Fluctuation in the intracellular concentration of Na⁺ and Cl^- but not of K⁺ or Mg²⁺ at mitosis of the first cell cycle in fertilized sea urchin eggs

This report concerns changes in the cytoplasmic concentration of Na⁺, Cl^-, K⁺ and Mg²⁺ during the first cell cycle and into the second cell cycle of fertilized sea urchin eggs of Lytechinus variegatus. The results of electron probe x-ray microanalysis showed a significant equimolar decrease in Na⁺ and Cl^- at the first cell division which was followed by a significant increase early in the second cell cycle. Neither K⁺ nor Mg²⁺ showed such significant changes. The loss of a large fraction of the Na⁺ ions from the egg at metaphase was not accompanied by a significant egg volume change. The observed pattern of ionic change was related to several previously reported chemical and metabolic changes at cell division in sea urchin eggs.