The distribution of sodium and potassium in amphibian embryos during early development.

1. Intracellular and intercellular concentrations of sodium and potassium have been measured in pregastrular embryos of Xenopus laevis and Amblystoma mexicanum. Calcium and magnesium contents have also been determined.2. Between egg and gastrula stages of development the potassium concentration is near to 60 m-mole/l. embryo. Up to the blastula stage the sodium concentration is near to 50 m-mole/l. embryo; the amount of sodium in the embryo begins to fall just before gastrulation.3. Embryo calcium and magnesium concentrations show no significant variations prior to gastrulation. Average calcium concentrations for the different stages range from 5 to 8 m-mole/l. embryo; magnesium concentrations lie between 12.4 and 17 m-mole/l. embryo.4. The intercellular fluid contains 100 mM sodium and 1 mM potassium; the majority of the sodium is ionically active. As no net uptake of sodium or potassium occurs before gastrulation these cations must have been transferred from the cells to the cavity.5. At all developmental stages studied, the intracellular potassium concentration is close to 100 m-mole/l. cell water. Intracellular sodium falls steadily from 80 m-mole/l. cell water in eggs to 30 m-mole/l. cell water at the beginning of gastrulation.6. The intracellular sodium activity, measured with sodium sensitive intracellular micro-electrodes, is relatively constant between egg and blastula stages at about 14 mM. During each cell division cycle the intracellular sodium activity rises transiently by 2-3 mM.