Reference for Chullin 135:23

איזהו בכור לנחלה ואינו בכור לכהנים הבא אחר נפלים אף על פי שיצא ראשו חי או בן ט' שיצא ראשו מת

Should you, however, say that [there] it was taught with regard to man, and [here] it is taught with regard to beasts, because we could not apply the principle as established in the case of beasts to man, inasmuch as there is no ante-chamber in beasts,<span class="x" onmousemove="('comment',' I.e., the forepart of the female genitals. So that as soon as the head emerges from the womb of the beast and sees the light of day it is forthwith regarded as born.');">¹²</span>

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