>one-celled human beings (zygotes) in a petri dish
hu·man be·ing
noun
1. a man, woman, or child of the species Homo sapiens, distinguished from other animals by superior mental development, power of articulate speech, and upright stance.
Wow, we know the conversation has sunk to low levels when the other pulls out a dictionary to win an argument, not even an appropriate medical or biological dictionary at that.
A zygote qualifies as a child of the species Homo Sapiens. If you tighten the definition to the actual presence of "superior mental development, power of articulate speech, and upright stance" as opposted to membership in the species, you must logically reach the conclusion reached by Michael Tooley, Peter Singer, and others, when they not only defended abortion but also infanticide, because newborn infants lack any traits, other than species, that might usefully distinguish them from animals philosophically. To accept that it's OK to kill a zygote for any purpose or even limited utilitarian purposes is to accept that it's OK to kill an infant for the same reasons. Is it OK for parent to have a second child, only to kill that child to fatally be used as an organ doner for a first child in failing health?
That, in a nutshell, defines the two logically defensible positions one may take concerning the personhood of an unborn child in the abortion debate because abortion is essentially prenatal infanticide and you can't logically separate the morality of the former from the later. There is no relevant trait that distinguishes the newborn from either an animal nor that same child moments befor birth, nor from the zygote. Oh, there are certainly differences, but none that would lead you to conclude an adult animal was a person if it exhibited the same characteristic. Animals have heartbeats, brains, feelings, and so on, too, and chimpanzees certainly look fairly human, so that the newborn has such characteristics and the zygote doesn't makes such things irrelevant as distinguishing criteria.
Sure, you can then argue that even if the unborn child is a person that a woman has no obligation to take care of it, even if the results of her removing it are fatal to the child, but then you wind up having to support child abandonment. Several years ago, a woman in New Jersey wanted to go out partying. After several attempts to find a babysitter failed (she did actually try to get others to take care of the child for her), she decided partying was more important than her infant son so she tossed him off a bridge into the Passaic river. If you do't think a child has the right to force a woman to take care of it, then I find it difficult to see how you could find fault in this woman's actions (Perhaps it might have been better if she'd just abandoned him on the bridge and it would have been find if he fell into the river himsef?).