Śatapatha Brāhmaṇa ("of a hundred paths") 9.3.1.2

9.3.1.2

yadvevaitaṃ vaiśvānaraṃ juhoti | vaiśvānaraṃ vā etamagniṃ janayiṣyanbhavati tamadaḥ purastāddīkṣaṇīyāyāṃ reto bhūtaṃ siñcati yādṛgvai yonau retaḥ sicyate tādṛgjāyate tadyattatra vaiśvānaraṃ reto bhūtaṃ siñcati tasmādayamiha vaiśvānaro jāyata upāṃśu tatra bhavati reto vai tatra yajña upāṃśu vai retaḥ sicyate nirukta iha niruktaṃ hi reto jātam bhavati

And, again, as to why he offers the Vaisvânara (cake),--it is as the Vaisvânara that he is about to produce that Agni: on that former occasion, at the initiation-offering, he pours him out in the form of seed: and what the seed is like that is poured. into the womb, such like (is the child that) is born; and inasmuch as there he pours out Vaisvânara in the form of seed, therefore he now is born as Vaisvânara. Silently it is (performed) there, for there seed (is implied) in the sacrifice, and silently seed is infused; but distinctly on the present occasion, for distinct (manifest) is the seed when born.