THE PHYSIOLOGY OF FERTILIZATION 175 



The action of the heterologous plasma may be sup- 

 posed to be due to a definite membrane effect enabling 

 interaction between sperm and egg substances. Its 

 interest in the present connection is to show that the 

 inhibiting effect of the species plasma cannot be due 

 merely to colloid content, which is common to both 

 kinds of plasma, but that it has something specific 

 in it. 



Does the inhibitor act on the egg or on the sperm 

 or by intervening in the reaction between the two ? 

 Experiments undertaken to answer this question showed 

 that both eggs and sperm exposed to the plasma and 

 washed free of it again were fertilizable. The plasma 

 therefore merely interferes in some way with the reac- 

 tion. The great variability of the effect of different 

 samples of plasma ranging all the way from o to 100 per 

 cent also shows that we are not deahng merely with 

 a general colloid effect. 



It cannot be supposed that the plasma operates by 

 preventing the adhesion of the spermatozoon to the egg 

 if this is brought about by agglutination, because it 

 was found that the agglutination of spermatozoa by 

 means of egg secretions takes place as readily in the 

 plasma as in sea-water. 



There are two other possibilities: (i) the plasma 

 might be supposed to harden the membrane or change 

 its permeable character; or (2) it might be supposed 

 to inhibit the action of the activable substance (ferti- 

 lizin) of the egg by deviation effect. If the latter were 

 true it should then be possible to prevent the inhibitory 

 action of the plasma by first saturating it with the 

 activable substance. This was found to be true. If a 



