Śatapatha Brāhmaṇa ("of a hundred paths") 6.5.2.2

6.5.2.2

yadvevāha makhasya śiro 'sīti | jāyata eṣa etadyaccīyate śīrṣato vai mukhato jāyamāno jāyate śīṣato mukhato jāyamāno jāyātā iti

And, again, as to why he says, 'Makha's head thou art!'--when he (Agni) is built up, then he is born, and it is by the head (issuing first), by the top, that he who is born is born: 'when he is born, may he be born by the head, by the top!' so he thinks.