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N-nitroso-N-methylurea, a chemical carcinogen was injected in the abdomen of Periplaneta americana at
an effective dose of 100 μg/g body weight on alternate days. The insects were vivisected after 4, 8, 12, 16
and 20 days of treatment. Histological changes were observed in midgut. The columnar cells were more
or less normal in shape but their nuclei were clumped. The regenerative nidi were also showing
degenerated chromatin material. The gut epithelium was distorted in shape and their nuclei were
comparatively very small and pycnotic. The cytoplasm was degenerated and weak in construction and the
circular muscle layer was thickened. Most of the cells of the gut epithelium had lost their normal
architecture and some epithelial cells were showing additional growth of cells towards the gut lumen due
to the proliferation of the cells. Longitudinal muscles were distorted in shape, vacuolated and obliterated.
Keywords: Periplaneta americana, Midgut, N-nitroso-N-methylurea.