Negaim, Chapter 6, Mishnah 6. From here the Mishnah begins presenting new combinations of baheres and michyah, arrangements in which one nega sits inside another, and we must sort out which patch of skin carries a siman tumah and which does not.
A preliminary correction about the shape of a michyah
Until now we have described the michyah as healthy flesh sitting in the center of the baheres, the size of an adashah (a lentil), and we have also described it as being whole, that is, a solid unbroken patch. That second detail turns out to be the subject of a machlokes Tannaim, and the opinion requiring a solid patch is not the halachah. According to the accepted view, the michyah qualifies so long as it contains the area of an adashah and has a square shape, even if the healthy flesh forms only the perimeter, the frame of that square, and something else fills the middle.
The case
Imagine an expansive baheres. Set into it is a michyah built like a hollow square, a frame of healthy flesh that meets every condition of a valid michyah. In the empty space at the center of that frame lies another baheres, one gris in area. The Mishnah takes up the innermost of these three areas first.
"Baheres k'gris u'michyah k'adashah makifaso, v'chutz lamichyah baheres": a gris of baheres, ringed by an adashah's worth of michyah, and beyond that ring a further baheres, again a gris in area.
The ruling of the Tanna Kamma
"Hapnimis l'haskir": the michyah wrapped around the inner patch does nothing to it. That inner patch is no more than a gris of baheres with no siman tumah present, so the kohen shuts it away for a week and observes what develops.
"V'hachitzonah l'hachlit": the outer baheres, however, is a baheres a gris in size with a michyah inside it, and a baheres with a michyah is declared muchlat by the kohen on the very first viewing. The person is a full metzora on account of the outer nega.
Rebbe Yosi disagrees
Rebbe Yosi takes issue. His words are: "ein hamichyah siman tumah" (the strip of healthy flesh does not serve as a sign of tumah) "lachitzonah" (for the outer patch), "shehabaheres l'sochah" (because a baheres rests within it). Rebbe Yosi maintains the definition given at the outset of our masechta, that a michyah be solid, a square of healthy flesh filled in throughout. Here the square is hollow, occupied by a baheres, so in his view it cannot render the outer baheres tamei. The halachah, however, sides with the Tanna Kamma rather than with Rebbe Yosi.
When the michyah disappears
"Nismatah v'halchah lah": the michyah gradually contracted until it was gone entirely. What caused its disappearance? Either the inner baheres expanded outward into it, or the outer baheres expanded inward into it.
Rabban Gamliel addresses the first possibility: "im mibifnim hichlah" (if the michyah was consumed from the inside), then "siman pisoyon lapnimis" (that counts as spreading for the inner nega). Rebbe Meir's position, that a baheres swallowing up a michyah situated within it constitutes pisoyon, is not the halachah. Even so, Rabban Gamliel teaches that in this particular arrangement, when the inner baheres grew, took over the area of the michyah and wiped it out, that growth is a genuine pisoyon for the inner baheres, which is accordingly declared muchlat.
"V'hachitzonah tehorah." The outer baheres, meanwhile, becomes tahor. The rule is that when a nega was declared muchlat on the strength of a siman tumah and that siman then vanishes, the person is tahor from that nega. Here the michyah, which was the outer baheres's siman tumah, is gone, so the outer nega no longer renders him tamei. The result is that the inner baheres is now tamei through pisoyon while the outer one has become tahor.
Now the reverse possibility: "im mibachutz" (if the erasure came from the outside), meaning that the outer baheres pushed inward and blotted out the michyah. Then "hachitzonah tehorah": the outer nega is pure in any event, both because its advance into the michyah is not reckoned as pisoyon (we do not follow Rebbe Meir) and because the michyah that had made it tamei no longer exists. As for the inner nega, "v'hapnimis l'haskir": it remains an ordinary gris of baheres and nothing more. Should this have happened as the first period of hesger ran out, another seven days of closure are required; should it have happened as the second period ran out, it too is pronounced tahor, having failed to spread over both stretches of time.
Rebbe Akiva
Rebbe Akiva's verdict: "bein kach uvein kach" (whichever of the two occurred), the skin is "tehorah", and this is the accepted halachah. His reasoning: if a baheres reaching inward over a michyah held within it is not pisoyon, then equally no expansion taking place within the borders of another baheres can be called pisoyon. The inner nega lies wholly inside the domain of the large outer baheres, so its enlargement carries no halachic weight as spreading. Since the inner one has no pisoyon and the outer one has been stripped of its siman tumah, neither renders the person tamei.
The Tiferes Yisrael draws out what this means in practice according to Rebbe Akiva. Had the entire development taken place as the first period of hesger closed, the outer baheres, now without its michyah, is fully tahor, while the inner one must still be shut away for another seven days. Had it taken place as the second period closed, both patches are tahor.