Formation of embryo sac is

Formation of embryo sac is Megagametogenesis.
Megagametogenesis is the process of maturation of the female gametophyte, or megagametophyte, in plants.
During the process of megagametogenesis, the megaspore, which arises from megasporogenesis, develops into the embryo sac, which is where the female gamete is housed.
These megaspores then develop into the haploid female gametophytes. This occurs within the ovule, which is housed inside the ovary.