The messy biology of human embryos.

The messy biology of human embryos.

The messy biology of human embryos.

Science, technology, and life.
Feb. 13 2008 9:16 AM

The Machine of a New Soul

The messy biology of human embryos.

Linda Hamilton. Click image to expand.
Linda Hamilton, former half-embryo

Are embryos morally equal to people? I say no. Robert George, a member of President Bush's bioethics council, and his colleague Christopher Tollefsen say yes. In their new book, Embryo: A Defense of Human Life, George and Tollefsen conclude not only that embryo-destructive stem-cell research should be defunded but that any research involving embryos should be banned if it even slightly risks an embryo's health. They propose to halt the common practice of producing extra embryos during in vitro fertilization and to require that every IVF embryo be transferred to a womb.

William Saletan William Saletan

Will Saletan writes about politics, science, technology, and other stuff for Slate. He’s the author of Bearing Right.

In Sunday's New York Times, I reviewed the book's arguments. A day later, the authors replied on National Review Online. This is a conversation worth pursuing. George and Tollefsen are pushing the discussion into an area—embryology—where, in contrast to the usual shrieking about abortion, real progress can be made. They're civil, logical, and smart. I've seen George pick apart fuzzy-thinking adversaries at meetings of the bioethics council. It's like watching a cat with mice. Today, unfortunately, I'll be the mouse.


The virtue of Embryo is that the authors stake their case on science and logic, not religion. What makes you a human being, they argue, isn't a soul, but "a developmental program (including both its DNA and epigenetic factors) oriented toward developing a brain and central nervous system." They believe that this program starts at conception and therefore, so does personhood.

I like this bet on science. It's scrupulous, brave, and constructive. Let's toss in our chips and call the bet. We'll have to accept what science shows: Conception is, as George and Tollefsen argue, the sharpest line we could draw to mark the onset of moral worth. But they, in turn, will have to accept the other side of what science shows: The lines of embryology are dotted, not solid. Such lines don't warrant severe categorical restrictions on stem-cell research or assisted reproduction.

Start with the line between embryo and mother. They send signals back and forth to facilitate the embryo's migration, implantation, and nutrition. The embryo carries the mother's RNA, which directs its growth. What's more, the embryo is already on her way toward motherhood, with primordial germ cells up and running in her second week of development. The same program that created her is creating her children. It will kill her, and later them, revising not just individuals but families and species. You can't isolate life's program in one body, any more than you can isolate the Internet on one computer. Indeed, life is far more fluid than the Internet, with shared software that remakes its hardware as well as itself.

George and Tollefsen assume a clear distinction between wholes and parts. Eggs and sperm are parts, they reason, while an embryo is a whole. At conception, the parts become a whole, the program launches, and personhood begins. But it isn't that simple. Some embryos divide after conception to become two or more people. Are those embryos, prior to twinning, an individual? Furthermore, all of us came from embryos that were part "embryoblast" (the segment that became a person) and part "trophoblast" (the segment that became placenta). The placental lineage grew you to birth, separated, and died. In computer terms, it's like a Zip file. In human terms, it's a bit like a mother. In these ways, the early embryo is simultaneously a whole, a part (of the mother-child system), and a dyad (of potential twins or of embryo and placenta).

The egg-embryo distinction, too, is permeable. George and Tollefsen write that eggs must combine with sperm or die. They say an organism "was never itself a sperm cell or an ovum." But look what just happened at a zoo in Kansas: another case of parthenogenesis—eggs becoming offspring without fertilization. This process has produced adults in dozens of vertebrate species, including sharks and turkeys.

And those are just nature's tricks. With biotechnology, we're adding our own. Through IVF, we've separated, for the first time, internal and external elements of the embryonic program. Through cloning, we've turned adult cells into embryos. Through viral injections, we've turned adult cells into embryonic stem cells. Through aggregation, we've made embryonic mouse stem cells grow into mice. By tweaking a single gene, we're learning to alter embryogenesis so that what would otherwise become an embryo becomes instead a disorganized bunch of stem cells.

In their rebuttal, George and Tollefsen try to sharpen these blurred lines. "The embryo is not a maternal body part," they observe. That's true, but it misses the point. The problem isn't that they put the embryo in the wrong category. It's that the embryo defies such neat categories. It's a part, a whole, and a dyad.

Maternal factors don't alter the embryo's genetic humanity, the authors write; they "merely enable it to continue to grow and develop." True again. But the embryo's dependence on these factors for its very life makes them more central to the embryonic program, not less. Indeed, this is the logic behind viability as a standard of abortion jurisprudence: The less the unborn human relies on its mother, the more it encompasses its own developmental program, and the more we should treat it like a born child.