Cleavage and division of the cell of an egg of a vertebrate (Remak, 1855).
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. The term "mesoderm" was introduced into English by Huxley in 1871, and "ectoderm" and "endoderm" by Lankester in 1873.