1. The functional megaspore is the first cell of female gametophyte of angiosperms.

2. It enlarges to form the female gametophyte, also called embryo sac.

3. Its nucleus undergoes a mitotic division and the two nuclei move to the opposite poles, forming the 2-nucleate embryo sac.

4. Two successive mitotic divisions in each of these two nuclei result in the formation of an 8-nucleate embryo sac.

5. Cell wall formation starts at the eight-nucleate stage, resulting in the formation of a typical female gametophyte.

6. Three cells are grouped together at the micropylar end to form the egg apparatus, consisting of two synergids and a female gamete/egg cell.

7. Three cells are grouped together at the chalazal end; they are called antipodal cells.

8. The remaining two nuclei are called polar nuclei; they move to the centre of the embryo sac (now called central cell) and fuse to form a diploid secondary nucleus.

9. Thus, a typical angiosperm embryo sac is 8-nucleate and 7-celled.