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H. TIMOURIAN and G. WATCHMAKER

Summary.

When male sea-urchins (Lytechinus pictus) are kept in hypo-osmotic sea-water, their spermatozoa are inactive in isosmotic sea-water but active in hypo-osmotic sea-water.

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H. TIMOURIAN, C. E. HUBERT, and R. N. STUART

Summary.

Fertilization of the sea-urchin egg has been determined for low sperm concentrations in various sperm-to-egg ratios. It was found that only about 2 % of the spermatozoa were successful in fertilizing the eggs. There was no way of distinguishing between the possibility that only 2 % of the spermatozoa were capable of fertilization, or that only 2 % of the surface of the egg was receptive to the spermatozoa. The information obtained by measuring fertilization as a function of sperm-to-egg ratio was discussed as a possible method for the study of factors affecting the capacity of either the spermatozoa or egg to participate in fertilization.