To understand how water moves in our world, attention must be directed to the hydrologic cycle. The hydrologic cycle is essentially a water continuum, representing the different paths through which water circulates and is transformed in the natural environment (Fig. 1). Being a cycle, it has no specific beginning or ending. Rather, liquid water from the Earth s surface, particularly the oceans, is evaporated into a gaseous form and enters the atmosphere as water vapor (clouds). The atmospheric moisture is eventually returned to the Earth s surface in the form of rain or snow.
Cell differentiation. In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Different tissues have different kinds of cells.
Differentiation means that one cell performs a different function than another cell, depending on where it is in your body. The example of this is your lung cells and your brain cells. The cells in your lung do a different job than the cells in your brain.