Explain the structure and development of the embryo sac.

(N/A) The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles,forming the $2$-nucleate embryo sac.
Two more sequential mitotic nuclear divisions result in the formation of the $4$-nucleate and later $8$-nucleate stages of the embryo sac. These mitotic divisions are strictly free-nuclear,meaning nuclear divisions are not followed immediately by cell wall formation.
After the $8$-nucleate stage,cell walls are laid down,leading to the organization of the typical female gametophyte or embryo sac.
Six of the eight nuclei are surrounded by cell walls and organized into cells; the remaining two nuclei,called polar nuclei,are situated below the egg apparatus in the large central cell. There is a characteristic distribution of the cells within the embryo sac.
Three cells are grouped together at the micropylar end and constitute the egg apparatus. The egg apparatus,in turn,consists of two synergids and one egg cell.
The synergids have special cellular thickenings at the micropylar tip called the filiform apparatus,which play an important role in guiding the pollen tubes into the synergid.
Three cells are at the chalazal end and are called the antipodals.
The large central cell,as mentioned,has two polar nuclei.
Thus,a typical angiosperm embryo sac,at maturity,although $8$-nucleate,is $7$-celled.
Panchanan Maheshwari has classified the female gametophyte into monosporic,bisporic,and tetrasporic embryo sacs based on how many megaspores take part in the development of the embryo sac.