Louis Pasteur used experiments to test the idea of spontaneous generation, which was the belief that living organisms could arise from non-living matter.
Robert Hooke observed in cork what we call cells. His work in the mid-17th century led to the coining of the term "cell" to describe the small, box-like structures he saw.
Antonie van Leeuwenhoek invented the microscope and called what he saw animalcules, which referred to the microorganisms he observed in samples of water, dental plaque, and other substances.