The Collaborative International Dictionary of English v.0.48:

Preformation \Pre`for*ma"tion\, n. (Biol.)
   An old theory of the pre["e]xistence of germs. Cf.
   Embo[^i]tement.
   [1913 Webster]

WordNet (r) 3.0 (2006):

preformation
    n 1: 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: preformation, theory of preformation]