Śatapatha Brāhmaṇa ("of a hundred paths") 4.4.2.9

4.4.2.9

sa yatrādhvaryuḥ | śālākairdhiṣṇyānvyāghārayati tatpratiprasthātā pātnīvataṃ grahaṃ gṛhṇāti yajñādvai prajāḥ prajāyante yajñātprajāyamānā mithunātprajāyante mithunātprajāyamānā antato yajñasya prajāyante tadenā etadantato yajñasya mithunātprajananātprajanayati tasmānmithunātprajananādantato yajñasyemāḥ prajāḥ prajāyante tasmātpātnīvataṃ gṛhṇāti

Now, while the Adhvaryu pours ghee on the hearths by means of fagots, the Pratiprasthâtri draws the Pâtnîvata cup. For from the sacrifice creatures are produced; and being produced from the sacrifice, they are produced from union; and being produced from union, they are produced from the hind part of the sacrifice;--hence he thereby produces them from a productive union, from the hind part of the sacrifice: therefore he draws the Pâtnîvata cup.