Question: Why must the cake be buried?

Answer: This is to publicize that the mother is exempt from Bechorah.

Inference: The cake is a proper child.

Question: Why isn't the cake Metamei?

Answer (R. Yochanan): The (dissolved) fetus is Batel in a majority of blood.

This is like he taught elsewhere;

(R. Yochanan): R. Eliezer ben Yakov (in our Mishnah) and R. Shimon taught the same thing.

(Mishnah): If a Shilya is in a house, the house is Tamei;

A Shilya is not considered a (dead) child (to be Metamei). Rather, it is always accompanied by a child.

R. Shimon says, perhaps the child was dissolved before the Shilya left the mother (the house is Tahor).

(Mishnah): Pesichas ha'Kever (once a woman begins to give birth to a Nefel, it is Metamei the house as if it were already born) applies only if the head was at least as big as a Pikah (lump).

Question: What Pikah does this refer to?

Answer (Rav Huna): It is the ball at the end of knitting needle for woolen thread.

Question (Chiya bar Rav): Do you refer to (a needle for) warp threads, or (for) woof threads? (The latter is larger.)

Answer (Rav Huna - Beraisa - R. Meir): The head must be like the Pikah of a warp needle;

R. Yehudah requires the size of a Pikah for woof;

R. Eliezer b'Rebbi Tzadok says, Pesichas ha'Kever applies (regarding of the head size), if Tefifiyos are seen in the womb.

Question: What are Tefifiyos?

Answer (Shmuel citing R. Eliezer b'Rebbi Tzadok): It is a crease twice the size of a Pikah, like when a mule bends to urinate.

(Rav Huna): I heard that two laws depend on a Pikah, one of warp and one of woof. I cannot explain this.

(Rav Dimi citing R. Yochanan): I heard that three laws depend on a Pikah. One is of warp, one is of woof, and one is of a large Pikah of sackmakers. I cannot explain this.

(Ravin citing R. Yochanan): Pesichas ha'Kever applies if the head is as big as a warp Pikah;

A Pikah of woof is the Shi'ur for Pesichas ha'Kever of animals. (A Nefel inside an animal is Balu'ah (swallowed). It is not Metamei (b'Maga u'Masa) until Pesichas ha'Kever.)

The last Shi'ur applies to the following Mishnah;

(Mishnah): If a clod of earth from a Beis ha'Pras (a field in which a corpse was plowed. We are concerned lest the plow dragged bones) or from Chutz la'Aretz is the size of a large Pikah of sackmakers, it is Metamei b'Ohel.

This is the same size as the seal of large sacks, and on the top cover of barrels of Beis Lechem.

(Reish Lakish): If one buys brine from an Am ha'Aretz (who is not trustworthy about Taharah), he may do Hashakah (connect it to a Mikveh). This is Metaher it, in any case:

If the majority (Tosfos -- or even half) is water, Hashakah is Metaher water;

If the majority is brine, brine is not Mekabel Tum'ah, and the water is Batel in the majority.

(R. Yirmeyah): This is Metaher the brine to use it for a dip (i.e. water will not be added);

If he will (add water, e.g.) cook it in a pot, and altogether there will be more water than brine, this arouses the Tum'ah. (Rashi -- it is Metamei the brine; Tosfos -- (only) the water returns to its Tum'ah.)

Question (Abaye): After Tum'ah was Batel, does it really return?!

Answer (Rav Dimi): Surely, you agree that it does!

(Mishnah - R. Eliezer): If a Se'ah of Tamei Terumah fell into 100 Sa'im of Chulin, we take one Se'ah from the mixture and let it rot;

(Mid'Oraisa, the Terumah was Batel. Chachamim require removing a Se'ah.) We consider the Se'ah removed to be the one that fell in.

Chachamim say, the Se'ah removed (is given to a Kohen. It and the mixture) may be eaten in any of the following ways (so it cannot become Tamei):

Dry (so it is not Huchshar to be Mekabel Tum'ah), parched, kneaded with fruit juice (which is not Machshir), or divided into small doughs, each less than the size of an egg. (Rishonim argue about whether an egg is the Shi'ur for Hechsher, or for a Tamei food to be Metamei food, or to be Mekabel Tum'as Ochlin mid'Oraisa.)

(Beraisa): According to R. Eliezer, the Chulin (the remainder after removing a Se'ah) may be eaten dry, parched, kneaded with fruit juice, or divided into small doughs.

Question (Ula): (Chachamim consider everything to be (Tahor) Chulin, through Bitul, and R. Eliezer considers the remainder to be Chulin.) Why must it be eaten dry (or parched, etc.)?

Answer (Ula): This is a decree, lest one join this Chulin to a slightly smaller volume of Tamei food to be Mevatel it (thinking that the Tum'ah will be the minority);

Really, since some of this Chulin is Tamei, it joins with the Tamei food, and the Tum'ah is aroused.