Reference for Bekhorot 43:3

ואמאי אין מטמא לא במגע ולא במשא

Then why does it not make unclean by contact nor make the carrier unclean? - R'Johanan answered: Because the principle of neutralisation by the larger portion is applied here.<span class="x" onmousemove="('comment',' The blood and the multi-coloured substance being the larger portion neutralizes the flesh of the embryo and therefore the latter is not susceptible of levitical uncleanness.');">¹</span>

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