Gemara

(Mishnah): If one slaughters an animal and finds inside an eight month fetus (Nefel) dead or alive, or a dead nine-month (viable) fetus), the fetus may be eaten without Shechitah, just its blood must be removed;

R. Meir says, if a live nine month fetus is inside, it must be slaughtered. The Isur of slaughtering a mother and her child on the same day applies to the child;

Chachamim say, Shechitah of the mother is Metaher (permits) the child;

R. Shimon Shezuri says, even if the child is five years old and plowing in the field, Shechitah of the mother was Metaher it;

(R. Elazar): Chachamim were lenient about a Ben Peku'ah only regarding Shechitah.

Suggestion: He excludes Chelev of the fetus and Gid ha'Nasheh.

Rejection: They argue about those, and Chachamim are lenient!

(Beraisa - R. Meir): The Gid ha'Nasheh and Chelev of a fetus are forbidden;

R. Yehudah permits them.

(R. Elazar): They argue about a live nine month fetus, just like they argue about Shechitah (whether a Ben Peku'ah is considered a live animal or permitted meat).

Rather, R. Elazar meant that Chachamim are lenient only about matters relating to eating, but they forbid bestiality with the calf, or making it work with a different species.

(Reish Lakish): R. Yehudah, who permits its Chelev, permits also its blood. R. Meir, who forbids its Chelev, forbids also its blood;

(R. Yochanan): All agree that the blood is forbidden.

75b - Question: R. Shimon Shezuri says just like Chachamim!

Answer (Rav Kahana): They argue about a Ben Peku'ah that stepped on the floor. (Chachamim decree that it must be slaughtered, lest people come to eat regular animals without Shechitah.)

Pesachim 21b (R. Avahu): Wherever the Torah says "Lo Yochal" it forbids eating and benefit, unless there is a source to permit benefit, like we find by Neveilah (a verse permits selling it to a Nochri).

22a: R. Avahu holds that when the Torah permitted (to give a Neveilah to a Ger to eat, this permits benefit from (every edible part, including the Chelev and) the Gid.

Question: It says "Kol Nefesh Michem Lo Sochal Dam," yet one may benefit from blood!

Rishonim

Rambam (Hilchos Ma'achalos Asuros 7:3): If one slaughtered an animal and inside was a fetus, all the Chelev is permitted. This is even if the fetus was alive, for it is like a limb of the mother. If it had a full term pregnancy and he found it alive, even if it did not step on the ground and it need not be slaughtered, its Chelev is Asur and one is Chayav Kares for it. We remove and veins and forbidden membranes from it, like from other animals.

Ra'avad: This is like R. Efrayim, and unlike the Rif.

Rosh (4:5): We hold like Chachamim. If it did not step on the ground, it need not be slaughtered. Its Chelev is permitted, but its blood is forbidden. If it stepped on the ground, its Chelev and blood are forbidden. Mid'Rabanan it must be slaughtered, lest it be confused (with a regular animal). We hold like R. Yehudah, who permits the Gid ha'Nasheh and Chelev of a Ben Peku'ah. We hold like R. Yochanan, that all forbid its blood.

Rosh: The fetus should not be permitted more than its mother. Why is the Chelev permitted? We learn from "Kol... bi'Vhemah... Tochelu" (Devarim 14:6). Its blood is forbidden, because it is absorbed in the entire body, so it is like blood of the limbs of the (mother) animal itself. The Rambam permits all the Chelev of a Nefel, and forbids Chelev of a nine month fetus. He holds that R. Yochanan and Reish Lakish argue like R. Meir and R. Yehudah, and he rules like R. Yochanan. This is unreasonable. If so, each would simply say 'I rule like Ploni.' Rather, each teaches according to all the Tana'im. The Halachah follows R. Yehudah.

Ran (25a DH ha'Shochet): R. Meir agrees that if one found a live eight month fetus or a dead nine month fetus inside a slaughtered animal, he removes the blood and eats the fetus without Shechitah. Since the fetus cannot live, "Kol bi'Vhemah Tochlu" includes it. The Chelev is permitted, but the blood is Asur. I explain that "Kol bi'Vhemah Tochlu" includes things that are eaten, such as Chelev and the Gid ha'Nasheh, but not liquids. Even though we find that eating applies to blood, since we can establish the verse to teach about foods, we do not establish it to discuss blood.

Tosfos (Pesachim 22a DH v'Harei): It seems that blood is not considered part of an animal, for the Gemara asked why we may benefit from blood, even though we said that the Heter for Neveilah included the Chelev and Gid. In Shabbos (77a), we say that a Revi'is of blood of a Neveilah is Metamei, for it could congeal to a k'Zayis. This shows that it is like the meat! The Ri says that really, it is not like the meat. A verse teaches that it is Tamei.

Poskim

Shulchan Aruch (YD 13:2): If one slaughtered an animal and it was found to be Kosher, and inside was a live or dead eight month fetus or a dead nine month fetus, one may eat (even) the fetus. It need not be slaughtered.

Beis Yosef (DH ha'Shochet): The Poskim rule like Chachamim, unlike R. Efrayim, who rules like R. Meir. The Torah permits what is inside a (slaughtered) animal.

Shach (1): Even if an eight month fetus is alive, it cannot live, so it is as if it is dead.

Pischei Teshuvah (1): She'alas Ya'avetz (1:25) says that regarding small animals (goats and sheep) we similarly distinguish between a four month fetus and a five month fetus, for the normal gestation of small animals is five months.

Pischei Teshuvah (2): The Radvaz (Teshuvos Chadashos 190) permits only if it was found dead inside, but if it died after entering the world, even if it did not step on the ground, it is forbidden. Avi ha'Ezri distinguishes in such cases of Tereifah. If it became Tereifah inside it is permitted, but not if it became Tereifah afterwards. The Radvaz agreed regarding death, but not regarding Tereifah.

Beis Yosef (DH u'Mah she'Chasav v'Chelbo): All the Poskim permit its Chelev, except for the Rambam, Ba'al ha'Itur and R. Efrayim. They forbid its Chelev and Gid even it did not step on the ground. Some Poskim permit the Chelev and forbid the Gid. it need not be slaughtered.

Taz (3): 'It need not be slaughtered' is extra, to teach that Ever Min ha'Chai does not apply to it. One may eat it even if he cut off a piece in its lifetime. Rashi (Shabbos 90b DH Iy) says similarly (regarding grasshoppers), but Bal Teshaktzu (do not make yourselves detestable) applies. Here Bal Teshaktzu does not apply, for he takes the meat when it is alive, but he cooks it. The Rosh forbids the blood because it is absorbed in the entire body, so it is like blood of the limbs of the animal itself. Why isn't the Chelev like Chelev of the animal itself? It seems that he holds that the blood of the fetus mixes with the mother's blood. The fetus comes into existence and is nourished through the mother, therefore its blood is no better than blood of the mother.

R. Akiva Eiger: I explain that "Kol bi'Vhemah" connotes the entire animal. We must say that the Chelev and Gid of the fetus are permitted. However, even if the blood is forbidden, one can fulfill (eating) the entire animal through cooking the blood. This can answer the question of Tosfos (Pesachim 22a). The Heter for Neveilah did not permit benefit from blood, for even if it were Asur b'Hana'ah, one could cook it and (eat it himself, so all the more so he may sell or) give the blood to a Ger (Toshav) along with the animal.

Shulchan Aruch (ibid.): If inside was a live nine month fetus, if it stepped on the ground, it must be slaughtered. If it did not step on the ground, it need not be slaughtered.

Taz (4): If it was not slaughtered, what is the law? How stringent were Chachamim about the decree, lest it be confused? The Tur says that whatever disqualifies Shechitah disqualifies it. This implies even b'Di'eved. Or, perhaps l'Chatchilah it disqualifies, like Tosfos (27a DH ha'Shochet) explains 'R. Yehudah disqualifies, unless he cuts the veins.' It seems that since R. Shimon Shezuri permits l'Chatchilah without Shechitah, we should not make such a great argument. Also, in Gitin (3a) we find concern for confusion regarding a Get brought from Chutz la'Aretz, for which the Shali'ach must say 'it was written and signed in front of me'.' Letter of the law, it should have been enough to say 'it was signed in front of me', but Chachamim were concerned lest this be confused with validation of documents (and people will think that one witness suffices for this). An extra clause (15a) teaches that if one did not say both, it is Pasul. If not for the extra clause, we would not disqualify b'Di'eved, even though there is concern for confusion. Here, we have no explicit source to disqualify b'Di'eved, so we are not stringent to say so. Also, the Beis Yosef brings from the Magid Mishneh and great authorities that a blemished knife does not disqualify.

R. Akiva Eiger (citing Pri Chodosh 61:11): One who slaughters it need not give Matanos (foreleg, jaw and stomach), for this is not "mi'Zovchei ha'Zevach." Even when it stepped on the ground, Chachamim were not stringent to consider it like a new animal to obligate Matanos.

R. Akiva Eiger: Tevu'os Shor (9) brings from the Rashba (525) that one blesses on the Shechitah, but Besamim Rosh (283) says that one does not bless.

Parshas Derachim (Derush Rishon, DH v'Ra'isi leha'Rav): R. S. Yafeh says that Yosef saw his brothers eating a Ben Peku'ah without Shechitah, which is permitted. He did not know that it is a Ben Peku'ah. I disagree. Ben Peku'ah is permitted only to us (after Matan Torah). For us, the Heter (of Ever Min ha'Chai) depends on Shechitah, and it applies even if it is still quivering. A Ben Peku'ah is like a quivering slaughtered animal. It is forbidden to a Ben No'ach, just like a quivering animal! Even those who permit a quivering animal to Nochrim say so only now that it is permitted to Yisrael.

Parshas Derachim (ibid. DH Od, cited in Pischei Teshuvah 4): Also, a Ben Peku'ah does not require Shechitah, but it requires Nechirah (i.e. one must kill it before eating it), lest one transgress "Lo Sochlu Al ha'Dam."