According to the myth, Leda produced four offspring from two eggs: Castor and Clytemnestra from one egg, and Helen and Polydeuces (Pollux) from the other. Therefore (says Wikipedia – I'm not sure how this follows), Castor and Clytemnestra were fathered by Leda's husband, the Spartan king Tyndareus, while Helen and Polydeuces were fathered by Zeus.
© Haydn Thompson 2019