Final shape to the cell theory was given by 

NCERT CLASS-XI : Page No. 126.

Schleiden and Schwann together formulated the cell theory. This theory however, did not explain as to how new cells were formed. Rudolf Virchow (1855) first explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula). He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape. 

Cell theory as understood today is: 

(i) all living organisms are composed of cells and products of cells.

 (ii) all cells arise from pre-existing cells