This paper presents a description of the embryonic development of Atlantic cod Gadus morhua. In addition, an equation describing the effect of temperature on developmental rate was developed. The whole period of embryogenesis was divided into 6 subperiods and 75 specific stages based on the distinctive characteristics. Temperature effects on the developmental rate of Atlantic cod embryos were studied at 5 different regimes of constant temperature in the range 1.9- 12.25ºC. During somitogenesis no less than 46 somites from a total number of 52-53 appear at equal time intervals. Embryos within single temperature batches showed very high synchronism of development: the deviation was no more than 1% from fertilization till hatching. In order to unify the developmental time under different temperature regimes, the age of embryos was measured in the relative units of τS (“tau somite”), which correspond to the time of formation of a single somite pair. The empirical τS values determined at different temperature regimes were used to define an equation describing the dependence of the developmental rate on temperature in Atlantic cod embryos. This dependence was expressed as: log τS (t) = 2.62025-0.06803t + 0.00087t2, where t is the temperature.