One of my favorite recent articles is Acorns and Embryos by Robert P. George and Patrick Lee. Here's a snippet:
But personhood is not an accidental characteristic, that is, a characteristic which one acquires at some point after he exists and may lose at another point. One is a human person by being a living member of the human community, a member of the human species. It is true that many people cannot immediately exercise the rational capacities characteristic of members of the speciessuch as the elderly person with dementia whose rational powers are gone forever, or the comatose person whose rational powers may or may not return, or infants, fetuses, and embryos whose rational powers are still developing. But such individuals are still morally valuable persons, at least to those who value all human beings equally. They are still members of the human community. Being a person is not a result of acquired accidental attributes; rather, it is being a certain type of individual, an individual with a rational nature. And human beings are individuals with a rational nature at every stage of their existence. We come into being as individuals with a rational nature, and we do not cease being such individuals until we cease to be (by dying). We did not acquire a rational nature by achieving sentience or the immediately exercisable capacity for rational inquiry and deliberation. We were individuals with a rational nature even during the early childhood, infant, fetal, and embryonic stages of our lives. If we are persons now, we were persons then. We were never human nonpersons.Read the entire article here.