The Pasuk (verse) gives us two criteria for an animal to be considered a Bechor. Number one, it has to be a Bechor. It has to be the oldest child, the firstborn from its mother. Secondly, it has to be peter rechem. It has to be the one that emerged out of the mother's womb. Here we'll talk about what happens if an animal possesses only one of these qualifications, not both. We'll talk about a case where the mother gave birth by C-section. The animal was cut out; it didn't go through the womb. If a firstborn animal is born by C-section and then another animal is born naturally afterward, both animals should be left to graze until they develop a blemish. After they develop that blemish, the owner could eat them in that state. Each animal may be the Bechor. The first one that was born may be the Bechor because it was born number one. The animal born second may also be a Bechor because it was the peter rechem. It's the one that opened up its mother's womb. So what do we do in this situation? We don't have to hand them over to the Kohen (priest) because Hamotzi me-chaveiro. I'll have the Kohen's obligation to prove that it's his, and he can't do that since it's a case of Safek (a doubt). So we allow them to graze, and they eventually develop a blemish.
The rationale behind this opinion is that we have a doubt. According to Rabbi Tarfon, we don't understand what makes a Bechor. Is it one of the two qualifications? Is it either being the oldest? Is it either being peter rechem or is it both? You need both qualifications. So we don't have clarity here. Therefore, we say that they are not definitely the Bechor, because it's possible you need both qualifications here. Therefore, the animals in this situation are left to develop a blemish, and the owner is allowed to eat them.
But Rabbi Akiva is going to disagree. Rabbi Akiva says that they are not a Bechor. Both animals here are definitely not a Bechor. The owner is allowed to eat them right away. They have nothing to do with the laws of Bechor. The animal that was born first is not a Bechor because it did not open the womb as well. Since it was not born in a natural way, it's not considered to be a Bechor. You need both qualifications. Likewise, the animal that was born second is not a Bechor because another animal preceded it. According to Rabbi Akiva, there's no doubt: a Bechor is only an animal that both opened up the womb and was the first one born. In this case, each animal is missing one of these two factors. Therefore, neither one of them is a Bechor.