In the phrase which he constantly repeats, decisio seminis, St Thomas interprets decisio as any scholar would interpret it, to be a derivative of decido (I cut off). Still, I suspect, whoever first used the phrase meant decisio to come from decido (I fall down), which would yield a more natural sense. To decisio, as meaning a 'cutting off,' is due to the false analogy of the divided earthworm.

Of God and His Creatures: 2.88