ABSTRACT
From the time of Aristotle there have been two competing views about the nature of the process by which individual organisms develop from fertilization to adulthood (= ontogeny). One of these, called preformation, held that a very tiny version of the complete individual is prepackaged in the egg-that is, all the parts and organs are preformed from the outset and individual development consists merely of the growth of these preexisting, fully formed parts until they reach their adult size. The alternative view, the one held by Aristotle and almost every other person who took the trouble to make observations on the growing embryo and fetus, holds that individual development takes place by transformations that bring each part and organ of the body into existence in a series of successive stages. This emergent view, called epigenesis, is the one we hold today. It signifies that individual development includes differentiation as well as growth, the hallmark of which is the progressive change from an initial relatively homogeneous state to a later highly heterogeneous state. We still do not completely
understand all the essential mechanisms whereby epigenesis takes place, but we do know that it is correct to say that individual development-psychologically, behaviorally, physiologically, and anatomically-is epigenetic and not preformative.1
