Hullin, Chapter Seven, Mishnah One
At the conclusion of the story of Jacob wrestling with the angel, Genesis 32:33 states, That is why the children of Israel to this day do not eat the thigh muscle that is on the socket of the hip, since Jacobs hip was wrenched at the thigh muscle. The thigh muscle is identified by the rabbis as the sciatic nerve. Our chapter is about this prohibition.
1) [The prohibition of] the sciatic nerve is in force both within the land and outside it, both during the existence of the Temple and after it, in respect of both unconsecrated and consecrated [animals]. It applies to cattle and to wild animals, to the right and left hip.
2) But it does not apply to a bird because it does not have a socket [on its hip].
3) It applies to a fetus.
a) Rabbi Judah says: it does not apply to a fetus. And its [forbidden] fat is permitted.
4) Butchers are not trustworthy with regard to the [removal of the] sciatic nerve, the words of Rabbi Meir.
a) The sages say: they are trustworthy with regard to it as well as with regard to the [forbidden] fat.
Section one: The prohibition of the sciatic nerve is applicable in all times and at all places (as was the prohibition of it and its young and the mitzvah of covering the blood). It applies to all types of mammals, both wild and domesticated and to both of their hips.
Section two: The prohibition does not apply to birds, because their hips do not have sockets as do those of mammals.
Section three: According to the first opinion, the prohibition of the sciatic nerve applies to a fetus found in its mothers womb when she was slaughtered. Rabbi Judah says it does not because this fetus is edible based on its mothers having been slaughtered. In other words, since this fetus is treated as if it were one of its mothers limbs, the prohibition of the sciatic nerve doesnt apply.
Rabbi Judah adds that the same holds true with regard to the forbidden fat, the helev, of a fetus. It can be eaten. Rabbi Judah, as we can see, does not accord to the fetus the status of a born-animal.
Section four: It is quite difficult to remove the sciatic nerve (this is why sometimes this cut of meat is not available at the kosher butcher). Therefore, Rabbi Meir holds that the butcher is not believed to have removed it. The other rabbis disagree and hold that the butcher is believed to have removed both the sciatic nerve and the forbidden fat.