IMMUNITY AND SUSCEPTIBILITY OF SEA-URCHIN EGGS. 235 



in acting on the cytoplasm the effect would be a repulsion of its 

 particles and an increase in pressure, the same as that of ether. 

 Accordingly it would a priori be probable that the period of least 

 immunity to HC1 would coincide with that of the greatest nor- 

 mal pressure, viz., just before cleavage. 



EXPERIMENTAL. 



Two preliminary experiments with seventeen different strengths 

 showed that the proper strength of solution, for an exposure of 

 one half hour, was between a and --i- normal HC1 soluion. 



Accordingly a ^ ^ n solution was next tried with the following 



results : 



EXPERIMENT IV. 



August 3, 1/450 n HC1 solution (by diluting with sea water); three series, 



2 5 / > 3 O/ an< i 35' exposure. 



August 5, Experiment V., -^-^ n HC1 solution, used every five 

 minutes for one half hour. The results are plotted on the next 

 page, the times after fertilization being the abscissae, the per- 

 centage of swimmers the ordinates, and are in general confirma- 

 tory of the previous experiments with HC1. 



Two other experiments were made with HC1, which gave a 

 general confirmation of the two described. These all agree in 

 giving a rise in immunity soon after fertilization, then a fall at 



1 The difference between A 1 , B 1 , and C 1 may be due to the effect of the necessary 

 manipulation on the eggs transferred so soon after fertilization, viz. A 1 ; B 1 and C 1 

 were necessarily transferred a little later. Cf. Mathevs, American Jour-nal of Physi- 

 ology, VIII. , IV., "Impcrterce cf RUcr.rr.ical Shcckon Protoplasmic Activity." 



