Leeuwenhoek's "Animalcules" (illustrated 1795)

    This 1795 color rendering is based on drawings made more than a century earlier by Leeuwenhoek. Note forms recognizable as spirochetes (3), rotifers (10-12), frog blastuae and gastrulae (14 & 15), vorticella (17-19), Daphnia (20), and sperm from various species (29 - 30, a-h)

    Leeuwenhoek first used the term Animalcules based on observation. However, Marcus Varro's Rerum Rusticarum Libri Tres (10 CE) speculated that "If there be any swampy ground, both for the reasons given above, and because certain minute animals [animalcules] invisible to the eye, breed there, and, borne by the air, reach the inside of the body by way of the mouth and nose, and cause diseases which are difficult to be rid of."