The cell theory was further expanded by ________.

The cell theory, which states that all living organisms are composed of one or more cells and that the cell is the basic unit of life, was first proposed by Matthias Jakob Schleiden and Theodor Schwann in the mid-19th century. The cell theory was later expanded by Rudolf Virchow, a German pathologist, in 1855. Virchow proposed that all cells arise from pre-existing cells, which is known as the principle of biogenesis. This principle overturned the previous notion of spontaneous generation, which stated that living organisms could arise spontaneously from non-living matter. Virchow's contribution to the cell theory further solidified the concept of the cell as the fundamental unit of life.