What might happen to cause biologists to modify the cell theory?

Rudolf Virchow modified the cell theory and gave the concept of 'Omnis cellule e cellulae'- which means new cells develop by the division of the pre-existing cells. This is called the theory of cell lineage or common ancestry.

Explanation:

Based on their studies on cells, 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 divide 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.

So, the correct answer is 'Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion'