STUDIES OF FERTILIZATION. 



II 



TABLE IV. 



THE MEASURE OF SPECIFICITY IN CROSSES OF STRONGYLOCENTROTUS FRANCIS- 



CANUS AND S. PURPURATUS. 



M. = percentage of membranes formed. 

 S. = percentage of eggs segmented. 



The above data show that the eggs of each species straight 

 fertilize well at a sperm concentration which has no effect in cross 

 fertilization. The fertilization is thus strikingly specific to an 

 extent that is not realized when higher sperm concentrations are 

 employed. If we take the lowest sperm concentration in each 

 case in which any cross fertilization occurs, viz., 0.25 in the case 

 of franciscanus eggs (Exp. 2a) and I in the case of purpuratus 

 eggs (Exp. 2c) we can give a mathematical expression to the 

 relative " ease " of straight and cross fertilization in a given lot 

 of eggs. Thus in the case of franciscanus where 100 per cent, 

 of the eggs fertilized with 0.25 franciscanus sperm we could say 

 that cross fertilization was 20 times more " difficult " than straight 

 fertilization. Or if we use Exp. ic for comparison, cross-fert.il- 



