POLAR IONIC CURRENTS AROUND EMBRYOS OF LYMNAEA-STAGNALIS DURING GASTRULATION AND ORGANOGENESIS

Embryos of Lymnaea stagnalis generate ionic currents which can be meas ured with the vibrating probe. Here we investigated the presence and o rigin of the currents during late embryonic development. During gastru lation the current pattern correlates with the animal-vegetal polarity and during organogenesis it is correlated to the newly formed antero- posterior axis. The origin of the ionic currents was studied by inhibi tion of the Na+/K+-pump with ouabain and by enzyme-cytochemical detect ion of the Ca2+-pump. Ouabain treatment resulted in a reduced current density around the embryo, indicating that the Na+/K+-pump contributes significantly to the net current. The Ca2+-pump was found to be local ized in the vegetal blastomeres during gastrulation and in the larval kidney during organogenesis. It seems likely that this Ca2+-pump rende rs only a minor contribution to the net current in late embryonic deve lopment. Ionic currents have now been described in Lymnaea from the un cleaved egg up to the juvenile snail. During this period the overall c urrent pattern changes only twice, demonstrating that the voltage grad ient generated by the embryo remains stable during prolonged periods i n development.