William Harvey (1578-1657)
English Physician  

By observing the action of the heart in small animals and fishes, Harvey proved that heart receives and expels blood during each cycle. Experimentally, he also found valves in the veins, and correctly identified them as restricting the flow of blood in one direction. He developed the first complete theory of the circulation of blood, correctly concluding that it was pushed throughout the body by the heart's contractions. He published his observations and interpretations in Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (1628), often abbreviated De Motu Cordis.

Harvey was the first to use quantitative and observation methods simultaneously in his medical investigations. In Exercitationes de Generatione Animalium (On the Generation of Animals, 1651), he was extremely skeptical of spontaneous generation and proposed that all animals originally came from an egg. His experiments with chick embryos were the first to suggest the theory of epigenesis, which views organic development as the production in a cumulative manner of increasingly complex structures from an initially homogeneous material.

Adapted from scienceworld.wolfram.com/biography/Harvey.html