The Talmud asks: And with regard to the opinion of the Rabbis, let us say the Rabbis hold that even a portion of the mouth of the womb sanctifies a firstborn despite the offspring never having touched the entire area. As, if only the entire mouth of the womb sanctifies the firstborn, then in a case where the heads emerged as one, granted that it is impossible for both births to coincide precisely and one certainly emerged first, but in any event there is an interposition, i.e., the head of the other fetus, between each fetus and the mouth of the womb.
אמר רב: מין במינו אינו חוצץ.
The Talmud answers that Rav says: No proof may be drawn from here that a portion of the womb sanctifies a firstborn, as a substance that is in contact with the same type of substance does not interpose. Therefore, each fetus is still considered to be touching the entire womb.
זכר ונקבה — מפריש טלה כו׳. וכיון דלעצמו הוא, למה לי לאפרושי? לאפקועי לאיסוריה מיניה.
§ The Mishnah teaches that if a female donkey that had not yet given birth gives birth to a male and a female, and there is uncertainty with regard to which was born first, the owner designates a lamb in case the male was born first, which he keeps for himself. The Talmud asks: But since he keeps it for himself, why do I need him to designate it? The Talmud answers: It is necessary in order to abrogate its forbidden status.
אלמא, כיון דלא מפקע לאיסוריה, אסור בהנאה. מתניתין מני? רבי יהודה היא, דתניא: פטר חמור אסור בהנאה, ורבי שמעון מתיר.
The Talmud comments: Apparently, since prior to the designation the sanctity is not yet abrogated, it is prohibited to derive benefit from the donkey at that stage. If so, whose opinion is expressed in the Mishnah? It is the opinion of R' Yehuda, as it is taught in a baraita: It is prohibited to derive benefit from a firstborn donkey, in accordance with the opinion of R' Yehuda; and R' Shimon deems it permitted.