Counting chickens

Hi! What is the origin of the phrase, "Don't count your chickens before they're hatched."? Mrs. Morrissey's class has struck again...(Saxon)

Mrs. Morrissey's students would be well advised to look at Aesop's Fables--in particular, "The Milkmaid and Her Pail" (link below or [Dead link removed - ed]. The version at that site looks suspiciously modern to me, referring as it does to a Farmer Brown, but it has the gist of the old story.

Confusingly enough, the phrase is from an Aesop Fable, but not this one! It should be from a poor couple who find an egg and imagine the riches they can get once the chicken is hatched. Only the egg ends up broken.