What does Macbeth mean when he says, "The worm that's fled Hath nature that in time will venom breed, No teeth for the present"?

Macbeth speaks that line as part of speech that's a bit longer in Act III, Scene IV. He's talking about the fact that Banquo is dead, but, unfortunately, his son Fleance got away. He refers to Fleance as a worm because he thinks of him as treacherous and likely to turn on Macbeth, but, because of his youth, doesn't think Fleance is likely to be a problem for a while. That's the part about having "no teeth for the present."  Pretty poetic response for a man hearing a report about paid assassination.