In 1651, William Harvey published "On the Generation of Animals" ("Exercitationes de Generatione Animalium"), a seminal work on embryology that contradicted many of Aristotle's fundamental ideas on the matter. Harvey famously asserted, for example, that "ex ovo omnia"—all animals come from eggs. Because of this assertion in particular, Harvey is often credited with being the father of ovist preformationism. However, Harvey's ideas about the process of development were fundamentally epigenesist. As gametes (male sperm and female ova) were too small to be seen under the best magnification at the time, Harvey's account of fertilization was theoretical rather than descriptive. Although he once postulated a "spiritous substance" that exerted its effect on the female body, he later rejected it as superfluous and thus unscientific. He guessed instead that fertilization occurred through a mysterious transference by contact, or contagion.