Self-organizing behavior is one of the most remarkable properties of regulative animal embryos. The reorganization of disarranged embryonic primordia to form an approximation of the "correct" structure by any number of abnormal pathways constitutes a form of goal-directed behavior. One might suppose that such anatomical "goals" are specified by genetic programs evolved through mutation and natural selection to produce useful structures. However, one might also ask the following questions: how can genes direct morphogenesis without specifying the pathways to be followed? how can genetic systems have evolved to specify the organization of the never-before-assembled structures reproducibly generated by abnormal tissue combinations? Experiments have shown that the layered structures generated in such experiments belong to the category of "inherently precise" machines, in which a specific pattern is generated with great precision by the constant repetition of a simple local behavior throughout the pattern-forming system. The organization characteristic of the chordate body plan-the "goal" of early development-also arises by very different developmental pathways in the various members of the phylum. Yet divergent evolution can hardly have altered the mechanisms governing gastrulation and neurulation while holding the end results essentially constant. Evidence suggests that the striking differences in these pathways may be understood less as fundamental alterations of morphogenetic mechanisms than as the physical consequences arising from heterochrony-differences in the times at which a shared set of underlying cellular changes are initiated.