In calling this opinion 'false,' St Thomas can never have meant to deny the intimate connexion of the vital acts of the embryo with those of the mother, so that separation from the mother at an early stage by abortion or miscarriage is death. He means only that the mother is not everything, -- that the embryo has vital acts of its own, though not independent acts; that the embryo lives and developes, which it could not do without a distinct vegetative soul to animate it. But when he presently goes on to attribute operations of sense to the embryo, and says that it feels, -- speaking of the embryo ante ultimum complementum, -- he outruns all probability. Feeling supposes an advanced development of the nervous system. It is impossible to believe that the merely cellular embryo, with only a potential nervous system, can feel.

Of God and His Creatures: 2.88