Preformationism
Preformationism, a philosophical theory of heredity, claimed that either the egg or the sperm (exactly which was a contentious issue) contained a complete preformed individual called a homunculus. Development was therefore a matter of enlarging this into a fully formed being.
The term homunculus was later used in the discussion of conception and birth, Nicolas Hartsoeker discovered "animalcules" in the semen of humans and other animals. This was the beginning of spermists' theory, who held the belief that the sperm was in fact a "little man" (homunculus) that was placed inside a woman for growth into a child. This seemed to them to neatly explain many of the mysteries of conception. It was later pointed out that if the sperm was a homunculus, identical in all but size to an adult, then the homunculus may have sperm of its own. This led to a reductio ad absurdum with a chain of homunculi "all the way down". This was not necessarily considered by spermists a fatal objection however, as it neatly explained how it was that "in Adam" all had sinned: the whole of humanity was already contained in his loins. The spermists' theory also failed to explain why children tend to resemble their mothers as well as their fathers, though some spermists believed that the growing homunculus assimilated maternal characteristics from the womb environment in which they grew.
Read more about this topic: Homunculus