Leeuwenhoek and Hartsoeker each claimed to have discovered spermatozoa. Fontenelle, in an obituary for Hartsoeker, reported that Hartsoeker saw 

“these little animals until then invisible, which must transform themselves into men, which swim in prodigious amounts in the liquor destined to carry them, which do not occur but among males, and which have the appearance of young frogs, with large heads and long tails and very vivid movements.”

In this work, Hartsoeker devoted his attention to extending the mathematics of optics and the mechanical philosophy of Descartes. According to Descartes, material bodies are comprised of smaller particles. Hartsoeker’s diagram of the “little man” (or homunculus) in the head of the sperm illustrated that living beings also are comprised of smaller entities.

This illustration became an iconic representation of Leeuwenhoek's spermist form of preformationism, according to which adults are already preformed in the sperm. On the other hand, Hartsoeker described the sperm, egg and womb as making up a single body during development. In later life, after the discovery of regeneration in polyps and crayfish, he explicitly distanced himself from Leeuwenhoek's preformationism.