Do not contaminate the remaining eggs by contact with the sperm.
The development of the sea urchin embryo occurs rather rapidly. Fertilization will probably occur before you can get the coverslip on the slide. Within about 1-2 minutes the fertilization membrane will lift off, and the first division should occur within 2-3 hours, depending on the temperature. The embryo will hatch in just over 24 hours and will complete its entire development within 72 hours.
If close examination of the fertilization process is desired, place the eggs on a slide with a support to keep the coverslip just off the eggs (a second coverslip will do), add a coverslip and focus on the eggs with darkfield illumination. Place a drop of sperm just at the edge of the coverslip while observing the eggs through the microscope. With darkfield illumination the sperm will be visible and minute changes in the cortical region of the egg will be observed as a sperm penetrates the outer membrane.
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