Changes in the potassium content of sea urchin eggs on fertilization.  In the eggs of Arbacia lixula and Paracentrotus lividus an uptake of K occurs during the first 10 minutes following fertilization. Between 10 and 40 minutes K is then released. Both in Arbacia and in Paracentrotus the minimum point of the curve coincides with the nuclear streak stage. A maximum loss of 25 per cent in Arbacia and 20 per cent in Paracentrotus with respect to the amount present in the unfertilized eggs has been found. From 40 minutes up to 1 hour K undergoes a further increase and when the first cleavage sets in the same amount of K is present as in the unfertilized eggs. By treating the eggs with K-free artificial sea water it has been established that about 60 per cent of the K content of the eggs is in a non-diffusible condition. Also under such conditions the eggs when fertilized are able to take up even the very small amount of K present in the medium that was released by them prior to fertilization.