Earlier models of the morphogenesis in graptoloid colonies can be improved by taking into account the rate of growth and budding. It is assumed that both these factors are controlled by a specific function of the morphogen, here called for convenience the blastogen, and both are responsible for the attenuation of the proximal part of the rhabdosome. The permanence of this attenuation may theoretically be explained by the universal advantage of differentiation between the proximal and the distal part of the colony. Some aspects of possible adaptive significance of such colony organization are discussed. □Graptoloid colonies, morphogenesis, mathematical model.