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  Embryology (from Greek bruon, ‘swelling’ + ology) is the study of the growth and development of the embryo of an animal. In the 4th century  BCE, Aristotle hypothesized that the embryo was formed by gradual assembly from menstrual fluid, but little investigative work was done until the 17th century with the rise of anatomy. René Descartes dissected embryos and noted elements of their structure, and William Harvey studied the embryos of chicks and deer and formulated his hypothesis that the egg is the origin of the embryo in all living things. Also common at this time was the ideas of preformation, in which the precursor of the embryo is a miniature adult called the homunculus, and epigenesis, where the environment in which the embryo is housed shapes its development.

The 18th-century German anatomist Caspar Wolff set the ball rolling for modern embryology with his observation that undifferentiated material became specialized as the embryo developed. Karl von Baer (1792 - 1876) compared the development of vertebrate embryos and noted that the embryos resembled each other very closely, so that embryonic reptiles could hardly be distinguished from those of bird or mammals. This stimulated the idea, popular in the 19th century, that ‘ontogeny recapitulates phylogeny’: in other words that a developing embryo passes through the forms of its evolutionary ancestors until it reaches the shape of its parent. It is true that closeness of evolutionary relationship is reflected in embryological development and embryology is important in the study of phylogeny. Modern embryology is concerned with the control of cell differentiation and the development of the embryo from the zygote (the single-celled product of the fusion of the sperm and egg) to the developed offspring. RB

See also developmental biology; germ layer theory; germ plasm; ovism.Further reading Salvador Luria, A View of Life.



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