I don’t get it either. It seems to revolve around the fact that only the sperm moved, so only the sperm could have a soul. Inheritability was centuries away from being understood at this time. There were conflicting ideas during Leeuwenhoek’s life. Particularly after it was proven women produce eggs (Leeuwenhoek called the “female testicles” a useless adornment at first) it was still thought that soul “irradiated” it’s way from the uterus to the ovaries. They had no understanding of ovulation at all. In mean, bear in mind, the two largest schools of thought in science were still the Aristotelians and the Hippocratics. One school thought menstration was the expulsion of an unformed human body that required sperm to form and the other thought women have an internal sperm that meets the male sperm in the uterus. When the cutting edge ideas in science are almost two thousand years old at the time then you can’t expect much. Here’s a good link giving some background on the debate during Leeuwenhoek’s life.
There are some subtle differences between Catholic and Jewish teachings, differences that a Jewish scholar might call a pilpul, an extremely casuistic hairsplitting distinction.
The Jewish approach is that under some circumstances, you can think of the fetus’ human life as a hostile entity and take action against it to stop its “attack.” The Catholic view is that you may never take action intended to harm the human life, but you may take action intended to save the life of the mother. If that action happens to cause the death of the fetus, it’s an unintended secondary consequence and therefore acceptable.
It’s called the double effect. According to Dr. Richard Geraghty of EWTN:
"The relevant text from the on-line Catholic Encyclopedia goes as follows:
However, if medical treatment or surgical operation, necessary to save a mother’s life, is applied to her organism (though the child’s death would, or at least might, follow as a regretted but unavoidable consequence), it should not be maintained that the fetal life is thereby directly attacked. Moralists agree that we are not always prohibited from doing what is lawful in itself, though evil consequences may follow which we do not desire. The good effects of our acts are then directly intended, and the regretted evil consequences are reluctantly permitted to follow because we cannot avoid them. The evil thus permitted is said to be indirectly intended. It is not imputed to us provided four conditions are verified, namely:
That we do not wish the evil effects, but make all reasonable efforts to avoid them; That the immediate effect be good in itself; That the evil is not made a means to obtain the good effect; for this would be to do evil that Good might come of it – a procedure never allowed; That the good effect be as important at least as the evil effect.
The text denies that that the killing of a child which would happen while removing a diseased ovary, for instance, would be a direct killing. It is the removal of the ovary that is the effect intended, not the death of the child. Yet as far as I know you, cannot remove the ovaries without killing a child which may be implanted in them."