What is meant by cell theory? Who presented its principles? Which improvements were done in it? Describe them.

(N/A) $\Rightarrow$ In $1838$,Matthias Schleiden and Theodore Schwann presented the cell theory.
$\Rightarrow$ Matthias Schleiden was a German botanist. He observed that all plants are composed of different kinds of cells which form the tissues of the plant.
$\Rightarrow$ $A$ British zoologist,Theodore Schwann,studied different types of animals and reported that cells had a thin outer layer,which is now known as the plasma membrane. He also concluded that the presence of a cell wall is a unique character of plant cells.
$\Rightarrow$ Both scientists together formulated the cell theory,which stated:
$(a)$ All living organisms are composed of cells and their products.
$(b)$ The cell is the structural and functional unit of all living organisms.
$\Rightarrow$ However,this theory did not explain how new cells were formed.
$\Rightarrow$ Rudolf Virchow (in $1855$) first explained that cells divide and new cells are formed from pre-existing cells (Omnis cellula-e cellula).
$\Rightarrow$ He modified the hypothesis of Schleiden and Schwann to give the cell theory its final shape:
$(i)$ All living organisms are composed of cells and products of cells.
$(ii)$ All cells arise from pre-existing cells.