Shatapatha Brahmana 8.6.3.11

8.6.3.11

tāṃ haike jaghanārdha upadadhati | jaghanārdhādvai retaḥ sicyate pucasaṃdhau pucādvai retaḥ sicyata iti na tathā kuryādbahirdhā ha te yone retaḥ prajātiṃ dadhati ye tathā kurvanti madhya evopadadhyāttatsamprati yonau retaḥ prajātiṃ dadhāti

Now some lay it down on the hind part (of the bird-like altar), because it is from the hind part that seed is introduced,--(to wit) on the juncture of the tail (and the body), for it is from (the part near) the tail that seed is introduced. Let him not do this, for they who do this lay seed and generative power outside the womb; but let him rather place it in the centre: he thus lays seed and generative power right into the womb.