234 RADIOACTIVITY AND POTASSIUM 



control. It was shown that death in the thorium experiments was due 

 to lack of K and not to the toxicity of ThCU, because when ThCU was 

 added to normal sea water in the same concentrations in which it was 

 used to replace KCl it in no way interfered with the formation of 

 normal larvae, owing probably tQ the fact that the Th was in suspen- 

 sion and not in true solution. The radioactivity of the ThCU was not 

 sufficient to interfere with the development of the eggs up to the 

 pluteus stage at least. 



E. Uranium Acetate Experiments. — A series of experiments with 

 the radioactive element uranium also gave negative results; i.e., in 

 no case were swimming larvae obtained where uranium acetate was 

 used to replace potassium and the eggs died before the sixteen cell 

 stage was reached. The concentrations of uranium acetate used in 

 the K-free sea water were m/125,000, m/62,500, m/42,000, m/32,000, 

 m/25,000, m/16,000, m/12,500, m/11,000, m/6,800, m/4,700, m/3,000, 

 and m/2,100. The pH in these solutions was brought to about 7.6 

 through the addition of NaHCOa and in the two highest concentra- 

 tions some uranium was visibly precipitated. The lack of develop- 

 ment of the eggs here was not due to the toxicity of the uranium salt 

 because, when the uranium acetate was added to sea water in the 

 same concentrations, normal larvae developed except in the two 

 highest concentrations and here the pH was found to be below 7. 

 The uranium as well as the thorium was probably in suspension in 

 the sea water and not in true solution but this, of course, did not influ- 

 ence the radioactivity. It is of interest to note that, here and in the 

 thorium experiments, the radioactive elements did not inhibit the 

 action of K as Zwaardemaker seems to assume. 



III. Theoretical Remarks. 



It appears from the foregoing experiments that in the development 

 of the fertilized egg of Arhacia up to the blastula-gastrula stage potas- 

 sium can be replaced by the non-radioactive Cs ion, the minimum con- 

 centration required for development being practically the same for 

 both cations (KCl minimum concentration = m/660, CsCl minimum 

 concentration = m/500). Furthermore, it is apparent that K cannot 

 be replaced in our experiments by suspensions of the radioactive 

 elements thorium and uranium. Hence, the action of potassium in the 



