Uktzin 2:3

A pomegranate or melon that’s partially rotten doesn’t combine; if it’s fresh at either end and rotten in the middle, it doesn’t combine. The “nipple” of a pomegranate does combine but the fibers that sprout around it don’t. Rabbi Eliezer says its “comb” (i.e., around the fibers) isn’t susceptible to impurity.

Uktzin 2:4

All peels are susceptible to impurity and convey impurity, and they combine. Rabbi Yehuda says regarding an onion’s three skins that the innermost combines whether it’s intact or perforated; the middle combines when it’s intact but not when it’s perforated; the outermost is insusceptible to ritual impurity regardless of its state.