1. A Zav must count seven consecutive clean days before he can become pure.
2. There is a difference between whether he has an emission of a Zav or of Keri during these seven clean days.
3. The Gemara explains why an emission of Keri disqualifies only one day from the count of the Zav.
4. Not everything that is red is due to menstrual blood.
5. Shmuel explains why Rebbi Meir says that even an animal-like fetus that is "born" is considered a Halachic birth.
A BIT MORE
1. He must have seven days without an emission of a Zav or an emission of Keri.
2. If he has an emission of a Zav, he must begin his count of seven clean days again after the emission. If he has an emission of Keri, that day is subtracted and the previous clean days count towards his seven clean days.
3. The reason why Keri interrupts the count of a Zav is that there is a concern that there is a partial Zav emission with the Keri. However, the Torah teaches that for a partial emission of Zav only one day is disqualified, as opposed to a clear Zav emission which makes him start the entire count again.
4. The Gemara quotes two incidents wherein doctors explained that a woman who had red scabs or hairs coming out of her probably had a wound or birthmark that was causing these emissions. The doctors said that if these scabs or hairs would disintegrate when placed in water, then they indeed were blood. If they would not disintegrate in water, then they were merely scabs from a wound or hairs from a birthmark.
5. The Torah uses the word "va'Yitzer" -- "and He created" -- with regard to the creation of birds and animals just as it uses the word with regard to the creation of man. Rebbi Meir therefore derives from a Gezeirah Shavah that a fetus that looks like a bird or animal is deemed a Halachic birth.