Reference for Chullin 143:33

הוי אומר זו רביעית דם

<big><b>MISHNAH: </b></big>IF AN ANIMAL WAS IN DIFFICULT LABOUR AND THE FOETUS PUT FORTH ITS FORE-LIMB AND A PERSON IMMEDIATELY CUT IF OFF AND THEN SLAUGHTERED THE DAM, THE FLESH [OF THE FOETUS]<span class="x" onmousemove="('comment',' It will not be rendered unclean from contact with the protruded limb because it is in the womb and is part of a living animal and it is established law that a living animal cannot contract uncleanness. V. supra ');">²¹</span>

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