STUDIES OF FERTILIZATION. 



2. Variability of Different Individual Cross Fertilisations. 



(a) Purpuratus Eggs Fertilised by Franciscanus Sperm. For 

 the purpose of bringing out the individual variability of cross 

 combinations we shall consider first the fertilizations made with 

 sperm of 10 units concentration (Table I) ; temperature approxi- 

 mately 15 C. Each entry stands for a separate female. 



TABLE I. 



VARIATION OF FERTILITY OF PURPURATUS EGGS FERTILIZED WITH 10 UNITS 



FRANCISCANUS SPERM. 



The variations of fertility in this cross, from o to 84 per cent, 

 of cleavage, is exceeding striking and shows a great range to 

 exist on the female side (cf. especially Nos. 2-7 where one fran- 

 ciscanus male was used). There is a probable considerable 

 variation also on the male side as appears when the group 2-7 

 fertilized with one franciscanus male is compared with the group 

 8-1 1 fertilized with another franciscanus male. But no system- 

 atic attempt was made to evaluate this side, as the franciscanus 

 material was relatively rare and difficult to obtain. It should also 

 be noted that such high percentages as Nos. 6 and 7 were never 

 approximated in any other combination (43 in all recorded and 

 many others tried). 



To correct the impression that this cross-fertilization is very 

 easy, which might readily be given by the exceptional percentages 

 in Nos. 2-7, the following record of the fertilization of the eggs 



