Cells were first described in 1665 by the English scientist Robert Hooke, who studied the dead cells of cork with a crude microscope. Living cells were first described in detail in the 1670s by the Dutch scientist Anton van Leeuwenhoek. These early descriptions were not improved on until the early 19th century, when better-quality microscope lenses were developed. In 1839 the German botanist Matthias Jakob Schleiden and the German zoologist Theodor Schwann formulated the basic cell theory of today. In 1858 the German pathologist Rudolf Virchow's theory, that all cells come from preexisting cells, led to the development of ideas about cell division and cell differiation.