Mishnah for Bekhorot 45:15

These things defile by overshadowing: a corpse, an olive-sized [portion of flesh] of a corpse, an olive-sized [portion] of nezel, a ladleful of corpse-mold, the spine or the skull, [a] full limb of a corpse, or [a full] limb [severed] from a living person with the appropriate amount of flesh, a quarter [of a kav] of bones from the structural majority or numerical majority, and the structural majority or numerical majority [of the bones] of a corpse even though they do not amount to a quarter [of a kab]; [all these] are unclean. How many [bones] form the numerical majority? One hundred and twenty-five.