לע"נ מרת חנה בת ר' יצחק יונה
תני רבי חייא אינה מטמאה לא במגע ולא במשא ומאחר דאינה מטמאה לא במגע ולא במשא אמאי תקבר כדי לפרסמה שנפטרה מן הבכורה למימרא דולד מעליא הוא ואמאי אין מטמא לא במגע ולא במשא אמר רבי יוחנן משום ביטול ברוב נגעו בה.
R. ELIEZER B. YAACOV SAYS: IF A LARGE DOMESTIC ANIMAL HAS DISCHARGED A CLOT OF BLOOD, IT [THE CLOT] SHALL BE BURIED AND IT [THE MOTHER] IS EXEMPTED FROM THE LAW OF THE BECHORA.
R. Chiyya taught: [The clot of blood] does not make unclean with contact, nor by being carried. Now since it does not make [a person] unclean by contact nor the carrier unclean, why is it buried? In order to make known that the mother is exempted from [the law of] the bechora. Does not this mean to say that it is a genuine embryo? Then why does it not make unclean by contact nor make the carrier unclean? — R. Yochanan answered: Because the principle of neutralisation [bitul] by the larger portion is applied here.
The question begs - If there is bitul, then why do we say that the animal is exempt from the law of bechora? We see from this gemara that even though bitul creates a situation where the laws that apply to the minority will no longer apply [and it is thus not tamei], nevertheless, it doesn't change the מציאות and regarding the mother we still consider it a firstborn.
Asked the Steipler [Bechoros 20]: The gemara in Yevamos teaches that if the woman performing chalitza spits blood the chalitza is valid because there was defitnitely some saliva mixed in. Asked the Mordechai - Why don't we say bitul bi-rov and consider it as if no saliva emanated from her mouth?
He answers that we don't use the law of bitul when it was נולד בתערובת - created as a mixture. This mixture of blood and saliva started out as a mixture and thus there is no din of bitul. This would imply that if not for this rule, then we would say that the saliva is batel in the majority of blood and the chalitza is invalid. But why??? asked the Steipler. We just proved from the gemara that bitul doesn't change the מציאות so even if we employ the rule of bitul, we still must acknowledge the fact that there is saliva here and the chalitza should be valid??
He bleibs with a צריך עיון גדול!