Negaim, Chapter 4, Mishnah 10. This stretch of the perek deals with a baheres that has spread (pissayon) and, at the same time, has lost part of its original body, the om of the nega. The question in every case is whether what remains is still the original nega with its spread attached to it, or whether the original nega has effectively vanished and we are looking at something entirely new.
One general observation belongs here before we open the mishnah itself. The rishonim and achronim disagree about the situation these mishnayos describe. Some understand them to be discussing a person who had already been pronounced tohor when his hesger ran out, with the changes occurring afterwards. Others understand them to be discussing the pissayon that shows itself while the hesger is still in force, in the initial seven days or in the second seven days, before the kohen has ruled either way. Both approaches hold up, and the commentators divide among themselves in how they carry them through the perek. The Rav, throughout the earlier mishnayos here, has taken the second route, treating these as cases examined when one of the two hesger periods expires.
The preceding case, as background
The mishnah immediately preceding ours discussed a baheres measuring one gris which afterwards spread by half a gris ("u'posasoh kachatzi gris"), while half a gris of the om itself faded away ("veholach min ha'om kachatzi gris"). Rebbi Akiva rules "teiraeh bis'chilah": the surviving half of the om is attached to the half gris that spread, and the whole is regarded as a brand new nega whose examination cycle starts over. The Chachamim rule that the man is tohor, and the Rav spells out why: a spread counts as pissayon only when it enlarges the nega beyond the dimensions it had when it was first seen. In that case the entire whitened area now measures a gris and nothing more, precisely the size it was at the beginning, so it is judged a baheres kagris that never changed at all, omed be'einov. A nega that remains exactly as it was when the hesger concludes, with no spread to speak of, leaves the person tohor.
The first case of our mishnah: a spread of half a gris and more
Our mishnah begins: "Baheres kagris, u'posasoh kachatzi gris va'od." A baheres measuring one gris spread by half a gris plus a further amount beyond that, and the word "va'od" carries the entire case. "Veholach min ha'om kachatzi gris": simultaneously, half a gris of the nega's original body faded from view.
Rebbi Akiva declares him tomei. Precisely as in the preceding case, he combines the half of the om that survived with the spread, but here the combination yields something larger than the original shiur, because of that extra bit, the "va'od." The half of the om together with the half gris and more of pissayon add up to the original nega with a genuine spread attached to it. The Rav puts it in these terms: Rebbi Akiva is metamei on account of the "va'od," since something of the pissayon remains over and above what was there at the start.
It is at this very line that the Rav writes that the spreading took place once petur had already been granted, and the shift is jarring. There are certainly rishonim, the Rosh included, who understand the whole perek as describing developments that occurred after the man had been sent away tohor. But the Rav has consistently been reading the earlier mishnayos as pissayon found while the person still sits in hesger, and now, without warning, he moves the case elsewhere. Rabbi Akiva Eiger is bothered by this, and the same objection is raised explicitly by the Tosfos Yom Tov: what compels the Rav to abandon his own approach precisely here? The Tiferes Yisroel, joined by others, keeps our mishnah within the Rav's earlier framework, treating it as a nega inspected when one of the two hesger periods comes to its close.
The Chachamim, for their part, declare him tohor. Their logic, as the Rav presents it: since a half gris departed from the om, the remaining half of the om is not joined to the spread at all. It does not combine with the chatzi gris va'od to confer upon the original nega the status of a nega that spread. The original nega is therefore treated as gone, and what stands before us is nothing but the pissayon on its own, a chatzi gris and a bit, which is less than the gris required for a nega altogether. Read within the framework of the second week: at the end of that week there is no valid pissayon, the original nega no longer exists, and there is not enough white in what remains to be a nega. Hence, tohor.
The second case: the om disappears entirely
The mishnah proceeds to a second case: "Baheres kagris, u'posasoh kagris va'od," a baheres measuring one gris that spread by a full gris and something more, "veholach lo ha'om," and on this occasion the om vanished in its entirety, so that not a trace of the original white patch is left and only the spread remains on the skin.
Rebbi Akiva declares him tomei. To his mind the spread never loses its connection to the nega from which it grew, even once that nega has itself disappeared. The pissayon issued from the original om, and it measures a gris and more, so it retains the status of a spread and renders the person tomei on the strength of the original nega.
The Chachamim say "teiraeh bis'chilah": once the om is gone, it is gone, and what stands there now is simply a white area of a gris and more which must be brought to the kohen as a nega in its own right, with the examination beginning from the start.
A practical kulah lies buried in the ruling of the Chachamim. Because this patch is classified as a fresh nega that has yet to undergo its first inspection, should the moment coincide with Yom Tov, with the intermediate days of the festival, or with the seven days of feasting that follow a chosson's wedding, the man does not present himself to the kohen at all. Earlier in the masechta we learned that a nega still awaiting its opening examination is not brought for viewing on such occasions. What emerges is a real, if only temporary, respite for the person concerned.