BT Men 42b(34) :ושקלינן פורת׳ בביעתא וטעמינן לה באוד{ר}<ד>א‏ : we take a small amount of (the tekhelet dye) inside an eggshell and examine it with a wad of wool
See the entire entry for bēˁā, bēˁṯā n.f. egg