Unbeknownst to Tyndareus, Leda was carrying on an affair with Zeus during the course of their marriage. The god would come to her regularly donning the shape of a swan, and before long Leda realized she was pregnant. She went on to give birth to four children in a most unusual way.
Leda laid two eggs; from one hatched Castor and Clytemnestra, who were fathered by Tyndareus and mortal. From the other egg hatched Helen and Polydeuces, who were fathered by Zeus and therefore half divine. Both Helen and Clytemnestra became famous for their roles in the Trojan War. Castor and Polydeuces went on to become well respected warriors known as the Discouri. They were later placed in the among the stars as the constellation Gemini.
Leda & the swan by Yeats
A sudden blow: the great wings beating still
Above the staggering girl, her thighs caressed
By the dark webs, her nape caught in his bill,
How can those terrified vague fingers push
The feathered glory from her loosening thighs?
And how can body, laid in that white rush,
And how can body, laid in that white rush,
But feel the strange heart beating where it lies?
A shudder in the loins engenders there
The broken wall, the burning roof
and tower And Agamemnon dead.
Being so caught up, so mastered by the brute blood of the air,
Did she put on his knowledge with his power
Did she put on his knowledge with his power
before the indifferent beak could let her drop?
Where the slow river meets the tide,
a red swan lifts red wings and darker beak,
and underneath the purple downof his soft breast uncurls his coral feet.
the level ray of sun-beam has caressed the lily with dark breast, and flecked with richer gold its golden crest.
and lifts the yellow flags, he floats where tide and river meet.
Ah kingly kiss--no more regret nor old deep memoriesto mar the bliss;
where the low sedge is thick, the gold day-lily outspreads
and rests beneath soft fluttering of red swan wings
and the warm quivering of the red swan's breast.
