Śatapatha Brāhmaṇa ("of a hundred paths") 8.2.4.18

8.2.4.18

sa vai purastādevāgra upadadhāti | śiro hi prathamaṃ jāyamānasya jāyate 'tha dakṣiṇata upadhāyottarata upadadhāti sārdhamayamātmā jāyātā ityatha paścātpucaṃ hyantato jāyamānasya jāyate

He first lays down those in front, for of an animal that is born the head is born first. Having then laid down those on the right (south) side, he lays down those on the left (north) side, thinking, 'Together with its sides this body shall be born.' Then those behind, for of (the animal) that is born the tail is born last.