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.]

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