Epigenesis

Epigenesis, the biological truth that the development of organisms is the progressive differentiation of a primarily homogeneous germ. This truth, propounded by Aristotle in his work on the generation of animals, was defended by Harvey in his Exercitationes, published in 1651, on the evidence of his observations on the incubation of the hen's egg. He says in effect that the formation of the new organism is not the sudden and simultaneous accretion of rudiments of all the organs of the adult, nor the sudden metamorphosis of formative substance into a miniature of the whole organism, but an epigenesis or successive development of the parts from the originally homogeneous rudiment. This view, disputed by Malpighi, Leibnitz, Bonnet, and Haller, was finally established by Caspar Frederick Wolff in his Theoria Generatiemis (1759).