Rudolf Virchow proposed that all cells arise from pre-existing cells. He was one of the three key figures, along with Theodor Schwann and Matthias Schleiden, who formulated the cell theory.
The cell theory is based on three concepts: all living organisms are made up of cells, cells are the fundamental units of all living things and cells originate from other cells. Major scientific inventions and breakthroughs by various scientists contributed to the development of the cell theory. The advent of the compound microscope led to the discovery of the cells by Robert Hooke in 1663. In 1674, Anton von Leewenhoek was the first to observe bacteria under the microscope. In 1838, Schleiden discovered plants are composed of cells and a year after, Schwann claimed that animals are comprised of cells.
In the 1850s, Virchow conducted a series of experiments to refute the theory of spontaneous generation, which was popularly held at the time. Virchow concluded that cells do not arise spontaneously, but instead come into existence from the division of already existing cells. Along with his observation, Virchow compiled Schwann and Schleiden's discoveries into one unified cellular theory. He published his findings in a book entitled "Cellular Pathology," including the now famous saying "Omnis cellula e cellula," meaning cells only arise from previously existing cells.