Developmental Processes

Until the mid-1800s, many naturalists supported a theory of development called epigenesis, which held that the eggs of organisms were undifferentiated, but had a developmental potential which could be directed by certain external forces. Other naturalists supported a theory of development called preformationism, which held that the entire complex morphology of mature organism is present in miniature form in the egg, a developmental form called the homunculus.

These early theories of development relied on little experimental evidence. Thus, biologists often criticized the original theory of epigenesis because it seemed to propose that mystical forces somehow directed development, a view clearly outside the realm of science. Biologists also rejected preformationism, since studies of cytology and embryology clearly showed that development is much more than the simple growth of a preformed organism.

The modern view is that developmental processes have certain general features of both preformationism and epigenesis. Thus, we know that the simple cells of an egg are preformed in the sense that they contain a preformed instruction set for development which is encoded in their genes. Similarly, we know that the egg is relatively formless, but has the potential to develop into a complex organism as it grows. Thus modern developmental biology views development as the expression of a preformed genetic program which controls the epigenetic development of an undifferentiated egg into a morphologically complex adult.