Śatapatha Brāhmaṇa ("of a hundred paths") 7.1.1.44

7.1.1.44

athāsyām paya ānayati | etadvā etadreto dhatte 'tha payo dhatte yoṣā vā ukhā tasmādyadā yoṣā reto dhatte 'tha payo dhatte 'dharāḥ sikatā bhavantyuttaram payo 'dharaṃ hi reta uttaram payastanmadhya ānayati yathā tatprati puruṣaśīrṣamupadadhyāt

He then pours milk into it,--it first receives seed, and now it receives milk; for the fire-pan is a female: hence when a female receives seed, then it receives milk. The sand is below, and the milk above, for the seed is below, and the milk above. He pours it into the middle, so that thereon he may place the human head.