For another century and a half, the general importance of cells was not appreciated by biologists. In 1809, Jean Baptist De-Lamarck proposed that “Nobody can have life if its constituent parts are not cellular tissues or are not formed by cellular tissues”.

In 1831, a British Botanist Robert Brown discovered a nucleus in a cell. In 1838, a German Botanist Matthias Schleiden studied plant tissues and made the first statement of the first theory. He stated that all plants are “aggregates of individual cells which are fully independent and separate beings”. One year later in 1839, a German Zoologist Theodor Schwann reported that all animal tissues are also composed of individual cells. Thus, Schleiden and Schwann proposed cell theory in its initial form i.e. “all living things are composed of living cells”.