Ideas: A History of Thought and Invention, from Fire to Freud

by Peter Watson

Then, in 1688, Leeuwenhoek trained his microscope on the transparent tail of the tadpole. ‘A sight presented itself more delightful than any mine eyes had ever beheld; for here I discovered more than fifty circulations of the blood in different places, while the animal lay quiet in the water, and I could bring it before my microscope to my wish. For I saw that not only in many places the blood was conveyed through exceedingly minute vessels, from the middle of the tail toward the edges, but that each of the vessels had a curve or turning, and carried the blood back toward the middle of the tail, in order to be again conveyed to the heart.’74 Nor should we overlook Leeuwenhoek’s discovery, in 1677, of spermatozoa, though it would be another century before their true role was identified. Leeuwenhoek was the first person to make biologists aware of the vast realms of microscopic life.