Uric acid is the least toxic nitrogenous waste. It is almost water-insoluble and tends to form a white paste or powder or remain in a semisolid form. The animals which excrete uric acid are referred to as uricotelic animals. As uric acid requires the least amount of water for its removal, it is one of the best ways to conserve water in uricotelic animals. Uric acid is excreted by birds, insects and most of the reptiles. Excretion of the nitrogenous wastes in the form of uric acid is specifically advantageous for land vertebrates which lay shelled eggs; the shelled eggs of reptiles & birds have numerous minute pores which allow the exchange of gases only. If the embryo had produced ammonia or urea inside the shelled egg, these toxic wastes would have accumulated to lethal concentration levels. But as the wastes are in the form of uric acid, which is a thousand times less soluble than the NH3 or urea, this uric acid precipitates out of the solution and can be stored in the shell as a solid waste which is left behind when the animal hatches.