Mishnah Oholot 2:2-3

[The following likewise defile:]A quarter [of a log] of blood, A quarter [of a log] of mixed blood from one corpse. Rabbi Akiva says: [even] from two corpses. [With regard to] the blood of a child that has completely flowed forth: Rabbi Akiva says: [it defiles] in even the smallest quantity, But the sages say: [there must be] a quarter [of a log]. [With regard to] an olive-sized [portion] of [corpse] worms whether alive or dead: Rabbi Eliezer declares [it] unclean, like the flesh, But the sages declare [it] clean. [With regard to] the ashes of burnt persons: Rabbi Eliezer says: the [minimum] quantity [for defilement is] a quarter [of a kav]. But the sages declare [them to be] clean. A ladleful and [a little] more of grave-dust is unclean. Rabbi Shimon declares [it to be] clean. A ladleful of corpse-mold mixed with water is not [regarded as] joined [into one mass] for [the purposes of] defilement.

The following defile by contact and carriage but not by overshadowing: A bone of barleycorn size, Earth from a foreign country, A bet peras, A limb of a corpse, or a limb [severed] from a living person which has no longer its appropriate flesh, A spine or a skull which is deficient. How much is [considered] a deficiency in the spine? Bet Shammai say: two vertebrae, But Bet Hillel say: even one vertebra. And in the skull? Bet Shammai say: [the size of a] hole [made] by a drill, But Bet Hillel say: as much as would be taken from a living person and he would die. Of what drill did they speak? Of the small one [used] by physicians, the words of Rabbi Meir. But the sages say: of the large one in the Temple-chamber.