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Trace the development of megaspore mother cell up to the formation of a mature embryo-sac in a flowering plant.

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Megagametogenesis: 

The formation of female gametophyte (embryo sac) is called megagametogenesis. 

(i) Megaspore is the first cell of the female gametophyte. 

(ii) The megaspore increase in size and its nucleus divides mitotically into two nuclei which move apart to opposite poles. Thus a 2-nucleate embryo sac is formed. 

(iii) Two daughter nuclei undergo another mitotic division giving rise to the 4-nucleate stage. 

(iv) The third mitotic division, which gives 8-nucleate embryo sac comprises of a micropylar and chalazal end with 4 nuclei at each end. 

 (v) One nuclei from each end proceeds towards the centre of the embryo sac and are called the polar nuclei. 

(vi) The three nuclei remaining at the micropylar region form the egg apparatus, without any cellular wall. 

(vii) In the egg apparatus, the middle one is the largest called oosphere/egg/ovum, while other two naked cells adjoining the egg cell are called synergids. 

(viii) The three remaining nuclei at the chalazal end are surrounded by mass of cytoplasm and cellular wall and are called antipodal cells. 

(ix) The above mentioned method of female gametophyte formation is known as normal 8-nucleate type, because 8 nuclei contribute in the formation of gametophyte and it is very common among angiosperms.

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