The ethical relevance of srudying human development appears when one asks which stages of the human life cycle embody significant ethical concerns. Between birth and death, the human organism is a persan, equipped with the full measure of basic human rights. This much is not really controversial, and the debate primarily concerns the prenatal phase of development. Do human rights accrue to the unborn all at once, for instance at fertilization? Do they instead arise in a graduai manner, based on the various progressive steps through which the prenatal human organ­ism acquires significant person-like properties? Besicles per­sona! rights, are there other ethically-significant values and properties that would justify a respectful treatment of em­bryos and fetuses? An understanding of prenatal develop­ment is a necessary, albeit in no way sufficient, condition for addressing these issues successfully.