Shatapatha Brahmana 7.1.1.42

7.1.1.42

athaināṃ vimuñcati | apradāhāya yaddhi yuktaṃ na vimucyate pra taddahyata etadvā etadyuktā reto 'bhārṣīdetamagniṃ tamatrājījanadathāparaṃ dhatte yoṣā vā ukhā tasmādyadā yoṣā pūrvaṃ retaḥ prajanayatyathāparaṃ dhatte

He then unlooses it, to keep it from chafing; for if that which is yoked is not unloosed it is chafed. Now when yoked there, it (the fire-pan) bore this Agni within it as seed, and him it has now brought forth. It now conceives a second time; for the 'Ukhâ' is a female, and hence when a female has brought forth the seed the first time, it conceives a second time.