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. This process can be studied using both embryos and organoids. 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, which is simply the process of forming new organs, occurs continuously and only stops when the plant dies. In the shoot, the shoot apical meristems regularly produce new lateral organs (leaves or flowers) and lateral branches. In the root, new lateral roots form from weakly differentiated internal tissue (e.g. the xylem-pole pericycle in the model plant Arabidopsis thaliana). In vitro and in response to specific cocktails of hormones (mainly auxins and cytokinins), most plant tissues can de-differentiate and form a mass of dividing totipotent stem cells called a callus. De novo organogenesis can then occur from those cells. The type of organ that is formed depends on the relative concentrations of the hormones in the medium.