Negaim, Chapter 6, Mishnah 3. This chapter deals largely with the michyah, a patch of healthy living flesh found within a baheres, and with the various configurations in which it appears. Our Mishnah takes up what happens when either the baheres or the michyah changes in size after the person has already been declared a metzora.
The starting case
The Mishnah opens: "Baheres kagris u'vah michyah k'adashah". There is a baheres of the minimum measure, a gris, and within it a michyah the size of an adashah, a lentil. That is the shiur of a michyah: one lentil.
Keep in mind that a baheres of a gris takes up the space of nine lentils, and the michyah is entitled to sit in one of those nine without eating into the measure that the white must have; the shiur is not reduced by it. We have already encountered that rule. Here, then, the kohen has no reason to shut the man up for a week: at his very first inspection he pronounces him muchlat, since michyah is a sign of tumah that takes effect with no period of waiting at all. He is a metzora at once, in every sense.
Ravsa habaheres, temeiah
The Mishnah proceeds: "ravsa habaheres, temeiah", meaning that growth of the white leaves him tamei. In the Eliyahu Rabbah, the Vilna Gaon points out that this is said lav davka, not that the expansion is what generates the tumah; the michyah on its own would have rendered the nega temeiah even had the white remained exactly as it was. If so, what is this clause coming to add?
This is precisely what troubles the commentators, and a number of answers are given. One approach reads the clause as referring back to the situation taught just a short while earlier: the white spread and at that very moment the living flesh disappeared. His original chalitah had rested on the michyah, and now the spreading takes over and holds him in that same status.
A second answer is that the Mishnah restates the halachah so that it should be firmly anchored in our minds. We might have supposed that spreading operates only in the familiar setting in which we normally meet it, at the close of the first or the second week of hesger, while the man's status was still undecided and he had not yet been ruled tamei; on that assumption pisayon would be a siman capable only of moving a person out of taharah. Our Mishnah therefore says it again: even where the nega had already been declared tamei on the strength of the living flesh, and afterwards the white spread while that flesh vanished, the metzora stays in his tumah. The tzaraas is temeiah still.
Nismata, tohorah
The Mishnah continues: "nismata, tohorah". This is a straightforward halachah. If the baheres was a gris with a michyah of an adashah inside it, and the baheres then shrank to less than a gris, then even though the man had already been declared muchlat on the strength of the baheres kagris with its michyah, he is no longer tamei. A discoloration smaller than a gris is not a siman tumah at all.
A dispute in the girsa: ravsa hamichyah
The Mishnah before us then reads: "ravsa hamichyah, temeiah. Nismata, tohorah." The closing phrase is clear enough: if the michyah shrank below the size of a lentil, the nega is tahor. But the first phrase requires attention, because the Rav had a different text of the Mishnah.
In the Rav's text, when the michyah expands and eats away at the surrounding white, pushing outward until a full lentil's width of white no longer remains on every side of it, the nega is tohorah: the living flesh has lost its standing as mevutzeres, hemmed in at the center of the whiteness. The Rav's reading is therefore: ravsa hamichyah, tohorah.
The Tosfos Yom Tov observes that the Rav transcribed the word as tohorah rather than temeiah, and that the Rosh reads this way as well, as do other nuschaos of the Mishnah and the girsa of the Rambam. (The Rosh here is the rishon Rabbeinu Shimshon ben Avraham of Sens, the baal Tosfos Sens, from Sens in France.)
Nevertheless, the Rosh writes that the girsa we have before us, temeiah, can be sustained. He explains it as a case of baheres kagris b'lo michyah: that is, setting the michyah aside entirely, there still remains a full gris of white, of whichever of the shades or red admixtures the baheres happens to be, beyond the area taken up by the michyah.
Imagine a wide baheres eight hairs across and eight hairs down, sixty four square hair breadths in all, which comes to four lentils by four lentils. Seated in the middle is a michyah that has expanded to the size of four lentils, two by two, and a full lentil's width of white still encircles it on all four sides. The living flesh has indeed outgrown the single lentil with which it began, yet the white that remains amounts to a gris in its own right, and the flesh is still enclosed. Under such conditions tumah applies to both of them, and our text of the Mishnah reads perfectly well.
The Mishnah then closes a second time with "nismata, tohorah". Should the michyah contract to less than a lentil, while the white has not spread at all and stands exactly as it stood, neither enlarging nor altering, the tumah lifts. Once the living flesh has fallen below its shiur there is no siman tumah left, and the nega is tohorah, pure.