The situation is quite the reverse in the case of inert matter, where the local accidents of morphogenesis are generally fairly uniform and little in number, but the global structure that arises is often indeterminate, chaotic, and rarely opens to interpretation. The conceptually simplest model of epigenesis is clearly that called mosaic, in which each part of the egg has its destiny fixed from the very beginning. Even primitive morphogenesis shows an enormous variety of epigenetical movements, according to the animal being considered, but much of this diversity might only be apparent, since seemingly very different movements might arise, one from the other, by very simple transformations. In embryology, some tissues move faster than others and give rise to shock waves on the surfaces of velocity discontinuity. Another problem that arises is the appearance of secondary organizing centers in the chreod of the basic organizing center.