OTHER ORGANS 53 



leucoplastids in the embryonic cells. By enlargement and differen- 

 tiation these give rise to the starch-builders (amyloplastids), to the 

 chlorophyll-bodies (chloroplastids), and to other pigment-bodies 

 (chromoplastids), all of which may retain the power of division. The 

 embryonic leucoplastids are also believed to multiply by division and 

 to arise by the division of plastids in the parental organism ; but it 

 remains an open question whether this is their only mode of origin, 

 and the same is true of the more highly differentiated forms of plas- 

 tids to which they may give rise. 



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 plastids 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 dis- 

 tinct Algae living symbiotically in the cell. This view is probably 

 correct in some cases, e.g. in the Radiolaria ; but it may be doubted 

 whether it is of general application. In the plants the plastids are 

 almost certainly to be regarded as differentiations of the protoplasmic 

 substance. 



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-propa- 

 gating units. 



G. THE CELL-MEMBRANE 



The structure and origin of the cell-wall or membrane form a 

 subject somewhat apart from our general purpose, since the wall 

 belongs to the passive or metaplasmic products of protoplasm rather 

 than to the living cell itself. We shall therefore treat it very briefly. 

 Broadly speaking, animal cells are in general characterized by the 

 slight development and relative unimportance of the cell-walls, while 



