POLYSPERMY 121 



but interpretation will not be easy. The difficulties imposed by 

 the use of high sperm densities did not arise in the experiments of 

 Hagstrom & Hagstrom (19546). They repeated Sugiyama's 

 experiments but used sea-urchin eggs which had been pre-treated 

 with trypsin, to remove the vitelline membrane, before fertiliza- 

 tion. In these conditions refertilization can be achieved at a com- 

 paratively low sperm density, 2-6 X lo^/ml. 



Conclusions regarding Type I Inhibition of Polyspermy. The 

 experiments described in this chapter make it possible to construct 

 a tentative picture, about which there will be disagreement, of the 

 operation of the block to polyspermy (Type I). At the moment of 

 attachment of the fertilizing spermatozoon to the egg surface, a 

 change in cortical structure passes over the egg in less, but prob- 

 ably not much less, than two seconds. This reduces the chance of 

 refertilization by a factor of twenty, and catalyses the production 

 of a sperm-impermeable layer at the egg surface. Exploding 

 cortical granules, discharging alveoli, or their equivalent near the 

 egg surface may contribute to the formation of this layer, which is 

 estabUshed in about sixty seconds, though this will undoubtedly 

 vary from species to species. The integrity of the layer depends, 

 inter alia, on the existence of divalent cations in the external 

 medium. If they are not present, the layer may 'dissolve' and 

 refertilization may be possible. 



An action potential may pass over the egg surface before these 

 changes, as so many cell physiologists have believed or hoped; but 

 experiments to establish the existence of such action potentials, or 

 electrical depolarization of the egg surface, are exceedingly difficult, 

 and claims to have observed them must be examined with caution, 

 if not scepticism. This subject is discussed in detail in the next 

 chapter. 



The block to polyspermy and the 'fertilization impulse', as the 

 early phases of the fertilization reaction are sometimes called, are 

 being actively investigated at the present time. Partial fertilization, 

 for example, first systematically examined by Allen (1954), will 

 undoubtedly shed light on these phenomena, and in a few years' 

 time the picture referred to above will be less hazy, though it may 

 require modification. 



'Heterologous' Polyspermy and Somatic Fertilization. An account 

 of polyspermy would be incomplete without some mention of 

 recent Russian work on this subject and, in particular, of a very 



