A zygote is the product of syngamy, which participate in the fusion of a haploid male gamete and a haploid female gamete,i.e., egg cell.
Male gamete(n)+Egg----->Zygote(2n)
A primary endosperm cell is the product of the fusion of a secondary nucleus (2n) and a haploid male gamete(n).
Secondary(2n)+male gamete(n)------> Endosperm nucleus.