1. A method is described for the direct counting of male pronuclei in recently fertilized sea-urchin eggs.

2. Using this method, fertilization rate determinations were made to compare 30% artificial sea water (A.S.W.), isotonic KCl, sea water containing lauryl sulphate, calcium-free and magnesium-free A.S.W. containing EDTA, and sea water containing uranyl nitrate, as agents blocking fertilization but permitting further development of previously fertilized eggs.

3. 30% A.S.W. was found to be less satisfactory than the other agents, lacking instant effect, and tending to promote polyspermy. The other agents all gave sigmoid rate curves, that of uranyl nitrate lagging 15-25 sec. behind the others.

4. Evidence was found that uranyl nitrate acts at a later stage in fertilization than the other agents.

5. Sigmoid rate curves were found, except with 30% A.S.W., when eggs with the bulk of the jelly coat removed, and nicotine-treated eggs, were fertilized.

6. Analysis of sperm distribution among eggs from samples fertilized for more than 40 sec. confirmed that re-fertilization takes place at a lower rate than primary fertilization.

7. The processes blocked by KCl and uranyl nitrate were found to precede the cortical responses to fertilization, and the termination of nicotine sensitivity.

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