OTHER ORGANS 37 



F. 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 proto- 

 plasts (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 cell- 

 substance, 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 chlorophyll- 

 bodies 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 vacu- 

 oles 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 appli- 

 cable 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 self- 

 propagating units. 



