Salon.com News | Everything you always wanted to know about the stem cell debate:
"There are also philosophical reasons to question the embryo's status. Does the embryo have a 'soul'? Bioethicists note that if it does, it's not an individual soul, since an embryo at this stage has not yet reached the point where it might split in two to become twins. An embryo can't be thought of as an individual person, some ethicists say, since it may actually become two different people."
I've seen this argument repeated over and over again and it is simply not a very good argument. This is akin to arguing that an adult can't have a soul because a child comes from biological material from it's parents. And it's parents recombined into three people.
The obvious reply is that when the embryo splits the soul that was there before is linked with one part and the new part gets a new soul. Just think of it as human reproduction being a bit more complicated.