Though the sperm is a nonviable product, and the egg is much the same, the sperm-egg combination is both viable and genetically distinct from either of its parents. Accidents and deliberate molestations notwithstanding, it will mature into recognizably protected human life. There is no qualitative change along the way that separates the zygote from the baby it will eventually become.

Aristotle would have had something to say here about "essence versus accident."

Now, before anyone puts words in my mouth, this is not an airtight case for banning all abortions. Among other considerations, that question must account for the enforceability of such a law, the cost of enforcement in terms of other rights sacrificed, and the loss of respect for all law that would follow if it were widely violated without penalty, as we have good reason to believe it would. (E.g., Brazil, which has the strongest anti-abortion laws in the world, suffers more than 1,000,000 abortions per year, by conservative estimates.)

However, it is a strong case against the deliberate creation of zygotes and embryos for the purpose of sacrificing them for the benefit of others.

Curmudgeon Emeritus, Palace Of Reason