During the Australian summers of 1996/1997 and 1997/1998, the embryonic development of Natator depressus was investigated in the laboratory. Eggs were incubated under different thermal and hydric conditions on vermiculite substrates. The thermal environment between 26 and 32 C significantly influenced the water exchange of eggs, incubation duration, nutrient mobilization of embryos, and the size, tissue hydration, and energy reserves of hatchlings. Hatchlings produced at 26 C and 29 C were larger but had lower energy reserves than those produced at 32 C. The influence of the hydric environment depended greatly on the range of substrate water potentials used in experiments. Nutrient mobilization of embryos and the size and energy reserves of hatchlings were dependent on total egg water exchange over the range of 2% gain to 29% loss (at approximately −180 to −3500 kPa incubation substrates) of initial egg mass but independent within the narrower range of 6% gain to 19% loss (at approximately −200 to −650 kPa substrates). Hatching was affected only when eggs lost more than 21% of their initial egg mass (at approximately −1300 kPa). A significant effect of clutch on the morphological and physiological aspects of developing embryos indicated that genetic/maternal factors influenced these traits. Natator depressus eggs seem to be laid with sufficient water reserves for successful embryonic growth and development. In most natural circumstances, this would make them independent of supplementary water from the environment.