Negaim, Chapter 10, Mishnah 1. Until now the mishnayos have dealt with nega'im that appear on the skin of the body and on a wound. With this chapter we move to an entirely separate parshah in the Torah, the nesek, the nega that appears on the head or in the beard, that is, in those areas of the body covered by hair. The Torah sets out its laws in Vayikra 13:29-37, and because the ground rules there are different from the tzaraas of ordinary skin, the mishnah opens the chapter by laying out those rules in a single sentence.
"Hanesakim mitamin bishnei shavuos bishnei simanim: bese'ar tzahov dak uvepisyon." Nesakim become tamei over the course of two weeks and through two signs: fine golden hair and spreading.
Two weeks: the Kohen who sees the nesek closes the person up for a week, examines him, and if nothing has developed he closes him up for a second week. Only after the second examination, if no sign of tumah has appeared, is he released. This is the same span of hesger we are familiar with from tzaraas of the skin, and it stands in contrast to a shechin or a michva, whose hesger runs no longer than a single week.
Two signs, and only two. In the tzaraas of ordinary skin there were three ways for the nega to become tamei: se'ar lavan, michya and pisyon. In the nesek the Torah gives us se'ar tzahov dak, a fine hair that has turned golden within the bald patch, and pisyon, the spreading of the nesek beyond the boundaries the Kohen marked. White hair and living flesh have no standing here at all. On the contrary, in the nesek it is black hair growing in the patch that serves as a sign of taharah.
The mishnah then turns to the precise definition of that golden hair. "Dak" is understood as "lakui," a hair that is thin and weak rather than full and robust, and the Tannaim of the mishnah go on to differ over whether such a hair renders tamei no matter its length, whether short or long, or only when it is short.
With these two principles in hand, the length of the hesger and the two signs that govern it, the mishnayos that follow can take up the details of how a nesek is examined, marked and judged.