what and where
Seeing it happen,
and naming the parts
Early cell biologists did not have many tools with which to examine the very complex process of cell division. They used fixatives to prevent the cells from moving or breaking down, colored dyes to stain and bring contrast to parts of the cell which looked similar, and visualized the results using simple light microscopes.
As a result they missed a lot of events modern cell and molecular biologists now understand and consider important. However, one major product of the work of these early scientists remains with us still; the names they invented for the structures they were seeing, and the phases of cell division they observed.
The starting material
Not all cells are dividing all the time. Early cell biologists quickly began to recognize that they could see three different and distinct populations of cells in different tissues and in different circumstances.
these are cells which are constantly growing, dividing and growing again in a more or less constant cycling pattern. Such cells would be found in certain embryos, the tips of plant stems and roots, and in tissues were there is a need for constant replacement - such as blood or skin.
these are cells in suspended animation. They are alive, performing certain internal maintenance functions, but held in a stage of the cell cycle often called Go, (pronounced, "G zero"). They await certain environmental stimulation, when they spring back to life and start dividing. A good example would be skin cells that repair wounds.
these are cells which have grown and divided for many generations and then stop. Usually they stop dividing so that they can take on specialized forms and functions, such as muscle cells in animals and xylem cells in plants. Normally, once fully differentiated into their specialized form they never divide again.