Figure 2.

Hourglass model and tempo. The hourglass model proposes that mid-embryonic stages (phylotypic period) in vertebrate development represent the period of highest conservation, and that early and late developmental stages are highly variable. We propose that proportional temporal scaling (allochrony) is prevalent from the early to mid-stages of development. At late stages, there are frequent changes in the onset and/or duration of developmental processes (heterochronies). Embryos adapted from icons by BioRender.com.