Explanation #1 (Rashi): He challenges Abaye.

Question: Why did he wait until now?

Explanation #2 (Rav Pores, citing the Rashbam): He challenges his father Rav Yehudah. He means as follows;

Since you said that one who is Motzi one wheat kernel for sowing is liable, even though it is not common, because it depends on his intent, likewise if he intended to be Motzi his entire house, we should follow his intent, and he is exempt for less, since less is not important in his eyes!

We answer that there, Batla Daito Etzel Kol Adam (we ignore his intent, since it is unlike that of normal people). No one thinks that less than this (his entire house) is not important. However, here there are people for whom one wheat kernel is important to them, for sowing!

Inference: If it depends on now, there is no question, since [Rava] asks after this "if you will say that it depends on now, if he was Motzi a k'Grogeres to eat, and it shriveled, and then it inflated again, what is the law?", and he did not ask the current question (if one was Motzi a k'Grogeres to eat, and it shriveled, then he decided to plant it).

Question: Why [didn't he ask]? He could still ask whether or not Dichuy applies to Shabbos, for once it shriveled, it was Nidcheh (it was less than a Shi'ur, until he reconsidered!

Answer: Only in the case below is considered Dichuy, for it was Nidcheh both from the first Shi'ur and the latter Shi'ur (i.e. both at the beginning and at the end, the Shi'ur was k'Grogeres);

However, here it is not Nidcheh from the latter Shi'ur (even after it shriveled, there is a Shi'ur for planting). He does not consider this to be Dichuy.

Inference: Food less than k'Beitzah is not Mekabel Tum'ah.

Implied question: Rashi brought from Toras Kohanim that any amount of food is Mekabel Tum'ah!

Citation (Toras Kohanim): "Kol ha'Ochel" teaches that any amount can become Tamei. Perhaps it is Metamei other [food or drink]! It says "Asher Ye'achel" - (it is Metamei others only if it is as much) food that one can eat (swallow) at once, i.e. [the size of] a hen's egg.

Rejection (R. Tam): It is an Asmachta. Toras Kohanim also includes Ma'aser of vegetables from "v'Chol Ma'aser ha'Aretz", and it is a mere Asmachta.

Implied question: It asks in Pesachim (33b) according to the opinion that liquids are absorbed [in the fruit from which they are squeezed] from the Mishnah of a Tamei Mes who squeezed olives or grapes. If they were exactly k'Beitzah, they are Tehorim. [We ask,] if liquids are absorbed, why are they Tehorim? (When the person first touched them, they became Tamei with the grapes or olives!)

[The Gemara] establishes [the Mishnah] to discuss grapes that were not Huchshar (liquid was never put on them, so they are not yet Mekabel Tum'ah). When are they Huchshar? When he squeezes them (and juice comes on them). Once he squeezes them, they are less than a Shi'ur!

Answer: This is fine also for Rashi. Even though any amount receives Tum'ah, [once he squeezes them, they are less than a Shi'ur to receive Hechsher], according to Rav Pores, who explained in the name of Rashi below (145a) that food less than k'Beitzah does not receive Hechsher;

The reason is because we learn from "Asher Ye'achel" that k'Beitzah can be Metamei others, and regarding this the Torah wrote "Asher Yavo Alav Mayim."

Implied question: Also in Pesachim (45a, it connotes unlike Rashi) regarding dough in a crack in a kneading trough, Abaye explains that there is less than k'Beitzah of food, and it touched this dough. During Pesach, the Isur makes it important, so it joins;

[If less than k'Beitzah is Mekabel Tum'ah, Abaye could have taught that the Isur makes it important] even without joining less than k'Beitzah to the dough! He could have established that a Sheretz touched a k'Zayis in a crack in a kneading trough. During Pesach, the Isur makes it important to receive Tum'ah;

The rest of the year, if he is adamant [about the dough], yes (it becomes Tamei). If not, it does not.

Answer: Whether the dough receives Tum'ah is not relevant to us. In any case it is forbidden due to Chametz! Therefore [Abaye] discusses a case of being Metamei other food.

Proof (R. Tam): Food less than k'Beitzah is not Mekabel Tum'ah [mid'Oraisa], for a Beraisa (Pesachim 44a) teaches that porridge of Terumah and garlic and oil of Chulin, if a Tevul Yom touched some of [the garlic and oil], he disqualified all of them;

If the porridge was Chulin and the garlic and oil were Terumah; and a Tevul Yom touched part [of the garlic and oil], he is Metamei only the place he touched. [The Gemara] asks, why does he disqualify it? It is less than k'Beitzah!

Rebuttal: Rashi's [and our] text there does not say so. Rather, it says that it is Batel to the majority! I.e. we ask why is the place he touched disqualified? The garlic and oil are Batel to the porridge. This is why the Reisha taught that he disqualified everything!

Rejection #1 (of rebuttal): We should not change [the text of] Seforim.

Rejection #2: Also, granted, in the Reisha they are Batel to be stringent. We do not say that they are Batel to be lenient!

Rejection #3: We cannot say that garlic and oil are Batel, for they are intact, since it says that he disqualified only the place he touched. (If they dissolved into the porridge, there is no distinction between one place and another!)

Also, the Seifa [of that Mishnah] teaches "what is the case? This is when it is a solid in the bowl." This implies that they are intact!

Conclusion: It is not proper to elaborate. Rashi retracted in Kerisus (21a), and regarding Parah [Adumah] receives Tum'as Ochlin (Chulin 82a).

Implied question: It says in Chulin (24b) that the Torah testified about a Kli Cheres (if Tum'ah entered its interior, all the contents are Tamei), even if it is full of mustard [seeds. I.e. we do not say that the Kli is a Rishon, and the seeds that touch the Kli are a Sheni, and those that touch them are a Shelishi..., for then seeds in the middle would be Tehorim! Rather, the Tum'ah is directly Metamei all the food inside. This shows that less than k'Beitzah is Mekabel Tum'ah!]

Rejection: "Mustard" is not precise (mid'Oraisa, the seeds do not receive Tum'ah). Rather, it is full of eggs (or pieces of food, each at least k'Beitzah);

Because regarding mustard seeds, [in the Havah Amina] there are many levels of Tum'ah - Rishon, Sheni... and they receive Tum'ah mid'Rabanan, it mentioned mustard.

Observation: The Gemara could have asked "why did it say that he threw?", and "why did it mention Terumah, and in a Tamei house?"

Explanation (Ri): The reason it says that he threw is because now, [the Melachah and Tum'ah] come together. He is not liable for Shabbos until it rests, like it says above (80a) that within three [Tefachim] of the ground, it must rest on something;

However, one who [carries and] enters [into a house], once it is within three there is an Isur Shabbos, but there is no Isur Tum'ah until it joins, and they do not come at once.

It specifically discussed a k'Zayis of Terumah, for it is forbidden to eat it b'Tum'ah (when it is Tamei), but less than a k'Zayis, is not [forbidden];

And similarly, [it did not discuss] a k'Zayis of Chulin. Since there is no Isur to eat it b'Tum'ah, we cannot say that since it joins for Tum'ah, it joins for Shabbos.

Question: How is a k'Zayis of Terumah different than a half k'Zayis? The Isur to eat Tamei Terumah is only an Aseh, like is proven in Yevamos (73b) that one who eats it when [the Terumah] itself is Tamei, he is not lashed;

Also a half k'Zayis (is forbidden mid'Oraisa). We hold like R. Yochanan, that the Torah forbids Chetzi Shi'ur (Yoma 74a)!

Answer: In any case, a k'Zayis is more severe, for it is the Shi'ur to be lashed everywhere [for other forbidden foods].

Explanation: It specifically discussed a Tamei house, but not a Tahor house. Even though [a k'Zayis of Tahor Terumah] is important to lash a Zar who eats it, this does not make less than k'Grogeres important, since the Isur to Zarim did not come through this Hotza'ah.

Explanation #1: Likewise, we could have said that there is k'Beitzah of food, which does not need to join, and a k'Zayis by itself needs to join to it.

Explanation #2 (Ri): It discusses specifically less than k'Beitzah, but perhaps k'Beitzah does not join, since before joining it is already Tum'ah. Perhaps Sava Lah Tum'ah (since it is satiated with Tum'ah, it does not join to a Shi'ur to receive Tum'ah);

This (Sava Lah Tum'ah) is a question in Menachos (24a) regarding an Isaron (a Minchah, i.e. the volume of 43.2 eggs of flour) that was divided, and one part became Tamei, and it was all put in a Kli, and a Tevul Yom touched the Tamei part. What is the law?

Do we say Sava Lah Tum'ah, and the Tahor does not become Tamei? Even though if both parts were Tehorim and a Tevul Yom touched one of them, both of them would be Temei'im, because a Kli joins its contents for Kodesh - however, now that Sava Lah Tum'ah, it does not join;

Or, perhaps we do not say Sava Lah Tum'ah!

Conclusion: Because there (in Menachos) we were unsure, it discusses here less than k'Beitzah.

Implied question: Why is this stronger than our Mishnah (which also says that he is exempt until he is totally Motzi it, and Chizkiyah explains that it discusses long gourds? He can explain the Beraisa similarly!)

Answer: A Stam spice peddler's box has [small] wrapped [fragrances. When part of the box is outside, some fragrances are totally outside.]

Support: Also the Beraisa connotes like this, for it says "there are many species in it";

The Gemara answers that it discusses bundles of long roots. It taught "and many varieties are in it" merely to discuss a common case. All the species are long.