The duration of embryonic development (e.g., egg incubation period) is a critical life‐history variable because it affects both the amount of time that an embryo is exposed to conditions within the nest and the seasonal timing of hatching. Variation in incubation periods among oviparous reptiles might result from variation in either the amount of embryogenesis completed before laying or the subsequent developmental rates of embryos. Selection on incubation duration could change either of those traits. We examined embryonic development of fence lizards (Sceloporus undulatus) from three populations (Indiana, Mississippi, and Florida) that occur at different latitudes and therefore experience different temperatures and season lengths. These data reveal countergradient variation: at identical temperatures in the laboratory, incubation periods were shorter for lizards from cooler areas. This variation was not related to stage at oviposition; eggs of all populations were laid at similar developmental stages. Instead, embryonic development proceeded more rapidly in cooler‐climate populations, compensating for the delayed development caused by lower incubation temperatures in the field. The accelerated development appears to occur via an increase in heart mass (and, thus, stroke volume) in one population and an increase in heart rate in the other. Hence, superficially similar adaptations of embryonic developmental rate to local conditions may be generated by dissimilar proximate mechanisms.