Negaim, Chapter 4, Mishnah 9. This chapter has been dealing with the interplay of the simanim of tumah, se'ar lavan (white hair), michyah (a patch of healthy flesh within the nega) and pisayon (spreading), and with the closely related question of when a nega before us is still the nega we examined earlier, and when it must be treated as something entirely new. Our mishnah presents a case that grows more intricate: a baheres that expands on one side while an equal portion of it withdraws on the other.
The case
"Baheres kagris u'pasasah k'chatzi gris." A baheres of one gris was present, and then it grew by another half a gris. The Rav locates that growth "l'tzad mizrach," on the eastern flank, meaning it pushed out to one side of where the original spot sat. The nega now covers a gris and a half, which is a full-fledged pisayon. Yet the story continues: "v'halach min ha'om k'chatzi gris," half a gris then vanished from the om, the original body of the blemish. The upshot is a baheres measuring precisely one gris once more, except that its position has slid half a gris eastward.
These cases are most easily understood, as the Rosh explains regarding this stretch of mishnayos, as happening after the p'tur, after the man had already been examined and released as tahor, and we now come to decide whether what stands before us is a new nega.
Rabbi Akiva and the chachamim
"Rabbi Akiva omeir: teira'eh bat'chilah." In Rabbi Akiva's view we must look at this as a nega acher, an entirely separate affliction. The gris of baheres now in front of us occupies ground half a gris to the east of where the earlier one sat, and that displacement makes it a fresh nega whose count opens from the beginning, with a first week of hesger.
"V'chachamim m'tarin," the chachamim rule that he is tahor. The Rav sets out their logic: growth qualifies as pisayon only where it leaves the blemish beyond the dimensions it had when it was first measured. In this case the total is unchanged. A gris was what we measured at the start, and a gris is what we measure now. The half-gris shift to the side does not alter the fact that we are looking at one gris and nothing more. And since the area is still just a gris, the Rav concludes, we treat it as the very same baheres that held steady without change and was pronounced tahor.
The practical result follows accordingly. If we are in the course of hesger, then once two weeks have passed with no additional growth, no pisayon, the man is tahor. And if this occurred after the p'tur, after he had already been released, he remains tahor, because what happened here is not reckoned as a pisayon at all.