You compare a embryo with several cells from you leg. Those cells won't grow and develop into a adult human.
A embryo will. That's a cruical difference.
Stem cells from my leg COULD grow into another human being under the correct circumstances, just like a totipotent embryo can become an adult human being under the correct circumstances. It's a matter fo probability (and for the record, the probability, even with medical intervention, is quite low in BOTH cases - extremely few preganancies actually make it to term).
When does it being then? When you say so? I find your definition of the begining of life lacking.
Just because totipotent stem cells CAN become a human being doesn't mean they will. Human life begins at the stage of adult viability - about 20 weeks gestation (about the same time that spontaneous muscle movement begins, indicating activation of the central nervous system). Prior to that, a fetus merely has the potential to grow into a viable organism; it isn't actually viable YET.
I'm not going to actually convince you because abortion debates typically are about how people feel about the issue, not what they rationally think.
As it happens, I do not personally agree with the use of abortion as just another method of birth control, but that it should be used only in extenuating circumstances to terminate pregnancy. That said:
1. I would never presume to tell a woman what she can and can not do with her body; ultimately, she is physically sharing her body and I don't think anyone should have to do that against their will;
2. I am well aware of the social havoc unwanted pregnancies can cause (health care, crime, social services intervention, child welfare, etc) and can recite some rather alarming statistics of what unwanted pregnancies do in developed nations.
In an ideal world, very few would get pregnant by accident and of those that did, a healthy support network would be there to help, guide, and support them and the child to produce a productive adult. In reality, that is EXTREMELY rare. Unwanted pregnancies contribute to fully 16% of all criminal activity, either directly or indirectly, and account for more than half of the children's services interventions.