According to the Vedic process, one can get a son through semen (sukra), one can get a bona fide disciple through initiation (savitra), or one can get a son or disciple through the fire of sacrifice (yajna). Thus when Maharaja Pururava generated the fire by rubbing the aranis, the fire became his son. Either by semen, by initiation or by yajna one may get a son. The Vedic mantra omkara, or pranava, consisting of the letters a-u-m, can call each of these three methods into existence. Therefore the words nirmanthanaj jatah indicate that by the rubbing of the aranis a son was born.