Timers in developing systems.

Conditional methods are proposed for investigating the number and relationships of processes that are rate-limiting for the genesis of consecutive stages in a developmental sequence. These methods depend on the differential sensitivity of "timer" pathways to small changes in temperature and can be applied to any developmental sequence in which discrete stages can be reproducibly monitored with time. We have applied the methods to multicellular morphogenesis in the slime mold Dictyostelium discoideum and have obtained an unexpected tentative scheme for timer relationships. A minimum of six timers has been delineated, each specific for at least one morphological stage. The majority of these timers appear to be in parallel.