38 PHYSIOLOGICAL TRIGGERS 



chemical methods (82). However, other positive evidence in support of the 

 view is lacking. Finally, the fact that the propagated response can apparently 

 be elicited by pricking with a fine needle (75) casts doubt on the necessity of a 

 substance from the sperm. 



Block to polyspermy. In eggs in general and marine eggs in particular, one 

 spermatozoan, and only one, ordinarily participates in the development of the 

 egg. This is true even though many sperms may be present in the vicinity of 

 the egg at the time of fertilization. Clearly some mechanism prevents more 

 than one sperm from entering the egg. This mechanism, the block to polyspermy, 

 has been the subject of extensive study. Earlier investigations led to the view 

 that the block does not depend upon the formation of the fertilization mem- 

 brane but to a change of some sort in the constitution of the egg surface. This 

 change is generally assumed to arise at the site of union with the activating 

 sperm and to spread over the egg surface. Just's (44) frequently quoted state- 

 ment describes the change as a 'wave of negativity.' 



On the basis of subjective observation most investigators concluded that 

 the block to polyspermy sweeps over the egg surface very rapidly. Gray (31), 

 for example, estimated that the time required was of the order of io~^ seconds. 

 However, recent investigations have considerably altered the earlier views 

 concerning both the time required for development of the block and the nature 

 of the block itself. 



Rothschild and Swann (88, 89, 91, 92) undertook to measure the time neces- 

 sary for the development of the block to polyspermy in the egg of Psammechinus 

 ■miliaris. They began their study by determining the rate of propagation of the 

 first visible change in the egg cortex, namely, the dark field color change. 

 From motion picture records they found that the propagation time of this 

 response was about 20 seconds at i8°C. In an attempt to relate this visible 

 cortical change to the physiological block to polyspermy, these workers per- 

 formed a number of elaborate experiments and subjected them to a searching 

 mathematical analysis. They first calculated the number of sperm-egg collisions 

 that would occur in the unaffected part of the egg during the passage of the 

 dark field change. At sperm densities of 10'', 10^, and 10^ per milliliter the number 

 of such collisions was estimated to be 1.6, 16 and 160 respectively. However, 

 no appreciable polyspermy was observed at these sperm concentrations in 

 spite of the number of collisions. This absence of polyspermy would be ex- 

 plained if the block swept over the egg much more rapidly than the visible 

 change, but it could also result if sufficiently few of the sperm-egg collisions 

 succeeded in fertilizing the egg. The studies of Rothschild and Swann indicate 

 that both of these factors contribute to the prevention of polyspermy. 



Several lines of evidence indicate that the number of successful sperm-egg 

 collisions constitutes only a fraction of the total collisions. Thus, the number 

 of blebs formed on immature eggs (oocytes) was far lower than the calculated 



