Orlah Chapter 3, Mishnah 3. In the previous Mishnah we dealt with someone who dyed a thread with orlah peels - with dyes produced from the peels of orlah fruit - and afterwards wove that thread into a garment. We learned that according to Rabbi Meir the dye is not nullified, while according to the Sages it is nullified in one part out of two hundred.
Our Mishnah departs from the subject of orlah and turns to the wool of a bechor (firstborn animal):
"Ha'oreg melo hasit mitzemer habechor babeged - yidalek habeged" - one who weaves a measure of "a full sit", which is the measure we mentioned in the previous Mishnah (the distance from the index finger to the middle finger), of thread that comes from the wool of a firstborn animal, and inserts it into a piece of clothing. The halachah is that one does not shear the wool of a firstborn animal, and this wool is forbidden to derive benefit from - no benefit at all may be taken from it. Therefore, if he wove it into a garment, the garment must be burned: the entire garment is destroyed.
At first glance this ruling fits the view of Rabbi Meir in the previous Mishnah, but the question is whether it was stated specifically according to his view, or whether even the Sages agree with it. There is a discussion about this among the commentators and even among the Rishonim.
The hair of a nazir and of a firstborn donkey:
The Mishnah continues with the same idea: "Umise'ar hanazir umipeter chamor basak - yidalek hasak" - and from the hair of a nazir or of a firstborn donkey into a sack - the sack must be burned. We have two cases before us:
The hair of a nazir: a person's hair, which is forbidden to derive benefit from.
A firstborn donkey: the hair of a firstborn donkey, which is likewise forbidden to derive benefit from. There is a discussion as to exactly when this applies: is the prohibition in force before it is redeemed - since the way it is redeemed is with a lamb - or is the case specifically one where a person chooses not to redeem it and breaks its neck, so that perhaps only under those circumstances is it forbidden to derive benefit from it.
In any case, whenever the hair of a firstborn donkey is forbidden to derive benefit from, if a person took some of it, or of the hair of a nazir, and worked it into a sack - the sack must be burned, exactly as we said regarding the garment, because it is not nullified. And the reason the Mishnah speaks specifically of a sack is this: hair, unlike wool, is not normally woven into a garment, but it can be made into a sack. That is simply the nature of the material.
"Uvamukdashin - mekadshin bechol shehu":
When we are dealing with something that is consecrated, other than a firstborn animal, even the smallest quantity is considered to carry the sanctity of consecrated items, and it is not nullified.
The reason for this, says the Yerushalmi, is that we are dealing with consecrated items that have a redemption, that can be redeemed - whereas a firstborn animal is not redeemed, and therefore it is not included here. And since we are dealing with consecrated items that can be redeemed, the halachah is that anything which can become permitted, here through redemption, is called "a thing that has a means of becoming permitted". And since it has the ability to become permitted, it is not nullified even in a thousand, and even the tiniest quantity causes the rest of the garment or the sack to be considered consecrated along with it, and it is not nullified.
In summary: in this Mishnah we learned that wool of a firstborn animal woven into a garment in the measure of "a full sit" causes the garment to be burned, and likewise the hair of a nazir or of a firstborn donkey worked into a sack causes the sack to be burned, since all of these are forbidden to derive benefit from and are not nullified. We considered the question of whether this ruling follows Rabbi Meir alone or holds even according to the Sages, and the reason for the distinction between a garment and a sack. Finally we learned the law of consecrated items, that they consecrate in any amount whatsoever, and its reason according to the Yerushalmi: consecrated items that have a redemption are "a thing that has a means of becoming permitted", and therefore they are not nullified even in a thousand.