a. Rudolf Virchow first explained that the cells divide and new cells are formed from the pre-existing cells. He gave the modern cell theory by modifying the old one given by Schleiden and Schwann –

• all living organisms are composed of cells and products of cells.

• all cells arise from pre-existing cells.

b. • In 1838, Schleiden conducted tests on different plants and concluded that all the plant are made up of different types of cells which together form plant tissues.

• In 1839, Schwann 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.

• So the cell theory with the contribution of the two stated that the bodies of animals and plants are composed of cells and products of cells.