Thirty-five years after Roe v. Wade, the pro-life movement faces a new challenge: biotechnology. The first human biotech issue, embryonic stem-cell research, looks like an easy call. Stem cells could save millions of lives. And the entity we currently sacrifice to get them — a sacrifice that may soon be unnecessary — is a tiny, undeveloped ball of cells. The question, like the embryo, seems a no-brainer.

For pro-lifers, that’s precisely the problem. Biotechnology is arguably more insidious than abortion. Abortions take place one at a time and generally as a response to an accident, lapse or nasty surprise. Their gruesomeness actually limits their prevalence by arousing revulsion and political opposition. Conventional stem-cell harvesting is quieter but bolder. It’s deliberate and industrial, not accidental and personal. In combination with cloning, it entails the mass production, exploitation and destruction of human embryos. Yet its victims don’t look human. You can’t protest outside a fertility clinic waving a picture of a blastocyst. You have to explain what it is and why people should care about it.

This is the task Robert George and Christopher Tollefsen undertake in “Embryo.” To reach a secular and skeptical public, they avoid religion and stake their case on science. George, a professor of jurisprudence at Princeton and a member of the President’s Council on Bioethics, and Tollefsen, a philosopher at the University of South Carolina, locate humanity not in a soul but in a biological program. “To be a complete human organism,” they write, “an entity must possess a developmental program (including both its DNA and epigenetic factors) oriented toward developing a brain and central nervous system.” The program begins at conception; therefore, so does personhood.