Mishnah for Nazir 52:5

A bone the size of a barley-corn that is divided into two: Rabbi Akiva declares it unclean But Rabbi Yohanan ben Nuri declares it clean. Rabbi Yohanan ben Nuri says: they did not say 'bones’ the size of a barley-corn, but ‘bone’ the size of a barley-corn. A quarter [of a kav] of bones crushed fine, and there is not a single [bone] the size of a barley-corn: Rabbi Shimon declares it clean But the sages unclean. A limb [severed] from a living person which has been divided into two is clean. Rabbi Yose declares [it] unclean; But he agrees that if it is taken from a living person by halves it is clean.