Yevamot 78B

והא אמר רב כהנא: לא שנו אלא רובו, אבל כולו ⁠— חוצץ! שאני עובר, דהיינו רביתיה.

However, this is difficult, as didn’t Rav Kahana say that they taught this halakha that if one is not particular about the substance it is not considered an interposition only when the substance covers just a majority of his body; but if it covers all of it, it is considered an interposition by Torah law, even if he is not particular about it. The Talmud answers: A fetus is different, as this is its natural manner of growth. Its mother’s womb cannot be considered an interposition, as it is the fetus’ natural place of development, and therefore the fetus itself is regarded as having undergone immersion.

כי אתא רבינא, אמר רבי יוחנן: באומות, הלך אחר הזכר. נתגיירו, הלך אחר פגום שבשניהם. באומות הלך אחר הזכר, כדתניא: מנין לאחד מן האומות שבא על הכנענית והוליד בן, שאתה רשאי לקנותו בעבד ⁠— שנאמר: ״וגם מבני התושבים הגרים עמכם מהם תקנו״.

When Ravina came from Eretz Yisrael to Babylonia, he said that R' Yoḥanan said: With respect to lineage, among the other non-Jewish nations, i.e., while they are still non-Jews, follow the male, but if they married after they converted, follow the more flawed in lineage of the two. The Talmud explains: Among the nations, follow the male, as it is taught in a baraita: From where is it derived that if one from the other nations had sex with a Canaanite woman and had a son from her, you are permitted to purchase him as a slave, and he is not considered a Canaanite who may not be allowed to remain in Eretz Yisrael? As it is stated: “And also from the children of the strangers that reside among you, of them may you buy, and of their families that are with you, which they have begotten in your land; and they may be your possession” (Leviticus 25:45).