In the process of fusion of gametes during fertilization, there is mainly the fusion of nuclei of the male and the female gametes, i.e. the sperm and the ovum respectively. When a sperm reaches near the ovum, it comes in contact with it and releases its nucleus into the egg cell. There is fusion, thus of the nuclei of sperm and egg cell, inside the egg cell. The rest of the sperm body degenerates. And hence, the product formed is a single cell called zygote.