How does the megaspore mother cell develop into 7-celled, 8 nucleate embryo sac in an angiosperm? Draw a labelled diagram of a mature embryo sac.

Explanation

(i) 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 4-nucleate and later the 8-nucleate stages of the embryo sac.
(ii) These mitotic divisions are strictly free nuclear, that is, nuclear divisions are not followed immediately by cell wall formation. After the 8-nucleate stage, cell walls are laid down leading to the organisation of the typical female gametophyte or embryo sac.
(iii) Six of the eight nuclei are surrounded by cell walls and 'organised into cells, the remaining two nuclei, called polar nuclei are situated below the egg apparatus in the large central cell.
(iv) There is charactertics distribution of the cell 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.
(v) The synergids have special cellular thickenings at the micropylar tip called filiform apparatus which play an important role in guiding the pollen tubes into the synergid.
(vi) Three cells are at the chalazal end are called the antipodals. The large central cell, as mentioned earlier, has two polar nuclei. Thus, a typical angiosperm embryo sac, at maturity, through 8-nucleate is 7-celled.