The Making of Twins
A constant claim issued by non-rationalists, such as Rav Aharon Feldman and Rabbi Moshe Meiselman, is that it's absurd to state that Chazal or the Rishonim could have been mistaken about scientific facts, because their knowledge was far ahead of their era; Chazal knew things that have only recently been discovered by modern science. Over the last twenty years I've examined many such claims, and in every single case, I discovered that the alleged modern scientific facts stated by Chazal are either
1) things that do not mean what they are claimed to mean, or are so ambiguous that they can be interpreted in all kinds of ways;
2) things that non-Jews knew also; or
3) things that are not actually true.
I recently came across a new such claim, proposed by our old friend "Rabbi" Yosef Mizrachi. He argues that until twenty years ago, nobody knew how identical twins are formed. They knew that non-identical twins are formed by two sperm combining with two eggs, but they did not know how identical twins are formed. Chazal, on other hand, did know: "The Gemara said, one seed went into one egg and split into two!" Mizrachi goes on to stress how eggs cannot be seen without a microscope, implying that Chazal could only have known this due to supernatural knowledge. (He then segues into mocking evolution, and asks why anyone would go to college and pay to be taught nonsense; he finishes by telling his audience that they don't know how lucky they are to be able to be listening to him.)
Is his claim true? Of course not. Let's leave aside the minor inaccuracy regarding when medical science discovered how identical twins are formed (it was not twenty years ago - it was already known in the nineteenth century). And let's leave aside the inaccurate claim that a human egg is too small to be seen without a microscope - it isn't. Let's just address his claim that the Gemara said that "one seed went into one egg and split into two."
There ain't no such Gemara.
What the Gemara (in Yevamos 98b and Niddah 27a) actually says is that twins are formed "when one drop (tipah) divides into two." The word "drop" refers to the male sperm, not to the female ovum. Similarly, Aristotle believed that twins result from an abundance of sperm; it is the intuitive, albeit incorrect, conclusion. Chazal did not know that females produce ova. Rather, they had a different idea as to the role that a woman plays in the formation of a fetus:
"Our Rabbis taught: There are three partners in the creation of man - God, the father and the mother. The father seminates (mazria) the white substance, from which are derived the bones, vessels, fingernails, brain and the white of the eye. The mother seminates (mezara'at) the red substance, from which are derived the skin, flesh, hair and the black of the eye. God provides the spirit, the soul, the beauty of the features, vision for the eyes, hearing for the ears, speech for the mouth... and intelligence." (Niddah 31a)
Ramban elaborates that the fetus is not formed from any female "seed," as there is no such thing; rather, the "red substance" to which the Sages are referring is uterine blood. Tashbetz writes similarly.
So, rather than Chazal knowing how twins are formed long before modern science discovered it, Chazal actually had a mistaken view of fetal development (as did everyone in antiquity). It is, of course, very psychologically reassuring to believe that Chazal knew modern science through supernatural means. Alas, there is no evidence for it, and overwhelming evidence against it.
For further discussion, see:
Jeremy Brown, On Twins, and the Sperm that Splits in Two
Edward Reichman, The Rabbinic Conception of Conception: An Exercise in Fertility