INFLUENCE OF HYDROGEN ION CONCENTRATION. 331 



Such comparisons have shown that some retardation of develop- 

 ment occurs even at pH 7.6 and 8.5, and that the effect of increas- 

 ing acidity or alkalinity does not take the form of an abrupt inhibi- 

 tion at any point, but manifests itself in almost imperceptible 

 gradations from normal development to no development at all. 

 In the acid solutions the inhibition culminates in coagulation with 

 little division ; and in alkaline solutions in either complete cytolysis 

 or in the formation of formless, ciliated masses of protoplasm 

 swimming within the fertilization membrane. It is doubtful if 

 normal development can be obtained throughout a period of 24 

 hours in solutions more acid than pH 7.8, or more alkaline than 



8.4- 



SUMMARY. 



The effect of acid and alkaline sea water on the rate of cell divi- 

 sion in normally fertilized Arbacia and Asterias eggs was ob- 

 served as far as the 128-cell stage. 



In Arbacia, the velocity of division is reduced to 50 per cent, of 

 the velocity in sea water (pH 8.15) at pH 5.2 and 9.4. Between 

 pH 5.8 and 8.2 these eggs divide normally both in respect to ve- 

 locity and quality of cell division. Between pH 8.2 and 9.2 the 

 velocity of division is increased from 15 to 25 per cent. 



Asterias eggs are more sensitive to both acid and alkaline sea 

 water during the precleavage period than at any subsequent time. 

 When these eggs are transferred to the acid and alkaline sea 

 water immediately after fertilization, the velocity of division is 

 reduced to 50 per cent, at pH 5.6 and 9.2; when transferred in 

 the two cell stage the corresponding limits are pH 5.4 and 10.2. 

 There is a slight decrease in the mean velocity of division between 

 pH 8.2 and 5.8, but no significant increase in solutions more 

 alkaline than sea water. 



In both species, when the developmental velocity is reduced be- 

 low 50 per cent, by either acid or alkali, the nucleus tends to di- 

 vide without division of the cytoplasm, and abnormal multi- 

 nuclear cells are formed. 



We are indebted to Mabel T. Studebaker for the statistical work 

 in the experiments recorded in this paper. 



