twenty-four hours. Doctors then use a microscope to see if there are two pronuclei (one from the egg and one from the sperm) in the egg, indicating that fertilization occurred. In some cases, the sperm are unable to penetrate the egg. They may be unable to swim correctly, or they may not have been capacitated successfully. In the past, the only solution was to use sperm from another man. Now, however, sperm can be injected directly into the cytoplasm the material egg's cytoplasm, in a process called microassisted fertilization. in a cell, excluding the , , , ,, , , , , nucieus There are three ways used successfully by doctors and researchers to micro-fertilize the egg. The first is "zona drilling," in which a hole is punched into the zona pellucida, letting sperm penetrate the egg. The second method is called "subzonal sperm insertion," in which a sperm is injected directly under the zona pellucida. A third, related method is "intracytoplas-mic sperm injection," in which a sperm is injected directly into the egg cytoplasm. The second method is reported to have a higher fertilization success rate (59%) than the third (13%).

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