In the next few Mishnayot, we'll talk about a machloket (dispute) between Beit Shammai and Beit Hillel. Beit means the school of, the students of Beit Shammai and Hillel. But Shammai the elder, Shammai himself, is going to disagree with both of them.
The first machloket will be about the bones that come from a dead body. A dead body is the highest possible level of Tum'ah (ritual impurity). If any person or object touches that body or carries it, they automatically become tamei (ritually impure). It also could spread Tum'at Ohel, through making a roof with that dead body. If a person goes and makes a roof over that body, then the person is tamei. Or if the body is on top of a person, that person is tamei as well. Or if the body and another person are under the same roof, then that person or object is tamei.
Of course, the entire body could spread Tum'at Ohel, but even a part of the dead body, if it's a certain size, could spread this Tum'ah as well. For example, human bones could spread Tum'at Ohel if there's a quarter-kav (a unit of volume) of bone, a certain measurement of bone; then it could spread the Tum'ah. Here we'll have a machloket about what kind of bones have to make up that quarter-kav in order to spread Tum'ah through Tum'at Ohel.
According to Beit Shammai, the quarter-kav of bone could spread Tum'ah through Ohel with any combination of bones. It doesn't matter what type of bones or what kind of bones from the person, whether it's from one corpse or from two or three corpses. No matter what, if you have a Rova HaKav, you have that amount of bone, of human bone, that would spread Tum'at Ohel.
Beit Hillel disagrees. Beit Hillel omrim, says Beit Hillel, first of all, that the quarter-kav of bone spreads Tum'ah through Ohel only if the bones all come from a single dead body. You can't have multiple dead bodies. And the bones can't just be any combination of bones from the body; they have to either come from the majority of the frame of the skeleton. For example, let's say they have two leg bones and a thigh bone. That is the majority of the skeleton's height. A combination of things that creates the majority of the skeleton's height would create the Tum'ah. Or they have to be a majority of the bones of the body. The human body has 248 bones. So a majority of the skeleton is 125 bones. If there are 125 bones, no matter what part of the body they come from, that would spread Tum'ah as well. Otherwise, they don't spread Tum'ah through Ohel. That is the opinion of Beit Shammai and Beit Hillel.
But they both agree that the quarter-kav has to come from more than one bone from the dead body. It can't just be one single bone. Shammai will disagree. Shammai omer afilu me'etzem echad. The quarter-kav that comes from a single bone could spread Tum'ah through Ohel as well. You don't need multiple bones. Even one bone that's a quarter-kav would spread Tum'ah to Ohel.