" Thus, when the eye-cups (the future retina), which grow out of the brain at the end of two stalks (the future optic nerves), make physical contact with the surface, the skin over the contact area folds into the concave cups and differentiates into transparent lenses (see arrows on the right of the diagram). The eye-cup induces the skin to form a lens, and the lens in its turn induces adjacent tissues to form a transparent horny membrane, the cornea. Moreover, if an eye-cup is transplanted under the skin on the belly of a frog embryo, the skin over it will obligingly differentiate into a lens. We may regard this obligingness or 'docility' of embryonic tissue, its readiness to differentiate into the kind of organ best suited to the tissue's position in the growing organism, as a manifestation of the integrative tendency, of the part's subordination to the interests of the whole. "

― Arthur Koestler , The Ghost in the Machine