What did Robert Virchow observe when he witnessed a cell dividing in half?

The correct answer is - all cells arise from pre-existing cells.

Explanation:

Robert Virchow observed during the cell division in half that a single cell divide into two new cells therefore he suggested that, a famous aphorism Omnis cellula e cellula which means all cells arise from pre-existing cells. His observation is one of the major points of the cell theory.

This observation made by Robert Virchow becomes the fundamental basis of the cell theory which describes that cell divides to produce new cells. The other greatest observation made by him was that all cells not get sick or affected but a group of cells only.