State the cell theory. Name the scientists who presented it.

(N/A) The cell theory states that:
$(i)$ All living organisms are composed of cells and products of cells.
$(ii)$ All cells arise from pre-existing cells.
The cell theory was originally proposed by Matthias Schleiden $(1838)$ and Theodor Schwann $(1839)$.
Later,Rudolf Virchow $(1855)$ expanded the theory by suggesting that all cells arise from pre-existing cells (Omnis cellula-e cellula).