The Brothers Grimm indicated the origin of Die Kristallkugel as Friedmund von Arnim's book, as tale nr.
14, Vom Schloss der goldnen Sonne ("The Castle of the Golden Sun").
She turned the oldest into an eagle and the second into a whale, and each could take his human form for only two hours a day.
The youngest son fled before he could suffer the same fate and went off to seek the king's daughter, bewitched and held prisoner in the Castle of the Golden Sun.
The bird sprang free, but his brother the eagle harried it until it dropped the egg.