Reference for Bekhorot 13:5

לאכילה נמי בעי ר' שמעון ראשו ורובו

May we not deduce from here<span class="x" onmousemove="('comment',' For it is R. Simeon who holds that an unclean animal born from a clean animal is forbidden, and since the prohibition of eating is put in the proximity of the expression of the head and the greater part of the body, we therefore may conclude that for eating purposes, as well as for the law of the firstling, the offspring must resemble the mother as regards its head and the greater part of the body.');">³</span>

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