Statius

Alcmeon, who, begged by his father, killed his own mother.

This is a reference to a story Dante mostly knows from Statius in the Thebeid.

Book two, lines 265 to 304.

Without going into the whole story here, Alcmian's mother revealed the hiding place of Alcmian's father during the Siege of Thebes.

and that Plato believes that it had detached from there when nature gave it its form, its body.

This is familiar language from Marco of Lombardy, familiar language from Statius, and his discourse in embryology, the body being the form of the soul.

She continues on.

Perhaps what he means is in another costume.