Jan Swammerdam. Illustration of Jan Swammerdam (1637-80), the Dutch naturalist and microscopist who discovered red blood cells. Swammerdam studied medicine but devoted his life to microscopy, despite financial pressure. Swammerdam showed with frogs' legs that muscular contraction involves a change of shape but not volume. In 1658, he discovered red blood cells in the blood of frogs. He also discovered valves in the lymph vessels. Swammerdam pioneered entomology, collecting and studying the anatomy of 3000 insect species. He suffered from mental illness during his later years. His work was mostly neglected until published by Boerhaave 50 years after his death.