6 FRANK R. LILLIE. 



which is far from being absolute, would appear to indicate that as 

 a rule the franciscanus egg is understimulated by the purpuratus 

 spermatozoon, and this may aid to explain their lesser viability as 

 compared with the reciprocal cross. 



When high sperm concentrations are used (5 units and above) 

 the jelly of the crossed franciscanus eggs becomes permanently 

 packed with purpuratus spermatozoa, thus producing the appear- 

 ance of halos varying in intensity with the concentration of the 

 sperm. Such halos do not appear in the reciprocal cross, nor yet 

 in either of the straight fertilizations. The difference in the 

 physical characters of the jelly in the two kinds of egg does not 

 explain this difference, for the franciscanus sperm does not form 

 comparable halos in franciscanus eggs. It would appear to be an 

 effect of some secretion contained in the franciscanus jelly on the 

 purpuratus sperm; but the franciscanus egg-water has no appar- 

 ent effect on the purpuratus sperm. 



We shall consider first the variability of different individual 

 combinations in the two crosses separately, classified again by 

 the sperm concentration ; secondly, we .shall consider the effect 

 of various sperm concentrations on the percentages of fertiliza- 

 tion in the eggs of one female ; third, we shall tabulate the com- 

 plete experiments in which both crosses and both controls were 

 run simultaneously. The first table will give the measure of 

 variability of different individual cross combinations. The 

 second will give the effects of sperm concentrations for given 

 combinations. The third will give the measure of specificity, 

 which cannot be evaluated without the other two. 



The percentages of fertilization may be measured either by the 

 percentage of membranes formed, or by the percentage of eggs 

 that actually segment. The latter determination is much more 

 accurate, for membrane formation is apt to be defective in the 

 cross-fertilized eggs, or even absent, more especially where fran- 

 ciscanus eggs are used. In some cases, both determinations were 

 made, and the difference gives approximately the percentage of 

 membraneless eggs that segment. Eggs that form membranes 

 sometimes fail to segment, though this is relatively rare ; it ap- 

 pears especially in the lower sperm concentrations. 



