Fertilization. Colored Scanning Electron Micro- graph (SEM) of sperm clustered on the surface of a human egg (ovum) during fertilization. The round egg (blue) has a furry surface. Sperm attached to it are hair-like structures (yellow); each sperm has a head and a long tail. Here, they are penetrating the thick surface of the egg that attracts sperm and enables them to attach. The human female usually produces a single large egg, and only one of the millions of male sperm released may penetrate the egg to fuse with the egg nucleus. Once fertilized, the egg begins its process of growth into a human embryo. Magnification: x1,000 at 6x7 cm size.