54 J. GRAY. 



remained rather closely applied to the egg-surface ; in such cases the 

 resistance of the fertilized eggs was, as in other cases, markedly lower 

 than that of the unfertilized eggs. It was also found that eggs from 

 which the fertilization membrane had been forcibly removed developed 

 quite normally. Now Loeb has shown that the fertilization membrane 

 is permeable to electrolytes, and I therefore conclude from the above 

 facts that the presence of a membrane round the eggs is equivalent to a 

 similar quantity of sea- water, and that when the eggs are reduced 

 to the same volume as before fertilization, the distance between the eggs 

 is identical. 



The fact that after the initial fall in resistance of the eggs after fertiliza- 

 tion there is a definite and well-marked rise in resistance, eliminates the 

 suggestion that the changes observed are due to the presence of a fertiliza- 

 tion membrane. Again, in a few cases (i.e. in experiments dealing with 

 high resistances) a considerable number of the membranes are removed 

 from the egg whilst determining the resistance of the fertilized eggs, so 

 that when sea-water is added the membranes are found floating in it. 

 If the esfffs are not allowed to settle at once tliev are found still to have a 

 resistance equal to the value obtained by the first determination after 

 fertilization. 



4. That the observed differences are due to the existence of free sper- 

 matozoa between the eggs. This is not the case, because (1) the addition 

 of as much sperm as was used in any of these experiments does not alter 

 the conductivity of a bulk of water equal to that of the eggs ; (2) the 

 conductivity of a concentrated emulsion of sperm is considerably lower 

 than that of pure sea-water. 



(Note. — McClendon states that by repeatedly washing the unfertilized 

 eggs of Arhacia he was able to prevent the formation of a fertilization 

 membrane when the sperm entered the egg. I have never observed this 

 in the eggs of Echinus, but prolonged washing tends to prevent the 

 membranes being pushed out to their normal extent.) 



