allantois allantoinuria • allantoxaidin · · · Info · Definitions · Characters · · allantois · · Text Latin · · Script 9 · · Length Definitions · · · (noun) the vascular fetal membrane that lies below the chorion and develops from the hindgut in many embryonic higher vertebrates (reptiles, birds and mammals). · · Noun Definition 1 Characters · · a latin small letter a U+0061 i latin small letter i U+0069 l latin small letter l U+006C n latin small letter n U+006E o latin small letter o U+006F s latin small letter s U+0073 t latin small letter t U+0074