Abstract
1. 1. Specimens of Xenopus laevis, when kept out of water for 1–3 weeks, excreted little nitrogen. On return to water, large amounts of urea, but only normal amounts of ammonia were excreted. 2. 2. Accumulation of urea in blood, liver and muscle during dehydration was demonstrated. 3. 3. Urea accumulation could also be induced by keeping the animals in 0·9 per cent sodium chloride. 4. 4. Bufo calamita kept in a dry tank also accumulated urea. 5. 5. The rate of excretion of accumulated urea in Xenopus appeared to follow an exponential plot, indicating passive excretion. In Bufo calamita, the excretion plot was not exponential. 6. 6. The adaptive significance of urea accumulation during dehydration is discussed. In Xenopus, urea represents an “emergency” waste product, replacing ammonia in dry conditions. 7. 7. A theory of the evolution of ureotelism in Amphibia is suggested.
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