Rav Asi did not come to the Beis Medrash. He encountered R. Zeira and inquired about what was discussed there.

R. Zeira: What is your question?

Question (R. Asi): In the Seifa, why is it Tamei? It is Tum'as Beis ha'Starim (the contact between the meat and the Tum'ah (the Ever it was cut from, at the time it was cut) is concealed)!

R. Zeira: I had the same question and asked R. Aba bar Memel;

Answer (R. Aba bar Mamal): The Beraisa is like R. Meir, who holds that Tum'as Beis ha'Starim is Metamei.

Objection (R. Asi): I already refuted R. Aba. R. Meir says so only regarding Tum'ah that does not need Hechsher!

Question (Rava): Perhaps the Beraisa discusses meat that was already Huchshar!

Question (Rabah bar Rav Chanan): Why is Hechsher needed? The meat already had severe Tum'ah (Ever Min ha'Chai)!

Answer (Rava): The meat was not considered a food at the time. (Therefore, it needs Hechsher.)

Answer (Abaye): Until the meat was cut off, it was not considered food. It could not receive Hechsher.

(Beraisa): If Se'or (sourdough) was designated to sit on, it is Batel. (One may keep it on Pesach. It receives Tum'as Mishkav.)

(Abaye): Its Tum'ah is mid'Rabanan, for food is never an Av ha'Tum'ah.

Rejection: (It can be an Av mid'Oraisa.) After designation, it is not considered a food.

(Abaye): Something offered to idolatry has Tum'as Ohel. This is mid'Rabanan, for a food is never an Av ha'Tum'ah.

Rejection: (It can be mid'Oraisa.) Once it is forbidden, (one may not eat it, so) it is no longer considered a food.

(Abaye): If food was left to remain in (a crack in) a Kli, it becomes like the Kli. (If it touches a Mes, it becomes an Av ha'Tum'ah);

This Tum'ah is mid'Rabanan, for a food is never an Av ha'Tum'ah.

Rejection: (It can be mid'Oraisa.) After abandoning it, it is not considered a food.

(Beraisa): In a village, Chelev of a Nevelah receives Tum'as Ochlim only after one intends to feed it to a Nochri and it is Huchshar.

(Rav Papa): This shows that Tum'as Nevelah of Chelev enveloping the kidney (of a Nevelah) must be mid'Rabanan, for food is never an Av ha'Tum'ah. (Rashi, according to Maharsha - something can be an Av ha'Tum'ah before it is a food, but this cannot apply to Chelev around the kidney, for if so it would not require Hechsher afterwards.)

Rejection (Rava): (It can be mid'Oraisa.) It is not Tamei like a food, rather, like a Shomer.

(Rav Masnah): If one made a roof from vegetation, the vegetation (becomes Tahor and - some texts delete this) is considered like a roof;

If the house gets Tzara'as, the roof is Tamei. This Tum'ah is mid'Rabanan, for food is never an Av ha'Tum'ah.

Rejection: (It can be mid'Oraisa.) After making a roof from it, it is not considered a food.

(Mishnah): R. Shimon is Metaher (the limb, when the animal dies).

Question: In any case, this is difficult!

If when it dies, we consider it as if the limb fell off (before death), it is Tamei like Ever Min ha'Chai;

If we do not consider it as if it fell off (before death), it is Tamei like a limb of a Nevelah!

Answer #1: R. Shimon refers to the Reisha;

(Mishnah): Dangling limbs or flesh of an animal receive Tum'as Ochlim (while dangling), and require Hechsher;

R. Shimon is Metaher.

Question: What is R. Shimon's reason?

Answer #1 (R. Asi citing R. Yochanan): "Any food that may be eaten" - only a food that you may feed to Nochrim is considered a food (to receive Tum'as Ochlim).

Objection (and Answer #2 - R. Zeira): He should be Metaher because it is still attached (like you yourself answered. He should not need a verse!)

(Beraisa - R. Yehudah): If a fig branch was cut and it is attached through the bark, it is Tahor;

Chachamim are Metaher only if the branch can heal.

Question: What is R. Yehudah's reason?

Answer (R. Asi): Any attachment to something alive is Metaher.

Answer #2 (R. Asi): Rather, R. Yochanan teaches that in the middle clause, R. Shimon learns from a verse:

(Mishnah): R. Meir says, if the animal is slaughtered, (the limb or meat) receives Hechsher through the blood;

R. Shimon says, it does not.

Question: What is R. Shimon's reason?

Answer #1 (R. Yochanan): "Any food that may be eaten" - only a food that you may feed to Nochrim is considered a food.

Objection (and Answer #2): Perhaps R. Shimon's reason is like Rabah or R. Yochanan above! (An animal is not a Yad for one limb, or because the larger piece does not come with the smaller piece. He does not need a verse!)

Answer #3: Indeed, R. Yochanan explained R. Shimon's reason in the Seifa. R. Shimon discusses only dangling meat, but not a limb:

(Seifa): If the animal dies, the flesh requires Hechsher;

R. Shimon says, it is Tahor.

Question: What is R. Shimon's reason?

Answer (R. Asi citing R. Yochanan): "Any food that may be eaten" - only a food that you may feed to Nochrim is considered a food.

(Mishnah - R. Meir): Dangling limbs and flesh of a person are Tehorim. If he dies, the flesh is Tahor. The limb has Tum'as Ever Min ha'Chai, but not like Tum'as Ever Min ha'Mes;

R. Shimon is Metaher.

(Gemara) Question: In any case, R. Shimon's opinion is difficult!

If when it dies, we consider it as if the limb fell off (before death), it is Tamei like an Ever Min ha'Chai!

If we do not consider it as if the limb fell off (before death), it is Tamei like an Ever Min ha'Mes!

Answer #1: R. Shimon comes to argue with R. Meir:

R. Meir said that the limb is Tamei like Ever Min ha'Chai, but not like Ever Min ha'Mes. This implies that Ever Min ha'Mes is Tamei;

R. Shimon is Metaher. (He holds like R. Eliezer below.)

(Beraisa - R. Eliezer): I heard that Ever Min ha'Chai is Tamei.

R. Yehoshua: Are you Metamei only of a Chai, but not of a Mes? A Kal va'Chomer is Metamei Ever Min ha'Mes!

A living person is Tahor, yet Ever Min ha'Chai is Tamei. A Mes is Tamei, so all the more so, Ever Min ha'Mes is Tamei!

R. Yehoshua: In Megilas Ta'anis (a list of days on which we do not fast or eulogize), it says that we do not eulogize on Pesach Sheni. All the more so, Pesach itself is forbidden!

You must admit that the list omits Pesach, for we can derive it through a Kal va'Chomer;

Similarly, the one who taught you that Ever Min ha'Chai is Metamei omitted Ever Min ha'Mes, because it follows from a Kal va'Chomer!

R. Eliezer: I heard that Ever Min ha'Chai is Tamei, to exclude Ever Min ha'Mes!

Question: What is the difference between the Tum'ah of Ever Min ha'Chai and Ever Min ha'Mes (according to R. Meir, in our Mishnah)?

Answer: The difference is a k'Zayis of flesh or a bone ka'Adashah (the size of a lentil) that separated from them:

(Mishnah - R. Eliezer): If a k'Zayis of flesh separated from Ever Min ha'Chai, it is Tamei;

R. Nechunyah ben Hakanah and R. Yehoshua are Metaher.

R. Nechunyah says, if a bone ka'Adashah separated from Ever Min ha'Chai, it is Tamei;

R. Eliezer and R. Yehoshua are Metaher.

Answer #2 (to Question (b)): Since Tana'im argue about a k'Zayis of flesh or a bone ka'Adashah that separated from Ever Min ha'Chai, we can say that R. Shimon likewise argues with R. Meir about this!