A model of pattern formation in insect embryogenesis.

A model is proposed in which the interaction of an autocatalytic substance with a short diffusion range--the activator--and its more diffusible antagonist--the inhibitor--leads to a local high concentration of activator at the posterior pole of the egg. The inhibitor, which is then produced mainly in this activated region, diffuses into the rest of the egg, where it acts as a 'morphogen', forming a concentration gradient which supplies positional information. This model can account quantitatively for the patterns resulting from a large number of different experiments performed during early insect development, including ligation, u.v.-irradiation and microsurgical manipulations. The formation of additional posterior structures is interpreted as the result of the appearance of a new activator peak. Omission of segments after ligation of the egg is explained as the result of accumulation of morphogen (the inhibitor) at the posterior side of the ligation and a decrease of morphogen on the anterior side. In order to account for certain quantitative features of the ligation experiments it is necessary to assume that determination in response to the morphogen gradient is a slow, stepwise process, in which the nuclei or cells first pass through determination stages characteristic for more anterior structures until they ultimately form a given structure. The mutual interactions of activator and inhibitor are expressed as a set of partial differential equations. The individual experiments are simulated by solving these equations by use of a computer.