In animal development, organogenesis (organo-genesis, compound of the Greek words όργανον "that with which one works", and γένεσις "origin, creation, generation") is the process by which the ectoderm, endoderm, and mesoderm develop into the internal organs of the organism. Internal organs initiate development in humans within the 3rd to 8th weeks in utero. The germ layers in organogenesis differ by three processes: folds, splits, and condensation. Developing early during this stage in chordate animals are the notochord, which induces the formation of the neural plate, and ultimately the neural tube. Vertebrate animals all differentiate from the gastrula the same way. Vertebrates develop a neural crest that differentiates into many structures, including some bones, muscles, and components of the peripheral nervous system. The coelom of the body forms from a split of the mesoderm along the somite axis.
In plants, organogenesis can occur from totipotent callus cells.