I ascribe this special significance to the chromacea among all the monera I have instanced because I take them to be the oldest phyletically, and the most primitive of all living organisms known to us. In particular their very simple forms correspond exactly to all the theoretic claims which monistic biology can make as to the transition from the inorganic to the organic. Of the chroococcacea, the chroococcus, glœocapsa, etc., are found throughout the world; they form thin, usually bluish-green coats or jelly-like deposits on damp rocks, stones, bark of trees, etc. When a small piece of this jelly is examined carefully under a powerful microscope, nothing is seen but thousands of tiny blue-green globules of plasma, distributed irregularly in the common structureless mass. In some species we can detect a thin structureless membrane enclosing the homogeneous particle of plasm; its origin can be explained on purely physical principles by "superficial energy"--like the firmer surface-layer of a drop of rain, or of a globule of oil swimming in water. Other species secrete homogeneous jelly-like envelopes--a purely chemical process. In some of the chromacea the blue-green coloring matter (_phyocyan_) is stored in the surface-layer of the particle of plasm, while the inner part is colorless--a sort of "central body." However, the latter is by no means a real, chemically and morphologically distinct, nucleus. Such a thing is completely lacking. The whole life of these simple, motionless globules of plasm is confined to their metabolism (or _plasmodomism_, chapter x.) and the resulting growth. When the latter passes a certain stage, the homogeneous globule splits into two halves (like a drop of quicksilver when it falls). This simplest form of reproduction is shared by the chromacea (and the cognate bacteria) with the chromatella or chromatophora, the green particles of chlorophyll inside ordinary plant-cells; but these are only parts of a cell. Hence no unprejudiced observer can compare these unnucleated and independent granules of plasm with real (nucleated) cells, but must conceive them rather as _cytodes_. These anatomic and physiological facts may easily be observed in the chromacea, which are found everywhere. The organism of the simplest chromacea is really nothing more than a structureless globular particle of plasm; we cannot discover in them any composition of different organs (or organella) for definite vital functions. Such a composition or organization would have no meaning in this case, since the sole vital purpose of these plasma-particles is self-maintenance. This is attained in the simplest fashion for the individual by metabolism; for the species it is effected by self-cleavage, the simplest conceivable form of reproduction.