Formalism for heterochronic processes and their morphological expressions.

Redrawn and Alberch et al. [15] and after Reilly et al. [18]. Paedomorphosis and peramorphosis can result from multiple perturbations of the three developmental parameters: rate, onset and offset. Each of the simple perturbation trajectories (shaded boxes) can be shifted by one, the other, or both of the other two parameters (circles and rectangles). a, the age at the onset of development; b, the age at offset of development; kr, the rate of development (i.e. the rate of change in shape); and ks, the rate of growth (i.e. the rate of change in size). See also Table 1.