Niddah 30b

Rabbi Yishmael and Rabanan argue about "the amount of time required for the initial formation of a human embryo from the period of postnatal tumah of the mother" (quotation from Levi, p. 233), and both try to prove their case from empirical observation (how could they argue if they were scientifically omniscient, and why would they bother with empirical proofs if their knowledge were based exclusively on kabbalah or drashah?):

נדה ל.-ל: (ש"ס ווילנא); ז"ל

מתני' המפלת ליום מ' אינה חוששת לולד ליום מ"א תשב לזכר ולנקבה ולנדה רבי ישמעאל אומר יום מ"א תשב לזכר ולנדה יום פ"א תשב לזכר ולנקבה ולנדה שהזכר נגמר למ"א והנקבה לפ"א וחכ"א אחד בריית הזכר ואחד בריית הנקבה זה וזה מ"א:  גמ'
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רבי ישמעאל אומר יום מ"א תשב לזכר ולנדה כו': תניא רבי ישמעאל אומר טימא וטיהר בזכר וטימא וטיהר בנקבה מה כשטימא וטיהר בזכר יצירתו כיוצא בו אף כשטימא וטיהר בנקבה יצירתה כיוצא בה אמרו לו אין למדין יצירה מטומאה אמרו לו לר' ישמעאל מעשה בקליאופטרא מלכת אלכסנדרוס שנתחייבו שפחותיה הריגה למלכות ובדקן ומצאן זה וזה למ"א אמר להן אני מביא לכם ראייה מן התורה ואתם מביאין לי ראייה מן השוטים מאי ראיה מן התורה אילימא טימא וטיהר בזכר וטימא וטיהר בנקבה כו' הא קאמרי ליה אין דנין יצירה מטומאה אמר קרא תלד הוסיף לה הכתוב לידה אחרת בנקבה ומאי ראיה מן השוטים אימר נקבה קדים ואיעבור ארבעין יומין קמי זכר ורבנן סמא דנפצא אשקינהו ור' ישמעאל איכא גופא דלא מקבל סמא אמר להם ר' ישמעאל מעשה בקלפטרא מלכת יוונית שנתחייבו שפחותיה הריגה למלכות ובדקן ומצאן זכר לארבעים ואחד ונקבה לפ"א אמרו לו אין מביאין ראיה מן השוטים מאי טעמא הך דנקבה אייתרה ארבעין יומין והדר איעבר ורבי ישמעאל לשומר מסרינהו ורבנן אין אפוטרופוס לעריות אימא שומר גופיה בא עליה ודילמא אי קרעוהו להך דנקבה בארבעין וחד הוה משתכחא כזכר אמר אביי בסימניהון שוין:  עכ"ל

MISHNAH.  If a woman miscarried on the fortieth day, she need not take into consideration the possibility of a valid childbirth; but if on the forty-first day, she must continue [her periods of uncleanness and cleanness as] for both a male and a female and as for a menstruant.  R. Ishmael ruled: [If she miscarried on] the forty-first day she continues [her periods of uncleanness and cleanness as] for a male and as for a menstruant, but if on the eighty-first day she must continue [these periods as] for a male and a female and a menstruant; because a male is fully fashioned on the forty-first day and a female on the eighty-first day.  The sages, however, maintain that both the fashioning of the male and the fashioning of the female take the same course, each lasting forty-one days.
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"R. Ishmael ruled: [If she miscarried on] the forty-first day she continues [her periods of uncleanness and cleanness as] for a male and as for a menstruant etc."  It was taught: R. Ishmael stated, Scripture prescribed uncleanness and cleanness in respect of a male and it also prescribed uncleanness and cleanness in respect of a female, as in the case of the former his fashioning period corresponds to his unclean and clean periods so also in the case of the latter her fashioning period corresponds to to her unclean and clean periods.  They replied: The duration of the fashioning period cannot be derived from that of uncleanness.  Furthermore, they said to R. Ishmael, A story is told of Cleopatra the queen of Alexandria that when her handmaids were sentenced to death by royal decree they were subjected to a test and it was found that both [a male and a female embryo] were fully fashioned on the forty-first day.  He replied: I bring you a proof from the Torah and you bring proof from some fools!  But what was his 'proof from the Torah'?  If it was the argument, 'Scripture prescribed uncleanness and cleanness in respect of a male and it also prescribed uncleanness and cleanness in respect of a female etc.', have they not already replied, 'The duration of the fashioning period cannot be derived from that of uncleanness'? – The Scriptural text says, She bear, Scripture thus doubles the ante-natal period in the case of a female.  But why [should the test spoken of by the Rabbis be described as] 'proof from some fools'? – It might be suggested that the conception of the female preceded that of the male by forty days.  And the Rabbis? – They were made to drink a scattering drug  [sic] And R. Ishmael? – Some constitution is insusceptible to a drug.  Then said R. Ishmael to them: A story is told of Cleopatra the Grecian queen that when her handmaids were sentenced to death under a government order they were subjected to a test and it was found that a male embryo was fully fashioned on the forty-first day and a female embryo on the eighty-first day.  They replied: No one adduces proof from fools.  What is the reason? – It is possible that the handmaid with the female delayed [intercourse] for forty days and that it was only then that conception occurred.  And R. Ishmael? – They were placed in the charge of a warden.  And the Rabbis? – There is no guardian against unchastity; and the warden himself might have intercourse with them.  But is it not possible that if a surgical operation had been performed on the forty-first day the female embryo also might have been found in a fully fashioned condition like the male? – Abaye replied: They were equal as far as these distinguishing marks were concerned.  [translation by Rev. Dr. Israel W. Slotki in The Babylonian Talmud, Rabbi Dr. I. Epstein, ed., Seder Tohoroth Vol. I (Niddah), London: Soncino, 1959]

(Reference from Levi, Facing Current Challenges, Chap. 33, fn. 39)