STUDIES OF FERTILIZATION. 



TABLE II. 



EFFECT OF SPERM CONCENTRATION, PURPURATUS 9. X FRANCISCANUS 



prove this better perhaps than the figures given in Table II : On 

 February 6 I tested the cross-fertilization of a given lot of pur- 

 puratus eggs with franciscanus sperm up to a concentration of 75 

 units with hardly any fertilization resulting at any concentration. 

 The purpuratus eggs thus resistant to franciscanus sperm at all 

 concentrations fertilized perfectly with purpuratus sperm (added 

 later), and the franciscanus sperm used fertilized franciscanus 

 eggs perfectly. 



Another experiment in which the eggs of five purpuratus 

 females were fertilized in each case with franciscanus sperm of 

 6.6 and 13.2 units will bring out once more the greater effective- 

 ness of the individual variations than the sperm concentration : 



TABLE III. 



EFFECT OF SPERM CONCENTRATION ON EGGS OF FIVE FEMALES. 

 S. purpuratus 5x5". franciscanus $. 



Percentage Segmented with 

 13.2 Units Sperm. 



6 



1.6 

 0.6 



O.I 



0.3 



There is a much greater difference between females I and 4 

 for instance than between the two lots of eggs of any one female. 



(b) S. franciscanus $ X S\ purpuratus . In the case of this 

 cross again the individual variation outruns the effects of sperm 

 concentration. Thus four samples of the eggs of one female 

 fertilized with i, 2, 4 and 8 units of the sperm of one purpuratus 



Percentage Segmented with 

 6.6 Units Sperm. 



