OTHER ORGANS The cell-substance is often differentiated into other more or less definite structures, sometimes of a transitory character, sometimes showing a constancy and morphological persistency comparable with that of the nucleus and centrosome. From a general point of view the most interesting of these are the bodies known as plastids or protoplasts (Fig. 5), which, like the nucleus and centrosome, are capable of growth and division, and may thus be handed on from cell to cell. The most important of these are the chromatophores or chromoplasts, which are especially characteristic of plants, though they occur in some animals as well. These are definite bodies, varying greatly in form and size, which never arise spontaneously, so far as known, but always by the division of pre-existing bodies of the same kind. They possess in some cases a high degree of morphological independence, and may even live for a time after removal from the remaining cellsubstance, as in the case of the "yellow cells " of Radiolaria. This has led to the view, advocated by Brandt and others, that the chlorophyll-bodies found in the cells of many Protozoa and a few Metazoa (Hydra, Spongilla, some Planarians) are in reality distinct Algae living symbiotically in the cell. This view is probably correct in some cases, e.g. in the Radiolaria ; but it may well be doubted whether it is of general application. In the plants the chlorophyllbodies and other chromoplasts are almost certainly to be regarded as differentiations of the cytoplasmic substance. The same is true of the amyloplasts, which act as centres for the formation of starch.
The contractile or pulsating vacuoles that occur in most Protozoa and in the swarm-spores of many Algae are also known in some cases to multiply by division ; and the same is true, according to the researches of De Vries, Went, and others, of the non-pulsating vacuoles of plant-cells. These vacuoles have been shown to have, in many cases, distinct walls, and they are regarded by De Vries as a special form of plastid ("tonoplasts ") analogous to the chromatophores and other plastids. It is, however, probable that this view is only applicable to certain forms of vacuoles.
The existence of cell-organs which have the power of independent assimilation, growth, and division, is a fact of great theoretical interest in its bearing on the general problem of cell-organization ; for it is one of the main reasons that, have led De Vries, Wiesner, and many others to regard the entire cell as made up of elementary selfpropagating units.