Just about anywhere you look in the natural sciences you'll find a cycle of some description. From cells to individuals to populations and beyond, cyclical patterns exist on every scale. The following collection of Web sites follows on this theme: The first site (1) is an excellent, animated introduction to the cell cycle from Cells Alive! Users can also get a closer look at the stages of mitosis...

Because Spring has officially arrived, this week's Topic In Depth focuses on events that occur with the arrival of the new season. Offered by the Howard Hughes Medical Institute, the first Web site (1) focuses on seasonal events like hibernation and migration, and the rhythms behind them. Next is a phenology site (2), where visitors can register to record their observations of Spring online or...