Kommentar zu Bekhorot 2:9

יוצא דפן (a fetus extracted by means of the cesarean section) – its mother was operated upon by a section and they removed the fetus through its walls.

Introduction Our mishnah deals with an animal born through what is today called a cesarean section. The term the Mishnah uses is “one who came out through the wall” because this animal was taken out through the wall of its mother’s stomach.

והבא אחריו – that which is born after it through the path of the womb.

With regard to [an animal] that was born through a cesarean section and the first born which came after it:
Rabbi Tarfon says: both go out to pasture until blemished and are eaten with their blemishes by the owners. Rabbi Tarfon believes that both animals are doubtful first borns. The one born through cesarean section was the first born and the second one was the one who opened its mother’s womb, the phrase the Torah uses to describe a first born (see Exodus 13). Therefore, both must go out to pasture until they become blemished and then they may be eaten (blemishes and all) by their owners.

ירעו עד שיסתאבו – for it is doubtful to Rabbi Tarfon if the firstling of the offspring is holy even though it is not a firstling to the womb, such as a fetus extracted by means of cesarean section which is the first that is born even though it did not open up the womb. If it is a firstling to the womb, it is holy even though it is not the firstling to the offspring, as for example, that which comes after one born by means of cesarean section which is the first that opened the womb bur is not the firstling of the offspring since the one born by means of cesarian section preceded it. But from doubt, both are sent to pasture until they develop blemishes and their owners will consume them with their blemishes for perhaps, we require both [that it is the first to open the womb and the first one born] of them to be firstlings of offspring and to [the opening of] their wombs, but neither one of them is holy. But he who wants to exact [compensation] from his fellow bears the burden of proof. But for Rabbi Akiva, it is obvious to him that we require both [of these prerequisites] require, and not a single one of them is holy. And the Halakha is according to Rabbi Akiva.

But Rabbi Akiva says: neither of them is a first born; the first because it did not open its mother’s womb, and the second, because another preceded it. According to Rabbi Akiva, in order for the rule of the first born to apply, the animal must be a first born and be the first to open its mother’s womb. The first born did not open its mother’s womb and the second was not the first born, so the law of the first born does not apply to either.