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In the history of biology, preformationism (or preformism) is a formerly-popular theory that organisms develop from miniature versions of themselves. Instead of assembly from parts, preformationists believed that the form of living things exist, in real terms, prior to their development.〔Maienschein, Jane, "(Epigenesis and Preformationism )", The Stanford Encyclopedia of Philosophy (Fall 2008 Edition), Edward N. Zalta (ed.).〕 It suggests that all organisms were created at the same time, and that succeeding generations grow from homunculi, or animalcules, that have existed since the beginning of creation. Epigenesis〔According to the Oxford English Dictionary: It is also worth quoting this adumbration of the definition given there (viz., "The formation of an organic germ as a new product"): 〕 (or neoformism),〔Callebaut, Marc, 2008: (Historical evolution of preformistic versus neoformistic epigenetic thinking in embryology ). Belgian Journal of Zoology 138(1): 20-35.〕 then, in this context, is the denial of preformationism: the idea that, in some sense, the form of living things comes into existence. As opposed to "strict" preformationism, it is the notion that "each embryo or organism is gradually produced from an undifferentiated mass by a series of steps and stages during which new parts are added." (Magner 2002, p. 154) 〔Magner, Lois. ''A History of the Life Sciences''. New York: Marcel Dekker, Inc, 2002〕 This word is still used, on the other hand, in a more modern sense, to refer to those aspects of the generation of form during ontogeny that are not strictly genetic, or, in other words, ''epigenetic''. Furthermore, apart from those distinctions (preformationism-epigenesis and genetic-epigenetic), the terms preformistic development, epigenetic development and somatic embryogenesis are also used in another context, in relation to the differentiation of a distinct germ cell line. In preformistic development, the germ line is present since early development. In epigenetic development, the germ line is present, but it appears late. In somatic embryogenesis, a distinct germ line is lacking.〔Buss, L.W. (1987). ''The Evolution of Individuality''. Princeton University Press, Princeton, NJ, p. 20 ().〕 Some authors call Weismannist development (either preformistic or epigenetic) that in which there is a distinct germ line.〔Ridley M (2004) Evolution, 3rd edition. Blackwell Publishing, p. 295-297.〕 The historical ideas of preformationism and epigenesis, and the rivalry between them, are obviated by our contemporary understanding of the genetic code and its molecular basis together with developmental biology and epigenetics. ==Philosophical development== Pythagoras is one of the earliest thinkers credited with ideas about the origin of form in the biological production of offspring. It is said〔e.g. by Ian Johnston, Malaspina University-College, Nanaimo, BC, in "...And Still We Evolve: A Handbook on the History of Modern Science", (Section Five: Heredity and Modern Genetics ), May 2000.〕 that he originated "spermism", the doctrine that fathers contribute the essential characteristics of their offspring while mothers contribute only a material substrate. Aristotle accepted and elaborated this idea, and his writings are the vector that transmitted it to later Europeans. Aristotle purported to analyse ontogeny in terms of the material, formal, efficient, and teleological causes (as they are usually named by later anglophone philosophy) – a view that, though more complex than some subsequent ones, is essentially more epigenetic than preformationist. Later, European physicians such as Galen, Realdo Colombo and Girolamo Fabrici would build upon Aristotle's theories, which were prevalent well into the 17th century.〔 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.〔Clara Pinto Correia, ''The Ovary of Eve: Egg and Sperm Preformation'', Chicago: University of Chicago Press, 1997. ISBN 0-226-66952-1〕 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.〔 Harvey's epigenesis, more mechanistic and less vitalist than the Aristotelian version, was, thus, more compatible with the natural philosophy of the time.〔 Still, the idea that unorganized matter could ultimately self-organize into life challenged the mechanistic framework of Cartesianism, which had become dominant in the Scientific Revolution. Because of technological limitations, there was no available mechanical explanation for epigenesis. 〔Gould, S. J. (1974) ("On Heroes and Fools in Science." ) ''Natural History'' 83 (7): 30-32.〕 It was simpler and more convenient to postulate preformed miniature organisms that expanded in accordance with mechanical laws. So convincing was this explanation that some naturalists claimed to actually see miniature preformed animals (animalcules) in eggs and miniature plants in seeds.〔 In the case of humans, the term homunculus was used. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Preformationism」の詳細全文を読む スポンサード リンク
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