One of the marvels of nature is the way in which a complex organism develops from a single cell. The fully formed organism, however, is not produced in an instant. It is the outcome of a number of increasingly intricate changes occurring over a protracted period of time. This sequence takes place in the very young organism—the embryo. The study of the embryo’s formation and development is a branch of biology called embryology.
The term for all the phases of embryonic development is embryogeny. During embryogeny, cells divide countless times to form the tissues and organs of the body. A human being, for example, begins as a single cell that could be lost in a pinhole. At birth, however, after a period of development that lasts about nine months, the human body contains some 200 billion cells, is 2 billion times larger, and has all the parts necessary for life.
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