in Sexual reproduction in flowering plants by

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  1. Female gametophyte is developed in the nucellus of the ovule.
  2. In the nucellus of a developing ovule, one diploid cell develops into a diploid megaspore mother cell.
  3. This megaspore mother cell divides by meiosis to produce four haploid megaspores which are arranged in a linear tetrad.
  4. Of these, the upper three degenerate and the lower chalazal megaspore is functional to produce the female gametophyte (embryo sac).
  5. The nucleus of the functional megaspore divides mitotically into two nuclei.
  6. These are called primary micropylar and primary chalazal nuclei.
  7. The second division produces one pair of nuclei at the micropylar end and one pair of nuclei at the chalazal end. The third division results in the formation of 4 nuclei at each pole.
  8. One nucleus from each pole then migrates towards the centre to become polar nuclei, which fuse together and give rise to the secondary nucleus (2n).
  9. Out of these, the central one is the egg cell while the two lateral cells are called synergids.
  10. The three nuclei towards the chalazal end also get organized into three thin walled cells called antipodal cells.
  11. The structure containing egg apparatus, secondary nucleus and antipodals is called female gametophyte or embryo sac.
  12. It is a seven-celled and eight-nucleate structure.
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