1. Unfertilized eggs of Psammechinus miliaris which have been allowed to stand in sea water containing nicotine and are then inseminated in normal sea water are polyspermic.
2. Polyspermy is not due to an increase in the speeds at which spermatozoa swim in these circumstances; nor to a decrease in the rate at which the change in cortical structure of the egg, the first sign of fertilization, is propagated over the egg surface.
3. Experiments to test alternative explanations of the effect of nicotine have been carried out.
4. The experiments do not enable a firm decision to be made between (a) a high-speed block to polyspermy (which has not been observed) with a high probability of a successful collision, and (b) a low-speed (20 sec.) block to polyspermy with a low probability of a successful collision.