Bekhorot 22A

כדי לפרסמה שנפטרה מן הבכורה.

The Talmud answers: It must be buried in order to publicize that the animal is exempt from having its future offspring counted a firstborn.

למימרא דולד מעליא הוא, ואמאי אין מטמא לא במגע ולא במשא? אמר רבי יוחנן: משום ביטול ברוב נגעו בה.

The Talmud notes with regard to the ruling of the Mishnah that the animal is exempt from having any future offspring counted as firstborn: That is to say that the expelled mass is a full-fledged offspring. But if so, why doesn’t it impart ritual impurity, neither through physical contact nor through carrying? R' Yoḥanan says: It is due to the halakhic nullification of a forbidden substance in a majority of permitted substances that the rabbis touched upon it, in order to exclude it from impurity through contact or carrying. In other words, the fetus is in fact considered a full-fledged offspring, but it is does not impart impurity because it is nullified by the rest of the congealed mass.

ואזדא רבי יוחנן לטעמיה, דאמר רבי יוחנן: רבי אליעזר בן יעקב ורבי שמעון אמרו דבר אחד, רבי אליעזר בן יעקב ⁠— הא דאמרן.

The Talmud comments: And R' Yoḥanan follows his line of reasoning in this regard, as R' Yoḥanan says: R' Eliezer ben Yaakov and R' Shimon both said the same thing, i.e., they both issued rulings based on the same principle. The relevant statement of R' Eliezer ben Yaakov is that which we said previously, i.e., that the expelled mass does not impart ritual impurity because the fetus is nullified by the majority.

רבי שמעון מאי היא? דתנן: השליא בבית ⁠— הבית טמא. לא שהשליא ולד, אלא שאין שליא בלא ולד. רבי שמעון אומר: ולד נימוק עד שלא יצא.

And what is the statement of R' Shimon to which R' Yoḥanan was referring? It is as we learned in a Mishnah (Nidda 18a): If a woman miscarried and the afterbirth is in the house, the house is impure. The reason is not that the afterbirth itself has the status of an offspring, but that there is no afterbirth without an offspring within it, even if it is undetected. R' Shimon says: The offspring disintegrated before it emerged with the afterbirth. Consequently, the house is not rendered impure, because the disintegrated fetus is nullified by the majority.