Hence, although the amoeba is nothing but a simple cell, it is evidently able to accomplish all the functions of the multicellular organism. It moves, feels, nourishes itself, and reproduces. Some kinds of these amoebae can be seen with the naked eye, but most of them are microscopically small. It is for the following reasons that we regard the amoebae as the unicellular organisms which have special phylogenetic (or evolutionary) relations to the ovum. In many of the lower animals the ovum retains its original naked form until fertilisation, develops no membranes, and is then often indistinguishable from the ordinary amoeba. Like the amoebae, these naked ova may thrust out processes, and move about as travelling cells. In the sponges these mobile ova move about freely in the maternal body like independent amoebae (Figure 1.17). They had been observed by earlier scientists, but described as foreign bodies--namely, parasitic amoebae, living parasitically on the body of the sponge. Later, however, it was discovered that they were not parasites, but the ova of the sponge. We also find this remarkable phenomenon among other animals, such as the graceful, bell-shaped zoophytes, which we call polyps and medusae. Their ova remain naked cells, which thrust out amoeboid projections, nourish themselves, and move about. When they have been fertilised, the multicellular organism is formed from them by repeated segmentation. It is, therefore, no audacious hypothesis, but a perfectly sound 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.).