ABSTRACT
Spatial differentiation, or pattern formation, is the process by which the individual cells within a population are specified to undergo a particular molecular differentiation, which results in a characteristic spatial pattern. Ideas on the mechanism of pattern formation are deeply rooted in the concepts of induction, field, individuation, gradient and dominance that were elaborated in the 1920s and 1930s. This chapter suggests that positional information is the main feature which determines the pattern of cellular differentiation, and that the mechanism of position determination is universal. Polarity may be defined in relation to the points with respect to which a cell’s position is being specified: it is the direction in which positional information is specified or measured. The concept of positional information will be shown to provide a unifying conceptual framework for a variety of systems including regeneration of hydroids, sea urchin development, and pattern formation in the insect epidermis.
