What is monosporic development of female gametophyte?

In angiosperms, during the development of female gametophyte, ovules are differentiated to form archesporium. The archesporium cell develops into megaspore mother cell. It is made up of a tetrad of 4 haploid megaspores. These megaspores are arranged in a linear fashion. Out of these 4 megaspores, the lower most megaspore enlarges in size and becomes fully functional. With time, the other three megaspores degenerate. Thus, it is called endosporous and monosporic development. It then fuses with microspore pollen nuclei and form an embryo sac. The mature embryo sac contains, 3 antipodal cells at the chalazal end, 3 synergid cells at the micropylar end, an egg cell at he entire and a 2 polar nuclei that fuse together to form a secondary nucleus.