Final answer:
Rudolf Virchow borrowed ideas from other scientists and discovered the third part of the cell theory, which states that all cells arise from other cells.
Step-by-step explanation:
Rudolf Virchow borrowed his ideas from other scientists and discovered the third part of the cell theory. Specifically, he popularized the concept that all cells arise from other cells, which is known as the principle of biogenesis. This idea was first published by Robert Remak, but Virchow's essay titled 'Cellular Pathology' in 1855 helped to disseminate and solidify this concept.