Development of dependence on oxygen in embryo salamanders

Survival times in anoxia and hypoxia were measured at various stages of development in Ambystoma embryos and larvae of two species. Survival times in anoxia at 20 degrees C shifted from more than 30 h at 2 days after fertilization to 20 h at 10 days of age, to only 4--2 h at 14 days of age. In hypoxia (oxygen pressure equivalent to 3.8% oxygen) similar shifts of survival times appeared about 7 days of age later. During anoxia heart rates decreased, less at younger stages than at older. At older stages the heart stopped beating, sometimes irreversibly. In hypoxia also, hearts at all stages whether in situ or isolated decreased their rates of beat. Oxygen uptakes of larvae diminished in oxygen pressures even as high as 11% oxygen. This critical oxygen pressure did not change between early stages without blood flow and later stages with blood flow. Oxygen uptake was probably not limited by oxygen delivery but presumably by properties of cellular masses. No oxygen debts were paid off. Some parallel changes of tolerances to anoxia in embryo birds and mammals are noted.