Which meshed nicely with an online discussion I had recently about fetal development. Some pro-choice folks put the embryo in the category of a tumor, and that is completely inaccurate, scientifically. They don't consider the baby to be human until it has developed the things that they consider make a being human - a heart, brain, fingers, etc. The discussion involved when does a human being begin to develop these things. Some pro-lifers gloss over the issue and say "heart beat - 21 days" and be done with it, but the reality is more complex. Just as a baby doesn't magically form from a mass of amorphous tissue as it exits the uterus, so there is no "bright line" where you can say there is a heart, or brain. Those organs develop over time.
So I began looking up information on fetal development. The information I'm about to present is available from embryology texts, although much of it is echoed here https://en.wikipedia.org/wiki/Embryogenesis and http://biology.kenyon.edu/courses/biol114/Chap13/Chapter_13B.html timetables can be found at http://www.nrlc.org/abortion/facts/fetaldevelopment.html and http://faculty.washington.edu/chudler/dev.html
Although most of us don't think about the development of specific features before the fetal stage, the zygote does indeed, have marked cellular differentiation. In fact, at fertilization the egg itself becomes asymmetric and the first two cells of the zygote are distinct, one going on to form ectoderm and mesoderm, the other to form the endoderm. So if we're using differentiation and organization as the criteria, then is it clear that from fertilization on these traits are present. The first cell of you was not a tumor.
Some people want to use something like the brain as a criteria for humanity. First off, note that the human species is quite accurately defined, scientifically. Any attempt to require more than membership in the species is an attempt to exclude some members of the species from attaining their legal rights. The question to be asked is "why is not being a human being sufficient to be treated as one?"
But let's talk about the brain anyway. How much brain is enough brain? The embryo begins forming the nervous system with the primitive streak at about 14-16 days. The nervous system has formed visibly at 21 days. The human brain isn't fully formed until about 21 years after birth. So those who argue on brain function don't really have a leg to stand on.
Some will argue on the basis of brain function, not brain development, but aside from the obvious "why" question, how much brain function is "enough" and how does one measure it?
Alberto Giubilini and Francesca Minerva, in their paper entitled “After-birth abortion: why should the baby live?” argue that a human has no rights until he can be "self aware". What does it mean to be self aware, and how can we know if someone is? Should we apply intelligence tests to everyone each day, and those who score below a certain rank lose the right to live?
The point I'm trying to make is that there is no "bright line" in development from conception to birth, or after. Any point we choose, whether it be the beginning of brain development at 21 days heartbeat at 22 days, neural waves detectable with current technology at 42 days, "awareness" at two years after birth etc. is merely an arbitrary point on a continuum. Attempts to justify abortion (or in the last case infanticide) based on development is an attempt to say "I can kill these humans but not those humans." Why should we accept killing humans at all?