INFLUENCE OF HYDROGEN ION CONCENTRATION. 337 



acid, none of the eggs fertilize. We have previously shown that 

 the unfertilized eggs of these species are uninjured by short ex- 

 posures to the solutions in which fertilization does not occur (3) ; 

 and that normally fertilized eggs will develop with normal ve- 

 locity at H-ion concentrations much greater than those at which 

 the block to fertilization occurs (4). 



The constancy of this acid block to fertilization is very marked. 

 To illustrate this a few experiments have been given below. 

 Table I. contains a resume of experiments performed in 1921, 

 1922 and 1923 on the fertilization of Asterias and Arbacia eggs. 

 Despite the probability of variable conditions in these experiments, 

 the point at which 50 per cent, of the eggs fertilized remained 

 constant to 0.2 pH. 



There are at least two factors which might be expected to shift 

 the block one direction or another ; first, the length of time which 

 the eggs or sperm have remained in the acid solution, and second, 

 the relative quantity of sperm used for insemination. Examina- 

 tion of the first factor has shown that the equilibrium between 

 the pH solution and the egg (or sperm) is reached with astonish- 

 ing rapidity. This can be illustrated by first adding the sperm to 

 the pH solution and then adding to the resulting sperm suspension 

 a drop of eggs suspended in sea water. Under these conditions 

 one would expect that the time required for the egg cortex to 

 come to chemical equilibrium with the pH solution would be long 

 enough to permit many more eggs to be reached by sperm and 

 fertilized than would be the case if the eggs were allowed to 

 come to equilibrium with the solution before adding the sperm. 

 The results of experiments of this kind with Asterias eggs are 

 given in Table II. Converse experiments were simultaneously 

 performed ; the eggs were added to the pH solution first and after 

 5 minutes a drop of comparatively concentrated sperm suspended 

 in sea water was added to these eggs. When the experiment is 

 performed as first described, the block appears at the same pH as 

 when both eggs and sperm are at equilibrium with the pH solu- 

 tions before insemination. When the experiment is reversed, the 

 block is shifted slightly towards the alkaline side. This indicates 

 that the essential equilibrium underlying the block involves the egg 

 cortex rather than the sperm. The difference is hardly great 



