Marsupial frog embryos develop under saline conditions typically found in the body in contrast with the aquatic mode of development found in "Xenopus" and many other frogs. Traditional frogs and marsupial frogs also differ in how their embryos excrete waste. Free-swimming tadpoles excrete ammonia, which would be toxic if accumulated in close quarters. Eugenia del Pino discovered that marsupial frog embryos excrete urea instead of ammonia. She discovered that addition of urea, which reaches high levels in the pouch, allows eggs to develop outside of the marsupial frog mother. Urea is a nitrogenous waste product that marsupial frog embryos use for water retention under the water stress conditions of the maternal pouch.