Explanation:

The cell theory was proposed by Schleiden (1838) and Schwann (1839). According to the cell theory, all the plants and animals are composed of cells and that the cell is the basic unit of life. In 1855, Virchow further expanded the cell theory and suggested that all cells arise from pre-existing cells.In biology, cell theory is the historic scientific theory, now universally accepted, that living organisms are made up of cells, that they are the basic structural/organizational unit of all organisms, and that all cells come from pre-existing cells.