(Latin, from pre- and formatio, from formare, "to shape, to form")
Theory of creation and conception, advanced in the 17th and 18th centuries. According to preformation theory, either the sperm cell or the egg cell contains, at the time of conception, a miniature human figure, fully formed but extremely tiny. Thus, the growth of the individual only consists of the enlargement of an already formed (preformed) body.
By extension, the generative cells (egg and sperm) were then supposed to contain all successive generations, nested in miniature, like a series of Chinese boxes, inside each other - each sperm or egg thus containing the fully formed shapes of all possible descendants of the individual. The first egg and sperm, from which all the others were supposed to be derived, would thus have been those of Adam and Eve.
Preformation theory was a rather naïve and unwieldy hybrid of theology and microscopy. It gained currency through the publication by Nicolas Hartsoeker (1656-1728) in 1678 in Paris, of his microscopic examinations of egg and sperm cells.
Essentially, this was a magical theory of conception, based in the principle that the microcosm is reflected in the macrocosm, and vice versa ("As above, so below.").