conclusion, to regard the amoeba as the particular unicellular
organism which offers us an approximate illustration of the ancient
common unicellular ancestor of all the metazoa, or multicellular
animals. The simple naked amoeba has a less definite and more original
character than any other cell. Moreover, there is the fact that recent
research has discovered such amoeba-like cells everywhere in the
mature body of the multicellular animals. They are found, for
instance, in the human blood, side by side with the red corpuscles, as
colourless blood-cells; and it is the same with all the vertebrates.
They are also found in many of the invertebrates--for instance, in the
blood of the snail. I showed, in 1859, that these colourless
blood-cells can, like the independent amoebae, take up solid
particles, or "eat" (whence they are called phagocytes =
"eating-cells," Figure 1.19). Lately, it has been discovered that many
different cells may, if they have room enough, execute the same
movements, creeping about and eating. They behave just like amoebae
(Figure 1.12). It has also been shown that these "travelling-cells,"
or planocytes, play an important part in man's physiology and
pathology (as means of transport for food, infectious matter,
bacteria, etc.).

The power of the naked cell to execute these characteristic
amoeba-like movements comes from the contractility (or automatic
mobility) of its protoplasm. This seems to be a universal property of
young cells. When they are not enclosed by a firm membrane, or
confined in a "cellular prison," they can always accomplish these
amoeboid movements. This is true of the naked ova as well as of any
other naked cells, of the "travelling-cells," of various kinds in
connective tissue, lymph-cells, mucus-cells, etc.