Uktzin, Chapter 2, Mishnah 3. This Mishnah continues the theme that occupies our chapter: the law of a shomer, that portion of a food which shields and preserves the fruit. Since such a portion performs a service for the fruit, halachah regards it as part of the fruit, so that it contracts tumah along with the fruit and is also counted together with it when we measure the k'beitzah, the quantity of food required before that food can transmit tumah onward. Here the Mishnah takes that rule and asks two things: how do we treat a portion of the fruit that has gone bad, and among the various small growths found on a pomegranate, which ones have the standing of a shomer.
"Harimon v'ha'avatiach shenimak miktzaso, eino mitztaref" - a pomegranate or a melon, part of which has rotted away. The decayed portion is no longer reckoned as fruit for the purposes of tumah, and it therefore does not join in toward the shiur of a k'beitzah that would enable the fruit to convey tumah further.
The Mishnah then presents a sharper case. "Shalem mikan u'mikan v'nimak me'emtza, eino mitztaref" - picture a watermelon whose two ends are perfectly sound and fresh, while the middle has rotted. Here not only is the rotted middle excluded; even the two healthy ends do not combine with one another. The spoiled section in the center effectively divides the fruit into two separate pieces, and two separate pieces are not joined together to reach the measure of a k'beitzah.
"Pitmo shel rimon mitztarefes" - the protrusion at the top of a pomegranate does combine with the fruit. Why? Because it functions as a genuine shomer: once it is removed, the pomegranate begins to decompose. Since it guards the fruit, it has the status of the fruit and counts toward the shiur.
"V'hanetz shelo eino mitztaref" - the thin, hairlike threads perched on that protrusion, however, are judged otherwise. Since they safeguard nothing at all, they cannot be called protectors of the fruit, and they contribute nothing toward the measure of a k'beitzah.
"Rebbi Eliezer omer, af hamasreik" - Rebbi Eliezer adds a further exclusion. At the very top of the pomegranate the skin forms a ring of little tooth-like points, and Rebbi Eliezer holds that this too does not qualify as a shomer for the fruit: were it to be cut away, the pomegranate would remain fresh, and a part whose removal does not harm the fruit is no shomer. Whether that portion nevertheless serves as a yad, a handle, and what precisely the Tanna Kamma maintains regarding it, is a matter of dispute among the meforshim.
The thread running through the whole Mishnah is one and the same: a piece of produce counts as food, and joins the shiur of a k'beitzah, only when it is either the fruit itself in sound condition or a part that genuinely stands guard over it. What has rotted away, what merely sits decoratively on top, and even sound fruit that has been split in two by decay, all fall outside that reckoning.