Negaim, Chapter 8, Mishnah 10. The subject of this chapter is pricha, the case of a baheres that expands until the whole body is covered in white, governed by the principle that when the spread begins from a condition of tumah it purifies, while a spread that begins from a condition of taharah defiles. The mishnah before us adds a further factor that was present from the very beginning: a michyah, an island of healthy flesh inside the white area, which is itself a siman tumah. The Tannaim then disagree about how to classify the man once the whiteness engulfs that michyah and the extremities afterward come into view once more.
The Case
"Habah kulo lavan uvo michyah ka'adashah": a man comes before the kohen entirely white, and within that whiteness there is a michyah the size of a lentil. A michyah of that size renders him a muchlat, and by right the kohen should declare him tamei the moment he examines him.
The Rav offers two ways of picturing the situation, and each one holds up. According to the first, no ruling of muchlat was ever issued: "shelo hispik lehachlito", the kohen had no chance to pronounce him tamei, "ad sheparcho bekulo", since events outran him and the whiteness had already covered his entire body. According to the second, the kohen did in fact rule him a muchlat on the strength of the michyah, and everything the mishnah goes on to describe unfolds after that ruling.
The mishnah continues: "parcha bekulo, ve'acharkach chazru bo rashei evarim." The nega expanded across all of his skin, and in the process it buried the michyah together with everything else. When the spread swallows the michyah as well, this is a pricha that issues from tumah, and such a pricha is metaher. Later the rashei evarim, the tips of the limbs, reverted: clean skin showed itself on them once again. The issue is whether that renewed appearance of healthy flesh brings the tumah back with it.
Rebbi Yishmael
Rebbi Yishmael says "kachaziras rashei evarim", that the reappearance of clean skin on the extremities in our case follows the law of "baheres gedolah", the large baheres, where such a reappearance does not defile the man. The nega is not turned into a source of tumah for this reason. The Rav points out, though, that hesger still applies: he is shut in and kept under observation, but he does not carry the status of a muchlat.
Rebbi Elozar ben Azaryah
Rebbi Elozar ben Azaryah rules otherwise, and the reason lies in the michyah that was there from the start. He compares "chaziras rashei evarim", the return of the extremities, to "baheres ketanah", a small baheres holding a michyah, since any baheres that contains a michyah, whatever its measurements may be, is judged under the heading of baheres ketanah uvo michyah. On that basis: the whiteness expanded and buried the michyah, so the man becomes a musgar; and when skin later surfaces inside that baheres, even if only at the tips of the limbs, that exposed flesh serves as a michyah in its own right, and he is tamei muchlat.
The dispute, then, comes down to a single practical question: is this man a musgar, held under quarantine, or a muchlat, definitively declared tamei?
The Rav's Second Explanation
Alongside the first reading, in which the spread and the return of the extremities came before the kohen had any opportunity to rule the man a muchlat, the Rav supplies an alternative: "ve'od yeish lefaresh", there is another way to explain it, "de'afilu hechlito bemichyah", even where the kohen already ruled him a muchlat on the strength of the michyah, a michyah of a lentil's size being enough to make a man muchlat, and then "u'parcho bekulo", the white covered all of him, the michyah included, "vetiharo", and the kohen declared him tahor.
Even in such a case, Rebbi Yishmael holds that there is no renewed tumah when the extremities reappear. His reasoning: when does the reappearance of healthy skin on the extremities after a pricha generate tumah? Only where the man's taharah came to him betoras pricha, through the mechanism of the spread itself, a pricha out of tumah that purified him. Here, though, his taharah did not come through the pricha. It came about because the michyah was reduced and disappeared: the baheres covered the michyah over, "de'ilu nisma'ato michyah tahor", for had the michyah simply diminished on its own he would likewise have been tahor. Since the taharah rests on the elimination of the michyah rather than on the pricha, the later exposure of the extremities does not undo it.
One further point emerges from this. The healthy skin now exposed on the extremities must be smaller than the size of a lentil, less than the measure of a michyah ka'adashah; otherwise it would constitute a michyah in its own right and would render him tamei on that basis alone.