Ohalos, Chapter 2, Mishnah 6. This chapter has been listing the various forms of tumas meis and the measures required for each of them to be metamei b'ohel. Our Mishnah now raises a question that follows naturally from all of those measures: what happens when the required amount is there, but it did not all come from a single body? Does the tumah of two separate meisim join together to make up one shiur, or must every shiur come from one and the same person?
The Cases in the Mishnah
The first case listed is that of "hashidrah ve-hagulgoles", the spine and the skull, when they are made up of material "mi-shnei meisim", coming from two corpses. Some portion of the vertebral column, or some portion of the cranium, is that of one niftar, while the balance belongs to a second body whose neshama has left it. Only when the two are taken together does a whole shidrah or a whole gulgoles emerge.
The next case is "o revi'is dam", or a quarter-log of blood, the amount the Mishnah has already established as a shiur, except that here it was drawn "mi-shnei meisim": one half of the liquid issued from one niftar and the other half from a second.
Then comes "ve-rova atzamos", a quarter of a kav of bones, which is the recognized measure of rova atzamos, but with the bones having been gathered "mi-shnei meisim", out of two distinct bodies rather than one.
Next, "ve-eiver min hameis", a limb taken from a corpse, when it too was put together out of two bodies. One section of the limb traces back to the first niftar and the remainder to the second, and only the two together add up to a single whole limb.
Finally, "ve-eiver min hachai", a limb detached from someone alive, in a case where the limb was assembled "mi-shnei anashim", from two living individuals: one of them supplied a portion of a hand and the second supplied the portion that was missing.
The Dispute
"Rebbe Akiva metamei": Rebbe Akiva holds that every one of these cases is subject to tumah. He maintains that the pieces join, and once they have joined, the required measure lies before you fully formed: an entire shidrah, an entire gulgoles, an entire eiver, a full quarter-kav of bones, or a full quarter-log of blood. The shiur exists, so the tumah takes effect, notwithstanding the fact that its source was two separate people.
"Ve-Chachamim metaherin": the Chachamim disagree and declare all of these cases tahor. The Bartenura explains precisely what this means. The Chachamim are not saying that there is no tumah here at all. Even a single bone the size of a barley grain, an etzem k'se'orah, carries tumas maga and tumas masa, and that much tumah is certainly present. What the Chachamim deny is tumas ohel. For a corpse to be metamei everything under one roof, the shiur must come from one person alone, and a measure assembled from two meisim does not create tumas ohel.
The Bartenura concludes that the halacha follows the Chachamim.