Śatapatha Brāhmaṇa ("of a hundred paths") 3.8.5.6

3.8.5.6

atha jāghanyā patnīḥ saṃyājayanti | jaghanārdho vai jāghanī jaghanārdhādvai yoṣāyai prajāḥ prajāyante tatpraivaitajjanayati yajjāghanyā patnīḥ saṃyājayanti

Thereupon they perform the Patnîsamyâgas with the tail (of the victim), for the tail is the hind-part, and from the hind-part of woman offspring is produced: hence offspring is produced by the Patnîsamyâgas being performed with the tail.