Fig. 7. Effects of embryo development (age) and position of external gills in the oxygen gradient within eggs on the time red-eyed treefrog embryos remained in a position, after spontaneous movements. Deep positions within the egg are hypoxic, and positions near the air-exposed surface are better oxygenated. No embryos remained long with their gills in hypoxic regions of the egg, and younger embryos were more active. Data were taken from videotapes (means ± s.e.m., N=40 embryos per age).