Thanks to Wikipedia for this:
Preformationism is the formerly popular theory that animals developed from miniature versions of themselves. Sperm were believed to contain complete preformed individuals called “animalcules“.
Development was therefore a matter of enlarging into a fully formed being. The term homunculus was later used in the discussion of conception and birth.
Nicolas Hartsoeker postulated the existence of animalcules in the semen of humans and other animals.
This was the beginning of spermists’ theory, which held that the sperm was in fact a “little man” that was placed inside a woman for growth into a child, a neat explanation for 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.
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https://en.wikipedia.org/wiki/Preformationism
