Aborting embryo. Light micrograph of a human embryo aborting at the 4-cell stage. The embryo is undergoing tissue death (necrosis), and its cells, or blastomeres, are breaking down. The membrane around the embryo is the zona pellucida. The 4- cell stage, reached after two divisions of the fertilised egg, occurs about 40 hours after fertilisation. It is thought that roughly half of all pregnancies end in spontaneous abortion at an early stage. Photographed at Cochin Hospital, Paris, France. Magnification unknown.