Chullin 77 - Amniotic Sac
If one slaughters an animal and finds inside an amniotic sac, one he is not squeamish can eat it, because the Torah permitted it with the phrase " Every animal (even an amniotic sac)... you may eat ." However, it is not really food, and therefore it does not carry the ritual impurity of foods. When one planned to eat it, this thought elevates it to the status of food, and now it can carry ritual impurity.
If an amniotic sac partially emerged from the womb before the the animal was slaughtered, the complete sac is prohibited for consumption even after the animal was slaughtered. There is a suspicion that the partially emerged part contained the head of the fetus, which would make it legally born, and then the shechitah of the mother would not make the sac permissible.
If an animal carrying its first miscarried an amniotic sac, it may or may not have been a male firstborn, and even if it was a male, it may have been deformed, therefore it can thrown to the dogs. However, if the mother was a sacrifice, the sac needs to be buried -- because both male and female young would be sanctified.
Art: Charles Émile Jacque - A Flock Of Sheep In A Barn