Changes in the plasma membrane complex following the injury of single cells of Amoeba proteus were examined with the electron microscope. Two types of injury were employed in this study; cells were either pinched ("cut") in half or speared with a glass microneedle, and quickly fixed. Speared cells, when fixed in the presence of the ruthenium violet (a derivative of ruthenium red), revealed the presence of an extra trilaminar structure outside of each cell. This structure, called the "new membrane," was separated from the plasma membrane complex by a distance of less than a micron. The trilaminar structure of the new membrane strikingly resembled the image of the plasma membrane in all cells examined, except for its increased width (30%). This new membrane appeared nearly to surround the injured amebae. Attempts were made to demonstrate the possible origin of the new membrane, its reality, and its sensitivity to calcium. Also, some evidence is shown concerning the role of the small dense droplets (100–1200 A in diameter) normally present in the cytoplasm of amebae. Their frequent contact with the plasma membrane of the cell as the result of injury is interpreted as indicating their involvement in the formation and expansion of the plasma membrane.

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