Here you see the birth of Helen, Castor, and Pollux from their respective eggs, watched over by Zeus, Hera and Eros (Cupid). Zeus took the form of a swan in order to seduce the lovely mortal woman Leda, and so the children emerge from eggs. Leda also slept with her husband Tyndareus on the same night, so she has a mix of human and mortal children from both of her consorts. The resulting children are very famous: lovely Helen (later "Helen of Troy"), her sister Clytemnestra (yes, the one who killed her husband Agamemnon when he came home from the war), and the famous twins Castor and Pollux. This artist has excluded Clytemnestra from the scene (presumably she was purely the offspring of Leda and Tyndareus and of no interest to the proud papa Zeus). Hera was normally not very happy about Zeus's love affairs, but she seems rather satisfied with the three children hatched here from the eggs.