Cell theory was developed in the early 19th century by three German scientists, Theodor Schwann, Matthias Jacob Schleiden and Rudolf Virchow. While all three scientists contributed to the generally accepted classical cell theory, each hypothesized different sections of it and had parts of their theory refuted and replaced.
The classical cell theory makes the assertion that all living organisms are made up of cells. Organisms can then be classified as either unicellular or multicellular. The theory also makes the claim that the cell is the smallest unit that makes up life. The last part of the theory also asserts that a cell can only arise from pre-existing cells.