A large number of unicellular plants and animals, and many of the cellular units which make up the complex tissues of man and more evolved animals, exhibit certain movements. These are termed “Amœboid,” as they are well seen, and were at an early period studied in the Amœba.

If an amœba or a white blood corpuscle of the newt be examined with a good lens, the following facts, among others, may be made out. The corpuscle looks like a granular lump of jelly containing two or three nuclei, and it is, we will suppose, spherical to begin with. Soon the shape changes, for a little process is seen to protrude at one side, which may become retracted, or go on elongating. In the substance of the cell and in the processes, movements may be observed, consisting evidently of a flowing of the protoplasm, as indicated by the embedded granules which are carried along. The little processes are termed pseudopodia, and vary much in shape, some being thick and either pointed or club-shaped, others are filamentous, as in the perforated Foraminiferæ, where large numbers of these are seen radiating from the test. These movements may be automatic, and they are modified by various external forces; for example, heat and electricity.