Dear student,
The process of formation of megaspores from megaspore mother cell is called as megasporogenesis.The megaspore mother cell undergoes a type of cell division called as meiosis and leads to the formation of megasporangium and megaspores.
Initially the ovules are made up of the diploid mother cell which is also called as megasporocyte.This will then form megaspores by meiosis which will divide by the mitosis process to form haploid female gametophyte. This megagametophyte or the haploid female gametophyte will then produce egg cell in the ovule.
Regards