Śatapatha Brāhmaṇa ("of a hundred paths") 6.6.2.9

6.6.2.9

taddhaike | yadi ciramarcirārohatyaṅgārānevāvapantyubhayenaiṣo 'gniriti na tathā kuryādasthanvānvāva paśurjāyate 'tha taṃ nāgra evāsthanvantamiva nyṛṣanti reta ivaiva dadhati reta u etadnasthikaṃ yadarcistasmādenāmarcirevārohet

Now, if the flame is too long in mounting up, some throw coals on (the pan), thinking, 'There is fire now on both sides.' But let him not do so; for the animal is indeed born with bones; but it is not forced in with bones, as it were, at first; but it is introduced only as seed. Now that flame is boneless seed: hence the flame alone should mount up to it.