Shatapatha Brahmana 7.1.1.16

7.1.1.16

tadvai yoniḥ pariśritaḥ | ulbamūṣā retaḥ sikatā bāhyāḥ pariśrito bhavantyantara ūṣā bāhyā hi yonirantaramulbam bāhya ūṣā bhavantyantarāḥ sikatā bāhyaṃ hyulbamantaraṃ reta etebhyo vai jāyamāno jāyate tebhya evainametajjanayati

The enclosing-stones, then, are the womb; the saline earth is the amnion, and the sand is the seed. The enclosing-stones are outside, and the saline earth is inside; for the womb is outside, and the amnion inside. The saline earth is outside, and the sand inside; for the amnion is outside, and the seed inside. He who is born is born from these: it is from them that he thus causes him (Agni) to be born.