Shatapatha Brahmana 10.4.2.29

10.4.2.29

sa yadagniṃ ceṣyamāṇo dīkṣate yathaiva tatprajāpatireṣu triṣu lokeṣūkhāyāṃ yonau reto bhūtamātmānamasiñcadevamevaiṣa etadātmānamukhāyāṃ yonau reto bhūtaṃ siñcati candomayaṃ stomamayam prāṇamayaṃ devatāmayaṃ tasyārdhamāse prathama ātmā saṃskriyate davīyasi paro davīyasi paraḥ saṃvatsara eva sarvaḥ kṛtsnaḥ saṃskriyate

Now when he (the Sacrificer), being about to build an altar, undergoes the initiation-rite,--even as Pragâpati poured his own self, as seed, into the fire-pan as the womb,--so does he pour into the fire-pan, as seed into the womb, his own self composed of the metres, stomas, vital airs, and deities. In the course of a half-moon, his first body is made up, in a further (half-moon) the next (body), in a further one the next,--in a year he is made up whole and complete.