334 



IIOMK.R \Y. SMITH AND G. II. A. CI.oWKS. 



per se influences the fertilization of eggs by sperm of the same 

 species. 



Further examination of the block to fertilization which is cre- 

 ated when the H-ion concentration of sea water is increased to a 

 critical point has convinced us of its physiological significance, 

 and we have extended our observations to include the effects of 

 increasing alkalinity on the fertilization of Arbacia and Asterias 

 eggs, and the effects of acid and alkaline sea water on the ferti- 

 lization of the eggs of Chcetopterus p erg amentaceous. 



THE ACID BLOCK TO FERTILIZATION. 



CO 2 -free sea water solutions were prepared as described in a 

 previous paper (3). Our experiments on fertilization were per- 

 formed as follows : A drop of concentrated egg suspension was 

 added to 50 or 100 cc. of each of the pH solutions, and a drop of 

 sperm suspension was added to about 5 cc. of the pH solutions. 

 After an interval of 3 to 5 minutes the sperm and eggs were 

 mixed and thoroughly agitated. (No precautions were taken to 

 remove body fluids which might be present around the eggs, other 

 than the routine washing which they were always given in prepar- 

 ing them for any experiment.) Subsequently the proportion of 

 fertilized eggs in each dish was carefully determined. It makes 

 no difference whether the counts are made 10 minutes or several 

 hours after insemination because every egg that is going to ferti- 

 lize will lift a membrane within the normal time of 3 to 5 minutes. 

 It has been our custom in performing experiments of this kind to 

 remove samples from the pH solutions 10 or 15 minutes after 

 insemination and return them to sea water with fresh sperm to 

 make sure that the eggs had not been irreversibly modified by the 

 action of the pH solutions or by contact with sperm in these solu- 

 tions. It may be said that this procedure has one invariable re- 

 sult; if the exposure is below that required for the acid to injure 

 the egg, then every egg which is not fertilized on the acid side of 

 the block will fertilize when returned to sea water with fresh 

 sperm. 



The influence of H-ion concentration on the fertilization of 

 Arbacia and Asterias is illustrated in Figs, i and 2. The solid 

 line in each figure indicates the range within which fertilization 



