The most common type of embryosac in angiosperms is 

Megaspore mother cell undergoes meiosis and forms four megaspores . Among four megaspores three will get degenerates and remaining one developed as functional megaspore . Functional megaspore undergoes three generations of mitotic divisions and forms embryosac . So if embryosac developed from one megaspore , such condition is called as monosporic development , if two megaspores are involved in embryosac formation , such condition is called as bisporic developement