It is, therefore, no audacious hypothesis, but a perfectly sound conclusion, to regard the amœba 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 amœba has a less definite and more original character than any other cell. Moreover, there is the fact that recent research has discovered such amœba-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 amœbæ, take up solid particles, or “eat” (whence they are called _phagocytes_ = “eating-cells,” Fig. 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 amœbæ (Fig. 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 amœba-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 amœboid 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. Fig.18. Ovum of a sponge. Fig. 18—Ovum of a sponge (_Olynthus_). The ovum creeps about in a body of the sponge by thrusting out ever-changing processes. It is indistinguishable from the common amœba.) We have now, by our study of the ovum and the comparison of it with the amœba, provided a perfectly sound and most valuable foundation for both the embryology and the evolution of man. We have learned that the human ovum is a simple cell, that this ovum is not materially different from that of other mammals, and that we may infer from it the existence of a primitive unicellular ancestral form, with a substantial resemblance to the amœba.