Summary: Milinkovitch et al. (2013)

Many amniotes (an animal possessing an amnion - sac enclosing the developing embryo) show keratinization of various skin appendages, such as hair, feathers and scales. These different appendages differentiate during embryonic development. The differentiation is genetically controlled by developmental units and reaction-diffusion mechanisms (RDMs) pattern the spatial organisation of these units. Milinkovitch et al. (2013) demonstrate that the face and jaw scales of crocodiles do not form under the influence of these genetically controlled elements. Rather, they are random polygonal domains of highly keratinized skin that form by cracking of the skin in a stress field (a physical self-organizing stochastic process). They suggest that this occurs because the embryonic facial and jaw skeleton undergoes rapid growth and is highly keratinized, which generates the mechanical stress that induces cracking.