Developmental models have been constructed for many plant and insect species, but not for reptiles. A developmental growth model can be useful to determine the factors limiting species' distribution or determining sex under naturally fluctuating temperatures. Development rate of embryos in many turtle species is positively correlated with incubation temperature. The growth model proposed in this study suggests that developmental stage, 's', is a function of accumulated heat units, 'u'('t'), and time, 't', since oviposition ('s' = k0 + 'k' u'u'('t') + 'k't' t'). The model built uses developmentally staged snapping turtle (' Chelydra serpentina') embryos collected from 17 females over 7 years in Algonquin Park, Ontario. The growth model explains 87.1% of the variation in developmental stage for all 7 years (n = 129, p < 0.0001). Year accounts for an additional 6.3% of variation in developmental stage. Using the growth model, accumulated heat units were estimated over the critical period for sex determination at constant temperature and suggest that total heat does not determine sex.