Wait a minute!
Why can't the soul be in the egg?
Or why isn't the soul a co-operative composite of the egg and the sperm?
An even more intriguing question is this: Suppose you have a fertilized egg (a zygote) that was produced by one egg and one sperm. The zygote will eventually become a baby with one soul. Suppose, however, that the zygote splits at an early stage (either by natural or artificial means) into two, each of which then goes on to become one of a set of identical twin babies. Where did the second soul come from, and which one was first and which one was second?