4 FRANK R. LILLIE. 



ized with the sperm of purpuratus." (Loeb, 1914, p. 136.) 



(3) In his "Artificial Parthenogenesis and Fertilization" (The 

 University of Chicago Press, 1913), p. 293, Loeb gives figures 

 of plutei from purpuratus eggs fertilized by franciscanus sperm. 

 These records of the occurrence of both reciprocal fertilizations 

 give, however, no measure of the degree of specificity; in fact, 

 there is only a bare hint that the straight fertilizations occur more 

 readily than the crosses. 



If one wishes to gain a correct idea of the degree of specificity 

 in fertilization, it is necessary to control the variable factors very 

 carefully. Assuming that only perfectly ripe gametes are used, 

 as was the case in all the recorded experiments, the principal pre- 

 cautions to be observed are the following: (i) The eggs should 

 be washed in at least two changes of sea-water to get rid of tissue 

 secretions, blood or detritus ; the eggs used should be uniform 

 in this respect, and similar quantities should be used in com- 

 parable experiments. (2) The sperm should be sufficiently abun- 

 dant so that measurable quantities of the dry sperm free from 

 any fluid or foreign particles may be used as a basis for cal- 

 culating the sperm concentration in any experiment. (3) The 

 range of individual variability with reference to cross-fertiliza- 

 tion should be understood; it is at least very considerable. 



(4) The method of mixing the sperm with the eggs should be 

 as uniform as possible, or very considerable differences in per- 

 centage results may occur from this cause alone. (5) It should 

 go without saying that the utmost precautions against contamina- 

 tion must be observed : abundance of sterilized pipettes and glass- 

 ware, washing of specimens in drinking water to destroy adherent 

 spermatozoa, sterilization of hands and instruments after handling 

 any male, etc. (6) In spite of all precautions there will remain 

 a certain degree of residual variability that shows clearly that all 

 the conditions of fertilization are not yet understood. 



Sperm Concentrations. The basic sperm suspension from 

 which fertilizations were usually made is one drop (o.i c.c.) dry 

 sperm thoroughly mixed in 5 c.c. sea-water; one drop (0.07 c.c.) 

 of this suspension added to eggs in 100 c.c. sea-water will be arbi- 

 trarily selected as it nit sperm concentration. Any measured in- 

 semination may be expressed as unity or as a fraction or multiple 



