Astanga Hrudaya
तेजो यथाऽर्करश्मीनां स्फटिकेन तिरस्कृतम्। नेन्धनं दृश्यते गच्छत्सत्वो गर्भाशयं तथा॥३॥
Simily to explain 'the invisibility of the soul entering the womb' Just as the heat of the rays of the sun intercepted by the lens is not seen getting on to the fuel (blades of grass, piece of wood or paper etc. on which it is focussed) kept underneath it, similarly is the entry of satva (atman-soul) into the womb (is not seen, but it can be known through the activities in the embryo.
english translation
tejo yathA'rkarazmInAM sphaTikena tiraskRtam। nendhanaM dRzyate gacchatsatvo garbhAzayaM tathA॥3॥
hk transliteration by Sanscript