Parah, Chapter 5, Mishnah 6. This chapter deals with the vessels used in preparing the chatas water: drawing the spring water and performing the kiddush, the placing of the ashes of the parah adumah into it. A vessel of almost any material is acceptable, but it must genuinely be a vessel. Our mishnah brings two borderline cases where the Tannaim disagree about whether the object in front of us has earned that name.
The Potter's Egg
The mishnah's first case: "Beitzas hayotzrim kesheirah" - the egg of the potters is valid. Against this, "Rabbi Yosi poseil", Rabbi Yosi holds that such an item may not be used.
What is this item? Craftsmen who work in earthenware prepare rounded balls of clay with a cavity scooped into the middle, roughly the size and form of an egg, and they keep a supply of them ready. When the potter wants to produce a particular utensil, he takes one of these prepared forms and molds it into the finished article. The first view maintains that since the cavity already holds liquid, the object qualifies as a vessel and the chatas water may be sanctified inside it. Rabbi Yosi rejects it: to his mind nothing here deserves the name vessel, since the clay is still material in progress, awaiting the shape its maker has in mind for it.
The Hen's Egg
The second case: "Beitzas hatarnegoles", the egg of a hen, meaning the shell itself. Here "Rabbi Meir veRabbi Yehudah machshirin", the two of them permit sanctifying the water in it, while "vaChachamim poslin", the Chachamim rule that it is unfit.
In the reasoning of Rabbi Meir and Rabbi Yehudah, the shell functions as a receptacle, so the chatas water may be sanctified within it. The Chachamim counter that a shell of this sort simply does not carry the status of a vessel, and their position is the accepted ruling.
Both halves of the mishnah turn on the same question. A hollow that holds water is not automatically a vessel. Clay that has not yet been fashioned into a utensil, and a shell that was never fashioned at all, lack the standing that the chatas water demands.