Full Idea
In Aquinas's view the fertilized egg is not, either at the moment of conception or for quite a while afterwards, endowed with an immortal soul. In fact, technically speaking, it is not even human.
Gist of Idea
Aquinas says a fertilized egg is not human, and has no immortal soul
Source
report of Thomas Aquinas (Summa Theologicae [1265]) by R Martin / J Barresi - Introduction to 'Personal Identity' p.20
Book Reference
'Personal Identity', ed/tr. Martin,R /Barresi,J [Blackwells 2003], p.20
A Reaction
It is pointed at that therefore Aquinas does not give good support for modern Catholic views on abortion. There is certainly no reason why a human zygote should be ensouled from the start, as God may do this whenever He wishes.