I'd suggest that Knox actually get some learning about basic human embryology and what an ovum, a blastocyte, a zygote, an embryo, and a fetus actually go through in developmental stages, but as a card carrying Republican I'm sure he's allergic to real science. Republicans: Ignorance is power!
However, for those who like their science hardcore and sans any religious bias, here's a great website on embryology:
http://www.embryology.ch/indexen.html
If we're going to pick arbitrary points at which a soul enters a body, I'm gonna have to go with the closing of the neural tube around day 29 myself. This is the point at which the spinal cord closes off, forming at once the lining of the brain and the hymen in girls. It's also the point when things tend to go horribly horribly wrong, and if the brain lining doesn't close properly you end up with a brainless baby. Urgh. No brain, no soul I say!
Or if we want to go with a scientific definition, the soul enters the body when the embyro becomes a fetus. Which is around the 11th week. At that point the proto-human is a whopping 1.25 inches long and still resembles a tadpole, but since the basal plates for the major organs are all in place, you could make the argument soundly that it's actually a little human being and not a lump of somewhat organized human cells like it was before.
Or perhaps the soul enters the body when the fetus first moves, usually around 16 weeks. Since obviously if something can't move on its own it's not alive.
Orrrrrr -- we could go to the other extreme, and this is actually the argument that I've heard from my Fundamntalist in-laws amazingly enough -- the soul enters the body on the first breath, so a baby that is stillborn or aborted was never tainted with original sin on earth. Cuz if you're Catholic, you know, if your soul is on the Earth, and you weren't baptized before you die, you go to hell and all. It's kinda sh*tty to tell a family who just had a stillborn that their baby's soul isn't going to heaven since it wasn't baptized!
You can argue that the soul enters the single-celled organism at the moment of fertilization all you want, but one third of all such fertilized ovum are flushed from a woman's body because meiosis (DNA recombination) didn't go right, so by that train of logic God kills more babies than anyone. DNA is some pretty delicate stuff, and one little transcription error can result in an inviable zygote. The female body detects that things aren't going right, and gives up pretty quick. This is why we don't have babies that are born with their skin inside out or feet growing out of their head.
Edit: And because I know this argument is going to come up . . . what if my mother had aborted me? She point blank told me when I was a teenager that she considered it, since she was 39 and already had three girls and really couldn't afford another one. But they opted to wait until I was far enough along developmentally to make sure I wasn't going to have massive heart defects or Down syndrome and such, then determined since it seemed like I was all right to go ahead and incur the cost of a fourth kid. Oddly enough, I'm the only one of my sisters that didn't end up with a mental illness, and landed closer to the genius end of the spectrum than the disabled one.
I'm glad of course that my mother chose to have me, but I'm also glad she made sure I wasn't going to suffer in pain and misery all my life first. That's the kind of decision making I can appreciate.
Edited, Aug 19th 2008 7:12pm by catwho