The whole life of these homogeneous globules of plasm consists of simple growth and reproduction by cleavage. When the tiny particle has reached a certain size by the continuous assimilation of inorganic matter, it divides into two equal halves, by a constriction in the middle. The two daughter-monera that are thus formed immediately begin a similar vital process. It is the same with the brown _Procytella primordialis_ (formerly called the _Protococcus marinus_); it forms large masses of floating matter in the arctic seas. The tiny plasma-globules of this species are of a greenish-brown colour, and have a diameter of 1/10,000 to 1/5000 of an inch. There is no membrane discoverable in the simplest _Chroococcacea,_ but we find one in other members of the same family; in _Aphanocapsa_ (Fig. 227) the enveloping membranes of the social plastids combine; in _Glœcapsa_ they are retained through several generations, so that the little plasma-globules are enfolded in many layers of membrane. 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).