258 PROBLEMS OF FERTILIZATION 



catalytic substances for the interactions that follow the 

 primary cortical effect; but their nature has remained 

 entirely unknown. 



Just's experiments (1915^) on heat parthenogenesis 

 in Nereis furnish an experimental basis for the view 

 that the activating substance is readily diffusible. He 

 found that the eggs could be stimulated to complete 

 parthenogenetic development by proper heat exposure, 

 provided that they were not first washed in sea-water. 

 If they were first washed, the eggs lost their capacity 

 for complete heat activation, although they were still 

 fertilizable. This difference can be readily understood 

 on the assumption that the spermatozoon is a more 

 efficient activator than heat, and that there remains 

 after washing a sufficient amount of activable substance 

 for action of the spermatozoon, but not of heat. We 

 have previously pointed out that the sea-water used 

 for washing the eggs contains a sperm-agglutinating 

 substance, and we have given the reasons for identifying 

 this with the activating substance (pp. 229 ff.). 



How can the spermatozoon exert an effect on the 

 cortex similar to that produced by heat or butyric 

 acid? Heilbrunn (191 5) suggests that the spermato- 

 zoon produces a partial liquefaction or swelling of the 

 vitelline membrane at the point of attachment, thus 

 lowering the surface tension at one point; there is an 

 immediate tendency for the tension to be equalized 

 everywhere, which results in a certain lowering of the 

 tension of the membrane around the entire egg, and 

 elevation of the membrane (usually called membrane 

 formation) results, owing to an overbalance of forces 

 acting outward on the membrane. Such an explana- 



