INFLUENCE OF HYDROGEN ION CONCENTRATION. 339 



tinctly opalescent suspension ; yet the block did not shift beyond 

 the limits pH 6.9 to 7.1. In general, increasing the quantity of 

 sperm used in insemination increases the proportion of eggs ferti- 

 lized in the acid solutions, but the shift to the acid side is not so 

 great as would be expected if the failure to fertilize in the 

 acid solutions were attributable to an impairment of the sperm. 

 Rather the slight magnitude of this shift favors the belief that 

 the block is due to an alteration of the properties of the egg. 



It may be stated here that unless the sperm are injured or at- 

 tenuated by the toxic action of egg secretions, all eggs which 

 fertilize in the pH solutions develop normally, indicating that the 

 fertilization reaction when once initiated in the neighborhood of 

 the block, is completed without impairment. 



THE ACID BLOCK IN Chcstopterus. 



The determination of the acid block to fertilization in Chcutop- 

 terus was made in the same manner as in Astcnas and Arbacia. 

 The egg sacks were cut in sea water and the eggs liberated from 

 the ovaries by teasing these to pieces. The ovary fragments were 

 removed by straining through cheese cloth, and when the eggs had 

 maturated they were concentrated by centrifuging. A drop of 

 the concentrated egg suspension was added to 50 cc. of the pH 

 solution; after 5 minutes the sperm, previously diluted with the 

 pH solutions, were added and the mixture agitated. The per cent, 

 of fertilized eggs was determined by counting the dividing eggs 

 one and a half to four hours after insemination. 



The scarcity of material made it impossible to get more than a 

 half dozen determinations ; of these, two were discarded since only 

 a small proportion of the eggs were fertilized in sea water. The 

 remaining four indicated that the block appeared between pH 7.0 

 and 7.3, and from the two most satisfactory experiments the block 

 was tentatively set at pH 7.1. 



The acid (pH 5.8) activation of the Chcetoptents egg, with the 

 consequent temporary block to fertilization, has been discussed in 

 a previous paper (3). This block, which is most effectually estab- 

 lished by short exposures to pH 5.2 to 6.4, was tentatively ascribed 

 to cortical changes which tend to persist after the eggs have been 

 removed from the acid solutions, and returned to sea water. It 

 is in no sense comparable to the physiological block occurring at 



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