Intriguing article in the BMJ by John Burn, a professor of genetics.

When does life begin? At fertilization? At conception? When the embryo implants in the uterus?

He points out that the early zygote is a clump of cells, which actually are not the embryo but give rise to an inner mass that then becomes the embryo, at about 14 days. You could argue that since it has the potential to become a human that it is deserving of personhood; but actually, any one of the blastomere cells retains that potential, which is why one can be removed for preimplantation diagnosis without causing any problems with the development of the others.

In fact, cultured skin cells can be genetically manipulated to revive their embryonic potential. Does that mean it suddenly develops a soul?

Sadly for the religiously minded, "the origin of human individuality must be defined with … pragmatic precision".