I have often wondered why certain Masechtos start with abstract concepts. Our Mishnah starts with the Halacha of one who purchases a fetus of a donkey that belongs to a gentile. It would seem to be more appropriate for the Mishnah to begin its discussion with the basic laws of bechor, a firstborn animal or human. Perhaps the Mishnah is coming to teach us indirectly the fundamental concept of kedushas bechor, the holiness of the firstborn. It is said (Shemos 13:15) ויהי כי הקשה פרעה לשלחנו ויהרג ה’ כל בכור בארץ מצרים מבכור אדם ועד בכור בהמה על כן אני זבח לה’ כל פטר רחם הזכרים וכל בכור בני אפדה, and it happened when Pharaoh stubbornly refused to send us out, that HaShem killed all the firstborn in the land of Egypt, from the firstborn of man to the firstborn of beats.
This verse and other similar verses (see Bamidbar 3:11-13) throughout the Torah teach us that the essence of the mitzvah of bechor is that HaShem killed the Egyptian firstborns and the Jewish firstborns remained alive and were redeemed from exile. Similarly, our Mishnah commences with the Halacha of a Jew who purchases the fetus of a donkey that belongs to a gentile. This seemingly abstract ruling teaches us that we must maintain our distinction from the gentiles, and then HaShem will once again redeem us, speedily, in our days.