Heterochrony, the change in the relative time of appearance or rate of development of a character during phylogeny, has been an important mechanism for generating morphological change during evolution. Retardation and acceleration are two processes which drive heterochrony. I have considered the developmental mechanisms which govern three parameters of heterochrony (when growth starts, growth rate, when growth stops) during skeletal development and conclude that the number and mitotic activity of the cells in the initial skeletal condensation emerges as a fundamental level of control. A developmental explanation for heterochrony should therefore focus on what controls the size of condensation and where and when they arise. Inductive tissue interactions regulate the formation of the condensations in which skeletal elements form, and the timing of these interactions can be modified genetically. As an example, evidence is presented to show that the timing of the interaction which leads to the formation of Meckel's cartilage in the lower jaw varies from one vertebrate group to another, being earlier in birds and later in mammals than it is in amphibians. Variation in the timing of inductive tissue interactions is identified as one mechanism controlling skeletal condensations, and therefore as one possible basis for heterochrony.