when was the cell theory developed

The cell theory was developed and formally articulated in 1839 by the German scientists Theodor Schwann and Matthias Jakob Schleiden. They proposed that all living organisms are composed of cells, which are the fundamental units of structure and function in living things. This theory laid the foundation for modern biology. Later, in 1855, Rudolf Virchow added the important tenet that all cells arise from pre-existing cells, completing the classical cell theory. The initial discovery of cells dates back to 1665, when Robert Hooke first observed cell walls in cork under a microscope, but the cell theory itself was not developed until the 19th century as microscopes and scientific understanding improved.