Color enhanced Scanning Electron Micrograph (SEM) showing a human sperm cell fertilizing a hamster egg. Human sperm can fertilize animal eggs from which the zona pellucida has been removed, but zygotes will not develop. The plasma membrane, containing microvilli, of the hamster egg is visible. Infertility clinics sometimes use hamster eggs to test the fertilizing capacity of human sperm in vitro. Magnification: 19,000x at 8 x 10".