Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.1.11

3.2.1.11

sā vai śāṇī bhavati | mṛdvyasaditi nveva śāṇī yatra vai prajāpatirajāyata garbho bhūtvaitasmādyajñāttasya yannediṣṭhamulbamāsītte śaṇāstasmātte pūtayo vānti yadvasya jarāyvāsīttaddīkṣitavasanamantaraṃ vā ulbaṃ jarāyuṇo bhavati tasmādeṣāntarā vāsaso bhavati sa yathaivātaḥ prajāpatirajāyata garbho bhūtvaitasmādyajñādevamevaiṣo 'to jāyate garbho bhūtvaitasmādyajñāt

It is made of hemp. Hempen it is in order to be soft. Now when Pragâpati, having become an embryo, sprung forth from that sacrifice, that which was nearest to him, the amnion, became hempen threads: hence they smell putrid. And that which was the outer membrane (and placenta) became the garment of the consecrated. Now the amnion lies under the outer membrane, and hence that (zone) is worn under the garment. And in like manner as Pragâpati, having become an embryo, sprung forth from that sacrifice, so does he become an embryo and spring forth from that sacrifice.