Quote:
Originally Posted by zuluking
What you seem to be suggesting is that any arbitrary developmental milestone is potentially valid except for the one cell stage.
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again, I don't know where the line should be drawn. Not to be overly scientific, I do think the line should be when there is structure, function and organization to the cluster of cells - i.e. the cells should begin some differentiation. Prior to differentiation, if the cells were to be separated from one another, each cell would be capable of forming a human being - essentially an embryonic stem cell. Once differentiation begins, the cells need one another to form the whole.
The four developmental milestones listed are not arbitrary to scientists.
(in layman terms)
1. The primitive streak is the embryo. Everything else will become the placenta, umblicus, yolk sac, amnion etc. Without development of the primitive streak, there will be no baby. That happens around 14 days after fertilization.
2. Formation of the neural tube is the future central nervous system. Without that, there is no viable baby - it is a critical step in embryology that dictates subsequent embryological events. That occurs around 25-28 days
3. Closure of the neural tube signifies development of the early brain. Some think that what distinguishes humans from other primates is higher brain function, therefore development of the human brain is thought by some to be the limit. This occurs around 28-30 days
4. When I said heartbeat, I meant development of the early circulatory system (~5 weeks), not just random pulsating heart muscle cells (which occurs around 20 days). At this stage the nervous system and the circulatory system begin to tie together the whole body, and their is the beginning of co-ordinated organ system functions
Obviously, there is far more to it than my message board blurb. Scientists spend their career studying the fine cellular and molecular details. Bioethicists use that information (and more) to have come up with these 4 milestones. Doesn't mean their opinions are correct, but I like that there is scientific justification for each.
to address BigBrodieFan, I don't think it's a useless debate. I think it's a facinating one that intelligent people should have, preferably without malice to towards those who may have a different point of view