Tractate Negaim - Chapter 3 - Mishnah 6 נגעים פרק ג׳ משנה ו׳

Scalp baldness or forehead baldness [negaim] become unclean for two weeks and by one of the following two signs: by quick flesh or by a spreading. "By quick flesh," in the beginning, at the end of the first week, at the end of the second week, or after they have been pronounced clean. "Or by a spreading," at the end of the first week, at the end of the second week, or after they have been pronounced clean. They become unclean for two weeks which are only thirteen days.