Preformationism

Preformationism is the theory that an organism develops from a miniature, fully formed version already present in an egg or sperm. It contrasts with epigenesis, which holds that structures emerge during development.

Homunculus: A tiny, fully formed human imagined by some preformationists to exist inside a sperm cell. The homunculus made sperm-based preformationism concrete, though it was not accepted by all proponents.
Nicolas Malebranche: A French philosopher and theologian who defended the view that organisms were preformed in the eggs created at the beginning of life. Malebranche gave ovist preformationism a prominent philosophical defense.
Embryology: The branch of biology that studies development from fertilization through the embryonic stages. Preformationism was an early framework that embryology later tested and replaced.
Generation of animals: The historical study of how animals reproduce and how new organisms arise. Preformationism was one influential answer to the problem of animal generation.
Ovism: The preformationist view that the embryo is already present in the egg. Ovism located the miniature organism in the egg rather than the sperm.
Jan Swammerdam: A Dutch naturalist whose microscopic studies of insects supported the idea that development unfolds from pre-existing structures. His observations were invoked in arguments for preformation, though they did not establish nested miniature organisms.
Charles Bonnet: A Swiss naturalist and philosopher who defended preformation and developed the idea of nested generations in germ cells. Bonnet's defense made encasement theory one of preformationism's best-known forms.
Spontaneous generation: The discredited belief that living organisms can arise directly from nonliving matter under ordinary conditions. It offered a different account of life's appearance than development from a preformed organism.
Spermism: The preformationist view that the embryo is already present in the sperm. Spermism made the sperm the site of the preformed organism.
Antonie van Leeuwenhoek: A Dutch microscopist who first described spermatozoa in detail after observing them through microscopes. Microscopic sperm observations helped fuel spermist interpretations of preformation.

Linked from 5 pages