Śatapatha Brāhmaṇa ("of a hundred paths") 7.5.1.26

7.5.1.26

athokhāmupadadhāti yonirvā ukhā yonimevaitadupadadhāti tāmulūkhala upadadhātyantarikṣaṃ vā ulūkhalaṃ yadvai kiṃ cāsyā ūrdhvamantarikṣameva tanmadhyaṃ vā antarikṣam madhyatastadyoniṃ dadhāti tasmātsarveṣām bhūtānām madhyato yonirapi vanaspatīnām

He then places the fire-pan thereon,--the fire-pan is a womb: a womb (birth-place) he thus gives to him (Agni). He places it on the mortar,--the mortar is the air, and everything that is above this earth is air; and the air is the middle: he thus places the womb in the middle; whence the womb of all beings, even of trees, is in the middle.