Not quite, sperm and egg cells are pluripotent gonadic cells while bone marrow cells are multipotent (less potent than pluripotent) somatic cells.
D'oh... how could such mixed up nonsense go unnoticed... *auto-facepalm* sorry 'bout that. Sperm cells don't divide anymore and their progenitor cells only build sperm cells. But gonads are the only cells that naturally regain totipotence after fusion.
An infant isn't a stand alone organism either.
I've seen some say that a fetus isn't a person... a baby isn't a person either... a 2 year old is arguably little more than lump of flesh that is growing in intelligence.
Well.. a 2 year old already has all it's organs and mostly only grows. A beginning foetus still has to built organ primordias and structure them. So I think it's wrong to say there's little difference. But I think I know what you mean; and anyway it's very subjective and not apt to determine something like abortion to decide from where on someone is a person or not. That's almost philosophical, so...
But your argument for children not being stand-alone organisms either, if applied consistently, would also say that a stand-alone organism can't possibly exist. We all rely on some sources to live. The difference between a child and a foetus is that it is physiologically independent of the mother; they don't share the same blood circulation anymore.
A little to think about concerning the difference between a baby and a foetus, and the moment to abort: what about caesareans?
If we draw a line for making life un-abortable then why at birth and not as teenagers?
The point about abortion and when to allow it, is to ensure that the future mom has time to realize that she is pregnant, and to make a decision. She doesn't need a decade to make that decision. It is legal to abort up to 12 weeks p.m. here, later only with a real good reason, I think that leaves women enough time to ponder about their feelings towards the child and the realizability of parenthood. If she changes her mind after that, either it's because external conditions have changed a lot, or because of a spontaneous decision which is not a well-founded enough reason for abortion.
1. @ conception
We can't possibly predict the moment of conception in individual cases so this is already ruled out.
2. @ birth
Again, what about cesareans? I think birth, or the earliest moment at which cesareans are possible, is already too late, since obviously the organism is viable out of the womb; at least I feel like that, don't ask me why exactly.
3. @ developed sentience and personality
This sounds more sensible, compared to the two others.
But abortion can't be the option every single time a family isn't "happy"
They could want the child, but not have a happy home life.
If they want the child, why even discuss about abortion?
Abortion should not be a forced-on solution against 'unhappyness'; shouldn't and can't, it can at most prevent happyness to fall even lower, not raise it. The method is not a reason for the situation, but the situation can be a reason for the method.