<P> The nucellus (plural: nucelli) is part of the inner structure of the ovule, forming a layer of diploid (sporophytic) cells immediately inside the integuments . It is structurally and functionally equivalent to the megasporangium . In immature ovules, the nucellus contains a megasporocyte (megaspore mother cell), which undergoes sporogenesis via meiosis . In the megasporocyte of Arabidopsis thaliana, meiosis depends on the expression of genes that facilitate DNA repair and homologous recombination . </P> <P> In gymnosperms, three of the four haploid spores produced in meiosis typically degenerate, leaving one surviving megaspore inside the nucellus . Among angiosperms, however, a wide range of variation exists in what happens next . The number (and position) of surviving megaspores, the total number of cell divisions, whether nuclear fusions occur, and the final number, position and ploidy of the cells or nuclei all vary . A common pattern of embryo sac development (the Polygonum type maturation pattern) includes a single functional megaspore followed by three rounds of mitosis . In some cases, however, two megaspores survive (for example, in Allium and Endymion). In some cases all four megaspores survive, for example in the Fritillaria type of development (illustrated by Lilium in the figure) there is no separation of the megaspores following meiosis, then the nuclei fuse to form a triploid nucleus and a haploid nucleus . The subsequent arrangement of cells is similar to the Polygonum pattern, but the ploidy of the nuclei is different . </P> <P> After fertilization, the nucellus may develop into the perisperm that feeds the embryo . In some plants, the diploid tissue of the nucellus can give rise to the embryo within the seed through a mechanism of asexual reproduction called nucellar embryony . </P> <P> The haploid megaspore inside the nucellus gives rise to the female gametophyte, called the megagametophyte . </P>

Development of embryo from the cells of the nucellus is called