This Mishnah, Bechoros 6:6, continues to deal with blemishes of the reproductive system, and this time with the testicles. In the hind part of the animal, between the hind legs, is the scrotum - a pouch of skin divided into two compartments, each called a 'pocket.' In each such compartment, one testicle is supposed to be present, and in both of them, two testicles. When this is not formed and developed properly, it is considered a permanent blemish.
The Mishnah lists two blemishes:
The animal does not have two separate pockets - The animal needs two separate pockets, each with its own testicle, and in their absence, it is blemished.
There are two separate pockets, but only one contains a testicle - The second testicle did not descend, and this too is a blemish.
The dispute between Rabbi Yishmael and Rabbi Akiva:
Rabbi Yishmael disagrees and holds: If two separate pockets are visible in the scrotum, it is a sign that the second testicle will also eventually descend, and although currently there is only one testicle there, this is not a blemish. But if there are no two separate pockets but rather only one pocket, it means there is only one testicle there, and the second one did not descend and will never descend.
Rabbi Akiva disagrees and establishes that one must check: They seat the animal on its buttocks, and squeeze the place where the scrotum connects to the animal's body, to feel if there is a testicle there that will eventually descend. If it is found - it will ultimately emerge and descend, and it is not a permanent blemish. And if a testicle is not found there - it is considered to have a permanent blemish, and it is permitted to slaughter this animal outside the Beis Hamikdash and eat it.
An incident that occurred:
An incident occurred where they acted according to the ruling of Rabbi Akiva: They seated the animal, squeezed to bring out the second testicle, and did not find it, and therefore they slaughtered the bechor as a blemished animal. After the slaughter, it was discovered that there was a second testicle there, but it had not yet descended, and it was attached high up, in the inner thigh of the animal.
They disputed the law in this case:
According to one opinion: The slaughter is valid and the meat is permitted to be eaten. Since the testicle was so high up, it was impossible to feel it while squeezing, and it would never have emerged - this is a permanent blemish, and there is nothing more to discuss about the matter.
Rabbi Yochanan says: Once it was discovered that there was a testicle there, it is proven that it would have descended later, and this is not a permanent blemish. While no prohibition was committed during the slaughter itself, since it became clear that it had a testicle that had not yet descended and would have descended in the future, it must be treated as an unblemished animal, and therefore it is obligatory to bury it and not eat it.
The Halachah: The halachah follows Rabbi Akiva in this Mishnah - one must check if there is a testicle present that has not yet descended and is waiting to descend. If it is found during the check - this is not a blemish; if it is not found - it is blemished. And if afterwards, when opening the animal, a testicle is found high up that could not be felt and could not be squeezed and brought out at all, this bechor is still permitted to be eaten.