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Definition of Theory of preformation
1. Noun. 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. 
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1. Noun. 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.