Śatapatha Brāhmaṇa ("of a hundred paths") 4.3.2

Now truly when the Hotri praises (recites the sastra), he sings, and to him thus singing the Adhvaryu responds (prati-â-gar), whence the name response (pratigara).

[The Hotri] calls upon that (Adhvaryu) seated (before him) with his face towards the east. For all others except the Udgâtri perform their priestly duties while facing the east, and in this manner that priestly duty of his is performed towards the east.

Now the Udgâtri is Pragâpati, and the Hotri, (being) the Rik (fem.), is a female. And when he chants, then the Udgâtri, Pragâpati, implants seed in the female Hotri, the Rik; this the Hotri brings forth by means of the sastra (recitation), he sharpens it even as this man is sharpened, and because he thereby sharpens (so) therefore it is called sastra.

Having turned round (so as to face the Hotri, the Adhvaryu) then responds: thereby he quickens that implanted seed. On the other hand, were he to respond while standing with his face turned away (from the Hotri), that implanted seed would assuredly perish away, and would not be brought forth; but thus facing each other (the male and female) bring forth the implanted seed.

Now the strength of the metres was exhausted by the gods, for it was by the metres that the gods attained the world of heaven. And the response (song) is ecstasy (mada)--what ecstasy there is in the rik and that which there is in the Sâman, that is sap: this sap he now lays into the metres, and thus makes the metres of restored strength; and with them of restored strength they perform the sacrifice.

Hence if (the Hotri) recites by half-verses, let (the Adhvaryu) respond at each half-verse; and if he recites by pâdas (hemistichs), let him respond at each pâda. For whenever, in reciting, he (the Hotri) draws breath, there the Asura-Rakshas rush into the sacrifice: there he (the Adhvaryu) closes it up by means of the response, so that the evil spirits, the Rakshas, cannot rush in; and thus he destroys the world of the sacrificer's enemies.

Now, in the beginning the metres consisted of four syllables. Then Gagatî flew up for Soma and came back, leaving behind three syllables. Then Trishtubh flew up for Soma and came back, leaving behind one syllable. Then Gâyatrî flew up for Soma, and she came back bringing with her those syllables as well as Soma. Thus she came to consist of eight syllables: wherefore they say, 'Gâyatrî is octosyllabic.'

With her they performed the morning feast of the Soma-sacrifice,--whence the morning feast pertains to Gâyatrî. With her they performed the midday feast. Trishtubh then said to her, 'To thee will I come with three syllables: invite me, and exclude me not from the sacrifice!'--'So be it!' she said and invited her. Thus the Trishtubh came to consist of eleven syllables, and therefore they say, 'The midday Soma feast pertains to Trishtubh.'

With her (Gâyatrî) indeed they performed the evening feast. Gagatî then said to her, 'To thee will I come with one syllable: invite me, and exclude me not from the sacrifice!'--'So be it!' she said and invited her. Thus the Gagatî came to consist of twelve syllables; and therefore they say, 'The evening Soma feast pertains to Gagatî.'

As to this they say, 'Surely all the Soma feasts pertain to Gâyatrî, since Gâyatrî alone went on increasing.' At the morning feast he should therefore respond with a complete (formula), for complete Gâyatrî returned. At the midday feast (he responds with a formula) containing once (the verb) 'to rejoice (mad),' for she (Trishtubh) came back, leaving one syllable behind; and with that same (formula) he then completes her, makes her whole,

When trishtubh verses were recited. At the evening Soma feast (the Adhvaryu responds with a formula) containing thrice (the verb) 'to rejoice,' for she (Gagatî) came back leaving three syllables behind; and with these (formulas) he then completes her, makes her whole,--

When (the hymn) to Heaven and Earth is recited. Now these creatures subsist on those two, the heaven and the earth--he thereby imbues those two, heaven and earth, with vigour; and upon those two, thus vigorous and affording the means of subsistence, these creatures subsist. Let him respond with 'Om!' only, for that is truth, that the gods know.

Now some respond with 'Othâmo daiva vâk,' saying, 'The response is speech (vâk): thus we obtain speech.' But let him not do this; for surely, in whichsoever way he may respond, speech is obtained by him, since he responds by speech. Let him therefore respond with 'Om!' only, for that is truth, that the gods know.