Antonie Von Leeuwenhoek first saw and described a live cell. Robert Brown later discovered the nucleus, Matthias Schleiden, a German botanist, examined a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant. Theodore Schwann a German Zoologist studied different types of animal cells and reported that cells had a thin outer layer which is today known as the 'plasma membrane'. He also concluded, based on his studies on plant tissues, that the presence of cell wall is a unique character of the plant cells. On the basis of this, Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells Schleiden and Schwann together formulated the cell theory. Rudolf Virchow first explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula).