Fig. 1. Examples of oxygen consumption rates measured at different oxygen levels for individual 5-day-old embryos and newly hatched tadpoles of red-eyed treefrogs with and without external gills. Embryo measurements stopped at hatching. Lines show regression fits used to estimate P_crit (arrows); data points above and below P_crit are indicated by filled and open circles, respectively. Note that the x-axis location for the gilless embryo is shifted relative to that for the other animals.