Hence, although the amœba 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 amœbæ can be seen with the naked eye, but most of them are microscopically small. It is for the following reasons that we regard the amœbæ 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 amœba. Like the amœbæ, 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 amœbæ (Fig. 17). They had been observed by earlier scientists, but described as foreign bodies—namely, parasitic amœbæ, 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 medusæ. Their ova remain naked cells, which thrust out amœboid 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 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 (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.
The statement that the earliest progenitors of the human race were simple cells of this kind, and led an independent unicellular life like the amœba, has not only been ridiculed as the dream of a natural philosopher, but also been violently censured in theological journals as “shameful and immoral.” But, as I observed in my essay On the Origin and Ancestral Tree of the Human Race in 1870, this offended piety must equally protest against the “shameful and immoral” fact that each human individual is developed from a simple ovum, and that this human ovum is indistinguishable from those of the other mammals, and in its earliest stage is like a naked amœba. We can show this to be a fact any day with the microscope, and it is little use to close one’s eyes to “immoral” facts of this kind. It is as indisputable as the momentous conclusions we draw from it and as the vertebrate character of man (see Chap. XI).
