Reference for Bekhorot 32:1

ולדותיהן פטורין ולדי ולדותיהן חייבין

THEIR OFFSPRING ARE EXEMPT [FROM THE LAW OF] THE FIRSTLING.<span class="x" onmousemove="('comment',' For, in the first place, a half of the offspring belongs to the heathen and secondly the latter will seize the offspring if he cannot have the flock');">¹</span>

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