The age-old doctrine that the entire adult organism is preformed within the egg or sperm, but only begins to grow to a visible size after fertilization occurs. It origins can be traced back at least to the pangenesis of Hippocrates of Cos (460 ca-377 BP), which held that everything necessary for development (or rather growth) is contained in the spermia, and is transported to all parts of the body by circulation of the blood (a view that persisted into the 19th century and even entertained by Charles Darwin). This preformationistic interpretation was opposed by Aristotle (384-322 BP) who on the basis of observations on the chick embryo maintained that the embryo was created through a mixture of spermia and menstrual blood (another view persisting into the 19th century). Preformationists separated into two opposing two camps: the ovists (growth determined by ova) versus the spermists (organic structures to be found in the spermia). Preformationism was fatally undermined by the work of Caspar Fredrich Wolff (1733-1794) on the chick embryo and plant seed and published in in Theoria Genrationis. In short, using a light microscope, he found no evidence for any organs or forms resembling an adult, but instead granules, layers and other structures that could not be related to any known biological structures at the time. Wolff’s claims were subsequently, and crucially, supported by Karl Ernst von Baer (1792-1876). The efforts of Wolff and von Baer paved the way for preformationism to be supplanted by epigenesis in the theorizing of embryologists. Nevertheless, a residue of preformanistic-like thinking still persists in some quarters of contemporary developmental psychology, with notions such as the ‘deep structure of language’ and ‘innate knowledge’ (of the physical and social worlds).
See Differentiation (embryology), Differentiation (general), Development, Epigenesis, Evolution, Generative grammar approach, Growth, Homunculus, Homunculus problem, Innate modularity hypothesis