Gemara

(Rav): If a woman miscarried a baby with two backs and two spines, it is not a child. If an animal miscarried such a creature, one may not eat it;

(Shmuel): Such a (human) baby is a child. Such an animal is permitted.

They argue about Rav Chanin's law;

(Rav Chanin): "(Es Zeh Lo Sochlu... ) ha'Shesu'ah" is a creature with two backs and two spines.

Rav understands that it is not a species. Rather, it is a mutation. The Torah forbids such a fetus found inside (the womb of) a slaughtered animal.

Shmuel holds that the Torah forbids it if it was born, but if it is found inside a slaughtered animal, it is permitted.

Question (against Rav - R. Simi - Mishnah - R. Chanina ben Antignos): A Kohen with two backs and two spines is blemished.

It is necessary to teach this only if such a child can live!

Answer (Rav): The Mishnah discusses one whose back is bent. It only looks like two backs.

A case occurred, and R. Yirmeyah bar Aba wanted to rule like Shmuel.

Rav Huna: Do you intend to be stringent? Your stringency leads to a leniency, i.e. Dam Tohar!

Rather, the Halachah follows Rav (against Shmuel) in Isurim (e.g. everything except for monetary laws), whether he is lenient or stringent.

Chulin 60b (Rav Chanan bar Aba): The Shesu'ah is a unique creation. It has two backs and two spines.

Question: Was Moshe a hunter, that he would know about such exotic creatures?!

Answer: Rather, this proves that he received the Torah from Hash-m.

Menachos 37a: A man said that his (firstborn) son was born with two heads, and asked how much he must pay to redeem him.

(An elder - Beraisa): You must pay 10 Shekalim (five for each head).

Rishonim

Rif (Chulin 25a): The Shesu'ah does not live from when it enters the world (birth).

Ran (DH v'Chi): The tradition teaches about inside the womb, for we do not find it in any other case.

Rambam (Hilchos Ma'achalos Asuros 1:6): If a Kosher animal gave birth to a creature with two backs and two spines, or such a creature was found in its womb, this is the Shesu'ah that the Torah forbids. I.e. it is Shesu'ah (cleaved) into two animals.

Magid Mishneh: We hold like Rav, therefore it is forbidden even if it is found inside the womb.

Kesef Mishneh: This is difficult. Rav holds that it is found only in the womb. Why did the Rambam discuss if it was born?

Pri Chodosh (18): Some misunderstood that the Kesef Mishneh asks that since it is not found in the world, why did the Rambam say that it gave birth? This is wrong. 'It is not a creation' means that it is not a species, for it does not live past eight days, like other Nefalim. The Kesef Mishneh asked that 'gave birth' connotes that it can live. The Rambam should have said 'miscarried', like the Gemara and Beraisa. 'It is not a creation' does not mean that it cannot be born alive. The Gemara says 'this excludes one who has two backs and two spines. It is forbidden even in the womb.' The word 'even' proves that it can be born alive. This is clear also from the fact that this was taught with others, and the Gemara concluded 'they cannot have a Bris Milah on day eight, i.e. they do not live eight days.

Poskim

Shulchan Aruch (YD 13:6): If one slaughtered an animal and found inside a creature with two backs and two spines, it is forbidden.

Beis Yosef (DH v'Da): The Tur mentions a miscarriage of such a creature. This is not precise. One may not eat any miscarriage, for it is a Nevelah! The tradition forbids when it is found in the womb. The Tur mentioned a miscarriage to teach about Tum'ah of a woman who miscarried a Shesu'ah.

Bedek ha'Bayis: I found a corrected text of the Tur that says 'was found inside' in place of 'miscarried.'

Bach (8): Rav holds that there is no Tum'as Yoledes for a Shesu'ah, since it is a Nefel that could not survive, so it is not a child. This is why Shechitah of an animal does not permit such a fetus inside.

Shach (21) and Levush (6): The tradition forbids a Shesu'ah in the womb, for it cannot live even a moment after it is born.

Pri Chodosh (18): I disproved this (above).

Machazik Berachah (of the Birkei Yosef, 5): Piskei R. Yeshayah ha'Rishon of Triani on Nidah says 'some Yisre'elim saw a woman with two backs, spines, and heads, and four eyes and hands. Each back and spine had a head, two eyes and two hands. From her hips and below she was one body. She was about 20 years old. Also Nochrim saw such a man. This shows that the Halachah follows Shmuel.' Even though he decided like Shmuel, surely he would not permit such a fetus in an animal. Firstly, these reports were about people. Even though Rav compares man to animals, perhaps we do not find them in animals, so we forbid like Rav. Also, even if the reports are true, we heard only about cases in which they have two heads. The Shesu'ah has only one head. The Gemara connotes that the only Chidush is that it has two backs and two spines. If it has two heads, Rav would agree, like the case in Menachos. Even though the Nochrim's report did not mention two heads, we can say that they were not precise, or that they lied.

Machazik Berachah (DH veha'Pri): The Pri Chodosh brought proofs from the Gemara that a Shesu'ah can live like other Nefalim. Tevu'os Shor rejected his proofs, and brought the Rif. The Ran said that the tradition teaches about inside the womb, for we do not find it in any other case. He did not say 'we do not need the tradition, for even though it lives, it does not survive. Chidushei ha'Ran (Nidah 24a DH u'Shmuel) said 'outside, we do not need the tradition, for it is a total Nefel.' This shows that the inference from the Ran in Chulin is not a valid inference. The Ramban in Nidah wrote like Chidushei ha'Ran. They support the Pri Chodosh. Also Rashi (Bechoros 43b DH Asur) says that it is a Nefel, and it is like a Mes. Piskei R. Yeshayah connotes similarly. Also the Gemara connotes like the Pri Chodosh. One can explain the Rif this way.

Malbim (Shemini 73 DH v'Da): Scientists mention a creature in the pig family that grows in herds in south Central America. In the middle of its back it is split the width of a finger, and it has a small pocket full of juice. This illuminates Rav Chanin's words. No one knew about America in the days of Moshe. This proves that he received the Torah from Hash-m. This is like Shmuel, who says that it is a species, according to Rashi, and unlike R. Tam. Rav must hold that this is not called two backs because they are connected inside. Rav must say that it is a subfamily of a Tahor species. The scientists say that it has teeth in the upper jaw like a pig, and chews the food, even though it has four stomachs, like ruminants (animals that chew the cud). This (the teeth) is why the Chacham Lina said that it is in the pig family. Chazal said that ruminants do not have teeth in the upper jaw. The Torah said "the pig will not chew the cud" to teach that also the species that is close to chewing the cud, through four stomachs, is not a ruminant, for it chews with its teeth. This also demonstrates the Divine wisdom of Oral Torah. The Gemara (Chulin 59a) says that any animal without upper teeth chews the cud and has split hooves. This, and Rav Chisda's rule that an animal is Tahor if and only if the flesh tears lengthwise and widthwise (Chulin 59b), can be known only from Hash-m.

Yismach Levav (YD 6): A case occurred in which a fetus had two whole bodies totally separated, but they joined at the neck. They had one common neck and one normal head. Perhaps this is the Shesu'ah. The Rambam and Shulchan Aruch rule like Rav, who forbids it. The Gemara connotes that it has only one head, like the Machazik Berachah said. Also Tashbatz (Yavin Shemu'ah, Hilchos Trefos 11:4) explicitly says that it has only one head. We cannot say that the two bodies are intertwined and cling together, for "Shesu'ah" connotes that they are separate. However, Targum Yonason ben Uziel says that it has two heads. Our Gemara is primary. However, since (the rest of) the Targum is like Rav, I suspect that there is a printing mistake )in the Targum), and it should say two backs in place of 'two heads.' Therefore, I forbade eating the fetus, for I think that it is the Shesu'ah.

Torah Temimah (Devarim 14:4, Ha'ara'ah 20): Why was Rav sure that the Shesu'ah cannot live? The Gemara asked how Moshe could know about such a creature. How could Rav know that no such creature can live?! Not seeing is not a proof! He answered with difficulty a Mishnah and Beraisa, which connote that it can live. If it was from Rav's knowledge of nature and medicine, his opponent, Shmuel, was known to be outstanding in these Chachmos. I say that Rav learned from this Drashah. Among all creatutres, why do we say specifically about the Shesu'ah that Moshe must have known it from Ru'ach ha'Kodesh? Rather, we must say that it does not live, and it is seen only in the womb.

Note: Rav Chanan bar Aba, Rav's son-in-law, expounded that Moshe must have known it from Ru'ach ha'Kodesh. If Rav would not rely on his own knowledge to argue with Shmuel why would he rely on Rav Chanan's? Perhaps Rav Chanan had a tradition from Tana'im for this. About which other rare creatures could we have expounded this? Perhaps he refers to the birds Peres and Ozniyah, which are not found in civilized areas. Still, I do not understand the proof that Moshe knew from Ru'ach ha'Kodesh. Perhaps someone who had seen such a fetus (or animal) told Moshe!

Torah Temimah: Ba'al ha'Mashbir mentions a creature found in Brazil with a deep split in its back. It looks like its back and spine are split. This is like Rav established the Mishnah in Bechoros, that his back is bent and looks like two.