"Die Entwickelung der Meduse" in "Das Meer" by M. J. Schleiden, 1869. Life cycle of scyphozoans. 1-8 planula attachment and metamorphosis to scyphistoma stage; 9-10 scyphistoma strobilation; 11 ephyra release; 12-14 transformation of the ephyra into an adult medusa. Matthias Jakob Schleiden (April 5, 1804 - June 23, 1881) was a German botanist and co-founder of cell theory, along with Theodor Schwann and Rudolf Virchow. While a professor of botany at the University of Jena, he wrote Contributions to our Knowledge of Phytogenesis (1838), in which he stated that all parts of the plant organism are composed of cells. Thus, Schleiden and Schwann became the first to formulate what was then an informal belief as a principle of biology equal in importance to the atomic theory of chemistry. He was one of the first German biologists to accept Darwin's theory of evolution and became a leading proponent of Darwinism in Germany. He died in 1881 at the age of 77.