The generation of action potentials in nerve and muscle requires cell membranes with steeply voltage-dependent ionic permeabilities. The voltage-dependent, ion-selective pathways responsible for excitation have been characterized for numerous excitable tissues such as nerve axon, muscle, electric organ, algae and epithelia (Aidley, 1971; Hodgkin, 1964; Cole, 1968). The process of activating ionic pathways by some stimulus, such as a change in membrane potential, is called gating.

Potassium conductance-voltage curves have been determined for a squid axon in high external potassium solution for a wide range of divalent cation concentrations. A decrease in divalent ion concentration shifts the conductance-voltage curve along the voltage axis in the direction of more hyperpolarized voltages by as much as 9 mv for an e-fold change in concentration. When the divalent ion concentration is less than about 5 mM, a further decrease does not cause a significant shift of the conductance-voltage curve. These results can be explained by assuming that on the outer surface of the membrane there is a negative fixed charge which can bind calcium ions, and that the axon is sensitive to the resulting double-layer potential. From our data, the best value for charge density was found to be one electronic charge per 120 square angstroms, and a lower limit to be one electronic charge per 280 square angstroms.

Embryogenesis of the free-living soil nematodeCaenorhabditis elegans produces a juvenile having about 550 cells at hatching. We have determined the lineages of 182 cells by tracing the divisions of individual cells in living embryos. An invariant pattern of cleavage divisions of the egg generates a set of stem cells. These stem cells are the founders of six stem cell lineages. Each lineage has its own clock-i.e., an autonomous rhythm of synchronous cell divisions. The rhythms are maintained in spite of extensive cellular rearrangement. The rate and the orientation of the cell divisions of the cell lineages are essentially invariant among individuals. Thus, the destiny of cells seems to depend primarily on their lineage history. The anterior position of the site of origin of the stem cells in the egg relates to the rate of the cell cycle clock, suggesting intracellular preprogramming of the uncleaved egg. We used a technique that allows normal embryogenesis, from the fertilized egg to hatching, outside the parent under a cover glass. The cell lineages reported here were analyzed from tapes of two eggs recording the complete development until the animals started moving inside the egg shell. In addition, embryogenesis was recorded and analyzed in 18 other individuals from fertilization to the 30-(10 cases), the 54-, 60-, 75-, and 87-(3 cases), and the 100-(2 cases) cell stages, respectively. We also analyzed the E-cell lineage in six more individuals to make certain that all 8 E-cells divide in going to 16. After recording was terminated, it was ascertained that, under the cover glass, all 26 animals hatched and moved normally.

When sufficiently small amounts of excitability-inducing material (EIM) arc added to a bimolccular lipid membrane, the conductance is limited to a few discrete levels and changes abruptly from one level to another. From our study of these fluctuations, we have concluded that the EIM-dopcd bilaycr contains ion-conducting channels capable of undergoing transitions between two states of different conductance. The difference in current between the "open" and "closed" states is directly proportional to the applied membrane potential, and corresponds to a conductance of about 3 X 10 -1° ohm -1. The fraction of the total number of channels that is open varies from unity to zero as a function of potential. The voltage-dependent opening and closing of channels explains the negative resistance observed for bimolecular lipid membranes treated with greater amounts of EIM.