Next to the Chromacea come the Bacteria, which have been evolved from them by the remarkable change in nutrition which gives us the simple explanation of the differentiation of plant and animal in the protist kingdom. The Chromacea build up their plasm directly from inorganic matter; the Bacteria feed on organic matter. Hence, if we logically divide the protist kingdom into plasma-forming Protophyta and plasma-consuming Protozoa, we must class the Bacteria with the latter; it is quite illogical to describe them—as is still often done—as Schizomycetes, and class them with the true fungi. The Bacteria, like the Chromacea, have no nucleus. As is well-known, they play an important part in modern biology as the causes of fermentation and putrefaction, and of tuberculosis, typhus, cholera, and other infectious diseases, and as parasites, etc. But we cannot linger now to deal with these very interesting features; the Bacteria have no relation to man’s genealogical tree.
We may now turn to consider the remarkable Protamœba, or unnucleated Amœba. I have, in the first volume, pointed out the great importance of the ordinary Amœba in connection with several weighty questions of general biology. The tiny Protamœbæ, which are found both in fresh and salt water, have the same unshapely form and irregular movements of their simple naked body as the real Amœbæ; but they differ from them very materially in having no nucleus in their cell-body. The short, blunt, finger-like processes that are thrust out at the surface of the creeping Protamœba serve for getting food as well as for locomotion. They multiply by simple cleavage (Fig. 228).
The next stage to the simple cytode-forms of the Monera in the genealogy of mankind (and all other animals) is the simple cell, or the most rudimentary form of the cell which we find living independently to-day as the Amœba. The earliest process of inorganic differentiation in the structureless body of the Monera led to its division into two different substances—the caryoplasm and the cytoplasm. The caryoplasm is the inner and firmer part of the cell, the substance of the nucleus. The cytoplasm is the outer and softer part, the substance of the body of the cell. By this important differentiation of the plasson into nucleus and cell-body, the organised cell was evolved from the structureless cytode, the nucleated from the unnucleated plastid. That the first cells to appear on the earth were formed from the Monera by such a differentiation seems to us the only possible view in the present condition of science. We have a direct instance of this earliest process of differentiation to-day in the ontogeny of many of the lower Protists (such as the Gregarinæ).
Fig. 228—A moneron (Protamœba) in the act of reproduction. A The whole moneron, moving like an ordinary amœba by thrusting out changeable processes. B It divides into two halves by a constriction in the middle. C The two halves separate, and each becomes an independent individual. (Highly magnified.)
The unicellular form that we have in the ovum has already been described as the reproduction of a corresponding unicellular stem-form, and to this we have ascribed the organisation of an Amœba (cf. Chapter VI). The irregular-shaped Amœba, which we find living independently to-day in our fresh and salt water, is the least definite and the most primitive of all the unicellular Protozoa (Fig. 16). As the unripe ova (the protova that we find in the ovaries of animals) cannot be distinguished from the common Amœbæ, we must regard the Amœba as the primitive form that is reproduced in the embryonic stage of the amœboid ovum to-day, in accordance with the biogenetic law. I have already pointed out, in proof of the striking resemblance of the two cells, that the ova of many of the sponges were formerly regarded as parasitic Amœbæ (Figure 1.18). Large unicellular organisms like the Amœbæ were found creeping about inside the body of the sponge, and were thought to be parasites. It was afterwards discovered that they were really the ova of the sponge from which the embryos were developed. As a matter of fact, these sponge-ova are so much like many of the Amœbæ in size, shape, the character of their nucleus, and movement of the pseudopodia, that it is impossible to distinguish them without knowing their subsequent development.
Our phylogenetic interpretation of the ovum, and the reduction of it to some ancient amœboid ancestral form, supply the answer to the old problem: “Which was first, the egg or the chick?” We can now give a very plain answer to this riddle, with which our opponents have often tried to drive us into a corner. The egg came a long time before the chick. We do not mean, of course, that the egg existed from the first as a bird’s egg, but as an indifferent amœboid cell of the simplest character. The egg lived for thousands of years as an independent unicellular organism, the Amœba. The egg, in the modern physiological sense of the word, did not make its appearance until the descendants of the unicellular Protozoon had developed into multicellular animals, and these had undergone sexual differentiation. Even then the egg was first a gastræa-egg, then a platode-egg, then a vermalia-egg, and chordonia-egg; later still acrania-egg, then fish-egg, amphibia-egg, reptile-egg, and finally bird’s egg. The bird’s egg we have experience of daily is a highly complicated historical product, the result of countless hereditary processes that have taken place in the course of millions of years.
The earliest ancestors of our race were simple Protophyta, and from these our protozoic ancestors were developed afterwards. From the morphological point of view both the vegetal and the animal Protists were simple organisms, individualities of the first order, or plastids. All our later ancestors are complex organisms, or individualities of a higher order—social aggregations of a plurality of cells. The earliest of these, the Moræada, which represent the third stage in our genealogy, are very simple associations of homogeneous, indifferent cells—undifferentiated colonies of social Amœbæ or Infusoria. To understand the nature and origin of these protozoa-colonies we need only follow step by step the first embryonic products of the stem-cell. In all the Metazoa the first embryonic process is the repeated cleavage of the stem-cell, or first segmentation-cell (Fig. 229). We have already fully considered this process, and found that all the different forms of it may be reduced to one type, the original equal or primordial segmentation (cf. Chapter VIII). In the genealogical tree of the Vertebrates this palingenetic form of segmentation has been preserved in the Amphioxus alone, all the other Vertebrates having cenogenetically modified forms of cleavage. In any case, the latter were developed from the former, and so the segmentation of the ovum in the Amphioxus has a great interest for us (cf. Fig. 38). The outcome of this repeated cleavage is the formation of a round cluster of cells, composed of homogeneous, indifferent cells of the simplest character (Fig. 230). This is called the morula (= mulberry-embryo) on account of its resemblance to a mulberry or blackberry.