Śatapatha Brāhmaṇa ("of a hundred paths") 8.2.2.6

8.2.2.6

sa yaḥ sa prajāpatirvyasraṃsata | ayameva sa yo 'yamagniścīyate 'tha yā asmāttāḥ prajā madhyata udakrāmannetāstā vaiśvadevya iṣṭakāstadyadetā upadadhāti yā evāsmāttāḥ prajā madhyata udakrāmaṃstā asminnetatprapādayati retaḥsicorvelayā pṛṣṭayo vai retaḥsicau madhyamu pṛṣṭayo madhyata evāsminnetāḥ prajāḥ prapādayati sarvata upadadhāti sarvata evāsminnetāḥ prajāḥ prapādayati

Now the Pragâpati who become relaxed is this very Agni (fire-altar) that is now being built up; and the creatures who went forth from the midst of him are these same All-gods’ bricks; and when he lays these down, he causes those creatures, which went forth from the midst of him, to enter him. In the range of the Retahsik (he places the Vaisvadevî bricks), for the Retahsik are the ribs, and the ribs are the middle: he thus causes the creatures to enter him in the very middle. He places them on all sides: on all sides he thus causes the creatures to enter him.