There are some organisms lower down in the scale, whose whole activity is confined within the narrow limits of a single cell.  Thus, the amoeba begins its life as a cell split off from its parent.  This divides in its turn, and each half is a complete amoeba.  When we come a little higher than the amoeba, we find organisms which consist of several cells, and a specialization of function begins to appear.  As we ascend in the animal scale, specialization of structure and of function is found continually advancing, and the various kinds of cells are grouped together into colonies or organs.

12.  Cells and the Human Organism.  If the body be studied in its development, it is found to originate from a single mass of nucleated protoplasm, a single cell with a nucleus and nucleolus.  From this original cell, by growth and development, the body, with all its various tissues, is built up.  Many fully formed organs, like the liver, consist chiefly of cells.  Again, the cells are modified to form fibers, such as tendon, muscle, and nerve.  Later on, we shall see the white blood corpuscles exhibit all the characters of the amoeba (Fig. 2).  Even such dense structures as bone, cartilage, and the teeth are formed from cells.

[Illustration:  Fig. 2.—­Amoeboid Movement of a Human White Blood Corpuscle. (Showing various phases of movement.)]

In short, cells may be regarded as the histological units of animal structures; by the combination, association, and modification of these the body is built up.  Of the real nature of the changes going on within the living protoplasm, the process of building up lifeless material into living structures, and the process of breaking down by which waste is produced, we know absolutely nothing.  Could we learn that, perhaps we should know the secret of life.

13.  Kinds of Cells.  Cells vary greatly in size, some of the smallest being only 1/3500 an inch or less in diameter.  They also vary greatly in form, as may be seen in Figs. 3 and 5.  The typical cell is usually globular in form, other shapes being the result of pressure or of similar modifying influences.  The globular, as well as the large, flat cells, are well shown in a drop of saliva.  Then there are the columnar cells, found in various parts of the intestines, in which they are closely arranged side by side.  These cells sometimes have on the free surface delicate prolongations called cilia.  Under the microscope they resemble a wave, as when the wind blows over a field of grain (Fig. 5).  There are besides cells known as spindle, stellate, squamous or pavement, and various other names suggested by their shapes.  Cells are also described as to their contents.  Thus fat and pigment cells are alluded to in succeeding sections.  Again, they may be described as to their functions or location or the tissue in which they are found, as epithelial cells, blood cells (corpuscles, Figs. 2 and 66), nerve cells (Fig. 4), and connective-tissue cells.