Explanation #1 (Rashi): It causes Tum'ah Masa to the Zav himself, like it says "v'Chol Asher Yihyeh Sachtav", and we establish this to discuss Elyono Shel Zav.

Objection #1: Elyono Shel Zav is not called Masa, and it is Metamei only food and drink.

Objection #2: Regarding Zov, we find such a case only when the drop is in pan of a balance scale, and it lifts the other pan.

Explanation #2: Rather, it causes Tum'ah Masa to the Zav himself, that one who carries the Zav becomes Tamei. One who carries the Zov, all the more so he becomes Tamei!

Implied suggestion: We could explain this to discuss Tum'os other than Masa, e.g. Mishkav and Moshav!

Rejection: It causes Tum'ah also to a Metzora [and the Gemara said that it does not! Rather, we must say that we discuss only Tum'as Masa.]

If one is only a Metzora (but not a Zav), he is Metamei a Mishkav or Moshav only to be Metamei food and drink, like I explained in Pesachim (66a);

If he is a Zav, his Mishkav is Metamei people and garments.

Question: In any case, we can resolve that it is not a Mayan! If it were a Mayan, why do we need a verse to be Metamei Zov?

Once he saw any amount of the second sighting, immediately he becomes a Zav, according to the opinion that any amount of Zov is Metamei, and does not require the amount of Chasimas Pi ha'Amah;

The next Mashehu (after the first Mashehu made him a Zav) is Tamei, like the law of a Mayan! [We can always say so, for] it is impossible that a drop [of Zov] cannot be split into two tiny drops. (Maharam, Maharsha - this is like the opinion above that Zov is a Mayan.)

Answer: Perhaps just like the first Mashehu, which makes him a Zav, is not Metamei through Masa, the same applies to [the entire sighting, i.e.] as much as is called one sighting, i.e. the time [to walk] from Geryon to Shilo (100 Amos, about a minute), it is not Metamei b'Masa;

Even though his spit and urine are Temei'im immediately after seeing the first Mashehu of Zov [until he becomes a Zav, his Zov does not get its full Tum'ah].

Question: If so, above what is the source to establish it to discuss Dam Tohar of a Metzora'as? We need the verse for like we expound here! (R. Akiva Eiger and Aruch l'Ner answer that since Beis Hillel use "l'Zachar" to teach about a Metzora'as, and they do not need it to teach about Zov, this shows that it is a Mayan.)

Question: What was the difficulty? Perhaps Rav Huna taught that he is Metamei through Ones, i.e. Tum'as Erev, like a Ba'al Keri;

Here (the Beraisa) discusses joining to a second sighting for severe Tum'ah to make him a Zav. Now the Reisha properly resembles the Seifa!

Answer: Since [Rav Huna] did not say "one who sees one sighting of Zivah", rather, "the first sighting of a Zav", this connotes that the case is that he saw two sightings, and the first was b'Ones;

He is called a Zav only after two sightings.

He learns from the Hekesh "Zos Toras [ha'Zav]" that [the verse] discusses a total Zav, and the first of the two sightings is equated to semen.

Suggestion: We infer from the wording. Since R. Eliezer needed to mention Korban, this implies that the first Tana discusses Tum'ah.

Rejection: If so, he did not answer the question at all!

Explanation: Rather, we infer that the first Tana does not require checking for Korban, rather, for Tum'ah;

If [he required checking] for Korban, he should obligate checking also the third [sighting], like R. Eliezer.

Question: Even without R. Eliezer, we could have asked from a Mishnah in Zavim (2:2). Rabanan explicitly say that we do not check the third [sighting]!

Answer: Perhaps we bring from R. Eliezer, for we want to explain what they argue about.

Question: What is Beis Hillel's reasoning, to bring a proof from Nidah? Beis Shamai's response is obvious (the blood would be Metamei her even if she had immersed)!

Answer: We bring [a proof] because Leidah is equated to Nidah - it says "ki'Ymei Nidas Devosah" and "v'Tam'ah Shevu'ayim k'Nidasah."

Implied question: There are many differences between them, i.e. all the challenges brought after this! (Why does the Gemara say that they argue about one matter?)

Answer: It gave the simplest matter that they argue about.

Explanation: In this case, even if she immersed, Beis Hillel are Metamei wet and dry. Our Mishnah discussed when she did not immerse, due to Beis Shamai [to teach that even then, they are Metaher].

Question: In Eduyos (5:4), this is taught among Halachos in which Beis Shamai are lenient and Beis Hillel are stringent;

When she spurts from within 40 days to after 40 days [after birth of a boy], Beis Hillel are lenient!

Answer: The question was asked regarding the beginning of [Yemei] Leidah [when she spurts from within seven days to after seven, or within 14 days to after 14. Then, Beis Shamai are stringent.]

Question: In any case, we need due to Beis Shamai (to teach that they distinguish Yoledes b'Zov from a regular Yoledes)! What did the Makshan think?

Also according to Rav, we need the Seifa only to teach that Beis Shamai agree that it is Metamei wet and dry, because she did not count!

Answer: According to Rav, it needs to teach that Beis Shamai agree about Yoledes b'Zov because it is one Mayan;

Had it taught only the Reisha, one might have thought that Beis Shamai hold that there are two Mayanos, and they are Metaher even Yoledes b'Zov, and Beis Hillel hold that it is one Mayan. Therefore, we need the Seifa;

However, according to Levi, who establishes the Reisha to discuss Shofa'as, and Beis Shamai hold that there is one Mayan, what does the Seifa teach?