138 FERTILIZATION 



while Hultin (1948) had the same experience with other sea- 

 urchins. 



(4) Increase n. This- is the classical way of achieving cross- 

 fertilization. An increase in sperm density involves an increase in 

 the number of sperm-egg collisions as the latter, Z, is equal to 

 Tia^nc. For any p, however low, the more collisions there are, the 

 greater the chance that an egg will be fertilized.* The most im- 

 portant investigation of cross-fertilization in which n was con- 

 trolled was made by Tyler (1949), though Fuchs (1914-1915) also 



TABLE 22 



Comparison betzveen cross-fertilization and cross-agglutination 



The upper figures in each pair refer to the dilution of a standard sperm 

 suspension necessary to achieve 2% fertilization (e.g., 3000 means i /3000). The 

 lower figures in each pair are the highest dilutions of fertilizin solution at which 

 visible agglutination occurred. 



realized the importance of controlling n, in his studies on self- 

 sterility in Ciona intestinalis. Tyler's results are reproduced in 

 Table 22. The experiments were done for a particular reason, to 

 compare cross-fertilization and cross-agglutination; the technique 

 of known sperm-egg interaction times had not been developed at 

 that time. Tyler's results are interesting as they show that the 

 correspondence between the degree of cross-fertilizability and 

 cross-agglutination is not particularly marked. This means 

 that there is more specificity in fertilization than in the fer- 

 tilizin-antifertilizin reaction, or that fertilization is not ex- 



* This is not the same proposition as the familiar but fallacious one about red 

 being 'bound' to turn up after a run of 20 blacks, at roulette. 



