Wiktionary
n. (context biology English) the theory that all organisms start their existence already in a predetermined form upon conception and this form does not change in the course of their lifetime (as opposed to epigenesis)
Wikipedia
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. 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.
Epigenesis (or neoformism), 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) This word is still used, on the other hand, 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.
Furthermore, 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. Some authors call Weismannist development (either preformistic or epigenetic) that in which there is a distinct germ line.
The historical ideas of preformationism and epigenesis, and the rivalry between them, are obviated by our contemporary understanding of the genetic code and its molecular basis together with developmental biology and epigenetics.
Usage examples of "preformationism".
Yet it is really preformationism that has triumphed, for there is no essential difference, but only one of mechanical details, between the view that the organism is already formed in the fertilized egg and the view that the complete blueprint of the organism and all the information necessary to specify it is contained there, a view that dominates modern studies of development.
Modern preformationism - the blueprint theory - holds that the DNA in a fertilized egg is equivalent to a blueprint of the adult body.
Quite apart from this problem of infinite regress, naive preformationism neglects the fact, which was hardly less obvious in the seventeenth century than now, that children inherit attributes from the mother as well as the father.
This doesn't happen but, if it did, it would be fair to say that modern molecular biology had vindicated the ancient theory of preformationism.