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1. Electrical connexions between mesoderm cells have been examined during the formation of somites in Xenopus laevis, Bombina orientalis and Ambystoma mexicanum. 2. In Xenopus the resting potentials of presumptive myotome cells (-65 + 2 mV, S.E. of mean) and somite muscle cells (-65 +/- 0-6 mV S.E. of mean) were 40 mV, greater than dermatome cells (-25 +/- 0-6 mV, S.E. of mean). Similar differences were found in Bombina and Ambystoma. 3. In all three species cells of the dermatome layer of the mesoderm were electrically coupled to each other. Cells of the presumptive myotome layer in the unsegmented region of the mesoderm were also electrically coupled. 4. In Xenopus dermatome and myotome layers of the mesoderm were not electrically coupled to each other either before or after somite formation. In the other two species dermatome and myotome layers were uncoupled once the somites had formed. 5. In all three species the position of the intersomite border in the unsegmented mesoderm region is marked by the breaking of electrical contracts between those cells destined to form the next somite and the rest of the unsegmented mesoderm. 6. In the axolotl each somite remains electrically insulated from its neighbour. In Xenopus and Bombina electrical connexions are re-established between somite muscle cells once the morphogenetic movements underlying somite formation are complete. 7. Presumptive myotome cells in Xenopus and Ambystoma acquire the membrane property of inward-going rectification before incorporation into a somite. 8. Once Xenopus and Bombina embryos show spontaneous movements large end-plate potentials are recorded from the head somites. Excitation spreads from somite to somite along the low resistance intercellular pathway allowing simultaneous contraction of several somites before extensive somite innervation. 9. The structure of developing somite muscle of Xenopus has been studied with the electron microscope. 10. Close membrane contacts of the gap junction type have been seen between undifferentiated presumptive myotome cells, muscle cells in the same somite and between muscle cells in adjacent somites. 11. Myofilament organization first begins in mesoderm cells when they are forming a new somite. Complete sarcomeres appear in the head somites when the embryo begins spontaneous flexion movements.

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