Studies were made with time-lapse motion pictures of the reactions of cells in culture to changes in their environment. The concentrations of H+, HCO3-and CO2 in the medium were altered in such a way that each, in turn, could be maintained constant while the others were varied. Observations were made on the shape of the cells, their activity, and their relation to the substratum. Characteristic reversible changes in the cells were observed whenever environmental pH was altered. Elevation of the pH accelerated cell movements and caused contraction of the cytoplasm, while lowering of the pH retarded and eventually stopped all cell activity, causing apparent gelation of the protoplasm. These responses did not occur when HCO3- and CO2 were varied without changing the pH. It is suggested that local pH changes in the micro-environment of a cell's surface may be a significant factor in controlling cell behavior in culture and in vivo.

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