Reference for Niddah 112:12

תא שמע דתניא א"ר מאיר

rule that if a dead creeping thing was found in an alley it causes uncleanness retrospectively to such time as one can testify, 'I examined this alley and there was no creeping thing in it', or to such time as it was last swept?<span class="x" onmousemove="('comment',' Sc. why does not the uncleanness begin prior to the sweeping? ');">¹²</span>

Jastrow

Jastrow