Śatapatha Brāhmaṇa ("of a hundred paths") 3.1.3.28

3.1.3.28

athainaṃ śālām prapādayati | sa jaghanenāhavanīyametyagreṇa gārhapatyaṃ so 'sya saṃcaro bhavatyā sutyāyai tadyadasyaiṣa saṃcaro bhavatyā sutyāyā agnirvai yoniryajñasya garbho dīkṣito 'ntareṇa vai yoniṃ garbhaḥ saṃcarati sa yatsa tatraijati tvatpari tvadāvartate tasmādime garbhā ejanti tvatpari tvadāvartante tasmādasyaiṣa sacaro bhavatyā sutyāyai

He (the Adhvaryu) then makes him enter the hall. He walks along the back of the Âhavanîya and the front of the Gârhapatya,--this is his passage until the Soma pressing. The reason why this is his passage until the Soma pressing is this. The fire is the womb of the sacrifice, and the consecrated is an embryo; and the embryo moves about within the womb. And since he (the sacrificer) moves about there (between the fires), and now turns round and now back, therefore these embryos move about, and now turn round and now back. Hence this is his passage till the Soma pressing.