Chullin, Chapter 9, Mishnah 6. This chapter has been working through the laws of tum'ah as they apply to the parts of a creature that are not the flesh itself, such as skins, and to the coverings and attachments that serve the flesh. Our mishnah applies those principles to two unusual cases involving the sheratzim, the eight creeping creatures whose carcasses convey tum'ah: an egg of a sheretz, and a mouse that is only partly formed.
An egg with a formed embryo
The mishnah opens: "Beitzat hasheretz hamerukemet tehorah" - the egg of a sheretz in which the embryo has already taken shape is tahor. Even though there is a sheretz developing inside, the shell seals it off completely. Tum'ah of a sheretz is transmitted through touch, and here there is nothing to touch. The embryo is entirely enclosed, so the egg conveys no tum'ah.
"Nikvah kol shehu teme'ah" - if the egg has been pierced, in any measure at all, it is tamei. Once there is an opening, even one so small that only a hair could be inserted through it to reach the embryo inside, the situation changes entirely. The embryo is now touchable, and the shell surrounding it functions as a shomer, a protective covering for the sheretz within. A shomer joins with what it guards, and so the egg itself now carries and transmits the tum'ah.
The mouse that is half flesh and half earth
The mouse that is half flesh and half earth
Next the mishnah turns to a second case, that of the "achbar", the mouse, which is counted among the eight sheratzim. The mishnah describes one "shechetzyo basar", half of whose body is real flesh, "vechetzyo adamah", while its other half is still plain soil. The mishnah is speaking of an extraordinary creature caught midway through coming into being: one portion of it has already developed into actual flesh and skin, while the remaining portion has not yet become anything more than ground.
So what happens to a person who makes contact with a creature like this? Regarding the developed side the mishnah rules "hanogea babasar tamei": whoever touches that half contracts tum'ah, for what he has handled is the flesh of a sheretz that is not alive. As for the undeveloped side, the ruling is "ba'adamah tahor": contact there leaves a person tahor, for soil simply does not qualify as sheretz flesh.
The view of Rebbi Yehudah
The view of Rebbi Yehudah
A dissenting opinion follows. "Rebbi Yehudah omer", and his ruling extends the tum'ah further: "af hanogea", even a person whose hand lands on the soil, provided it is the soil "shekeneged habasar", the portion situated straight across from the fleshy half, is rendered "tamei". As Rebbi Yehudah sees it, that ground is no ordinary dirt sitting next to the animal. It is the material actively being transformed into this mouse's body, a process still under way, and consequently even the dirt already has the status of the sheretz's flesh and passes on tum'ah.