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.