Megasporogenesis is the process of formation of megaspore in Flowering plants. Diploid MMC (Megaspore Mother Cell) undergoes meiosis and forms four haploid Megaspore tetrads. Unlike Microspore tetrad, which were formed from PMC (Pollen Mother Cell) after meiosis and all four Microspores were functional in a tetrad, in most of the common cases, only one out of four Megaspores remain viable. This is called Functional Megaspore. Only the functional megaspore develops further into mature embryo sac. Such development is called Monosporic Embryo Sac development. The remaining three megaspores, which are situated towards the Chalazal end of ovule, degenerates. The degenerating cells provide energy to the functional megaspores for further division and developments.
The functional megaspore divides three successive mitosis and forms a 7-celled, 8-nucleated, Polygonum type embryo sac, which contain one egg cell or the female gamete.
Thus, one MMC forms one functional megaspore and one megaspore has one embryo sac developed from it. Each embryo sac contains one egg cell. Hence, to produce 100 embryo sacs, 100 megaspore cells are required, which can be derived from 100 MMC.