Reference for Chullin 178:22

ומי מצית מוקמת לה בשליל והא מדקתני סיפא נוהג בשליל מכלל דרישא לאו בשליל עסקינן

And he is of the opinion that it [sc. the prohibition of the sciatic nerve] applies to a foetus, and also that the young of a consecrated animal is holy even when in its dam's womb; accordingly the prohibition of the nerve and the prohibition of consecrated things come into force simultaneously.<span class="x" onmousemove="('comment',' I.e., the moment the foetus was formed within the dam's womb.');">¹³</span>

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