The modern cell theory states that living organisms are made of cells, that all cells come from other cells and that cells are the most basic life form. As scientific advancements are made, definitions of theories, like the cell theory, are updated. The modern cell theory is not the same as cell theory of the past.
Cell theory was created upon the discovery of cells, which was done through a primitive microscope by Robert Hooke in the 17th century. Hooke made the discovery while observing plant tissues. As microscopes became more powerful over the years, it became possible not only to identify which type of cells are being observed, but what organelles and other structures are contained in each cell.