Chandogya Upanishad 6.3

'Of all living things there are indeed three origins only, that which springs from an egg (oviparous), that which springs from a living being (viviparous), and that which springs from a germ.

'That Being (i.e. that which had produced fire, water, and earth) thought, let me now enter those three beings (fire, water, earth) with this living Self (gîva âtmâ), and let me then reveal (develop) names and forms.

'Then that Being having said, Let me make each of these three tripartite (so that fire, water, and earth should each have itself for its principal ingredient, besides an admixture of the other two) entered into those three beings (devatâ) with this living self only, and revealed names and forms.

'He made each of these tripartite; and how these three beings become each of them tripartite, that learn from me now, my friend!