Gemara

(Mishnah - R. Meir): If one had bundles of clover of Kilai ha'Kerem (crossbreeds in a vineyard), they must be burned. If they became mixed with (permitted) bundles, all must be burned;

Chachamim say, they are Batel if there are 201 bundles in all.

R. Meir says, all that it is normally counted, forbids (it is never Batel);

Chachamim say, only six things are never Batel: nuts of Parech, pomegranates of Badan, sealed barrels (of wine), leeks, large stalks of cabbage, and Greek gourds;

R. Akiva adds a seventh, loaves of a Ba'al ha'Bayis (not a professional baker).

Some of these (the first three) are forbidden due to Orlah (fruits within the first three years). The others are forbidden due to Kilai ha'Kerem.

R. Yochanan says, the text (of R. Meir's words) is 'That which is counted';

Reish Lakish says, it says 'All that is counted'.

Avodah Zarah 74a - (Mishnah): These are forbidden, and if they became mixed (b'Mino and the Isur is not recognizable), they forbid a mixture of any amount:

Yayin Nesech, idolatry, hides with a hole (from which the heart was removed, an idolatrous practice), an ox sentenced to be stoned, a calf beheaded (when a murdered body is found), birds used to Metaher a Metzora, a Nazir's hair, a firstborn donkey (before it is redeemed), meat cooked with milk, the goat sent to Azazel on Yom Kipur, and Chulin slaughtered in the Azarah.

(Gemara) Question: If the Tana lists Devarim sheb'Minyan (things that are sold by number), he should also list pieces of a Neveilah. If he lists Isurei ha'Na'ah, he should also list Chametz during Pesach!

Answer (R. Chiya bar Aba): The Tana lists only Devarim sheb'Minyan that are also Isurei ha'Na'ah.

The Mishnah says 'these are forbidden' to exclude Devarim sheb'Minyan that are not Isurei ha'Na'ah, and Isurei ha'Na'ah that are not Devarim sheb'Minyan.

Chulin 99b - (Mishnah): If a Gid ha'Nasheh or a piece of Neveilah was cooked with other Gidim or pieces, if we cannot recognize the Gid ha'Nasheh or Neveilah, they are all forbidden.

Question: The Gid ha'Nasheh or Neveilah should be Batel in the majority!

Answer: A Gid ha'Nasheh is a Briyah, so it is never Batel. A piece of Neveilah is a Chatichah ha'Re'uyah Lehiskaved (a piece fitting to honor guests with). It is important, so it is never Batel.

Rishonim

Rif (Chulin 35b): A Gid is not Batel because it is a Briyah. A piece of Neveilah is not Batel because it is fitting to honor guests.

Ran (36b DH v'Garsinan): The Gemara says according to R. Meir that Chatichah ha'Re'uyah Lehiskaved is not Batel. Even though it is not Devar sheb'Minyan, it is important. However, Chachamim disagree, for they say that only six things are not Batel. It is not better than bundles of clover, which Chachamim say are Batel. The Halachah follows R. Meir, for the Stam Mishnah is like him, and R. Yochanan and Reish Lakish argue according to him. However, the Rambam rules like Chachamim. He holds that Chatichah ha'Re'uyah Lehiskaved is like one of the six things.

, Mar'eh ha'Panim Orlah 8b DH ha'Orlah and Nazir 27a Kol ha'Isurim

Rambam (Hilchos Ma'achalos Asuros 16:3): Something important forbids any amount b'Mino. There are seven such things: nuts of Parech, pomegranates of Badan, sealed barrels, leeks, large stalks of cabbage, and Greek gourds, and loaves of a Ba'al ha'Bayis.

Rambam (9): It seems to me that anything that is important to people in a place, such as nuts of Parech or pomegranates of Badan in Eretz Yisrael at those times, forbids a mixture of any size. Chachamim listed those that forbid any amount in every place. The same applies to everything similar in other places. It is clear that these Isurim are mid'Rabanan.

Rosh (Avodah Zarah 5:30): The Tana lists only Devarim sheb'Minyan that are Isurei ha'Na'ah. Rashi says that even though a piece of Neveilah is Devar sheb'Minyan, since it is not Isurei Hana'ah it is Batel in a majority. Rashi holds that our Tana argues with the Tana in Chulin who says that Gid ha'Nasheh and Neveilah forbid any amount because a Briyah and Chatichah ha'Re'uyah Lehiskaved are not Betelim. One can explain differently, that the Tana agrees that Devar sheb'Minyan is forbidden even if it is not Isurei ha'Na'ah, just he does not list it. Even though Chametz b'Pesach is Asur b'Hana'ah, if it is not Devar sheb'Minyan it can be Batel.

Question (Pilpulei Charifta 300): This is unlike what the Rosh said above, that the Tana only lists what has both properties, but he forbids even without both!

Rosh (35): Mid'Oraisa an Isur is Batel in a majority of Heter even if it is important. Mid'Rabanan it is not Batel.

Poskim

Shulchan Aruch (110:1): Something important forbids any amount Min b'Mino. There are seven such things: nuts of Parech, pomegranates of Badan, sealed barrels (of wine), leeks, large stalks of cabbage, Greek gourds and loaves of a Ba'al ha'Bayis. Other things are Batel in the appropriate Shi'ur, even if they are normally counted.

Shach (4,6): Only big barrels are important. Small barrels are Batel in a majority (Rema 134:2). Obviously, where bakers' loaves are important like those of a Ba'al ha'Bayis they are not Batel.

Question (Taz 1): Sha'arei Dura and the Maharshal rule like Reish Lakish. The Shulchan Aruch itself (86:3) says that eggs are not Batel, unlike its ruling here! Since this is mid'Rabanan, one may rely on the lenient opinion.

Taz (1): The Mordechai says that intestines are Batel even though they are sold only by number, because they are eaten only when filled (with something else).

Rema: Some say that anything b'Minyan, i.e. it is always counted, is not Batel. This is our custom.

Bedek ha'Bayis (DH veha'Rishon): This opinion seems better, but since this is a mid'Rabanan matter, we are lenient like the Rambam.

Beis Yosef (DH v'Chen Kosav): Many say that the Halachah follows R. Meir, for R. Yochanan and Reish Lakish argue according to him. The Ritzva says that the Halachah follows R. Yochanan. Eggs can be Batel, because sometimes they are sold by estimation. Sefer ha'Terumos disagrees.

Beis Yosef (ibid.): The Rambam does not learn from the Amora'im's argument that the Halachah follows R. Meir against Chachamim and R. Akiva. The Rambam rules like R. Akiva. He also rules that Chatichah ha'Re'uyah Lehiskaved, a Briyah and a living being are not Batel. The Rambam can explain why the Mishnah did not mention these: it lists the seven to be examples of important things that are not Batel. The seven are not to exclude everything else.

Gra (2): The Rambam rules like R. Akiva because a Stam Mishnah like R. Akiva follows the argument.

Question: If R. Akiva (and Chachamim) agree that similarly important things are not Batel, why do they argue about whether there are six or seven? Also, 'there are only six' implies that there are no others!

Answer (Kesef Mishneh): R. Meir discussed things that grow from the ground. Chachamim say that there are only six such things that are not Batel. R. Akiva holds that loaves of a Ba'al ha'Bayis are just as important as the six. Alternatively, even though they are less important, since they are more important than everything else that grows, they forbid any amount.

Shach (9): The custom is to be stringent about anything sometimes sold by number. Nowadays everyone sells eggs by number, so one must be stringent.

Shulchan Aruch: Anything that is important to people in a place, such as nuts of Parech in Eretz Yisrael at those times, forbids a mixture of any size. Chachamim listed those that forbid any amount in every place. The same applies to everything similar in other places.