Fig. 1. Effects of the partial pressure of oxygen (PO2) on (A) thickness (L), (B) effective surface area (ESA) and (C) oxygen conductance (GO2) of Ambystoma annulatum egg capsules. Filled circles represent the mean proportional change in capsule dimensions from the beginning of the experiment to developmental stage 37–38. Open circles represent the mean proportional change in capsule dimensions from the beginning of the experiment to the conclusion of the experiment on day 12. Values are means ±1 S.E.M. (N=4–6). Egg capsule thickness decreased over the developmental period. However, the decrease in capsule thickness did not vary statistically with PO2. ESA generally increased as development proceeded, and the increase in ESA was inversely related to PO2. GO2 also increased over the developmental period, and the increase was inversely related to PO2.