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Aristotle (384–322 B.C.) described the two historically important models of development known as preformation and epigenesis. According to preformationist theories, an embryo or miniature individual preexists in either the mother’s egg or the father’s semen and begins to grow when properly stimulated. Some preformationists believed that all the embryos that would ever develop had been formed by God at the Creation. Aristotle actually favored the theory of epigenesis, which assumes that the embryo begins as an undifferentiated mass and that new parts are added during development. Aristotle thought that the female parent contributed only unorganized matter to the embryo. He argued that semen from the male parent provided the “form,” or soul, that guided development and that the first part of the new organism to be formed was the heart. Aristotle’s theory of epigenetic development dominated the science of embryology until the work of physiologist William Harvey (1578–1657) raised doubts about many aspects of classical theories. In his studies of embryology, as in his research on the circulation of the blood, Harvey was inspired by the work of his teacher, Girolamo Fabrici (ca.1533–1619). Some historians think that Fabrici should be considered the founder of modern embryology because of the importance of his embryological texts: On the Formed Fetus and On the Development of the Egg and the Chick. Harvey’s On the Generation of Animals was not published until 1651, but it was the result of many years of research. Although Harvey began these investigations in order to provide experimental proof for Aristotle’s theory of epigenesis, his observations proved that many aspects of Aristotle’s theory of generation were wrong.
Using deer that had mated, Harvey dissected the uterus and searched for the embryo. He was unable to find any signs of a developing embryo in the uterus until about six or seven weeks after mating had taken place. In addition to his experiments on deer, Harvey carried out systematic studies of the developing chick egg. His observations convinced him that generation proceeded by epigenesis, that is, the gradual addition of parts. Nevertheless, many of Harvey’s followers rejected epigenesis and turned to theories of preformation.
K. Lee Lerner and Brenda Wilmoth Lerner ed., 2004. The GALE ENCYCLOPEDIA of Science, 3rd ed. v. 2. Thomson/Gale. Publisher’s

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