Commentary Purg XXV 61

What Dante has not yet heard (and thus cannot understand) is how this 'animal' embryo can and does become a human being, i.e., how it receives its rational soul.  The word 'fante,' here translated 'human,' strictly speaking means 'one who speaks.'  Thus an 'infant' (in-fans) is a human who cannot yet speak.  Here Dante, through Statius, is speaking precisely, but not technically.  He means to indicate that the rational soul, once it is joined to the embryo, only then makes this new creature potentially fully human.  And this third capacity of the soul we share with no other mortal beings (angels are nothing but rational soul, having no bodily form).  For these three faculties as found in each single human soul, see C.Purg.IV.1-15.