What happens to the egg cell and sperm cell during fertilization?
Actually, once the sperm reach the egg in the fallopian tubes, they have to break through the outer layer of the egg, the zona pellucida. They do this by releasing digestive enzymes from the acrosome on top of the sperm head. Once the first one gets through, it sets off a change in the cell membrane so that no others can get through. This prevents an uneven number of chromosomes that can lead to serious genetic problems. Identical twins happen if during the first half dozen cell divisions of the resulting zygote, the identical cells fail to stay together and divide into two separate embryos. Half identical twins are possible of the ovum divides before it’s fertilized and and both daughter cells are fertilized and implant, but it’s really, really rare.
