Bechorot 28: Blemishes, Experts, Blemishes Changing After Death

The priest must give a bechor to an expert to determine whether or not it has a mum, a blemish.  This ensures that a priest does not benefit himself when he might want to accept the blemished animal.  If the priest does not show the bechor to an expert, today's Mishna teaches: If one slaughtered the bechor and then showed its blemish to an expert, Rabbi Yehuda permits, whereas Rabbi Meir says since it was not slaughtered by the instructions of the expert, it is forbidden. 

In the Gemara, rabbis agree that an animal's blemish may change after it has died and thus it cannot be eaten.  This applies to blemishes in the body itself but not other parts of the body, like the ear, which would not change after death.  The rabbis argue about when one type of blemish affects another type of blemish.

The rabbis use dukin sh'ba'ayin, a minor eye condition, as their main example of a blemish that changes after death.  Steinsaltz offers us information about possible meanings of dukin sh'ba'ayin.