preformation n : a theory (popular in the 18th century and now discredited) that an individual develops by simple enlargement of a tiny fully formed organism (a homunculus) that exists in the germ cell [syn: theory of preformation]
Preformationism is the theory that all organisms were created at the same time, and that succeeding generations grow from homunculi, animalcules, or other fully-formed but miniature versions of themselves that have existed since the beginning of creation.
In the case of humans, philosophers and theologians have conjectured that every individual existed as a homunculus in Adam's testicles (spermism) or Eve's ovaries (ovism).
Epigenesis, in contrast, is based on the idea 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)
Pythagoras was one of the first to elaborate a theory of generation, the biological production of offspring. He advocated a patrocline theory whereas males contributed the essential characteristics of their offspring while females contributed only a material substrate. Pythagorus' theory heavily inspired Aristotle, who elaborated both upon preformation and epigenesis, two distinct theories of generation. Subsequent theorists such as Galen, Realdo Colombo and Girolamo Fabrici built upon Aristotle's theories, which were prevalent well into the 17th century.
Leeuwenhoek discovered that the origin of semen was the testicles and was a committed preformationist and spermist. He reasoned that the movement of spermatozoa was evidence of animal life, which presumed a complex structure and, for human sperm, a soul. (Friedman 79)
Ova were known in some non-mammalian species, and semen was thought to spur the development of the preformed organism contained therein. The theory that located the homonculus in the egg was called ovism. But when spermatozoa were discovered, a rival camp of spermists sprang up, claiming that the homunculus must come from the male. In fact, the term "spermatozoon," coined by Karl Ernst van Baer, means "seed animals."
In the 18th century, some animalculists thought that an animal's sperm behaved like the adult animal, and recorded such observations. Some, but not all, preformationists at this time claimed to see miniature organisms inside the sex cells. But about this time, spermists began to use more abstract arguments to support their theories.
Jean Astruc, noting that parents of both sexes seemed to influence the characteristics of their offspring, suggested that the animalcule came from the sperm and was then shaped as it passed into the egg. Buffon and Pierre Louis Moreau also advocated theories to explain this phenomenon.
- Clara Pinto Correia, The Ovary of Eve: Egg and Sperm Preformation, Chicago: University of Chicago Press, 1997. ISBN 0-226-66952-1
preformation in French: Théorie de la préformation
preformation in Polish: Preformacja