Embryonic Debate

A Reply to William Saletan, liberal bioethics writer, former embryo.

By Robert P. George & Christopher Tollefsen

In yesterday’s New York Times Book Review, William Saletan of Slate magazine reviewed our new book Embryo: A Defense of Human Life. Saletan is a deservedly respected bioethics journalist. While he is a determined defender of legal abortion and the public funding of embryo-destructive research, he is not unsympathetic to the concerns of those opposed to these practices. Unsurprisingly, then, his review of our book, though critical, was neither ungracious nor even unyielding on some important points. Saletan praised the book’s “essential and timely message.” He conceded that embryos have a certain moral standing—one that is, presumably, not enjoyed by mere gametes, tissues, or organs. “We should never create and destroy embryos lightly. We owe them our respect.” Yet the respect to which embryos are entitled, Saletan evidently believes, is not inconsistent with what he himself describes (in considering cloning) as “the mass production, exploitation, and destruction of human embryos.”

In attempting to resist our conclusion that human embryos ought not to be exploited and killed, while at the same time acknowledging their moral standing and the special respect they are owed, Saletan gets himself into a jam. To meet our argument that a human embryo is, as a matter of scientific fact, a developing human being—i.e., a living member of the species Homo sapiens in the earliest stages of development—and thus, as a matter of basic justice, a possessor of inherent dignity and a right to life, Saletan is driven to deny that human embryos are whole entities, as opposed to mere parts (such as gametes, tissues, or organs). He denies that embryos are determinate individuals, and he seems to doubt that they are organisms at all. But if these denials and doubts are warranted, then there is no rational basis for believing that human embryos “deserve our respect” or that “we should never create or destroy them lightly.” Saletan is trying to find a plot of solid ground lying between the views of radical liberal bioethicists, on the one side, and defenders of the pro-life view, on the other. The failure of his effort shows that the middle ground is nothing but quicksand.


Saletan’s denial that human embryos are human beings in the embryonic stage of development cannot be sustained in light of the scientific facts. Modern embryology and human developmental biology establish beyond any doubt that human embryos are wholes and not mere parts, that they are indeed determinate individuals; and that they are organisms that endure throughout the developmental process, that is, both during gestation and after birth.

Consider any adult human being—William Saletan, for example. He is the same whole living individual human organism—i.e., the same human being—that was at an earlier stage of his life an adolescent. And the adolescent Will was the same whole living individual human organism that was at earlier developmental stages a child, an infant, a fetus, and an embryo. By contrast, he was never an ovum or a sperm cell. The gametes whose felicitous union brought the embryonic Will Saletan into existence were parts of other organisms, his mother and father. But Will was once an embryo, just as he was once a fetus, an infant, a child, and an adolescent. From the embryonic stage forward, Will was a complete (though in the beginning developmentally immature) and distinct (both genetically and functionally) organism. He developed by an internally directed and gapless process from the embryonic into and through the infant, child, and adolescent stages and ultimately into adulthood with his organismic determinateness, distinctness, and unity intact.


Will, meet Will
The argument against our view being advanced by the adult Will Saletan is confounded by the fact that Will Saletan, like the rest of us, really was once an embryo. In telling the story of Will’s life, it would be a howler of a scientific mistake to say that once upon a time there was an embryo that was something distinct from the living human organism that is now Will Saletan, but that got transformed from whatever it was into the organism that is Will Saletan at some point after the embryo came into existence. The true story is that the organism that is Will Saletan is the same organism that, at an earlier stage of Will’s development, was that embryo.

Let’s now examine the specific claims about embryogenesis and early intrauterine human development on the basis of which Saletan tries to make his case against our position. He quotes us saying that “nothing extrinsic to the developing organism itself acts on it to produce a new character or direction of growth.” But despite having quoted us on this point in full, Saletan mistakenly argues against the quite different claim that nothing acts on the embryo at all. We do not make this claim and it is unnecessary to make in order to establish that human embryos are, as a matter of biological fact, new and distinct individuals of the human species. We did not claim comprehensively that nothing acts on the embryo. Such a claim would be false of the human embryo and perhaps every other developing organism, whether that organism is in utero, if viviparous, or in an egg, if oviparous. Developing organisms (including humans during gestation and even after birth) are dependent in various ways and often depend upon environmental cues and prompts for certain aspects of their development. In some cases, development will even stall while the organism waits for environmental signals indicating, say, receptivity for implantation, in the case of embryos of certain species.

In the early development of a human being, the embryo requires maternal signaling of receptivity for successful implantation. Yet, as every human embryology text affirms, even during the process of implantation the embryo is acting as a distinct biological unit—an organism. The embryo is not a maternal body part. No text of modern embryology even remotely suggests such a thing. As Saletan notices, the embryo acts on the mother, just as she acts on him or her. He quotes an embryology text saying that “the early embryo and the female reproductive tract influence one another.” Indeed, they do. (For example, the embryonic human secretes human chorionic gonadotropin, which helps to maintain the maternal secretion of progesterone and estrogen without which menstruation would begin and the embryo would be expelled.) But the interactions of mother and developing child in no way warrant the conclusion Saletan seems to want to draw, namely, that the embryo is not a whole, distinct, living organism.

Consider the Science
Our claim was not that nothing acts on the embryo in the developmental process. It was that nothing acts on the embryo in such a way as to “produce a new character or new direction of growth.” This is a straightforward fact fully established by embryological science. Nothing in the developmental process (certainly no action of the mother) transforms the developing organism from one kind of entity (say a nonorganismic entity or a nonhuman organism) into another kind of entity (a human). Human development is the development of an entity that comes into existence as, and remains until death, a complete, self-integrating, determinate human organism—a human being. Indeed, we can see this in the fact that the effect had by the mother on her developing child is species specific: maternal signaling, the provision of nutrition and an environment hospitable to the child’s life and development, and other maternal factors help to enable the embryo and fetus to continue along the distinctive developmental pathway determined by the embryo itself. When zebra embryos are experimentally transferred to horse mares, such trans-species pregnancies can proceed successfully to term, but invariably result in the birth of baby zebras, not baby horses or zebra-horse hybrids. The maternal environment supports and influences the development of the embryo, but does not control development. Similarly, no maternal or other extrinsic action changes the human embryo from a human being or into a human being; they merely enable it to continue to grow and develop as a human being.

Similar points could be made about another feature of embryonic development to which Saletan alludes in trying to resist our defense of the embryo, i.e., the influence of maternal RNA on early embryonic development. The RNA is “maternal” only in the sense that it is contributed by the oocyte. But as human embryologist Maureen Condic explains, “once an embryo has come into existence, the maternally-derived RNA, like the embryo’s genome, belong to the embryo itself. They are not components of the mother, somehow acting at a distance, but components of the embryo acting to further its own development.” They form aspects of the complete developmental program of the embryo and are neither extrinsic, nor distinct agents. (Nor do they cause the embryo of some early stage to become a numerically different being.) These facts discredit Saletan’s claim—central to his case against our position—that “maternal factors don’t just facilitate the embryo’s program; they direct it.” The truth is that the embryo’s development is internally directed. The embryo directs not only its own integral organic functioning, but also its development in the direction of maturity as a member of the human species.

If a human embryo were something other than a human being in the embryonic stage of development—an embryonic human being—what could it be? Saletan’s suggestion seems to be, not simply that the embryo is less than a human being, but that the mother and embryo taken together form the relevant biological unit. Writing of the mother’s relation to the developing embryo, he says that “[h]er body sustains it, guides it and affects its direction of growth. Mother and child are a system.” Later, Saletan casts an even wider net in search of the relevant unit: The biological program for humanity “doesn’t run on one body. It runs on the network of humanity. In fact, it runs on the entire Internet of evolving species.”
Here Saletan is veering away from straightforward biology toward metaphysical speculation of considerable abstraction. Human embryologists focus strictly, and rightly, on the life of a developing human, and his or her developmental program. Their business is not with the biological program for humanity. From their work, we can understand when the life of a human being begins, while fully acknowledging that “human life” is transmitted and has been transmitted from human beings to new human beings from the time of the appearance of the first members of the species.

Saletan points out (as do we, in Embryo) that “Within two weeks of conception, a female embryo’s primordial germ cells begin the assembly of her future children. Her primary oocytes are complete at birth.” But Saletan’s own words should make him pause: Who is this entity to whom Saletan (rightly) refers as “her”? Surely he is here describing precisely what we have described: a new and complete (whole) human organism—a new individual of the human species, already female, who is herself providing for her future reproductive success—not, of course, in the way of a voluntary agent (for in the embryonic stage, as in the fetal, infant, and early childhood stages, humans do not yet exercise agentic capacities), but rather in the way of a biological organism.

Three Last Things
We shall conclude by addressing three arguments Saletan tries to run against our view: The first concerns the phenomenon of twinning; the second the relationship between embryo and placenta; and the third the phenomenon of parthenogenesis.

Some people have argued that until the point at which monozygotic (i.e., identical) twinning is no longer possible a human embryo is not yet an individual member of the human species. If one embryo can split into two, then it lacks individuality. We are by no means the first writers to notice the flaws in this argument, but we provided in Embryo a highly detailed refutation of it. We were therefore surprised that Saletan proposed it in his review as if we had not answered it—and at length—in the book he was reviewing. Of course, it is possible that our answer is wrong; and Saletan is perfectly entitled to point to defects if he can find any. But he failed to do that. So we have no idea of the grounds on which he thinks the “twinning argument” remains defensible despite the points we and others have made against it.

Saletan presented the argument as a response to our claim that the cells of an embryo “function together to develop into a single, more mature member of the human species.” He pointed out that in “one of every 300 cases, the embryo splits to become two or more people, at least one of whom wasn’t a distinct organism at conception.” But that fact in no way establishes that the embryo lacks individuality. If A splits into B and C, that provides no evidence at all that prior to that splitting A was not a determinate individual. For example, a flatworm can be split and the result will be two whole flatworms: but that of course does not show that prior to that division there was not a determinate individual flatworm. With human embryos, it is clear that at fertilization, a new and complete organism comes into existence—a distinct, actively self-developing human organism—for he or she exhibits internally directed, complex development between fertilization and the last point in time at which twinning may occur. So, the original embryo A lives until twinning occurs, and at that point, either A continues to exist and a new embryo comes to be by “budding” from the original one, or (less likely, given recent findings) A ceases to be and two new embryos, B and C, come to be.

Since most twinning occurs after day 5, and since in many cases one of the twins has more qualitative likeness to the original than the other, the “budding” scenario is more likely what occurs. Twin B is a sort of natural clone of Twin A. (Of course, A and B might never know which of them is A and which is B.) Twin B comes into being as an embryo, just as twin A did, though twin B was not produced by the union of gametes. Even in the less likely possibility—that is, if the embryo “splits” and gives rise to two distinct new embryos (its material constituents entering into their composition)—the fact of twinning does nothing to show that the original embryo was not a determinate, individual human organism. Twinning in this case would be akin to what happens in the case of cells that divide by mitosis, for example, or in the dividing of an amoeba.

A second argument of Saletan’s concerns the placenta, a structure generated by the embryo to provide the nourishing environment needed for 8 months of the developing child’s life. Focusing on this organ, Saletan concludes “The embryo, too, is collective.” This is a bit opaque, but Saletan’s point seems to be that because the entire embryo is, or becomes, two distinct parts, one of which will eventually be discarded, the embryo is therefore not an individual. But this argument fares no better than Saletan’s attempt to revive the twinning argument. Organisms do have parts—organs, cells, extracellular matrices—the existence and functional roles of which are subordinated to the existence and needs of the organism itself. Organisms are unities of multiple structures—biological wholes which, in fact, precede the existence of many of their organic parts: organisms contain the developmental program to establish the parts they need to continue to grow and develop.

Some of these parts, such as the brain, are, once they exist, more or less permanent and co-extensive with the remainder of the organism’s life. But some parts, such as cells, are not permanent, and, indeed, in the early stages of the organism’s existence, programmed cell death is essential for proper development. Somewhat similarly, the placenta is, not just a collection of cells, but an entire organ whose role in the organism’s biological economy is temporary. The embryo generates through its own activities that organ, as it develops its other organs, but its reliance on the placenta is bound by narrow temporal conditions, and the placenta will (like baby teeth) be discarded in time.

How could this possibly demonstrate that the embryo is not an individual? It cannot—unless Saletan is holding that any form of biological complexity jeopardizes individuality. And surely he would not want to say that. As we see it, the fact that the embryo itself is capable of generating even temporary organs to foster its own growth and development strongly supports the claim that it is a distinct living organism of the human kind, precisely the same kind of organism, that is to say, as you and we are and William Saletan is. Indeed, as we’ve observed, all of us adult human organisms were once in the embryonic stage of our lives, just as we were once in the adolescent and before that in the child and infant stages. We were human organisms then, as we are now. Indeed, we were then the same human organism we are now. It is that organism that has experienced human development.

Finally, Saletan takes issue with our sharp distinction between the embryo, which, again, we claim is a whole and distinct (both genetically and functionally) biological organism of the species Homo sapiens, and the sperm and oocyte, which we claim are biological parts of the parental organisms. As we stated the case in Embryo, sperm and ova are “parts of the men and women whose gametes they are. Their union can generate another organism, an entity that is not merely part of another organism. But that organism was never itself a sperm cell or an ovum.” Saletan, however, thinks he might be able to call into question this distinction between embryos and gametes. “In some 70 vertebrate species,” he reports “unfertilized eggs have developed into offspring.” Of course, these 70 species do not include human beings, which was our subject for discussion. Nevertheless, we think our claim is true even in the unrestricted context: no organism was ever a sperm or ovum.

In parthenogenesis, embryogenesis occurs in an unusual way: there is no fertilization of an ovum by sperm. In ‘ordinary’ fertilization, one copy of the chromosomes contributed by the egg is rapidly expelled into a small polar body that contains very little cytoplasm, leaving only a haploid number of maternally-derived chromosomes. Thus, the nuclear material of the already haploid sperm and egg can combine to form a new diploid cell (one with all the necessary chromosomal material). In most cases of parthenogenesis, by contrast, whether natural or induced in a lab, all the chromosomes of the newly developing embryo are derived from one source only, the egg. The mechanisms by which the ovum is transformed into a one celled zygote are various; in some cases, haploid nuclei are duplicated; in other cases, the ova never undergo a meiotic process to become haploid, and remain diploid. But in all such cases there is a critical change from an entity that acts only as a part of the larger biological whole to which it belongs to an entity that acts in the way characteristic of an embryo.

The oocyte as such will not continue along the developmental path characteristic of its species. But a single-celled parthenote does begin the process of cell division characteristic of an embryo. There is, in other words, a transition from oocyte to embryo just as there is in fertilization by sperm (and just as there is, we should add, in cloning by use of somatic cell nuclear transfer.) Neither the egg, nor the genetic material of the somatic cell as such are an embryo. But they can be acted on (i.e., induced either by human intervention or some other cause) to produce a single, living, one-celled organism, capable of its own self-directed growth and development.

We reiterate that there is no documented evidence of this ever happening in human beings. But even if it did, our analysis would stand: parthenogenetic reproduction would involve a transformation of the oocyte from being a part of a human being (the woman whose oocyte it is), to being a new, distinct, biological individual. The “egg-embryo distinction” is not suspended, as Saletan claims.

We thank Saletan for his review of Embryo and commend him for focusing (for the most part) on just the right question: Is the human embryo a whole living member of the species Homo sapiens—a human being—in the earliest stage of his or her natural development? We say yes, that is exactly what a human embryo is; he says no. The question is not metaphysical or religious, but rather scientific. But it is a scientific question with profound moral consequences for those who believe, as we do, and as we are sure Saletan does, in the profound, inherent, and equal dignity of all members of the human family. So we urge readers to consider what Saletan has said, what we have said in reply, and to consult any of the major textbooks in human embryology to determine who has gotten the science right. It is worth the effort, for what is ultimately at stake, if we are right, is a true moral nightmare: “the mass production, exploitation, and destruction of human embryos.”

—Robert P. George is McCormick Professor of Jurisprudence at Princeton and a Member of the President’s Council on Bioethics. Christopher Tollefsen is Professor of Philosophy at the University of South Carolina. They are co-authors of Embryo: A Defense of Human Life.
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