The document discusses the history and principles of cell theory. It describes how early microscopists like Hooke, Levenguk, and Purkinje made early observations of cells. In 1839, Schwann and Schleiden formulated the first principles of cell theory: (1) all living things are made of cells and (2) animal and plant cells have similar structure and composition. Later, Virchow added that cells only arise from preexisting cells. The modern cell theory includes four principles: (1) cells are the basic unit of life, (2) cells have similar composition and function, (3) new cells arise from existing cells, and (4) cell specialization leads to tissue and organ formation