Who proposed the theory that cells arise only from pre-existing cells?

The theory that cells arise only from pre-existing cells was proposed by Rudolf Virchow. Virchow, a German physician, and scientist, put forward this concept as part of his cellular pathology work in the mid-19th century. 

He emphasized the idea that cells can only arise from pre-existing cells, stating "omnis cellula e cellula," meaning "every cell originates from a cell." This principle is a fundamental tenet of the cell theory and highlights the continuity and interconnectedness of cells in living organisms.