Hooke could not have seen what were actually the nonliving cell walls of his cork preparation were it not for the microscope, an instrument that uses a system of magnifying lenses to reveal the features of objects too small to be seen by the unaided eye. As early as 300 B.C., the Greeks had used curved glass containers filled with water to magnify nearby objects, but it was not until the seventeenth century that Anton van Leeuwenhoek refined the grinding process to produce lenses that could be used effectively in simple microscopes.
In 1809, Lamarck recognized that all living things show cellular structure. In 1824, Dutrochet stated unequivocally that all living tissues are made up of tiny globular cells. Further, he realized that growth involved both an increase in the size of existing cells and an increase in the number of cells. In 1831, Robert Brown described the nucleus, which is a feature of almost all eukaryotic cells. In 1838, Schleiden published his studies of the cellular structure of plants, and Schwann released his parallel findings on the cellular makeup of animal tissue the following year. Because of the clarity of their description and the vigor with which they campaigned for acceptance of their notions, Schleiden and Schwann are generally credited with formulating the cell doctrine and placing the cell in the center of investigations into the nature of life. When Rudolph Virchow asserted in 1858 that all cells come from preexisting cells, the cell assumed the role of a continuous living chain in time by which life was to be understood.
