1. The reactions of sea-urchin eggs (Psammechinus miliaris) to high concentrations of homologous spermatozoa have been investigated by a new method.
2. The method was to inseminate eggs with high concentrations of spermatozoa (108/ml.), which ensured that all eggs were fertilized for the first time at the beginning of the experiment; and then functionally to separate the eggs and spermatozoa at known times after the original insemination.
3. The proportion of polyspermic eggs in the suspension rises from zero at t=0 to a constant value at a time T, which is the conduction time of the block to polyspermy. In fourteen experiments on eggs from different sea-urchins, the estimated values of T were 17, 69, 72, 74, 94, 85, 26, 60, 31, 91, 85, 34, 75 and 67 sec. (arithmetic mean 63 sec.). The standard errors of the estimates of T, which are tabulated in the text, ranged from 5 to 15 sec.
4. These results confirm previous experiments which strongly suggested that the conduction time of the block to polyspermy was of the order of seconds; these earlier experiments were incompatible with a block to polyspermy lasting a fraction of a second.
5. At the same time experiments in this paper made it possible to estimate the fertilization parameter or sperm-egg interaction rate α, during the conduction of the block to polyspermy, and to compare it with the pre-fertilization α. In a given sperm suspension, α is a measure of the receptivity of the egg surface to spermatozoa. During the conduction of the block to polyspermy α was found to be markedly lower (1/20) than in unfertilized eggs.
6. This suggests that at fertilization there is a fast but incomplete block to polyspermy, whose conduction time may be 1 sec. or less. This is followed by the slower block which finally makes the egg impermeable to spermatozoa. This also confirms previous observations on the small incidence of polyspermy at normal sperm densities, which is not consistent with the concept of a slow block to polyspermy.
