Answer:During sexual reproduction, two haploid cells, called gametes, fuse to form a diploid cell called a zygote. In animals, the gametes are sperm and eggs, and the process by which they fuse is called fertilization. In plants, the gametes are produced by the male and female reproductive cells, called the microgametes and the megagametes, respectively, and the process is called syngamy.
During fertilization in animals, the sperm penetrates the egg, and the nuclei of the sperm and egg fuse, combining their genetic material. This forms the diploid zygote, which then undergoes cell division to form a multicellular organism.
In plants, the fusion of the microgametes and megagametes occurs at the tip of the pollen tube and the egg cell. This process is called double fertilization, which leads to the formation of a diploid zygote and a triploid cell called the endosperm, which provides nourishment for the developing seed.
In both case, sperm and egg or micro and mega gametes are the structure fusing to form diploid zygote during sexual reproduction.