Answer:
Anton van leeuwenhoek with the help of the microscope observed the stagnant water. He observed protozoans and at that time he named them "Animalcule" . He observed bacteria, sperm cells, blood cells, rotifers, and much more. He was the first to observe live microorganisms.
He did important observations such as the life cycle weevils, fleas and mussels, and concluded that they developed from tiny eggs refuting arguments of spontaneous generation