The nuclear envelopes of amoebae were damaged microsurgically, and the fate of the lesions was studied with the electron microscope. Amoebae were placed on the surface of an agar-coated slide. Using a glass probe, the nucleus was pushed from an amoeba, damaged with a chopping motion of the probe, and reinserted into the amoeba. Cells were prepared for electron microscopy at intervals of between 10 min and 4 days after the manipulation. Nuclear envelopes studied between 10 min and 1 h after the injury displayed extensive damage, including numerous holes in the nuclear membranes. Beginning 15 min after the manipulation, pieces of rough endoplasmic reticulum intruded into the holes in the nuclear membranes. These pieces of rough endoplasmic reticulum subsequently appeared to become connected to the nuclear membranes at the margins of the holes. By 1 day following the injury, many cells had died, but the nuclear membranes were intact in those cells that survived. The elaborate fibrous lamina or honeycomb layer characteristic of the amoeba nuclear envelope was resistant to early changes after the manipulation. Patches of disorganization of the fibrous lamina were present 5 h to 1 day after injury, but the altered parts showed evidence of progress toward a return to normal configuration by 4 days after the injury. It is proposed that the rough endoplasmic reticulum participates in the repair of injury to the nuclear membranes. The similarity of this repair process to reconstitution of the nuclear envelope in telophase of mitosis is noted, and the relationship between the nuclear envelope and the rough endoplasmic reticulum is discussed.

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