The Torah does not inform us that Moshe divided the blood into two. Who divided it?
Rashi: An angel came and divided it.
What is the significance of the division of the blood?
What were the "Aganos" into which Moshe placed the blood?
Rashi: They were bowls into which he placed half the blood, one for half the blood of the Olos; the other, for half the blood of the Shelamim - to sprinkle on the people. 1
Ramban #1: They were receptacles made not in the shape of Mizrekos (the bowls that were generally used on the Mizbe'ach), which were used to sprinkle on the people. 2
Ramban #2 (citing Ibn Ezra and Targum Onkelos) and Targum Yonasan: Moshe used the regular basins to sprinkle both on the people and on the Mizbe'ach.
Why did Moshe sprinkle half the blood on the Mizbe'ach?
Rashi: Refer to 24:6:3:1.
Seforno: He appointed the Mizbe'ach as the Shali'ach of Hashem, and the blood that was due to be sprinkled on Hashem (Kevayachol) in His capacity as the other side entering into the covenant, he sprinkled on the Mizbe'ach.
Kerisus, 9a: This teaches us that Yisrael entered into covenant with the sprinkling of the blood.
What is the significance of the sprinkling of the blood on the people?
QUESTIONS ON RASHI
Rashi writes that an angel divided the blood. Why was this needed? We hold that a person can be precise!
Rosh: The angel divided the blood while it was in the animal, something that only an angel can do. 1
Moshav Zekenim: The angel closed the place of Shechitah of each animal after exactly half the blood left, and after Moshe threw the first half, the angel opened the wound, and Moshe collected the latter half.
Rashi writes that Avoseinu entered into the covenant with Milah, Tevilah and Haza'ah. Yevamos 46a says that Avoseinu circumcised, but did not immerse!
Riva: There, 'Avoseinu' refers to Avraham, Yitzchak and Yakov (but those who left Egypt immersed).
Tosfos Yeshanim (Yevamos 46a): R. Yehoshua holds that half the blood was sprinkled on the people, so we must say that they immersed. According to R. Eliezer, it was sprinkled on the Mizbe'ach to atone for the people. He said that they did not immerse.