The Fertilization Reaction in the Sea-Urchin

Abstract

1. Unfertilized eggs of the sea-urchin (Psammechinus miliaris) were left for known but varying times in contact with homologous sperm suspensions containing known numbers of spermatozoa. Counts were made of the numbers of fertilized and unfertilized eggs at times ranging from 0 to 300 sec. after mixing. 2. If spermatozoa are considered as particles moving in random directions, the frequency of sperm-egg collisions can be calculated if the density and mean speed of the sperm suspension are known. 3. The information in (1) and (2) enables an estimate to be made of the probability of a successful sperm-egg collision. 4. The estimated probability of a successful collision, p, varies with sperm density, d. At the lowest density used, 7.44x104/ml., p was found to be 0.226. At the highest density, 9.62/106/ml., p was about 0.011. The inverse relationship between p and d may be due to sperm-sperm interactions of a physical nature. 5. The presence of jelly round the eggs increases p. This disposes of the possibility, raised in the past, that egg jelly may have an adverse effect on the fertilizing capacity of homologous spermatozoa under normal conditions of fertilization. 6. The technique of subjecting eggs to a pre-determined number of collisions facilitates investigation of the conduction time of the block to polyspermy. Preliminary experiments suggest that the conduction time may be of the order of seconds rather than fractions of a second. 7. Sources of error arising from the ‘kinetic’ treatment of sperm suspensions are discussed in detail. One source of error concerns the alleged chemotaxis of spermatozoa towards eggs and egg secretions. No chemotaxis was observed.