The first two tenets of the Cell theory were postulated by Schleiden and Schwann at the first half of the 19th century. Those tenets are that "all organisms are composed of one or more cells" and that "the cell is the basic unit of life". In 1855, Rudolf Virchow added the third tenet "Omnis cellula e cellula" which means that "all cells come from cells". In other words, cells can be formed only in the division of other cells.
