246 FRANK R. LILLIE. 



was always employed throughout any experiment, it is obvious 

 that the effect to be attributed to the egg-concentration employed 

 in the preceding experiments is very small. 



III. Discussion. 



Within a wide limit of egg-concentration the important factors 

 in fertilizing power of sperm suspensions are: (i) concentration, 

 (2) time. A third factor, which is not of equal significance to the 

 other two, is the given variability of the reproductive elements. 

 Such variability attaches of course both to ova and spermatozoa ; 

 in general it will affect only absolute values for given combina- 

 tions, and not at all the relative values found in any single experi- 

 ment. Moreover, as it is a chance factor, it will tend to be 

 eliminated in a series of determinations. Fortunately both eggs 

 and spermatozoa of Arbacia are relatively very constant mate- 

 rials if care be taken to wash the eggs thoroughly, and if the 

 factors of concentration and time are fairly constant for the 

 sperm. For the eggs these two latter factors are of such slight 

 importance within the given limits as to be practically negligible. 

 The significance of the concentration factor for the fertilizing 

 power of sperm is of course obvious without discussion. We 

 therefore turn to the time factor. 



The most significant aspects of the time factor are, first, the 

 unexpectedly rapid rate of loss of fertilizing power of sperm 

 suspensions, and second the increase of rate of loss with dilution. 

 There are but two ways of explaining these facts: either (i) the 

 motility of the spermatozoa is quickly reduced in sperm suspen- 

 sions to such an extent that they cannot bore into the egg or (2) 

 the spermatozoa lose some substance essential for the fertilization 

 reaction. 



The following are the objections to the first alternative: (a) 

 Microscopical examination lends it no support; I have repeatedly 

 observed, that fertilizing power of sperm cannot be expressed 

 either in terms of motility, or of success in penetrating the jelly 

 of the egg and coming in contact with the membrane. In the 

 experiments on successive half dilutions (p. 238) I kept records, 

 in several series, of the numbers of spermatozoa in the jelly of 

 unfertilized eggs, and found in some cases an average of 9 



