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.
Thus, cell differentiation is an essential and ongoing process at all stages of life. The visible differentiation of cells is only the last of a progressive sequence of states. In each state, the cell becomes increasingly committed toward one type of cell into which it can develop.