Free calcium is rendered visible by the luminiscence of aequorin, a photoprotein. The content of free calcium in the cytoplasm of the egg rises steeply during insemination. The rise starts at the animal pole, where the sperm penetrates, and spreads over the whole egg, with the reaction tapering off at the vegetative pole. The calcium wave activates the egg, inducting it to further development. With the aid of a low light level camera, these processes can easily be observed on a video monitor.