The principle of non-equivalence in development.

Cells that look alike to the histologist but are in different positions in the body may have different intrinsic characters; they may have positional information, making them non-equivalent. This idea, familiar in the context of neuronal specificity and of embryonic determination, goes against some well-known theories which view development at the cellular level simply in terms of choices of histological type. Non-equivalence is nonetheless demonstrable in, for example, the development of the nervous system, skeleton, musculature and skin of vertebrates, and of the epidermis of insects. The non-equivalence of the parts of the body makes them independently modifiable, and defines a fundamental scheme upon which evolution can work its variations.