Briefly describe the contributions of the following scientists in the formulation of the cell theory:
$(a)$ Rudolf Virchow
$(b)$ Schleiden and Schwann

(N/A) Rudolf Virchow $(1855)$: He modified the hypothesis of cell theory by explaining that cells divide and new cells are formed from pre-existing cells,famously stated as 'Omnis cellula-e cellula'.
$(b)$ Schleiden and Schwann: Matthias Schleiden $(1838)$ observed that all plants are composed of different kinds of cells which form the tissues of the plant. Theodor Schwann $(1839)$ reported that animal cells had a thin outer layer which is today known as the 'plasma membrane'. Together,they formulated the cell theory,which states that all living organisms are composed of cells and products of cells.