Sea urchin eggs have a "jelly" coat around them that serves them in many ways.

• It prevents too many sperm from getting to the egg at the same time, because of its viscosity.
• Proteins in the jelly initiate the acrosome reaction in sperm so they are ready to fertilize the egg.
• It provides a sort of "shock absorber" to prevent injury to the egg from wave action.

It is colorless, so it is difficult to see.

1. Add a small amount of "sumi ink" to the sea water surrounding the eggs to make the jelly layer more visible.
2. The ink particles are too big to get through the jelly layer and are excluded.
3. Where the ink is excluded, more light can get through and we see the jelly layer as a clearing around the eggs.