422 



J. GRAY 



that th(' t'ggs of Arbacia punctulata can be activated 

 by means of highly active minute Infusoria. It is, therefore, 

 the degree of activity of the sperm which determines one condi- 

 tion of fertihzation and not its structure or chemical constitu- 

 tion. 



Whereas the normal activity of a spermatozoon is usually 

 adequate for the activation of eggs of the same species, yet it 

 requires to be increased to an abnormal degree to fertilize the 

 eggs of another species. Now the activity of spermatozoa can 

 readily be controlled by the hydrogen-ion concentration of the 

 medium ((iray, 9), and correspondingly it is found that the 

 addition of hydroxyl ions removes the block which normally 

 exists between the eggs of Strongylocentrotus and the 

 sperm of Sphaerechinus. These and similar facts are 

 difficult to explain on the basis that fertilization depends on 

 the existence of specific chemical substances in the egg or sperm 

 (see Loeb, p. 204). They appear to the writer to be less formid- 

 able when subjected to physical arguments. According to the 

 present physical hypothesis, if it be mechanically possible for 

 the sperm of one species to gain contact with the egg of another, 

 then activation will take place if the E.M.F. set up between the 

 two cells reaches a certain critical value within a minimum 

 time. Consider two species, A and B. When normal fertilization 

 of egg A is effected by sperm A, let the E.M.F. be Ei and let 

 it be developed in unit time ; similarly when egg B is fertilized 

 by sperm B let the E.M.F. at contact be E2. Let Ei > E2 ; 

 then when egg B comes into contact with sperm A, the E.M.F. 

 will probably be more than enough to activate the egg ; when 

 egg A comes into contact with sperm B, no activation will 

 occur unless the activity of the sperm is artificially increased, 

 since otherwise the requisite E.M.F. will not be reached. Such 

 irreciprocal hybridizations have already been mentioned and 

 are by no means uncommon [see Vernon (28), Doncaster (5)]. 

 Further, the conditions which affect the ease ^vith which 

 hybridization can occur are such as support the view that 

 some such physical factors are involved, e. g. seasonal variation 

 of gametes, degree of maturity, staleness or freshness of gametes, 



i 



