Śatapatha Brāhmaṇa ("of a hundred paths") 4.6.3

Now as to the manner of animal offerings. One may perform with the (ordinary) set of eleven victims. He seizes one for Agni as the first victim, and one for Varuna (as the last); then again one for Agni.: in this way let him perform with the set of eleven victims.

Or one may day after day seize a victim for Indra and Agni; for all the gods are Agni, since in Agni offering is made to all the deities; and Indra is the deity of the sacrifice: thus he neither offends any of the deities, nor does he offend him who is the deity of the sacrifice.

Then as to the manner (of animal offering) in accordance with the Stoma. At the Agnishtoma let him seize a (victim) for Agni; for it is befitting that at the Agnishtoma ('Agni's praise') he should seize a victim for Agni. If it be an Ukthya sacrifice, let him seize as the second (victim) one for Indra and Agni, for the hymns (uktha) belong to Indra and Agni. If it be a Shodasin sacrifice, let him seize as the third (victim) one for Indra; for the Shodasin (graha) is Indra. If it be an Atirâtra, let him seize as the fourth (victim) one for Sarasvatî; for Sarasvatî is Vâk (speech), and Vâk is a female, and so is râtri (fem., 'night') female. Thus he duly distinguishes between the sacrificial performances. Such are the three manners (of animal offering): he may perform in whichever manner he pleases. Two victims must needs be seized,--for Sûrya he seizes the second en the Vishuvant day, and for Pragâpati at the Mahâvrata.