דהוא מיית ברישא ואין הבן יורש את אמו בקבר להנחיל לאחין מן האב
The reason is that we presume that the fetus died first, before its mother died, and the son does not inherit from his mother while in the grave, in order to bequeath the inheritance to his paternal brothers. The halakha is that if a son dies, and afterward his mother dies, the deceased son does not inherit from his mother and subsequently bequeath the inheritance to his paternal brothers, who are not related to the mother. But in other cases, where it is not his mother’s estate, a fetus inherits property.
למימרא דהוא מיית ברישא והא הוה עובדא ופרכס תלתא פרכוסי אמר מר בר רב אשי מידי דהוה אזנב הלטאה שמפרכסת
The Gemara asks: Is this to say that it is certain that the fetus died first? But there was an incident where the mother died and the fetus made three spasmodic motions afterward. Apparently, a fetus can die after the mother. Mar bar Rav Ashi said: That incident was just as it is with the tail of the lizard, which jerks after being severed from the lizard, but it is just a spasmodic motion which does not indicate that it is still alive.
מר בריה דרב יוסף משמיה דרבא אמר לומר שממעט בחלק בכורה ודוקא בן יום אחד אבל עובר לא מאי טעמא (דברים כא, טו) וילדו לו אמר רחמנא
Mar, son of Rav Yosef, says in the name of Rava: The mishna comes to say that a one-day-old child reduces the portion of the firstborn. The firstborn is entitled to a double portion of the inheritance, and this is calculated taking into account the portion due to his dead brother. And it is in this context that specifically the portion of a one-day-old child is taken into account, but the portion of a fetus is not taken into account, even though a fetus also inherits property. What is the reason for this? The Merciful One states concerning the portion of the firstborn: “If a man has two wives, the one beloved, and the other hated, and they bore him children, both the beloved and the hated; and if the firstborn son be hers that was hated” (Deuteronomy 21:15). The term “children” excludes a fetus.
דאמר מר בריה דרב יוסף משמיה דרבא בן שנולד לאחר מיתת אביו אינו ממעט בחלק בכורה מאי טעמא וילדו לו אמר רחמנא והא ליכא
This is similar to another halakha, as Mar, son of Rav Yosef, says in the name of Rava: A son who was born after his father’s death does not reduce the firstborn’s portion. What is the reason for this? The Merciful One states in the Torah: “And they bore him children,” and this term “children” does not apply to a fetus.
בסורא מתנו הכי בפומבדיתא מתנו הכי אמר מר בריה דרב יוסף משמיה דרבא בכור שנולד לאחר מיתת אביו אינו נוטל פי שנים מאי טעמא יכיר אמר רחמנא והא ליתא דיכיר והלכתא ככל הני לישני דאמר מר בריה דרב יוסף משמיה דרבא
The Gemara notes: In Sura they taught Mar’s statement that way, but in Pumbedita they taught it this way: Mar, son of Rav Yosef, says in the name of Rava: A firstborn who was born after his father’s death does not receive a double portion. What is the reason for this? The Merciful One states in the Torah: “But he shall acknowledge the firstborn, the son of the hated, by giving him a double portion” (Deuteronomy 21:17), and in this case the father is not there that he can acknowledge him. The Gemara concludes: And the halakha is in accordance with all these versions of that which Mar, son of Rav Yosef, says in the name of Rava. Accordingly, a one-day-old child reduces the portion of the firstborn, a son born after his father’s death does not reduce the firstborn’s portion, and a firstborn born after his father’s death does not receive a double portion.
אמר רבי יצחק אמר רבי יוחנן המזכה לעובר לא קנה ואם תאמר משנתינו הואיל ודעתו של אדם קרובה אצל בנו
§ Rabbi Yitzḥak says that Rabbi Yoḥanan says: With regard to one who transfers ownership of an item to a fetus, the fetus does not acquire the item. And if you say that the statement of our mishna (140b), with regard to one who gives a gift to his unborn child, indicates that an item can be transferred to a fetus, the circumstances there are unique. Since the disposition of a person is to be inclined toward his son, the Sages validated such a transfer, but one cannot transfer an item to the unborn child of another.
אמר ליה שמואל לרב חנא בגדתאה פוק אייתי לי בי עשרה ואימר לך באפייהו המזכה לעובר קנה והלכתא המזכה לעובר לא קנה
Shmuel said to Rav Ḥana of Baghdad: Go out and bring me an assembly of ten men, and I will say a halakha to you in their presence, so that it will be well publicized. The halakha was: With regard to one who transfers ownership of an item to a fetus, the fetus acquires the item. The Gemara concludes: And the halakha is that with regard to one who transfers ownership of an item to a fetus, the fetus does not acquire the item.
ההוא דאמר לדביתהו נכסי לבני דיהוו לי מיניך אתא בריה קשישא אמר ליה ההוא גברא מאי תיהוי עליה אמר ליה זיל קני כחד מברא הנך ודאי לא קנו דאכתי ליתנהו
§ The Gemara relates: There was a certain man who said to his wife, before she conceived: My property is given to the sons that I will have from you. His older son from a previous marriage came and said to him: That man, i.e., me, what will become of him, i.e., will I receive nothing? He said to his son: Go and acquire a portion like one of the sons who will be born, i.e., you will receive a share as well. The Gemara comments: Those sons who were not yet born certainly did not acquire the property, and do not receive more than their share as heirs, as they are not yet in existence.
האי אית חולק לטליא במקום בניא או לית ליה חולק לטליא במקום בניא רבי אבין ורבי מיישא ורבי ירמיה דאמרי אית חולק לטליא במקום בניא רבי אבהו ורבי חנינא בר פפי ורבי יצחק נפחא דאמרי לית חולק לטליא במקום בניא
The Gemara asks: With regard to this son, does the young man [letalya] receive an additional share of the inheritance in a case where there are other sons from the second wife, since his father gave him an additional share, or does the young man not receive an additional share of the inheritance where there are other sons? There are Rabbi Avin, and Rabbi Meyasha, and Rabbi Yirmeya, who all say: The young man does receive an additional share of the inheritance where there are other sons. And there are Rabbi Abbahu, and Rabbi Ḥanina bar Pappi, and Rabbi Yitzḥak Nappaḥa, who all say: The young man does not receive an additional share of the inheritance where there are other sons.
אמר ליה רבי אבהו לרבי ירמיה הלכתא כוותן או הלכתא כותייכו אמר ליה פשיטא דהלכתא כוותן דקשישנא מינייכו ולאו הלכתא כוותייכו דדרדקי אתון אמר ליה מידי בקשישותא תליא מילתא בטעמא תליא מילתא וטעמא מאי זיל לגביה דרבי אבין דאסברתה ניהליה
Rabbi Abbahu said to Rabbi Yirmeya: Is the halakha in accordance with our opinion, or is the halakha in accordance with your opinion? Rabbi Yirmeya said to Rabbi Abbahu: It is obvious that the halakha is in accordance with our opinion, as we are older than you, and the halakha is not in accordance with your opinion, as you are youngsters [dardekei]. Rabbi Abbahu said to Rabbi Yirmeya in reply: Does the matter depend upon age? The matter depends upon the reason behind the ruling. Rabbi Yirmeya asked him: And what is your reason? Rabbi Abbahu replied: Go to Rabbi Avin, as I explained this halakha to him,