230 RADIOACTIVITY AND POTASSIUM 



and certain radioactive substances might help in deciding definitely 

 whether or not the physiological influence of potassium is dependent 

 on its radioactive properties. 



II. EXPERIMENTAL. 



The method used in this work was as follows. Eggs of Arbacia were 

 fertihzed in potassium-free artificial sea water of the following compo- 

 sition: 100 cc. of m/2 NaCl + 7.8 cc. of m/2 MgClz + 3.8 cc. of m/2 

 MgS04 + 1.75 cc. of m/2 CaCl2. To this solution enough m/10 

 NaHCOa (usually 0.2 cc. per 25 cc. of solution) was added to bring the 

 pH to between 7.4 and 8.0. The chemicals used were Kahlbaum's 

 purest preparations. Both glass- and metal-distilled water were 

 used in preparing the solutions. After fertilization, the eggs were 

 washed three times in similar K-free artificial sea water and were 

 then placed in various solutions to observe development. In all 

 experiments, some of the fertilized eggs were placed in (a) normal 

 sea water, and (b) alkaline K-free sea water for control. In no case 

 where eggs were placed in K-free sea water did the development go 

 beyond the sixteen cell stage, and in a few hours the eggs were disin- 

 tegrated. All sea water controls developed into normal plutei. 



The criterion used in these experiments for the adequacy of the 

 substitutes for potassium was the formation of normal swimming 

 blastulae within 24 hours. In reality development went beyond this 

 stage in most instances. 



A. Potassium Chloride Experiments. — Eggs were fertiHzed and 

 washed as described, and were then placed in various dishes all con- 

 taining 25 cc. of K-free sea water to which was added enough KCl to 

 make the total concentration of KCl in the dishes m/4,600, m/2, 300, 

 m/1,300, m/850, m/660, m/550, m/470, m/370, m/330, etc., up to m/18, 

 the latter concentration of KCl being about 5.5 times that present in 

 normal sea water. The pH in these and succeeding experiments was 

 about 7.4. 



From Table I we see that fertilized eggs do not form swimming 

 larvae in artificial sea water with a concentration of KCl lower than 

 m/660. We also see that a certain excess of potassium, i.e. 5.5 times 

 its normal concentration in sea water, does not interfere with develop- 

 ment up to the gastrula stage. 



