The Fertilization Reaction in the Sea-Urchin Egg

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Abstract
1. After insemination of unfertilized eggs of Psammechinus miliaris and before the elevation of the fertilization membrane, a change in cortical structure is propagated over the egg surface or through the cytoplasm. When the fertilizing spermatozoon has become attached, the cortex progressively scatters more light when viewed under dark ground illumination. The cortex, which before fertilization is scarcely distinguishable from the cytoplasm, is white after the reaction is completed. 2. At 18° C. the change covers the egg surface in about 20 sec. 3. The conduction velocity is not uniform, there being a reduction in rate when the change is about halfway round the egg, after which there is a marked increase. 4. The cortex of the unfertilized egg shows a weak positive radial birefringence which disappears at the same time as the dark ground scattering appears. 5. Other changes in the egg structure occur at the same time, and are visible under dark ground illumination. These are: (a) a conical erection appears transiently at the site of sperm attachment; (b) the shape of the egg changes during the reaction; (c) there is a localized and transient decrease in light scattering at the point of entry of the spermatozoon, after the initial increase in cortical light scattering; (d) the fertilization membrane first appears at the point on the surface where the cortical change is initiated. 6. To investigate the possibility that the observed change might be the block to polyspermy, measurements were made of the speeds at which sea-urchin spermatozoa swim. A new technique has been evolved for this purpose. Spermatozoa were photographed under a special dark ground illumination system, with exposures of 0.25 sec. The heads of the spermatozoa trace out tracks on the film, the mean translatory speed being 190 µ/sec. The motion is helicoidal, the frequency of oscillation of the illuminated sperm heads being about 40/sec. 7. By treating the sperm suspension as an assembly of gas molecules, a rough estimate has been made of the frequency of collisions between the spermatozoa and an egg. For sperm densities of 105, 106 and 107/ml., and for the observed translatory sperm speeds, the number of collisions between spermatozoa and those parts of the egg surface unaffected by the propagated change, after it has been initiated, is 1.6, 16 and 160 respectively. 8. Some preliminary experiments on the insemination of oocytes have been carried out to assess the probability of a sperm-egg collision being successful. The results were: (a) oocytes react to insemination by the extrusion of blebs or papillae, each of which is associated with a spermatozoon; (b) this reaction is inhibited by insemination in Ca-free sea water and partially inhibited by acidified sea water. It is concluded that the extrusion of papillae represents an abortive fertilization reaction, the oocyte cortex being unable to propagate a block to polyspermy. The number of papillae extruded is less than the number of sperm-oocyte collisions. 9. The experiments on the insemination of oocytes favour the possibility that attachment of the spermatozoon to an egg is not followed by fertilization unless there exists a particular orientation, on a molecular scale, between the egg and sperm surfaces, and provided there has been no previous interaction between the spermatozoa and Gynogamone II. The low probability of fertilization that this implies is compatible with the hypothesis that the observed cortical change may be the block to polyspermy. Further experiments are needed to resolve this question.