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.] Up to the present it has been assumed that the environment of the amoeba is that to which it has become adapted and which is favorable to its existence. Under these conditions its structure conforms to the type of the species, as do also the phenomena which it exhibits, and it can assimilate food, grow and multiply. If, during the observation, a small crystal of salt be placed in the fluid, changes almost instantly take place. Motion ceases, the amoebae appear to shrink into smaller compass, and they become more granular and opaque. If they remain a sufficiently long time in this fluid, they do not regain their usual condition when placed again in fresh water. None of the phenomena which characterized the living amoebae appear: we say they are dead. After a time they begin to disintegrate, and the bacteria contained in the water and on which the amoebae fed now invade their tissue and assist in the disintegration. By varying the duration of the exposure to the salt water or the amount of salt added, a point can be reached where some, but not all, of the amoebae are destroyed. Whether few or many survive depends upon the degree of injury produced. Much the same phenomena can be produced by gradually heating the water in which the amoebae are contained. It is even possible gradually to accustom such small organisms to an environment which would destroy them if suddenly subjected to it, but in the process of adaptation many individuals will have perished.