Schematic representation of chick embryos as representative of amniotes (birds, reptiles, and mammals). Dorsal views and transverse sections taken at the level of the dotted lines. (A) The embryonic layers are defined during gastrulation. Mesodermal (green) and endodermal cells (pink) are internalized at the primitive streak through a process of EMT, while ectodermal cells remain epithelial (yellow). (B) The mesodermal cells condense to form various derivatives (blue) along the medio-lateral axis of the embryo. The axial mesoderm gives rise to the notochord; paraxial mesoderm epithelializes through a process of MET to form the somites; the intermediate mesoderm will later form the urogenital system; and the lateral mesoderm condenses to form somatopleure and splanchnopleure. The ectodermal cells contribute to the neural tube or the epidermis. The endoderm is shown in pink.