Development of Female Gametophyte or Embryo Sac

The female gametophyte of angiosperms is called the embryo sac. It is the haploid (n) structure inside the ovule that contains the egg cell and all supporting cells required for fertilisation and early seed development.

The mature female gametophyte (embryo sac) thus contains two polar nuclei, three antipodal cells, one egg cell, and two synergids - making it 8-nucleated but only 7-celled.

The functional megaspore undergoes three successive free nuclear mitotic divisions (nuclear divisions without immediate cell wall formation):

1. Functional Megaspore → 2-Nucleate Stage (Mitosis I)
   The haploid nucleus of the functional megaspore divides mitotically. The two nuclei move to opposite poles of the enlarging megaspore → 2-nucleate embryo sac.
2. 2-Nucleate → 4-Nucleate Stage (Mitosis II)
   Both nuclei divide again simultaneously → 4-nucleate stage. Two nuclei are now at each pole.
3. 4-Nucleate → 8-Nucleate Stage (Mitosis III)
   All four nuclei divide once more → 8-nucleate stage. Four nuclei at each pole. These divisions are strictly free nuclear, with no cell walls between them.
4. Polar Nuclei Migration: One nucleus from each pole migrates towards the centre. These two centrally positioned nuclei are called polar nuclei. They are situated below the egg apparatus in the large central cell.
5. Cell Wall Formation → 7-Celled Embryo Sac
   After the 8-nucleate stage, cell walls are laid down. Six of the eight nuclei are individually enclosed by cell walls. The remaining two (polar nuclei) remain in the large central cell without individual walls.

Development of the female gametophyte 

At maturity, the typical angiosperm embryo sac is 7-celled and 8-nucleate.

A mature embryo sac

The cells are distributed as follows: