In the history of cell biology, what was Antonie van Leeuwenhoek’s key contribution that helped advance ideas leading to cell theory?

• A. He formulated the three main tenets of cell theory and presented them as a unified theory.
• B. He was among the first to observe and describe living single-celled organisms (“animalcules”) such as bacteria and protists using improved microscopes.
• C. He proposed that all cells arise only from preexisting cells (biogenesis).
• D. He identified the cell nucleus and described it as a defining cellular structure.

Answer: (B)