I08 FERTILIZATION 



the cone is seen at the equator, the egg was fertiHzed at the equator. 

 An experiment of this sort shows that the conduction time is about 

 twenty seconds at i8° C in the eggs oi Psamtnechinus miliaris (Roths- 

 child & Swann, 1949), with quite a significant induction period 

 before the cortical change begins to pass over the egg surface (Kacser, 

 1955). It is natural to ask whether the change in surface structure 

 underlying this colour change or increase in light scattering is the 

 block to polyspermy, but this at once raises another question — 

 how many spermatozoa collide with the egg and, in particular, with 



FIG. 21. — Passage of the cortical change over the egg surface. The observer at 

 O sees the egg in optical section, the periphery of the section being the 

 equatorial great circle E. (a) Fertilization at the north pole. The whole of 

 the cortical great circle E changes colour instantaneously, (b) Fertilization 

 at the equator. The cortical great circle E changes colour at the true rate at 

 which the change is propagated, (c) Fertilization at 2.0 o'clock. The change 

 affects the cortical great circle E more quickly than the whole egg and the 

 conduction rate appears to the observer to be higher than it is. The con- 

 centric circles Cj, C2, etc. represent the leading edge of the propagated change 

 at times ^1, (2, etc. (Rothschild & Swann, 1949). 



parts of the egg surface unaffected by the surface change until the 

 end of the twenty second period ? When eggs are inseminated with 

 fairly dense sperm suspensions, 10'^ /ml., swarms of spermatozoa 

 are normally seen round every egg in the suspension ; yet only one 

 spermatozoon fertilizes each egg. This has led people to believe 

 that the block to polyspermy passes over the egg in an incredibly 

 short time. But this presupposes that every spermatozoon which 

 collides with an egg is capable of fertilizing it, and this presupposi- 

 tion requires examination. 



The number Z of spermatozoa, moving in random directions — 

 sea-urchin spermatozoa do move in random directions, even when 

 near homologous eggs (Rothschild & Swann, 1949) — which will 

 collide with an egg each second is given approximately by the 

 equation 



Z = Tra^nc . . . • (0 



