Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.1.28

3.2.1.28

tām pratiparāmṛśyaveṣṭyācinat | tāṃ yajñasya śīrṣanpratyadadhādyajño hi kṛṣṇaḥ sa yaḥ sa yajñastatkṛṣṇājinaṃ yo sā yoniḥ sā kṛṣṇaviṣāṇātha yadenāmindra āveṣṭyācinattasmādāveṣṭiteva sa yathaivāta indro 'jāyata garbho bhūtvaitasmānmithunādevamevaiṣo 'to jāyate garbho bhūtvaitasmānmithunāt

Having seized and pressed it tightly, he tore it off and put it on the head of Yagña (sacrifice);for the black (antelope) is the sacrifice: the black deer skin is the same as that sacrifice, and the black deer's horn is the same as that womb. And because it was by pressing it tightly together that Indra tore out (the womb), therefore it (the horn) is bound tightly (to the end of the garment); and as Indra, having become an embryo, sprang from that union, so is he (the sacrificer), after becoming an embryo, born from that union (of the skin and the horn).