'Seed' in the Bible

and to Abraham were the promises spoken, and to his seed; He doth not say, 'And to seeds,' as of many, but as of one, 'And to thy seed,' which is Christ;

Why, then, the law? on account of the transgressions it was added, till the seed might come to which the promise hath been made, having been set in order through messengers in the hand of a mediator --

and if ye are of Christ then of Abraham ye are seed, and according to promise -- heirs.