Hullin, Chapter Four, Mishnah Seven
Our mishnah deals with the status of the amniotic sac found in a slaughtered animal.
1) If a person slaughtered an animal and found in it an amniotic sac, he who is not fastidious may eat it.
2) It does not contract uncleanness, either food uncleanness or the uncleanness of nevelah.
3) If he intended to eat it, it can contract food uncleanness but not the uncleanness of nevelah.
4) If part of the amniotic sac emerged [before the slaughtering of the mother], it may not be eaten;
a) For it is a sign of birth in a woman and also a sign of birth in an animal.
5) If an animal which was pregnant for the first time miscarried an amniotic sac, it may be thrown to dogs.
a) But in the case of a consecrated animal it must be buried.
b) It may not be buried at cross-roads or hung on a tree, for these are amorite practices.
Section one: When the mother is slaughtered, her amniotic sac is permitted to be eaten by virtue of the slaughtering of the mother. However, the mishnah notes that only one who is not fastidious and doesnt mind eating yucky parts of the animal will eat it. [I guess they didnt have hot dogs back then.]
Section two: Although it is edible, the amniotic sac is not considered food. Nor is it considered to be the flesh/meat of the animal. Therefore, it cannot become ritually unclean in the way that food becomes ritually unclean. Furthermore, if the animal is not slaughtered properly and therefore becomes a nevelah, it is not unclean as is a nevelah. The sac is not considered to the edible flesh of the animal, and only such flesh is unclean.
Section three: If a person intends to eat it, his thoughts cause it to become food and it is susceptible to uncleanness. However, intention cannot cause the sac to become a nevelah because it is categorically not considered part of the edible flesh of the animal.
Section four: Once the sac begins to emerge it is considered as if the woman has begun to give birth, for the head might be inside the sac. Therefore, even if the sac goes back inside the mothers womb, it still cannot be eaten, even if found in the mother after she was slaughtered. In this sense is treated as if it was offspring and once offspring has emerged from the womb it cannot be eaten by virtue of the mother being slaughtered. However, the main difference is that while the offspring cannot be eaten based on its mother being slaughtered, the offspring itself can be slaughtered. In contrast, when the sac cannot be eaten based on the mother being slaughtered, it can never be eaten.
Once the sac begins to emerge, it is a sign of childbirth in a woman as well, again because the head might be inside the sac. From this point she has the uncleanness of a woman who gave birth (see Leviticus 12:2).
Section five: The amniotic sac itself does not have the halakhic status of offspring. Therefore, the amniotic sac of an animals first birth does not have the sanctity of a firstling. It can be thrown to the dogs, whereas if it was sacred it would have had to have been buried.
While it is not considered offspring, the sac is still part of the animal. Therefore, if it comes from a sanctified animal, one that was dedicated to the Temple, it must be buried because it is holy.
The mishnah forbids practices that are deemed idolatrous or magical. It seems that some people bestowed amniotic sacs with magical powers and either hung them in a tree or buried them at crossroads. Magical practices are deemed to be the ways of the Amorites and are prohibited.