Embryosac generally develops  

The tissue present inside the ovule is called nucellus . One of the nucellus situated towards micropylar end differentiates into megaspore mother cell .Megaspore mother cell undergoes meiosis and forms four megaspores. Among four megaspores three will get degenerates , one megaspore develops as functional megaspore . Embrysac develops from functional megaspore inside the ovule.