The effect of varying substrate moisture contents and incubation temperatures on developing snapping turtle embryos was addressed. Mass change was measured for several clutches of snapping turtle eggs within field nests. Throughout the incubation period, a subset of the total number of nests was measured on either a weekly or a bi-monthly interval. Mass of each egg within a particular nest was separately weighed and inspected. Nest microclimate measurements, such as soil water potential and volumetric water content, were also measured on a regular basis. The patterns of egg mass change, which have previously been determined to be due to water exchange of the egg with the nesting environment, were compared with soil water content. Viable eggs, which produce living hatchlings, have positive water balances, with a net water uptake over the course of incubation. Eggs which are infertile or in which the embryo dies have negative water balances. Furthermore, egg water exchange has been determined to be independent of those soil water contents experienced at the present test site;The phenomenon of temperature dependent sex determination was also analyzed in the common snapping turtle Chelydra serpentina. Field and laboratory treatments were undertaken to investigate the mechanism of action. Copper constantan thermocouples were placed within field nests to record hourly temperature averages over the course of the field incubation period. Similar thermocouples were placed within climate-controlled growth chambers programmed to alter temperature treatments. Following incubation, offspring sex ratios were determined and compared among the different treatments. The influence of varying egg geographic origin on resulting offspring sex ratios was also analyzed. Both field and laboratory-incubated eggs required approximately 150 degree-hours to produce at least 50% female offspring. Those eggs originating from more southern locations required fewer degree hours to produce a given percentage of female turtles. In other words, more female offspring from lower latitudes were produced within a particular temperature treatment than resulted from more northern latitudes.