Preformationism was a popular theory in the 17th and 18th centuries. It allowed people to believe in a clockwork world, where there were tiny people running around in sperm. And those tiny people also had tiny people in their sperm. But there was a peculiar kind of logic to it all.
Throughout the 17th and 18th centuries there was a scientific war going on about the origins of man. Not the evolutionary origins, but the biological ones. Everyone had made the connection between sex and babies, but exactly how babies formed from the, shall we say, products of sex, was anyone's guess. There were two main theories. One was epigenesis, which stated that groups of undifferentiated cells slowly acquire their eventual mature characteristics. The other was preformationism, which held that, depending on who you asked, either the sperm or the egg contained a little person that would begin to grow once conception occurred.
Although the idea seems ridiculous to us now, preformationism was considered the more coldly rational theory of its time. People were embracing the idea of a mechanical universe. Epigenesis got in the way of that. It held that neither the sperm nor the egg were human or independently alive, but somehow, during the pregnancy, acquired a "vital essence" that no one could define or pin down. Preformationism merely held that existing copies enlarged under the right circumstances.
Nor were preformationists only going on theory. Scientists had dissected chicken embryos and caterpillars to find tiny wing-like structures that seemed to enlarge as the animals developed. One of the most famous preformationists, Nicolaas Hartsoeker had a flawless pedigree. He was the student of Antonie van Leeuwenhoek, also known as the "father of microbiology," and the mentor of Christian Huygens, who became a famous astronomer. He was in business (in different capacities and at different times) with both of these men as a lens maker. They worked all day, making lenses, telescopes, and microscopes more powerful.
It's never been exactly worked out who provided the material when he, another student, and Leeuwenhoek became the first to observe sperm in human semen, but what they saw turned Hartsoeker into a performationist for life. He believed that the observable individual cells within what had seemed to be a mass of fluid showed that there was no vital essence. There were only individuals, folded up inside of sperm cells, trying to get to a place where they could grow. His description of what he believed to be in sperm cells, and his drawings of the "little infant" inside the sperm cell, provided the argument for performationists for nearly a century.
In the end, it was Hartsoeker's day job that ended things for the preformationists. As people were able to make more and more accurate observations of sperm cells, ova, and animal embryos, there was no getting around that nothing like a fully formed individual was in there. His advances started the idea of performation, and they finished it.