Epigenesis

Epigenesis is a theory in developmental biology proposing that organisms develop through the progressive differentiation of cells and tissues from an initially undifferentiated state, rather than existing as preformed structures within the embryo. In classical epigenesis, new anatomical features emerge as development proceeds, guided by genetic and biochemical signals. The doctrine stood in contrast to preformationism, which held that miniature, fully formed organisms or organs exist within sperm, eggs, or fertilized eggs. Early advocates included Aristotle, and in the 18th century Caspar Friedrich Wolff provided empirical support by showing that organs arise from simple tissue rather than preformed organs.

Today, epigenesis as a term is used in two related senses. In developmental biology, it refers to the generative process by which complex organisms arise from simple tissues through patterning and differentiation. In contemporary molecular biology, epigenetics describes heritable changes in gene expression that do not involve changes to the DNA sequence itself. Mechanisms include DNA methylation, histone modification, chromatin remodeling, and non-coding RNAs, which can regulate when and where genes are active. Epigenetic marks can respond to environmental cues and, in some cases, be transmitted through cell division or across generations, as in imprinting or X-chromosome inactivation.

Epigenesis remains central to understanding development, cell lineage specification, and developmental disorders, as well as the interface between genetics and the environment. Although the modern emphasis is often on epigenetic regulation, the term itself retains historical significance about how organismal form emerges during embryogenesis.