Shatapatha Brahmana 6.5.2.1

6.5.2.1

atha mṛtpiṇḍamapādatte | yāvantaṃ nidhaye 'lam makhasya śiro 'sīti yajño vai makhastasyaitacira āhavanīyo vai yajñasya śira āhavanīyamu vā etaṃ ceṣyanbhavati tasmādāha makhasya śiro 'sīti

He then takes a lump of clay, as much as he thinks sufficient for the bottom part, with, 'Makha's head thou art!'--Makha, doubtless, is the sacrifice, and this is its head; for the Âhavanîya fire is the head of the sacrifice, and that Âhavanîya (fire-altar) he is now about to build: hence he says, 'Makha's head thou art!'