https://en.wikipedia.org/wiki/Homunculus#Preformationism
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".
Why does this just sound like something Calvin's dad would have BSed in a lost "Calvin and Hobbes" strip???