Figure 6. Differences among models.

Hypothetical stress (left), creep compliance (middle), and deformation (strain, ε; right) in the tissue. (A) The generalized model (Model 1) assumes the relative timing (ô) and the strains, of all events (1, 2, 3,…) are independent of temperature (cool (blue) vs warm (red)), as in a movie played faster. The generalized model does not specify the developmental course of strain or stress, only that timing scales with temperature. The generalized model predicts how stress and compliance vary together as temperature changes (Model 1, eqn.16). (B) Step and ramp models (Models 2A & 2B). The step and ramp models assume morphogenetic stresses are stepped (top) or ramped (bottom) with time. For a step stress (upper), the change in t_P with temperature does not affect the time t_C to reach strain ε_C (when the blastopore closes) because peak stress and compliance are unchanged (Model 2A, eqns. 21–22). A ramp is the sum of stress increments (gray lines; bottom left). Stress timing (hence the slope of the ramp) scales with t_P, and therefore with temperature (red, warm; blue, cool). The time t_C varies with t_P (and therefore temperature) for the ramp model (upper; Model 2B, eqns. 25–26), because strain increments follow the change in timing of stress increments (gray lines).