Chullin, Chapter 4, Mishnah 7. This chapter has been dealing with the fetus found inside a slaughtered animal and the ways in which the shechitah of the mother does or does not permit what is inside her. Our mishnah turns to the shilya, the amniotic sac in which the fetus develops, and asks what its status is: may it be eaten, can it become tamei, and what happens when part of it emerges before the mother is slaughtered.
May the sac be eaten?
The mishnah begins with "hashochet et habeheimah," one who slaughters an animal, "umatzah bah shilya," and discovers a shilya within it. The ruling: "nefesh hayafah tochelenah." Since the sac is a part of the mother's own body, the shechitah performed on her renders it permitted exactly as it renders her flesh permitted, and on the level of strict halachah there is no prohibition whatsoever. Why then does the mishnah speak of a "nefesh hayafah," one whose constitution is up to it? Because the average person finds such a thing revolting and would never treat it as something to put in his mouth.
Tum'ah: food or meat?
"Ve'einah mitama'ah lo tum'at ochalin velo tum'at neveilot." Permitted though it is, the general revulsion toward it carries halachic weight. There are two categories of tum'ah one might have expected to apply here. Foodstuffs are susceptible to tum'at ochalin, and the flesh of an animal that expired without valid shechitah is neveilah and transmits tum'ah in that capacity. Neither category takes hold of the shilya. Because human beings ordinarily treat it as something inedible, halachah does not classify it as food at all; and since it is not counted as the animal's meat, the death of the mother without shechitah does not render the sac a neveilah.
"Chishev aleha, mitam'ah tum'at ochalin, aval lo tum'at neveilot." If a person made up his mind to eat it, his intention changes the picture, but only partially. By deciding that this is what he wants to eat, he has taken it out of the category of the repulsive and turned it, for himself, into food. Therefore it can now contract tum'at ochalin. It still cannot become tamei as a neveilah, because a person's intention has the power to designate something as food; it does not have the power to turn something into meat.
A sac that partially emerged
"Shilya sheyatz'ah miktzatah, asurah ba'achilah." Here the mishnah presents a case in which the sac is prohibited. Picture a shilya, part of which protruded outside while the animal was still alive and had not yet undergone shechitah, and inside the sac no fetus can be seen. Such a shilya may not be eaten at all, and the prohibition extends even to the section that remained inside the animal.
"Siman valad ba'ishah vesiman valad babeheimah." A shilya is the telltale sign of a fetus, both in a woman and in an animal. Even though we see nothing inside it, we assume that it did contain a fetus which has since dissolved to the point where it can no longer be detected. Once we assume that, we must worry that the portion of the sac which emerged contained the head of that fetus. And as we learned in the first mishnah of the chapter, once the head has emerged the fetus is considered born. A born animal that then died without shechitah is a neveilah, and here that neveilah is enveloped within the sac and inseparable from it. The result is that the whole sac is forbidden, and the mother's shechitah does not permit it.
The firstborn and the sanctified animal
"Hamevakeret shehipilah shilya, yashlichenah laklavim." A mevakeret is an animal that has not yet given birth, so any offspring it produces would carry the status of a bechor. Should such an animal miscarry a shilya, one may toss that sac to the dogs. The ruling is surprising: if a fetus had been inside and later dissolved, that fetus may have been a bechor, and a bechor is forbidden to derive benefit from, so feeding it to dogs should be out of the question. The mishnah nonetheless permits it, because the likelihood is that no kedushah is involved here at all. Bechor kedushah takes hold only in a male firstborn of sheep or cattle, and about half of all births produce females. Beyond that, even a male might have come out deformed in a way that made it resemble another species, and such an animal has no bechor status. We are left with a sfeik sfeika, two layers of doubt: it may well have been a female, and should it have been a male, it may well have been deformed. On that strength the sac goes to the dogs.
"Uvamukdashin, tikaveir." With a sanctified animal, for example a female shelamim that aborted its sac, the ruling is the opposite: it must be buried and no benefit may be taken from it, since in most cases it is indeed sacred. Unlike a bechor, whose sanctity depends on the offspring being male, the offspring of a sanctified female carries the sanctity of its mother whether it is male or female. The only way it would escape sanctity is if it were malformed and resembled another species, and that is a minority case. Therefore we cannot rely on doubt here, and the sac must be buried.
Where it may not be placed
"Ve'ein kovrin otah befarashat derachim ve'ein tolin otah be'ilan, mipenei darchei haEmori." A shilya may not be buried at a crossroads, that is, in a public thoroughfare, nor may it be hung on a tree. Burying an aborted sac at a crossroads or suspending it from a tree was done as a charm to protect the mother from further miscarriages, and such practices are darchei haEmori, the ways of the idol worshippers. Even where the burial itself is required, it must not be carried out in a manner that imitates them.