8 FRANK R. LILLIE. 



of five purpuratus with the sperm of one franciscanus at 13.2 

 units will be useful: percentages segmented o.i per cent., 0.3 per 

 cent., O.6 per cent., 1.6 per cent., 6 per cent. 



It should be realized that sperm concentrations of 10 and more 

 units are very much higher than would ordinarily be used in 

 straight fertilizations, where I unit concentration is adequate to 

 fertilize every egg. 



(&) Franciscanus Eggs Fertilised by purpuratus Sperm. The 

 determinations for this cross are not nearly so numerous as for 

 the reciprocal on account of the greater rarity of the material in 

 its best condition. The eggs appear to be not quite so variable 

 in their cross-fertilization capacity as those of purpuratus, but 

 with a lower concentration of sperm they commonly show a 

 higher percentage of cleavage after cross-fertilization than cross- 

 fertilized purpuratus eggs; on the other hand, I have never 

 secured so high a percentage with higher sperm concentrations 

 as in exceptional lots of purpuratus eggs. Moreover, the cross- 

 fertilized franciscanus eggs appear to be less viable than the 

 cross-fertilized purpuratus eggs. These matters will be dealt 

 with beyond. The data given in Table IV. throw some light on 

 the question of variability. 



3. The Effect of Sperm Concentration. 



In the case of both species a higher concentration of sperm is 

 required for any degree of cross fertilization than for practically 

 perfect straight fertilization (see Table IV.). Beyond such 

 minimum concentration, the result of increased sperm concen- 

 tration is to increase the percentage of eggs fertilized up to a cer- 

 tain point; however, such increase is by no means proportional 

 to the sperm concentration and sometimes it is very strikingly 

 absent. 



(a) 5. purpuratus 5 X S. franciscanus <$. Though the higher 

 percentages of fertilization were never secured with sperm con- 

 centration below about 6 units, yet it must be said that the factor 

 of individual variability of combinations is much more important 

 than that of sperm concentration. An isolated illustration will 



