Functional megaspore in an angiosperm develops into an
 

In majority of flowering plants one of the megaspore is functional while the other three degenerate. Only one functional megaspore develops into female gametophyte i.e. embryo sac. This method of embryo sac development from a single megaspore is called monosporic development.

Endosperm develops from PEC, embryo develops from egg and ovule develops from ovule primordia.