The central questions regarding the ethics of stem cell research are 2 in number: One, do embryos have any moral status? In other words, are embryos entities that deserve moral respect and are they owed some duty by moral agents due to their intrinsic properties or relationships with moral agents? Two, if embryos have some moral status, what kind, or degree, of respect are they entitled to and what particular duties are they owed by moral agents? I answer these questions in summary form as follows: embryos have a modest but genuine moral status and should not be either created or destroyed for insubstantial reasons, embryos must be used or destroyed by persons who possess a sincere attitude of respect toward them and act in accordance with that attitude, and embryos can only be used or destroyed with the informed consent of the persons whose gametes created them (I will call them, for want of a better and more euphonious term, the "gamete sources").
Embryos are morally considerable; they are not just bunches of cells having no link to the moral community. Specifically, embryos have a modest moral status because they are alive, because they have a special ontological, biological, and moral relationship with the persons whose gametes literally constitute them, and because they are valued—sometimes as highly as any entity can be morally valued—by sincere moral agents whose attributions of moral status must be given serious consideration as well as some deference and weight.
All living things deserve at least some minimal moral status because all living things have a good of their own. For example, it is morally wrong for me to take to the sidewalks and try to stomp every ant I can find simply to satisfy a whim. However, if ants invade my home and start eating my sugar and flour—or me, I am entitled to kill them.
Embryos are in a morally unique relationship with the persons who deliberately used their gametes to bring them into existence. For those persons and only those persons, embryos are genetically a literal part of them and have the potential to become their children. Consequently, these embryos have some moral status due to their unique relationship with the gamete sources. But this potential has moral significance only if the woman who provided the egg, or some other woman, voluntarily chooses to gestate the embryo with the consent of both gamete sources. Embryos do not have moral status simply because they have theoretical potential to develop into born persons. Embryos have true potential only if they are in the process of being gestated by some particular woman. Moreover, no woman, including the female gamete source, has any moral duty to create an embryo or to gestate any embryo, whether hers or someone else's. Put differently, no embryo has a right to be either created or gestated. Embryos are not the kind of entity that has a justifiable claim on a woman for the type of personal investment and involvement (including physical risk) necessarily generated by gestation, childbirth, and child rearing.
Finally, embryos have some moral status because some persons in fact value them very highly, like Father Coleman and those who adhere to the view that after the process of conception is complete, a human being has come into existence who has all the same rights, including a right to life, and is owed all the same duties as a born human person. What I call the principle of evaluative respect and accommodation requires moral agents to give careful consideration and some deference to the sincere attributions of moral status made by other moral agents out of respect for them as fellow members of the moral community, and they owe others this even if they completely disagree with the attribution of moral status in question. However, this principle does not require us to honor any and all attributions of moral status made by others. If it did, then we all might be morally required to refrain from eating apples picked and not fallen from a tree, to worship and reverence cows, or refrain from swatting pesky mosquitoes tasting our blood.
I do not accept the argument that embryos have any moral status, much less the full and equal rights of persons, simply because they are human. I do not see how this claim is rationally defensible. If it were true, then a petri dish teeming with living human cells cultured from someone's cancerous tumor would be full of persons because these cells are human, and this is absurd. Of course, the response to this objection is that an embryo is different from the cancer cell because it has the potential to develop into a person. However, I have already argued the embryos outside a woman's body have no potential in the true meaning of the word, just like a seed in the packet from the store does not have the same potential as the seed in fertile, tended ground.
Furthermore, the claim that a living entity which happens to be human has special moral significance runs into another difficulty. If such a creature as Speilberg's ET of movie fame existed (and I do not see why the existence of nonhuman intelligent life is inconceivable), then he would have full and equal moral status just like any other paradigmatic person because he was sentient, rational, could communicate, had emotions, possessed a future valued by him, and he was involved in relationships—after all, he just wanted to go home. ET would not be of lesser moral status than one of us simply because we were human and he was not, nor would he be of greater moral status because he could fly or restore withered plants to life. Also, think of the more powerful and intelligent aliens who might come to earth someday and believe they are morally justified in destroying us because we are not of their species. Of course, a theological case may be made for the specialness of being human (i.e., the Bible says God made humans morally special or the incarnation of Jesus makes humans morally special), but the philosophical case must fail.
Unfortunately, scientists must destroy embryos in order to obtain primordial stem cells. This destruction is ethically defensible because embryos have only a modest moral status and can be destroyed for substantial reasons. While we are not soon going to see miraculous cures for terrible human diseases like Alzheimer's or Parkinson's, nor will we see fixes for spinal cord injuries that cause paralysis any time soon, nevertheless there are sufficient scientific reasons to believe that research with stem cells could someday result in significant benefits for suffering and injured persons. This is a great moral good; it is not mere caprice or frivolity that underlies the promise of stem cell research. Therefore, it is ethically permissible for embryos, which have a modest moral status but not the status of persons, to be destroyed in the course of responsible stem cell research—provided they are destroyed with a sincere attitude of respect, for there is a moral loss here—something morally valuable is being destroyed, and provided they are used only with the voluntary, informed consent of both gamete sources.