On the Generation of AnimalsWilliam Harvey

Harvey's late work, written in his seventies and published reluctantly in 1651 after friends rescued his manuscripts from the wreck of the English Civil War. A treatise on embryology rather than physiology, it collects the observations he had made over decades on the developing eggs of hens and on deer obtained from the royal hunt.

Harvey argues for epigenesis against preformation: the embryo is not a fully formed miniature unfolded from the egg but develops gradually by the successive appearance of parts, the heart first, then the blood vessels, then the other organs. He watches the blastoderm form, the first drop of blood begin to beat, and traces the emergence of the chick through the twenty-one days of incubation. The dictum omne vivum ex ovo, every living thing from an egg, is attributed to this book. Harvey extended the principle from birds to mammals, where no egg could then be seen, by inference from his dissections of deer.

On the Generation of Animals founds embryology as an observational science. Its anatomical claims would be confirmed and extended once the microscope reached the laboratory, but the framework of epigenetic development, laid out by Harvey without a microscope, remained.