Negaim, Chapter 3, Mishnah 7. This perek lays out the six types of nega'im: four kinds that appear on a person's body, one that appears in clothing, and one that appears in houses. The mishnah before us takes up the nega'im found on a head that has lost its hair, in the two regions called karachas and gabachas.
The mishnah begins: "Hakarachas v'hagabachas." The Rav directs us forward, observing that these two terms receive their full definition later in the tractate, in the tenth perek, where the Mishnah itself lays out exactly which area counts as karachas and which counts as gabachas. For now we can rely on what has already emerged: karachas is the hairless region toward the rear of the head, while gabachas is the hairless region toward the front.
Two weeks and two signs
"Mitam'os bishnei shavu'os uvishnei simanim: b'michyah uv'fisayon." These nega'im bring tumah upon a person across a span of two weeks, and by means of two signs.
The case under discussion is a man whose hair is gone, with a discolored patch surfacing on the exposed scalp, either in front or behind. Such a nega has two simanim of tumah. One is michyah: a spot of sound, living flesh that survives inside the discolored area, precisely as we find in the nega'im of ordinary skin, basar, where a square of live flesh sits within the nega. The other is pisayon: the discoloration widening beyond its original borders.
When each siman takes effect
"B'michyah bit'chilah, uv'sof shavua rishon, uv'sof shavua sheni, l'achar hapetur." Michyah, that island of living flesh, functions as a siman of tumah at four different junctures: when the kohen first inspects the nega; when the initial week of hesger draws to a close; when the following week draws to a close; and even once the kohen has pronounced the man tahor, should the michyah surface afterward, l'achar hapetur.
"Uv'fisayon b'sof shavua rishon, b'sof shavua sheni, l'achar hapetur." Expansion of the discolored patch, however, cannot function as a siman at the opening inspection, for nothing has yet been measured against which growth could be recognized. It brings tumah when week one concludes, when week two concludes, or once the petur has been given.
Thirteen days
The mishnah closes: "U'mitam'os bishnei shavu'os shehein shloshah asar yom." The tumah may surface across a stretch of two weeks, and those weeks add up to thirteen days rather than fourteen. Seven days make up the first period, and that seventh day, on which the kohen looks again, doubles as day one of the following period. This structure of two weeks governs both simanim alike, the pisayon as well as the michyah, that patch of raw living flesh, upon the hairless portions of the head.