Early Concepts: Preformation Versus Epigenesis

Early Concepts: Preformation Versus Epigenesis
Early scientists and laypeople alike speculated at length about the mystery of development long before the process was submitted to modern techniques of biochemistry, molecular biology, tissue culture, and electron microscopy. An early and persistent idea was that young animals were preformed in eggs and that development was simply a matter of unfolding what was already there. Some claimed they could actually see a miniature of the adult in the egg or sperm (Figure 8-1). Even the more cautious argued that all parts of the embryo were in the egg, needing only to unfold, but so small and transparent they could not be seen. The concept of preformation was strongly advocated by most seventeenth- and eighteenth-century naturalist-philosophers.

In 1759 German embryologist Kaspar Friedrich Wolff clearly showed that in the earliest developmental stages of the chick, there was no preformed individual, only undifferentiated granular material that became arranged into layers. These layers continued to thicken in some areas, to become thinner in others, to fold, and to segment, until the body of the embryo appeared. Wolff called this process epigenesis (“origin upon or after”), an idea that a fertilized egg contains building material only, somehow assembled by an unknown directing force. Current ideas of development are essentially epigenetic in concept, although we know far more about what directs growth and differentiation.

Development describes the progressive changes in an individual from its beginning to maturity (Figure 8-2). In sexual multicellular organisms, development usually begins with a fertilized egg that divides mitotically to produce a many-celled embryo. These cells then undergo extensive rearrangements and interact with one another to generate the animal’s body plan and all of the many kinds of specialized cells in the body. This generation of cellular diversity is not defined all at once but is formed as the result of a hierarchy of developmental decisions. The many familiar cell types that make up the body do not simply “unfold” at some point, but arise from conditions created in preceding stages. At each stage of development new structures arise from the interaction of less committed rudiments. Each interaction is increasingly restrictive, and the decision made at each stage in the hierarchy further limits developmental fate. Once cells embark on a course of differentiation, they become irrevocably committed to that course. They no longer depend on the stage that preceded them, nor do they have the option of becoming something different. Once a structure becomes committed it is said to be determined. Thus the hierarchy of commitment is progressive and it is usually irreversible. The two basic processes that are responsible for this progressive subdivision are cytoplasmic localization and induction. We will discuss both processes as we proceed through this section.