In fact, the Dutchman had discovered bacteria and protozoans by using a more powerful microscope than anyone, up until that point, had been able to construct."I then most always saw, with great wonder, that in the said matter there were many very little living animalcules, very prettily a-moving. The biggest sort. . . had a very strong and swift motion, and shot through the water (or spittle) like a pike does through the water. The second sort. . .oft-times spun round like a top. . . and these were far more in number."
In 1665, the British microscopist Robert Hooke hailed the dawn of a new generation of discoveries which would become possible: "By the help of Microscopes there is nothing so small as to escape our inquiry; hence there is a new visible world discovered to the understanding".
Van Leeuwenhoek also became involved in the debate at the time as to whether life began with the egg or the sperm (it rarely occurred to people at the time that it could derive from both). He came down on the side of the spermists and against the ovists due to observations he made through his microscope:
"having found spermatozoa in the male seminal fluid of animals, birds, fishes and even insects, I assert much more certainly than before that man arises, not for from the egg, but from the spermatozoa in the male semen"
Van Leeuwenhoek left 272 microscopes on his death but most have now disappeared. Since he kept his glass blowing and lens grinding skills secret in his lifetime, his contempories and successors were unable to repeat his discoveries.