Q.Why does the zygote divide only after the division of Primary endosperm cell?

After fertilization, every floral part transforms: sepals, petals, stamens, style and stigma usually wither; the ovary becomes the fruit and the ovule the seed; the egg becomes the zygote; the funicle becomes the seed stalk; the ovule's micropyle becomes the seed's micropyle (for O2/water uptake in germination); surviving nucellus persists as perisperm (black pepper, beet root); the outer integument becomes the testa and the inner the tegmen; and the synergids and antipodals degenerate while the secondary nucleus develops into the endosperm (some species also show unusual transformations - a fleshy receptacle in apple, a persistent calyx in Solanum melongena, an edible pedicel in cashew, a fleshy caruncle from the integument tip in Ricinus, a fleshy aril from the funiculus in Myristica). The triploid primary endosperm nucleus (PEN) itself develops into one of three endosperm types depending on whether/when wall formation follows division - nuclear (free nuclei only, e.g. Coccinia), cellular (a wall follows every division from the first, e.g. Helianthus) or helobial (a wall divides a large micropylar chamber of free nuclei from a small chalazal chamber, e.g. Hydrilla) - plus the irregular-surfaced ruminate variant (Areca catechu); a seed may be non-endospermous/ex-albuminous (endosperm fully consumed, e.g. pea) or endospermous/albuminous (endosperm persists to nourish germination, e.g. paddy), and specialised haustoria in some endosperms absorb nutrients from surrounding ovary tissue. Endosperm nourishes the embryo, and in most angiosperms the zygote only starts dividing after endosperm development has begun, so endosperm also regulates the timing/mode of embryo development. Embryogenesis in the dicot Capsella bursa-pastoris proceeds zygote to terminal+basal cell, to a four-celled proembryo, to a quadrant, to an eight-celled octant (epibasal/hypobasal tiers), to a sixteen-celled stage (outer dermatogen forming epidermis; inner periblem forming cortex and plerome forming st …