Bechoros 2:5 - 2:6

רחל שילדה כמין עז – even though that both of them are species they are liable for [the law] of the firstborn, nevertheless, if it is not similar to its mother, it is exempt from [the law] of the firstborn. As is it written (Numbers 18:17): “But the firstlings of cattle, [sheep or goats may not be redeemed; they are consecrated],” until it [the cattle/ox] and its first-born is a cattle/ox.

ואם יש בו מקצת סימנים – that it is similar to its mother, until that it is recognized in them that it is from the same species, it is liable.

שנאמר הזכרים לה' – [the word] "זכרים"/males – implies two.

אי אפשר – to ascertain exactly their two heads came out as one (see Tractate Bekhorot 17a), but rather, the one came out first and we don’t know which of them it was.

בורר לו את היפה – that in general, the better and [more] healthy one goes out at the beginning.

משמנים ביניהם – the owners and the Kohen do not have an advantage one on the other in their division other than the [permitted] fat, for the Israelite takes the fat and the leaves the lean one to the Kohen, for he who wants to exact [compensation] from his fellow bears the burden of proof (see Tractate Bava Kamma, Chapter 3, Mishnah 11). And the Halakha is according to Rabbi Akiva (see Talmud Bekhorot 18a).

והשני – that remains to the Israelite, it should pasture until a blemish befalls it and afterwards it may be consumed, for it is a doubtful firstling , therefore, it is not slaughtered as pure. But that which belongs to the Kohen, it is not necessary to state that certainly he does not slaughter it until there should be a blemish, but we are speaking about at this time.

וחייב במתנות – the Israelite, when he slaughters it, he would give to the Kohen the foreleg, jaw and maw. For whichever way you turn, if it is firstling, all of it goes to the Kohen, but if it is not a firstling, he (i.e., the owner) is liable to the priestly gifts.

ורבי יוסי פוטר – for this is as if the Kohen was worthy of it and it reached his hand but if a blemish befell it, it is given to an Israelite. And it is found that it is exempt from the priestly gifts, for since we consider it as if it came from the hand of a Kohen to an Israelite. And the Halakha is according to Rabbi Yossi.

רבי טרפון אומר יחלוקו – for Rabbi Tarfon retracted from what he said above [in this Mishnah] that the Kohen selects for himself the better part, for he holds that while alive also we compromise with both of them (i.e., the Kohen and the Israelite) and divide the proceeds. For both of them have a part in both of them (i.e., the animals – the two males). Therefore, if one of them died, they should divide the living one.

רבי עקיבא אומר המוציא מחבירו עליו הראיה – and the Halakha is according to Rabbi Akiva.

זכר ונקבה אין לכהן כלום – for perhaps a female [animal] came out first and he who wants to exact [compensation] from his fellow bears the burden of proof. But here, even Rabbi Tarfon admits, that there, when he disagrees because certainly that one [animal] goes to the Kohen, therefore it strengthens one’s right to divide it equally, but here, his power is lessened for perhaps the issue of first-born status is not connected at all. But however, it should go to pasture until a blemish befalls it and afterwards, he can consume it, for it’s a doubt.