A Detailed Analysis of X-Ray-Induced Prophase Delay and Reversion of Grasshopper Neuroblasts in Culture

Effects of x-ray exposures of 8, 16, 32, 64, 128, 512, and 1024 R on mitosis in hanging-drop preparations were determined by repeated observations of selected prophase cells before and at short intervals after irradiation. Prophase was subdivided into eight microscopically distinguishable stages: very-early, early, initial-middle, intermediate-middle, terminal-middle, initial-late, intermediate-late, and terminal-late. With one exception, cells treated at intermediate- or terminal-late prophase with any of the doses were neither delayed nor reverted. After 8 to 16 R nearly all cells from early to initial-late prophase are delayed in reaching prometaphase, but none reverts. After 32 to 64 R all cells in early to initial-late prophase are delayed and most revert; after 128 to 1024 R nearly all revert. Delay or reversion is preceded by a postirradiation inertia, during which the cell progresses mitotically at an almost normal rate for a short time. Duration of inertia is inversely related to dose. The degree of reversion depends on the exposure; at 128 R and greater, all cells that revert go back to early prophase. The relations of the G₁, S, and G₂ periods of the mammalian cell to the morphologically defined stages of the cell cycle of the acridian neuroblast are considered.