ii6 



FERTILIZATION 



block to polyspermy by chemical treatment of eggs was examined 

 in detail by the Hertwigs in 1887. In recent years Clark (1936) is 

 the only person who has made a systematic study of this subject. 

 Although a number of different agents, which appear to have no 

 common denominator, such as heat, cold, acid sea water, excess 

 magnesium, alkaloids, fat solvents, chloral hydrate, and, in some 

 circumstances, extracts from eggs and spermatozoa, cause poly- 

 spermy, nicotine and magnesium are probably the most efficacious 





Proportion of isotonic MgCl^ in sea water iv/v) 



FIG. 24. — Effect of addition of isotonic MgCL to sea water on incidence of 

 polyspermy in eggs of Arbacia punctulata (after Clark, 1936). 



agents. Fig. 24, showing the effect of increasing the magnesium 

 content of sea water, is adapted from Clark's paper. As regards 

 nicotine, Clark makes the interesting observation that the degree 

 of polyspermy is a function of the time of exposure and the con- 

 centration of nicotine in the sea water. I have confirmed this 

 observation, but it would be of interest (and comparatively easy) to 

 examine the form of this strength-duration curve in greater detail. 

 The existence of this relationship means that neither the block to 

 polyspermy nor the receptivity of the egg surface is an all-or-none 

 phenomenon ; they are capable of being varied in a continuous way 



