Introduction

The size and proportions of an animal are reliable morphological features but rather little is understood about the ways in which they are achieved through the control of cell proliferation and cell size throughout development. Typically, each organ and each part of an organ starts to form in its appropriate position at a particular developmental stage, follows a characteristic pattern of growth and, in many animals, stops growing at a predictable time and with a reliable final size.

Spatial organisation is not completely preformed in the egg, although in most cases the cytoplasm is heterogeneous and this may determine the location and orientation of the embryo. The results of experimental embryology show that the detailed patterns of cell fate in most (perhaps all) embryos are formed gradually, through interactions by which cells acquire ‘positional information’ (Wolpert, 1971), leading them to develop as befits their location. These interactions can be studied by altering the spatial arrangements within the developing embryo, depriving cells of their normal neighbours or placing them against cells which are normally far away. The ensuing alterations in cell fate are often associated with changes in proliferation, and give information on the normal processes whereby cells acquire fates and rates of division appropriate to their positions in the undisturbed embryo.

Pattern formation and growth control have been extensively studied in the insect epidermis which, being the surface layer of cells secreting the cuticle, determines the overall size and surface features of the animal.