1. the theory, now accepted, that prenatal development is a gradual, complex, and cumulative process involving successive differentiation of morphological structures from fertilized ovum to embryo to fetus. Compare preformism.

2. the theory that characteristics of an organism, both physical and behavioral, arise from an interaction between genetic and environmental influences rather than from one or the other. See also nature–nurture.

3. in genetics, the occurrence of a heritable change in gene function that is not the result of a change in the base sequence of the organism’s DNA. See also epigenetics.

4. in the theory of Erik Erikson, the emergence of different goals at each stage of ego and social development. See Erikson’s eight stages of psychosocial development. —epigenetic adj.