Śatapatha Brāhmaṇa ("of a hundred paths") 3.3.3.12

3.3.3.12

athātrāporṇute | garbho vā eṣa bhavati yo dīkṣate prāvṛtā vai garbhā ulveneva jarāyuṇeva tamatrājījanata tasmādaporṇuta eṣa vā atra garbho bhavati tasmātparivṛto bhavati parivṛtā iva hi garbho ulbeneva jarāyuṇeva

He now uncovers (his head); for he who is consecrated becomes an embryo, and embryos are enveloped both in the amnion and the outer membrane: him (the sacrificer or sacrifice) he has now brought forth, and therefore he uncovers himself. Now it is he (Soma) that becomes an embryo, and therefore he is enveloped, since embryos are, as it were, enveloped both in the amnion and the outer membrane.