Fig. 2. Effect of external gills on oxygen levels that (A) limited metabolic rate in red-eyed treefrog embryos and tadpoles and (B) induced hatching, measured using closed-system respirometry. External gills improved oxygen uptake for embryos in terrestrial eggs, lowering both P_crit and the P_O2 that induced hatching. Embryos without gills were oxygen limited even in air (20.5 kPa). External gills had no effect on the P_crit of hatched tadpoles in water. Data are means ± s.e.m.; N=11 animals per treatment.