cho,
You said: “you claim the soul enters the zygote at conception. At conception there is just one zygote… therefore, one “soul”. Does the soul split into two when the zygote splits?”
Catholic theology teaches that a human being comes into existence at the moment of conception. That moment is defined as the penetration of the egg by a sperm. Implantation is not necessary for the the “process” to be complete. A human being exists before implantation.
We know that in the case of identical twins, a zygote (the name given to the egg that has been penetrated by the sperm) can, for a certain number of days after its creation, divide into two or more zygotes.
Medical science does not yet know how or why a zygote divides. Because of the absence of knowledge on this point, the Catholic Church (based on the research I conducted) has no official teaching. Therefore, Catholics are free to speculate what might occur, provided their speculation does not violate a teaching already validated as true by the Church.
I think one of two things can occur:
A. One soul is created and infused at the moment a zygote is created. Upon fissioning by the zygote, the existing soul also fissions and goes into the various zygotes (twins, triplets, quadruplets, etc.).
OR
B. Multiple souls are created and infused into the zygote upon its creation, and upon fissioning, the mutiple souls are “parceled out” into the various zygotes.
I am partial to the second theory, as I personally don’t like the idea of “soul dividing” or “soul splitting”. I think that, upon penetration of the egg by the sperm, something occurs between that sperm and egg that sets in motion the setting for zygote fissioning. That something - again, must my opinion - also sets into motion the creation of the individual souls based on the number of zygote divisions.