Yeah, after reading this I had the same question as Clovis. Augustine's "generationism" sounds like it is intrinsically bound to his development of the concept of original sin. If you take that away what you have is God expressly creating souls at the moment of conception, out of nothing, with original sin.
Then again, you can see that "generationism" leaves the door wide open for Creatianism ( is that a misspelling? ) "...The soul of the offspring originates from the parental soul in some mysterious way..." The mysterious way is that God infuses it Himself. If you don't come to that conclusion, the only other option, "traducianism," is that souls are produced like sausage links on a factory floor from the original material of the parents, which is certainly a crude and gross theory.
So the problem we are left with is, how can embryos simultaneously share the sin of their first parent, Adam, while being infused directly by God? Why would God directly create souls with original sin? There must be two simultaneous processes happening: The infusion of the soul by God, and the contamination of said soul by its human host. That way, God is not creating tarnished souls ex nihilo.
Clovis, the Catholic Encyclopedia can mostly be trusted. Just beware of any articles signed "Pohle" or "Solliers."