Formation of Plant Egg Cells

The female gamete of a flowering plant, the egg cell, forms inside an ovule in the ovary of the carpel. Each ovule contains a mass of tissue (the nucellus) surrounded by protective layers called integuments, which leave a small gap at one end, the micropyle.The stages are:
• One large diploid cell in the nucellus, the megaspore mother cell (2n), divides by meiosis to produce four haploid cells.
• Three of these degenerate. The one that survives, the megaspore (n), grows to form the embryo sac.
• The nucleus of the embryo sac divides by mitosis three times, giving 2³ = 8 haploid nuclei.The eight nuclei are arranged in a typical pattern:
• at the micropyle end: the egg cell (the female gamete) with two synergids beside it;
• in the centre: two polar nuclei, which often fuse to form a diploid central nucleus;
• at the opposite end: three antipodal cells.At fertilisation one male nucleus fuses with the egg cell nucleus to form the diploid zygote, and the other fuses with the two polar nuclei to form the triploid (3n) endosperm nucleus, which develops into the food store of the seed.Because all eight nuclei come from one haploid megaspore by mitosis, they are genetically identical to each other.Worked example: a plant has 2n = 20. The megaspore mother cell has 20 chromosomes, the egg cell and each polar nucleus have 10, and the endosperm nucleus formed at fertilisation has 10 + 10 + 10 = 30.