Reference for Bekhorot 13:6

לא לבכורה דוקא

that even as regards permission to eat, R'Simeon requires the head and the greater part of the body to be [similar to its mother? ] - No, only as regards [the law] of the firstling.<span class="x" onmousemove="('comment',' Does R. Simeon require that the head and the greater part of the body must be similar to its mother.');">⁴</span>

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