In the history of cell biology, what was Zacharias Janssen’s key contribution that helped enable later development of cell theory?

• A. He is often credited with constructing one of the first compound microscopes, making it possible to observe cells and microorganisms.
• B. He discovered the cell nucleus and proposed it controls heredity and cell division.
• C. He formulated the three main tenets of cell theory, stating that all organisms are made of cells and cells arise from preexisting cells.
• D. He first observed bacteria and named them “animalcules” using a simple microscope.

Answer: (A)