Number the events from earliest (1) to most recent (6) to outline the development of the cell theory. Anton van Leeuwenhoek observed small organisms he called “animalcules.” Robert Hooke was the first to use the word “cell.” Matthias Schleiden theorized that plants are made of cells. Theodor Schwann theorized that animals are made of cells. Hans and Zacharias Janssen created the first microscope. Rudolf Virchow theorized that cells come from other pre-existing cells.