He means, in the language of a more familiar proverb, that he is counting his chickens before they are hatched; that is, he is too hasty in reckoning upon the success of his plans.
THE FAIR
In 1 Henry VI. (iii. 2) Joan of Arc gets into Rouen with her soldiers in the guise of countrymen bound for market:—
"Enter La Pucelle, disguised, and Soldiers dressed like countrymen, with sacks upon their backs.
Pucelle. These are the city gates, the gates of Rouen,
Through which our policy must make a breach.
Take heed, be wary how you place your words;
Talk like the vulgar sort of market-men,
That come to gather money for their corn.