A simple method of whole-mount radioautography is devised to investigate aspects of the cell cycle during metamorphosis in the epidermis of the milkweed bug, Oncopeltus. Tritiated thymidine is used to indicate DNA synthesis. As the label only lasts in the insect for about 2 h, a wave of labelled cells passes through the different phases of the cell cycle. The S period is found to overlap with an exceptionally long prophase and there is thus no G2 period. The length of prophase (408±10 min) is estimated from a plot of the fraction of labelled prophases against time after injection of label. By an equivalent method the length of the S period is found to be 289±12 min. No labelled cells divide again until about 24 h after the previous mitosis, when some cells embark on a second mitosis. The minimum interphase (G1 period is approximately 16 h. In the area studied, the cell number more than doubles during the proliferative mitoses; and it is thus possible, but not certain, that every cell divides at least once.

Fifth-stage larvae injected during the differentiative divisions (which are involved in the development of dense hairs) show that each of the three kinds of differentiative divisions has its own peculiar timing. The timing of the very first division, that of the epidermal cell which will become the hair mother cell, suggests that the cell is already different from its progenitors prior to prophase.

Department of Genetics, Milton Road, Cambridge.