have a gastrulation stage and possess no true germ layers. They do though have two layers of cells separated by a gel-like mesohyl. Some aquatic invertebrates such as cnidarians, and comb jellies, develop from only two germ layers, an ectoderm and an endoderm, and are known as diploblasts. But most animals have a bilateral symmetry, and develop from three germ layers, an ectoderm, endoderm, and a middle layer of mesoderm, and are triploblastic. Germ layers eventually give rise to all of an animal's tissues and organs through the processes of histogenesis, and organogenesis. Caspar Friedrich Wolff observed organization of the early embryo in leaf-like layers. In 1817, Heinz Christian Pander discovered three primordial germ layers while studying chick embryos. Between 1850 and 1855, Robert Remak had further refined the germ cell layer (Keimblatt) concept, stating that the external, internal and middle layers form respectively the epidermis, the gut, and the intervening musculature and vasculature.