sthali-sthanam gato ’svattham
sami-garbham vilaksya sah
tena dve arani krtva
urvasim mantrato dhyayann
atmanam ubhayor madhye
yat tat prajananam prabhuh
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
When the process of fruitive yajña became manifest within his heart, King Pururava went to the same spot where he had left Agnisthali. There he saw that from the womb of a sami tree, an asvattha tree had grown. He then took a piece of wood from that tree and made it into two aranis. Desiring to go to the planet where Urvasi resided, he chanted mantras, meditating upon the lower arani as Urvasi, the upper one as himself, and the piece of wood between them as his son. In this way he began to ignite a fire.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The Vedic fire for performing yajña was not ignited with ordinary matches or similar devices. Rather, the Vedic sacrificial fire was ignited by the aranis, or two sacred pieces of wood, which produced fire by friction with a third. Such a fire is necessary for the performance of yajña. If successful, a yajña will fulfill the desire of its performer. Thus Pururava took advantage of the process of yajña to fulfill his lusty desires. He thought of the lower arani as Urvasi, the upper one as himself, and the middle one as his son. A relevant Vedic mantra quoted herein by Visvanatha Cakravarti Thakura is sami-garbhad agnim mantha. A similar mantra is urvasyam urasi pururavah. Pururava wanted to have children continuously by the womb of Urvasi. His only ambition was to have sex life with Urvasi and thereby get a son. In other words, he had so much lust in his heart that even while performing yajña he thought of Urvasi, instead of thinking of the master of yajña, Yajñesvara, Lord Visnu.