[Illustration:  Fig. 1.—­Diagram of cell. 1.  Cell membrane. 2.  Cell substance or cytoplasm. 3.  Nucleus. 4.  Nuclear membrane. 5.  Nucleolus.]

A simple conception of health and disease can be arrived at by the study of these conditions in a unicellular animal directly under a microscope, the animal being placed on a glass slide.  For this purpose a small organism called “Amoeba” (Fig. 2), which is commonly present in freshwater ponds, may be used.  This appears as a small mass, seemingly of gelatinous consistency with a clear outline, the exterior part homogeneous, the interior granular.  The nucleus, which is seen with difficulty, appears as a small vesicle in the interior.  Many amoebae show also in the interior a small clear space, the contractile vesicle which alternately contracts and expands, through which action the movement of the intracellular fluid is facilitated and waste products removed.  The interior granules often change their position, showing that there is motion within the mass.  The amoeba slowly moves along the surface of the glass by the extension of blunt processes formed from the clear outer portion which adhere to the surface and into which the interior granular mass flows.  This movement does not take place by chance, but in definite directions, and may be influenced.  The amoeba will move towards certain substances which may be placed in the fluid around it and away from others.  In the water in which the amoebae live there are usually other organisms, particularly bacteria, on which they feed.  When such a bacterium comes in contact with an amoeba, it is taken into its body by becoming enclosed in processes which the amoeba sends out.  The enclosed organism then lies in a small clear space in the amoeba, surrounded by fluid which has been shown to differ in its chemical reaction from the general fluid of the interior.  This clear space, which may form at any point in the body, corresponds to a stomach in a higher animal and the fluid within it to the digestive fluid or gastric juice.  After a time the enclosed organism disappears, it has undergone solution and is assimilated; that is, the substances of which its body was composed have been broken up, the molecules rearranged, and a part has been converted into the substance of the amoeba.  If minute insoluble substances, such as particles of carmine, are placed in the water, these may also be taken up by the amoeba; but they undergo no change, and after a time they are cast out.  Under the microscope only the gross vital phenomena, motion of the mass, motion within the mass, the reception and disintegration of food particles, and the discharge of inert substances can be observed.  The varied and active chemical changes which are taking place cannot be observed.

[Illustration:  Fig. 2.—­Amoeba. 1.  Nucleus. 2.  Contractile vesicle. 3.  Nutritive vacuole containing a bacillus.]