One of the breakdown products of nitrogen metabolism is ammonia. However, ammonia is very toxic to tissues, and it must be excreted or removed as rapidly as it is formed. This presents no problem to many aquatic non-vertebrate organisms which simply excrete the ammonia to the surrounding water as it is formed. Terrestrial animals cannot do this, obviously; they and the marine fish could flush out the ammonia with a large volumes of urine but they must conserve water. How do they solve this dilemma? Thera are two principal ways of getting around this problem. Animals like man, that can afford to expend some water, detoxify ammonia to urea, which can be excreted with a moderate amount of water loss through the kidneys. Some animals, however, cannot afford to lose even that small amount of water; these include snakes and lizards, most birds, and insects. They convert ammonia to non-toxic uric acid which can be stored in the body for relatively long periods and can be excreted as a paste with little water or in a dry form with almost no water.
An animal may exrete nitrogen in various forms at different times during its life. For example, the tadpole, which is aquatic, excretes ammonia, but the adult frog, faced as it is with water loss through its very permeable skin, cannot spare enough water to exrete ammonia, and so it excretes urea. Interestingly the nitrogenous waste product which an animal excretes depends on where its embryo develops. Birds and reptiles develop in eggs which have tough, water-proof coverings; they can manage this because the embryo secretes non-toxic uric acid which can be stored until they animal hatches, and no harm is done. The embryos of man and of many other mammalian vertebrates that develop within the uterus excrete urea, which in turn is carried away from the placenta by the mother's blood to be excreted in water by the mother's kidneys.