88 PIGMENT CELLS. 



miglit be altogether insufficient to effect the slightest 

 motion in the thicker basal portion connected with 

 the surface of the cell, and in very close relation 

 with the bioplasm. In this way we may explain the 

 fact of movement ceasing first at the base of the 

 cilium without resorting to the hypothesis of gradual 

 death from base to apex in a structure which is pro- 

 bably not alive, and the movements of which are not 

 vital movements, but rather the consequence of vital 

 changes in living matter immediately adjacent to it. 

 Cilia are indeed modifications of formed material, — of 

 tissue or of cell wall. 



131. Pigment Cells. — In various parts of the frog, 

 newt, and otlier batrachia are numerous very large 

 and branched cells containing fluid, in which are sus- 

 pended multitudes of very minute particles of pig- 

 ment formed from the bioplasm of the cell which is 

 situated in the central part, and is usually obscured 

 by the quantity of pigment present. As in other 

 cases, currents of fluid set to and from this mass of 

 living forming bioplasm. The radiating tubular pro- 

 longations of the cells communicate with one another, 

 and are sometimes filled with fluid having the pigment 

 granules evenly diffused through it, while sometimes 

 the minute dark granules become aggregated round 

 the bioplasm in the centre of the cell, and the radii 

 are destitute of them. The movement of the visible 

 particles of pigment results from the movements of 

 the invisible fluid, which at one time fills the cavities 

 and tubular prolongations of the cells, and at another 

 permeates the delicate walls, and becomes diffused 

 into the surrounding textures. When the tissues are 

 saturated with fluid, the radii also contain it, and the 

 pigment spreads into the tubular network, but in the 

 opposite condition the particles become aggregated in 

 the centre of the cell, and the walls of the tubular 

 processes approach one another. The opposite con- 

 ditions of saturation of the tissues with fluid and its 



