IO FRANK R. LILLIE. 



gave 0.3 per cent., 0.6 per cent., 1.3 per cent., and 4 per cent, 

 cleavage respectively. Two samples of the eggs of another 

 female fertilized with i and 2 units of the sperm of another pur- 

 puratus gave in each case 5 per cent, cleavage. In a third case 

 two samples of the eggs of one female fertilized with 8 and 32 

 units of purpuratus sperm gave about 6 and 5 per cent, mem- 

 branes respectively; and in a fourth case three samples of the 

 eggs of one female fertilized with 10, 20 and 40 units purpuratus 

 sperm gave 30 per cent., 5o per cent., and 65 per cent, cleavage. 



4. The Measure of Specificity. 



From the preceding data it will be seen that the measure of 

 specificity varies greatly with the individuals used, and this is 

 a fact that must be born in mind in considering the following 

 data. But however much variation there may be in the crosses, 

 they contrast forcibly with the straight fertilizations whether 

 they are considered individually or collectively. 



The experiments tabulated below (Table IV.) were set up as 

 follows : the usual precautions were observed for sterilization of 

 animals, instruments, hands, glassware ; each dish had a separate 

 sterilized pipette used for it alone. The eggs of one female of 

 each species were always washed at least twice, and an unfertil- 

 ized control of each lot was kept, which never showed any mem- 

 branes or segmented eggs. One hundred c.c. of sea water was 

 placed in the bowls and about 1.5 c.c. of eggs was transferred to 

 each, and the bowls then covered with glass plates. When the 

 eggs were all ready for fertilization, the sperm was prepared in 

 another room, and brought in and added as rapidly as possible ; 

 one lot of each kind of sperm thus always served for all the 

 fertilizations of a given experiment. My assistant removed the 

 cover, stirred in the sperm as added and replaced the cover before 

 proceeding to the next bowl. In each experiment in the table 

 only one male and one female of each species was used; straight 

 fertilizations with the same male and female thus serve for 

 control of the condition of the material used, and also for esti- 

 mates as to the degree of specificity. 



