Parah, Chapter 2, Mishnah 1. This chapter takes up the question of which conditions in a cow disqualify it from serving as a parah adumah and which conditions we may safely ignore. Our mishnah opens with two disputes between Rabbi Eliezer and the Chachamim, and then broadens out into a general rule about where korbanos may be brought from.
The pregnant cow
"Rabbi Eliezer omer: paras chatas hamuberes ksheirah": Rabbi Eliezer holds that a cow designated as the parah adumah remains fit even if it is carrying a calf. "Vachachamim poslin": the Sages rule that such a cow is unfit.
The two views turn on how we regard the fetus. To Rabbi Eliezer the calf inside the cow is simply part of the mother's own body, and a cow does not become disqualified on account of its own limbs. The Sages view the fetus as something distinct from the mother. Once it is a separate entity, the cow that carries it is at this very moment engaged in labor, the labor of bearing a burden, and a parah adumah that is put to work is thereby rendered unfit.
Purchasing the cow from a gentile
A second argument follows in the same mishnah. Rabbi Eliezer maintains "einah nilkachas min hagoyim," that such an animal is not to be bought from a non-Jew, while the mishnah records "vachachamim machshirin," that the Sages consider a cow obtained this way perfectly fit.
Rabbi Eliezer's concern is that a gentile owner may have allowed his animal to be mated with a beast of another species. Such a union counts as a defect in the cow, and a defective cow cannot serve as the parah adumah. The Sages are not worried. Mating of that sort leaves the animal sterile, and no owner willingly ruins his own herd by making his cow barren. Since it is against the owner's own interest, we do not suspect that it happened.
The broader rule: from where may korbanos come?
The mishnah adds that this disagreement is not confined to the parah adumah: "velo zu bilvad, ela kol korbanos hatzibbur vehayachid ba'in meiha'aretz umichutz la'aretz, min hechadash umin hayashan." Not only in the case of the red cow do they differ, but the same debate extends to every offering, communal and individual alike. According to the Sages these may all come from within Eretz Yisrael or from outside the land, and from the new crop or from the old, even though in practice many of them would be obtained from non-Jews outside the land.
Two items are removed from this leniency, "ha'omer ushtei halechem," concerning which the mishnah states "she'einan ba'in ela min hechadash umin ha'aretz." The two loaves of Shavuos and the barley offering of the omer are limited on both counts: only grain that grew in Eretz Yisrael, and only grain of the current season, may be used for them.
Each of these limitations has its own scriptural anchor. In the passage about the omer the Torah introduces the mitzvah by speaking of your entry into the land, which ties the offering to produce of Eretz Yisrael. As for the two loaves, the Torah directs that fifty days be counted, reaching the day after the seventh week, and that a minchah chadashah, a meal offering of the fresh grain, then be brought before Hashem; the very name of that offering shows that older grain will not do.
Thus a single mishnah moves from the private question of one cow's fitness to a principle governing the entire order of korbanos in the Beis Hamikdash.