The electrokinetic potential of fertilized sea-urchin eggs, without the fertilization membrane and hyaline layer, was investigated by measuring the electrophoretic mobility of the eggs from fertilization to the second cleavage. A cyclic change in mobility was found t o accompany the division cycle: the peak of the change was observed about 15 min before the appearance of both the first and second cleavage furrows.A smaller peak was observed at 20-30 min after fertilization, but such a peak was not repeated between the first and the second cleavage.Fertilized eggs with the fertilization membrane intact did not show a significant change in electrophoretic mobility throughout the division cycle.Several workers have measured the electrophoretic mobility of various cells during the division cycle. MAYHEW (1966) found that human sarcoma cells have a higher electrophoretic mobility during mitosis than at other phases in the mitotic cycle. BRENT and FORRESTER ( 1 967) observed a similar increase in mobility during mitosis in HeLa cells. On the contrary, SHANK and BURKI (1971) found no change in the electrophoretic mobility of synchronized mouse lymphoblasts throughout the division cycle. Similarly, DAN'S classical data (1933) indicate that sea-urchin eggs do not show any change in mobility from fertilization to the first cleavage. It should be noted, however, that the apparent electrophoretic mobility of fertilized sea-urchin eggs may be due, not only to the protoplasmic surface potential, but also to the surface potential of the fertilization membrane. In this paper, measurements were made on fertilized sea-urchin eggs without the fertilization membrane in order to study the protoplasmic surface potential itself.
MATERIALS AND METHODSEggs of the sea-urchins, Anthocidaris crassispina, Pseudocentrotus depressus and Hemicentrotus pulcherrimus, were used.The eggs were deprived of the fertilization membrane by 1 M urea and thereafter kept in calcium-free artificial sea water in order t o prevent the formation of a hyaline layer.The microelectrophoresis chamber, after MORt (1968), is shown in Fig.1. The chamber is made of glass with rectangular cross-section (26 mm X 0.9 mm). Both ends are closed by blocks 19
