Claim: Cells are formed by pre-existing cells.
Evidence: Cell theory , first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann , states that all organisms are composed of one or more cells , that cells are the fundamental unit of structure and function in all living organisms , that all cells come from preexisting cells , and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.