 Urea is a nitrogenous waste product that marsupial frog embryos use for water retention under the water stress conditions of the maternal pouch.Eggs of these frogs are very large, ranging from 3 to 10 mm in diameter in different species, and contain the nutrients needed for development up to metamorphosis.She found that "Gastrotheca" develops much like a chick embryo on the surface of the yolk.