Cell theory states that all living organisms are made up of cells and that the cell is the basic unit of life. It also states that all cells arise from preexisting cells. The formulation of this theory is not attributed to Anton van Leeuwenhoek, but his pioneering work with microscopy was essential in its development.
Anton van Leeuwenhoek, born in Holland in 1632, is generally considered to be the first microbiologist. Although magnifying lenses were already known, they were relatively crude devices. Leeuwenhoek refined and improved the lens-making process dramatically. He crafted hundreds of microscopes, nine of which survive. The best of these are capable of magnification up to 275 times. While his devices were well-known even in his own day, Leeuwenhoek never shared his lens-making secrets with his contemporaries and kept his best microscopes for his own use. Based on his recorded observations, scientists speculate that the level of magnification achieved by his best microscopes must have been closer to 500 times. Leeuwenhoek was only an amateur scientist, but his investigation of the microscopic world was exacting and exhaustive. He was the first to see microorganisms in pond water and bacteria in saliva. His work was expanded upon by botanist Matthias Schleiden and zoologist Theodor Schwann, both of whom came to the conclusion that all living things are made up of cells.