Embryonic development time has been of long-standing interest to physiologists and ecologists because of its importance in understanding the behaviour and ecology of reproduction, and the evolution of reproductive strategies in animals. Yet, the relationship of embryonic development time to both neonate mass and incubation temperature remains poorly understood for most taxonomic groups. Here we define the relationship of embryonic development time to neonate mass (embryo mass at birth) and incubation temperature for a broad range of animals, ectothermic and endothermic, invertebrate and vertebrate, microscopic and gigantic. We begin by establishing the relationship of embryonic development time to neonate mass for a broad array of zooplankton, fishes and amphibians for temperatures from 5 to 20°C. Next, we compare the relationships of embryonic development time to neonate mass in these aquatic ectotherms to those for terrestrial ectotherms (reptiles) and endotherms (birds and mammals) in terms of degree-days. The similar nature of these relationships allows us to define embryonic development time as a function of neonate mass for many of the species considered here with a single equation in terms of degree-days for incubation temperatures ranging from 5 to 38°C. This relationship establishes a basis by which to compare differences in embryonic development time among species.