In this essay, we discuss the contribution of local cell interactions, as opposed to global cues, such as hormones and morphogens, to the determination of the size and shape of organs. This internalistic notion is based on the integration of physiological experiments, clonal analysis and genetic mosaics of morphogenetic mutants. Physiological experiments such as regeneration, cell dissociation and re-aggregation, and feeder layers, reveal the existence of regional specificities related to regional patterning, positional values and cell polarity. On the other hand, clonal analysis shows homogeneous and exponential growth by mitosis associated with oriented cell divisions along two axes of growth. Preferential mitotic orientations are also regional characteristic. Finally, genetic mosaics of morphogenetic mutants help to understand the genetic and eventually the molecular bases of morphogenesis, revealing the modes of cell behavior (such as positional accommodation, mitotic orientations, cell affinities and cell recognition) at work in the generation of the constant sizes and shapes of Drosophila imaginal structures. These classical and recent findings contribute to a more updated Entelechia model of morphogenesis.