Reference for Chullin 148:51

אם כן שה שה מאי אהני לך

R'Johanan said: We do reckon here the first and second degree of uncleanness;<span class="x" onmousemove="('comment',' The foetus and dam are two separate entities; the former would therefore be unclean in one degree less than the latter.');">²⁶</span>

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