During the first week of a human being’s life, it is a free-floating cell, not yet attached to the uterine wall.  It grows rapidly; after only three or four weeks, before a woman even knows about it, the embryo has brain waves: neural activity. At five weeks, the embryo moves, shows sensitivity to pain, and has a heart beat.  Does that make it a person? Frank Zindler mentions that dogs, frogs, and earthworms have all those characteristics.
     Mr. Zindler describes the growth as a miniature evolution reenactment (Ontogeny recapitulates phylogeny, etc.). It starts out like algae. At five weeks
it has fish-like features; the heart has two chambers like a fish’s heart, and the embryo appears to have gill-clefts and fins. It even has a “tail” that gives it an animal-like appearance. “Hemisphere [brain] development reaches reptile-grade during the fourth month, and primitive mammal-grade (opossom) during the sixth month.”  Zindler contends that rather than feeling pain the way an adult would, “behavior is entirely reflexive as in earthworms. Only long after birth will the nervous system be developed sufficiently for the perception of [mortal danger].”
     Actually, the nervous system develops during the earliest stages, within two weeks of conception, so the embryo or fetus does feel pain (and better than worms can), but perception of mortal danger depends on mental development, not just nerves.  According to current science, the part of the cerebellum that controls thinking and emotions does not develop until later.  Robert Wennberg says, “It is not until the tenth day after birth that the neocortex (that part of the cerebral cortex peculiar to mammals and man that is responsible for the higher mental functions) shows signs of change, becoming excitable in a weak and diffuse way. Gradually thereafter, automatic movements are brought under cortical control.” The embryo, fetus, and newborn are subcortical beings.
     By definition, a human being is not a person until it becomes aware of itself so that it can relate to its surroundings. The fetus becomes conscious as early as ten weeks after conception or as late as thirty-two weeks, according to Peter Singer, but the fetus does not become “rational or self-aware, let alone capable of seeing itself as existing in different times and places.” By using this concept of “personhood” an abortion advocate can convince people that abortion is no worse than killing an animal. It’s not a good thing, but it seems to be something we can live with.
     The acorn analogy is popular. Clifford Grobstein explains, “Extending personhood to an individual cell that is barely visible makes no more sense than declaring acorns to be oak trees and selling them at oak tree prices.” Clearly, a fertilized acorn does not attain much worth until it has become a full-grown tree. According to this view, just as an acorn must grow into a tree to have much value, a human being must grow and develop to gain value.
     Garett Hardin wrote a science-fiction story to show his judgement that we should not value a fetus as a person. In the story, a 28th amendment to the Constitution declares a human zygote to be a person.  During a court case a clever lawyer hands a flask of twenty trillion fertilized eggs to the judge. The judge then has the responsibility of caring for them. To let them die would be manslaughter. To destroy them would be the genocide of more people than have ever lived before.
human embryo, 5 weeks