Tosefta do Chullin 9:1

Rabbi Yehuda says, [inedible] meat residue (האלל) that collects [together] and contains an olive's bulk, behold, it is liable [to convey impurity (see Hul. 9:1)]. The hide of the hooves of a small animal, behold, it is like its flesh, and [with regard to] all of these [skins] which were tanned, or trodden in order to tan them [afterwards], they are [ritually] pure if [these acts are done] for the purpose of the skin, but [ritually] impure if for the purpose of the flesh -- except for the skin of a person, which always imparts impurity. From this they said, [with respect to] a legion (ligyon) passing from place to place, [if a member of the legion] stands over it (i.e., the skin of a human corpse), behold, he is impure. There is no legion [whose soldiers do not carry around] scalps, and [indeed] all of them choose to do so. Rabbi Yehuda says, the lizard (leta'ah) does not have skin. Rabbi Yochanan ben Nuri says, the eight sheratzim (i.e., creeping or swarming creatures, of which the leta'ah is one, see Lev. 11:29-30) all have skin, [and] one who traps them or kills them or wounds them, or one who leaves a mark on them [causing discoloration (=חבורה, see Jastrow)], behold, he is liable. But the Sages say, there is no [creature that is considered to have] skin except those which the Sages enumerated. One who flays a domesticated animal or a wild animal or an impure animal, or a small animal, or a large animal, in order to [use the skin as a] carpet, [its halakhic status remains that of flesh until he has flayed] a sufficient quantity to grasp it. And how much is "sufficient quantity to grasp it"? Two handbreadths. [With respect to] one who flays this amount, one who [then] touches the flayed skin is pure, [but one who touches] the skin of the neck is impure. Rabbi Yochanan ben Nuri says, one who touches the skin of the neck is pure, as he did nothing but separate them (i.e., the flayed skin from the skin of the neck). Rabbi Dostai ben Yehuda says in the name of Rabbi Shimon, one who flays [any of the] sheratzim, all [of the skin is considered] to be connected [for purposes of impurity]. Rabbi Yehuda says, [with respect to flayed] skin that has an olive's-bulk of flesh [connected to it], one who touches the [area of the] skin which is connected to the flesh on the other side, is impure. [With respect to] the egg of a sheretz in which [fetal] tissue developed, [one who touches it is] impure. And how much [fetal] tissue must develop? [Large] enough that one can see a sheretz inside of it. [But if the egg] is perforated, no matter how small, [one who touches the egg is] impure. And how large must be the perforation? The size of a hairbreadth. A limb or a piece of flesh that hangs down from a [live] animal is susceptible to food-impurity. Rabbi Shimon says, it is not susceptible to food-impurity.