In the history of biology, preformationism (or preformism) is a formerly popular theory that organisms develop from miniature versions of themselves. Instead of assembly from parts, preformationists believed that the form of living things exist, in real terms, prior to their development.[1] It suggests that all organisms were created at the same time, and that succeeding generations grow from homunculi, or animalcules, that have existed since the beginning of creation, which is typically defined by religious beliefs.

A tiny person inside a sperm, as drawn by Nicolaas Hartsoeker in 1695
Jan Swammerdam, Miraculum naturae sive uteri muliebris fabrica, 1729

Epigenesis[2] (or neoformism),[3] then, in this context, is the denial of preformationism: the idea that, in some sense, the form of living things comes into existence. As opposed to "strict" preformationism, it is the notion that "each embryo or organism is gradually produced from an undifferentiated mass by a series of steps and stages during which new parts are added" (Magner 2002, p. 154).[4] This word is still used in a more modern sense, to refer to those aspects of the generation of form during ontogeny that are not strictly genetic, or, in other words, epigenetic.

Apart from those distinctions (preformationism-epigenesis and genetic-epigenetic), the terms preformistic development, epigenetic development and somatic embryogenesis are also used in another context, in relation to the differentiation of a distinct germ cell line. In preformistic development, the germ line is present since early development. In epigenetic development, the germ line is present, but it appears late. In somatic embryogenesis, a distinct germ line is lacking.[5] Some authors call Weismannist development (either preformistic or epigenetic) that in which there is a distinct germ line.[6]

The historical ideas of preformationism and epigenesis, and the rivalry between them, are obviated by the contemporary understanding of the genetic code and its molecular basis together with developmental biology and epigenetics.

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