STUDIES OF FERTILIZATION. 13 



5. Fertilisation with Mixtures of Eggs and Sperm. 



Loeb has stated that if mixed eggs of the two species are 

 fertilized separately with the two kinds of sperm the eggs of the 

 same species as the sperm form their membranes more quickly 

 than those of the other species. This would appear to be a 

 promising method of measuring specificity provided that normally 

 the membrane reaction is equally rapid in the two species. There 

 is, however, a considerable difference in this respect : the eggs of 

 purpuratus form membranes about 30 seconds after straight in- 

 semination, whereas the larger eggs of franciscanus require about 

 90 seconds (at about 17 C.)- Thus when mixed eggs of the 

 two species are fertilized with purpuratus sperm, the purpuratus 

 membranes naturally form first ; when the franciscanus sperm is 

 used the question is difficult to answer decisively, because the pur- 

 puratus eggs that fertilize are usually so few that it is difficult to 

 find them in the time available. My impression is that a small 

 percentage forms membranes in about the usual time, but others 

 more slowly so that after the franciscanus membranes are all 

 formed some purpuratus membranes still arise. In this respect, 

 as in others, there is great variability in the crosses. 



The question involved would be more simply answered by 

 timing the formation of membranes in straight and cross fertiliza- 

 tions. The latter determinations were not made systematically, 

 but in those cases in which I have records of membrane forma- 

 tion of crosses taken some minutes apart, I find the percentages 

 higher in later than in earlier determinations. Thus it would 

 appear that while in a straight insemination membranes appear to 

 form in most of the eggs close together in time, in cross insemina- 

 tions membrane formation is spread over a longer period of time. 



The above observations suggested the fertilization of mixed 

 eggs not only with the sperm of each species separately but also 

 with mixed sperm of the two species in order to determine 

 whether there was any antagonism between the sperm of the two 

 species. A rather elaborate experiment of this kind showed con- 

 clusively that no antagonism exists, such as is found sometimes 

 between sperm suspensions of different phyla. Thus a mixture 



