From pages 69 to 73 in The Clash of Orthodoxies by Robert P. George:
A human being is conceived when a human sperm containing 23 chromosomes fuses with a human egg also containing 23 chromosomes (albeit of a different kind) producing a single-cell human zygote containing, in the normal case, 46 chromosomes that are mixed differently from the 46 chromosomes found in the mother or father. Unlike the gametes (that is, the sperm and egg), the zygote is genetically unique and distinct from its parents. Biologically, it is a separate organism. It produces, as the gametes do not, specifically human enzymes and proteins. It possesses, as they do not, the active capacity or potency to develop itself into a human embryo, fetus, infant, child, adolescent, and adult.
Assuming that it is not conceived in vitro, the zygote is, of course, in a state of dependence on its mother. But independence should not be confused with distinctness. From the beginning, the newly conceived human being, not its mother, directs its integral organic functioning. It takes in nourishment and converts it into energy. Given a hospitable environment, it will, as Dianne Nutwell says, "develop continuously without biological interruptions, or gaps, throughout the embryonic, fetal, neo-natal, childhood and adulthood stages--until the death of the organism."
Some claim to find the logical implication of these facts--that is, that life begins at conception--to be "virtually unintelligble." A leading exponent of that point of view in the legal academy is Jed Rubenfeld of Yale Law School... Rubenfeld argues that, like the zygote, every cell in the human body is "genetically complete"; yet nobody supposes that every human cell is a distinct human being with a right to life. However, Rubenfeld misses the point that there comes into being at conception, not just a clump of human cells, but a distinct, unified, self-integrating organism, which develops itself, truly himself or herself, in accord with its own genetic "blueprint." The significance of the genetic completeness for the status of newly conceived human beings is that no outside genetic material is required to enable the zygote to mature into an embryo, the embryo into a fetus, the fetus into an infant, the infant into a child, the child into an adolescent, the adolescent into an adult. What the zygote needs to function as a distinct self-integrating human organism, a human being, it already possesses.
At no point in embryogenesis, therefore, does the distinct organism that came into being when it was conceived undergo what is technically called "substantial change" (or a change of natures). This is the point of Justice Byron White's remark in his dissenting opinion in Thornburgh v. American College of Obstetricians and Gynecologists that "there is no non-arbitrary line separating a fetus from a child." Rubenfeld attacks White's point, which he calls "[t]he argument based on the gradualness of gestation," by pointing out that, "[n]o non-arbitrary line separates the hues of green and red. Shall we conclude that green is red?"
White's point, however, was not that fetal development is "gradual," but that it is continuous and is the (continuous) development of a single lasting (fully human) being. The human zygote that actively develops itself is, as I have pointed out, a genetically complete organism directing its own integral organic functioning. As it matures, in utero and ex utero, it does not "become" a human being, for it is a human being already, albeit an immature human being who will undergo quite dramatic growth and development over time.
These considerations undermine the familiar argument, recited by Rubenfeld, that "the potential" of an unfertilized ovum to develop into a whole human being does not make it "a person." The fact is, though, that an ovum is not a whole human being. It is rather, a part of another human being (the woman whose ovum it is) with merely the potential to give rise to, in interaction with a part of yet another human being (a man's sperm cell), a new and whole human being. Unlike the zygote, it lacks both genetic distinctness and completeness, as well as the active capacity to develop itself into an adult member of the human species. It is human cellular material, but, left to itself, it will never become a human being, however hospitable its environment may be. It will "die" as a human ovum, just as countless skin cells "die" daily as nothing more than skin cells. If successfully fertilized by a human sperm, which, like the ovum (but dramatically unlike the zygote), lacks the active potential to develop into a human adult member of the human species, then substantial change ( that is, a change of natures) will occur. There will no longer be merely an egg, which was part of the mother, sharing her genetic composition, and a sperm, which was part of the father, sharing his genetic composition; instead, there will be a genetically complete, distinct, unified, self-integrating human organism whose nature differs from that of gametes--not mere human material, but a human being.
These considerations also make clear that it is incorrect to argue (as some pro-choice advocates have argued) that, just as "I" was never a week-old sperm or ovum, "I" was likewise never a week-old embryo. It truly makes no sense to say that "I" was once a sperm (or an unfertilized egg) that matured into an adult. Conception was the occasion of substantial change (that is, change from one complete individual entity to another) that brought into being a distinct self-integrating organism with a specifically human nature. By contrast, it makes every bit as much sense to say that I was once a week-old embryo as to say I was a week-old infant or a ten-year old child. It was the new organism created at conception that, without itself undergoing any change of substance, matured into a week-old embryo, a fetus, an infant, a child, an adolescent, and finally, an adult.
But Rubenfeld has another argument: "Cloning processes give the non-zygotic cells the potential for development into distinct, self-integrating human beings; thus to recognize the zygote as a human being is to recognize all human cells as human beings, which is absurd."
It is true that a distinct, self-integrating human organism that came into being by a process of cloning would be, like a human organism that comes into being as a monozygotic twin, a human being. That being, no less than human beings conceived by the union of sperm and egg, would possess the human nature and the active potential to mature as a human being. However, even assuming the possibility of cloning human beings from non-zygotic human cells, the non-zygotic cell must be activated by a process that effects substantial change and not mere development or maturation. Left to itself, apart from an activation process capable of effecting a change of substance or natures, the cell will mature and die as a human cell, not as a human being.