496 Physiology by E. B. Mayrs and T. M. Watt, who have described tests on experimental animals performed by them, having regard to the fact that urea is less efficiently eliminated by the kidneys than sulphates. phosphates, and creatinine. Urea is probably not a useless waste product, as is generally supposed, but a substance which is retained in the body until its concentration reaches some low threshold value, at which the excess begins to appear in the urine.