LAEVAE OF ARBACIA PUNCTULATA 425 



first general conclusion, that equally adverse conditions do not 

 produce equally great reduction in size, and agrees with the re- 

 sults of the experiments with increased hydroxyl concentration, 

 where the injury inflicted may be ascribed to over-rapid growth. 



In Experiment 2, we have two experiments made with the same 

 control, one of dilution and one of concentration of the sea- 

 water. The specimens in the solution of 30 per cent dilution 

 (gf) and those in the solution of 120 cc. sea- water concentrated 

 to 100 (e +), lived only 96 hours, and hence we may look upon 

 them as having undergone practically the same extent of injury, 

 yet a comparison of their growth-curves reveals the widely dif- 

 ferent effects of the two modes of treatment. We may conclude, 

 therefore, that each modification of the medium produces its 

 specific effect upon growth. 



In those instances in which two constituents of the medium 

 are changed, the effects of one may completely mask the effects 

 of the other. Thus increased alkalinity tends to produce irregti- 

 larity of structure and inhibition of growth, while the carbonates, 

 although alkaline in reaction, are associated with a high degree 

 of symmetry. Moreover, in cultures in which seawater was 

 diluted with isotonic NaCl solution, dilution of the other con- 

 stituents with increase of Na, produced a size above the nor- 

 mal. But when the NaCl solutions were hypertonic, the phy- 

 sical effects of increased osmotic pressure neutralized the physio- 

 logical effects of the Na, and a growth-curve scarcely different 

 from the normal was produced. As the solutions became still 

 more hypertonic, all stimulating effect of the Na was lost, and 

 a decreased size resulted. 



In other instances, when both factors tend to produce the 

 same result, the combined effect may be greater than either one 

 alone. In the solutions of high degree of dilution, those in the 

 first experiment, such as / (30 per cent dilution) possessed a 

 much lower vitality than did those in the solution of the same 

 dilution in the second ig). 



This leads to our next general conclusion, that where the 

 control is below the average size, the experimental solutions 

 exert a more unfavorable influence. In the two experiments in 



