Demai, Chapter Five, Mishnah 4. This mishnah continues the ideas of the previous mishnah, dealing with buying bread and with the question of whether we must be concerned that part of the bread was tithed and part of it was not.
What is a 'palter':
A palter is a bread dealer who does not bake the bread himself. The question under discussion is whether the palter buys his merchandise from several different bakers or from only one baker.
The opinion of Rabbi Meir:
"HaLokei'ach min hapalter - me'aser mikol tefus vatefus, divrei Rabbi Meir" - one who buys bread from a palter must tithe from each and every mold separately. The different shapes may well have come from different bakers, and therefore we must be concerned that one of them tithed and the other did not.
The opinion of Rabbi Yehudah:
"Rabbi Yehudah omer: me'echad al hakol" - in his view the buyer may tithe from one loaf on behalf of all the loaves he bought from a single palter, and there is no need for concern, since the palter receives his merchandise from one baker, and that is the full extent of the concern.
Even according to Rabbi Yehudah, however, this holds true only when everything is from the same day, as we learned in the previous mishnah: even with a single baker, if he has bread from today and from yesterday, we must be concerned that they came from two different sources of grain. It emerges that his words "me'echad al hakol" were said only regarding what was baked on that same day, even if the loaves are of different types and shapes.
Rabbi Yehudah's concession regarding one who buys from a nachtom:
"Modeh Rabbi Yehudah belokei'ach min hanachtom shehu me'aser mikol echad ve'echad" - the nachtom was a bread dealer who held the exclusive right to sell all the bread in the city, and his name says it all: a monopoly on the sale of bread. Naturally he receives his merchandise from all the bakers in the city, and therefore Rabbi Yehudah concedes to Rabbi Meir that one must tithe from each and every shape separately, since each shape may have come from a different baker, one who tithed and one who did not.
In summary: in this mishnah Rabbi Meir and Rabbi Yehudah disagree about one who buys bread from a palter: Rabbi Meir requires tithing from each shape separately, out of concern that the shapes came from different bakers; and Rabbi Yehudah permits tithing from one on behalf of all, assuming that the palter buys from a single baker, and provided that everything is from the same day. But regarding one who buys from a nachtom, who holds the monopoly on the sale of bread in the city and receives from all the bakers, Rabbi Yehudah concedes that one must tithe from each and every one.