<P> In biology, cell theory is the historic scientific theory, now universally accepted, that living organisms are made up of cells . Cells are the basic unit of structure in all organisms and also the basic unit of reproduction . With continual improvements made to microscopes over time, magnification technology advanced enough to discover cells in the 17th century . This discovery is largely attributed to Robert Hooke, and began the scientific study of cells, also known as cell biology . Over a century later, many debates about cells began amongst scientists . Most of these debates involved the nature of cellular regeneration, and the idea of cells as a fundamental unit of life . Cell theory was eventually formulated in 1839 . This is usually credited to Matthias Schleiden and Theodor Schwann . However, many other scientists like Rudolf Virchow contributed to the theory . </P> <P> The three tenets to the cell theory are as described below: </P>

Who said all living things are composed of cells